COMPOSITES - Materials for Civil and Construction Engineers - page 455

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11.2.1 Plain Portland Cement Concrete

Plain portland cement concrete is a composite material consisting of ce-
ment paste and aggregate particles with different physical and mechanical
properties, as discussed in Chapter 7 (Figure 11.11). Aggregate particles in
concrete act as a filler material, since it is cheaper than the portland ce-
ment. In addition, since cement paste shrinks as it cures, aggregate increas-
es the volume stability of the concrete. When the concrete structure is
loaded, both cement paste and aggregate share the load. Both the strength
of aggregate particles and the bond between the aggregate and cement
paste play an important role in determining the strength of the concrete
composite, which is limited by the weaker of the two. The bond between
cement paste and aggregate is affected by roughness and absorption of the
aggregate particles, as well as by other physical and chemical properties of
aggregate.

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11.2.2 Reinforced Portland Cement Concrete

Steel-reinforced concrete can be viewed as a composite material, consisting
of plain concrete and steel rebars, as shown in Figure 11.12. Since concrete
has a very low tensile strength, which is typically ignored in designing con-
crete structures, steel rebars are usually placed in areas within the structure

FIGURE 11.11 Cross section of portland
cement concrete showing cement paste and
aggregate particles.

FIGURE 11.12 An example of steel-reinforced concrete
beam and slab details.