What technique is employed to reduce free water present in plastic concrete after it has been placed?

Vacuum dewatering is a technique specifically designed to remove excess free water from plastic concrete after it has been placed. This process involves placing a vacuum blanket over the concrete surface, which creates a negative pressure that draws out water from within the concrete matrix. By reducing the amount of free water, vacuum dewatering enhances the strength, durability, and surface finish of the concrete. It is particularly useful in situations where a high-quality finish is required or when the concrete needs to be ready for subsequent operations sooner.

In contrast, the other options serve different purposes in concrete work. Finishing involves the process of smoothing and shaping the surface of the concrete but does not specifically address free water removal. Pretensioning involves the application of tension to steel tendons before the concrete sets, which is related to improving the tensile strength of concrete elements but does not directly address water content. Consolidation refers to the process of ensuring that the concrete mix is dense and free of air pockets, primarily achieved through methods like vibrating, which aids in achieving the desired density but does not involve the removal of free water as effectively as vacuum dewatering does.