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The level of shrinkage strains depends on several factors, including
temperature, humidity, mixture proportions, type of aggregates, etc. Shrinkage
strain in hardened concrete induces elastic tensile stress. Cracks appear in
concrete when the induced tensile stress exceeds the tensile strength of the
concrete. Creep may reduce the induced tensile stress to a certain extent, but
the resultant stress can be large enough for cracking concrete.
Using sufficient steel reinforcement has traditionally controlled cracking.
However, using reinforcement does not solve this problem completely. By
using reinforcement, fewer large cracks may be reduced to numerous invisible
and immeasurable microcracks (23). Transverse cracks seen in bridge decks
are typical examples. Cracking in concrete is the first step to deterioration, as
it results in the migration of harmful ions into the interior of concrete and to
the reinforcement.
Several preventive and mitigating measures can be used to minimize the
degradation of concrete due to corrosion of reinforcing steel. The use of fly
ash as a partial replacement of cement is a cost-effective solution (inclusion of
fly ash in a mixture provides the same workability at a lower water content
and lower cement content both of which reduces the concrete shrinkage). In
several states across the country, the Department of Transportation (DOT) has
made it mandatory to include fly ash as an ingredient. The heat of hydration is
substantially reduced when fly ash is used in concrete as a partial replacement
to cement.
Durability of concrete is very critical in most DOT applications, especially in
regions subject to cold weather conditions. In such cases, the incorporation of
fly ash in concrete is advantageous, even though the setting and hardening
process may be slightly slower than ordinary Portland cement concrete.
Fly ash has been used in concrete for several decades. Research work on
short-term and long-term behavior of concrete containing fly ash has been
conducted by several research agencies. However, the properties of fly ash
vary with the specific coal burned as well as the process of coal preparation,
firing and collection.
Hence, We Energies has conducted research on the actual fly ash generated at
its coal-fired plants. This research has been conducted with the aid of
universities and research institutions in conjunction with concrete producers to
develop mix designs that can be readily used for construction. Several
parameters, both short-term and long-term, have been studied, and their
performances evaluated to identify the suitability of the particular mixture
design for a specific field application. One important point is the spherical
shape of fly ash with its lubricating effect for pumping and the same
workability with a lower water to cementitious materials ratio. Also, fly ash is
finer than Portland cement and thus produces a denser concrete with lower
permeability.
53 We Energies
Coal Combustion Products
Utilization Handbook