COMPRESSIVE STRENGTH 
 OF CLAY BRICK WALLS 
 
 BY 
 
 A. H. STANG, Engineer 
 D. E. PARSONS, Associate Civil Engineer 
 J. W. McBURNEY. Research Associate 
 Bureau of Standards 
 
 Research Paper No. 108 
 
 Reprint from Bureau of Standards Journal of Research 
 
 VoL 3 , October. 1929 
 
RP-108 
 
 COMPRESSIVE STRENGTH OF CLAY BRICK WALLS 
 
 (:£■(? i / By A. H. Stang, D. E. Parsons, and J. W. McBurney 
 
 ABSTRACT 
 
 Compressive tests of 168 walls of common brick, each 6 feet long and about 
 9 feet high and of 129 wallettes, about 18 inches long and 34 inches high, were 
 made. Four kinds of brick, 3 mortar mixtures, 2 grades of workmanship, 
 different curing conditions, and 10 different types of masonry (3 solid and 7 
 
 hollow) were the variables. 
 
 ** _ 
 
 Wall strengths were more closely related to the shearing strength of the single 
 brick than to any other strength property of the brick. On the average, the com¬ 
 pressive strength of the wallettes was by far a better measure of the strength of 
 the walls than any of the brick strength values. The use of cement mortar gave 
 higher w T all strengths than of cement-lime mortar and much higher than if lime 
 mortar was used. For the solid walls the strength varied about as the cube root 
 of the compressive strength of the mortar cylinders, 2-inch diameter and 4-inch 
 length, cured on the walls. Large differences in strength due to differences in 
 workmanship were found. The walls having smoothed-off spread-mortar beds 
 and filled joints were much stronger than walls in which the horizontal mortar 
 beds were furrowed by the mason’s trowel. Some of this difference in strength 
 might be ascribed to difference in the filling of the vertical joints. Some of the 
 walls laid in cement mortar were kept damp for seven days after construction. 
 These walls were not stronger than similar walls cured under ordinary conditions 
 in the laboratory. 
 
 The solid walls were stronger than the hollow types. With bricks of rectangu¬ 
 lar cross section the hollow wall strengths varied about as the net areas in com¬ 
 pression. When the bricks were not truly rectangular in section, the strength 
 of the hollow walls was found to be less than that expected from the net area. 
 
 0 Construction data are given which show the relative saving in materials and time 
 
 f. for the hollow types. The results of the wallette tests confirm, in general, the 
 
 . conclusions deduced from the wall tests. 
 
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 CONTENTS 
 
 A 
 
 g I. Introduction_ 
 
 Ja II. Scope and purpose of the tests_ 
 
 STII. Description of specimens and the testing methods 
 
 1. Bricks_ 
 
 (а) Description of the bricks_ 
 
 (б) Test methods_ 
 
 (1) Compressive tests_ 
 
 (2) Transverse tests_ 
 
 (3) Tensile tests_ 
 
 (4) Shear tests_ 
 
 (5) Absorption tests_ 
 
 A 
 
 v 
 
 
 2. Mortar_ 
 
 3. Walls_ 
 
 (а) Types 
 
 (б) Size_- 
 
 727687 
 
 Page 
 
 508 
 
 509 
 511 
 511 
 
 511 
 
 512 
 
 512 
 
 513 
 513 
 513 
 
 513 
 
 514 
 
 515 
 515 
 517 
 
 507 
 
508 Bureau of Standards Journal of Research [Voi. s 
 
 III. Description of specimens and the testing methods—Continued. 
 
 3. Walls—Continued. p age 
 
 (c) Workmanship_ 518 
 
 (1) Series 1_ 518 
 
 (2) Series 2_ 518 
 
 ( d ) Method of construction_ 519 
 
 (e) Construction data_ 520 
 
 (1) Rate of building_ 520 
 
 (2) Materials used_ 523 
 
 (3) Comparative construction data for different 
 
 types of walls_ 523 
 
 (4) Thickness of mortar joints_ 525 
 
 (/) Aging conditions_ 525 
 
 (1) Schedule of building_ 526 
 
 (2) The laboratory_ 526 
 
 (3) Damp-cured walls_ 526 
 
 (g) Age- 526 
 
 (h) Testing machine_ 526 
 
 (i) Method of testing_ 528 
 
 4. Wallettes_ 528 
 
 IV. Results of the tests with discussion_ 528 
 
 1. Brick_ 528 
 
 2. Mortar_ 531 
 
 3. Waffs___ 532 
 
 (а) Basis of computations_ 532 
 
 (б) Deformation of the waffs_ 532 
 
 (1) Stress-strain curves_ 532 
 
 (2) Secant modulus of elasticity_ 534 
 
 (3) Permanent set_ 539 
 
 (c) Behavior of the waffs under load_ 539 
 
 ( d ) Compressive strength_ 540 
 
 (e) The comparative strengths of solid and of hollow 
 
 walls of brick_ 540 
 
 (/) The effect of mortar on the strength of walls other¬ 
 wise similar_ 545 
 
 ( g ) The effect of damp curing on the compressive strength 
 
 of brick walls_ 548 
 
 (/i) The effect of the quality of workmanship on wall 
 
 strength_ 551 
 
 ( i ) The relation of stress at first crack to the maximum 
 
 stress_ 555 
 
 4. Wallettes_ 556 
 
 5. Relations between the strengths of the waffs and the strengths 
 
 of the bricks and wallettes_ 563 
 
 V. Conclusions_ 568 
 
 VI. Appendix_ 569 
 
 1. Reports on workmanship_ 569 
 
 (а) Comments by J. W. Ginder_ 570 
 
 (б) Comments by A. L. Harris_ 571 
 
 I. INTRODUCTION 
 
 At a conference held at the Bureau of Standards in 1921, attended 
 by representatives of the clay, sand-lime, and cement brick industries, 
 
^Bumey om ’] Compressive Strength of Clay Brick Walls 
 
 509 
 
 it was agreed that the bureau should make tests which would afford 
 a comparison of the strength of masonry built of these three kinds of 
 brick. 
 
 Tests of walls built of sand-lime brick have been made and the 
 results reported in Technologic Paper No. 276. The present paper 
 reports and discusses the results of the tests on clay brick masonry. 
 In this report walls numbered 1 to 17 are of such construction as to 
 permit comparison with those built of sand-lime brick, but it should 
 be noted that the sand-lime brick used were of slightly higher com¬ 
 pressive strength than those used in the construction of walls 1 to 
 17, inclusive. The effect of brick strength on wall strength is dis¬ 
 cussed herein. The tests on masonry of cement brick have not yet 
 (1929) been made. 
 
 The scope of this investigation has been extended much beyond 
 the original plan, for it seemed desirable to discover, if possible, some 
 of the factors which influence or measure masonry strength. It is 
 believed that much pertinent information, not heretofore known and 
 analyzed, has been disclosed and that this information may be useful 
 to engineers and architects and to the construction industry generally. 
 
 The present construction trend is toward a more economical use of 
 material, but it is obvious that a handicap is imposed upon any 
 structural material if the “factor of uncertainty” is large. Brick 
 masonry has been under such a handicap in the past, for few tests 
 have been made on specimens which fairly represent brick walls as 
 used in construction. Tests of brick masonry made heretofore have 
 been principally of piers and small wall sections, and results have not 
 been conclusive for two reasons: (1) Uncertainty has existed as to 
 the relationship between the stress producing failure in these small 
 specimens and that in brick walls as used in structures; (2) previous 
 reports have given but little data on the effect of the various physical 
 properties of individual bricks, and especially on the effect of work¬ 
 manship on wall strength. 
 
 Realizing these conditions, the Bureau of Standards, cooperating 
 with the Common Brick Manufacturers’ Association of America, 
 undertook the investigation reported herein. 
 
 II. SCOPE AND PURPOSE OF THE TESTS 
 
 This investigation deals with the compressive tests under central 
 loading of 168 brick walls each 6 feet long and about 9 feet high and 
 of 129 wallettes or small walls each about 18 inches long and 34 inches 
 high. A view of several of the walls in the laboratory is shown in 
 Figure 1. Four kinds of common brick, three mortar mixtures, and 
 ten types of wall construction were included. Table 1 gives the 
 wall numbers for the different variables. 
 
510 
 
 Bureau of Standards Journal of Research 
 
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 1 No wallette built with these walls. 
 
fidBuFney* 071 *’] Compressive Strength of Clay Brick Walls 
 
 511 
 
 The walls are divided into two series based on the grade of work¬ 
 manship. A contract for building the walls of series 1 was let on a 
 lump-sum basis to a brick mason who specialized in small contracts • 
 and who laid the bricks for all walls in the series. The walls of series 2 
 were built under rather careful supervision by a mason who was 
 hired by the day. 
 
 It is hoped that the data obtained from these tests will be valuable 
 for the purposes listed below. 
 
 1. To determine the relation between the several physical proper¬ 
 ties of the bricks and the strength of walls built therefrom. 
 
 2. To obtain the comparative strengths of different types of solid 
 and of hollow walls of brick. 
 
 3. To determine the effect of damp curing on the compressive 
 strength of brick walls. 
 
 4. To determine the relation between the strength of the mortar 
 and the strength of the wall. 
 
 5. To determine the effect of the quality of workmanship on wall 
 strength. 
 
 6. To compare the compressive strengths of masonry specimens of 
 different sizes. 
 
 III. DESCRIPTION OF SPECIMENS AND THE TESTING 
 
 METHODS 
 
 1. BRICKS 
 
 (a) DESCRIPTION OF THE BRICKS 
 
 Four kinds of common bricks were used in building the specimens 
 for this investigation. As the principal differences between these 
 bricks are in the method of manufacture, a short description of the 
 methods of forming is here introduced. 
 
 Bricks are usually formed by the soft-mud, dry-press, or stiff-mud 
 processes. In the soft-mud process the clay is reduced with water to 
 a semiliquid consistency and formed in molds with slight pressure. 
 Usually the molds are first dampened and then sanded before use, 
 thus giving the bricks a finish which is commonly termed “sand 
 struck.” The dry-press process resembles the soft-mud process 
 except that much less water is used, resulting in a stiffer mixture, 
 and the bricks are formed in molds under considerable pressure. 
 
 In the stiff-mud process the clay mixture is extruded from an orifice 
 or die in a continuous column which is cut to form the individual 
 bricks. If the cross section of the column is approximately 8 by 3% 
 inches and the cuts are 2}{ inches apart, there results what are known 
 as “side-cut” bricks. If the column has a cross section approximately 
 2)i by 3% inches and the cuts are 8 inches apart, the term “end-cut” 
 is applied to the bricks. Sometimes double or triple dies are used on 
 
512 Bureau of Standards Journal of Research [Voi. s 
 
 one machine, but the columns are cut in the same manner as when 
 but one is used. 
 
 For convenience these bricks are designated by the name of the 
 region in which they were made and are described as follows: 
 
 Chicago .—These bricks were made from surface clay and formed by 
 the end-cut, double-column, stiff-mud process. They are rather 
 irregular in shape and contain lime nodules. 
 
 Detroit .—These bricks were formed by the soft-mud process from 
 surface clay. Like many soft-mud bricks, they were formed with a 
 frog or depression in one face approximately 0.4 inch deep. These 
 Detroit bricks resemble the soft-mud bricks from the Cleveland 
 (Ohio) region. 
 
 Mississippi .—These were surface-clay bricks formed by the dry- 
 press process. Regularity of size and shape was the outstanding 
 characteristic of these specimens. 
 
 New England .—These bricks were formed by the soft-mud process 
 from surface clay and were “sand struck.” The specimens possessed 
 a shallow frog or depression, were very hard burned, and rather 
 irregular in size and shape. 
 
 The average sizes and weights of these bricks, from measurements 
 of 50 specimens of each kind, are given in Table 2. 
 
 Table 2. —Average size and weight of the bricks 
 
 Kind of brick 
 
 Chicago 
 
 Detroit 
 
 Missis¬ 
 
 sippi 
 
 New 
 
 England 
 
 Length... ___ _inches.. 
 
 Width_ _ ... . . ..... . ..do ... 
 
 Thickness. . ._ .. .. _ _____ .do_ 
 
 Dry weight per brick_ ... .... . _ .. ...pounds.. 
 
 Weight _ .. _pounds per cubic foot.. 
 
 7. 84 
 3. 60 
 2. 21 
 3.94 
 109 
 
 8.16 
 3.60 
 2.46 
 4.16 
 99 
 
 8.20 
 3.80 
 2. 34 
 4.20 
 99 
 
 7.88 
 
 3.54 
 
 2.24 
 
 4.56 
 
 126 
 
 (b) TEST METHODS 
 
 The usual tests as outlined by the American Society for Testing 
 Materials, 1 of compressive strength, modulus of rupture, and water 
 absorption, were made on about 50 bricks of each kind. In order to 
 obtain a more complete knowledge of the properties of the brick, still 
 other tests were carried out as outlined below. 
 
 (1) Compressive Tests. —The compressive tests of half bricks on 
 edge are the only tests described in the specifications for building 
 brick of the American Society for Testing Materials. It was decided, 
 however, to include also compressive tests of whole bricks edgewise 
 and flatwise. Besides the half bricks tested flatwise and edgewise in 
 a dry condition, an equal number of half bricks were tested when 
 wet. For these tests they were prepared as for the dry tests, immersed 
 
 1 A. S. T. M. Standards, p. 665; 1924. 
 
B. S. Journal of Research, RP108 
 
 Note the mortar specimens on the walls, 
 
,v 
 
 B. S. Journal of Research, RPI08 
 
 .» 
 
 
 512—2 
 
 Figure 2.— Apparatus for tensile tests of brick 
 
f!c n Bur J n% som ’] Compressive Strength of Clay Brick Walls 513 
 
 in water at room temperature for 48 hours, taken out, and tested 
 within 10 minutes. 
 
 The compressive strength was obtained by dividing the maximum 
 load by the sectional area. 
 
 (2) Transverse Tests. —The bricks for the transverse test were 
 tested on a 7-inch span with a central load and with apparatus rec¬ 
 ommended by the American Society for Testing Materials. Since 
 in some of the walls the bricks are laid on edge or rowlock, transverse 
 tests were made on brick edgewise as well as flatwise. The modulus 
 of rupture in pounds per square inch was computed from the formula 
 
 T> _ 3 wl 
 U ~2 bd 2 
 
 in which 
 
 l is the distance between supports, 7 inches, 
 
 b is the width of the brick perpendicular to the line of load 
 application, in inches, 
 
 d is the depth of the brick parallel with the line of load application, 
 in inches, and 
 
 w is the maximum center load in pounds. 
 
 (3) Tensile Tests. —Tensile tests of the whole brick were made 
 in the apparatus 2 shown in Figure 2. The bricks for these tests 
 were dried and shellacked. The sides of the bricks were than bedded 
 in plaster of Paris to give smooth and parallel surfaces as shown in 
 this figure. The spherical bearings of the apparatus assisted in 
 producing a uniform loading over the cross section of the specimen, 
 and nearly all of the specimens broke in the free length between the 
 ends of the gripping wedges. 
 
 The tensile strength was obtained by dividing the maximum load 
 by the product of the width and depth of the brick. 
 
 (4) Shear Tests. —Punching shear tests were made with the 
 apparatus designed by H. H. Dutton and described and pictured in 
 Kessler and Sligh’s paper on “Physical Properties of the Principal 
 Commercial Limestones Used for Building Construction in the 
 United States.” 3 All tests except those on the New England brick 
 were made on halves from the tensile test. Due to the strength of the 
 New England brick, it was found necessary to use thinner test speci¬ 
 mens, and consequently the tests were made with slabs 1 inch in 
 thickness sawed from whole bricks. 
 
 (5) Absorption Tests. —Water-absorption tests were made by the 
 5-hour boiling test and by a 48-hour immersion test. For the former, 
 dry bricks were submerged in water at room temperature, the water 
 
 2 For a more complete description of this apparatus, see J. Am. Ceram. Soe., 11, No. 2, pp. 114-117; Feb¬ 
 ruary, 1928. 
 
 3 D. W. Kessler and W. H. Sligh, B. S. Tech. Paper No. 349, p. 508. 
 
514 
 
 Bureau of Standards Journal of Research 
 
 l Vol.S 
 
 heated to boiling within 1 hour, boiled continuously for 5 hours, and 
 allowed to cool to room temperature. In the 48-hour immersion test 
 they were immersed in water at room temperature for that period of 
 time. The percentage of absorption was calculated on the dry weight 
 according to the relation 
 
 Percentage of absorption = 100 
 
 (B-A) 
 
 A 
 
 where A is weight of the dry brick and B is the weight of the saturated 
 brick. 
 
 2. MORTAR 
 
 The mortar mixes represent certain commonly used volume pro¬ 
 portions. Measurements by volume, however, would have resulted 
 in wider variations in the mortar compositions than seemed desirable 
 so that equivalent proportions by weight were used, assuming that 1 
 cubic foot of lime weighs 40 pounds and 1 cubic foot of cement 
 weights 94 pounds. The weight of the dry materials in a cubic foot, 
 loose measure, of the damp sand used on this work was determined by 
 preliminary tests to be about 73 pounds. Since the weight of a cubic 
 foot of damp sand, loose measure, varies with the moisture content, 
 the moisture in a sample of sand was determined each day during the 
 construction of the walls and the weight necessary to make the desired 
 amount of dry sand was computed. This value was used in propor¬ 
 tioning the mortar for the day. Water was added to give the consist¬ 
 ency desired by the mason and the amount of water recorded. All the 
 mortar used was proportioned by these equivalent weights. 
 
 The mortar for walls 1 to 18 and 160 to 162, built of Chicago brick, 
 was mixed in the same proportions as had been used for the sand-lime 
 brick walls, already referred to. These mixtures were as follows: 
 
 Lime mortar_By volume, l/\ parts of hydrated lime to 3 parts of 
 
 damp sand; weight equivalents, 50 pounds of lime to 
 220 pounds of dry sand. 
 
 Cement-lime mortar_By volume, 1 part of Portland cement, 1/ parts of 
 
 hydrated lime, to 6 parts of damp sand; weight 
 equivalents, 94 pounds of cement, 50 pounds of 
 lime, to 440 pounds of dry sand. 
 
 Cement mortar_By volume, 1 part of Portland cement to 3 parts of 
 
 damp sand; weight equivalents, 94 pounds of cement 
 to 220 pounds of dry sand. 
 
 The mortar mixtures used in the other walls were cement lime and 
 cement mortars. They differed slightly from the above mixes so 
 as to be in conformity with the mortars recommended for solid walls 
 by the Building Code Committee of the Department of Commerce. 4 
 
 4 Report of Building Code Committee, Elimination of Waste Series, Recommended Minimum Require¬ 
 ments for Masonry Wall Construction, Department of Commerce, Washington, D. C. 
 
