An article about " Importance of curing in building construction "

Importance of curing in building construction

In concrete construction, the importance of curing cannot be over-emphasized. Concrete which is not adequately cured and loaded before it attains full strength is very likely to develop cracks. The cracks in concrete adversely affect its durability by corrosion of reinforcement.

IS 456-2000 (plain and reinforced concrete – code of practice) defines curing as “the process of preventing the loss of moisture from the concrete while maintaining a satisfactory temperature regime”. The moisture loss takes place in two ways. One is by evaporation caused by the heat generated during hydration of cement. Second is by evaporation aided by atmospheric temperature and wind. The evaporation caused by wind and atmospheric temperature increases when the exposed surface area of concrete increases. 

In the case of horizontal surface such as top of flat slabs, moisture loss is prevented by ponding – making a pond 8 cm to 10 cm deep and keeping it filled with water. If the surface is sloped or vertical, ponding is not possible. Such surfaces are covered by a layer of sacking, canvas, hessian or similar materials and kept constantly wet. IS 456-2000 stipulates that exposed surfaces shall be kept constantly wet at least for seven days if ordinary Portland cement is used, and at least for 10 days, where mineral admixtures or blended cement are used.

IS 456-2000 permits removal of vertical form-work of column, beams and walls after a lapse of 24 hours of placing concrete. It also permits removal of form work from the underside of slabs after three days if propos (vertical supports) are re-fixed immediately. Removal of formwork from the underside of slabs exposes the entire surface to atmosphere and abets moisture loss.  This part cannot be kept wet by ponding or by covering with sacking or hessian. Spraying of water is also not effective to keep the surface constantly wet. Thus, removal of formwork conforming to provisions of is code makes it virtually impossible to comply with the provision regarding continuous curing of exposed surfaces for seven or 10 days.

To speed up construction, the columns above the slab are raised the very next day of casting the slab. When column raising and shuttering for the upper slab are in progress, the ponding of the top surface of the slab is often neglected as it inconveniences the workers engaged in form work erection. The ultimate result is inadequate curing adversely affecting the strength and durability of concrete.

Now-a-days, there is a craze for shorter cycle time for slab casting. The cycle time is the interval between casting one slab and the slab immediately above it. A 10-day cycle time means once the slab at a particular level is cast, the slab immediately above it will be cast within 10 days. The concrete attains design strength in 28 days. Within that period, two upper slabs will be cast if the cycle time is 10 days. Form work blow the lower slab will have been removed, because IS code permits removal of vertical support below the slab (spanning upto 4.5 metre) after a lapse of seven days. The stipulation that forms shall not be released until the concrete has achieved a strength of at least twice the stress to which concrete may be subjected to at the time of removal of form work is conveniently overlooked. Thus, a 10-day-old slab, which, in all probability is not properly cured, carries the weight of the slab immediately above it. It is also subjected to impact load caused by the vibration of concrete and movement of trolleys carrying concrete.

Another instance of speed affecting quality of construction is in masonry and plastering. Bricks required for brickwork in cement mortar should be soaked in water at least for six hours. Earlier, the practice was to keep the bricks overnight in a tank filled with water and take them out in the next day for use in masonry. This practice is now seldom followed. The present day practice is to spray water on bricks kept in stacks. By this, the bricks in the inner layers sometimes even do not get wet. Brick, if not properly soaked in water, absorbs water from the mortar in the joints and it reduces the bonding between bricks and results in masonry with poor strength. Brick, if not properly soaked, will absorb water from the plaster mortar also. This will result in development of cracks in the plaster, which might also break up. The brickwork in cement mortar should be cured for seven days and plastering should be done only after the curing.  But, to save time, it is a common practice to start plastering earlier. This too results in development of cracks in plaster. Concrete blocks are being widely used in place of brick.  These blocks should be cured for 28 days for attaining full strength. It is now a common practice to use it much earlier.

Owners should resist the temptation to set unrealistic deadlines. Contractors in their eagerness to get the order should resist the temptation to accept such deadlines. Time is money, no doubt.  But trying to save money by sacrificing quality will prove only counter-productive.