[C M Dordi]

Curing 
Taken from : Ambuja Technical  Literature Series -11

The chemical action between cement and water, which results in the setting and hardening of concrete or mortar, is dependent on the presence of water. Although there is normally adequate quantity of water for full hydration when the concrete or mortar mix is prepared , it is important to ensure that the water is either retained or replenished to enable the chemical action to be continued till such time the required
strength is gained.
A significant loss of water due to evaporation from the concrete or mortar surface ay result in slowing down or stopping the hydration process and resulting in consequent reduction of strength and durability. In order to help the hydration process to continue, water in the capillaries should be prevented from evaporating. It is therefore necessary to maintain an environment of high humidity around the freshly placed concrete or mortar till it attains reasonably good strength. This process is called curing of concrete. 
Curing of concrete or mortar is the last step required to be taken in the process of concrete or masonry construction. This last step plays a very significant role in concrete and mortar performance and needs full attention of the persons involved in construction.
Concrete or mortar in its early life needs to be carefully looked after like parents looking after their new born baby. If concrete or mortar is not nurtured properly by carrying out the required curing or protected against wind and extreme ambient conditions then the structure will lose its strength and durability, and its performance will be far below the required level. In other words it will behave like a child born beak. Therefore if you do not CURE your concrete masonry adequately you will have to ENDURE it's poor performance throughout it's life time. 
Since cement hydration is more rapid in the first few days after fresh concrete or mortar is placed, it is important for enough water to be retained within the concrete or mortar mass during this period. This can be achieved by either reducing the evaporation losses or by replenishing the water continuously on the concrete or mortar surfaces for an adequate period of time. Curing has a strong influence on various properties of concrete and therefore it should not be taken lightly.  Strength, durability, watertightness, wear resistance, volume stability, chemical attacks and resistance to freeze-thaw cycle is much superior of a well cured concrete or mortar than that of a concrete
wherein curing was neglected, all other parameters being indentical. 
Besides this advantage, curing reduces shrinkage, gives better resistance to wear and impact and improves long term
appearance. This is due to the fact that adequate curing gives to the thin exposed surface a case hardened property which otherwise would not be obtained if curing is neglected. In every manner a well cured concrete/mortar is a better concrete/mortar.

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[Manish Jain]

CURING METHODS AND MATERIALS

There are two general ways by which concrete can be kept moist and humid or kept at a favourable temperature:

 By maintaining the presence of mixing water in the concrete during the early hardening period. Methods generally deployed are pending or
immersion, spraying, sprinkling or fogging, wet covering using hessian cloth, gunny bags etc.
 By preventing loss of mixing water from the concrete by sealing the exposed surfaces of concrete. The exposed surfaces can be general'
covered by membrane formed by curing compound, impervious paper, plastic sheets or even by leaving formwork in place.

However at times there is a need to accelerate curing or there is necessity to expedite gain to strength due to adverse weather conditions or there is an
intent to handle or put to use the concrete structure earlier. This is achieved by using live steam, heating coils or electrically heated forms or pads.
The curing method or combination of curing methods are generally selected depending on some of the following factors.

• Specifications.
  
• Availability of curing materials.
  
• Economics
  
• Type of concrete structure (precast I cast-in-situ).
  
• Shape and size of concrete surface.
  
• Aesthetic appearance
  

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 CURING FD.pdf

[Manish Jain]

Curing methods 
PONDING OR IMMERSION IN WATER
The best curing method is total immersion of concrete in water . This method of total immersion is practically
not possible unless the concrete is a laboratory test specimen or a small precast member which can be easily handled and placed in the curing pond.
However, curing of flat surfaces such as those of pavements, concrete roads, sidewalks, floor slabs can be best cured
by ponding water on the exposed top surface . However, following precautions are necessary :

• Ponding water lost due to evaporation should be continuously replenished.
  
• Ponding water should also cover the corners and edges and should be able to cover the entire surface uniformly to avoid dry spots wherein curing would be deficient 
  
• Water used for curing should have identical properties as that of water used for manufacture of concrete.
  
• Water and the materials used for bunding should be free of substances that will stain or discolour the concrete surfaces.
  
• The difference in the curing water temperature and the concrete temperature should not be more that 11 °C to prevent thermal stresses that could result in cracking.
  
• Adequate water should be available at site throughout the curing period.
  
• Bunds of impervious earth or cement mortar to retain water should be maintained throughout the curing period.
  
• The height of the bunds and area of concrete surface to be ponded must be so selected that there is at least about 25 mm of water ponded on the highest surface.
  

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 CURING FD.pdf