Building Materials, Third Edition

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 The higher cost of the expansive cement used can be balanced by the reduced
amount of drying shrinkage reinforcement required, large placement sections, and fewer
construction joints and water stops compared with PCC construction. Since 1990s, expansive
cements have been used in several countries for the purpose of producing both shrinkage-
compensating concrete and self-stressing concretes. Most of the applications have been in
structural elements, such as slabs, payments, prestressed beams and roofs.

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The heavy weight concrete also known as high density concrete is used in the construction of
radiation shields. These concretes are produced generally by using natural heavyweight
aggregates. The unit weights are in the range of 33.60 to 38kN/m^3 which is about 50% higher
than that of normal weight aggregates. Concrete is usually the most economical material when
it comes to use in biological shielding in nuclear plat, medical units, and atomic research and
testing facilities. However, where the usable space is limited, the reduction in the thickness of
the shield is accomplished by the use of heavyweight concrete. Concrete is an excellent shielding
material that possesses the needed characteristics for both neutron and gamma ray attenuation,
has satisfactory mechanical properties, and has a relatively low initial as well as maintenance
cost. Since concrete is a mixture of hydrogen and other light nuclei, and nuclei of higher atomic
number, and can be produced within a relatively wide range of density, it is efficient in
absorbing gammas, slowing down fast neutrons, and absorbing resonance and slow neutrons.
The hydrogen and oxygen, contained in chemically combined form in the hydrated cement,
moderate the neutron flux satisfactorily.
Because of the high density of aggregate particles, segregation of fresh concrete is one of the
principal concerns in mix proportioning. From the standpoints of high unit weight and less
tendency for segregation, it is desirable that both fine and coarse aggregates be produced from
high-density rocks.

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Roller-compacted concrete (RcC) presents a relatively recent development in the construction
technology of dams and locks. It is based on the concept that a no-slump concrete mixture
transported, placed, and compacted with the same construction equipment that is used for
earth and rockfill dams, can meet the design specifications for conventional mass concrete. It
is a mixture of aggregates, cement (with or without puzzolanas), water and sometimes water
reducing admixtures, proportioned to support external compaction equipment. The stiff (dry
or lean), zero slump concrete mixture has consistency of damp gravel. The air entraining, water
reducing and set controlling admixtures are effective in reducing the vibration time required
for full consolidation of RcC. This lowers the entrapped air-void content, increases strength
and lowers the permeability of concrete.
RcC differs from conventional concrete principally in its consistency requirement; for effective
consolidation, the concrete must be dry to prevent sinking of the vibratory roller equipment
but wet enough to permit adequate distribution of the binder mortar throughout the material
during the mixing and vibratory compaction operations. The conventional concept of minimizing
water-cement ratio to maximize strength does not hold; the best compaction gives the best
strength, and best compaction occurs at the wettest mix that will support an operating vibratory