Frequently Asked Questions

How does anti-washout technology in Conbextra UW resist water dilution?

The anti-washout admixtures in Conbextra UW are water-soluble polymer thickeners (typically cellulose derivatives or polysaccharide gums) that dissolve in the mixing water and increase the viscosity of the water phase of the grout. This increased viscosity makes it much harder for the surrounding water to penetrate and dilute the grout — the grout behaves like a cohesive paste rather than a suspension of cement in free water. When water contacts the surface of the anti-washout grout, only a very thin surface layer is diluted; the core of the grout remains coherent and at the designed water-cement ratio. Once the grout begins to set (typically within 2 to 4 hours), the cementitious structure becomes self-supporting and further washout is not possible even without the anti-washout polymer.

What is the tremie method and why is it the preferred placement technique for Conbextra UW?

The tremie method involves placing a pipe or tube (the tremie pipe) with its outlet at the base of the void, annulus, or space to be grouted. Conbextra UW grout is pumped or poured through the tremie pipe from the surface and discharged at the bottom of the void, where it displaces the water upward and out through a vent at the top. The key principle is that the fresh grout flows upward, always in contact with previously placed grout rather than with the surrounding water — this minimises the surface area of fresh grout exposed to water at any moment and therefore minimises washout. Placing grout from the top into a water-filled void (by pouring or dumping) is the worst method for underwater grouting as the falling grout passes through the maximum possible depth of water and is maximally exposed to washout.

What compressive strength can be expected from Conbextra UW cured underwater?

The compressive strength of Conbextra UW cured in water is typically somewhat lower than the equivalent product cured in air, due to the dilution effect at the surface of the grout mass and the higher water content in the near-surface zone. However, the bulk of the grout (away from the surface) cures at close to the designed water-cement ratio and achieves strengths typically above 25 MPa at 28 days. For critical structural grouting applications underwater (such as offshore pile grouting where the grout must transfer very high axial and lateral loads from the pile to the jacket leg), specialised underwater grout specifications with higher cement content and verified underwater test strength data are used. The acceptance testing of underwater grout strength should be carried out on samples cast and cured in water to replicate the field conditions, not in standard air-cured moulds.