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. Here we are going to discuss the types or method of curing of concrete
The quantity of the product of hydration and consequently, the amount of gel formed depends upon the extent of hydration.
It has been mentioned earlier that cement requires a water/cement ratio about 0.23 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.
Types or Methods of Curing of Concrete
This is by far the best method of curing as it satisfies all the requirements of curing, namely, promotion of hydration, elimination of shrinkage and absorption of the heat of hydration. It is pointed out that even if the membrane method is adopted, it is desirable that a certain extent of water curing is done before the concrete is covered with membranes. Water curing can be done in the following ways
(c) Spraying or Fogging
(d) Wet covering
The precast concrete items are normally immersed in curing tanks for a certain duration. Pavement slabs, roof slab, etc. are covered underwater by making small ponds. A vertical retaining wall or plastered surfaces or concrete columns etc. are cured by spraying water.
In some cases, wet coverings such as wet gunny bags, hessian cloth, jute matting, straw, etc., are wrapped to a vertical surface for keeping the concrete wet. For horizontal surfaces saw dust earth or sand are used as a wet covering to keep the concrete in a wet condition for a longer time so that the concrete is not unduly dried to prevent hydration.
2. 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. It has been pointed out earlier that curing does not mean only application of water, it means also creation of conditions for the promotion of uninterrupted and progressive hydration. It is also pointed out that the quantity of water, normally mixed for making concrete is more than sufficient to hydrate the cement, provided this water is not allowed to go out from the body of concrete.
For this reason, concrete could be covered with a membrane which will effectively seal off the evaporation of water from concrete. It is found that the application of membrane or a sealing compound, after a short spell of water curing for one or two days is sometimes beneficial.
Sometimes, concrete is placed in some inaccessible, difficult or far off places. The curing of such concrete cannot be properly supervised. The curing is entirely left to the workmen, who do not quite understand the importance of regular uninterrupted curing. In such cases, it is much safer to adopt membrane curing rather than to leave the responsibility of curing to workers.
3. Application of heat
The development of 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.
Concrete cannot be subjected to dry heat to accelerate the hydration process as the presence of moisture is also an essential requisite. Therefore, subjecting the concrete to a higher temperature and maintaining the required wetness can be achieved by subjecting the concrete to steam curing
4. Curing by Infra-red Radiation
Curing of concrete by Infra-red Radiation has been practiced in very cold climatic regions in Russia. It is claimed that much more rapid gain of strength can be obtained than with steam curing and that rapid initial temperature does not cause a decrease in the ultimate strength as in the case of steam curing at ordinary pressure.
The system is very often adopted for the curing of hollow concrete products. The normal operating temperature is kept at about 90°C.
5. Electrical Curing.
Concrete can be cured electrically by passing alternating current (Electrolysis trouble will be encountered if direct current is used) through the concrete itself between two electrodes either buried in or applied to the surface of the concrete.
Care must be taken to prevent the moisture from going out leaving the concrete completely dry. As this method is not likely to be adopted in this country, for a long time to come, this aspect is not discussed in detail.
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