Page 456 - Forensic Structural Engineering Handbook

13.6 MATERIAL-SPECIFIC FORENSIC ANALYSES

Clay Brick Masonry in Flexure. Flexure cracks in masonry form primarily at the unit-mortar
interface. Resistance to such cracks depends on tensile bond strength of the unit-mortar combi-
nation. Higher bond strength reduces cracking, leaking, staining, and spalling. Bond strength is a
function of (1) initial rate of water absorption, texture, and cleanliness of clay brick; (2) water
retentivity, flow, and cement and air content of mortar; and (3) quality of workmanship.
Allowable stress in flexural tension with non-air-entrained portland cement–lime mor-
tars is about twice as great as that with masonry cement or air-entrained portland
cement–lime mortar. Masonry built prior to the mid-twentieth century with lime mortars is
assumed to have no resistance to flexural tension perpendicular to the bed joints.
The 28-day flexural strength of clay brick masonry walls built with ASTM C 270, Mortar
63
for Unit Masonry, type S, portland cement–lime, non-air-entrained mortar, and inspected
2
workmanship, with stress perpendicular to bed joints, has a mean value of 140 lb/in with a
2
standard deviation of 31 lb/in , when tested in accordance with ASTM E 72, Methods of
Conducting Strength Tests of Panels for Building Construction. 63
Walls with type M mortar are about 10 percent stronger in flexure. Use of type N mor-
tar reduces strength about 23 percent. Increasing air content reduces flexural bond strength.
Uninspected workmanship reduces mean strength by about 23 percent. Other things being
equal, use of type N mortar with air content of 18 percent and uninspected workmanship
typically reduces flexural strength about 60 percent below that attained by type S mortar.
For 4-in (100-mm) wythes of standard modular clay brick, flexural strength is about
3.7 times greater when stress is parallel rather than perpendicular to bed joints.
First crack in clay brick masonry in flexure occurs at about 80 percent of ultimate
strength. Out-of-plane cracking occurs at a deflection of about 0.05 percent of wall span
with a coefficient of variation of 26 percent.

Clay Brick Masonry in Shear. Modulus of rigidity (shear modulus E ) is about 40 percent of
v
E . In the absence of compressive stress, the ultimate shear strength of clay brick masonry f sb
m
2
can be approximated as follows with a coefficient of variation of about 30 percent: f (lb/in )
sb
≈ 40 exp (f′ /1780). Compressive stress increases shear strength.
m
Concrete Masonry
Concrete Masonry Wall Weight. Weights of concrete masonry walls are given in Table 13.2.
Concrete Masonry in Compression. Compressive strength of concrete masonry is a func-
tion of concrete masonry unit (CMU) compressive strength, mortar type, stress direction, and
63
workmanship. For ASTM C 270, Mortar for Unit Masonry, type M or S non-air-entrained
mortars, stress perpendicular to bed joints, and inspected workmanship, the mean 28-day
compressive strength on net area of a standard prism of concrete masonry f ′ can be estimated
m
2
as f ′ (lb/in ) ≈ . 0.635 (f 1120), where f = compressive strength of concrete masonry units
cu
cu
m
in pounds per square inch.
Strength is reduced about 25 percent for type N mortar, 35 percent for uninspected
workmanship, 10 percent with increasing the air content from 5 to 18 percent, and 25 percent
for stress parallel to bed joints.
Elastic Modulus of Concrete Masonry. Mean chord modulus of elasticity of ungrouted
concrete masonry with a prism height/length ratio of 2 is 615 f ′ with a standard deviation
m
of 365 f ′.
m
Concrete Masonry in Flexure. Concrete masonry walls 28 days old constructed with
hollow concrete masonry units, non-air-entrained ASTM C 270, Mortar for Unit Masonry, 63
type S mortar, inspected workmanship, and stress perpendicular to bed joints have an estimated