Curing of Concrete

Here it is discussed about why curing of concrete is important? By the hydration of cement particles the strength of the concrete is achieved. The hydration of cement is a process that takes a long time i.e., it is not a momentary action. For sure, the rate of hydration continues over a very long time at a decreasing rate but it is fast to start with. The amount of the result of hydration and thusly the measure of gel framed relies on the degree of hydration.

It has been known that cement requires a water / cement ratio about 0.23 for hydration and a water / cement ratio of 0.15 for making up for the shortfalls in the gel pores. At the end of the day a water / cement ratio of about 0.38 would be needed to hydrate all the particles of cement and furthermore to consume the space in the gel pores. Theoretically, for a concrete made and contained in a fixed holder a water / cement ratio of 0.38 would fulfil the necessity of water for hydration and simultaneously no capillary pits would be left.

However, it is seen that basically a water / cement ratio of 0.5 will be needed for complete hydration in a fixed compartment for keeping up the advantageous relative humidity level. In the field and in genuine work, it is an alternate story. Despite the fact that a higher water / cement ratio is utilized, since the concrete is available to atmosphere, the water utilized in the concrete dissipates and the water accessible in the concrete won’t be adequate for compelling hydration to happen especially in the top layer.

On the off chance that the hydration is to proceed unbated, additional water should be added to recharge the deficiency of water because of absorption and evaporation. Then again a few measures should be taken via arrangement of impervious covering or utilization of curing mixtures to keep the deficiency of water from the outside of the concrete.

Therefore, the curing of concrete can be considered as production of an ideal environment during the early period for continuous hydration. The advantageous conditions are a reasonable temperature and plentiful moisture.

Curing of concrete can likewise be depicted as keeping the concrete damp and warm enough with the goal that the hydration of cement can proceed. In a more elaborate way, it tends to be portrayed as the way toward keeping a good moisture content and a great temperature in concrete during the period promptly following placement, with the goal that hydration of cement might proceed until the ideal properties are created to an adequate degree to meet the necessity of administration.

Curing is being given a position of expanding significance as the interest for top notch concrete is expanding. It has been perceived that the quality of concrete shows all round progress with proficient continuous curing of concrete. In case curing is dismissed in the early time of hydration, the quality of concrete will encounter a kind of hopeless misfortune. An effective curing in the early time of hydration can measure up to a good and healthy taking care of given to another conceived child.

The speedy surface drying of concrete outcomes in the development of moisture from the inside to the surface. This steep moisture gradient can cause high interior stresses which are likewise responsible for inside miniature cracks in the semi-plastic concrete.

Concrete while hydrating delivers high heat of hydration which is harmful according to the perspective of volume stability. In the event that the heat produced is eliminated by certain means, the unfriendly impact because of the age of heat can be decreased. This should be possible by a careful water curing.

Methods of Curing of Concrete

The methods of curing of concrete can be classified into four following categories:

• Water Curing
• Membrane Curing
• Application of Heat
• Miscellaneous

Water Curing

According to the concrete technology theories, water curing is the best method of curing as it covers all the necessities of curing of concrete, such as, elimination of shrinkage, absorption of the heat of hydration and promotion of hydration. In some cases, even if the membrane method is adopted, before the concrete is covered with membranes the water curing is done to a possible extent. There are generally four ways to practice the water curing which are as follows:

• Wet Covering
• Immersion
• Spraying or Fogging
• Ponding

The immersion can be followed with the precast concrete items which are immersed in large curing tanks for a certain duration. The pavement slabs, roof slabs, etc. are cured by making small ponds by giving the slabs some boundaries of plaster and filled with water. The walls, concrete columns, etc. are cured by spraying and fogging of water . In some cases, the vertical concrete structures are wrapped by wet coverings like gunny bags, jute matting, hessian cloths, straw, etc. for keeping the structure wet. For the horizontal surfaces, the structures are kept wet for a long time by sand, saw dust, earth, etc. are used as wet covering so that the concrete do not dries to prevent hydration.

Membrane Curing

The construction works are sometimes carried out in a place where there is shortage of water where there is not possible to use water abundantly for curing due to economic reasons. It has been noted that the quantity of water that is used for mixing the concrete is normally more than sufficient to hydrate the cement, provided this water is not allowable to go out from the concrete body, for which the concrete can be covered by membrane that will seal off the evaporation effectively of water from concrete.

Sometimes, the curing of concrete cannot be properly supervised in some places where the concrete is placed in inaccessible or difficult or far off places. Since there is no alternative, the curing is entirely left to the workmen, who don’t quite understand the importance of curing of concrete in a regular uninterrupted way. In such cases, the membrane method of curing of concrete is more appreciable and safer to adopt than to leave the responsibility of curing to the workers.

For an adequate state of wetness in the concrete body membrane method of curing of concrete is good for maintaining to promote continuous hydration when original water / cement ratio used is not less than 0.5. In some cases, after the actual wet curing i.e., water curing, membrane is applied to achieve best results. When following the membrane method of curing, the polyethylene film and waterproofing paper are used as membranes which should be taken care properly to see if these are not puncture less and also see at the junctions whether satisfactory lapping is given and the lap is effectively sealed.

