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CONCRETE TECHNOLOGY

LECTURE NOTES-CHAPTER 8CURING OF CONCRETE

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CURING OF CONCRETEEvaporation of water from concrete soon after placing depends on the temperature and relative humidity of the surrounding air and on the velocity of the wind. The difference between the temperatures of concrete and of surrounding air also affects the loss of water.

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CURING METHODS-2Concrete for flat surfaces

such as pavements, sidewalks, and slabs can be cured by ponding. However, it is often impractical except for small jobs. Furthermore, ponding is undesirable if the concrete will be exposed to early freezing.

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CURING METHODS-3

Sprinkling is an excellent method of curing. If it is done at intervals, care must be taken to prevent the concrete from drying the applications of water.

When a fine spray of water is continuously applied, the possibility of "crazing" or cracking due to alternate cycles of wetting and drying can be minimized.

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CURING METHODS-4

Wet coverings such as burlaps, cotton mats, or other moisture-retaining fabrics are extensively used for curing. Treated burlaps that reflect light and are resistant to rot and fire are available. Wet coverings of earth or sand are effective for curing, too. However, it is expensive and may be useful only in small jobs.

Moist earth or sand ~ 5cm (on previously moistened surface)

Moist hay or straw ~ 15 cm (keep continuously wet)

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Burlaps

Treated burlaps

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CURING METHODS-5

Sealing the concrete surface may be done by means of waterproof papers, plastic sheets, liquid membrane-forming compounds, and forms left in place. One important advantage of this group of methods is that periodic additions of water are not required. These methods assure the hydration of cement by preventing loss of water from the concrete. They should be applied as soon as the concrete has hardened sufficiently to prevent surface damage, and after concrete has been thoroughly moistened.

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Liquid membrane-forming compounds sprayed onto the surface

Polyethylene sheet

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CURING METHODS-6

Steam curing can be used to advantage where early strength gain in concrete is important or where additional heat is required to accomplish hydration, as in cold-weather concreting. There are two methods of steam curing:(1) curing in live steam at atmospheric pressure (for enclosed cast in place structures and manufactured precast units)(2) curing in high pressure steam autoclaves (for small manufactured units)

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CURING METHODS-7

A steam curing cycle consists of

(1) an initial delay prior to steaming;

(2) a period for increasing temperature;

(3) a period for holding the maximum temperature constant;

(4) a period for decreasing temperature.

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CURING METHODS-8

Mostly, time lapses from casting to steam shut off is constant at 18 hrs.

Steam curing at atmospheric pressure is generally done in a steam chamber or other enclosure to minimize moisture and heat losses. Tarpaulins are frequently used to form enclosure. Applications of steam to the enclosure should be delayed at least two hours after final placement of concrete to allow for some hardening. Maximum steam temperatures above 80°C should be avoided because they may result in low ultimate strength and they are uneconomical.