Structural Concrete - Hassoun

CHAPTER2
PROPERTIES OF
REINFORCED
CONCRETE
IBM Building, Montreal Canada (the highest concrete
building in Montreal, with 50 stories).
2.1 FACTORS AFFECTING STRENGTH OF CONCRETE
In general, concrete consists of coarse and fine aggregate, cement, water, and—in many cases—
different types of admixture. The materials are mixed together until a cement paste is developed,
filling most of the voids in the aggregates and producing a uniform dense concrete. The plastic
concrete is then placed in a mold and left to set, harden, and develop adequate strength. For the
design of concrete mixtures, as well as composition and properties of concrete materials, the reader
is referred to Refs. 1 through 6.
The strength of concrete depends upon many factors and may vary within wide limits with the
same production method. The main factors that affect the strength of concrete are described next.
2.1.1 Water–Cement Ratio
The water–cement ratio is one of the most important factors affecting the strength of concrete. For
complete hydration of a given amount of cement, a water–cement ratio (by weight) equal to 0.25 is
needed. A water–cement ratio of about 0.35 or higher is needed for the concrete to be reasonably
workable without additives. This ratio corresponds to 4 gal of water per sack of cement (94 lb)
(or 17.8 lb per 50 kg of cement). Based on this cement ratio, a concrete strength of about 6000 psi
may be achieved. A water–cement ratio of 0.5 and 0.7 may produce a concrete strength of about
5000 and 3000 psi, respectively.
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