Concrete Curing and Strength Gain

Curing of Concrete

• Strength of concrete increases with age under favorable curing conditions.
• Early-age strength gain is rapid; slowing down as time progresses.
• At 28 days, concrete reaches about 60% of its maximum compressive strength.
• By 3 months, compressive strength can reach approximately 80%.
• Curing keeps concrete from drying out too quickly, essential for strength.
• Effective curing methods include:
  • Covering with wetted paper sacking, burlap, sand, or earth.
  • Removing forms only after the specified time has elapsed.
  • Sprinkling or spraying surfaces with water immediately after form removal.
  • Using curing compounds to retain moisture.

Admixtures

• Substances added to improve specific properties of cements, mortars, and concrete.
• Types of admixtures include:
  • Hydrated lime for improved workability.
  • Air-entraining agents to enhance durability.
  • Calcium chloride as an accelerator for quick setting.
  • Retarders to slow the setting process.
  • Additives for wear resistance and waterproof qualities; e.g., kaolin, celite.
  • Pigments for color enhancement.
• Dropped concrete should not exceed 5 ft for unexposed work and 3 ft for exposed work to prevent damage.

Shrinkage and Temperature Changes

• Concrete shrinkage occurs due to hardening and temperature contraction, leading to cracks.
• Crack size is influenced by the volume of concrete; large cracks can often be subdivided with reinforcement.
• Use of shrinkage or contraction joints is advisable in long continuous concrete placements.
• Shrinkage cracks typically occur at joints between fresh and set concrete; proper joint placement is crucial.

Hardening Process

• Curing allows concrete to harden, as cement and water unite to form strong durable compounds.
• Hardening is influenced by three main factors:
  • Age of the concrete.
  • Temperature during the curing process.
  • Availability of moisture.
• Protect fresh concrete from excessive vibration, loads, extreme temperatures, rapid drying, and contamination during the first 7 days of curing to ensure optimal strength development.