Page 79 - Handbook of Thermal Analysis of Construction Materials

62 Chapter 2 - Introduction to Portland Cement Concrete

tensile strength, flexural strength, static modulus of elasticity, Poisson’s
ratio, mechanical properties under triaxial loads, creep under compression,
abrasion resistance, bond development with steel, penetration resistance,
pull out strength, etc.
The mechanical behavior of concrete should be viewed from the
point of view of a composite material. A composite material is a three
dimensional combination of at least two chemically and mechanically
distinct materials with a definite interface separating the components. This
multiphase material will have different properties from the original compo-
nents. Concrete qualifies as such a multiphase material. Concrete is
composed of hydrated cement paste (C-S-H, CH, aluminate, and ferrite-

based compounds) and unhydrated cement, containing a network of a
mixture of different materials. In dealing with cement paste behavior,
basically it is considered that the paste consists of C-S-H and CH with a
capillary system. The model of concrete is simplified by treating it as a
matrix containing aggregate embedded in a matrix of cement paste. This
model provides information on the mechanical properties of concrete.
The factors that influence the mechanical behavior of concrete are:
shape of particles, size and distribution of particles, concentration, their
orientation, topology, composition of the disperse and continuous phases,
and that between the continuous and disperse phase and the pore structure.
The important role played by cement paste is already described.

12.0 DURABILITY OF CONCRETE

One of the most important requirements of concrete is that it should
be durable under certain conditions of exposure. Deterioration can occur in
various forms such as alkali-aggregate expansion reaction, freeze-thaw
expansion, salt scaling by de-icing salts, shrinkage and enhanced attack on
the reinforcement of steel due to carbonation, sulfate attack on exposure to
ground waters containing sulfate ions, sea water attack, and corrosion
caused by salts. The addition of admixtures may control these deleterious
effects. Air entrainment results in increased protection against freeze-thaw
action; corrosion inhibiting admixtures increase the resistance to corrosion;
inclusion of silica fume in concrete decreases the permeability; conse-
quently, the rate of ingress of salts and the addition of slags in concrete
increases the resistance to sulfate attack.