B. S. Journal of Research, RPI08 
 
 Figure 3. —Types of brick walls 
 
 514—1 
 
B. S. Journal of Research. RP108 
 
 12 r' all rolok 
 
 IN FLEMISH BOND 
 
 iIlTrolok] 
 TTwI-lEH ISH bond 
 
 8" ALL ROLOK 
 IN FLEMISH BOND 
 
 i 75 ^ 
 
 10LOKBAK 
 
 ROLOK BAK 
 
 ECONOMY] 
 
 2j“ ROLOK BAK 
 {heavy duty type) 
 
 4'ECONOMY 
 
 Figure 4 . — Types of brick walls 
 
 514—2 
 
AkBum/y 0713 ’] Compressive Strength of Clay Brick Walls 515 
 
 The mixtures for walls 19 to 159 and 163 to 168 were as follows: 
 
 Cement-lime mortar_By volume, 1 part of Portland cement, 1 part of 
 
 hydrated lime, to 6 parts of damp sand; weight 
 equivalents, 94 pounds of cement, 40 pounds of 
 lime, to 440 pounds of dry sand. 
 
 Cement mortar_By volume, 1 part of Portland cement to 3 parts of 
 
 damp sand (hydrated lime equal to 10 per cent of 
 the volume of the cement was added); weight equiv¬ 
 alents, 94 pounds of cement, 4 pounds of lime, to 
 220 pounds of dry sand. 
 
 It is not believed that these mortars differed to an appreciable 
 extent in their effect on brick-wall strength from the corresponding 
 mortars used in the walls of Chicago brick, but a comparison of 
 the cement mortars may be obtained from the results of tests of the 
 two groups, 160 to 162 and 163 to 165. 
 
 The Portland cement and the hydrated lime were purchased on the 
 open market and conformed to the requirements of the United States 
 Government purchase specifications, Federal Specifications Board 
 specification Nos. la and 249, respectively. The sand was Potomac 
 River sand. All the lime and sand were on hand before any walls 
 were built. The cement was obtained as needed from Government- 
 inspected bins. 
 
 Six cylinders (2 inches diameter, 4 inches long) for compressive 
 tests were made from the mortar of each wall, except that for each 
 wall built with lime mortar only three cylinders were made. After 
 they had been taken from the molds, three cylinders were placed on 
 the wall they represented, as shown in Figure 1, and allowed to age in 
 this position. These cylinders will be described as “dry.” The other 
 cylinders were placed in water and will be termed “wet.” The 
 three cylinders made for each of the lime mortar walls were aged dry 
 because lime mortars disintegrate when placed in water. The cylinders 
 were tested on the same day as the corresponding wall. 
 
 3. WALLS 
 
 (a) TYPES 
 
 Ten different types of brick masonry were included in this investi¬ 
 gation. They consisted of three types of solid brickwork, and seven 
 types of hollow walls as follows: 
 
 A, 8-inch solid. 
 
 B, 12-inch solid. 
 
 C, 8-inch all-rolok. 
 
 D, 12-inch all-rolok. 
 
 E, 8-inch all-rolok in Flemish bond. 
 
 F, 12-inch all-rolok in Flemish bond. 
 
 G, 8-inch rolok-bak. 
 
 H, 12-inch rolok-bak, heavy duty. 
 
 I, 12-inch rolok-bak, standard. 
 
 J, 4-inch economy walls. 
 
 Partially completed walls of each of these types, except that of the 
 12-inch rolok-bak, standard type, are shown in Figures 3 and 4. 
 Sectional views of the 10 types of construction are shown in Figure 5. 
 
516 
 
 Bureau of Standards Journal of Research 
 
 [ Vol. s 
 
 The 8-inch and the 12-inch solid walls were laid in common Ameri¬ 
 can bond, having headers each sixth course. The bricks in these 
 walls were laid flat. These types are shown in Figure 3. 
 
 The solid walls built with Chicago brick (walls 1 to 18) began and 
 ended with a header course. None of the other solid walls had header 
 courses at the top or bottom. 
 
 The hollow walls had header courses at the top and bottom. In 
 the all-rolok walls the stretchers are laid on edge. Every third 
 
 8" SOLID 
 
 D 
 
 □ □ 
 
 D 
 
 □ □ 
 
 □ 
 
 12? all rolok 
 
 IN FLEMISH BOND 
 
 
 
 
 
 
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 12? ALL ROLOK 
 
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 12? ROLOK BAK 
 (HEAVY DUTY) 
 
 12?“ ROLOK BAK 
 
 {STANDARlS) 
 
 □ □ 
 
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 8' ALL ROLOK 
 IN FLEMISH BOND 
 
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 4 ECONOMY 
 
 Figure 5. —Sectional views of the types of brick-wall construction tested 
 
 course is a header course with the brick flat. Figure 3 shows that in 
 the 8-inch all-rolok wall the headers are side by side and, of course, 
 extend completely through the wall. The back of a 12-inch wall of 
 this type is also shown in Figure 3. The center withe of brick on 
 edge is not centered in this wall, but is lined up with the outside 
 headers as shown in Figure 5. The header courses are of “ basket 
 weave/’ in which two header bricks joining the face withe with the 
 central withe alternate with two header bricks joining the back withe 
 with the central withe, the spaces opposite each pair of headers being 
 occupied by a single stretcher. 
 
] Compressive Strength of Clay Brick Walls 
 
 517 
 
 The all-rolok in Flemish bond walls are shown in Figure 4. In 
 these walls all the bricks are on edge laid in Flemish bond for the 
 outside 8-inch width. For the 12-inch wall a withe of stretchers is 
 added, three courses high. The appearance of the front of the wall 
 in the next course is still of the Flemish bond, but the headers of this 
 course are bats and the backing is a continuous course of rowlock 
 headers. 
 
 The exterior 4-inch thickness of the rolok-bak walls, shown in 
 Figure 4, is laid with the brick flat and the backing is laid of brick on 
 edge. On the exterior, therefore, the brickwork has the usual ap¬ 
 pearance of ordinary brickwork having headers each seventh course. 
 Four courses of brick on edge bring the backing courses to the same 
 height as that of the six flat courses of the face. The 12-inch rolok- 
 bak wall, shown in Figure 4, is the heavy-duty type recommended by 
 the Common Brick Manufacturers’ Association for heavy load- 
 bearing construction. In this type the fourth backing course con¬ 
 sists of rolok-headers. Two 12-inch rolok-bak walls, standard type, 
 were included in this investigation. The standard type differs from 
 the heavy-duty type by having the four backing courses all stretchers, 
 the flat header course being of “ basket weave.” This difference is 
 shown in Figure 5. 
 
 The economy wall shown in Figure 4 is essentially a 4-inch wall 
 with pilasters 8 inches wide and 4 inches thick built into the wall at 
 intervals of about 5){ stretcher lengths. The pilasters were tied to 
 the 4-inch withe with headers each sixth course. All bricks were 
 laid flat. The “economy” walls of this investigation were plastered 
 on the back between the pilasters with the same kind of mortar as 
 that in which they were laid, since this method of construction is 
 recommended for weatherproofing the single withe. 
 
 A more extended description of the hollow walls is given in the 
 literature of the Common Brick Manufacturers’ Association. 5 
 
 (b) SIZE 
 
 The walls were 6 feet long. When necessary, the end bricks were 
 chipped so as to project only slightly beyond the end of the wall, as 
 shown in Figure 1. The height of the walls was about 9 feet. 
 
 The thickness of each wall was measured at three different heights 
 on each end and was considered as the average of these six measure¬ 
 ments. The length of each wall was considered to be 72 inches, since 
 this was their minimum length and equaled the lengths of the base 
 channels and the platen of the testing machine. 
 
 5 Hollow Walls of Brick and How to Build Them, Common Brick Manufacturers’ Association of Amer¬ 
 ica, Cleveland, Ohio. 
 
518 Bureau of Standards Journal of Research t voi.s 
 
 (c) WORKMANSHIP 
 
 The walls of this investigation have been divided into two series, 
 each of which was built by a different mason and with a different 
 grade of workmanship. There is at present no scale by which the 
 workmanship can be measured, and, consequently, differences in the 
 quality of the work are difficult to evaluate. The following descrip¬ 
 tion may aid in judging the grades of workmanship. 
 
 (1) Series 1 . —Walls 1 to 17 of series 1 were built so as to have 
 the quality of the workmanship directly comparable to that obtained 
 with the sand-lime brick walls which had already been tested at the 
 Bureau of Standards. 6 Bids were obtained from a number of rep¬ 
 utable local masons, and the work of building the walls was awarded 
 the lowest bidder, who happened to be the same mason who had 
 built the sand-lime brick walls. This mason received neither instruc¬ 
 tion nor supervision. Only on wall 159 of series 1 were instructions 
 given and supervision exercised. Characteristics of the work were 
 that there was practically no mortar in the longitudinal vertical 
 joints, the horizontal mortar beds were deeply furrowed, and the 
 brick were laid at a high rate. Figure 6 is an end view of several 
 walls of series 1. 
 
 Figure 7 shows a horizontal mortar bed of one of these walls after 
 the test of the wall. The furrows made by the trowel point are plainly 
 visible, and the uneven bedding caused by them must have weakened 
 the wall. Figure 8 shows an end view of a similar wall, built with ce¬ 
 ment-lime mortar. The trowel marks may be plainly seen. In 
 this connection, it should be pointed out that the mason thought that 
 he eliminated the furrows by pressing down and tapping the brick. 
 
 Walls 154 to 158 were built by the same mason and with the same 
 grade of workmanship. The hollow brick walls included among these 
 were the first of those types that this mason had built. Wall 159 was 
 also built by the same person. For this wail he was instructed to 
 fill the vertical mortar joints between the withes and not to furrow 
 the horizontal mortar beds with the point of the trowel. 
 
 (2) Series 2. —The walls of series 2 were built by the other mason, 
 who was employed by the day without regard to output. His work 
 was characterized by complete filling of all vertical joints and smooth¬ 
 ing of beds. The filling of vertical joints was accomplished not by 
 the use of “shoved” work but by heavy “buttering” and “slushing” 
 or “dashing” after completion of the course. His original instruc¬ 
 tions were to use his best workmanship without “shoving” the brick. 
 Near the end of the program he was instructed to use shoved work on 
 the two 12-inch solid walls 48 and 108. It may be of interest to note 
 that he did not require instructions in filling vertical joints, but he 
 
 • Whittemore and Stang, Compressive Strength of Sand-Lime Brick Walls, B. S. Tech. Paper No. 276 
 
B. S. Journal of Research, RP108 
 
 518—1 
 
 Figure 6.— End view of several walls of series 1 
 
B. S. Journal of Research, RP108 
 
 Figure 7. —The mortar bed in a wall of series 1 
 "Wall 14, an 8-inch solid wall built of Chicago brick, laid in cement mortar. 
 
 518—2 
 
B. S Journal of Research, RPI08 
 
 Figure 8.— End view of a wall of 
 series 1 after test 
 
 518—3 
 
 Wall 8, an 8-inch solid wall built of Chicago 
 brick, laid in cement-lime mortar. 
 
B. S. Journal of Research, RP108 
 
 Figure 9. —End view of several walls of series 2 
 
 518—4 
 
B. S. Journal of Research, RP108 
 
 Figure 10. —The mortar bed in a wall of series 2 
 Wall 101, a 12-inch solid wall built of New England brick, laid in cement-lime mortar. 
 518—5 
 
B. S. Journal of Research, RP108 
 
 
 Figure 11. —Characteristics of the horizontal mortar bed {the furrowing) in the 
 
 walls of series 1 
 
 518—6 
 
B. S. Journal cf Research, RP108 
 
 Figure 12. —Carefully leveled horizontal mortar beds in the walls of series 2 
 518—7 
 
B. S. Journal of Research, RP108 
 
 Figure 13 . — A portion of wall 18 after test 
 Note that the mortar joints are completely filled. 
 
 518—8 
 
 t 
 
McBurney Compressive Strength of Clay Brick Walls 
 
 519 
 
 had to be cautioned not to furrow the horizontal beds. However, he 
 later reported that laying smooth spread beds was as easy as laying 
 furrowed ones. He worked at all times very carefully and slowly; 
 in fact, his work would be characterized as “fussy” compared with 
 that of the mason who built the walls of series 1. No difference 
 was noted between the general plumbness of the two men’s work. 
 
 Figure 9 shows an end view of several walls and Figure 10 a typical 
 mortar bed for the walls for series 2. Figures 11 and 12 are views 
 showing a furrowed bed and a carefully leveled horizontal bed. 
 
 It was originally planned to have 18 walls of Chicago brick built in 
 series 1, but sufficient bricks were not available to build the last wall 
 (wall 18). Enough bricks were, however, salvaged from the speci¬ 
 mens after the tests of the first 17 walls for another wall. Wall 18 
 was, therefore, built afterwards by the mason of series 2. He used 
 great care in its construction, and it is doubtful whether better con¬ 
 struction could have been obtained. Figure 13 shows a portion of 
 this wall after test and gives an idea of how well all joints were filled. 
 
 Two 12-inch solid brick walls of this series laid in cement mortar 
 (walls 48 and 108) were built with “shoved” workmanship. Wall 
 48 was built of Mississippi brick and wall 108 of New England brick. 
 The object of building these walls was to compare the strength of 
 the shoved work with that of equally careful work not shoved. 
 
 The difference in the rate of laying brick by the two masons is 
 believed to be due more to the characteristics of the men than to the 
 methods employed. The reason for this belief is that the mason 
 who built the walls of series 1 worked as fast on the supervised wall 
 (No. 159) as he did on the others. 
 
 (d) METHOD OF CONSTRUCTION 
 
 The Chicago brick used for walls 1 to 18 were stored in the labora¬ 
 tory and were consequently rather dry. The bricks were placed in 
 a wheelbarrow and a pail of water poured over them. On the average, 
 the brick of these walls were drier when laid than those in the other 
 walls. 
 
 The other bricks were piled in the open without arranging them in 
 regular order. Twenty-four hours before they were laid in the wall 
 they were thoroughly sprinkled until the water flowed continuously 
 from every portion of the pile. They were again sprinkled in the 
 same manner just before laying. 
 
 Each wall was built on a steel channel, as shown in Figure 6, so it 
 could be moved into the testing machine. Starting on the level 
 channel, the wall was kept plumb and the courses level as the work 
 progressed. 
 
520 
 
 Bureau of Standards Journal of Research 
 
 l Vo*. 3 
 
 (e) CONSTRUCTION DATA 
 
 The average construction data for the walls are given in Table 3. 
 The walls were constructed under careful supervision as to the 
 mixture of the mortar materials and records were kept of the time 
 and materials required. Since these records give construction data 
 which may be used in making estimates for comparing the cost of 
 the several types of walls, it is thought that they are of value to 
 contractors and architects. 
 
 (1) Rate of Building. —The time required to build each wall 
 was recorded, beginning when the base plate was level and ending 
 when the last brick was laid. The time, about 10 minutes, required 
 to erect the scaffold, was included. Table 3 gives the rate of building 
 in square feet of wall surface per hour per mason and also the rate 
 of laying brick per hour. This table shows that the walls of series 1, 
 which were built under contract, were built at a much faster rate 
 than those of series 2, built by day labor under careful supervision. 
 
Table 3. —Average construction data of brick walls 
 A. WALLS OF SERIES 1 
 
 Stung, Parsons, 1 
 McBurncy J 
 
 Compressive Strength of Clay Brick Walls 
 
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Table 3 .—Average construction data of brick walls —Continued 
 
 B. WALLS OF SERIES 2—Continued 
 
 522 
 
 Bureau of Standards Journal of Research 
 
 [Vol.S 
 
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McBurney Compressive Strength of Clay Brick Walls 
 
 523 
 
 (2) Materials Used. —Table 3 also shows the amounts of the 
 various mortar materials used for each square foot of wall surface. 
 The amount of water used in the mortar is also reported. The num¬ 
 ber of bricks per square foot of wall surface was obtained by dividing 
 the counted number of bricks in the wall by the area of the wall 
 surface. 
 
 (3) Comparative Construction Data for Different Types 
 of Walls. —In order to study the comparative construction data 
 for different types of walls, only the walls built of Mississippi and 
 New England brick will be considered, since no hollow walls were 
 built of either the Chicago or the Detroit brick. In order to limit the 
 variables still further, only walls of series 2 will be considered first. 
 These walls were all built by one mason and included 60 walls each 
 of Mississippi and New England brick. It should be remembered 
 that the solid walls of this group had. full horizontal and vertical 
 mortar joints. 
 
 Although two different mortars were used, it is possible to bring 
 them to a common basis for comparing the quantities of mortar 
 materials by a study of the amounts of sand. This is permissible, 
 since each mortar is essentially 1 part of cementing materials to 3 
 parts of sand. 
 
 Since the same number of walls was built of each kind of brick, 
 the number of bricks per square foot of wall surface has been averaged 
 for all the walls of one type and these values are given in Table 4 (A). 
 
 The rate of building will, of course, vary with the mason, the kind 
 of structure, and other factors. The data given in Table 4 (A) were 
 obtained with walls which were all of the same size, were built in 
 the same laboratory and by the same mason and helper. A compara¬ 
 tive study of building time can, therefore, be made from these data 
 directly, although it should be remembered that there were no open¬ 
 ings, jambs, or corners which might make some difference in the 
 respective rates of building the different types of walls. 
 
 The 8-inch and the 12-inch solid walls have been used as the basis 
 for a comparison of the materials and the time required for building 
 the different types of walls. Since it seems more logical to compare 
 the time required to build walls of equal size rather than the rate of 
 building, the time ratios, which are proportional to the reciprocals 
 of the rate of building, are also given. These ratios are plotted in 
 Figure 14. 
 
524 Bureau of Standards Journal of Research [Voi.s 
 
 Table 4. —Average construction data of solid and hollow walls of brick 
 
 (A) WALLS OF SERIES 2 
 COMPARISON WITH 8-INCH SOLID WALLS 
 
 
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 Brick 
 
 Rate of 
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 Ratios to solid walls 
 
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 12 
 
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 13.5 
 
 13.0 
 
 Sq.ft./hr. 
 
 7.4 
 
 1.00 
 
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 8-inch all-rolok.. ___ 
 
 12 
 
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 8-inch A. r. F. b. 1 _ 
 
 12 
 
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 9.7 
 
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 8-inch rolok-bak . __ 
 
 12 
 
 8.6 
 
 11.1 
 
 8.0 
 
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 1.08 
 
 .93 
 
 4-inch economy .. _.... 
 
 12 
 
 9.1 
 
 7.9 
 
 7.8 
 
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 .61 
 
 1.05 
 
 .95 
 
 COMPARISON WITH 12-INCH SOLID WALLS 
 
 12-inch solid ... 
 
 18 
 
 20.8 
 
 19.8 
 
 5.6 
 
 1.00 
 
 1.00 
 
 1.00 
 
 1.00 
 
 12-inch all-rolok __... . ... 
 
 12 
 
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 6.7 
 
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 1.20 
 
 .83 
 
 12-inch A. r. F. b. 1 _ 
 
 12 
 
 10.9 
 
 15.3 
 
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 .77 
 
 1.18 
 
 .85 
 
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 18 
 
 12.6 
 
 16.5 
 
 6.3 
 
 .61 
 
 .83 
 
 1.12 
 
 .89 
 
 (B) WALLS OF SERIES 1 
 COMPARISON WITH 8-INCH SOLID WALLS 
 
 8-inch solid..... 
 
