Building Materials, Third Edition

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physical processes of degradation. The first known use of HSC was in the construction of the
53 storey Helmsley Palace Hotel in New York in 1979. The HSC was used to reduce the size of
columns.
As is well known, an important advantage of the use of puzzolanas in concrete is that
relatively less heat of hydration is generated per unit strength; therefore the rise of thermal
cracking is reduced. The construction of New Tjorn Bridge in Sweden in 1981 is documented
to be the first known use of condensed silica fume in HSC and where heat of hydration was the
main consideration. HSC is being increasingly selected for the construction of islands in ocean
due to the high durability in sea water environment.

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Shrinkage compensating concrete (SCC) is an expansive cement concrete which, when properly
restrained by reinforcement or other means, will expand an amount equal or slightly greater
than the anticipated drying shrinkage. Because of the restraint, compressive stresses will be
induced in the concrete during expansion. Subsequently drying shrinkage will reduce these
stresses. Ideally, a residual compression will remain in the concrete, eliminating the risk of
shrinkage cracking. As the expansive cement used in the concrete hydrates, large amount of
ettringite are formed. When the concrete sets and develops strength, it will offer bond to the
reinforcement and at the same time start expanding if sufficient quantity of curing water is
present. Since the concrete is bonded to steel, its expansion under the restraining influence of
steel induces tension in the latter while the concrete itself goes into compression. At the end of
moist curing, when the element is exposed to drying conditions, it will shrink like a PCC.
However, the shrinkage will first relieve the precompression before inducing tensile stress in
concrete. By preventing the build up of high tensile stress, the risk of cracking of concrete due
to drying shrinkage is reduced.
Current design and construction practices assume that concrete will crack, and try to get
around the problem in many ways, such as selection and proportioning of concrete mixes that
will shrink less, provision of adequate joints in floor slabs or pavements, and reinforcement of
concrete elements with steel. The advent of expansive cement has offered an alternative and
cost effective approach. The same basic materials and methods necessary to produce high
quality PCC are required to produce satisfactory results in the use of SCC. Additional care,
however, is necessary to provide continuous moist curing for at least 7 days after placement,
in order that the expansion does take place, and the structural design must be such as to ensure
adequate expansion to offset subsequent drying shrinkage. A comparison of PCC with SCC is
given in Table 20.6.

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‡—˜ Because of the water-imbibing characteristics of ettringite, which forms in relatively
large quantities during very early stages of hydration, the concrete mix tend to stiff but is
highly cohesive. Compared to OPC, the use of a somewhat higher w/c ratio (without the
possibility of strength impairment) than recommended by the standard water-cement strength
relationships of PCC is, therefore, permitted with expansive cements for achieving a reasonable
consistency. The slumps in the range of 100 to 150 mm are recommended for most structural
members, such as slabs, beams, reinforced walls and columns.