First of all it is outlined that shrinkage as measured on drying concrete is not a simple material property but the complex response of a given specimen to long lasting time-dependent internal stresses. Then the physical basis of two frequently cited approaches to explain the origin of hygral shrinkage is briefly described: capillary action and disjoining pressure.
The predictions of the two concepts are compared with experimental findings. The following examples have been selected for a critical comparison:
(1) direct observation of the interaction of water in a narrow gap,
(2) sorption and length change isotherms of different materials,
(3) capillary shrinkage of fresh and young concrete,
(4) shrinkage of water repellent concrete, and
(5) influence of ion concentration on shrinkage.
From the presented results it can be concluded that capillary action plays a minor role in the process of shrinkage of cement-based materials.
We may consider shrinkage and swelling of concrete to be the volume change imposed by a change of moisture content and by chemical and physical reactions of the solid skeleton with the pore solution.
From this definition follows that several mechanisms may act simultaneously or consecutively to generate macroscopically observed volume changes. This complexity is probably one of the reasons, why no general agreement could be found up to now on dominant shrinkage and swelling mechanisms.
In this contribution we will essentially concentrate on drying shrinkage. Drying can be due to loss of water to an environment, which is drier than the pore space of the drying material, or it can be due to water consumption by hydration of cement (autogenous drying). Although the underlying mechanisms are the same, the development of shrinkage under water loss differs in a characteristic way from autogenous shrinkage under water consumption (Alvaredo & Wittmann 1995, Wittmann 2008). Drying shrinkage is provoked by time dependent moisture gradients. In most cases the stress distribution in drying specimens overcomes the tensile strength. As a consequence drying shrinkage is accompanied in most cases by strain softening and surface crack formation.
A multitude of mechanisms have been proposed for hygral shrinkage. Some are simplistic and based on phenomenological observations exclusively. They cannot be verified nor can they be falsified; therefore they are of limited significance.
Two approaches, however, are based on physical fundamentals. Some authors tried to establish a link between drying shrinkage and capillary action (Coussy et al. 2004, Baroghel-Bouny et al. 1999, Hua et al. 1995). An alternative approach is based on the change of surface energy and disjoining pressure as function of moisture content (Wittmann 1968, Wittmann 1973, Wittmann 1976a, Wittmann 1977, Setzer 1991, Setzer 2008, Churaev & Derjaguin 1984, Weimann & Li 2003).
In the following predictions of the two different approaches shall be critically compared with experimental findings.
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