653
FLEXURAL STRENGTH OF JOINT REINFORCED BLOCK MASONRY WALLS
by
Ahmad A. Hamid
Assoc. Professor of Civil Engineering
Drexel University
Philadelphia, PA, USA
Catherine Chia
Structural Engineer
Yardley Consultants Inc.
Yardley, PA, USA
Harry G. Harris
Professor of Civil Engineering
Drexel University, Philadelphia, PA, USA
ABSTRACT
Wire bed joint reinforcement has been used over the years for
control of cracks due to temperature and shrinkage, for continuity in
multiple wythe walls and to satisfy arbitrary minimum code
requirements in the horizontal direction.
However, the structural
significance of joint reinforcement in block masonry walls is not
well established. This pape r presents an experimental study of the
strength of horizontally spanning joint reinforced block masonry
walls under out-of-plane lateral loading.
Three wall panels were
tested to determine the effect of amount of horizontal
reinforcement on the wall cracking moment and flexural stength. It
is concluded that joint reinforcement does not influence the
cracking load but the flexural strength, however, is increased
depending on the type and spacing of the reinforcing steel.
654
INTRODUCTION
Wire bed joint reinforcement has been used over the years for crack
control due to temperature and shrinkage (1, 2), for continuity in
multiple wythe walls (3-5) and even to satisfy arbitrary minimum
co de requirements in the horizontal direction.
However, the
structural significance of joint reinforcement in block masonry
walls, particularly for seismic resistance, is not well established .
There has been relatively few documented tests (6-14) addressing
The test
the function of joint reinforcement in masonry walls .
results indicate that its contribution to load carrying capacity
ranges from -10 to 300 percent. This wide range is attributed to
variation in. material and geometric properties as well as
construction details.
Test data are scarce and conflicting.
Ali
available data are for vertically unreinforced walls, and as such
Also, the
does not represent reinforced masonry wall behavior.
tests were conducted under force control which cannot predict the
post-yield behavior required to determine wall ductility.
A
comprehensive experimental program was conducted at Orexel
University to investigate the effects of amount and type of
reinforcement, block size, bond type, grouting, and geometric
parameters on the behavior of horizontally spanned joint reinforced
walls using displacement control input.
Results of three walls
tested to determine the effect of amount of joint reinforcement on
wall flexural capacity are presented.
EXPERIMENTAL PROGRAM
Materiais
The materiais used in the construction of the walls are typical of
those commonly used in building construction in the United States.
Concrete Blocks: The physical and mechanical properties of
the 6 in. nominal hollow concrete blocks used are listed in Table 1.
Morta r:
Cement-lime mortar consisting of 1 part Type I
Portland cement, 3/8 part hydrated lime, and 3-1/2 parts sand,
conforming to requirements for type S mortar described in ASTM
C270-82 (15), was used in the construction of the wall panels. An
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655
averag e water/cement ratio of 0.74 was used to obtain the average
initial flow of 110 percent.
Cylindrical control specimens 2-in.
diameter by 4 in. high were obtained following procedures similar to
ASTM C109-80 (15).
The cylinders were air-cured alongside the
companion walls and tested under axial compression at
approximately the same age as the walls. The average compressive
strength obtained for the mortar specimens was 2890 psi.
TABLE 1
Properties of Concrete Masonry Units
Description
ASTM
Property
Density, pcf
Absorption, pcf
0/0
Moisture Content, %
Initial Rate of Absorption,
gm/min/30 in. 2
Saturation Coefficient
Axial Compressive Strength, psi,
for Net Area
for Gross Area
Splitting Tensile Strength, psi
C140-75
C140-75
102
11.0
10.8
3.83
C140-75
C67-83
C67-83
C140-75
C1006-84
43.9
0.721
2920
1550
280
Grout: Coarse grout consisting of 1 part Type 11 Portland
cement, 3 parts sand and 2 parts 3/8-in. pea gravei; conforming to
the requirements described in ASTM C476-83 (15), was used in the
construction of the wall panels. A grout aid from Sika Corporation
(Sika mix 119/120) was premixed in the water and added to the
base mix to provide a more workable grout (9-10 in. slump) which is
easy to pour and consolidate.
Core drilled cylinders 1-3/4-in.
diameter by 3-1/2 in. high were used as control specimens. The
average compressive strength of the grout was 1790 psi.
Reinforcement:
Prefabricated truss type wire reinforcement
from Dur-O-Wal, Inc. conforming to ASTM A82-79 (16) was used as
horizontal reinforcement. Properties of the two types (standard and
656
extra heavy) are listed in Table 2.
