Page 12 - Masonry and Concrete

Understanding Concrete
11
UNDERSTANDING CONCRETE

LOAD LOAD LOAD

COMPRESSION
WEAK
STRONG
WEAK

TENSION
CRACK
LOAD LOAD

COMPRESSION TENSION FLEXURE
FIGURE 2-3
Tension and compression in concrete.

equipment and procedures. On commercial projects, numerous samples
are tested throughout construction to verify that the concrete being put
into place actually has the specified strength. Laboratory testing is not
often required in residential work, except perhaps on large, high-end
projects or on projects with difficult sites where special foundation
designs make concrete strength critical. For most residential projects,
required concrete strength will be in the range of 2,500 to 4,000 psi,
depending on the intended use (Figure 2-4). A concrete that is stronger
than necessary for its intended use is not economical, and one that is not
strong enough can be dangerous. The primary factors affecting concrete
compressive strength are the cement content, the ratio of water to
cement, and the adequacy and extent of hydration and curing, all of
which are discussed later in this chapter.
Durability might be defined as the ability to maintain satisfactory
performance over an extended service life. Satisfactory performance is
related to intended use. Concrete that will be walked or driven on
must be abrasion resistant so that it doesn’t wear away. Concrete that
will be exposed on the outside of a building must be weather resistant
so that it doesn’t deteriorate from repeated freezing and thawing. Con-
crete in which steel reinforcement is embedded must resist excessive
moisture absorption in order to protect the metal from corrosion. Nat-
ural wear and weathering will cause some change in the appearance of
concrete over time, but in general, durability also includes the mainte-
nance of aesthetic as well as functional characteristics. Just as concrete

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