Water Entrainment in Concrete: Techniques and Optimum Size of Inclusions

This report gives a survey of different techniques for incorporation of designed, water-filled cavities in concrete: Water entrainment. Also an estimate of the optimum size of the water inclusions is given. Water entrainment can be used to avoid self-desiccation and self-desiccation shrinkage during hydration [1,26]. What is needed is some sort of container which retains the shape of the water when mixed into the concrete. The container may function based on several different physical or chemical principles. Cells and gels are examples of containers found in nature. A cell membrane provides a boundary to water, whereas a polymer network incorporates water in its intersticious space with its affinity due to interaction energy and polymer entropy. Such containers allow water to be stored as an entity. In relation to concrete the water encapsulation may be accomplished either before or after start of mixing. If encapsulated water is added to the concrete as particles they must be strong enough to withstand the mixing process. If the encapsulation takes place after start of mixing it must be finished before the setting. After the setting the entrained water must be accessible for the cement hydration, i.e. it is not allowed to be confined or bound. This means the water should have an activity close to 1. Despite these partly conflicting demands several techniques may, potentially, be used. Water absorbing materials is a group of candidates for this purpose [27]. In the following some of these are presented. Special emphasis has been put on super absorbent polymers since these seem to be the most promising way to entrain water in concrete.