Ser TH1 N21r2 no. 192 c. 2 I BLDG RESEARCH NATIONAL COUNCIL CANADA DIVISION O F BUILDING RESEARCH FACTORS AFFECTING BOND STRENGTH AND RESISTANCE TO MOISTURE PENETRATION OF BRICK MASONRY BY I T. RITCHIE AND J. I. DAVISON ! REPRINTED FROM AMERICAN SOCIETY F O R TESTING A N D MATERIALS SPECIAL T E C H N I C A L PUBLICATION N O . 3 2 0 . 1962. P. 16 - 30. RESEARCH PAPER NO. 192 OF THE DIVISION O F BUILDING RESEARCH OTTAWA PRICE 2 5 CENTS JULY 1963 NRC 7217 55905 This publication is being distributed by the Division of Building Research uf the Natiorlal Research Council as a contribution towards better building in Canada. It should not be reproduced in wholeor i r l part, without permission of the original publisher. The Division would be gIad t o be of assist:rnce in obtaining such permission. 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Authorized Reprint from the Copyrighted Symposium on Masonry Terting Spccial Tcchrcical P~rblicalio,~ 320 No. Published b y the .\XERICAX SOCIETY rOR TESTING i\f \TERI.\LS AXD 1962 ANALYZED FACTORS AFFECTING BOND S T R E N G T H A N D RESISTANCE TO MOISTURE PENETRATION OF B R I C K MASONRY Small panels of brick lnasoilry have been used to study the resistance to moisture penetration and the strength of bond of brickwork in relation to f several factors. Results o tests are presented to show the influence of prop- erties of bricks and inortars on leakage and bond strength. Information is presented on the effects of several other factors that are governed by the technique used by the bricklayer in constructing masonry. These factors include the consistency of the mortar when used, the time interval between spreading a mortar bed and laying a brick in it, and the extent to which a brick is tapped into the mortar. The effects of tooling of mortar joints and varying the thickness of mortar joints are also considered. ii'Iany studies of the weather-resisting Review of the work done in other and load-bearing properties of masonry laboratories, a s well as results of early walls have been reported in the technical studies made in the Division on moisture literature in the last forty years. How- penetration of relatively large test walls ever, problems of rain penetration and of (1); drew attention to the importance inadequate strength of brick masonry of the condition of the upper surface of a continue to arise. I n recent years the mortar bed a t the moment a brick is Division of Building Research of the laid in it. T h e condition of mortar after National Research Council of Canada i t is spread depends on such factors as has studied two main aspects of the the absorption of the bricks on which it problem of rain penetration: (1) the is placed, its initial water content and influence of properties of the materials- inherent resistance to loss of moisture, bricks and mortars, alone and in combi- the thickness of the mortar bed, and the nation-on the permeability of brick- time of contact between mortar and work, and (2) the influence of the method brick. T h e energy expended in forcing of assembly of the brickwork. I n many the brick into contact with the mortar of these studies. resistance to moisture bed also affects the nature of the bond. penetration and strength of brickwork Small panels of brick and mortar, have been studied jointly, since both consisting of 5 bricks in stack bond, have apparently depend in large measure on been used to investigate the influence of the nature of the bond between mortar these factors on moisture penetration and brick. and strength of bond. T h e type of 1 Building Materials Section and Atlantic sample and the method of test have been Regional Station, respectively, Division of Building Research. National Research Council, The boldface numbers in parentheses refer Ottawa, Canada. t o the list of references appended t o this paper. RITCHIE .4ND DAVISONON LEAKAGEAND BONDSTRENGTH 17 described (2), but test results from theattach a reservoir of water to the surface use of the metl~ocl of the masonry and to measure the move- have not been reported until now. Nearly 1000 panels have ment of water into i t (3); brickworlr been constructed anc! tested in this "chimneys" filled with water have also series, and only representative test been used (4,s). Since the depth of water results have been selected to illustratevaried in such tests, the pressure did the influence of various factors on the also. TOobtain constant ~ressure.a test properties of masonry. Conclusions are wall was placed horizontally with a pond based on a larger number of tests than of water maintained on t h e surface (6.71. - I n, several studies water has been those t o which direct reference is made. ~ The program was designed primarily sprayed against the test wall, in some instances with no attempt t o obtain an for