Factors affecting bond strength and resistance to moisture

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Factors affecting bond strength and resistance to moisture Powered By Docstoc
    no. 192
    c. 2

I   BLDG                         RESEARCH
                          NATIONAL      COUNCIL

                          DIVISION O F BUILDING RESEARCH


                                T. RITCHIE AND J. I. DAVISON

                                       REPRINTED FROM
              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


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                               Authorized Reprint from the Copyrighted
                                    Symposium on Masonry Terting
                                 Spccial Tcchrcical P~rblicalio,~ 320
                                           Published b y the
                          .\XERICAX   SOCIETY  rOR TESTING i\f \TERI.\LS


FACTORS AFFECTING BOND S T R E N G T H A N D RESISTANCE                                          TO

        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
      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).

                 -              A     D R Y - P R E S S BRICK
                                @     EXTRUDED BRICK
                 &    300   -
                                      SOFT-MIID      BRICK


                 $ 200-
                 3              J O I N S POINTS FOR
                 IT             SIX B R I C K S F R O M                   A
                 -              THE SAME LOT
                 a              OF B R I C K S

                      loo   -   JOINS POINTS
                 3              FOR T W O BRICKS

                                FROM T H E
                                SAME L O T              \t
                 0                                       /

                                10    20    30     40    50     60   70
                                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

                         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
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  -
                                         - mixed to a flow of 120 per cent, was
                                                 placed on the brick. After 1 min the
                                                 brick was bedded in the mortar by the
Wet . . . . . . . . . .
Wet.. . . . . . . . .
                                 514      1
                                                 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
                                                                                                             -      -

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.

                                                                                     u   W
                                                                                     >   Z
                                                                           50   .-
                                                                                     a   w
                                                                                a G=     s

                             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
TAR ON PROPERTIES OF MASONRY.                                         wth the technique of laying the bricks
                                                                      inav have a considerable influence on the
                                             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
                                        /   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
                                .               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 S HIGH IN FLOW AS
                          IN OTHER PANELS                                //

                                                :               /'        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
                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
                                                                               /                        - -

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

    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
                                               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.

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