Page 11 - Masonry and Concrete

Understanding Concrete
10 CHAPTER TWO

but adjustments must be made to the over-
all mix to maintain the proper proportion
of all ingredients. A sticky mix may have a
high cement content (fat mixes) or large
amounts of rock dust, fine sand, or similar
fine materials (oversanded mixes). Sticky
mixes do not segregate easily, but because
they require a lot of water to achieve even
minimal workability, sticky mixes often
develop excessive shrinkage cracking. A
plastic mix is cohesive without being
FIGURE 2-2
either sticky or harsh, and the ingredients
Concrete workability is best at a slump between 3–6 do not easily segregate unless the concrete
inches (From Waddell, Concrete Manual, Interna-
tional Conference of Building Officials, Whittier, is handled improperly.
California).
2.1.2 Properties of Hardened
Concrete

Fully cured, hardened concrete must be strong enough to withstand
the structural and service loads which will be applied to it and must
be durable enough to withstand the environmental exposure for which
it is intended. When concrete is made with high-quality materials and
is properly proportioned, mixed, handled, placed, and finished, it is
one of the strongest and most durable of building materials.
When we refer to concrete strength, we are generally talking about
compressive strength which is measured in pounds per square inch
(psi). Concrete is strong in compression but relatively weak in tension
and bending. It takes a great deal of force to crush concrete, but very
little force to pull it apart or cause bending cracks (Figure 2-3). Com-
pressive strength is determined primarily by the amount of cement
used but is also affected by the ratio of water to cement, as well as
proper mixing, placing, and curing. Tensile strength usually ranges
from 7 or 8% of compressive strength in high-strength mixes to 11 or
12% in low-strength mixes. Both tensile strength and flexural bending
strength can be increased by adding steel or fiber reinforcement.
Structural engineers establish required compressive strengths for var-
ious building elements based on an analysis of the loads which will be
applied and the soil conditions at the project site. Actual compressive
strength is verified by testing samples in a laboratory using standardized

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