Underwater Concrete Part 2: Proper Mixture Proportioning

This article discusses the factors that must be considered when properly portioning underwater concrete mixtures. Concrete placed under water must flow easily, retain adequate cohesion against washout and segregation, and possess self-consolidating characteristics. Thermal stress is a special concern for massive underwater concrete placements. Mixture proportioning is essentially an optimization process, which must be determined using sound technical knowledge and thorough consideration of specific project requirements. Recommended ranges are given for water-cementitious material ratio (0.35 to 0.45 by mass); fines content (360 to 500 kg/m3); water-to-fines ratio (0.85 to 1.0 by volume); sand-to-total aggregate ratio (45 to 50%); nominal maximum size aggregates (10 to 19 mm for structural concrete, 19 to 25 mm for mass concrete); silica fume (0 to 6%); and limestone powder (0 to 20% of cement content). Anti-washout admixture is not essential for tremie seal/drilled shafts, but a moderate dose is recommended for reinforced concrete with long flow distance. A high dose is recommended for self-leveling, thin layer placements over large areas or placements in flowing water. Anti-washout admixture should be used with high range water-reducing admixtures (HRWRA). The HRWRA dosage should be adjusted to produce a proper slump flow in the range of 350 to 680 mm.