 1 
 
 11.8 
 
 11.25 
 
 17.4 
 
 1.00 
 
 1.00 
 
 1.00 
 
 1.00 
 
 8-inch all-rolok. __ . 
 
 1 
 
 7.4 
 
 8. 73 
 
 19.2 
 
 .63 
 
 .78 
 
 1.10 
 
 .91 
 
 8-inch A. r. F. b. 1 _ 
 
 1 
 
 12.2 
 
 8. 90 
 
 17.5 
 
 1.03 
 
 .79 
 
 1.00 
 
 1.00 
 
 8-inch rolok-bak_ . 
 
 1 
 
 8.8 
 
 10.00 
 
 20.5 
 
 .74 
 
 .89 
 
 1.18 
 
 .85 
 
 1 All-rolok-in-Flemish-bond type of wall. 
 
 Tables 4 (A) and Figure 14 show that there is a considerable 
 saving in brick and mortar materials in all the hollow walls as com¬ 
 pared to solid walls of the same thickness. The all-rolok walls and 
 the all-rolok walls in Flemish bond show a saving of at least 45 per 
 cent in mortar materials and of about 25 per cent in the number of 
 brick required. The rolok-bak walls, on the average, show savings 
 of about 40 per cent in mortar materials and 15 per cent in brick. 
 Of the eighteen 12-inch rolok-bak walls mentioned in Table 1, 16 
 were of the heavy-duty type and 2 of the standard type described 
 in the reference. The time required to build the hollow walls is also 
 less than for solid walls of equal thickness. Fewer brick are needed 
 for the 4-inch “economy” walls than for the 8-inch hollow walls, but 
 because of the back plastering the amount of mortar (per square foot 
 of wall surface) was about the same as for the 8-inch hollow walls. 
 On account of the time taken for plastering the back, the average 
 amount of wall area built per hour was only 5 per cent more than 
 with the solid 8-inch walls. 
 
 These walls, referred to in Table 4 (A) were built by a mason who 
 was hired by the day, and they may be classed as construction under 
 careful supervision. In order to have a comparison between this 
 quality of work and work without careful supervision, the four walls (155 
 
525 
 
 McBurney ] Compressive Strength of Clay Brick Walls 
 
 to 158) of series 1 may be considered. These were all 8-inch walls of 
 series 1 built of Mississippi brick with cement mortar. The con¬ 
 struction data for these walls are given in Table 4 (B). 
 
 (4) Thickness of the Mortar Joints. —The average thickness 
 of the horizontal mortar joints is given in Table 3. These values 
 
 Figure 14 .—Comparative construction data of walls of 
 series 2. The data from the 8-inch and 12-inch solid 
 walls have been used as the basis of comparison 
 
 were found by subtracting the total brick thickness (number of 
 courses times average thickness of brick as given in Table 2) from 
 the height of the wall and dividing by the number of mortar joints. 
 The mortar joints in the walls of series 1 were much thicker than in 
 those of series 2. 
 
 (f) AGING CONDITIONS 
 
 The walls remained in the laboratory, as shown in Figure 1, until 
 they were tested. 
 
526 Bureau of Standards Journal of Research \ v 0 i: s 
 
 (1) Schedule of Building. —On account of the long time (more 
 than one year) required for these tests, Table 5 is given to show the 
 dates at which the walls were built and tested. 
 
 In order to minimize the effect of changes which might occur in 
 curing conditions or change in workmanship, the walls 34 to 153 of 
 Mississippi and New England brick, series 2, were built in the order 
 shown by going down the columns of Table 1—that is, 34, 37, 40-91, 
 94-151, 35, 38, etc. 
 
 Table 5. —Schedule of walls 
 
 Series 
 
 Wall Nos. 
 
 Brick 
 
 Building 
 
 started 
 
 Testing 
 
 completed 
 
 1 _ 
 
 1 to 17_ 
 
 Chicago.. _ - . 
 
 Apr. 4,1926 
 May 3,1926 
 May 27,1926 
 July 31,1926 
 Jan. 19,1927 
 Aug. 19,1927 
 Sept. 1,1927 
 
 June 21,1926 
 July 23,1926 
 Mar. 4,1927 
 Sept. 27,1926 
 Mar. 22,1927 
 Oct. 29,1927 
 Feb. 16,1928 
 
 2 _ 
 
 19 to 33_ 
 
 Detroit.. __ __ 
 
 2_ 
 
 34 to 153_ 
 
 Mississippi and New England __ . 
 
 2 _ 
 
 18__ 
 
 Chicago . _ . _ _ 
 
 1_ 
 
 154 to 159_ 
 
 New England and Mississippi _ . 
 
 2 
 
 160 to 167_ 
 
 Chicago.--. .- _ .- . - . 
 
 2 
 
 168 
 
 _do .... 
 
 
 
 
 (2) The Laboratory. —The curing conditions in the laboratory, 
 shown in Figure 1, in which all the walls were built and tested, was 
 generally about the same as out of doors during the spring, summer, 
 and autumn, since large doors were nearly always open. During the 
 colder part of the year it was heated. Being indoors, the walls were, 
 of course, protected from precipitation and from the direct rays of 
 the sun. 
 
 (3) Damp-Cured Walls. —In order to determine whether brick 
 walls, built with cement mortar, kept damp for several days after 
 construction are stronger than similar walls allowed to age under 
 normal conditions in the laboratory, the walls listed in Table 1 as 
 “wetted” were built. These walls, which were all laid in cement 
 mortar, were covered with burlap, as shown in Figure 15, as soon as 
 they had been completed. The burlap was kept wet for one week 
 after the walls had been built and was then removed. These wet 
 walls were similar as to the kind of brick, mortar, workmanship, and 
 size to an equal number of walls which were not covered with wet 
 burlap, as may be seen in Table 1. 
 
 (g) AGE 
 
 The walls were tested from 57 to 62 days after construction, with 
 the exception of wall 168, built of Chicago brick with cement mortar, 
 which was tested when 169 days old for the purpose of exhibiting the 
 test to a group of visitors. 
 
 (h) TESTING MACHINE 
 
 The walls were tested in the 10,000,000-pound capacity compres¬ 
 sion machine, shown in Figures 1 and 16. This machine is of the 
 vertical type and is capable of testing specimens whose height does not 
 exceed 25 feet and whose width in the clear does not exceed 6 feet. 
 
B. S. Journal of Research, RPI08 
 
 Figure 15. —A ivall covered with burlap for damp curing 
 
 526—1 
 
B. S. Journal of Research, RP108 
 
 Figure 16 .—A wall in the 10,000,000-pound capacity testing machine ready 
 
 for test 
 
 526—2 
 
McBurney ] Compressive Strength of Clay Brick Walls 
 
 527 
 
 The upper head during a test remains stationary, but for the purpose 
 of adjusting the machine to the height of the specimen this head is 
 moved by large nuts turning on the four 13%-inch diameter screws of 
 the machine. These nuts are turned by a gearing mechanism which 
 is driven by an electric motor placed on the head. 
 
 The lower platen, 6 feet square, rests on a spherical base of 5 feet 
 radius which is mounted upon a vertically acting piston of 50 inches 
 diameter and 24-inch stroke. There is attached to the bottom of 
 this ram a guiding plunger 14 inches in diameter and 2 feet 5 inches 
 long which fits into the large base casting of the testing machine and 
 serves for proper centering of the parts. 
 
 The lower, or straining platen, is subject to the oil pressure in the 
 cylinder, which, at the capacity of the machine, is approximately 
 5,000 lbs./in. 2 The oil pressure communicates through auxiliary 
 piping with a smaller piston of 5 9 / 16 inches diameter. This piston 
 has a “knife-edge ” bearing on the main lever, M, of the testing machine 
 lever system. (See fig. 17 for reference to the various parts by letters.) 
 The ratio of the areas of the large and small piston is approximately 
 as 80:1, the weighing lever, L, being graduated up to 2,000,000 pounds 
 to read the actual load on the specimen. For loads greater than 
 2,000,000 pounds four weights, W, are provided, which may be 
 placed on the end of the lever as desired. Each of these weights 
 when attached to the end of the lever has a moment about the balance 
 point B of the lever equal to that of the poise when it is at the 
 2,000,000-pound graduation, 2 M. Thus, if one weight is used and 
 the poise, P, indicates a given load, the force on the specimen is 
 equal to 2,000,000 pounds plus the poise reading. 
 
 The pressure in the cylinder of the machine is obtained by means 
 of a triple plunger oil pump, 0, having an air dome, A, for equalizing 
 the pulsations of the three pistons. The pump is belt driven by an 
 electric motor, E. The piping from the pump leads to the bottom 
 of the cylinder and is directed downward toward its base, while the 
 piping which leads to the weighing piston is taken from the top of 
 the cylinder, as far as possible from the other pipe, and where there 
 is the least disturbance. 
 
 The passage of oil from the pump is controlled by two valves, V, 
 and the system is protected from dangerous pressures by an automatic 
 overload relief valve. The oil can also be passed directly from the 
 pump back to the reservoir by means of a hand-controlled relief 
 valve, R. The speed of the lower platen may be varied in the follow¬ 
 ing manners: By varying the lengths of stroke of the pump pistons 
 through the handwheel, E; by operating a by-pass needle valve, N, 
 which controls the size of the passage in the valve block; by manip¬ 
 ulating the globe valves, V, or the relief valve, R; or by varying the 
 operating speed of the motor through the motor-controller switch. 
 
528 Bureau of Standards Journal of Research ivoi.s 
 
 The speed of the main piston may in this manner be varied from zero 
 to % inch per minute at no load. 
 
 (i) METHOD OF TESTING 
 
 The walls were tested in compression under central loading. For 
 these tests the channel at the base of the wall was bedded in plaster 
 of Paris. The lower platen was then tilted, if necessary, until the 
 wall was plumb and the top of the wall was nearly parallel to the 
 lower surface of the upper head of the testing machine. A cap 
 of plaster of Paris (calcined gypsum) was then spread on the top of 
 the wall and the upper head lowered until the space between it and 
 the wall was filled with plaster. The gypsum was allowed to set 
 for at least an hour before the test was begun. 
 
 Vertical compressometers having a gage length of about 100 inches 
 were attached near each corner, as shown in Figure 16. Horizontal 
 extensometers were also fastened along the length on each side at 
 mid height of the wall. These had a gage length of about 48 inches. 
 The dial micrometers were graduated to 0.001 inch, and readings 
 were taken at each 50 or 100 lbs./in. 2 increment of load on the wall. 
 The vertical speed of the lower platen was about 0.06 inch per minute 
 during the application of load. When readings were being taken, the 
 load was held constant. 
 
 4. WALLETTES 
 
 One hundred and twenty-nine wallettes or small walls, each about 
 18 inches long and 34 inches high, were built, which differed only 
 in size from the wall of the same number. The walls and wallettes 
 of the same number corresponded as to brick, mortar, type, curing 
 conditions, and workmanship. No wallettes of the “economy ” type 
 were built, since that design is not adapted for specimens of wallette 
 size. A group of wallettes is shown in Figure 18. The wallettes 
 were built to determine whether walls smaller than the 6 by 9 foot 
 specimens possessed compressive strengths which had a definite 
 relation to those of the wall specimens. All but the strongest wall¬ 
 ettes were tested in a 600,000-pound capacity universal testing 
 machine using a spherical bearing, as shown in Figure 19. One of 
 the wallettes (No. 104) tested in the 600,000-pound capacity machine 
 could not be broken. It was then retested in the 10,000,000-pound 
 machine. All other wallettes of this type which were tested later 
 were then tested in the 10,000,000-pound machine. 
 
 IV. RESULTS OF THE TESTS WITH DISCUSSION 
 
 1. BRICK 
 
 The results of the tests of the single bricks are given in Table 6. 
 Each average value, except those for shearing strength, is the average 
 result from 50 tests. 
 
B. S. Journal of Research, RP108 
 
 Figure 17 .—Pump and weighing mechanism of the 10,000,000-pound capacity 
 
 testing machine 
 
 Figure 18 .—A group of wallettes of series 2, built of Detroit brick 
 
 528—l 
 
B. S. Journal of Research, RPI08 
 
 Figure 19. —A wallette in the 600,000-pound capacity testing machine 
 
 Wallette 28, of series 2, built of Detroit brick, laid in cement mortar. The maximum stress with' 
 stood by this specimen was 1,605 lbs./mA 
 
 528—2 
 
Table 6. —Physical properties of the brick 
 
 S™" 0 "*’] Compressive Strength of Clay Brick Walls 
 
 Shearing 
 
 strength 
 
 ^ ICO 
 
 3 
 
 11,100 
 
 1,500 
 
 810 
 
 § 
 
 T“H 
 
 2,190 
 
 1,155 
 
 8 
 
 H 
 
 4,255 
 
 2,460 
 
 CO 
 
 e« 
 
 Tensile 
 
 strength 
 
 c 3 
 
 oo 
 
 3 
 
 rH 
 
 88 
 CO rH 
 
 222 
 
 CO 00 
 
 rH Tt< 
 lO rH 
 
 317 
 
 1,160 
 
 325 
 
 601 
 
 Compressive strength 
 
 Edgewise 
 
 Half 
 
 brick 
 
 (wet) 
 
 O O 
 C» N H 
 •g©Tt< 
 
 ^■<o'‘c<r 
 
 3, 620 
 
 6,560 
 
 1,680 
 
 3,165 
 
 5,630 
 
 2,160 
 
 3,620 
 
 18,750 
 
 5,510 
 
 11,020 
 
 Half 
 
 brick 
 
 (dry) 
 
 •S 
 
 1 -t 
 
 § 
 
 3, 350 
 
 4, 230 
 
 2,410 
 
 o 
 
 CO 
 
 7,330 
 
 2,320 
 
 3,625 
 
 15, 500 
 
 6,390 
 
 11,470 
 
 Whole 
 
 brick 
 
 (dry) 
 
 Lbs./in. 2 
 6,660 
 1,330 
 
 o 
 
 *H 
 
 00 
 
 csf 
 
 4,360 
 
 2,690 
 
 3,380 
 
 5, 760 
 
 1,460 
 
 3,200 
 
 16,400 
 
 5,380 
 
 10,300 
 
 Flatwise 
 
 Half 
 
 brick 
 
 (wet) 
 
 **.©© 
 
 CO 
 
 H Tt 
 
 -o 
 
 3, 670 
 
 4, 320 
 1,740 
 
 2,520 
 
 7,100 
 1,740 
 
 3,520 
 
 11,830 
 
 3, 710 
 
 6,990 
 
 Half 
 
 brick 
 
 (dry) 
 
 °\© o 
 
 ,§NH 
 
 ^tjTcsT 
 
 •Q 
 
 3,280 
 
 5, 950 
 2,110 
 
 3, 540 
 
 6, 510 
 2,390 
 
 3,410 
 
 OO 
 t- oo 
 co 
 
 <M~iO 
 
 rH 
 
 8 
 
 O 
 
 00 
 
 Whole 
 
 brick 
 
 (dry) 
 
 e-as 
 
 GO H 
 
 3 
 
 3,460 
 
 4,850 
 
 2,270 
 
 3,255 
 
 6,460 
 
 2,590 
 
 3,625 
 
 14,160 
 5,180 
 
 9,420 
 
 Modulus of rupture 
 
 Edgewise 
 
 N .op 
 
 .gss 
 
 •o 
 
 o 
 
 CO 
 
 r—i 
 
 1,055 
 226 
 
 680 
 
 1,480 
 
 387 
 
 760 
 
 2,340 
 
 448 
 
 1, 640 
 
 Flatwise 
 
 (wet) 
 
 ®v o o 
 
 ,gS 88 
 
 •Q 
 
 1, 470 
 
 <N 
 
 CM 
 © CO 
 
 632 
 
 1,390 
 441 
 
 750 
 
 O 00 
 
 T+l rH 
 
 00 
 
 CO 
 
 1,400 
 
 Flatwise 
 
 «© © 
 
 oo ^ 
 
 •Q 
 
 1,225 
 
 <M<N 
 
 CO <M 
 
 © ^ 
 
 rH 
 
 670 
 
 1,540 
 324 
 
 820 
 
 2,330 
 
 940 
 
 O 
 
 Jo 
 
 lO 
 
 rH 
 
 Absorption 
 
 i 
 
 < 
 
 X 
 
 o 
 
 immer- 
 
 siod 
 
 Per cent 
 22.0 
 4.9 
 
 11.7 
 
 22.5 
 
 16.4 
 
 20.7 
 
 21.8 
 
 12.3 
 
 16.7 
 
 11.9 
 
 2.4 
 
 6.9 
 
 
 6-hour 
 
 boiling 
 
 Per cent 
 23.0 
 10.5 
 
 16.5 
 
 25.5 
 
 19.4 
 
 22.3 
 
 25.1 
 
 18.4 
 
 !>• 
 
 rH 
 
 <N 
 
 14.9 
 
 3.1 
 
 9.2 
 
 Value 
 
 [Maximum_ 
 
 Minimum ... 
 
 Average_ 
 
 [Maximum_ 
 
 Minimum _ 
 
 Average_ 
 
 [Maximum_... 
 
 Minimum... 
 
 Average.. 
 
 [Maximum_ 
 
 1 Minimum... 
 
 l Average_ 
 
 ,Q 
 
 1 
 
 M 
 
 *-> 
 
 3 
 
 o 
 
 JO 
 
 o 
 
 a 
 
 ■u 
 
 ■g 
 
 .9 
 
 3 
 
 3 
 
 «*-» 
 
 o 
 
 <3> 
 
 | 
 
 > 
 
 <1 
 
 JO 
 
 « 
 
 •H 
 
 J2 
 
 o3 
 
 X3 
 
 3 
 
 o 
 
 ® 
 
 bo 
 
 (0 
 
 Ih 
 
 ® 
 
 > 
 
 JO 
 
 o 
 
 o 
 
 TJ 
 
 O 
 
 3 
 
 D 
 
 o 
 
 t-. 
 
 P 
 
 a 
 
 a 
 
 1 
 
 s 
 
 T) 
 
 S 
 
 bO 
 
 H 
 
 I 
 
 
 529 
 
530 
 
 Bureau of Standards Journal of Research 
 
 [Vol.S 
 
 The number 50 represents a complete sampling of the shipment. 
 While most determinations were checked by two or more independent 
 samplings and tests, for the purposes of this paper a particular set 
 of samples was selected and the others disregarded. On the basis of 
 the large number of tests made, it can be stated that the strengths of a 
 given lot as determined by averages of 50 specimens may differ by as 
 much as 10 per cent. Hence, small differences, such as appear between 
 the wet and dry compressive and transverse strengths of the Chicago 
 brick, for example, should be disregarded, being within the variations 
 due to sampling alone. 
 
 While, in general, there is a fair correlation between the different 
 measures of strength, comparing one land of brick with another, 
 certain divergencies are evident which may well have an important 
 effect on masonry strength. This will be discussed later in the paper. 
 For the present, attention is called to Table 7, where the ratios of 
 these various physical properties are given. 
 
