Development of Strength

• Development of strength: development of strength after placing of concrete.

• Curing: It is the procedures used for promoting the hydration of cement, and thus, the development of strength of concrete. Curing control the temperature and the moisture movement from and into the concrete which affects strength and durability.

• The objective of curing is to prevent evaporation of water from the surface and keep the concrete saturated as long as possible to allow the hydration of cement products to continue.

• Evaporation of water depends on:1) temperature,2) relative humidity and3) velocity of the wind.

• Poor curing reduce rate of chemical reaction, reduce strength , affect durability and result in shrinkage cracking.

• Relation between curing and strength(fig10-1)

• Methods of curing:1. Leaving the forms in place for sometime and wetting during

hardening.2. Wrapping with polythene sheets.3. In case of dry weather or rain, suspend a covering above the

concrete surface.4. Spraying or flooding(ponding), or covering the concrete with

wet sand, hessian or cotton mats.5. Seal the concrete surface with membrane. However, this will

reduce the degree and rate of hydration compared with wet curing.

- review fig. 10.2

• Period of curing, a minimum of 7 days curing should be maintained.

• Curing must start as soon as possible as early drying lead to shrinkage and cracking.

• Influence of temperature: - It is important to reduce the temperature of fresh concrete

in hot climate because the higher the temperature at placement the greater the initial rate of strength development but the lower the long term strength.

- With a high initial temperature, there is insufficient time available for the products of hydration to diffuse away from the cement grain to fill the space causing concentration of hydration products in the vicinity of the hydrating cement grains which retard subsequent hydration preventing development of longer- term strength.

- Concrete cast and made in summer can be expected to have a lower strength than the same mix in winter.

Steam curing:It is accelerated curing or moist curing used

when high early age strength is required in concrete and when it is still wet. It is mainly used with precast products.

• Effects of poor curing:1. Lack of curing increase the degree of cracking

within a concrete structure.2. Early termination of curing will allow for

increased shrinkage.3. Decrease in long term strength.4. Reduction in durability of the structure.5. Increase amount of gel pores and capillary

pores.