Building Materials, Third Edition

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Coarse aggregates in HPC occupy 35–70% of the volume of concrete, their properties influence
the properties of hardened concrete significantly. Usually an aggregate with specific gravity
more than 2.55 and water absorption less than 1.5% (except for light weight aggregates) is
desirable.
Chemical admixtures such as high-range water-reducers are needed to ensure that the
concrete is easy to transport, place and finish. Retarding admixtures are used to check the early
setting problems. For high-strength concretes, a combination of mineral and chemical admixtures
is nearly always essential to ensure achievement of the required strength. Typically, high-range
water-reducing admixtures are used. Concrete for bridge decks typically include water-reducing
admixtures. Natural wood resins and sulphonated compounds are used for air entrainment.
Most high-performance concretes have a high cement content and a water-cement material
ratio of 0.40 or less. However, the proportions of the individual constituents vary depending
on local preferences and local materials. Mix proportions developed in one part of the country
do not necessarily work in a different location. Many trial batches are usually necessary before
a successful mix is developed.
High-performance concretes are also more sensitive to changes in constituent material
properties than conventional concretes. Because many characteristics of high-performance
concrete are interrelated, a change in one usually results in changes in one or more of the other
characteristics. Consequently, if several characteristics have to be taken into account in producing
a concrete for the intended application, each must be clearly specified. Variations in the
chemical and physical properties of the cementitious materials and chemical admixtures need
to be carefully monitored. Substitutions of alternate materials can result in changes in the
performance characteristics that may not be acceptable for high-performance concrete. This
means that a greater degree of quality control is required for the successful production of high-
performance concrete.

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Some of the examples of HPC are: High workability concrete, Self compacting concrete (SCC),
Foamed concrete, High strength concrete, Lightweight concrete, No-fines concrete, Pumped
concrete, Sprayed concrete, Waterproof concrete, Autoclaved aerated concrete, Roller compacted
concrete.

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Cracks in concrete are inevitable and are one of the inherent weaknesses of concrete. Water and
other salts seep through these cracks, corrosion initiates, and thus reduces the life of concrete.
So there was a need to develop an inherent biomaterial, a self-repairing material which can
remediate the cracks and fissures in concrete. Bacterial concrete is a material, which can
successfully remediate cracks in concrete. It has been found that use of bacteria improves the
stiffness and compressive strength of concrete.
The “Bacterial Concrete” can be made by embedding bacteria in the concrete that are able to
constantly precipitate calcite. Bacillus Pasteurii and sporosorcina, a common soil bacterium,
can continuously precipitate a new highly impermeable calcite layer over the surface of an
already existing concrete layer. The favorable conditions do not directly exist in a concrete but
have to be created for the bacteria not only to survive in the concrete but also to feel happy and
produce as much calcite as needed to repair cracks.