Curing Of Concrete – How & why

Definition

Curing of concrete is defined as providing adequate moisture, temperature, and time to permit the concrete to gain the specified properties for its intended use. In other words watering the concrete to make it stronger. The more the better.

Concrete derives its strength by the hydration(releasing heat after reaction of cement with water) of cement particles. The hydration of cement is not a momentary action but a process continuing for a long time. Of course, the rate of hydration is fast to start with, but continues over a very long time at a decreasing rate. The quantity of the product of hydration and consequently the amount of gel formed depends upon the extent of hydration. It has been mentioned earlier that cement requires a water/cement ratio about 0.23 for hydration and a water/cement ratio of 0.15 for filling the voids in the gel pores. In other words, a water/cement ratio of about 0.38 would be required to hydrate all the particles of cement and also to occupy the space in the gel pores. Theoretically, for a concrete made and contained in a sealed container a water cement ratio of 0.38 would satisfy the requirement of water for hydration and at the same time no capillary cavities would be left. However, it is seen that practically a water/cement ratio of 0.5 will be required for complete hydration in a sealed container for keeping up the desirable relative humidity level. 

In the field and in actual work, it is a different story. Even though a higher water/cement ratio is used, since the concrete is open to the atmosphere, the water used in the concrete evaporates and the water available in the concrete will not be sufficient for effective hydration to take place particularly in the top layer. The drying behaviour of concrete. If the hydration is to continue unabated, extra water must be added to replenish the loss of water on account of absorption and evaporation. Alternatively, some measures must be taken by way of provision of impervious covering or application of curing compounds to prevent the loss of water from the surface of the concrete. Therefore, the curing can be considered as creation of a favourable environment during the early period for uninterrupted hydration. The desirable conditions are a suitable temperature and ample moisture. 

Curing can also be described as keeping the concrete moist and warm enough so that the hydration of cement can continue. More elaborately, it can be described as the process of maintaining a satisfactory moisture content and a favourable temperature in concrete during the period immediately following placement, so that hydration of cement may continue until the desired properties are developed to a sufficient degree to meet the requirement of service. 

 

Curing is being given a place of increasing importance as the demand for high quality concrete is increasing. It has been recognized that the quality of concrete shows all round improvement with efficient uninterrupted curing. If curing is neglected in the early period of hydration, the quality of concrete will experience a sort of irreparable loss. An efficient curing in the early period of hydration can be compared to a good and wholesome feeding given to a new born baby. 

 

A concrete laid in the afternoon of a hot summer day in a dry climatic region, is apt to dry out quickly. The surface layer of concrete exposed to acute drying conditions, with the combined effect of hot sun and drying wind is likely to be made up of poorly hydrated cement with inferior gel structure which does not give the desirable bond and strength characteristics. In addition, the top surface, particularly that of road or floor pavement is also subjected to a large magnitude of plastic shrinkage stresses. The dried concrete naturally being weak, cannot withstand these stresses with the result that innumerable cracks develop at the surface Fig, shows plastic shrinkage cracks on concrete surface due to quick drying and inadequate early curing. The top surface of such hardened concrete on account of poor gel structure, suffers from lack of wearing quality and abrasion resistance. Therefore, such surfaces create mud in the rainy season and dust in summer.

Quick drying of concrete is bad

The quick surface drying of concrete results in the movement of moisture from the interior to the surface. This steep moisture gradient causes high internal stresses which are also responsible for internal micro cracks in the semi-plastic concrete. Concrete, while hydrating, releases high heat of hydration. This heat is harmful from the point of view of volume stability. If the heat generated is removed by some means, the adverse effect due to the generation of heat can be reduced. This can be done by a thorough water curing. Fig, shows the influence of curing by ponding and wet covering.

Curing methods

Curing methods may be divided broadly into four categories:

(a) Water curing 

(b) Membrane curing 

(c ) Application of heat 

(d) Miscellaneous

When to Start Curing and how Long to Cure

Many a time an engineer at site wonders, how early he should start curing by way of application of water. This problem arises, particularly, just in case of weather concreting. In an arid region, concrete placed as a road slab or roof slab gets dried up during a very short time, say within 2 hours. Often questions are asked whether water are often poured over the above concrete within two hours to stop the drying. The associated problem is, if water is applied within say two hours, whether it’ll interfere with the water/cement ratio and cause harmful effects. In other words, question is how early water are often applied over concrete surface in order that uninterrupted and continued hydration takes place, without causing interference with the water/cement ratio. the solution is that first of all, concrete shouldn’t be allowed to dry fast in any situation. Concrete that are susceptible to quick drying is required to be covered with wet gunny bag or wet hessian cloth properly squeezed, in order that the water doesn’t drip and at an equivalent time, doesn’t allow the concrete to dry. This condition should be maintained for twenty-four hours or a minimum of till the ultimate setting time of cement at which duration the concrete will have assumed the ultimate volume. albeit water is poured, after this point , it’s not getting to interfere with the water/cement ratio. However, the simplest practice is to stay the concrete under the wet gunny bag for twenty-four hours then commence water curing by way of ponding or spraying. Of course, when curing compound is employed immediately after bleeding water, if any, dries up, the question of when to start out water curing doesn’t arise in the least .Many a time an engineer at site wonders, how early he should start curing by way of application of water. This problem arises, particularly, just in case of weather concreting. In an arid region, concrete placed as a road slab or roof slab gets dried up during a very short time, say within 2 hours. Often questions are asked whether water are often poured over the above concrete within two hours to stop the drying. The associated problem is, if water is applied within say two hours, whether it’ll interfere with the water/cement ratio and cause harmful effects. In other words, question is how early water are often applied over concrete surface in order that uninterrupted and continued hydration takes place, without causing interference with the water/cement ratio. the solution is that first of all, concrete shouldn’t be allowed to dry fast in any situation. Concrete that are susceptible to quick drying is required to be covered with wet gunny bag or wet hessian cloth properly squeezed, in order that the water doesn’t drip and at an equivalent time, doesn’t allow the concrete to dry. This condition should be maintained for twenty-four hours or a minimum of till the ultimate setting time of cement at which duration the concrete will have assumed the ultimate volume. albeit water is poured, after this point , it’s not getting to interfere with the water/cement ratio. However, the simplest practice is to stay the concrete under the wet gunny bag for twenty-four hours then commence water curing by way of ponding or spraying. Of course, when curing compound is employed immediately after bleeding water, if any, dries up, the question of when to start out water curing doesn’t arise in the least.

Incidentally, it’s seen that test cubes cast at site are allowed to dry without covering the highest with wet covering. they’re allowed to dry within the hot sun. Such cubes develop cracks and show low strength when crushed. it’s usual that they complain about poor quality of cement or concrete. 

Regarding how long to cure, it’s again difficult to line a limit. Since all the desirable properties of concrete are improved by curing, the curing period should be as long as practical. For general guidance, concrete must be cured till it attains about 70% of specified strength. At lower temperature curing period must be increased. 

Since the speed of hydration is influenced by cement composition and fineness, the curing period should be prolonged for concretes made with cements of slow strength gain characteristics. Pozzolanic cement or concrete admixed with pozzolanic material is required to be cured for extended duration. Mass concrete, heavy footings, large piers, abutments, should be cured for a minimum of 2 weeks.

Drying of Concrete

Drying of concrete is defined as providing the right conditions to permit the concrete to realize a moisture condition appropriate for its intended use. The moisture condition of a concrete slab is of serious importance for the appliance of moisture sensitive floor finishes like vinyl composition tile, linoleum, wood flooring, and non-breathable coating like epoxy.

Recommended Posts