8.8 : Preplaced Aggregate Concrete

Preplaced aggregate concrete is ideal for construction environments that are not easily accessible. The process begins by properly wetting the gap-graded coarse aggregates to remove the dirt, then placing it in the form and compacting it. Voids are filled with a mortar mix pumped under pressure through slotted pipes. This mortar typically consists of Portland cement, pozzolan, fine aggregates, water, and a fluidizing aid. The pozzolan helps reduce bleeding and segregation while improving the fluidity of the mortar, whereas the fluidizing aid delays the setting of the mortar. A cement-fine sand mortar is also used. It is prepared in a high-speed colloid mixer to keep the cement suspended until the mortar is pumped. As the mortar rises to the top, the pipe is gradually withdrawn. Applying external vibration to the top layer of the mortar can enhance the finish of the exposed surfaces.

Preplaced aggregate concrete is favored for its uniformity and reduced risk of segregation, which is essential for constructing nuclear shields and underwater projects. Due to its low shrinkage and permeability, preplaced concrete is used in water-retaining structures, large monolithic blocks, and repair works. In large-scale construction, refrigerated water is circulated around the aggregates before adding the mortar to control the heat from cement hydration. In contrast, steam is circulated to prevent frost damage in cold conditions.