1. 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 0.38, 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 µ strains. 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 review of strategies for eliminating or minimizing autogenous shrinkage cracking.