Role of water; high water content

Role of water

Water hydrates the cement, gives workability by lubricating the particles and is required for curing. About 0.23 of the cement weight is chemically combined and 0.15 is physically held in the gel pores (total w/c of about 0.38 for full hydration); anything more only supplies workability.

Advantages of high water content

• Higher workability, easy mixing, placing and compaction, which helps in congested reinforcement and long pumping distances.
• Easier finishing; lower labour effort.
• Provides enough water for complete hydration and reduces plastic cracking in very dry mixes.

Drawbacks

• Lower strength: strength falls rapidly with w/c (Abrams' law, $f = A/B^{w/c}$).
• Excess water leaves capillary pores on evaporation, so concrete is more porous and permeable, less durable (more chances of corrosion, frost and chemical attack).
• Increases bleeding and segregation, laitance and a weak top surface; water pockets under aggregate and bars reduce bond.
• Larger drying shrinkage and creep, causing cracks.
• Longer setting time and higher formwork pressure.
• Wastes cement when the cement is added to keep w/c constant.

Workability should be increased with plasticisers, not extra water.