Curing, Finishing, and Early-Age Behavior
This domain tests the link between early-age moisture and temperature history and long-term performance: strength gain, permeability, cracking and surface durability. Questions usually ask what to do, when to do it, and what goes wrong if you are early or late.
Curing
ACI 308 requires moisture retention and adequate temperature. As a rule: 7 days of moist curing for Type I cement at about 20 degrees C, 3 days for Type III, and 14 days where SCMs are used or durability is critical. Curing compounds must be applied at the specified rate to a damp surface; never over bleed water. Maintain concrete above about 10 degrees C.
Finishing sequence
Screed, bull float, then wait for bleed water to disappear before troweling. Troweling while bleed water is present traps water, causing delamination, blistering and dusting. Fog spray and windbreaks control evaporation. Saw joints as soon as the concrete can take the saw without raveling, typically 6 to 24 hours, to at least one quarter of the slab thickness.
Early-age cracking
Plastic shrinkage occurs when evaporation exceeds roughly 0.5 kg/m2 per hour with wind, low humidity or high concrete temperature. Thermal cracking follows high heat of hydration; keep peak temperature below about 70 degrees C and differentials within about 20 degrees C in mass concrete.
Worked example
Maturity using Nurse-Saul: M = sum of (T + 10) times time. Two days at 25 degrees C plus one day at 15 degrees C gives (25 + 10)(48) + (15 + 10)(24) = 1680 + 600 = 2280 degree-hours, which can be compared with a strength-maturity curve to decide formwork removal.
Traps
Finishing before bleed water clears; applying curing compound to a wet surface so it dilutes; sawcutting by clock rather than by strength; stripping formwork by schedule rather than by maturity or strength.