 Disregarding comparisons of strength wet with strength dry, and 
 considering the mean deviation divided by the mean as given in 
 Table 7 as a measure of agreement, the most constant ratios are those 
 of tensile strength to modulus of rupture. The ratio of modulus of 
 rupture to shearing strength shows the greatest deviation. 
 
 Table 7. —Ratios of various physical properties of brick 
 
 Kind of brick 
 
 Absorp¬ 
 
 tion 
 
 Compressive 
 stength of half 
 brick 
 
 Modu¬ 
 lus of 
 rupture 
 (flat, 
 dry) 
 
 Tensile 
 
 strength 
 
 Modu¬ 
 lus of 
 rupture 
 (flat, 
 wet) 
 
 Modu¬ 
 lus of 
 rupture 
 (flat, 
 dry) 
 
 Tensile 
 
 strength 
 
 Com¬ 
 pressive 
 strength 
 of half 
 (flat, 
 dry) 
 
 Shearing 
 
 strength 
 
 Com¬ 
 pressive 
 strength 
 of half 
 (flat, 
 dry) 
 
 48-hour 
 
 cold 
 
 Edge, 
 
 dry 
 
 Elat, 
 
 wet 
 
 Modu¬ 
 lus of 
 rupture 
 (flat, 
 dry) 
 
 Com¬ 
 pressive 
 strength 
 of half 
 (flat, 
 dry) 
 
 Modu¬ 
 lus of 
 rupture 
 (flat, 
 dry) 
 
 Shearing 
 
 strength 
 
 5-hour 
 
 boiled 
 
 Flat, 
 
 dry 
 
 Flat, 
 
 dry 
 
 1 
 
 2 
 
 3 
 
 4 
 
 5 
 
 6 
 
 7 
 
 8 
 
 9 
 
 10 
 
 Chicago... 
 
 0.71 
 
 .93 
 
 .77 
 
 .75 
 
 .08 
 
 1.02 
 .92 
 1.06 
 1.33 
 
 .11 
 
 1.12 
 .71 
 1.03 
 .81 
 
 .17 
 
 0.37 
 
 .19 
 
 .24 
 
 .18 
 
 .24 
 
 0. 34 
 .33 
 .39 
 .39 
 
 .08 
 
 1.20 
 
 .94 
 
 .92 
 
 .90 
 
 .10 
 
 1.11 
 .58 
 .52 
 .44 
 
 .34 
 
 0.13 
 .06 
 .09 
 .07 
 
 .22 
 
 0. 34 
 .33 
 .47 
 .41 
 
 .14 
 
 Detroit _ 
 
 Mississippi_ 
 
 New England_ 
 
 Mean deviation 
 
 Mean 
 
 A study of Tables 6 and 7 will show that the Chicago brick is 
 different from the other three kinds in that its ratio of modulus of 
 rupture and of tensile strength to compressive strength and to shearing 
 strength is markedly higher than are the corresponding strength 
 ratios for the other three bricks, as may be noted in columns 5, 8, and 
 9 of Table 7. This is possibly explained by the structure of the 
 brick, end cut and laminated, which gives the effect of a bundle of 
 fibers running lengthwise of the brick. 
 
 Of the other three kinds of brick, two represent the soft-mud and 
 one the dry-press method of manufacture. 
 
aSST* 4 ’] Compressive Strength of Clay Brick Walls 
 
 531 
 
 In the original plan for these tests it was the intention to use bricks 
 corresponding to the four grades in the American Society for Testing 
 Materials’ Specification for Building Brick (C21-24). On the basis 
 of the average values here given these brick would classify as follows: 
 
 1 
 
 Absorption, 5-hours boil¬ 
 ing 
 
 Modulus of rupture flat¬ 
 wise 
 
 Compressive strength 
 halves on edge 
 
 Chicago... . . . . . 
 
 Medium_ _ 
 
 Vitrified 
 
 Medium. 
 
 Detroit.... 
 
 Soft___ 
 
 Hard.. . ... 
 
 Do. 
 
 Mississippi._ . . .. 
 
 _do... 
 
 . . do.. 
 
 Hard. 
 
 New England ... _ 
 
 Hard___ 
 
 Vitrified 
 
 Vitrified. 
 
 
 
 
 Under the American Society for Testing Materials new Tentative 
 Specification for Building Brick (made from clay or shale) (C62-28T) 
 these brick classify as follows: 
 
 
 Com¬ 
 
 pressive 
 
 strength 
 
 halves 
 
 flatwise 
 
 Modulus 
 
 of 
 
 rupture 
 
 flatwise 
 
 Chicago... _ 
 
 B 
 
 A 
 
 Detroit..__ 
 
 B 
 
 A 
 
 Mississippi_ 
 
 B 
 
 A 
 
 New England_ 
 
 A 
 
 A 
 
 For both these classifications it is provided that the grading “ shall 
 be determined by the results of the tests for that requirement in which 
 it is lowest unless otherwise specified * * 
 
 Figures 20, 21, 22, and 23 give the distribution of the individual 
 tests, averages of which are given in Table 6. The 50 tests in each 
 group are not enough to give a satisfactory distribution, but these 
 graphs give an idea of the variability of the brick. 
 
 2. MORTAR 
 
 The average results of the tests of the mortar specimens are given 
 in Table 8. 
 
 Table 8. —Compressive strength of mortar specimens 
 
 [Cylinders 2 inches in diameter, 4 inches long] 
 
 REPRESENTING WALLS 1 TO 18 AND 160 TO 162 
 
 Mortar 
 
 Proportions (by volume) 
 
 Strength 
 
 Cured 
 
 wet 
 
 Cured 
 
 dry 
 
 
 IX L: 3S_ 
 
 Lbs.,tin : 2 
 
 Lbs.tin? 
 90 
 500 
 1,460 
 
 Oatti ati t-1 im A _ 
 
 1 ‘ C: IX L: 6S_ 
 
 1,070 
 3,580 
 
 
 1 C: 3S_ 
 
 
 
 REPRESENTING WALLS 19 TO 159 AND 163 TO 168 
 
 
 1 C: 1 L: 6S_ 
 
 1,100 
 
 750 
 
 
 1 C: X V L: 3S_ 
 
 3,260 
 
 1,950 
 
 
 
532 
 
 Bureau of Standards Journal of Research 
 
 [Vol.S 
 
 3. WALLS 
 
 (a) BASIS OF COMPUTATIONS 
 
 The sectional area of the wall was obtained by multiplying the 
 average thickness by the length (72 inches). All wall stresses, except 
 some of those in Table 10, were obtained by dividing the load by this 
 measured (not nominal) gross area. The stresses in the hollow walls 
 are also based on the gross measured areas. 
 
 Half Brick. Dry Half Brick. Mr Whole Brick. Dry 
 
 Chicaao 
 
 Figure 20. — Results of the tests on brick 
 
 Compressive strength, fiat, half brick, half brick dry, half brick wet, 
 and whole brick dry, and tensile strength 
 
 (b) DEFORMATION OF THE WALLS 
 
 (1) Stress-Strain Curves. —The average stress-strain curves of 
 the solid walls of series 1 built of Chicago brick are shown in Figure 24. 
 These curves show the differences in stiffness due to differences in the 
 mortar. Both the average vertical compression and the horizontal ex¬ 
 tension of the wall at mid height have been plotted against the com- 
 
AfcBume™ 0713 '] Compressive Strength of Clay Brick Walls 
 
 533 
 
 pressive stress. The horizontal extension is much less than the vertical 
 compression, but no constant relation between them, such as the nearly 
 constant Poisson’s ratio for some metals, appears to exist. The ratio of 
 extension to compression, however, increases with increase in stress. 
 
 The relative stiffness of the different types of walls is shown in 
 Figure 25. This shows average stress-strain curves for the 8-inch 
 
 Half Bnck, Dry 
 
 Chicago 
 
 Half Brick, Wet 
 
 Detro/r 
 
 35 
 
 
 20 
 
 3* 
 
 IOOOO 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 J_ 
 
 IOOOO 
 
 
 11, 
 
 /odoo 
 
 
 
 
 
 
 
 
 
 
 
 
 JL 
 
 L_ 
 
 20 
 
 IS 
 
 Mississippi 
 
 IS 
 
 IOOOO 
 
 Nen Ena land 
 
 A 
 
 
 
 lli 
 
 _dl. 
 
 
 
 
 
 
 
 
 J_ 
 
 moot) 
 
 
 i|l,il 
 
 /odoo 
 
 .1 ll i,a 6,1— 
 
 Compressive strength on edge -tx/inf 
 
 Whole Brick. Dry 
 
 
 
 
 
 
 
 - 
 
 
 
 1 . 
 
 b /odoo 
 
 
 
 • 
 
 
 
 
 
 
 
 
 
 
 
 
 1_ 
 
 i • IOOOO 
 
 
 
 
 
 
 1 
 
 JL 
 
 ll_ 
 
 'todoo 
 
 
 _ill 
 
 luL, 
 
 if IOOOO 
 
 Figure 21. — Results of the tests on brick 
 
 Compressive strength on edge, half brick dry, half brick wet, and 
 
 whole brick dry 
 
 walls of series 2 of Mississippi brick laid in cement mortar. These 
 curves for the different types of walls are typical of those found in the 
 other groups of walls. The solid walls deform less than the hollow 
 walls and the different types of hollow walls appear to deform about 
 equally for equal stresses. 
 
 Average stress-compression curves for 12-inch solid walls of series 2 
 built with cement mortar and of different kinds of brick are shown in 
 
534 
 
 Bureau of Standards Journal of Research 
 
 [Vol.S 
 
 Figure 26. Since the same mortar mixture was used for all of these 
 walls the differences in stiffness are due in large part to differences in 
 the properties of the brick, although differences in the thickness of the 
 mortar joints doubtless had an influence. 
 
 (2) Secant Modulus of Elasticity. —For the walls laid in cement- 
 lime and cement mortars the stress-strain curves are, for low stresses, 
 approximately straight lines. In general, however, there is no value 
 
 Flat On Edge 
 
 ^ntcago 
 
 
 
 
 
 
 
 II 
 
 1 
 
 
 
 . 
 
 J 
 
 hill . 
 
 2000 
 
 jX 
 
 ,1 1,1 u lii.i 
 
 2000 
 
 
 
 
 
 
 
 
 lL 
 
 . _LJL_ 
 
 2000 
 
 
 
 
 
 
 
 
 
 . 1 
 
 
 
 
 J-L. 
 
 >000 
 
 Modulus of rupture - Uo/n i 
 
 Figure 22.— Results of tests on brick 
 Modulus of rupture, flat and on edge 
 
 for a modulus of elasticity which is constant over a large stress range. 
 For computing the shortening of a wall under the first application of 
 working loads, the secant modulus of elasticity, obtained by dividing 
 the stress by the corresponding value of the compressive strain, may 
 be of use. These values are given in Table 9. The stress ranges are— 
 
 0 to 125 lbs./in. 2 for lime mortar, 
 
 0 to 200 lbs./in. 2 for cement-lime mortar, and 
 0 to 250 lbs./in. 2 for cement mortar. 
 
Stang, Parsons, ] 
 McBurney J 
 
 Compressive Strength of Clay Brick Walls 
 
 5 Hr Boil 4 $ Hr. Immersion 
 
 535 
 
 Chicago 
 
 
 
 --- 
 
 
 
 
 
 
 
 , 11 
 
 1 
 
 | 
 
 -1 
 
 
 UL 
 
 20 
 
 il 
 
 I s i .l 1 r j -. .,._. 
 
 20 
 
 Nt» England 
 
 
 • 
 
 
 | 
 
 
 
 
 
 L 
 
 
 
 
 
 
 
 J 
 
 L± 
 
 
 
 d sb ' i 
 
 ) ' ' ' 2'0 
 
 Absorption -per cent 
 
 Figure 23. — Results of tests on bricks 
 Absorption per cent, 5-hour boil and 48-hour cold immersion 
 
 Mortar 
 
Table 9. —Results of compressive tests of brick walls 
 (A) WALLS OF SERIES 1 
 
 536 
 
 Bureau of Standards Journal of Research 
 
 [Vol. S 
 
 <N 
 
 CO 
 
 W 
 
 s 
 
 w 
 
 CO 
 
 PR 
 
 O 
 
 CO 
 
 £ 
 
 PQ 
 
Stang, Parsons, 1 
 
 McBurney J 
 
 Compressive Strength of Clay Brick Walls 
 
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 69882°—29- 
 
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538 
 
 Bureau of Standards Journal of Research 
 
 [Vol. 3 
 
 solid 
 
 ail-roiok 
 
 a!I-roI ok in 
 Flemish bond 
 
 rofok-bak 
 
 Figure 25.- 
 
 £)efdrmotion - in/in. 
 
 o—o = stress- horizontal extension 
 c—o = stress-vertical compression 
 
 —The average stress-strain curves for different types of 8-inch 
 
 walls of series 2 
 
 The walls were built of Mississippi brick laid in cement mortar 
 
 Figure 26. —Average stress-compression curves for walls built 
 
 of different kinds of brick 
 
 The walls were 12-inch solid walls of series 2 laid in cement mortar 
 
TicBurne T y Sons ’] Compressive Strength of Clay Brick Walls 
 
 539 
 
 (3) Permanent Set. —The elastic behavior of these brick walls was 
 found to be similar to that found for the masonry piers tested at 
 Columbia University. 7 Upon release of load, a permanent set was 
 found, and upon reapplication of the load, if the former load was not 
 exceeded, the vertical deformation in the wall measured from its 
 initial condition was only a little more than it was at the initial appli¬ 
 cation of the load. Figure 27 shows the stress-compression curves 
 for wall 42 which was subjected to repeated loadings. The slopes of 
 the lines, which represent an increase of load, gradually decrease as 
 higher loads are applied, although for the smaller stresses there is 
 very little difference in the modulus of elasticity for consecutive 
 loads. The deformation found on again attaining a certain load after 
 a load release is slightly greater than the initial deformation, but upon 
 proceeding to a higher load, the compression at that load appears to 
 be equal to what it would have been if no release had taken place. 
 In other words, the envelope of the stress-compression relations rep¬ 
 resented by the dotted line of Figure 27 with repeated loading appears 
 to represent approximately the primitive stress-compression properties 
 of the wall. 
 
 One wall in each group of three was subjected to a release of load 
 test. Compressometer readings were taken as the load increased 
 until the stress was reached for which the secant modulus of elas¬ 
 ticity was computed. The stress was then reduced to 50 lbs/in. i 2 , 
 readings w T ere taken, and the load was reapplied. These second- 
 loading moduli were in all cases larger than the corresponding values 
 of the secant modulus of elasticity. For the lime-mortar walls the 
 second loading moduli were more than three times as great as the 
 primitive secant moduli, while with the other mortars the increase 
 was from 10 to 50 per cent. Upon the release of load from a higher 
 stress to 50 lbs./in. 2 , a permanent set was always found. A typical 
 stress-set curve is shown in Figure 27. The ordinates represent the 
 stress from which release took place and the abscissas, the compres¬ 
 sion set at 50 lbs./in. 2 , the stress to which the load was reduced. 
 These set curves are fairly straight for the lower stresses, but for 
 higher stresses the set increases more rapidly than the load. 
 
 (c) BEHAVIOR OF THE WALLS UNDER LOAD 
 
 In the solid lime-mortar walls (walls 1 to 6), as the load was 
 increased the mortar was crushed and squeezed out of the horizontal 
 beds. Soon after this occurred stretchers and headers broke, and at 
 the maximum load more of the headers were broken and cracks 
 appeared in many of the stretchers. These walls were all built of the 
 end-cut Chicago brick and many of the stretchers split longitudinally. 
 
 i A. H. Beyer and W. J. Krefeld, Comparative Tests of Clay, Sand-Lime, and Concrete Brick Masonry, 
 
 Bulletin No. 2, Department of Civil Engineering, Columbia University; 1923. 
 
540 
 
 Bureau of Standards Journal of Research 
 
 [Voi.S 
 
 In the solid walls built with the cement-lime and the cement mor¬ 
 tars longitudinal failure of the stretchers predominated in the walls 
 
 
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 of Chicago brick, while with the other kinds of brick, all of which 
 were molded, the failure of the headers was characteristic. At the 
 
B. S. Journal of Research, RP108 
 
 
 ft VJ 
 
 
 !i w f 
 
 
 Figure 28 .—The 8-inch solid wall 37 of series 2, built of Mississippi 
 
 brick, laid in cement mortar , after test 
 
 The strength of this wall was 1,335 lbs./in. 2 . 
 540—1 
 
B. S. Journal of Research, RP108 
 
 Figure 29 .—The 12-inch all-rolok in Flemish bond, wall 70 of series 2, built 
 of Mississippi brick, laid in cement mortar, after test 
 
 The strength of this wall was 559 lbs./in. 2 . 
 
 540—2 
 
Mc&LrnYy 8071 *’] Compressive Strength of Clay Brick Walls 
 
 541 
 
 maximum load nearly all headers and many stretchers had broken, 
 and in some cases, especially with the cement mortar, spalling of the 
 brick and vertical cracks also occurred, as shown in Figure 28. No 
 crushing of the mortar was observed except with the lime mortar. 
 
 The failure of the hollow walls was characterized by broken headers. 
 Many of the 8-inch walls of the all-rolok and the all-rolok in Flemish 
 bond types continued to take load until all the headers had been 
 broken and total collapse took place. In the 12-inch walls of these 
 types the rear withes often fell, as may be seen in Figure 29. 
 
 The failure of the rolok-bak walls was also characterized by header 
 failures and the collapse of the rear rowlock withe. 
 
 In the “ economy ” walls the headers broke first, then the mortar 
 plastering loosened, and finally stretcher cracks appeared. At the 
 maximum load only the 4-inch withe was withstanding the load. The 
 area of the pilasters has, however, been included in the area of the 
 walls used to calculate the maximum stress. 
 
 The average values for the stresses when the first crack was seen 
 are given in Table 9. There seems to be no definite relation between 
 these stresses and the maximum stresses that the walls withstood, 
 although there was, of course, a tendency for the load at failure to be 
 larger if the load at first crack was large. 
 
 The loading was not continued after a marked decrease in the load 
 showed that the maximum had been attained. 
 
 (d) COMPRESSIVE STRENGTH 
 
 The individual wall strengths, as well as the average value for any 
 group of walls, are given in Table 9. The value of this table is largely 
 as a compilation of data on the strength of brick walls, which differ 
 as to the materials and the quality of workmanship used in their 
 construction, and which may be duplicated in structures. 
 
 Since the selection of proper working stresses for brick masonry 
 requires a knowledge of the principal factors that govern strength, 
 the results given in Table 9 afford useful comparisons between the 
 strength of the walls and the variables featuring their construction. 
 
 (e) THE COMPARATIVE STRENGTHS OF SOLID AND OF HOLLOW WALLS OF BRICK 
 
 In order to study the effect of design on the strength of brick 
 masonry, the walls of series 2, alike in all respects except the type of 
 wall built with spread mortar joints and aged under ordinary con¬ 
 ditions, will first be considered. 
 