study of moisture penetration. Panels were cured after construction under con-air pressure difference across the wall trolled conditions (70 F, 50 per cent (8); in others the water spray has been relative humidity) for two weeks before accompanied by an air pressure differ- the leakage test was made. Bond strengthence across the test wall (9). Air pressure tests were made two weeks after the and wetting have been combined to obtain a useful representation of actual leakage test. I n the case of leaky panels, wall exposure conditions (lo). The thick- the water in the panel after the test may have affected the curing of the mortar ness of the test wall is also important, and the strength of bond. Bond strength since it may influence the difference in values given are comparative within a pressure across the units of the outer part of the wall, thereby affecting the given series of tests and are not absolute values. rate of penetration of moisture into the wall. The use of thin samples for rain TEST METHODS FOR RAIN PENETRATION penetration tests may, therefore, provide penetration of rain througll lvalls results difficult to apply in predicting usually occurs when rain is accompanied the performance of thicker walls. by high wind. T h e wind has a twofold TEST METIIODS FOR BOND STRENGTH effect: (1) it changes the vertical direc- tion of fall of the rain drops so that they The most obvious forces t h a t a brick strike the wall surface, frequently form- wall must resist are those resulting from iiig a film of water that flows down the loads applied vertically, such as roof surface, and (2) it causes regions of and floor loads. Detailed studies of the differential air pressure to be set up in load-bearing capacity of brick masonry and around a building. The air pressure have been made by the British Building patterns depend on the wind speed, the Research Station (11) and b y the U. S. shape of the structure, and its height. I t National Bureau of Standards (12,13). is probable that a wall wetted by rain Considerable information has been ob- has an air Dressure difference across it, tained about the influence of brick and with the outside pressure, when it is mortar properties, and of other factors, higher than that inside, having the effect on the compressive strength of brick of increasing the rate of passage of water masonry. I n addition to supporting through the wall. vertically applied loads, a wall must I1 laboratory studies of rain penetra- 1 resist lateral forces such as those caused tion, various methods of simulating the by lvind pressure. Tests of the trans- combined eflects of wetting and pressure verse strength of brick walls were in- have been used. One method was t o cluded in the studies mentioned (11,13). I n transverse tests of brick walls, failure recently been adopted as a n ASTM usually results from a breakdown of the standard method."ond strength tests brick-mortar bond rather than from reported here were made by a method failure in tension of brick or mortar. adapted from the crossed-brick couplet The strength of adhesion between brick method, except that the entire bedding and mortar has been studied in many surfaces of the bricks were in contact laboratories, particularly since a trend with mortar (2). LEGEND: - A D R Y - P R E S S BRICK E @ EXTRUDED BRICK & 300 - I SOFT-MIID BRICK u N 5 Y - U IT m , $ 200- 3 J O I N S POINTS FOR 0 IT SIX B R I C K S F R O M A x i - THE SAME LOT a OF B R I C K S E v, v, a =I LT loo - JOINS POINTS =I 3 FOR T W O BRICKS C LL o z FROM T H E SAME L O T \t 3 0 / z =I -.I.~<-~L 10 20 30 40 50 60 70 I 80 I 90 . I N I T I A L R A T E OF ABSORPTION OF BRICK, G PER M I N PER 3 0 SQ IN. FIG. 1.-Leakage f of Bricks in Relation to Initial Rate o Absorption. toward the use of thinner walls has stressed the importance of high resistance to lateral loading. Various methods have been used to Bricks: determine the strength of bond of brick Early moisture penetration tests in- to mortar. A detailed account of the dicated that the ~erformanceof brick- development of such tests and the cliffi- work could be varied by changing the culties of obtaining consistent results from them was presented in 1943 (14). f Method of Test for Bond Strength o Mor- tar to Masonry Units (E 149 - 59 T), 1961 The "crossed-brick couplet" method has Book of ASTM Standards, Part 5, p. 1071. type of briclc used. I t is generally ac- sq in. of brick bedding surface when the cepted that two properties of bricks brick is placed in water to a depth of $ in. may affect the resistance of a wall to for 1 min. Since this property is to some moisture penetration: (1) the permea- extent a measure of the ease of move- bility of the brick to water, and (2) ment of water into a brick, i t was ex- the ability of the brick to bond with pected that i