Concrete derives its strength from the hydration of cement particles. The hydration of cement is not a momentary action but a process continuing for a long time. Of course, the rate of hydration is fast to start with but continues over a very long time at a decreasing rate. cement requires a water/cement ratio of about 0.25 for hydration and a water/cement ratio of 0.15 for filling the voids in the gel pores. In other words, a water/cement ratio of about 0.38 would be required to hydrate all the particles of cement and also to occupy the space in the gel pores. Theoretically, for concrete made and contained in a sealed container a water-cement ratio of 0.38 would satisfy the requirement of water for hydration and at the same time, no capillary cavities would be left. However; it is seen that practically a water/cement ratio of 0.5 will be required for complete hydration in a sealed container for keeping up the desired relative humidity level. Curing can also be described as keeping the concrete moist and warm enough so that the hydration of cement can continue. More elaborately, it can be described as the process of maintaining satisfactory moisture content and a favorable temperature in concrete during the period immediately following placement, so that hydration of cement may continue until the desired properties are developed to a sufficient degree to meet the requirement of service.
- The ideal temperature of curing is 270
Curing methods may be divided broadly into four categories:
a. Water curing
b. Membrane curing
c. Application of heat
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- Water curing can be done in the following ways: a. Immersion b. Ponding c. Spraying or Fogging d. Wet covering
- Membrane Curing: Sometimes, concrete works are carried out in places where there is an acute shortage of water. The lavish application of water for water curing is not possible for reasons of economy, concrete could be covered with a membrane that will effectively seal off the evaporation of water from concrete.
- Application of Heat: The development of the strength of concrete is a function of not only time but also that of temperature. When concrete is subjected to higher temperature it accelerates the hydration process resulting in faster development of strength. The exposure of concrete to higher temperature is done in the following manner: a. Steam curing at ordinary pressure. b. Steam curing at high pressure. c. Curing by Infra-red radiation. d. Electrical curing.
- In India’, steam curing is often adopted for precast elements, especially pre-stressed concrete sleepers. The strength obtains by ordinary curing after 28days is achieved by steam curing in 3 days.
- High-pressure steam curing is something different from ordinary steam curing, in that the curing is carried out in a closed chamber. The superheated steam at high pressure and high temperature is applied to the concrete. This process is also called “Autoclaving‖ this high-pressure steam curing is practiced in the manufacture of cellular concrete products, such as Siporex, Celcrete.
- Curing of concrete by Infra-red Radiation has been practiced in very cold climatic regions in Russia, The system is very often adopted for the curing of hollow concrete products.
- Miscellaneous Methods of Curing: Calcium chloride is used either as a surface coating or as an admixture. It has been used satisfactorily as a curing medium. the fact that calcium chloride being a salt shows an affinity for moisture. The salt not only absorbs moisture from the atmosphere but also retains it at the surface. This moisture held at the surface prevents the mixing water from evaporation and thereby keeps the concrete wet for a long time to promote hydration.
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