Frequently Asked Questions

What is the pre-packed aggregate concrete technique and why is Conbextra HF used for it?

Pre-packed aggregate (PPA) concrete, also called pre-placed aggregate concrete, is a method where the void or formwork is first filled with clean, well-graded coarse aggregate, which is compacted and settles under gravity to fill the maximum possible proportion of the volume. Fosroc Conbextra HF grout is then injected under low pressure from the lowest point, rising upward through the pore spaces between the aggregate particles by displacement. The high fluidity of Conbextra HF is essential to allow it to flow through narrow interparticle channels without the grout segregating or stopping short of filling all the voids. PPA concrete is used where vibration to compact conventional concrete is impossible, and for mass concrete where using a large proportion of stone aggregate reduces the heat of hydration.

How does Conbextra HF differ from the other Conbextra grades such as GP2 and EP10?

The Conbextra range covers different chemistries and consistencies for different grouting applications. GP2 is a standard fluid cementitious grout for general base plate grouting at moderate flow. Conbextra HF is formulated for much higher flow — sufficient to be pumped through the interparticle voids of a coarse aggregate mass. EP10 is an epoxy grout for very high strength requirements. For the pre-packed aggregate technique and large void injection, only Conbextra HF (or similar very high fluidity products) provides the flow needed to penetrate through the aggregate pore network without blocking. Standard GP2 grout is too viscous for PPA injection.

What aggregate should be used with Conbextra HF in the pre-packed aggregate technique?

Clean, hard, single-size or gap-graded coarse aggregate with a minimum size of 10 mm is recommended for use with Conbextra HF in the PPA technique. The aggregate must be clean (washed, free of silt, clay, and organic matter) because any coating on the aggregate surface will impair the bond between the hardened grout and the aggregate. Angular crushed aggregate is preferred over rounded gravel as it provides better interlock. The aggregate should be pre-wetted before placement to avoid the dry aggregate absorbing water from the grout and reducing its fluidity during injection. For underwater applications, the aggregate placement method must prevent segregation and contamination before grout injection.