Mixture Proportioning for Internal Curing.
Mixture Proportioning for Internal Curing.
(1552 K)
Bentz, D. P.; Lura, P.; Roberts, J. W.
Concrete International, Vol. 27, No. 2, 35-40, February
2005.
Keywords:
concretes; chemical shrinkage; hydration; internal
curing; aggregates; lightweight aggregate; mixture
proportioning; equations
Abstract:
The concept of internal concrete curing is steadily
progressing from the laboratory to field practice. In
terminology currently being considered by ACI Committee
308, Curing Concrete, "internal curing refers to the
process by which the hydration of cement occurs because
of the availability of additional internal water that is
not part of the mixing water." The additional internal
water is typically supplied by using relatively
small amounts of saturated, lightweight, fine aggregates
(LWA) or superabsorbent polymer (SAP) particles in the
concrete. Benefits of internal curing include increased
hydration and strength development, reduced autogenous
shrinkage and cracking, reduced permeability, and
increased durability. The impact of internal curing
begins immediately with the initial hydration of the
cement, with benefits that are observed at ages as early
as 2 days. Internal curing is especially beneficial in
low water cement ratio (w/c) concretes because of the
chemical shrinkage that accompanies portland cement
hydration and the low permeability of the
calcium-silicate hydrates. Because the water that is
chemically bound and adsorbed by the cement hydration
products has a specific volume less than that of bulk
water, a hydrating cement paste will imbibe water (about
0.07 g water/g cement) from an available source. While
in higher w/c concretes, this water can be, and often
is, supplied by external (surface) curing, in low w/c
concretes, the permeability of the concrete quickly
becomes too low to allow the effective transfer of water
from the external surface to theconcrete interior. This
is one justification for internalcuring. Additional
water that can be distributed somewhat uniformly
throughout the concrete will be more readilyable to
reach unhydrated cement.
