What is curing of concrete and it's types

Introduction.

Cement gains strength and hardness because of the chemical action between cement and water. This chemical reaction requires moisture, favourable temperature and time referred to as the curing period. Curing of freshly placed concrete is very important for optimum strength and durability. The major part of the strength in the initial period is contributed by the clinker compound C3S and partly by C2S, and is completed in about three weeks. The later strength contributed by C2S is gradual and takes long time.

 As such sufficient water should be made available to concrete to allow it to gain full strength.  The process of keeping concrete damp for this purpose is known as curing. The object is to prevent the loss of moisture from concrete due to evaporation or any other reason, supply additional moisture or heat and moisture to accelerate the gain of strength. Curing must be done for at least three weeks and in no case for less than ten days. Approximately 14 litres of water is required to hydrate each bag of cement. Soon after the concrete is placed, the increase in strength is very rapid (3 to 7 days) and continues slowly thereafter for an indefinite period. Concrete moist cured for 7 days is about 50 per cent stronger than that which is exposed to dry air for the entire period. If the concrete is kept damp for one month, the strength is about double than that of concrete exposed only to dry air. 

Method of curing.

Concrete may be kept moist by a number of ways. The methods consist in either supplying additional moisture to concrete during early hardening period by ponding, spraying, sprinkling, etc. or by preventing loss of moisture from concrete by sealing the surface of concrete by membrane formed by curing compounds. Following are some of the prevelent methods of curing. 

1.Water curing.

Water curing is done by covering the concrete surface with gunny bags and then sprinkling water over them regularly or with water proof paper. In membrane curing the surface is coated with a bitumen layer to prevent loss of moisture by evaporation. Sealing compounds such as rubber latex emulsion, resins, varnish and wax may also be used as an alternative to bitumen layer. However, the concrete here may not achieve full hydration as in moist curing. The horizontal surfaces are kept wet by storing water over them (ponding) or by damp gunny bags, straw, etc. Ponding, may, affect the strength if the concrete is flooded too soon. When sprinkling of water is done at intervals, care must be taken that the concrete does not dry out between applications to prevent the possibility of crazing—the fine cracks that may occur in the surface of new concrete as it harden.

2.Steam curing.

Curing can be also accomplished by artificial heat while the concrete is maintained in moist condition. Both of these conditions can be fulfilled by the use fo steam curing. This method of curing is also known as  accelerated  curing since an increased rate of strength development can be achieved. The accelerated process of curing has many advantages in the manufacture of precast concrete products; (a) The moulds can be removed within a

shorter time.

(b) Due to shorter period of curing, production is increased and cost reduced.

(c) Storage space in the factory. 

The temperature can be raised by placing the concrete in steam, hot water or by passing an electric current through the concrete. In the hydration process of cement at higher temperatures, the released calcium hydroxide reacts with finely divided silica, present in the coarse and fine aggregates and forms a strong and fairly insoluble compound which results in higher strengths. Since free calcium hydroxide content is reduced, the leaching and efflorescence are minimised. The hydrating dicalcium silicates and tricalcium alluminates react together at high temperatures to form sulphate resisting compounds. Consequently autoclaved products show higher resistance to sulphate attack. The initial drying shrinkage and moisture movements are also considerably reduced. However, high-pressure steam-curing reduces the bond strength by about 50 per cent.

 The concrete members are heated by steam at 93 °C either at low pressure or high pressure. In low pressure steam curing about 70 per cent of the 28 day compressive strength of concrete can be obtained in about 16-24 hours. High pressure steam curing is usually applied to precast concrete members and gives 28 day compressive strength at 24 hours. The effect of curing temperature on compressive strength

It also results in increased resistance to sulphate action and to freezing and thawing. 

The mixes with low water-cement ratio respond more favourably to steam curing than those with higher water/cement ratio. An early rise in temperature at the time of setting of concrete may be detrimental because the green concrete may be too weak to resist the air pressure set up in the pores by the increased temperature. 

The rate of increase or decrease of temperature should not exceed 10 to 20° C per hour to avoid thermal shocks. The higher the water/cement ratio of concrete, the more adverse is the effect of an early rise in temperature. 

Therefore, to meet the requirement of compressive strength of concrete, the temperature and/or time required for curing can be reduced by having a lower water/cement ratio. While in identical time cycle, the higher the maximum temperature, greater is the compressive strength. The advantages of curing above 70°C are negated by dilational tendencies due to the expansion of concrete. Steam curing should be followed by water curing for a period of at least 7 days. This supplementary wet curing is found to increase the laterage strength of steam-cured concrete by 20 to 35 per cent. In most cases, steam curing is employed only for achieving 50 to 70 per cent of specified strength in a short period instead of full treatment for 2 to 3 days required to obtain specified strength.

Low pressure steam curing at atmospheric pressure can be continuous or intermittent. The maximum curing temperature is limited to 85 to 90°C.

 In the normal steam curing procedure, it is advisable to start the steam a few hours after casting. A delay of two to six hours, called the presteam or presetting period, depending upon the temperature of curing, is usual. The presetting period helps to achieve a 15 to 30 per cent higher 24 hour strength than that obtained when steam curing is resorted to immediately. The rate of initial temperature rise after presetting period is of the order of 10 to 20°C per hour. In the case of normal steam-curing at atmospheric pressure, the ultimate strength of concrete may be adversely affected if the temperature is raised rapidly. 

This difficulty can be overcome by employing the steam at a pressure of 8 atmospheres. The process is termed high-pressure steam-curing. The increase in temperature allowed is up to 50°C in the first hour, up to 100°C in second hour and up to 185°C in the third hour. The period of treatment under full pressure depends upon the strength requirements. This period of treatment is 7 to 10 hours for hollow block products and 8 to 10 hours for slab or beam elements the period may be increased with the thickness of concrete

Curing by Infra red radiation:

A much more rapid gain of strength can be obtained with the help of infra red radiation than even with steam curing. The rapid initial rise of temperature does not affect the ultimate strength. It is particularly suitable for the manufacture of hollow concrete products in which case the heaters are placed in the hollow spaces of the product. The normal operative temperature is 90°C.

Electrical  Curing: 

Concrete products can be cured by passing alternating current of low voltage and high amperage through electrodes in the form of plates covering the entire area of two opposite faces of concrete. Potential difference between 30 and 60 V is generally adopted. Evaporation is prevented by using an impermeable rubber membrane on the top surface of the concrete. By electrical curing, concrete can attain the normal 28-day strength in a period of 3 days. The technique is expensive.

Chemical Curing:

 Chemical membranes can be sprayed on to cure concrete. Liquid membraneforming curing compounds such as sodium silicate (water glass) solution retard or prevent evaporation of moisture from concrete. They form a film, fill the pores, seal the surface voids and prevent evaporation. The application should be made immediately after the concreting has been finished. If there is any delay, the concrete should be kept moist until the membrane is applied. Membrane curing compound should not be applied when there is free water on the surface, because this water will be absorbed by the concrete and the membranes broken. Nor should the compound be applied after the concrete has dried out since it will be absorbed into the surface of the concrete and a continuous membrane will not be formed. The correct time to apply the membrane is when the water sheet disappears from the surface of the finished concrete. Adequate and uniform coverage of curing compounds is essential. In most cases two applications are required.
Chemical membranes are suitable not only for curing fresh concrete but also for further curing of concrete after removal of forms or after initial moist curing.

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