Curing
In all but the least critical applications, care must be taken
to properly cure concrete, to achieve best strength and
hardness. This happens after the concrete has been placed. Cement requires a
moist, controlled environment to gain strength and harden fully. The cement
paste hardens over time, initially setting and becoming rigid though very weak
and gaining in strength in the weeks following. In around 4 weeks, typically
over 90% of the final strength is reached, though strengthening may continue
for decades.[46] The conversion of calcium
hydroxide in the
concrete into calcium
carbonate from
absorption of CO2 over several decades further strengthens
the concrete and makes it more resistant to damage. However, this reaction,
called carbonation,
lowers the pH of the cement pore solution and can cause the reinforcement bars
to corrode.
Hydration and hardening of concrete during the first three days
is critical. Abnormally fast drying and shrinkage due to factors such as evaporation
from wind during placement may lead to increased tensile stresses at a time
when it has not yet gained sufficient strength, resulting in greater shrinkage
cracking. The early strength of the concrete can be increased if it is kept
damp during the curing process. Minimizing stress prior to curing minimizes
cracking. High-early-strength concrete is designed to hydrate faster, often by
increased use of cement that increases shrinkage and cracking. The strength of
concrete changes (increases) for up to three years. It depends on cross-section
dimension of elements and conditions of structure exploitation.[47]
During this period concrete must be kept under controlled
temperature and humid atmosphere. In practice, this is achieved by spraying or
ponding the concrete surface with water, thereby protecting the concrete mass
from ill effects of ambient conditions. The picture to the right shows one of
many ways to achieve this, ponding – submerging setting concrete in water
and wrapping in plastic to contain the water in the mix. Additional common
curing methods include wet burlap and/or plastic sheeting covering the fresh
concrete, or by spraying on a water-impermeable temporary curing membrane.
Properly curing concrete leads to increased strength and lower
permeability and avoids cracking where the surface dries out prematurely. Care
must also be taken to avoid freezing or overheating due to the exothermic setting of cement. Improper curing can
cause scaling, reduced strength, poor abrasion resistance
and cracking.
