Page 13 - Masonry and Concrete

Understanding Concrete
12 CHAPTER TWO

Construction Element Compressive Strength, psi

Basement and foundation walls and 2,500–3,000
slabs

Driveways, garage slabs 2,500–3,500
Reinforced concrete beams, slabs, 2,500–3,500
patios, sidewalks, and steps

FIGURE 2-4
Typical compressive strength requirements for residential concrete.

mix designs can be adjusted to produce a variety of strengths, appro-
priate concrete ingredients, mix proportions, and finishes can and
should be adjusted on the basis of required durability.
In cold climates, exterior concrete is exposed to repeated freeze-
thaw cycles which can potentially be very damaging. Freeze-thaw
deterioration, in fact, is one of the most serious threats to concrete
durability, but resistance to damage can be significantly increased by
air entrainment. A network of fine voids formed by air entrained
cement or an air-entraining admixture absorbs the expansive force of
freezing water to prevent the hardened concrete from fracturing or
scaling over repeated cycles of winter freezing and thawing. Air
entrainment improves the durability of horizontal elements such as
sidewalks, driveways, patios, and steps, which are most frequently
exposed to rainwater, melting snow, and deicing salts. For vertical ele-
ments, which are less often saturated with rain, and in mild climates
where freeze-thaw cycles are infrequent, air entrainment adds little
value to hardened concrete but still may be used to increase the work-
ability of fresh concrete. Air entrainment is sometimes credited with
increasing the watertightness of concrete, but this is probably because
the increased workability of the mix is conducive to better placement,
consolidation, and finishing.
Another important aspect of concrete durability is volume stability.
All materials expand and contract with changes in temperature, and
porous materials like concrete also expand and contract with changes
in moisture content. In addition to reversible thermal expansion and
contraction, cement-based products such as concrete, concrete

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