TABLE 2
Properties of Joint Reinforcement
Type
Description
Standard
Extra Heavy
Yield Strength, psi
Ultimate Strength, psi
Modulus of Elasticity, psi
Strain, in./in
Reduction in Area, %
Elongation, %
98100(a)
,
103,000
29 ,700 ,000
0.005(b)
48.7
2.3
97,700(a)
102,000
28,700,000
0.005(b)
47.5
2.3
(a) at 0.5% extension under load
(b) at yield load
Test Specimens
Three wall panels of four courses high by 5 units long were
constructed in running bond with face shell mortar bedding . The
typical panel was made up of three full courses in addition to a half
course at the bottom and at the top (see Figure 1), to represent a
typical wall strip cut from center-of-block to center-of-block.
The
panel size was 32 in . wide by 80 in . long. Horizontal reinforcement
was present in the three walls (W1, W2, W3) in the following
configurations , . respectively:
a) standard Dur-O-Wal Truss placed in every course,
b) standard Dur-O-Wal Truss placed in every other course,
c) extra heavy Dur-O-Wal Truss placed in every course.
The amount of horizontal reinforcement ranged from 0.056 percent
(Iess than code minimum requirement) to 0.16 percent (greater than
code minimum requirement).
The quantity of steel reinforcement
used was based generally on the guidelines presented in the 1985
UBC (17) and the 1983 ACI 531-79 (18) , which specify the minimum
amount of wall reinforcement in either the vertical or horizontal
direction to be 0 .07 percent of the respective gross cross-sectional
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1-
80"
~I
Dur-Q-Wa 1
(Typ . )
#4 Rebar
(Typ . )
(a) Joint reinforcement every course
(Walls Wl and W3)
Dur-O-Wal
(Typ.)
#4 Rebar
(Typ.)
(b) Joint reinforcement every other course
(Wall W2)
Figure 1. Test specimens
658
area of the wall. In addition, the sum of the percentages of vertical
and horizontal reinfo rcement is specified to be at least 0.2 percent.
Four No. 4 bars (0 .18%) we re used for vertical reinforcement.
Grouting was performed at 2-ft intervals at the hollow cores that
contained the vertical reinforcement.
Test Set-up and Instrument ation
The walls were simply supported at the end and had a span of 74 in .
from centerline to centerline of the supports (one pinned and the
other roller support) . Two equal line loads, transverse to the span
length, were applied to the face of each wall (with the wall in a
vertical position) at the third points .
Deflection and strain
instrumentation were placed at specific locations on the walls.
Figure 2 shows a schematic of the experimental set-up. A 55 kips
MTS hydraulic actuator was operated under displacement control at
a constant rate through a computer-based measurement and control
system. Data acquisition was carried out on the the same computer
system.
A displacement ramp was appl ied slowly and load was
measured by the actuator precision load cell.
DISCUSSION OF TEST RESULTS
Crack Pattern and Mode of Failure
Crack patterns of the three walls are shown in Figure 3. The basic
pattern of cracking was one of head joints separation and vertical
Cracking
splitting of the masonry units on the tension face.
occurred first in the mortar joints at low loads in the constant
moment region between the lines of applied load . The cracks then
propagated into the face-shell area in the same region. Once the
crack pattern in the pure moment region had been established, load
redistribution in the wall panel occurred , as evidenced by the
appearance of a set of vertical cracks just beyond the pure moment
region. The load at which these cracks in the shear span occurred
was close to the failure load. Failure was preceeded by widening of
cracks with no further addition to the crack pattern .
A loud
snapping noise was heard shortly after the ultimate load was
reached in the wall panels indicating the rupture of the joint
reinforcement.
659
r-
í
Support Tube (Typ.)
J
r--
r
r
Wall Panel
Load Tube (Typ.)
,.-!-
f-
Distribution
Beam
~
~
N
M
/L
-
'--
HydraullC Actuator
I,
.1
80"
(a) Elevation view (not to scale)
Hydraulic Actuator
Strain Instrumentation
Beam
Wall Panel
Roller
Support
Instrumentation
25"
I-
24"
.1
e
(b) Top view (not to scale)
Figure 2. Test set-up
25"
~I
I
3"
,
sl
\
SlAEHdij9
2M LLI?M
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SlAZH<Rl9
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661
Wall Flexural Strength
The ultimate loads of the three walls are given in Table 3. Ultimate
load increased when the amount of horizontal reinforcement in the
wall increased, and joint reinforcement increased the flexural
strength over walls without joint reinforcement.
For walls without
joint reinforcement the ultimate strength is assumed to be the
cracking strength of the masonry assemblage.
In comparing the
ultimate load with the cracking load, it is found that joint
reinforcement in every course increased the flexural strength by
220 to 280 percent depending on the type of reinforcement
(standard or extra heavy) and joint reinforcement in every other
course increased the flexural strength by just 64 percent (Table 3).
