Concrete repairs sometimes fail because bond between new and old concrete fails. Although repairs may bond well at first, the new concrete shrinks and puts a strain on the bond face. This strain is often large enough to break the bond. Preplaced aggregate (PA) concrete is an alternative repair material that bonds well but shrinks very little. PA concrete is made by placing coarse aggregate directly into the formed space to be filled. The mortar part of the mix is then pumped into the voids between the particles of coarse aggregate. Ingredients are the same as for conventional concrete, but they are mixed and placed differently.
CHOOSING THE AGGREGATE
The coarse aggregate must meet the same requirements as for conventional concrete except that it must be screened to remove particles smaller than 1/2 inch.
CHOOSING THE GROUT
The grout consists of portland cement, usually a pozzolan such as fly ash, fine aggregate (sand), grout fluidifier, and enough water to produce a fluid slurry.
PREPARING THE SURFACE FOR REPAIR
To prepare the surface for repair, workers remove the weakened old concrete back to sound material or a bit more. Edges of cutout areas are roughly square cut .
PROVIDING GROUT-TIGHT FORMWORK
Forms are designed to withstand the anticipated head of fluid grout. The forms must be grout-tight (not necessarily watertight) and coated with a form release agent.
SATURATING THE AGGREGATES
At least a few hours, but preferably a day or two, before grouting, workers should fill the forms with clean water. This saturates aggregate and old concrete so they don't absorb water from the grout.
PUMPING THE GROUT
Always introduce the grout at the bottom of preplaced aggregate and let it rise through the voids.