As high-performance concrete (HPC) has moved from the laboratory to field use, one problem sometimes encountered is its propensity for undergoing extensive self-desiccation and autogenous shrinkage [1]. Due to the chemical shrinkage that occurs as the cement hydrates, empty pores are created within the cement paste, leading to a reduction in its internal relative humidity and a measurable shrinkage that may cause early-age cracking. This situation is intensified in HPC, relative to conventional concrete, due to its generally higher cement content, reduced w/c ratios, and incorporated pozzolanic mineral admixtures such as silica fume. The empty pores created during self-desiccation not only induce shrinkage stresses but also influence the kinetics of the hydration process, limiting the final degree of hydration, and thus strength, that can be achieved relative to that obtainable under saturated curing conditions [1].
This "curing" problem was recognized nearly ten years ago by Philleo
[2], who suggested incorporating saturated
lightweight fine
aggregate (LWFA) into the concrete mixture to provide an internal source of
water needed to replace that consumed by chemical shrinkage during hydration
(curing). As the cement hydrates, this extra water will be drawn from the
relatively "large" pores in the LWFA into the much smaller ones in the
cement paste. This will minimize the development of autogenous shrinkage as
the shrinkage stress is controlled by the size of the empty pores, via the
Kelvin-Laplace equation [1]. Unfortunately, little was done to follow
up on Philleo's idea until the mid-1990's when Weber and Reinhardt
[3] once again proposed the use of saturated
lightweight aggregates
to support the curing of concrete. In their experimental program, replacing a
portion of the fine aggregates by their saturated LWA counterparts resulted in
concretes which were "considerably less sensitive to the curing process"
[3]. More recently, Bentur and his colleagues [4,5]
have prepared high-strength concretes with a mixture of LWFAs and normal weight
aggregates and observed that the initial autogenous shrinkage measured for HPC
can be eliminated by the judicious replacement of a portion of the fine
aggregates with either saturated or air-dried LWFA. In this communication,
equations are derived to estimate the replacement level necessary to avoid
autogenous shrinkage as a function of mixture proportions. Additionally, a
3-D concrete microstructural model is applied to determining, for various
replacement levels and aggregate gradations, the fraction of the hydrating
cement paste within a given distance of the LWFA surfaces. We focus on
replacement of the fine aggregate component as opposed to the coarse,
due to both strength considerations and the fact that the much higher
surface area of the fine relative to the coarse aggregate will result
in a more uniform distribution of this additional curing water within
the 3-D concrete microstructure. The results of the 3-D microstructural model
are compared with an approximation based upon the equations developed
previously by Lu and Torquato [6] for the generic hard core/soft shell
microstructural model. This paper only considers the availability and
spatial proximity of the additional water introduced via the LWFA and
does not address the processing (rheology) issues which may need to be
addressed to offset the effects of the generally rougher surfaces of
the LWFA on the flow properties of the concrete.