9. Other types of additives

9.1 Shrink-proof

Shrinkage is termed "plastic" because it occurs as soon as the concrete begins to set. The mechanisms involved are complex. The first is related to the surface tension of the water in the concrete:

• when the water contained in the concrete's very small pores (2.5 to 50 nm in diameter) begins to drain away, curved menisci are formed that pull on the pore walls;

• on the other hand, these forces become negligible if the pore diameter is greater than 50 nm ;

• even smaller pores won't let the water meniscus form.

Admixtures act chemically by reducing the surface tension of water to counteract this initial mechanism that causes concrete...