t would be related in a mortar. I n the first case water may pass general way to the permeability of the through the brick in the wall and come brick. The results of the tests are shown out the other side. I n the second case, in Fig. 1, in which the amount of water certain properties of the brick may passing through a brick in 24 hr has adversely affect the bonding between been plotted against initial rate of ab- mortar and brick, so that channels sorption. form a t the brick-mortar interface which Six extruded bricks were tested. For enable water penetration. I n addition five of them there was no leakage, and to the influence of the initial rate of three had no mark of dampness on the water absorption or suction of the back surface. The amount of leakage of brick on bonding between brick and the sixth brick was very slight (0.5 ml mortar, interface channels may result in 24 hrj. Ten dry-press bricks were from other flaws such as warpage, sur- tested; all leaked and in general the face texture, deep scratches or cracks in permeability increased with increasing the bedding surfaces, and dust on the initial rate of absorption, particularly bedding surfaces. The effect of cores and for six bricks taken from the same lot frogs in bricks must also be considered. that covered a wide range of initial rate Permeability.--All bricks have a porous of absorption. As shown in Fig. 1, the structure, the nature of which depends amount of water that passed through upon the raw material, the method of these bricks in 24 hr of test varied from formation, and the firing temperature 3 to 349 ml, with considerable increase during the manufacturing process. Sev- in leakage when the initial rate of ab- eral investigators (3,11,14) have found sorption exceeded about 55 g per min that where materials are of good grade it per 30 sq in. is rare that bricks are permeable to such As a result of these moisture penetra- an extent that water will pass through tion tests on individual bricks, together them during a normal rain. with the leakage test results from many I n the present study some of the panels, it is considered that the per- moisture penetration tests indicated that meability of extruded (stiff-mud) bricks leakage had occurred through the bricks is not likely to be significant in the as well as at the brick-mortar interface. problem of moisture penetration of To obtain information on their per- masonry, but the permeability of dry- meability bricks of various kinds were press and soft-mud bricks may be im- tested individually for moisture pene- portant. tration in the same way as panels are Absorptio~zof Water.-The absorption tested, by spraying water on the face properties of a brick determine the and simultaneously maintaining an air amount of water that mav be extracted pressure differential of 2 in. of water in a given time from mortar placed on across the brick. One series of tests used the brick. T h e loss of water influences 17 bricks covering a wide range in the the state of plasticity of the mortar property of initial rate of absorption or when another brick is laid in it and thus suction, which is defined as the number affects the nature of the bond established of grams of water absorbed through 30 between the two (15). 1 1 1 1 ! 1 1 1 1 1 1 Q Q - Q APPROXIMATE CURVE FOR APPROXIMATE CURVE FOR BOND STRENGTH VS BRICK - - I i I I I I I 10 20 3 0 4 0 5 0 6 0 7 0 8 0 9 0 100 110 I N I T I A L R A T E OF ABSORPTION OF BRICK, G PER M I N PER 3 0 S Q I N . (AVERAGE O F VALUES OF A L L B R I C K S USED I N A PANEL ) FIG. 2.-Influence of Brick Suction on Bond Strength and Permeability. TABLE I .--EFFECT OF WETTING strength of bond of masonry. Results are BRICKS ON PROPERTIES OF BRICK MASONRY. illustrated b y a series of tests of 22 I panels involving bricks of various suc- Tensile tion values made into panels with a masonry cement mortar. T h e mortar, Dry. . . . . . . . . . D r y . . . .. - 1 1 - 5441 4948 - mixed to a flow of 120 per cent, was 11.3 14.6 placed on the brick. After 1 min the brick was bedded in the mortar by the Wet . . . . . . . . . . Wet.. . . . . . . . . 