Application of Heat

The strength of concrete development is a function of not only time but also of temperature. In higher pressure the concrete accelerates the hydration process which gives an outcome in faster development of strength. There are many other advantageous which can be contributed by a faster attainment of strength, such as,

• Concrete is helpless against harm just for brief time frame.
• Concrete member can be taken care of rapidly.
• Less space will be adequate in the casting yard.
• A smaller curing tank will be adequate.
• A higher outturn is feasible for a given capital expense.
• At a much early time the work can be put on to service.
• A minimum number of formwork will be satisfied or alternatively with the given number of formwork more outturn will be attained.
• Pre-stressing bed can be delivered right on time for additional casting.

Here it is seen that, from the above mentioned advantages, steam curing will give not only economical benefits but also technical benefits in the matter of prefabrication of concrete elements. In the higher temperature the exposure of concrete can be done in the following manner, such as, steam curing at high pressure, curing by infrared radiation, steam curing at ordinary pressure and electrical curing.

Steam Curing at High Pressure

When steam curing is done at atmospheric pressure, the steam temperature is naturally below 100°C. The steam is transformed into water which is generally known as hot water curing and in the open atmosphere it is done. From the ordinary steam curing, the high pressure steam curing is different which is generally carried out in a closed chamber. On the concrete the process of superheated steam at high temperature and pressure is known as “Autoclaving”. Some of the advantages from the high pressure steam curing process are :

• High pressure steam cured concrete gets development in one day or less the strength as much as the 28 days’ strength of typically cured concrete. The strength that is developed does not show any retrogression.
• From sulphate assault, thawing and freezing action and chemical action high pressure steam cured concrete shows higher resistance. Likewise it shows less efflorescence.
• The lower drying shrinkage and moisture movement is seen in high pressure steam cured concrete.

Curing by Infrared Radiation

In the very cold climatic regions of Russia the process of curing by infrared radiation of concrete is practiced. It is claimed that a more quick gain of strength can be achieved than of steam curing and that quick initial temperature does not cause a diminishing in the maximum strength like of steam curing at ordinary pressure. This system is generally adopted for the curing of hollow concrete products. The temperature is kept normally at about 90°C.

Steam Curing at Ordinary Pressure

For the most part of prefabricated concrete components the method of steam curing at ordinary pressure is adopted. In the in-situ condition the application of steam curing is a little difficult task. In some in-situ constructions the engineers have practiced steam curing by giving a cover of steam jacket with tarpaulin or thick polyethylene sheets. But this method of application of steam curing for the in-situ work is found out to be not economical and wasteful and the targeted strength and benefit is not really gained.

Electrical Curing

Another mostly practiced method of curing of concrete in the very cold climatic regions is the electrical curing. This method is generally not used in the ordinary climate due to its economic reasons. Curing of concrete can be done electrically by passing an alternating current through the actual concrete between two electrodes either covered in or applied to the surface of the concrete. There must be a good care to prevent the moisture of the concrete going out and leaving the concrete completely dry.

Miscellaneous Methods of Curing

As an admixture or as a surface coating, Calcium chloride is used in the concrete which has been used as a curing medium. On the fact that Calcium chloride being a salt it shows affinity of moisture, both these methods are used where, salt not only absorbs moisture from the atmosphere but also leaves it at the surface. In this way, the moisture presents at the surface preventing the mixing of water from getting evaporate and keeps the concrete wet for a long period to promote hydration.

Likewise, the formwork of the structure prevents escaping of moisture from the concrete in case of columns and beams an the lower part of the slabs. The formwork should be intact and joints should be sealed with wax or any other sealing compound to prevent the evaporation of moisture from the concrete. This process is one of the miscellaneous method of curing for promoting hydration.

When Should Curing of Concrete Start

Generally, it is difficult to set time period for how early water curing can be started. It basically depends on the humidity, temperature and wind velocity and it also depends on the type of cement, water/cement ratio used, fineness of cement, size of member, etc. The extreme point to observe is that the surface top of the concrete should not be allowed to dry. To promote hydration there must be enough moisture present. To make sure that surface of the concrete does not dry some practical steps can be undertaken like fine spray or fogging without disturbing the surface finish. It is pointed out that for the 53 grade of cement early curing is important.

How Long Concrete Takes to Cure

Likewise, regarding how long to cure, it is again difficult to set time period. The curing period should be as long as practical since all the desirable properties of the concrete are improved by curing. Generally, the concrete must be cured until it attains 70% of specified strength. The curing time frame must be increased at lower temperature. The Pozzolanic cement or concrete admixture mixed with pozzolanic material should be cured for a longer period of time. Structures like heavy footings, large piers, mass concrete work, abutments should be cured for minimum 2 weeks. It is pointed that, normally for 28 days curing is done in concrete slabs.

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