MASONRY - Materials for Civil and Construction Engineers - page 312

The compressive strength of clay bricks is an important mechanical
property that controls their load-carrying capacity and durability. The com-
pressive strength of clay bricks is dependent on the composition of the clay,
method of brick manufacturing, and the degree of firing. The compressive
strength is determined by capping and testing a half unit “flatwise” (load ap-
plied in the direction of the height of the unit) and is calculated by dividing
the load at failure by the cross-sectional area (ASTM C67). In determining
the compressive strength, either the net or gross cross-sectional area is used.
Net cross-sectional area is used only if the net cross-section is less than 75%
of the gross cross section. A quarter of a brick can be tested if the capacity of
the testing machine is not large enough to test a half brick. Other mechanical
properties of bricks include modulus of rupture, tensile strength, and modu-
lus of elasticity. Most clay bricks have modulus of rupture between 3.5 MPa
and 26.2 MPa (500 psi and 3800 psi). The tensile strength is typically be-
tween 30% to 49% of the modulus of rupture. The modulus of elasticity
ranges between 10.3 GPa and 34.5 GPa ( and ).
Building bricks are graded according to properties related to durability
and resistance to weathering, such as compressive strength, water absorp-
tion, and saturation coefficient (ASTM C62). Table 8.3 shows the three avail-
able grades and their requirements: SW, MW, and NW, standing for severe
weathering, moderate weathering, and negligible weathering, respectively.
Grade SW bricks are intended for use in areas subjected to frost action, espe-
cially at or below ground level. Grade NW bricks are recommended for use
in areas with no frost action and in dry locations, even where subfreezing
temperatures are expected. Grade NW bricks can be used in interior con-
struction, where no freezing occurs. Note that higher compressive strengths,
lower water absorptions, and lower saturation coefficients are required as the
weathering condition gets more severe, so as to reduce the effect of freezing
and thawing and wetting and drying.

10 6 psi 10 6 psi 1.5 * 5 *

T ABLE 8.3 Physical Requirements for Building Bricks (ASTM C62) (Copyright ASTM,
reprinted with permission).

Min. Compressive Strength, Max. Water Absorption Max. Saturation

Gross Area, MPa (psi) by 5-hour Boiling, % Coefficient

Average of Average of Average of

Grade Five Bricks Individual Five Bricks Individual Five Bricks Individual

SW 1

20.7 (3000) 17.2 (2500) 17.0 20.0 0.78 0.80

MW 2

17.2 (2500) 15.2 (2200) 22.0 25.0 0.88 0.90

NW 3 10.3 (1500) 8.6 (1250) No limit No limit No limit No limit

1 Severe weathering
2 Moderate weathering
3 Negligible weathering