TABLE 3
Summary of Test Results
Wall
% Steel
Pcr(lbs)
Pu(lbs)
PiPcr
W2
W1
W3
0.056
0.11
0.16
1030
938
1290
1700
3010
4910
1.64
3.20
3.80
CONCLUSIONS
The following conclusions are drawn from observation of the tests
and analysis of the test results:
(1) The walls were observed to share a common mode of
failure, that of head joints separation and splitting of the
face-shells in the tension zone.
The complexity of
the crack pattern was found to be dependent on the spacing of
the joint reinforcement .
Walls with joint reinforcement in
every course had a more developed crack pattern than walf
with joint reinforcement in just every other course.
(2) Joint reinforcement did not influence the cracking load
significantly but it did act to control and distribute the cracks
within the failure region.
A minimum amount of joint
662
reinforcement is required to maintain a safe ratio between the
cracking load and the ultimate load.
Joint reinforcement in
every course adequately provides this ratio.
(3) Joint reinforcement increased the flexural strength of block
masonry walls as compared to unreinforced walls.
Joint
reinforcement in every other course increased the flexural
strength by about 64 percent while joint reinforcement in every
course increased the flexural strength by 220 to 280 percent.
Therefore, the code minimum steel requirement in the
horizontal direction (0.07%) has the desirable effect of
increasing the wall flexural strength by over 64 percent as
compared to unreinforced walls.
ACKNOWLEDGMENTS
The support of this research by the National Science Foundation
through grant No. ECE-8319178 to Drexel University is gratefully
acknowledged. Tests were conducted using an MTS loading system
funded partially by the National Science Foundation through grant
No. ECE-8412474. Mason's time was made available through the
Delaware Valley Masonry Institute and D.M. Sabia and Company and is
gratefully acknowledged.
REFERENCES
1.
Bartlett, W.H., "Masonry Wall Reinforcement - When, Where, How
Much?," C/M Magazine, March 1965.
2.
----, "Ladur Type Masonry Wall Reinforcement, DUR-O-WAL,"
ICBO Evaluation Report No. 2292, International Conference of
Building Officials, Whittier, California, January 1982.
3.
Bortz, S.A., "Investigation of Continuous Metal Ties as a
Replacement for Brick Ties in Masonry Walls," Technical
Bulletin No. 67-5, DUR-O-WAL, Inc., Northbrook, IlIinois, 1967 .
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663
4.
Randall, F.A. and Panarese, W.C., Concrete Masonry Handbook for
Architects. Engineers, Builders. Portland Cement Association,
Skokie, IlIinois, 1976.
5.
Schneider, R.R. and Dickey, W.L., Reinforced Masonry Design,
Prentice-Hall, Inc., Englewood Cliffs, N. J., 1980.
6.
Cox , F.W. and Ennenga, J.L. , "Tranverse Strength of Concrete
Block Walls," Proceedings of American Concrete Institute, Vol.
54, 1958, pp. 951-960.
7.
Hedstrom, R.O., "Load Tests of Patterned Concrete Masonry
Walls," Proceedings of American Concrete Institute, Vol. 57,
1961, pp. 1265-1286.
8.
Mattison, E.N. and Churchward, G., "Some Tests on the Bending
Strength of Concrete Masonry in Running Bond," Division of
Building Research, Melbourne, Australia, 1969.
9.
----, "The Structural Role of Joint Reinforcement in Concrete
Masonry," Technical Bulletin No. 99, National Concrete Masonry
Association, Herndon, Virginia, 1978.
10. DeVekey, R.C and West, H.W., "The Flexural Strength of Concrete
Blockwork," Building Research Establishment, London, October
1979.
11. Anderson, D.L., Nathan, N.D., Cherry, S. and Gajer, R.B., "Seismic
Design of Reinforced Concrete Masonry Walls," Proceedings of
the Second Canadian Masonry Symposium, Ottawa, Canada, June
1980, pp. 181-196.
12. Cajdert, A., "Laterally Loaded Masonry Walls," Ph .D Thesis,
Chalmers University of Technology, Goteberg, Sweden, 1980.
13. Dickey, W.L., "Joint Reinforcement and Masonry," Proceedings of
the Second North American Masonry Conference, College Park,
Maryland, August1982, pp. 15-1 - 15-15.
14. Dickey, W.L. and Catani , M., "Joint Reinforcement is Also
Structural," Proceedings of Concrete International. ACI , Vol. 5,
664
No. 6, June 1983, pp. 32-39.
15. American Society for Testing and Materiais
Standards, Section 4, Vol. 04.05, 1984.
1984
Annual
16. American Society for Testing and Materiais
Standards, Section 1, Vol. 01.04, 1984.
1984 Annual
17. International Conference of Building Officials, "Masonry Codes
and Specifications," Uniform Building Code, Chapter 24,
Whittier, California, 1985.
18. American Concrete Institute, "Building Code Requirements for
Concrete Masonry Structures," ACI Standard 531-79, Detroit,
Michigan, 1983.
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