Civil Engineering Reference
In-Depth Information
as precast concrete, for which a faster strength development rate is required,
as well as when concreting in low temperatures, type III cement is the most
suitable choice of Portland cement. The increased strength development
rate in type III Portland cement is achieved by increasing the content of C
3
S
and grinding cement to finer particles. The latter strategy is deemed to be
more effective because the higher surface area of the smaller cement par-
ticles increases the surface area that will come in contact with water. The
24-hour strength development rate of a type III Portland cement is expected
to be almost twice that of a type I cement. Type IV cement has been devel-
oped mainly to deal with concerns associated with the high heat liberation
of type III cement, which could be problematic in mass concrete and when
casting large concrete components, for which subsequent cooling of the
concrete may cause cracking. Temperature increases as high as 30°C have
been reported in the case of mass concrete. In type IV cement, the amount
of liberated heat has been reduced by limiting the relative amounts of C
3
S
and C
3
A, which are responsible for most of the heat liberation in early ages.
Type V Portland cement was developed to deal with the problem of sulfate
attack, deterioration of concrete exposed to water or soils containing sul-
phates at early ages. In type V cement, this is achieved by lowering the C
3
A
content to below 5% as sulphate attack only involves interactions between
the hydration products of C
3
A. Good but relatively less sulphate resistance
than type V cement is also achievable by using type IV cement, which has
about 8% C
3
A. However, because the lower amount of C
3
A can result in a
considerable decrease in the early strength development rate, type II cement
was developed for this purpose by increasing the C
3
S content. The compres-
sive strength development trends of different ASTM cements are compared
in Figure 2.3.
The ASTM cements described have been developed to control specific
aspects of the hydration of cement by varying the proportion of different
constituents. However, because the constituents used are similar, the prop-
erties of hardened mortar or concrete made with any ASTM cement tend
to be similar. Therefore, although the initial rate of strength development
is different, the ultimate strength achievable using different cement types
tends to be within a similar range, although the lower initial strength devel-
opment rate usually leads to slightly higher ultimate strength (Figure 2.3).
2.2.2 Hydration of cement
Understanding the hydration process of cement constituents can be ben-
eficial in understanding the effects of the curing processes on the strength
development rate in concrete. As discussed, C
3
S and C
2
S are the main con-
stituents contributing to the development of strength in concrete. Therefore,
in this section, focus is placed on the hydration of these compounds. The
Search WWH ::
Custom Search