1 47 44 436 514 1 20.7 36.4 tap of a 4-lb hammer dropped 14 in. Each panel was tested for moisture pene- tration, after which the tensile strength " Average of all joints of the panel. of bond between brick and mortar was Some 200 panels were tested in this measured. T h e permeability of the panels series to study the effect of the suction of is compared in Fig. 2 on the basis of the brick on the permeability and the amount of water that passed through a panel in 24 hr of test. Highly per- Table I indicates the improvements meable panels resulted from the use of obtained in the greater strength of bond high suction briclcs. For some, the leakage and greater resistance to nloisture pene- may have consisted, in part, of water tration resulting from wetting the bricks. passing through bricks, but it is con- Those used in these tests were high in sidered that most of the leakage was a t suction, varying from 38 t o 73 g per the brick-mortar interface where the min per 30 sq in. Two test panels were effective bond between the two was made of this brick laid dry, and two influenced by the suctioli of the brick. others were made of i t after soaking in Variation in bond strength with change water for 10 min. The mortar was pre- of brick suction is also evident in Fig. 2. pared from masonry cement and sand in The relationship follows the same pat- proportions 1 :3 by volume, and its tern described in earlier studies (16). flow was 115 per cent. The mortar was TABLE 11.-COMPARISON OF PANELS MADE OF CORED BRICKS AND SOLID BRICICS. - - Mortar ,-omposition, by volurnea T~~~of ~ [ i ' l ; First Appearance of Total Leakage in Bond Strength, Dampness, rnin 24 111, ml psi& cored after 7 hr nil cored after 7 hr nil solid 30 702 solid nil solid nilC cored 9 cored nilC solid 8 solid nilc cored nilc cored nil solid nil solid nil - - a MC = masonry cement, S = sand, C = portland cement, L = lime. Average of all joints of the panel. Free water appeared on the back of the panel but none ran off. Bond strength increased to a maximum placed on a brick, and after 4 min a value a t brick suction between 10 a n d second brick was placed on the mortar 20 g and decreased sharply when the and bedded into it by tapping with a 4-lb suction exceeded 30 g, thus substan- hammer falling 13 in. Table I shows that tiating the generally accepted principle much less water passed through the that optimum bonding occurs when panels in which soaked bricks had been brick suction is less than 20 g. used and t h a t the strength of bond of the Welli~zg o Bricks.-Wetting f high- mortar joints of these panels was con- suction bricks before they are laid in siderably greater than that of the panels mortar improves the properties of the in which the bricks had been laid dry. brickwork, probably because the brick Some leakage occurred through the suction is reduced to a lower value con- bricks and was common to all fourpanels. ducive to better bonding with mortar. Results indicate that wetting of high- Wetting of bricks is popular with brick- suction briclcs can be an effective method layers because i t makes it easier to set of reducing moisture penetration by the bricks in place in the mortar. lowering the suctioli of bricks to a level a t which better bonding with mortar hr, while the panels of solid bricks showed will occur. dampness in less than 11 hr. For panels Core Holes and Texture of Bricks.- of all three mortars the strength of bond I n addition to the properties of water of those of cored bricks was less than the absorption and permeability of bricks, strength of the panels of solid bricks. The other properties such as the presence of decrease in strength mas more than core holes and the type of surface tex- could be accounted for by the decreased ture have been studied in relation to the area available for contact between brick performance of brick masonry. Core and mortar. holes conceivably can provide channels Although no specific studies were for the movement of water within the made of the effect of surface texture of mall, although only minor differences bricks on properties of masonry, it was have been observed in the permeability observed in several tests t h a t leakage and bond strength of masonry made of had occurred a t the brick-mortar inter- cored and solid bricks otherwise having face where a deep scratch o r depression the same properties (17,18). across the brick had not been filled hy I n this study, only a limited number mortar, thus producing a channel along of panels were constructed. Cored and which moisture penetration occurred. solid bricks of the same materials manu- factured a t the same plant were used, Mortar: with the only apparent difference being Studies (3,6,9,16) show the importance the presence of core holes in some of of properties of mortar on the perform- the bricks. A comparison of properties ance of brick masonry. Two properties of panels of the two types of brick is have received particular attention: (1) given in Table 1 . The bricks ranged in 1 the consistency of fluidity of a particular suction from 2 to 7 g per min per 30 mortar when i t is being used in laying sq in. They were used with three mor- bricks, and (2) its water retention value. tars: (1) 1:3 masonry cement:sand, (2) The former property is frequently ex- 1: 1 :6 cement :lime: sand, and (3) 1 :2 :9 pressed as the flow of the mortar, which cement: lime: sand. Each mortar had is the increase in diameter of a cone of a flow value of 120 per cent. I n making mortar when it