 The average results of the compressive tests of these brick walls 
 are given in Table 10 and are shown graphically in Figures 30 and 31. 
 Each value is the average for three similar walls. 
 
B. S. Journal of Research, RPI08 
 
 Figure 29.- —The 12-inch all-rolok in Flemish bond wall 70 of series 2, built 
 of Mississippi brick, laid in cement mortar, after test 
 
 The strength of this wall was 559 lbs./in. 2 . 
 
 540—2 
 
ficBurnel 8071 *’] Compressive Strength of Clay Brick Walls 
 
 541 
 
 maximum load nearly all headers and many stretchers had broken, 
 and in some cases, especially with the cement mortar, spalling of the 
 brick and vertical cracks also occurred, as shown in Figure 28. No 
 crushing of the mortar was observed except with the lime mortar. 
 
 The failure of the hollow walls was characterized by broken headers. 
 Many of the 8-inch walls of the all-rolok and the all-rolok in Flemish 
 bond types continued to take load until all the headers had been 
 broken and total collapse took place. In the 12-inch walls of these 
 types the rear withes often fell, as may be seen in Figure 29. 
 
 The failure of the rolok-bak walls was also characterized by header 
 failures and the collapse of the rear rowlock withe. 
 
 In the “ economy ” walls the headers broke first, then the mortar 
 plastering loosened, and finally stretcher cracks appeared. At the 
 maximum load only the 4-inch withe was withstanding the load. The 
 area of the pilasters has, however, been included in the area of the 
 walls used to calculate the maximum stress. 
 
 The average values for the stresses when the first crack was seen 
 are given in Table 9. There seems to be no definite relation between 
 these stresses and the maximum stresses that the walls withstood, 
 although there was, of course, a tendency for the load at failure to be 
 larger if the load at first crack was large. 
 
 The loading was not continued after a marked decrease in the load 
 showed that the maximum had been attained. 
 
 (d) COMPRESSIVE STRENGTH 
 
 The individual wall strengths, as well as the average value for any 
 group of walls, are given in Table 9. The value of this table is largely 
 as a compilation of data on the strength of brick walls, which differ 
 as to the materials and the quality of workmanship used in their 
 construction, and which may be duplicated in structures. 
 
 Since the selection of proper working stresses for brick masonry 
 requires a knowledge of the principal factors that govern strength, 
 the results given in Table 9 afford useful comparisons between the 
 strength of the walls and the variables featuring their construction. 
 
 (e) THE COMPARATIVE STRENGTHS OF SOLID AND OF HOLLOW WALLS OF BRICK 
 
 In order to study the effect of design on the strength of brick 
 masonry, the walls of series 2, alike in all respects except the type of 
 wall built with spread mortar joints and aged under ordinary con¬ 
 ditions, will first be considered. 
 
 The average results of the compressive tests of these brick walls 
 are given in Table 10 and are shown graphically in Figures 30 and 31. 
 Each value is the average for three similar walls. 
 
542 
 
 Bureau of Standards Journal of Research 
 
 [FoZ.S 
 
 The values of compressive strength, which are based on gross area, 
 are the same as those in Table 9 and were obtained by dividing the 
 maximum load on the specimens by the gross area. The net areas 
 of the walls were calculated as the product of the actual width of the 
 brick withes and the length of the walls. They are less than the 
 gross areas because the thickness of the longitudinal vertical joint 
 was not included. For the hollow walls these net areas represent the 
 minimum area in compression and for the solid walls they represent 
 the sums of the areas of the brick withes under compression. The 
 gross and net areas are the same for the 4-inch economy wall. 
 
 Table 10. —Average strength of the different types of brick walls of series 2 
 
 CEMENT-LIME MORTAR 
 
 Mississippi brick 
 
 New England brick 
 
 Type of wall 
 
 8-inch solid..... 
 
 12-inch solid____ 
 
 8-inch all-rolok_ 
 
 12-inch all-rolok_ 
 
 8-inch all-rolok in Flemish bond.. 
 12-inch all-rolok in Flemish bond. 
 
 8-inch rolok-bak... 
 
 12-inch rolok-bak.. 
 
 4-inch economy.. 
 
 Compressive strength based on— 
 
 Gross 
 
 area 
 
 Lbs./in. 2 
 1,160 
 1,300 
 765 
 705 
 750 
 675 
 940 
 850 
 1,435 
 
 Net area 
 
 Lbs./in. 2 
 1,265 
 1,450 
 1,345 
 1,270 
 1,320 
 1,220 
 1,265 
 1,280 
 1,435 
 
 Gross 
 
 area 
 
 Lbs./in. 2 
 1,790 
 1,890 
 875 
 740 
 640 
 830 
 880 
 965 
 1,940 
 
 Net area 
 
 Lbs./in. 2 
 2,030 
 2,180 
 1,570 
 1,350 
 1,130 
 1,560 
 1,210 
 1,480 
 1,940 
 
 CEMENT MORTAR 
 
 8-inch solid ...... . ..... . 
 
 1,380 
 
 1,515 
 
 2,635 
 
 2,980 
 
 12-inch solid __ ..... ... ... ... . ... 
 
 1,640 
 
 1,820 
 
 2,790 
 
 3,200 
 
 8-inch all-rolok. _ . . _ .. 
 
 920 
 
 1,630 
 
 955 
 
 1,710 
 
 12-inch all-rolok . ...... 
 
 800 
 
 1,450 
 
 870 
 
 1,610 
 
 8-inch all-rolok in Flemish bond ___ . ... . . .. 
 
 SOO 
 
 1,405 
 
 775 
 
 1,380 
 
 12-inch all-rolok in Flemish bond. 
 
 710 
 
 1,285 
 
 940 
 
 1,760 
 
 8-inch rolok-bak.. ....... . _ 
 
 920 
 
 1,240 
 
 1,205 
 
 1,670 
 
 12-inch rolok-bak. . . _ ... 
 
 940 
 
 1,420 
 
 1,590 
 
 2,460 
 
 4-inch economy_... ... .. __ 
 
 1,625 
 
 1,625 
 
 3,145 
 
 3,145 
 
 The compressive strengths in pounds per square inch of the three 
 types of solid walls are about equal. It is especially noteworthy 
 that the thin 4-inch econonrp wails, when concentrically loaded, 
 withstood as high compressive stresses as did the walls 8 and 12 
 inches thick. 
 
 For the walls built of either Mississippi or New England brick 
 the solid specimens withstood greater loads and greater stresses, 
 based on gross area, than did the hollow walls. If the wall strength 
 is a function of the area in compression, as may reasonably be sup¬ 
 posed, the strengths, based on net areas, should be the same for 
 all walls built of the same kind of brick and laid in the same kind of 
 mortar. This is apparently true for the walls of Mississippi brick, 
 
McBumey 80719 ' ] Compressive Strength of Clay Brick Walls 
 
 543 
 
 as may be seen from Figure 30. The strength values listed under 
 Mississippi brick, net area, in Table 10 are remarkably uniform for 
 each mortar mixture, and it is apparent that the net areas of the walls 
 built of Mississippi brick may be used as a basis of design for the 
 hollow walls. These bricks were not warped, were fairly uniform 
 in size, and had nearly perfectly rectangular cross sections. 
 
 The results of the tests of the hollow walls of New England brick, 
 however, show that they do not have a constant compressive strength 
 
 Figure 30. —Average strength of the walls of series 2 
 built of Mississippi brick 
 
 based on net area. Table 6 indicates that the various strength 
 properties are from two to three times as much for the New England 
 as for the Mississippi brick, while Table 10 shows that the solid walls 
 of New England brick are considerably stronger than those of Mis- 
 sissippi brick. The strengths of the hollow walls are, however, not 
 greatly different for the two kinds of brick. 
 
 The New England brick were not as regular in their shape as the 
 other. The cross section of these brick was similar in shape to that 
 shown in Figure 32, although the difference in width between top 
 and bottom is much exaggerated. The difference in the top and 
 bottom widths of these molded brick amounted to about 0.1 inch, 
 
544 
 
 Bureau of Standards Journal of Research 
 
 [Vol.S 
 
 and it may be that this lack of a truly rectangular cross section 
 affected the wall strength when the brick was laid on the narrow 
 edge as in the hollow walls. 
 
 Since the hollow walls of the strong New England brick are not 
 much stronger than those built of the Mississippi brick, it must be 
 
 Figure 31 .—Average strength of the walls of series 2 
 built of New England brick 
 
 concluded that the compressive and tensile strength and the modulus 
 of rupture of the individual brick do not completely define the factors 
 that have a determining influence on the strength of hollow masonry. 
 Further research will be necessary before the strength of a hollow 
 wall can be predicted from the strength of the brick and mortar. 
 
 In order to study the effect of type on wall strength when the walls 
 were built with less care, those walls of series 1 listed in Table 11 will 
 be considered. 
 
fi?Bur P ne T v° ns '\ Compressive Strength of Clay Brick Walls 
 
 Table 11 . —Strength of different types of walls of series 1 
 
 545 
 
 [Brick, Mississippi; mortar, cement] 
 
 Type of wall 
 
 Wall 
 
 Wall strength 
 based on— 
 
 No. 
 
 Gross 
 
 area 
 
 Net area 
 
 8-inch solid.._ ... ... ... 
 
 155 
 
 Lbs./in. 2 
 870 
 440 
 
 Lbs./in. 2 
 950 
 780 
 
 8-inch all-rolok . . ._ ___ ... ___ 
 
 156 
 
 8-inch all-rolok in Flemish bond... . ... .... 
 
 157 
 
 535 
 
 940 
 
 1,010 
 
 8-inch rolok-bak. _ _____ 
 
 158 
 
 745 
 
 
 Since only one wall of each type was tested, the results are not very 
 conclusive, but, in general, they show the same trend as those for the 
 
 Figure 32 .—An exaggerated view of a cross section of an 
 
 irregular brick 
 
 walls of series 2 built of Mississippi brick. The solid wall withstood 
 the greatest compressive stress, based on gross area, and the hollow 
 walls were practically as strong as the solid, based on net area. 
 
 The walls of series 1 had lower strengths than those of series 2, but 
 the values given in Table 11 show that the hollow walls of brick 
 may be safely used for many construction purposes. 
 
 (f) THE EFFECT OF MORTAR ON THE STRENGTH OF WALLS OTHERWISE SIMILAR 
 
 In series 1 the tests of the solid walls of Chicago brick afford com¬ 
 parisons from which the effect of the mortar on wall strength can be 
 determined. The three mortar mixtures had very different com- 
 
546 Bureau, of Standards Journal of Research [voi.s 
 
 pressive strengths as may be noted from Table 8. The strength of the 
 solid walls (Table 9) varied about as the cube root of these mortar 
 cylinder strengths. Although this is by no means to be taken as an 
 exact relationship, the ratio of average wall strength to cube root of 
 dry mortar cylinder strength is more constant for the three mixtures 
 of mortar than any other simple relation which was tried. 
 
 The strengths of the mortar cylinders varied considerably from wall 
 to wall with the same mixture, and only averages from a large number 
 of cylinders can be said to have a significant value. In several of the 
 groups of three walls which were alike in type of wall, mortar, work¬ 
 manship, brick, curing conditions, etc., the difference in wall strength 
 did not always follow in the same order as the differences in mortar 
 cylinder strength. Furthermore, the increase in wall strength due to 
 an increase in mortar strength was usually hot as great with the 
 Mississippi as with the stronger New England brick. 
 
 An attempt was made to determine the difference in wall strength 
 produced by the addition of a small amount of lime to a cement mor¬ 
 tar. For this comparison the two groups of 8-inch solid walls of 
 Chicago brick in series 2 were made. Table 12 gives the results of 
 these tests. 
 
 There was no significant difference either in average wall strength 
 or in average mortar cylinder strength. The values of wall strength 
 for the two groups overlap, as may be seen from Table 9. 
 
 In Table 13 are collected the results of the tests of the solid walls 
 of series 2 for which the variables are kinds of brick and mortar. 
 
 The increase due to difference in mortar is greater for the strong 
 New England brick than for the other lands. 
 
 The ratio of the cube root of the dry mortar cylinder strengths 
 (see Table 8) for the mortars of these walls is 1.38, a value within the 
 range of variation of the wall strength ratios given in Table 13. 
 
 Table 12. —Results of tests of walls with slight differences in the mortar mixtures 
 [Wall type, 8-inch solid; brick, Chicago; series 2; workmanship, spread joints; curing conditions, ordinary.] 
 
 Wall Nos. 
 
 Mortar proportions (by volume) 
 
 Average 
 
 wall 
 
 strength 
 
 Average mortar 
 cylinder strength 
 
 Wet 
 
 Dry 
 
 160,161,162___ 
 
 1C: 3S... 
 
 Lbs.fin . 2 
 880 
 855 
 
 Lbs./in . 2 
 3,050 
 3, 580 
 
 Lbs./in . 2 
 2,180 
 2,350 
 
 163, 164,165__ 
 
 1C: 0.1L: 3S_ 
 
 
 
ficBume™ 0718 ’] Compressive Strength of Clay Brick Walls 
 
 Table 13. —Results of tests of solid walls of series 2 
 
 547 
 
 [Smooth spread mortar joints; ordinary curing conditions] 
 
 Type of wall 
 
 8 -inch solid_ 
 
 12 -inch solid... 
 4-inch economy 
 
 Average wall strength 
 
 Kind of brick 
 
 Cement- 
 
 lime 
 
 ( C-L.) 
 
 Cement 
 
 (O 
 
 Ratio 
 
 C 
 
 C-L 
 
 Detroit .. ... 
 
 Lbs. tin . 2 
 910 
 
 Lbs./in.* 
 1,080 
 
 1.19 
 
 Mississippi.. _ 
 
 1,160 
 
 1,380 
 
 1.19 
 
 New England. .... .. 
 
 1, 790 
 
 2, 635 
 
 1.47 
 
 Detroit. ___ 
 
 985 
 
 1,210 
 
 1.23 
 
 Mississippi_ . ._ 
 
 1,300 
 
 1,640 
 
 1 . 26 
 
 New England.. 
 
 1,890 
 
 2, 790 
 
 1. 48 
 
 Mississippi.. ... .. _ 
 
 1, 435 
 
 1,625 
 
 1.13 
 
 New England_ 
 
 1, 940 
 
 3,145 
 
 1.62 
 
 The effect of mortar on the strength of walls of different types is 
 shown graphically in Figures 33 and 34 for the walls built of Mississippi 
 
 Mississippi Brick 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 © ce> 
 o cer 
 
 Legend 
 ment mori 
 nent-//me / 
 
 "or 
 
 nortar 
 
 
 
 
 
 126 4 
 
 ► 
 
 
 
 
 
 
 1/5 ® 
 
 | 
 
 //?< 
 
 ( 
 
 » 
 
 < 
 
 ) 
 
 ) 
 
 
 
 
 
 
 O 
 
 
 
 1.20 < 
 
 c 
 
 » 
 
 j < 
 
 * 1.07 < 
 
 > ' 
 
 > 
 
 > t.05\ 
 
 .96 t 
 
 s 
 
 i w < 
 
 c 
 
 » 
 
 > 
 
 
 
 
 
 * 
 
 * 
 
 
 
 
 3000 
 
 2500 
 
 2c 
 
 I 
 
 I 
 
 /ooo 
 
 500 
 
 
 I 
 
 <50 
 
 * a// ro/ok in Flemish bond 
 
 “so/id 
 
 
 1 
 
 1 
 
 85 
 
 g 
 
 5 
 
 1 
 
 I 
 
 
 «o 
 
 '1 
 
 <*> 
 
 
 
 N<\, 
 
 | 
 
 J 
 
 Figure 33. —The differences in wall strength due to differences in mortar 
 for the walls of series 2 built of Mississippi brick 
 
 and New England brick, respectively. The numbers placed beside the 
 values for the cement mortar walls are the ratios of the wall strengths 
 of cement mortar to those of cement-lime mortar. There appears to 
 be no definite and consistent relation for the increase in strength due 
 to the use of cement mortar, the increase being different from, the 
 
548 
 
 Bureau of Standards Journal of Research 
 
 [Vol.S 
 
 different types of walls and usually being less for the Mississippi 
 than for the stronger New England brick. 
 
 (g) THE EFFECT OF DAMP CURING ON THE COMPRESSIVE STRENGTH OF BRICK WALLS 
 
 One of the problems studied in this series of tests was to determine 
 the effect of damp curing on the compressive strength of brick walls. 
 At the time these tests were outlined there was a more or less general 
 opinion that walls kept damp for a time after they had been built 
 would be stronger than others allowed to age without wetting. 
 Cylinders of Portland cement mortar (size 2 by 4 inches) aged for 60 
 
 
 
 
 
 
 
 
 
 752 0 
 
 r47i 
 
 7.48 k 
 
 > 
 
 ► 
 
 
 
 
 
 
 
 
 
 
 © o 
 
 Legend 
 
 nentmortcv 
 
 nent-Umem 
 
 vrtor 
 
 
 
 
 C 
 
 ( 
 
 ) 
 
 > 
 
 
 
 
 
 7.65K 
 
 O 
 
 0 
 
 
 
 
 
 
 
 137 ( 
 
 > 
 
 
 
 
 70S 
 
 < 
 
 s 
 
 > 7.78 ( 
 
 C 
 
 » 
 
 < 
 
 US 4 
 
 ) 
 
 
 X 
 
 ) 
 
 f- 
 
 4 
 
 
 
 
 
 -* 
 
 * 
 
 
 
 
 3000 
 
 £500 
 
 2000 
 
 ISOO 
 
 /ooo 
 
 500 
 
 
 $ 
 
 <0 
 
 i 
 
 >1 
 
 I 
 
 ,1 
 
 "JN 
 
 Is 
 
 1 
 
 I 
 
 * y 
 
 IS* 
 
 1 
 
 «o 
 
 I 
 
 'Its 
 
 Is 
 
 <=0 
 
 ^ * 
 
 *all ro!ok in Flemish bond 
 
 Figure 34 .—The differences in wall strength due to differences in mortar 
 for the walls of series 2 built of New England brick 
 
 days in water are about twice as strong as those kept in air, as may be 
 noted from Table 8, and, consequently, it might be expected that 
 conditions which tend to keep the mortar in the wall moist would also 
 tend to increase its strength and therefore increase the strength of 
 the wall. 
 
 Some walls of series 2, built with cement mortar, were allowed to 
 age in the laboratory under ordinary conditions. Other similar walls 
 listed in Tables 1 and 9 as “wetted” were covered with burlap, as 
 shown in Figure 15. The burlap was kept damp for one week after 
 the walls had been built and was then removed. 
 
 The results of these compressive tests of brick walls are given in 
 Table 14. 
 
fi?Bu?ne T y 0ns ’ ] Compressive Strength of Clay Brick Walls 
 
 549 
 
 It is seen from these data that there was no increase in the strength 
 of the walls due to damp curing. Whether the same results would 
 have been obtained on specimens built out of doors is, perhaps, open 
 to question. 
 
