2. 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 Figure 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 by different mechanisms. As an example, the growth of certain salt crystals, such as ettringite, may cause expansion during hydration. As was mentioned, the more common example originates in the fact that the hydration products formed by portland cement hydration occupy less space than the reacting water and cement. This phenomenon, chemical shrinkage, causes a successive emptying of the pore structure (as shown in Figure 2) and leads to tensile stresses in the pore water through the formation of menisci. Menisci formation causes the relative humidity to drop, and self-desiccation occurs in the cement paste. The build-up of tensile stresses in the pore solution, furthermore, results in a bulk shrinkage of the hardening concrete, so-called self-desiccation shrinkage. The concrete may crack globally (macrocracking, if restrained) or locally at the surface of non-shrinking aggregates (microcracking). Experimental evidence suggests that self-desiccation shrinkage is an important type of autogenous deformation [12,13].

  

Figure 1: Graphical representation of applied terminology; self-desiccation shrinkage and salt expansion are proper subsets of autogenous deformation [20]. Autogenous relative humidity change is a phenomenon closely related to autogenous deformation.

 

Strategies to mitigate autogenous shrinkage cracking may be of either an autogenous or a non-autogenous nature. Basically, autogenous strategies concern the concrete mixture proportions, whereas all other strategies are non-autogenous in nature. In the following sections, examples of these strategies are discussed.

  

Figure 2: Two-dimensional image from a three-dimensional porous microstructure showing solids (white), water-filled pores (grey), and vapor-filled (self-desiccated) pores (black). The vapor-water menisci generate tensile stresses within the water which will result in an overall shrinkage of the porous solid. Image is periodic from left to right.