Mitigation strategies for autogenous shrinkage cracking
Introduction
While concrete can undergo dimensional changes for a variety of reasons, one of the most common is due to the loss of water during environmental exposure. Commonly referred to as drying shrinkage, this phenomena can lead to cracking of concrete members if not properly accounted for during the design and construction process. Typically, early-age moisture loss is minimized by specification of a curing regimen for the concrete [1]. In addition to preventing evaporation and the accompanying drying-type shrinkage, good curing practices also maximize the degree of hydration achieved by the cement within the concrete, potentially leading to stronger and more durable concrete.
Additional complications with respect to curing, shrinkage, and cracking appear when one considers so-called high-performance concretes (HPCs). While many definitions exist for HPC, typical HPC mixtures are characterized by a low (<0.4) water-to-cement ratio (w/c), an increased cement content, and the incorporation of silica fume (or other pozzolans) and a superplasticizer. In these concretes, a dense microstructure can form within a few days or less, preventing the introduction of external curing water to complete the hydration. For w/c below about 0.40, there is insufficient water in the initial mixture to complete the “potential” hydration. Because, as observed by Le Chatelier [2], the reaction products formed during the hydration of cement occupy less space than the corresponding reactants (i.e., chemical shrinkage occurs), a cement paste hydrating under sealed conditions will self-desiccate (creating empty pores within the hydrating paste structure). If external water is not available to fill these “empty” pores, considerable shrinkage can result. In 1934, Lynam [3] was perhaps the first to define such shrinkage as autogenous shrinkage, that is, shrinkage that is not due to thermal causes, to stresses caused by external loads or restraints, or to the loss of moisture to the environment.
As use of HPC has increased, so has research on self-desiccation and autogenous shrinkage [4], as evidenced by the now yearly conferences devoted to this subject [5], [6], [7], [8]. Autogenous shrinkage, however, is not only a negative attribute of a cement-based material. For example, it may provide a beneficial clamping pressure on fibers incorporated into concrete mixtures [9] or on aggregates (leading to interfacial strength increases), and it may offset thermal expansion due to temperature increases during hardening. But, generally, autogenous shrinkage is unwanted because it may cause cracking, as the deformations produced during autogenous shrinkage may easily exceed 1000 microstrains. Cracking is a complex phenomenon dependent on several factors including shrinkage rate, restraint, and stress relaxation [10]. Compared with long term drying shrinkage that generally occurs from the outside surface of the concrete inward, autogenous shrinkage develops uniformly through the concrete member, but can be more likely to cause cracking, because it develops more rapidly and occurs when the cement paste is young and has poorly developed mechanical properties (modulus of elasticity, fracture energy, etc.) [11]. Cracking due to autogenous shrinkage may lead to reduced strength, decreased durability, loss of prestress in prestressing applications, and problems with aesthetics and cleanliness. Therefore, the focus of this paper is to provide a structured overview of strategies for eliminating or minimizing autogenous shrinkage cracking. Also apparently impracticable ideas for mitigating autogenous shrinkage cracking are presented in the paper, since this may be the basis of useful further development in this area.
Section snippets
Terminology
In this paper, the different strategies to mitigate autogenous shrinkage are classified relative to the terminology in current use. For this reason, the terminology is described briefly here (see Fig. 1).
The autogenous deformation of concrete is defined as the unrestrained, bulk deformation that occurs when concrete is kept sealed and at a constant temperature. When the autogenous deformation is a contraction, it may be referred to as autogenous shrinkage. Autogenous deformation may be caused
Autogenous strategies
At a given temperature, autogenous shrinkage is determined by the concrete mixture composition. Changes in mixture composition (cement type, silica fume fineness, aggregate content, slag [14], etc.) will therefore change autogenous shrinkage [15]. Shrinkage cracking can be mitigated either by reducing shrinkage or by increasing the strain capacity of the concrete. When a mitigation strategy relies on the use of chemical admixtures, compatibility with other chemical admixtures is required [16].
Exchange of water with the surroundings
If concrete is allowed to freely imbibe water from the surroundings during curing, self-desiccation shrinkage will be delayed and cracking may be avoided. The water may be supplied to the concrete, for example, through membrane or fleece-lined forms [60], which may be considered as a reverse type of a controlled permeability formwork [61], [62]. Also, fog misting or water ponding of the concrete surface may be used. Wet curing should be started as soon as hydration begins [63]. In HPCs,
Conclusions
To produce long lasting durable concrete, early-age cracking must be minimized. Unfortunately, many of the recent trends toward higher early-age strengths also result in an increased propensity for early-age cracking. As demonstrated in this paper, the concrete materials engineer has numerous options available as mitigation strategies. In each case, a basic understanding of the underlying physical phenomena will aid the engineer in either selecting appropriate materials or adapting the
Acknowledgements
D.P. Bentz would like to acknowledge the HYPERCON/Partnership for High Performance Concrete Technology program at NIST for funding a portion of this work.
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