1 INTRODUCTION 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 reac- tions 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 speci- mens overcomes the tensile strength. As a conse- quence drying shrinkage is accompanied in most cases by strain softening and surface crack for- mation. 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 predic- tions of the two different approaches shall be criti- cally compared with experimental findings. 2 INFINITESIMAL SHRINKAGE Drying is at the origin of shrinkage. But drying is an extremely slow process. In many concrete structures a hygral gradient exists for many years. That means the outer layers have reached hygral equilibrium with the relative humidity (RH) of the environment quickly while the inner part may remain water satu- rated for decades. Shrinkage of the infinitesimal thin surface near layers, however, is hindered in the monolithic material. Under high imposed tensile strain, cracks are formed after damage of the materi- al due to strain softening in the surface near zone. This situation is schematically shown in Fig. 1. Heresies on Shrinkage and Creep Mechanisms F. H. Wittmann Aedificat Institute Freiburg, Germany ABSTRACT: 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 de- scribed: capillary action and disjoining pressure. The predictions of the two concepts are compared with ex- perimental findings. The following examples have been selected for a critical comparison: (1) direct observa- tion 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.