is bumped 25 times on a these panels the mortar remained in dropping table, thereby indicating the contact with the brick for 30 sec before ease with which the mortar spreads or the next brick was placed on it and flows. Water retention value is defined bedded by the tap of a 4-lb hammer as the ratio of two flow values of a dropping 1 in. ; mortar, that of the mortar after it has Although there was no significant had water extracted from it b y a certain difference in the total amount of water suction applied for 1 min (to correspond that leaked from panels of cored and solid to the loss of water from mortar in bricks in 24 hr of test, there was a differ- contact with an absorptive brick), and ence in the rapidity with which damp- that of the freshly mixed mortar. Water ness appeared on the back of the panels retention value, therefore, provides a after Ehe start of the test. I n all cases measure of the ability of a mortar to dampness was delayed in penetrating retain its flow when in contact with a the panels of cored brick. This was brick. particularly noticeable for panels of Flow oof Mortar.-Studies (17,19,20) in- cored brick and masonry cement mortar; dicate that bonding between bricks and these showed no dampness for over 7 a particular mortar is improved when that mortar is used a t high ilow values. hours for d:lmpness to appear on the The upper limit occurs when segregation backs of the two panels of high-flow of water from mortar takes place or the mortar, and for the latter panels no mortar is so fluid that it runs from the water leaked off the back surface during joints. the test. The strength of bond decreased The effects of variation in flow of mor- from about 60 psi for the high-flow tar on the permeability and bond mortar to about 10 psi for t h e low-flow strength of masonry are illustrated by mortar. 60 W 3 J - 1 u W > Z a 50 .- m g a w a G= s wt- FLOW OF MORTAR, PER CENT FIG.3.-Influence of Flow of Mortar on Resistance to Moisture Penetration and Strength of Bond. the results in Fig. 3 obtained from tests LVater Rete~rliort Value.-The water of six panels. The panels were made retention value of a particular mortar is from bricks of suction in the range 19 dependent on its composition. To change to 22 g per min per 30 sq in., with mortar this value requires a change in composi- composed of 1:1:6 portland cement: tion, which may also change other 1ime:sand. The flow of the mortar was properties of the mortar. T h e effects on varied from 104 to 136 per cent. Moisture the properties of masonry of variation in penetration of the two panels of low-flow water retention value are, therefore, mortar occurred immediately after the difficult t o establish. start of the test, but it took several Few tests were made in the present program on the effect on the properties of masonry of changing the water reten- I n addition to the influence of the tion value of mortar. I n one test series, properties of bricks and mortars on the six panels were constructed of bricks in performance of masonry, there are other the suction range 42 to 44 g per min per factors connected with the method of 30 sq in. Two panels were made of a bringing bricks and mortar together masonry cement mortar with a water which have important effects on the retention value of about 70. I n the other resistailce to moisturc penetration and panels, part of the masonry cement was the strength of masonry. Careless work, replaced by lime, which increased the in which mortar joints are not com- water retention value to about 74 and 75 pletely filled with mortar or where bricks depending on the amount of lime. For are moved after the mortar around them all panels the flow of the mortar was has stiffened, affects the strength and between 116 and 119 pcr cent. Resistance tightness of brick masonry (4,8,18). Even when bricklaying is done with TABLE 111.-EFFECT OF CHANGE I N great care, certain factors connected WATER RETENTION VALUE OF MOIL TAR ON PROPERTIES OF MASONRY. wth the technique of laying the bricks inav have a considerable influence on the Watef Amount of Water quality of the brickwork. For example, R e t e n t ~ o n Passing Bond the length of time that elapses between Panel Value of through Strength, PSI'" laying a bed of mortar on a course of Mortar / Panel in 24 hr, ml bricks and placing bricks in i t may vary No. 1 . .. . . . . . . over a wide range, depending on (1) No. 2.. ....... the distance the bricklayer strings out No. 3 . .. . . . . . . the mortar bed in advance of this work No. 4.. . . . . . . . No. 5 . .. . . . . . . and (2) how rapidly he places the bricks. No. 6.. . . . . . . . A brick may be set in t h e mortar by a shove of the hand, or more usually, by " Average of all joints of the panel. one or more taps of a trowel. The energy expended in bedding the