Curing Time

Curing :

What is curing of concrete

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      Curing of concrete is defined as the process of maintaining the moisture and the temperature conditions of concrete for hydration reaction to normally so that concrete develops hardness properties over times.

Duration of cement

     The curing of concrete can make the concrete stronger and increase it's strength. It can increase the durability of concrete. The duration of curing depends upon the weather conditions. If the weather of such are is warm or highly temperature then it take more time and days for curing some time curing time become double with respect to weather conditions. If the weather is cool or low temperature it will manage differently

       OPC = Ordinary Portland cement = 10 days
       Super sulfate cement                    = 7   days
       Mineral cement                            = 14 days

Types or methods of curing

There are two types of curing

1.  Ponding method
2.  Wet covering

Ponding method

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This method is adopted in floor slab. Make some ponds on floor slab depending on the size of floor and fill it with water continuously for 14 days for curing concrete.

Wet covering method

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This method is usually use for the columns, footings and bottom surface of slab where ponding method did not work or possible. In this method the gunny bags and hessian are use to cover the concrete and keep wet them with water.    

Why curing is necessary

               The curing of concrete is requires for the following reasons :

• Beginning immediately after concrete placement it should be safe from prematurely drying, excessively hot or cold temperatures and mechanical injury, and shall be maintained with minimum moisture loss at a relative constant temperature for the period necessary for hydration of cement and hardening of concrete.
• To prevent the concrete from drying out prematurely due to solar radiation and wind.
• To maintain the concrete temperature by allowing the hydration process.
• These following procedure shall be applied immediately after completion of placement and finishing.

1.   Ponding or continuous sprinkling
2. Application of absorptive mats or fabric kept continuously wet.
3. Application of other moisture-retaining covering as approved.
4. Application of other waterproof sheet materials approved by engineer.

• To prevent the concrete from drying out prematurely due to solar radiation and wind. The prevents plastic shrinkage of concrete.
• It helps to maintain the concrete temperature.  It require water to carry on and release heat.
• It helps the concrete to bond with other internal material and reinforcement.
• This helps the development of impermeable, crack-less and long time reliable.
• Moisture loss from surface placed against wooden forms or metal forms exposed to heating by the sun shell be minimized by keeping the forms wet until they can be safely removed , after form removal .
• Curing shall be continued for at least 10 days in the case of all concrete except concrete with rapid-hardening Portland Cement for which the period shall be at least 3 days. Alternatively, if test are cured by the same methods, moisture retention measure may be terminated when the average compressive strength has reached 70 % of the minimum specified works cube strength.
• When the mean daily outdoor temperature is less than 5 degree C then the temperature of concrete shall be maintained between 10 to 20 degree C.
• When necessary arrangement for heating, covering insulation or housing the concrete work shall be made in advance of placement and shall be adequate to maintain the required temperature without injury to concentration of heat, Combustion heaters shall not be used during the first 24 hours unless precautions are taken to prevent exposure of the concrete to exhaust gasses which contain carbon dioxide.
• When necessary, provision for wind brakes, shading for spraying, ponding or wet covering with a light coloured material shall be made in advance of placement, and such protective measures shall be taken as quickly as concrete hardening and finishing operation will allow.
• Change in temperature  of the air immediately adjacent to the concrete during and immediately following the curing period shall be kept as uniform as possible and shall not exceed 3 degree in any hour or 10 degree C in any 24 hour period.60 degree may  be 

Work at Extreme Weather

Unless adequate protection is provided and approval is obtained, concrete shall not be replaced during rain. Rain water shall not be allowed to increase the mixing water nor to damage the surface finish.

When the temperature of the surrounding air is expected to be below 5 degree placing or within 24 hours thereafter, the temperature of plastic concrete, as placed shall be no lower than 13 degree for sections less than 200 mm in any dimension nor 10 degree for other section. When necessary concrete material should be heated before mixing and carefully protected after placing, in general heating or mixing water alone to about 60 degree may be sufficient for this purpose. Dependence should not be placed on salt or other chemical for the prevention of freezing, calcium chloride up to 1 to 0.5 percent of weight of cement may be used to accelerate the rate of hardening only with prior written permission of the engineer, use of calcium chloride in excess of 1 to 0.5 percent is harmful. No frozen material containing ice shall be used. All concrete damage by frost shall be removed. It is recommended that concrete exposed to the action of freezing weather should have entrained air and the water content of the mix should not exceed 25 liters per bag of cement. If water or aggregate is heated above 18 degree the water shall be combined with the aggregate in the mixer before cement is added. 

Cement shall bot be mixed with water or with mixtures of water and aggregate having a temperature grater than 35 degree centigrade.

   During hot weather the temperature of the concrete as placed shall not be so high as to cause difficulty from loss of slump, flash set or cold joint and should not exceed 32 degree centigrade. For massive concrete temperature should not exceed to 21 degree centigrade. when the temperature is exceed to 32 degree centigrade precautionary measures approved by the engineer shall be put in to effect. When temperature of steel is greater than 50 degree, steel forms and reinforcement should be sprayed with water with water just prior to placing the concrete. The ingredients shall be cool before mixing, or flaked ice or well crushed ice of a size that will melt completely during mixing may be substituted for all part of the mixing water if due to high temperature, low slump, flash set or cold joint are encountered.

Factors which depend on curing

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1. Specified the strength of concrete
2. Grades of concrete
3. Atmospheric temperature
4. Shape and size of concrete