Frequently Asked Questions

How is underwater concrete incorporating Conplast UW-L placed and what special equipment is needed?

Concrete incorporating Conplast UW-L is placed underwater using the same methods as standard underwater concrete — the admixture reduces washout but does not eliminate the need for proper placement technique. The standard methods are: tremie pipe placement (a sealed steel pipe with an internal diameter of 200 to 250 mm, lowered to the bottom of the pour and concrete pumped through it — the pipe is raised as concrete fills from the bottom upward, keeping the pipe end always embedded in the fresh concrete to exclude water); concrete pump with a flexible pump hose used similarly to the tremie pipe; and bucket placement for smaller volumes (a sealed bottom-dump bucket containing the concrete is lowered to the pour face and the base opened to discharge the concrete at the bottom of the water column). Conplast UW-L does not allow concrete to be simply tipped into water from the surface — the admixture reduces but does not eliminate washout from free-fall through a water column. The cohesion improvement is most effective in the discharge zone around the tremie or pump outlet where the concrete is displacing water at the fill front.

What is the typical compressive strength achieved by concrete with Conplast UW-L placed underwater compared to reference concrete?

The ratio of underwater in-place strength to reference air-cured strength (the UW-to-air strength ratio) is a key performance indicator for underwater concrete. Plain concrete without an anti-washout admixture placed through water by tremie or pump typically achieves 60 to 80 per cent of the reference air-placed strength, due to cement washout reducing the effective cement content of the in-place concrete and water dilution of the water-cement ratio. Concrete with Conplast UW-L at the recommended dosage typically achieves 85 to 95 per cent of the reference strength in tremie or pump placement through standing water — a significant improvement that provides better confidence in the structural performance of the foundation or underwater structure. The actual strength ratio achieved depends on the placement method, the water flow velocity at the placement face, the admixture dosage, and the concrete mix proportions. Trial placement tests in a water-filled column should be conducted as part of the mix design verification for critical underwater concrete applications.

Can Conplast UW-L be used with a superplasticiser in the same concrete mix?

Yes. Conplast UW-L is designed to be used in combination with superplasticisers from the Conplast SP or Fosroc Auramix range. Underwater concrete typically requires both high workability (to flow and fill the pour by gravity under the tremie or pump head without vibration, which cannot be applied through water) and anti-washout resistance. The superplasticiser provides the high workability (target slump of 180 to 220 mm for tremie concrete is common), and Conplast UW-L provides the anti-washout cohesion. The two admixtures are compatible and can be added to the mix water at the batching plant in the normal way. The combined effect is a highly workable, self-compacting underwater concrete with high cohesion and resistance to cement washout — providing the best combination of placeability and in-place quality for marine and underwater foundation concrete.