brick, therefore, to moisture penetration and strength of may vary and undoubtedly results in bond are compared in Table 1 1 As 1. variations in the properties of the the water retention value of the mortar masonry. Changes in the properties of increased. resistance to moisture pene- mortar caused by moisture loss as it tration and the strength of bond also stands on the board before being used, tended to increase. Two factors may have and the inevitable retempering to more influenced the streneth of bond. T h e suitable flow, may also affect the prop- improved extent of bond resulting from erties of the brickwork. the increased water retention value of the mortar (as reflected in greater tightness of panels) would tend to in- There are few references in the litera- crease the bond strength because of ture to the length of time that elapses greater contact area between brick and between spreading out a mortar bed mortar. Replacement of masonry ce- and placing bricks in it. I n a recent ment by lime, however, ~vouldresult in study (21) the time interval varied from lower strength of the mortar, thus tend- a minimum of 20 sec to a maximum of ing to reduce the tensile bond strength. 2 min and 25 sec. The influence of this time interval time interval of 60 sec; and two others factor on resistance to moisture penetra- with a time interval of 90 sec. The mois- tion and strength of bond was studied in ture penetration and bond strength prop- the present program. I t s importance is erties are shown in Fig. 4. I n the test shown by the results obtained from the for moisture penetration, there was no tests of the group of sis panels made of leakage for the first two panels (30-sec t APPROXIMATE GRAPH I . FOR BOND STRENGTH - \. ,' - NOTE /* \. 1 -GREATER STRENGTH AND "TIGHTNESS " O F $. / \. - THIS PANEL WOULD - HAVE RESULTED I F \. MORTAR H A D B E E N \A - A S HIGH IN FLOW AS IN OTHER PANELS // z.- - - I : /' LEGEND: ' 1 r BOND - / STRENGTH - / / e PERMEA- - / / BILITY / I I ' ; - I 1 30 60 90 T I M E I N T E R V A L , SEC , BETWEEN PLACING M O R T A R B E D A N D SETTING BRICK I N IT FIG.4.-Effect of Time Variation on Properties of Masonry. bricks ranging in suction from 11 to 17 g time interval) during the 24-hr test. The per min per 30 sq in. and mortar com- second two (60-sec time interval) leaked posed of cement, lime, and sand in the slightly. T h e two panels assembled with proportions 1: 1:6 by volume (having a a 90-sec time interval were considerably flow of 110 per cent). Two panels were more permeable. The strength of bond made with a time interval of 30 sec be- decreased sharply as the time interval tween placing the mortar and laying the increased. T h e properties of one of the next brick; two panels were made with a panels of 60-sec time interval were affected by the flow value of the mortar, the bricks properly in mortar, it was which was considerably lower for this suggested that one heavy tap rather panel than for the other five. Additional than several light ones be given to save tests indicate that the importance of the time in thc bricklaying operation. T h e time factor depends to some extent on need for adjustment in the technique of the particular combination of brick and bricklaying in relation to the properties mortar being used. I n conjunction with of brick and mortar is a n interesting the time factor, the brick-mortar combi- point raised in this early study, suggest- nation influences the plastic condition ing the dependence, to some extent, of of the mortar a t the time a brick is the bricklayer's technique or morkman- placed in it, thus affecting the nature of ship on the properties of the materials the bond established. being used. he effect of variation in the tapping VARL~TION TAPPING IN IMPACT impact ,aiven to bricks when are After a bricklayer places a brick on a n bedded Y inortar was studied i i the bed of mortar he may tap i t with his present program. I n making certain TABLE 1V.-EFFECT OF VARIATION I N M E T H O D OF BILICKLAYING ON BOND STRENGTH. Type of Brick Uortar Composition by Volume" / - - Extruded . . . . . . . . . . . . . . . . . . . . . 2-lb hammerC Extruded . . . . . . . . . . . . . . . . . . . . . 4-lb hammerC Extruded. . . . . . . . . . . . . . . . . . . . . bricklayer Extruded . . . . . . . . . . . . . . . . . . . . . 2-lb hammer Extruded . . . . . . . . . . . . . . . . . . . . . 