Concrete derives its strength by the hydration of cement particles. The quality of the product of hydration and consequently the amount of gel formed depend on the extent of hydration. Theoretically, water–cement ratio of 0.38 is required to hydrate all the particles of the cement and to occupy the space in the gel pores. In the field, even though higher water–cement ratio is used, since the concrete is open to atmosphere, the water used in the concrete evaporates and the water available in the concrete will not be sufficient for effective hydration to take place, particularly in the top layer.
Curing can be considered as creation of a favourable environment during the early period for an uninterrupted hydration. The desirable conditions are a suitable temperature and ample moisture content. Concrete while hydrating releases heat of hydration. This heat is harmful from the point of view of volume stability. The heat generated can also be reduced by means of water curing.
The curing methods may be broadly divided into the following categories:
- Water curing
- Membrane curing
- Application of heat
Water curing
This is considered as 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. Water curing can be done in the following ways:
- Immersion
- Ponding
- Spraying or Fogging
- Wet covering
The precast concrete items are normally immersed in curing tanks for a certain duration. Pavement slabs, roof slabs, etc. are covered under water by making small ponds. Vertical retaining walls or plastered surfaces or concrete columns, etc. are cured by spraying water. In some cases, wet coverings such as wet gunny bags, jute matting and straw are wrapped to the vertical surface for keeping the concrete wet. For horizontal surfaces, saw dust, earth or sand are used as wet coverings to keep the concrete in wet condition for a longer time.
Membrane curing
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 that will effectively seal off the evaporation of water from concrete. In addition, if concrete works are carried out in places where there is acute shortage of water, the lavish application of water for water curing is not possible due to reasons of economy.
Sometimes, the concrete is placed in some inaccessible, difficult or far off places. The curing of such concrete cannot be properly supervised. In such cases, it is much safer to adopt membrane curing than to leave the responsibility of curing to workers.
Large number of sealing compounds have been developed in recent years. The idea is to obtain a continuous seal over the concrete surface by means of a firm impervious film to prevent moisture in the concrete from escaping by evaporation. Some of the materials that have been used for this purpose are bituminous compounds, polyethylene or polyester film, waterproof paper, rubber compounds, etc.
Application of heat
The development of the strength of concrete is a function of not only time, but also 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 manners:
- Steam curing at ordinary temperature
- Steam curing at high temperature
- Curing by infra red radiation
- Electrical curing
Steam curing at ordinary temperature
This method is often adopted for prefabricated concrete elements. Application of steam to in situ construction will be a difficult task. For steam curing, the concrete elements are stored in a chamber. The chamber should be large enough to hold a day’s production. The door is closed and steam is applied. The steam may be applied either continuously or intermittently. An accelerated hydration takes place at this higher temperature and concrete attains the 28-day strength of normal concrete in about 3 days. In large prefabricated factories, they have tunnel curing arrangements. However, concrete subjected to higher temperature at the early period of hydration is found to lose some of the strength gained at a later stage.
It has been emphasized that a very young concrete should not be subjected suddenly to high temperature. A certain delay period after casting the concrete is desirable. In India, steam curing is often adopted for precast elements, especially precast concrete sleepers.
Steam curing at high temperature
The 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 temperature and high pressure is applied on the concrete. This process is also called ‘autoclaving’. The following advantages are derived from the high-pressure steam curing process:
- High pressure steam cured concrete develops in 1 day or less, the strengths developed at 28 days of normally cured concrete. In addition, it does not lose the strength at a later stage.
- High-pressure steam cured concrete exhibits higher resistance to sulphate attack, freezing and thawing action and chemical action. It also shows less efflorescence.
- High-pressure steam cured concrete exhibits lower drying shrinkage and moisture movement.
Curing by infra red radiation
Curing of concrete by infra red radiations has been practised in very cold climatic regions of Russia. It is claimed that much more rapid gain of strength can be obtained than with steam curing and does not cause a decrease in the ultimate strength as in the case of steam curing at ordinary pressure.
Electrical curing
Another method of curing concrete, which is applicable mostly to very cold climatic regions, is by the use of electricity. This method is not likely to find much application in ordinary temperatures due to economic reasons.
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