Reducing Thermal and Autogenous Shrinkage Contributions to Early-Age Cracking.
Reducing Thermal and Autogenous Shrinkage Contributions
to Early-Age Cracking.
(317 K)
Bentz, D. P.; Peltz, M. A.
Title No. 105-M48;
ACI Materials Journal, Vol. 105, No. 4, 414-420,
July/August 2008.
Keywords:
cracking (fracturing); shrinkage; concretes;
construction; hydration; costs; curing agents; ASTM
C150; ASTM C305; mortar; compressive strength;
temperature rise; cement pastes; deformation; durability
Abstract:
Early-age cracking continues to be a significant problem
for new concrete construction. Two of the major
contributors to such cracking are the heat released by
cement hydration during the first few days of curing and
the autogenous shrinkage that often occurs during the
same time frame. In this paper, three potential
alternatives for reducing these contributions by
modifying the concrete mixture proportions are
investigated, namely increasing the water-cement ratio
(w/c), using a coarser cement, or replacing a portion of
the portland cement with a coarse limestone powder. Each
alternative reduces the heat generated per unit volume
by either reducing the volumetric cement content or its
early-age reactivity, and reduces autogenous shrinkage
by increasing the interparticle spacing between grains
in the three-dimensional microstructure. These
reductions are quantified for paste and mortar systems
by measuring their semi-adiabatic temperature rise and
autogenous deformation along with measurements of
compressive strength to indicate the strength trade-off
that will be experienced in reducing the risk of
early-age cracking. These mixtures each have the
additional advantage that they should result in a cost
savings in comparison with an initial (control) mixture.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899