4-lb hnmmer Extruded . . . . . . . . . . . . . . . . . . . . . bricklayer 2-lb hammer 4-lb hnmmer bricklayer a M C = masonry cement. S = sand, C = portland cement, L = lime. Average of nll joints of three panels. Dropped 1j.i in. trowel to bring it to line, or, depending panels, the brick was placed on the on the circumstances, he may shove the mortar bed and tapped by a 2-lb ham- brick into place without tapping it. I n mer, which fell from a height of 13 in. the former case the tapping may vary I n other panels, a 4-lb hammer was in both heaviness and number of taps, dropped from the same height. Compari- and there may also be variation in the son of the test results indicated a general rapidity with which they are made. An increase in the resistance t o moisture early study of the bricklaying operation penetration and in the strength of bond made by a pioneer in the field of time when the tapping impact was increased and motion study (22) found that tapping by the use of the heavier hammer. The bricks is not necessaly if (1) the mortar heavier tap tends to force the brick into has the correct temper, (2) the bricks more con~plete contact with the mortar. have been wetted sufficiently, and (3) The estent of improvement of properties the size of the joints is suitable. If for of masonry resulting from a greater some reason tapping is nccessary to bed ta.pping impact appeared to depend t o some extent 011 the properties of the biicks on bond strength. Bond strength materials being used. between the extruded brick and 1: 1 :6 The influence of the method of brick- mortar was considerably higher for all laying on properties of masonry is three methods of construction than the indicated by Table IV. Panels for this bond strength between the dry-press series of tests were constructed in three brick and the same mortar. wavs: 2-lb and 4-lb hammers were used as described previously, and the third set of panels were built by a bricklayer. His Another variable in the bricklaying panels differed from the others in the operation is introduced by the practice method of placing the bricli in the of retempering mortar. After it is mixed, mortar. The bricklayer used his normal there is an interval of time before mortar method of work, spreading a bed of f is placed in the wall. I this waiting mortar on the brick(to a depth greater period is sufficiently long (and this than used in the other panels), pressing the brick into the mortar and tapping T A B L E V.-EFFECT O F T I M E INTER- VAL B E F O R E R E T E M P E R I N G O F MOR- it into place with a rapid series of taps TAIt O N P R O P E R T I E S O F MASONRY of a trowel. Three combinations of brick and mor- Mortar Total Bond Panel Retempered Leakage in Strength, tar were studied. An extruded brick of After Time, 24 hr, m l hr psi" suction in the range 31 to 55 g per min . - -- per 30 sq in. was used with a masonry No. 1.. . . . . . . . 0 I nil 46.7 cement mortar (1:3 by volume) and No. 2... . . . . . . 0 nil 44.7 No. 3......... 2 nil 43.3 with a cement-lime mortar (1: 1:6). A No. 4......... 2 2353b 42.0 dry-press brick of suction in the range No. 5.. . . . . . . . 3 nil 39.0 46 to 87 g per min per 30 sq in, was used No. 6......... 3 4068" 22.5 No. 7......... 4 438 16.8 with the 1: 1:6 mortar. Three panels No. 8.. . . . . . . . 4 600 22.0 \\.ere built bv each method. The test of these panels for resistance to moisture " Average of all joints of panel. Leakage believed due t o greater warpnge of penetration revealed only slight differ- some of t h e bricks of these panels. ences, which could be attributed to the method of construction. Strength of probably depends on the particular mor- bond, however, was clearly affected by tar mix and the conditions of storage), the method of bedding the brick in the the plasticity of the inortar may change mortar, as is shown by the test results so much that the bricklayer may have listed in Table IV. The bond strength to rework the stiffened mortar with given in the table for each combination water to restore it to a suitable con- of brick and mortar is the average of all sistency. the mortar joints of three panels. I n this To determine the eflect of retempering series of tests the bricklayer-constructed mortar on properties of masonry, several panels were considerably higher in series of tests were made. In one of them, strength of bond than were the others, briclis of suction 13 to 17 g per min per while those made by the tap of the 4-lb 30 sq in. were used with 1: 1 :6 cement- hammer were of greater bond strength lime mortar. Two panels were made of than those made with the lighter ham- this mortar immediately after it had mer. been mixed to a flow of 120 per cent. The results of these tests also demon- Other panels were made of the same strate the influence of properties of mortar after it had stood after mixing for periods of 2, 3, and 4 hr. After these strength of bond and an increase in periods of time the mortar was remixed, permeability of the panel, but the effect with sufficient water to restore the flow of changing the thickness of the bed to 120 per cent. The results of leakage depended also on the particular mortar and bond strength tests for the series being used. In one series