 Although the strength of small mortar cylinders is increased by 
 keeping them wet, there are several reasons why much greater 
 strength in the wet walls than in the dry ones would not be expected. 
 
 In the first place, the failure in all of these walls laid in cement 
 mortar occurred in the brick and not in the mortar. While walls 
 similar in all respects, except as to mortar, will generally have the 
 higher strengths with stronger mortars, the strength of the brick is 
 probably the determining factor in wall strength when the mortars are 
 of about the same strength. 
 
 Table 14 . — Effect of wetting on wall strength 
 [Walls of series 2, cement mortar] 
 
 Type of wall 
 
 8 -inch solid 
 
 12 -inch solid 
 
 12 -inch rolok-bak 
 
 Kind of brick 
 
 Wall Nos. 
 
 f 
 
 Curing conditions 
 
 Average 
 
 wall 
 
 strength 
 
 Ratio of 
 wall 
 
 strengths, 
 
 wetted/ 
 
 ordinary 
 
 /Detroit... .. 
 
 22, 23, 24... 
 
 Ordinary. 
 
 Lbs./in. 1 
 1,080 
 1,055 
 
 1,210 
 1,105 
 
 1, 785 
 
 1, 590 
 2,835 
 2,510 
 
 935 
 
 
 \_do_ 
 
 33--.... 
 
 Wetted.. 
 
 0.98 
 
 f. ..do__ 
 
 28, 29, 30_ 
 
 Ordinary_ 
 
 . ..do ... . .. ._ 
 
 31, 32 .... 
 
 Wetted_... 
 
 .91 
 
 Mississippi .. . . 
 
 43, 44 _ 
 
 Ordinary_ 
 
 _do... ___ 
 
 46', 47..... 
 
 Wetted.'_ 
 
 .89 
 
 New England_ 
 
 103, 104 
 
 Ordinary_ . 
 
 _do... ..... 
 
 106^ 107... 
 
 Wetted... 
 
 .89 
 
 [Mississippi.._ 
 
 82, 83__ 
 
 Ordinary_ . 
 
 1 _do__ 
 
 85, 86 _ 
 
 Wetted.'_ 
 
 965 
 
 1.03 
 
 ) New England . 
 
 142, 143 
 
 Ordinary.... __ 
 
 1,580 
 
 1,325 
 
 [.. .do... ..... 
 
 145' 146 _ 
 
 Wetted.'_ 
 
 .84 
 
 
 
 
 In the second place, it has been found that an increase in mortar 
 strength is accompanied by a relatively smaller increase in wall 
 strength if the same quality of brick is used. 8 Hence, a large increase 
 in mortar strength is necessary to produce a notable increase in wall 
 strength. This should not, however, be taken to mean that the 
 mortar is not a very important factor in wall strength. 
 
 It is probable, moreover, that the wetting of the walls did not in¬ 
 crease greatly the strength of the mortar, for although the wetting 
 would tend to retard the evaporation of moisture, the loss of mois¬ 
 ture from the walls in dry storage proceeded at a slow rate. The 
 moisture present in the brick and mortar at the time the walls were 
 built evaporated at such a slow rate as to provide sufficient moisture 
 at early ages for favorable curing conditions, since in all of these 
 
 8 See data od wall strength and mortar strength in B. S. Tech. Paper No. 276, Compressive Strength 
 of Sand-Lime-Brick Walls, by Whittemore and Stang. 
 
550 Bureau of Standards Journal of Research [voi.s 
 
 walls the brick were probably wetter when laid than is the case in 
 ordinary commercial practice. 
 
 Finally, a few samples of the mortar taken from these walls after 
 test were found to contain, on the average, about 8 per cent of mois¬ 
 ture by weight. This damp condition would tend to decrease the 
 strength of the brick and of the mortar and, consequently, the 
 strength of the damp cured walls. There are no definite data to 
 show how much the strength of either the mortar or the brick was 
 changed by the indefinite or rather nonuniform moisture content of 
 the walls after they had aged. Numerous tests 8 9 have shown that 
 Portland-cement mortars and concrete are considerably weakened 
 when wetted just previous to testing. 
 
 The strength of some bricks is decreased by moisture. Others do 
 not appear to have their strength influenced by this condition. Table 
 15 gives the compressive strengths of bricks used in this investiga¬ 
 tion when tested flatwise in the wet and dry condition, each value 
 begin the average of 50 tests. The wet bricks had been immersed in 
 water for 48 hours before test and were, of course, much wetter than 
 were those in the walls at the time of test. 
 
 Table 15. —Compressive strength of half brick wet and dry, flatwise 
 
 Kind of brick 
 
 Compressive 
 
 strength 
 
 Ratio 
 
 wet/dry 
 
 Wet 
 
 Dry 
 
 Detroit_____ 
 
 Lbs. /in . 8 
 2,520 
 3, 520 
 6 , 990 
 
 Lbs./in* 
 3,540 
 3,410 
 8,600 
 
 0. 71 
 1.03 
 .81 
 
 Mississippi________ 
 
 New England______ ___ 
 
 
 The values given in Table 15, purporting to show the effect of 
 moisture on the compressive strength of brick, probably indicate a 
 somewhat greater decrease in strength due to moisture than actually 
 existed because of the wet plaster of Paris cap. All of the bricks 
 were capped with plaster of Paris prior to testing and, since the 
 strength of gypsum is greatly decreased by moisture, 10 the weakening 
 of the caps may have affected the results. 
 
 The Detroit and New England bricks were considerably weaker 
 when wet, while the strength of the Mississippi brick did not appear 
 to be affected by moisture. The slightly low compressive value for 
 these brick dry is probably due to the sampling error and not to any 
 real difference in their compressive strengths when in different moist¬ 
 ure conditions. Reference to Table 14, however, shows that the wet 
 
 8 Herbert J. Gilkey, The Effect of Varied Curing Conditions Upon the Compressive Strength of Mortar 
 
 and Concrete, Proc. Am. Concrete Inst., 22; 1926. 
 
 10 W. A. Slater, Some Structural Properties of Gypsum and of Reinforced Gypsum, Trans. Ill. Acad. 
 Sci., 9, 
 
AicBurncy Sons ’ ] Compressive Strength of Clay Brick Walls 
 
 551 
 
 walls built of Detroit and New England bricks were only a little 
 weaker relative to the companion dry walls than those built of 
 Mississippi brick, the average ratios being 0.905 for the Detroit and 
 New England and 0.960 for the Mississippi bricks. 
 
 Thus we have opposing effects which tend to keep the wall strength 
 constant as between damp cured and dry walls. The dampness 
 may tend to produce a stronger mortar, but the moisture present in 
 the brick at the time of test weakens the wall. 
 
 It seems reasonable, then, to suppose that brick walls built with 
 strong cement mortar and fairly wet brick will be as strong when 
 aged in air for 60 days as they will be if they are kept damp for a 
 short time after construction. 
 
 (h) THE EFFECT OF THE QUALITY OF WORKMANSHIP ON WALL STRENGTH 
 
 The workmanship typical of each of the two masons who built 
 these brick walls has already been described in some detail and may 
 be summarized as follows: 
 
 For walls of series 1, relatively high rate of laying brick, absence of 
 mortar in the longitudinal vertical joints, and pronounced furrowing in 
 the horizontal mortar beds; for walls of series 2, slow rate of laying brick, 
 careful filling of the vertical joints, and smooth spread bed joints. 
 
 In Table 16 are listed those walls which show the effect of differ¬ 
 ences in workmanship. For the 8-inch solid walls of Chicago brick, 
 the average thickness of the mortar joints was almost the same for 
 both groups, and the 30 per cent increase in strength for the walls of 
 series 2 must be attributed to the differences in workmanship. 
 
 The important differences as far as they could be observed consisted 
 of the absence of furrowing of the horizontal mortar joints in series 
 2 and the better filling of the vertical joints. With the Mississippi 
 brick, wall 155 of series 1 had exceptionally thick mortar joints 
 (0.73 inch), arid it may be that the greater strength of walls 37, 
 38, and 39, of 59 per cent, was due to their having thinner joints. 
 The absence of furrowing and the filling of the vertical joints ap¬ 
 parently made the 8-inch solid walls of New England brick and the 
 12-inch solid walls of Chicago brick of series 2 decidedly stronger 
 than the companion walls of series 1. 
 
 The large increases in strength for the hollow walls of series 2 over 
 the strengths of walls of series 1 was unexpected. It was supposed 
 that the use of brick on edge in the construction of the hollow walls 
 would not permit furrowing. However, demolition of these walls 
 after testing showed that the mason furrowed the horizontal mortar 
 bed on the narrow edge of the brick as consistently as on the regular 
 solid construction. The furrow was continuous throughout the 
 entire length of the mortar bed, but was broken up in much the same 
 manner as were the furrows in the solid walls as shown in Figure 7, 
 
552 
 
 Bureau of Standards Journal of Research 
 
 [ Vol.S 
 
 The uneven and irregular bedding probably produced bending 
 stresses in the bricks, which caused local concentrations of load and 
 resulted in failure at lower loads than would have been the case with 
 more even joints. 
 
 Table 16. —Result of tests of brick walls with variations in workmanship 
 
 [Cement mortar throughout] 
 
 Type of wall 
 
 Brick 
 
 Series 1 (furrowed joints) 
 
 Series 2 (spread joints) 
 
 Increase 
 in wall 
 strength, 
 series 2 
 to series 1 
 
 Wall 
 
 No. 
 
 Average 
 thickness 
 of mortar 
 joints 
 
 Wall 
 
 strength 
 
 Wall 
 
 No. 
 
 Average 
 thickness 
 of mortar 
 joints 
 
 Wall 
 
 strength 
 
 
 
 
 Inch 
 
 Lbs./in. 2 
 
 
 Inch 
 
 Lbs./in. 2 
 
 Per cent 
 
 
 
 ( 13 
 
 0. 56 
 
 660 
 
 160 
 
 0.57 
 
 800 
 
 
 
 Chicago_ 
 
 1 14 
 
 .56 
 
 750 
 
 161 
 
 .59 
 
 870 
 
 
 
 
 1 15 
 
 .57 
 
 580 
 
 162 
 
 .55 
 
 965 
 
 
 
 Average... 
 
 
 .56 
 
 665 
 
 
 .57 
 
 880 
 
 32 
 
 
 
 
 
 
 f 37 
 
 .52 
 
 1,335 
 
 
 
 Mississippi_ 
 
 155 
 
 .73 
 
 870 
 
 \ 38 
 
 .44 
 
 1, 405 
 
 
 8 -inch solid.. 
 
 
 
 
 
 ( 39 
 
 .40 
 
 1,405 
 
 
 
 Average—. 
 
 
 
 
 
 .45 
 
 1, 380 
 
 59 
 
 
 
 
 
 
 f 97 
 
 .36 
 
 2,040 
 
 
 
 New England... 
 
 154 
 
 .46 
 
 2, 030 
 
 98 
 
 .22 
 
 2,950 
 
 
 
 
 
 
 
 ( 99 
 
 .27 
 
 2, 915 
 
 
 
 Average. . 
 
 
 
 
 
 .28 
 
 2, 635 
 
 30 
 
 
 (■Chicago... 
 
 f I 6 
 
 .57 
 
 655 
 
 166 
 
 .49 
 
 890 
 
 
 
 l 17 
 
 .56 
 
 655 
 
 167 
 
 .51 
 
 1 , 080 
 
 
 19-infh snlirl 
 
 
 
 
 
 
 
 
 
 
 [ Average... 
 
 
 .56 
 
 655 
 
 
 .50 
 
 985 
 
 50 
 
 
 
 
 
 
 52 
 
 .46 
 
 850 
 
 
 
 (Mississippi_ 
 
 156 
 
 .66 
 
 440 
 
 ■ 53 
 
 .40 
 
 735 
 
 _ _ _ 
 
 S-inch all-rolok_ 
 
 
 
 
 
 54 
 
 .41 
 
 1,175 
 
 
 
 Average .. 
 
 
 
 
 
 .42 
 
 920 
 
 109 
 
 
 
 
 
 
 f 64 
 
 .47 
 
 825 
 
 
 
 (Mississippi... 
 
 157 
 
 .65 
 
 535 
 
 \ 65 
 
 .43 
 
 720 
 
 
 8 -inch all-rolok in 
 
 
 
 
 
 1 66 
 
 .43 
 
 855 
 
 
 
 
 
 
 
 
 
 
 
 
 Average.. - 
 
 
 
 
 
 .44 
 
 800 
 
 50 
 
 
 
 
 
 
 f 76 
 
 .48 
 
 925 
 
 
 
 (Mississippi.. . 
 
 158 
 
 .62 
 
 745 
 
 j 77 
 
 . 42 
 
 890 
 
 
 8 -inch rolok-bak__ _ __ 
 
 
 
 
 
 1 78 
 
 .'47 
 
 950 
 
 
 
 Average... 
 
 
 
 
 
 .46 
 
 920 
 
 24 
 
 These comparisons of strengths of hollow walls lead to the belief 
 that full spread horizontal mortar beds without furrowing are more 
 important from a strength standpoint than the complete filling of 
 vertical joints. In the hollow walls the longitudinal space between 
 the rowlock wythes, of course, contained no mortar, and differences 
 in strength were, therefore, almost entirely due to differences in the 
 horizontal mortar beds. 
 
 The apparent effect of furrowing and unfilled vertical joints on wall 
 strength is strikingly brought out by a comparison of the strengths 
 of walls 155 and 159, as shown in Table 17. Both walls were built 
 by the mason of series 1. Although wall 159, with no furrowing in 
 the mortar beds, was built at an exceptionally fast rate, its strength 
 was 70 per cent greater than that of wall 155 with the typical furrows. 
 
McBumey Sons ’\ Compressive Strength of Clay Brick Walls 553 
 
 \Table 17 .—Effect of furrowing and unfilled vertical joints in walls of series 1 
 [Type of wall, 8-inch solid; mortar, cement; brick, Mississippi] 
 
 Workmanship (type of bed joints) 
 
 Wall No. 
 
 Brick 
 laid per 
 hour 
 
 Average 
 thickness 
 of mortar 
 joint 
 
 Wall 
 
 strength 
 
 Furrowed. .. _ ....... __ 
 
 155 
 
 195 
 
 Inch 
 
 0.73 
 
 Lbs./in. 2 
 870 
 
 Spread, no furrowing... ___ ..... 
 
 159 
 
 264 
 
 .62 
 
 1,480 
 
 
 Ratio of wall strengths..... ... 
 
 
 
 
 1.70 
 
 
 
 
 
 The rate of laying wall 159, as given in Tables 3 and 17, and the 
 statement of the mason who built the walls of series 2 show that 
 spreading the mortar bed without furrowing does not reduce the rate 
 of laying brick. The comparative results given in Tables 16 and 17 
 show the remarkable increase in strength obtained by the better 
 filling of the mortar joints. It is believed that the realization of the 
 importance of having smooth and level horizontal mortar beds in a 
 brick wall is the most important conclusion of these tests. This con¬ 
 struction practice, which apparently involves no increase in labor 
 cost, but which results in a very significant increase in strength, is 
 strongly recommended to builders of masonry structures. 
 
 A comparison of “spread” and “shoved” workmanship for walls of 
 series 2 is afforded in Table 18. Wall 18 of Chicago brick and 
 “shoved” workmanship had thinner mortar joints than did the com¬ 
 panion walls 166 and 167 built of the same kind of brick with the full 
 spread joints of series 2. In the three groups listed in Table 18 the 
 walls of Chicago brick constitute the only group for which the 
 “shoved” workmanship was superior to the other. The table shows 
 that shoved work introduces no improvement in strength over the 
 spread and fully slushed joint workmanship which was typical of 
 series 2. 
 
 Table 18 .—Comparison of “spread” with “shoved” joints for walls of series 2 
 
 [Type of walls, 12-inch solid; mortar, cement] 
 
 Kind of brick 
 
 “Spread” joints 
 
 “Shoved” joints 
 
 Ratio of 
 wall 
 strength 
 “shoved” 
 to 
 
 “spread” 
 
 Wall No. 
 
 Average 
 thickness 
 of mortar 
 joint 
 
 Wall 
 
 strength 
 
 Wall No. 
 
 Average 
 thickness 
 of mortar 
 joint 
 
 Wall 
 
 strength 
 
 Chicago.... 
 
 f 166 
 
 \ 167 
 
 Inch 
 
 0.49 
 .51 
 
 Lbs./in. 2 
 890 
 1,080 
 
 18 
 
 Inch 
 
 0.31 
 
 Lbs./in. 2 
 1,165 
 
 1.18 
 
 Average__ _ 
 
 
 
 
 
 
 .50 
 
 985 
 
 
 
 
 
 Mississippi____ 
 
 
 
 
 
 
 [ 43 
 
 \ 44 
 
 l 45 
 
 .51 
 
 .37 
 
 .42 
 
 1, 710 
 1, 855 
 1,350 
 
 48 
 
 .42 
 
 1,465 
 
 .89 
 
 Average. 
 
 
 
 
 
 
 .43 
 
 1, 640 
 
 
 
 
 
 New England .. 
 
 
 
 
 
 
 f 103 
 
 \ 104 
 
 [ 105 
 
 .28 
 
 .23 
 
 .23 
 
 2,440 
 3,230 
 2, 695 
 
 108 
 
 .22 
 
 2, 720 
 
 .97 
 
 Average. 
 
 
 
 
 
 
 .25 
 
 2,790 
 
 
 
 
 
 
 
 
 
 
 
 69882°—29 - 4 
 
554 
 
 Bureau of Standards Journal of Research 
 
 [Vol.S 
 
 Reference has been made several times to the thickness of the 
 mortar joints as having a possible effect on wall strength. Although 
 this investigation was not planned with a view to having thickness 
 of mortar joint a variable for the walls, Table 3 shows that some 
 variation did exist. A study of the data for the single walls shows 
 that joint thickness was by no means a controlling factor in strength 
 for the hollow walls. The thickness of the beds in the groups of 
 solid walls of series 1 was practically uniform from wall to wall and 
 the results of these tests throw no light on this subject. 
 
 The thickness of the mortar joints and the wall strength are given 
 in Table 19 for 18 groups of solid walls of series 2 built with com¬ 
 parable workmanship and aged under ordinary conditions. The 
 walls of each group have been arranged in the order of wall strengths, 
 and it might be expected that the wall having the least thickness of 
 mortar joint would have the greatest strength. In the column des¬ 
 ignated “order” of* this table those groups have been classed as 
 “regular,” for which increase in joint thickness is accompanied by 
 decreases in wall strength, as “reverse,” if increase in joint thickness 
 is accompanied by increase in wall strength, and as “irregular” for 
 the others which follow neither of these classifications entirely. 
 
 Table 19. —Effect of thickness of mortar joints on strength of solid walls of series 2 
 
 A. CEMENT-LIME MORTAR 
 
 Type of wall 
 
 8 -inch solid. 
 
 12 -inch solid. 
 
 4-inch economy. 
 
 Kind of brick 
 
 Detroit_ 
 
 Mississippi_ 
 
 New England. 
 
 Detroit_ 
 
 Mississippi..., 
 
 New England. 
 
