Preplaced aggregate concrete (PAC) is not a new concept. In fact it is a method of concrete placement that has been around since the late 1930’s. However, it is often overlooked as an option to repair dams and bridge piers. In fact, it is one of the most effective forms of underwater concrete repair.
What’s the difference?
Conventional concrete is placed as a composite aggregate and cement-grout mixture. This means the aggregate and cementitious grout components are combined before the concrete is placed into formwork. PAC is different than conventional concrete methods because its components are placed in separate steps. Like the name implies, the aggregates are placed before the grout mixture. This leads to several advantages.
Because the aggregate is washed and placed into the formwork before the grout is injected, the aggregate-to-cement ratio is maximized. According to the American Concrete Institute this reduces shrinkage which can lead to higher bondability with existing concrete surfaces. Higher bondability and less shrinkage results in less cracking!
Another advantage is that PAC disallows material separation when being placed, especially when the formwork is partially or fully submerged in water. Once the aggregate is placed, grout injection begins at the bottom of the formwork, and progresses vertically at separate injection points. Any water in the formwork is displaced as the grout fills all the voids between the aggregate, creating a homogenous mixture. A homogenous mixture leads to a stronger repair!
Finally, when placing PAC, there is little need for heavy equipment. The aggregate can literally be washed into place using a sluice pipe. Then grout can be injected using a grout pump. This is especially beneficial on small bridge pier or dam repair projects because most of the equipment can be left onshore. Materials are transported to the repair site through temporary PVC sluice pipes and grout hoses. Fewer pieces of equipment will lead to a lower cost repair!