of tests in are presented in Table V. For each panel which mortar joints 5 in. and 5 in. thick the total amount of water (in milliliters) were used, panels were free of moisture which passed through in 24 hr of test is penetration, whereas in panels made with listed, along with the average tensile $-in. and $-in. joints, moisture penetra- bond strength of the joints of the panel. tion occurred. At the same time, the The panels made with mortar used strength of bond decreased as the thick- immediately after mixing were free of ness of the mortar joint decreased. leakage, but leakage occurred in those panels made of the mortar which had stood for 4 hr. The results of the leakage The resistance of brick masonry to tests for panels built of mortar re- moisture penetration and the strength tempered after 2 and 3 hr were incon- of bond between brick and mortar pri- sistent., and the permeability of these marily depend on the properties of the panels was believed to have been affected materials used and the manner in which by excessive warpage of a few of the brick and mortar are brought together bricks. Change in bond strength with to form masonry. Leakage may take increasing time interval before retem- place through the brick, depending on pering ranged from a value well over 40 its permeability, but more usually it psi for mortar used immediately after occurs through channels a t the brick- mixing to a value of about 20 psi for mortar interface. I n this connection the mortar retempered 4 hr after mixing. extent of bond between brick and mortar is critical and depends largely on the condition of the upper surface of the The thickness of the mortar spread by mortar bed when a brick is laid in it. the bricklayer before laying bricks in it is Factors affecting the condition of the usually not controlled carefully and mortar a t the time of bricklaying include varies along the length of the inortar the rate of water absorption of the bricks bed. The a.verage thickness may be on which the mortar is spread, the in- influenced to some extent by variation in herent resistance of the fresh mortar to properties of brick and mortar and also loss of moisture (water retention value) by the finished thickness required for the and the amount of water in the fresh joints. The thickness of the freshly laid mortar, the thickness of the mortar bed, mortar may be expected to have some the length of time that elapses before a effect on the bond with the briclr laid brick is placed in the mortar, and the in it, since in a given length of time after energy used to bed a brick. the mortar is placed the plasticity of The strength of bond between brick its upper surface will probably be greater and mortar also depends on the nature for the thicker mortar bed. of the bond between the two. A particular I n several tests in this program, the brick and mortar combination may, thickness of the mortar bed was varied however, have a complete extent of bond from 5 to $ in. in steps of $ in. The result at interface and have relatively low of decreasing the thickness of the mortar L. strength of bond, whereas another com- joint was, in general, a decrease in bination may have a '(patchy" or in- complete extent of bond with greater and partially compensated for by the strength. The extent of contact between bricklayer. For example, he may call brick and mortar and the tensile strength for a very fluid mortar to offset high of the mortar appear to influence the suction of the brick being used, or he bond strength. may shorten the "string-out" of a mor- A selection of typical test results in- tar bed to compensate for its low water volving almost 1000 panels illustrates retention properties. Inconsistent re- the effects of various factors on the sults in some tests, however, indicate properties of masonry. The extent of that there are unrecognized factors, or these effects is frequently difficult to combinations of factors, affecting prop- assess because the factors are inter- erties of masonry. dependent. A certain factor may be important under one set of conditions and have little significance when these conditions are varied by using materials This paper is a contribution from the having different properties or by altering Division of Building Research of the bricklaying techniques. National Research Council of Canada I n practice, deficiencies in the prop- and is published with the approval of erties of materials may be recognized the Director of the Division. REFERENCES (1) T. Ritchie and W. G. Plewes, "Moisture Ceramic Soc., Vol. 13, Feb., 1930, pp. 315- Penetration of Brick hfasonry Panels," 324. ASTN BULLETIN, NO. 249, Oct., 1960, p. (9) C. C. Fishburn, D. Watstein, and D. E. 39. Parsons, "Water Pern~eabilityof Masonry (2) T. 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