 Mississippi.... 
 
 New England. 
 
 Wall 
 
 No. 
 
 19 
 
 20 
 21 
 
 36 
 
 35 
 
 34 
 
 95 
 94 
 
 96 
 
 25 
 27 
 
 26 
 
 40 
 
 41 
 
 42 
 
 101 
 
 102 
 
 100 
 
 90 
 
 88 
 
 89 
 
 149 
 148 
 
 150 
 
 Average 
 thickness 
 of mortar 
 joint 
 
 Inch 
 0. 39 
 * .40 
 .44 
 
 .40 
 
 .44 
 
 .56 
 
 .28 
 
 .35 
 
 .27 
 
 .38 
 
 .44 
 
 .38 
 
 .50 
 
 .40 
 
 .44 
 
 .20 
 
 .25 
 
 .33 
 
 .42 
 
 .44 
 
 .40 
 
 .30 
 
 .36 
 
 .22 
 
 Wall 
 
 strength 
 
 Lbs. /in . 2 
 1,060 
 905 
 760 
 
 1, 340 
 1,175 
 965 
 
 1, 875 
 1,785 
 1, 710 
 
 1,050 
 
 965 
 
 940 
 
 1, 375 
 1, 305 
 1,225 
 
 1, 990 
 1, 855 
 1, 815 
 
 1, 565 
 1, 370 
 1, 365 
 
 1, 975 
 1 , 960 
 1,880 
 
 Order 
 
 I Regular. 
 
 Do. 
 
 jlrregular. 
 
 Do. 
 
 Do. 
 
 Regular. 
 
 jlrregular. 
 
 Do. 
 
McBumey 80 ™’] Compressive Strength of Clay Brick Walls 555 
 
 Table 19 .—Effect of thickness of mortar joints on strength of solid walls of series 2 — 
 
 Continued 
 
 B. CEMENT MORTAR 
 
 Kind of brick 
 
 Wall 
 
 No. 
 
 Average 
 thickness 
 of mortar 
 joint 
 
 Wall 
 
 strength 
 
 
 
 Inch 
 
 Lbs./inJ 
 
 
 1 C 2 
 
 0.55 
 
 965 1 
 
 Chicago... 
 
 161 
 
 .59 
 
 870 
 
 
 160 
 
 .57 
 
 800 J 
 
 
 r 164 
 
 .57 
 
 925 ) 
 
 _do..... 
 
 163 
 
 .62 
 
 885 \ 
 
 
 ( 165 
 
 .66 
 
 755 J 
 
 
 f 22 
 
 .38 
 
 1,305 1 
 
 Detroit..... 
 
 j 24 
 
 .43 
 
 1,050 
 
 
 ( 23 
 
 .40 
 
 895 J 
 
 
 f 38 
 
 .44 
 
 1,405 ) 
 
 Mississippi_ 
 
 39 
 
 .40 
 
 1,405 \ 
 
 
 ( 37 
 
 .52 
 
 1,335 ] 
 
 
 98 
 
 .22 
 
 2,950 1 
 
 New England.. 
 
 99 
 
 .27 
 
 2,915 \ 
 
 
 97 
 
 .36 
 
 2,040 j 
 
 
 f 29 
 
 .37 
 
 1,345 1 
 
 Detroit____ 
 
 1 28 
 
 .39 
 
 1,165 \ 
 
 
 ( 30 
 
 .41 
 
 1,115 j 
 
 
 ( 44 
 
 .37 
 
 1,855 1 
 
 Mississippi_ 
 
 43 
 
 .51 
 
 1,710 
 
 
 ( 45 
 
 .42 
 
 1,350 J 
 
 
 f 104 
 
 .23 
 
 3,230 1 
 
 New England ... __ 
 
 105 
 
 .23 
 
 2,695 \ 
 
 
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 2,440 J 
 
 
 f 93 
 
 .46 
 
 1,875 1 
 
 'Mississippi... 
 
 91 
 
 .44 
 
 1,650 
 
 
 ( 92 
 
 .41 
 
 1,350 j 
 
 
 [ 152 
 
 .29 
 
 3,520 1 
 
 New England.. 
 
 153 
 
 .23 
 
 3,160 \ 
 
 
 ( 151 
 
 .35 
 
 2,755 j 
 
 Type of wall 
 
 8 -inch solid. 
 
 12 -inch solid. 
 
 4-inch economy. 
 
 Order 
 
 Do. 
 
 Do. 
 
 •Regular. 
 
 ^Reverse. 
 
 •Irregular. 
 
 If there is only a chance relation between joint thickness and wall 
 strength, it would be expected that the “regular” and the “reverse” 
 order would each occur three times in the 18 groups. The table 
 shows that, of the 18 groups, 7 are regular, or two and one-third times 
 as many as would be expected from a chance distribution. Only one 
 reverse group is present instead of the three to be expected. The 
 tendency is therefore for walls with thin joints to have somewhat 
 higher strengths, although it has been impossible to find a definite 
 relationship between these two factors. 
 
 (i) THE RELATION OF STRESS AT FIRST CRACK TO MAXIMUM STRESS 
 
 Table 20 gives the ratio of the stress at first crack to the maxi¬ 
 mum stress for the walls of series 1 with furrowed joints and for those 
 of series 2 with full-spread joints which were cured under ordinary 
 conditions. 
 
 For the solid walls of series 1 it appears that these ratios become 
 greater as the strength of the mortar increases. The average ratio 
 
556 
 
 Bureau of Standards Journal of Research 
 
 [Vol. 3 
 
 for the Chicago brick walls of series 2 built with cement and cement- 
 lime mortars is greater than for those of series 1 built with the same 
 mortars. 
 
 The solid walls of Detroit and of New England brick, both of 
 which had frogs, had initial signs of failure at considerably lower 
 ratios than for the solid walls of the other kinds of brick. Whether 
 this relatively low initial failure is due in part to the presence of the 
 frog in these bricks and to the fact that they were more irregular in 
 size and shape than the Chicago and Mississippi bricks is unknown. 
 
 Table 20. —Relation of stress at first crack to wall strength 
 
 [Ordinary curing conditions] 
 
 Description of groups compared 
 
 Number 
 of walls 
 averaged 
 
 Stress at 
 first 
 crack/ 
 wall 
 strength 
 
 Series 1 (furrowed joints): 
 
 Lime-mortar walls, Chicago brick..... 
 
 6 
 
 Ratio 
 
 0. 71 
 
 Cement-lime mortar walls, Chicago brick.- -------- 
 
 6 
 
 .88 
 
 Cement mortar walls, Chicago brick_- _ _ _ _ .. ... _ 
 
 5 
 
 .94 
 
 All walls, Chicago brick_ -____ 
 
 17 
 
 .84 
 
 Series 2 (full spread joints): 
 
 Solid walls of Chicago brick_ __ _ __ __ 
 
 8 
 
 .96 
 
 Solid walls of Detroit brick_ - - . ... ___ _ 
 
 12 
 
 .64 
 
 Solid walls of Mississippi brick -- --- - __ - _ ___ 
 
 18 
 
 .92 
 
 Solid walls of New England brick-- _ -_ ----- - _ ... - 
 
 18 
 
 .68 
 
 Solid walls___ ... ... ... .. 
 
 56 
 
 .79 
 
 All-rolok walls__ _ 
 
 24 
 
 .70 
 
 All-rolok in Flemish bond walls_ -- _ _ ._ _ .. _ 
 
 24 
 
 .74 
 
 Bolok-bak walls.__ . _ ___ 
 
 24 
 
 .72 
 
 Walls of Mississippi biick_ _ _ _ ....__ . _ __ 
 
 54 
 
 .83 
 
 Walls of New England brick_ __ . ..._ ... - - _ 
 
 54 
 
 .67 
 
 Walls built with cement-lime mortar___ ... _ 
 
 60 
 
 .72 
 
 Walls built with cement mortar....__ _ __ .... 
 
 68 
 
 .78 
 
 
 Table 20 shows that the solid walls had a somewhat higher ratio 
 between stress at first crack and maximum stress than the hollow 
 walls. The ratios were about the same for the different types of 
 hollow walls. 
 
 The walls of series 2 built with cement mortar had a higher ratio 
 than those built with cement-lime mortar, the relative values being 
 much the same as for walls of series 1. 
 
 It is evident from a study of Table 20 that the relation of stress at 
 first crack to wall strength is not a constant, but varies with many of 
 the conditions that affect wall strength. 
 
 4. WALLETTES 
 
 The results of the compressive tests of the wallettes are given in 
 Table 21. The wallettes were built by the same mason and cured 
 under the same conditions as the walls of the corresponding numbers. 
 Most of the wallettes were built either on the same day as the walls 
 or on the day after. 
 
Table 21. —Results of compressive tests of brick wallettes (gross area ) 
 
 A. WALLETTES OF SERIES 1 
 
 Stang, Parsons ,1 
 McBurney J 
 
 Compressive Strength of Clay Brick Walls 
 
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ficBumey 0718 ’] Compressive Strength of Clay Brick Walls 
 
 559 
 
 The area of the wallettes was taken as the product of the average 
 thickness of the wallette by the minimum length. Figure 18 shows 
 that in small specimens, with the brick clipped as little as possible, 
 the length of the wallette may vary considerable from course to course. 
 The values of wallette strength given in Table 21 are therefore based 
 on minimum gross areas for the hollow as well as for the solid wallettes. 
 
 The compressive strength of the wallettes varied with the physical 
 properties of the brick, with the type of construction, with the mortar, 
 curing conditions, and workmanship very much as did the wall 
 strengths. 
 
 The variation in strength from wallette to wallette in any group 
 was much less for the specimens of series 1 than of series 2. The 
 mason who built the specimens of series 2 did not seem to like to 
 build the small walls and on several occasions had to be told to use 
 as much care on them as he used on the larger specimens. The 
 average fractional deviation of the strength values within a given 
 group from the average was for the wallettes of series 1, 0.04, and 
 for those of series 2 it amounted to 0.13. In drawing conclusions 
 from the tests of series 2 it is therefore to be remembered that rela¬ 
 tively large variations have occurred in many of the groups. 
 
 The differences in strength due to differences in the kind of brick 
 are found only for the wallettes of series 2. All bricks for the wallettes 
 of series 2 were soft-mud or dry-press bricks, except in the wallette 18 
 of Chicago brick. In all cases where groups of solid wallettes differing 
 only in the kind of brick are compared, the New England specimens were 
 strongest, followed in order by those of Mississippi and of Detroit brick. 
 
 As with the larger walls, the solid wallettes were stronger, based on 
 gross area, than were any of the hollow types. The strengths based 
 on net area were nearly the same irrespective of the type of 
 construction for wallettes built of Mississippi brick and, while not so 
 uniform for the wallettes of New England brick, were much more 
 constant than for the walls previously discussed. 
 
 Table 22 shows the relative strength of the different types of walls 
 and wallettes, the strongest being listed at the top in each group and 
 the others in the order of their strengths. In six of the eight wall 
 groups the types rank solid, rolok-bak, all-rolok, and all-rolok in 
 Flemish bond, and in the two cases where this order does not hold 
 the inversions are due to relatively small differences in strength—less 
 than 100 lbs./in. 2 . From the wallette tests, however, this arrange¬ 
 ment in the order for the strengths of the different types would not be 
 evident. In fact, only two groups of wallettes show this same order 
 while four wallette groups rank—solid, all-rolok in Flemish bond, 
 rolok-bak, and all-rolok. The fact that the solid wallettes were 
 stronger than any of the hollow types shows, however, that larger 
 strength differences are readily recognized from wallette tests. 
 
560 Bureau of Standards Journal of Research [Voi.s 
 
 Table 22. —Relative strength of different types of walls and wallettes 
 
 Strength rank 
 
 Wall thickness 
 
 Mortar 
 
 Mississippi 
 
 New England 
 
 Wall 
 
 Wallette 
 
 Wall 
 
 Wallette 
 
 
 
 [Solid... _ 
 
 Solid_ 
 
 Solid... 
 
 Solid. 
 
 
 Cement-lime_ 
 
 Rolok-bak . .. 
 
 A. r. F. b_ 
 
 Rolok-bak _ _ 
 
 All-rolok. 
 
 
 All-rolok. 
 
 Rolok-bak _ 
 
 All-rolok_ 
 
 Rolok-bak. 
 
 
 
 A. r. F. b_ 
 
 All-rolok_ 
 
 A. r. F. b_ 
 
 A. r. F. b. 
 
 8-iiicn—__— 
 
 
 Solid _ 
 
 Solid_ 
 
 Solid .. 
 
 Solid. 
 
 
 Cement.. 
 
 Rolok-bak _ 
 
 Rolok-bak_ 
 
 Rolok-bak_ 
 
 Rolok-bak. 
 
 
 All-rolok_ 
 
 All-rolok__ 
 
 All-rolok... 
 
 All-rolok. 
 
 
 
 A. r. F. b_ 
 
 A. r. F. b_ 
 
 A. r. F. b_ 
 
 A. r. F. b. 
 
 
 
 [Solid 
 
 Solid___ 
 
 Solid_ 
 
 Solid. 
 
 
 
 J Rolok-bak_ 
 
 A. r. F. b.. 
 
 Rolok-bak_ 
 
 A. r. F. b. 
 
 
 ocmenwime_ 
 
 All-rolok_ 
 
 Rolok-bak _ 
 
 A. r. F. b_ 
 
 Rolok-bak. 
 
 12-inch.. 
 
 
 A. r. F. b.. 
 
 All-rolok_ 
 
 All-rolok_ 
 
 All-rolok. 
 
 
 [Solid_ 
 
 Solid_ 
 
 Solid_ 
 
 Solid. 
 
 
 Cement_ 
 
 Rolok-bak_ 
 
 A. r. F. b_ 
 
 Rolok-bak_ 
 
 Rolok-bak. 
 
 
 All-rolok._ 
 
 Rolok-bak_ 
 
 A. r. F. b_ 
 
 A. r. F. b. 
 
 
 
 A. r. F. b_ 
 
 All-rolok_ 
 
 All-rolok_ 
 
 All-rolok. 
 
 A. r. F. b.=all rolok in Flemish bond. 
 
 For the solid wallettes the results given in Table 21 show that the 
 mortar had about the same effect on wallette strength as on wall 
 strength. The lime-mortar specimens were weakest and the cement- 
 mortar specimens strongest for all groups of solid wallettes which 
 differed only in the mortar mixtures used. 
 
 Keeping the wallettes damp for seven days after construction did 
 not make them stronger than similar specimens cured under ordinary 
 conditions except for the group for 12-inch rolok-bak wallettes of 
 New England brick (Table 23). Specimens 142 and 143, cured under 
 ordinary conditions, had an average strength of 1,755 lbs./in. 2 , while 
 the similar wallettes, 145 and 146, which were damp cured, had a 
 strength of 2,320 lbs./in. 2 . In the other five comparable groups the 
 ratio of wallette strengths, damp cured to ordinary, ranged from 
 0.64 for the 8-inch solid wallettes of Detroit brick to 0.99 for the 
 12-inch solid specimens of Mississippi brick. 
 
 Table 23. —Effect of wetting on wallette strength 
 
 [Wallettes of series 2, cement mortar] 
 
 i 
 
 Type of walette 
 
 Kind of brick 
 
 Wallette Curing 
 Nos. conditions 
 
 8-inch solid 
 
 Detroit 
 
 22 , 23,24 
 33 ... 
 
 Ordinary_ 
 
 Wetted- 
 
 Average 
 
 wallette 
 
 strength 
 
 Lbs./in . 2 
 1 , 350 
 865 
 
 Ratio of 
 wallette 
 strengths 
 wetted/ 
 ordinary 
 
 0.64 
 
 12-inch solid 
 
 _do- 
 
 Mississippi.... 
 New England 
 
 / 28 , 29 , 30 
 131 , 32 — 
 143 , 44 .... 
 146 , 47 .... 
 1103 , 104 . 
 \ 106 , 107 . 
 
 Ordinary_ 
 
 Wetted_ 
 
 Ordinary_ 
 
 Wetted_ 
 
 Ordinary.... 
 Wetted_ 
 
 1,335 
 1,165 
 1,890 
 1,865 
 3,330 
 3,075 
 
 .87 
 
 .99 
 
 .’92 
 
 12-inch rolok-bak (heavy-duty)_ 
 
 Mississippi- 
 New England 
 
 / 82 , 83 _. 
 185 , 86 ... 
 / 142,143 
 \ 145,146 
 
 Ordinary_ 
 
 Wetted_ 
 
 Ordinary_ 
 
 Wetted_ 
 
 905 .90 
 
 815 . 
 
 1,755 . 
 
 2,320 1 . 32 
 
SSrneJ S07W ’] Compressive Strength of Clay Brick Walls 
 
 561 
 
 The onty comparison due to difference in the workmanship is 
 between wallette 18 of series 2 built with shoved workmanship and 
 wallettes 16 and 17 of series 1 with the typical furrowed joint work¬ 
 manship of this series. The relative strengths are as 1,190 is to 755, 
 giving an increase in strength with shoved joints of 58 per cent over 
 that obtained with furrowed joints. 
 
 A study of the relation between stress at first crack and wallette 
 strength shows that the same conclusions may be drawn as from the 
 wall tests. The main difference between the wall and wallette results 
 is that the hollow wallettes had considerably lower ratio of stress at 
 first crack to final strength than was found from the wall tests. 
 
 The lowering of the ratio for strength at first crack to ultimate 
 strength of hollow wallettes is brought about not by the strength at 
 first crack being low, but by the ultimate strength being high as 
 compared with the corresponding hollow walls. The difference in 
 behavior of hollow walls and wallettes is apparently explainable 
 by considering the relative slenderness ratios of the withes when the 
 connecting headers are broken. For hollow walls (all-rolok or all- 
 rolok in Flemish bond) when the headers are broken, there results 
 two unsupported sections 9 feet high by 2 % inches thick. Through 
 column action these collapse. The wallettes, however, give sections 
 34 inches by 2} 4 inches. This gives a slenderness ratio so low that 
 column action does not take place and the load increases to the point 
 where crushing of the brick results. The values for slenderness ratios 
 (1/r) for wall and wallette sections (108 inches by 2 }{ inches and 
 34 inches by 2 % inches) are 166 and 52.4, respectively. 
 
 Table 24 gives average values of the ratio of wall strength to wal¬ 
 lette strength. These values show that the ratio (for the solid speci¬ 
 mens for series 2) becomes less as stronger brick are used. In other 
 words, the use of strong brick gives a greater increase in wallette 
 strength than in wall strength. 
 
 In Figure 35 are plotted the solid wall strengths against the solid 
 wallette strengths for each group in both series for which direct 
 comparisons can be made. The average ratio from Table 24, repre¬ 
 sented by the diagonal line in the figure, of the wall strength to the 
 wallette strength is 0.86, but the plotted points indicate the trend 
 toward proportionately greater wallette strength for the stronger 
 specimens. The ratio for the solid and rolok-bak specimens is greater 
 than for those types in which all the withes are laid on edge. 
 
