PORTLAND CEMENT CONCRETE - Materials for Civil and Construction Engineers - page 294

FIGURE 7.44 Steel used as ag-
gregate for heavy weight concrete.

during construction. The microstructure of concrete with a compressive
strength greater than 40 MPa is considerably different from that of conven-
tional concrete. In particular, the porosity of the cement paste and the tran-
sition zone between the cement paste and the aggregate are the controlling
factors for developing high strength. This porosity is controlled by the
water-cementitious materials ratio. The development of superplasticizers
has permitted the development of high-strength concrete that is workable
and flowable at low water-cementitious materials ratios. In addition, high-
strength concrete has excellent durability due to its tight pore structure. In
the United States, high-strength concrete is used primarily for skyscrapers.
The high-strength and corresponding high elastic modulus allow for reduced
structural element size.

■ 7.6.7 Shrinkage-Compensating Concrete

Normal concrete shrinks at early ages, especially if it is not properly cured,
as discussed earlier in this chapter. The addition of alumina powders to the
cement can cause the concrete to expand at early ages. Shrinkage-compen-
sating cement is marketed as Type K cement. Expansive properties can be
used to advantage by restraining the concrete, either by reinforcing or by
other means, at early ages. As the restrained concrete tries to expand, com-
pressive stresses are developed. These compressive stresses reduce the ten-
sile stresses developed by drying shrinkage, and the chance of the concrete
cracking due to drying shrinkage is reduced. Details on the design and use
of shrinkage-compensating concrete are available in ACI Committee 223 re-
port Recommendations for the Use of Shrinkage-Compensating Cements .

■

Polymers and Concrete
Polymers can be used in several ways in the production of concrete. The poly-
mer can be used as the sole binding agent to produce polymer concrete . Poly-
mers can be mixed with the plastic concrete to produce polymer-portland
cement concrete . Polymers can be applied to hardened concrete to produce
polymer-impregnated concrete .

7.6.8