What is the primary reason for using shrinkage-reducing admixtures in concrete slabs?

The primary reason for using shrinkage-reducing admixtures (SRAs) in concrete slabs is to minimize drying shrinkage and reduce the potential for cracking. Concrete, as it hardens and dries, undergoes shrinkage due to the loss of moisture. This shrinkage creates tensile stresses within the concrete. If these stresses exceed the concrete's tensile strength, cracking will occur. Slabs are particularly susceptible to shrinkage cracking due to their large surface area-to-volume ratio, which promotes rapid moisture loss. SRAs work by reducing the surface tension of the pore water within the concrete. Surface tension is the force that causes water to contract and create tensile stresses as it evaporates. By reducing the surface tension, SRAs reduce the magnitude of these tensile stresses, thereby minimizing shrinkage and the likelihood of cracking. SRAs are especially beneficial in large concrete slabs, slabs with restrained movement (e.g., slabs bonded to a rigid base), and slabs where crack control is critical. For example, in a large warehouse floor, SRAs can significantly reduce the number and width of shrinkage cracks, improving the floor's appearance, durability, and service life. While SRAs do not eliminate shrinkage entirely, they substantially reduce it, providing a cost-effective means of crack control.