Page 18 - Masonry and Concrete

Understanding Concrete
17
UNDERSTANDING CONCRETE

Characteristic Significance in Concrete
Soundness, volume stability Strength, durability, appearance
Chemical reactivity Alkali-silica reaction, popouts, disintegra-
tion, appearance
Abrasion resistance Wear resistance of floors, hardness

Particle shape Workability, economy, shrinkage, strength
Surface texture Bond, strength, durability
Grading Workability, density, economy, shrinkage

Maximum size of aggregate Economy, shrinkage, density, strength
Percentage of crushed particles Workability, economy, strength

Specific gravity Durability, density, needed for mix compu-
tations

Absorption Durability, needed for mix computations
and control

Moisture content Needed for mix computations and
control

FIGURE 2-5

Characteristics of concrete aggregate. (Based on Waddell, Concrete Manual, Interna-
tional Conference of Building Officials).

and more economical (Figure 2-6). Maximum recommended aggregate
size is based on the dimensions of the finished concrete and the spac-
ing of reinforcing steel. Maximum aggregate size should not exceed
any of the following (Figure 2-7):

■ One-third the depth of a concrete slab
■ Three-fourths the minimum clear distance between reinforcing
bars or between reinforcing bars and forms
■ One-fifth the narrowest dimension between sides of forms

For a 4-in.-thick concrete slab, maximum aggregate size should be
about 1 in., and for a 6-in. slab, maximum aggregate size could be as

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