 The average values of the ratio of wall strength to wallette strength 
 are about the same when the specimens are laid in cement-lime mortar 
 as in cement mortar. 
 
562 Bureau of Standards Journal of Research [Voi.s 
 
 Table 24. —Relation of wall to wallette strengths 
 
 Description of groups compared 
 
 Wall strength 
 Wallette strength 
 
 Average for all solid specimens of both series____ _ 
 
 Ratio 
 
 0.86 
 
 Series 2: 
 
 Solid specimens of Detroit brick....... 
 
 .93 
 
 Solid specimens of Mississippi brick,. ...... .. _ 
 
 .90 
 
 Solid specimens of New England brick. . _. ... 
 
 .79 
 
 Solid specimens. ... _ 
 
 .87 
 
 All-rolok specimens_ ... .... ... .. ._ __ 
 
 .75 
 
 AU-rolok in Flemish bond specimens 
 
 .63 
 
 Rolok-bak specimens. ... _ 
 
 . 86 
 
 Specimens laid in cement-lime mortar.. . . .... 
 
 . 78 
 
 Specimens laid in cement mortar _ ... . .. ._ __ 
 
 .81 
 
 
 In general, then, it may be stated that solid wallette strengths are 
 affected by differences in brick, mortar, curing conditions, and work- 
 
 Figure 35. —Relation of wall to wallette strengths for solid specimens 
 
 manship in very much the same way as are solid wall strengths. In 
 fact, the general tendencies of strength changes for the hollow speci¬ 
 mens are also indicated by wallette strengths. For the hollow speci¬ 
 mens the relation between the strengths of the wallettes and the walls 
 is not as well defined, but the wallette strengths may serve as a meas¬ 
 ure of the strengths of both types of walls. 
 
TiJbuw/v 071 *'] Compressive Strength of Clay Brick Walls 
 
 563 
 
 5. RELATIONS BETWEEN THE STRENGTHS OF THE WALLS AND THE 
 STRENGTHS OF THE BRICKS AND WALLETTES 
 
 The object of the tests of these brick walls was to find, if possible, 
 some strength property of the bricks or of smaller walls which will 
 be a measure of the strength of the solid walls. The ratios of the 
 compressive strengths of the walls to the various properties of the 
 bricks and to the compressive strengths of the wallettes are given 
 in Table 25. This table is divided into seven groups, in each group 
 of which the variable is the kind of brick. The ratio of wall strength 
 to brick or wallette strength, of course, varies from group to group 
 because of differences in mortar or in workmanship, but in any one 
 group that ratio is the best measure of wall strength which is most 
 nearly constant for the different kinds of bricks. A measure of the 
 constancy of this ratio is given by dividing the mean deviation of 
 the separate ratios by the mean of the ratios in a group. The smaller 
 this value is the better this property of the brick serves as a measure 
 of wall strength. Actually the magnitudes of the deviations of these 
 ratios from their mean are measures of the degree to which the values 
 for wall strength deviate from a linear relation with the particular 
 brick strength to which they refer. If a much wider range in brick 
 strengths had been involved, it is possible that the determination 
 of the properties of brick which were the most consistent measures 
 of wall strength should be based upon the consistency of the data 
 with respect to a more general form of relationship that might not 
 necessarily be linear. 
 
Table 25 .—Relation of wall strength to strength of bricks and wallettes 
 
 564 
 
 Bureau of Standards Journal of Research 
 
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 Averaged from values given in Table 9 of this paper for all but the sand-lime brick. The values for sand-lime brick are given in Table 5, p. 65, Technologic Paper No. 276. 
 Averaged from values given in Table 6 of this paper for all but the sand-lime brick. The values for sand-lime brick are given in Table 3, p. 62, Technologic Paper No. 276. 
 Averaged from values given in Table 21 of this paper for all but the sand-lime brick. The values for sand-lime brick are given in Table 7, p. 70, Technologic Paper No. 276. 
 
fteBuMy 0718 ’] Compressive Strength of Clay Brick Walls 
 
 565 
 
 For the range in strengths found in these tests, however, it is 
 believed that the basis of comparison used does not involve errors 
 sufficiently large to affect the conclusions stated. 
 
 The walls of series 1, which were built of Chicago brick, are directly 
 comparable to the sand-lime brick walls (see footnote 6, p. 518) which 
 had previously been tested at the Bureau of Standards. These 
 comparisons are given in groups A, B, and E for the different mortars. 
 The ratios for compressive strength of half brick flatwise, shearing 
 strength, and wallette strength are more constant than for the other 
 brick strength ratios. 1 11 
 
 In groups C, D, and G, where the bricks are all molded, the ratio 
 of wall strength to modulus of rupture flatwise is the most constant 
 of all the brick-strength ratios, and this would also be true for groups 
 E and F if the end-cut Chicago bricks were excluded. 
 
 For the latter, the transverse strength is high in comparison with 
 the compressive strength (Table 7), and with this brick the wall 
 strength was quite apparently determined more by the compressive 
 strength than the other. 
 
 The weighted averages of the mean deviations divided by the mean, 
 calculated by giving the value for each group a weight proportional to 
 the number of ratios in that group, are also given in Table 25 for the 
 different ratios. A comparison of these weighted average values 
 shows that the shearing strength appears to be the best brick-strength 
 property for predicting the strength of the wall. It must be pointed 
 out, however, that the values given in Table 6 for shearing strength 
 represent only a small number of samples as compared to the other 
 brick-strength values and that the shearing tests were made on 
 specimens which differed in thickness. 
 
 The compressive strength of the half brick flatwise is the next best 
 measure of wall strength after the shearing strength, for its mean 
 deviation divided by mean of 0.14 is considerably less than for the 
 compressive strength edgewise, modulus of rupture, or tensile 
 strength. 
 
 On the average, however, the compressive strength of the wallettes 
 is by far the most consistent value for determining wall strengths. 
 
 1] In the paper by J. W. McBurney entitled “ The Effect of Strength of Brick on Compressive Strength 
 of Brick Masonry,” Proc. Am. Soc. Testing Materials, 28, Pt. II, p. 605, 1928, somewhat similar compar¬ 
 
 isons were made, but the conclusion was drawn that the compressive strength (flatwise) of the bricks was 
 the most consistent measure of the strength of the walls. Because of an error in calculations, which was 
 discovered after that paper had been published, the values given for the shearing strengths for all bricks 
 except the sand-lime were double the true valuer, and for this reason, the strengths of the bricks in shear 
 was not found to be closely related to the strength of the walls. The values given in McBurney’s paper 
 for the other strength properties of the bricks differ slightly in some cases from those given in the present 
 paper. This, however, is due to the fact that, in the previous paper, a study was made of methods of 
 sampling and some of the values given were the averages of more than one sample, whereas those given 
 in the present paper are the results of tests of samples selected specifically for the investigation reported 
 herein. Had there been no error in the data on the shearing strength of the bricks, there would have been 
 no essential difference in the numerical data of the two reports. 
 
566 
 
 Bureau of Standards Journal of Research 
 
 [Vol.S 
 
 The mean deviation divided by the mean value is consistently low for 
 each of the five groups as well as having a weighted average value 
 of only about half that of the best brick-strength property. 
 
 A large number of testing machines are now available in the United 
 States which may be used for wallette tests. The tests of wallettes 
 built of the brick to be used and laid in the mortar mixture specified 
 can confidently be expected to give strength values which will seldom 
 exceed the strength of a large wall by as much as 25 per cent. The 
 prediction of wall strength from any brick strength value appears to 
 be very uncertain because of differences in the method of manufac¬ 
 ture of the brick and because different factors must be used for differ¬ 
 ent mortars and workmanships. 
 
 Table 26 gives the relation between wall stress at first crack and 
 the various strength properties of the bricks and to the strength of 
 the wallettes. The weighted average values of mean deviation to 
 mean show that the wallette strength is a better measure of wall 
 stress at first crack than are any of the brick strengths. Of the 
 brick strengths the shearing strength was the most consistent measure 
 and there was no important difference in the consistencies obtained 
 for the other strength properties. 
 
Table 26 .—Relation of wall stress at first crack to strength of bricks and wallettes 
 
 Slang, Parsons ,1 
 McBurney J 
 
 Compressive Strength of Clay Brick Walls 
 
 567 
 
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 1 
 
 1 
 
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 1 
 
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 devia¬ 
 
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 mean 
 
 0.06 
 
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 Mean 
 
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 0.34 
 
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 Mean 
 
 devia¬ 
 
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 mean 
 
 0.37 
 
 .37 
 
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 X- 
 
 05 
 
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 0.17 
 .37 
 
 .42 
 
 .91 
 
 .83 
 
 1.39 
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 1.68 
 
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 1.06 
 
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 1.25 
 
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 1.19 
 
 1.62 
 
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 Averaged from values given in Table 6 of this paper for all but the sand-lime brick. The values for sand-lime brick are given in Table 3, p. 62, Technologic Paper No. 276. 
 Averaged from values given in Table 21 of this paper for all but the sand-lime brick. The values for sand-lime brick are given in Table 7, p. 70, Technologic Paper No. 276. 
 
568 Bureau of Standards Journal of Research i voi.s 
 
 V. CONCLUSIONS 
 
 The results of these compressive tests of 168 walls and of 129 
 wallettes, built of 4 kinds of common brick laid in 3 mortar mixtures, 
 with 10 types of wall construction, with differences in workmanship 
 and in curing conditions lead to the following conclusions: 
 
 1. The average strengths of solid walls built of end-cut Chicago 
 brick (average compressive strength of half bricks flatwise, 3,280 
 lbs./in. 2 ) were as follows: 
 
 Lime mortar walls, 287 lbs./in. 2 . 
 
 Cement-lime mortar walls, 587 lbs./in. 2 . 
 
 Cement mortar walls, 661 lbs./in. 2 . 
 
 A contract for building these walls was let, on a lump-sum basis, 
 to a brick mason who specialized in small contracts. The work was 
 done without supervision and was characterized by absence of mortar 
 in the longitudinal vertical joints and deep furrowing of the horizontal 
 beds. 
 
 2. With carefully supervised workmanship, the average strengths of 
 solid walls, which were built by another mason hired by the day 
 without regard to output and which had completely filled vertical 
 joints and smooth spread horizontal mortar beds, were as follows: 
 
 Kind of brick 
 
 Compressive 
 strength of 
 half brick, 
 flatwise 
 
 Average compressive 
 strength of solid walls 
 
 Cement- 
 
 lime 
 
 mortar 
 
 Cement 
 
 mortar 
 
 Chicago . . ___ 
 
 Lbs. /in.* 
 3,280 
 3, 540 
 3, 410 
 8,600 
 
 Lbs. /in. 2 
 
 Lbs./in* 
 895 
 1,145 
 1,550 
 2, 855 
 
 Detroit _ _ _ _ __ ____ __ _ 
 
 945 
 
 1,300 
 
 1, 875 
 
 Mississippi .. ..__ ... ... -- ... 
 
 New England_ 
 
 
 3. The strengths of the solid walls were more closely related to the 
 shearing strength of the bricks than to any other strength property 
 measured. The compressive strength of the half bricks flatwise 
 appeared to be the next best measure and was better than the com¬ 
 pressive strength on edge, the modulus of rupture, or the tensile 
 strength of the bricks. 
 
 4. On the average, the compressive strength of the wallettes was 
 by far a more consistent measure of the strength of the walls than any 
 of the brick strength values. In predicting wall strengths from brick 
 strengths, effects of mortar and workmanship must be taken into 
 account, while from wallette strengths only a single value need be 
 determined. These tests show that the average strength of the walls 
 was from about 60 to 90 per cent of the average wallette strength. 
 
 5. The strength of the solid walls, which were built by contract, 
 varied about as the cube root of the compressive strength of the mor¬ 
 tar cylinders (2 inches in diameter and 4 inches long) which were made 
 
McBiimey 0 ™’] Compressive Strength of Clay Brick Walls 
 
 569 
 
 from the mortar of the walls and cured under the same conditions. 
 For the solid walls, built under careful supervision, the increase in 
 strength for cement-mortar walls over those laid in cement-lime mor¬ 
 tar was about 20 per cent for walls of Detroit and Mississippi brick 
 and about 50 per cent for walls of New England brick, while the aver¬ 
 age ratio of the cube roots of the mortar cylinder strengths (cement 
 and cement-lime mortars) was 1.38. 
 
 6. With brick the cross sections of which closely approximated 
 rectangles the strengths of the hollow walls varied about as the net 
 areas in compression. When the brick were warped, the strength 
 of the hollow walls was found to be less than that expected from the 
 net area. 
 
 7. Construction data show that there is a saving in materials and 
 in time for hollow walls of brick as compared with solid walls. 
 
 8. The condition of the horizontal mortar beds in the walls affected 
 the wall strength. Walls in which the beds were smooth were stronger 
 than walls in which the mortar beds were furrowed by from 24 to 109 
 per cent. 
 
 9. Walls laid in cement mortar and kept damp for seven days 
 after construction were not stronger at the age of 60 days than 
 similar walls cured in the laboratory under ordinary conditions. 
 
 10. The results of the wallette tests, in which the same variables 
 occurred as in the larger walls, lead in general to the conclusions 
 deduced from the wall tests. 
 
 VI. APPENDIX 
 
 1. REPORTS ON WORKMANSHIP 
 
 It will be observed that no opinions have been offered in the paper as 
 to the grading or classification of the two types of workmanship used. 
 It was made a regular procedure to ask the opinions of such architects, 
 engineers, and contractors who visited the bureau during the progress 
 of this work as to their grading of the workmanship. The opinions 
 expressed were quite variable. The only conclusion warranted was 
 that the workmanship used in different parts of the country varied. 
 
 However, in the original program for these tests provision was 
 made for inspection by three different bodies—the American Institute 
 of Architects, the Supervising Architect’s Office of the Treasury De¬ 
 partment, and the International Bricklayers, Masons and Plasterers’ 
 Union. 
 
 The American Institute of Architects designated A. L. Harris, munic¬ 
 ipal architect for the District of Columbia. J. W. Ginder represented 
 the Supervising Architect’s Office. No report was received from the 
 International Bricklayers, Masons and Plasterers’ Union. 
 
 Their reports, in so far as they deal with workmanship, are here 
 reproduced. 
 
 69882°—29-5 
 
570 Bureau of Standards Journal of Research [Voi.s 
 
 (a) COMMENTS BY J. W. GINDER 
 
 The physical characteristics of the units employed, the mortar 
 mixtures, the strengths developed, and the technical deductions 
 having been fully covered by others, the following comments are 
 intended to bear only upon the practical aspect of the investigation. 
 
 Considering first the manner in which the test units were con¬ 
 structed: Series No. 1 was performed under contract with a mason 
 who also performed the work, so that his every incentive both as a 
 contractor and workman was to complete the work in as short a 
 time as was consistent with obtaining acceptable results. The work 
 was without supervision, except as understood in the preparation of 
 mortar and in the laying up of one wall, so that the mason was free to 
 adopt those methods which appeared to best serve his own interests. 
 
 This, it is understood, was one of the purposes of this line of pro¬ 
 cedure. The resulting work was characterized, as shown by sub¬ 
 sequent tests, by furrowing of the mortar beds, the almost complete 
 absence of mortar in the vertical longitudinal joints, and only such 
 mortar in the end joints as was forced in by the “shoving” process of 
 bedding and by depositing the subsequent mortar bed. 
 
 All circumstances considered, I believe that this very closely 
 approximates the quality of work generally obtained in commercial 
 construction, where close supervision is not to be expected and the 
 most cogent consideration is economy. 
 
 Series No. 2, being built by day labor, exactly the opposite incentive 
 was pro vided, and it was to the interest of the mason to work deliber¬ 
 ately and to prolong the job, in addition to which, it is understood 
 that he was instructed, except in certain instances, to eliminate shov¬ 
 ing, that furrowing the beds was prohibited, and that he was to use 
 his best workmanship. 
 
 Under these instructions he carefully leveled the beds and slushed 
 all joints full. The subsequent tests showed the wall sections laid 
 up by him to be without voids, and in every way, so far as workman¬ 
 ship was concerned, to be of the highest type for common brickwork. 
 
 I believe that such work as of the type which could be expected 
 only under a definite specification, followed by careful supervision, 
 such as would obtain for a high-class public or private structure, 
 where considerations of cost were more or less subordinate. 
 
 It should be noted, however, that neither mason was under any 
 restriction as to the thickness of bed to be maintained. I believe 
 that if such a restriction had been imposed, say to a half inch or under, 
 that much of the work performed under series No. 1 might have been 
 of better character with regard to bedding, in that “tapping home” 
 the brick to a reasonably narrow point would have tended to more 
 completely eliminate the furrow in the bed. 
 
fi?BL™t T y 0n *' ] Compressive Strength of Clay Brick Walls 
 
 571 
 
 As to the curing of the walls, it is noted that no increase in strength 
 was found as a result of damp curing where this was employed, in 
 contrast with those units which were cured in the laboratory under 
 ordinary conditions. 
 
 I believe that owing to the saturation of the brick prior to laying 
 and the subsequent protection of the walls from the direct heat of 
 the sun, that all walls were in reality damp cured, the difference being 
 one of degree only, and that if the damp-cured walls were placed in 
 comparison with others laid up under ordinary conditions of outside 
 exposure, a difference in favor of the damp curing might be found. 
 
 This theory would seem to be supported to some extent at least by 
 the demonstrated increase in strength of damp-cured mortars over 
 others not so treated. 
 
 (b) COMMENTS BY A. L. HARRIS 
 
 The walls were laid up in different kinds of mortar and erected by 
 two different mechanics, one of whom laid the bricks to conform with 
 the highest standards of bricklaying, the other laying the bricks 
 according to ordinary every-day methods common to investment 
 building and ordinary commercial work. 
 
 The brickwork done by the first mechanic was characterized by 
 deep scoring of the mortar bed with the trowel, buttering a portion 
 of the header only, and the omission of mortar in the center of the 
 walls, except that which is forced into the joints by bedding the 
 brick in the mortar. This gave a very porous or cellular construction 
 to the interior of the wall and was comparable to the kind of brick¬ 
 work found in ordinary investment and commercial work. 
 
 The brickwork done by the second mechanic was characterized by 
 smooth, level bed joints, buttering the entire end of the header and 
 slushing full all vertical interior joints, making a job comparable with 
 what is known as “solid construction.” 
 
 The class of workmanship in the walls built by the second mechanic 
 (series 2) compares favorably with that obtained by the District of 
 Columbia in its building contracts. The specifications prepared in 
 the municipal architect’s office call for all brickwork to be executed 
 with hard-burned red bricks laid in 1 to 3 Portland cement mortar, 
 to which is added hydrated lime not to exceed 15 per cent of the vol¬ 
 ume of the cement. The brickwork is laid solid throughout, slushing 
 all joints full. The bricks in the interior of the wall are shoved up 
 in a full bed of mortar. This work is inspected by a superintendent 
 of construction, who is constantly on the job during working hours. 
 
 Washington, May 11, 1928. 
 

 
 
 
 
 
 
 
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UNIVERSITY OF ILLINOIS-URBANA