Page 14 - Masonry and Concrete

Understanding Concrete
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UNDERSTANDING CONCRETE

masonry, and stucco experience initial shrinkage as the cement
hydrates and excess construction water evaporates. This initial shrink-
age is permanent, and is in addition to reversible expansion and con-
traction caused by later temperature or moisture changes. Excessive
shrinkage can cause concrete to crack. The cracks allow moisture to
penetrate, and a vicious cycle of deterioration may begin. Shrinkage
cracking can be restrained to some extent by steel or fiber reinforce-
ment, and the location of shrinkage cracks can be controlled through
the use of special joints that divide the concrete into smaller panels or
sections. However, the mix design and ingredient proportions also
have an effect on the potential for shrinkage cracking. The higher the
cement content, the greater the tendency for shrinkage cracks to form
while the concrete is curing and hardening.

2.2 Concrete Ingredients

The basic ingredients in concrete are cement, aggregates, and water.
The type, quality, and proportioning of these ingredients affect the
curing rate, compressive strength, and durability of the concrete.
Chemical admixtures can be used to enhance one or more properties of
the concrete or to improve its handling and placing characteristics.
2.2.1 Cement

Cement is not the same thing as concrete. Many people mistakenly
refer to “cement” sidewalks or “cement” driveways and the like, but
cement is only one of the ingredients in concrete. It is also an ingredi-
ent in masonry mortar, stucco, and other materials.

■ Cement water cement paste
■ Cement water sand cement mortar

■ Cement water sand lime masonry mortar
■ Cement water sand coarse aggregate concrete

Cement is a powdery substance which reacts with water to form a
cement paste, which is the actual cementing or binding medium in
concrete. The cement paste must completely coat each aggregate parti-
cle, and as it cures in a process called hydration, the concrete hardens
into a strong, stonelike mass.

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