In this article we will discuss about:- 1. Process of Hydration for Concrete 2. Objects of Concrete Curing 3. Necessity 4. Methods 5. Period of Concrete Curing 6. Maturity of Cured Concrete.
Process of Hydration for Concrete:
Concrete derives its strength by the hydration of cement particles. The process of hydration is not a momentary action, it continues for long. Ofcourse the rate of hydration is fast in the beginning, but its rate decreases as the time passes. It is observed that about 90% hydration is complete within 28 days and the rest 10% hydration takes years together. The quantity of the product known as gel depends upon the degree of hydration.
For hydration cement requires about 23% of water by weight of cement i.e. for hydration water/cement ratio of 0.23 is sufficient. 15% water by weight of cement is required for filling the voids in the gel pores. Thus a water/cement ratio of 0.38 is required to hydrate all particles of cement and also to occupy the space in the pores of the gel. Though theoretically a water/cement ratio of 0.38 would satisfy the requirement of water for hydration and no capillary cavities would be left, but practically a water/cement ratio of 0.5 will be required for complete hydration in a sealed container for keeping the desirable relative humidity level.
In the actual work, though higher water/cement is used, but the water used in the concrete evaporates leaving insufficient water in the concrete for the effective hydration to take place, particularly in the top layer. Thus to prevent the loss of water from surface of the concrete some measures must be taken by way of provision of impervious covering or by the application of curing compounds. Therefore curing can be considered as creation of favourable conditions during the early period for uninterrupted hydration.
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The desirable conditions for hydration are:
1. Suitable temperature and
2. Sufficient moisture
Thus curing can be defined as the process of maintaining satisfactory moisture content and a favourable temperature in concrete during the period immediately after the placement of concrete, so that hydration of cement may continue till the desired properties are developed sufficiently to meet the requirements of service.
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It has been observed that the quality of concrete shows all round improvement with efficient uninterrupted curing. If the curing is neglected in the early period of hydration, the quality of concrete will suffer an irreparable loss.
A concrete laid in the afternoon of a hot summer day in a dry climate will dry out quickly. The combined effect of hot sun and dry wind will produce a poorly hydrated cement concrete with inferior gel structure, which will not give the desired bond and strength. The quick surface drying of concrete results in the movement of moisture from the interior to the surface. This steep moisture gradient develops high internal stresses. These internal stresses also develop internal micro cracks in the semi plastic concrete.
During hydration, concrete generates a high quantity of heat of hydration. The generation of this heat is harmful for the stability of concrete. If the heat generated is removed by any means, the adverse effect of this heat can be minimised. This can be done by thorough curing.
Objects of Concrete Curing:
Following are the objects of curing:
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1. The main object of curing is to keep the concrete saturated or as nearly saturated as possible, until the originally water filled space in the fresh cement paste has been filled to the desired extent by the product of hydration of cement.
2. To prevent the loss of water by evaporation and to maintain the process of hydration. In case of site concrete the active curing stops long before the maximum possible hydration has taken place.
3. To reduce the shrinkage of concrete.
4. To preserve the properties of concrete.
Necessity of Concrete Curing:
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The necessity of curing arises from the fact that hydration of cement can take place only in water filled capillaries. For this reason, a loss of water by evaporation from the capillaries must be prevented. Further water lost internally by self-desiccation has to be replaced by water from outside.
Water required for chemical reaction with cement i.e. for hydration is about 25 to 30% of the water added to the cement, rest of the water is used for providing workability and help to continue hydration. Thus hydration of sealed specimens can proceed only if the amount of water present in the paste is at least twice that of the water already combined. Self-desiccation (drying up) is thus of importance in mixes with water/cement ratio less than 0.5.
For higher water/cement ratios the rate of curing of sealed specimens is same as that of saturated specimen. It has been observed that only half the water present in the paste can be used for chemical combination, even if the total amount of water present is less than the water required for combination. This statement is important in view of the fact that formerly it was believed that a concrete mix containing water in excess of that required for the chemical reaction with cement, a small loss of water during hardening would not adversely affect the process of hardening and the gain in strength.
Now it is known that hydration can take place only when the vapour pressure in the capillaries is sufficiently high, about 80% of saturation pressure. Hydration at a maximum rate can proceed only under conditions of saturation. From experiments it has been observed that below a vapour pressure of 0.8 of the saturations pressure, the degree of hydration is low and below 0.3 of the saturation pressure, hydration is negligible. For satisfactory development of strength it is not necessary that all cement should hydrate, the quality of concrete depends on gel/space ratio of the paste.
Methods of Concrete Curing:
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As the common object of all methods of curing is to prevent the loss of moisture from the exposed surface of concrete and to keep the surface continually damp, following methods may be adopted for achieving this objective.
Actually the method of curing depends upon the nature of work and atmospheric conditions.
Usually following methods may be adopted:
1. Shading of concrete works.
2. Covering concrete surfaces with gunny bags or Hession, burlaps etc.
3. Sprinkling water on the concrete surface.
4. Ponding of concrete surface.
5. Membrane curing.
6. Steam Curing.
7. Electrical Curing.
8. Infra Red Radiation Curing Method.
1. Shading of Concrete:
This method is useful for large surfaces as roads. The object of this method is to protect the newly laid fresh concrete from direct sun rays and dry hot wind to avoid rapid drying and non-uniform temperature development in the mass of concrete. This objective can be achieved by using canvas by stretching it over frames. In cold weather, these canvases prevent freezing of the concrete under frost condition.
2. Covering Surfaces with Gunny Bags or Burlaps etc.:
This method is widely used for structural concrete. In this method the concrete surface is covered with empty cement bags or burlaps etc. and water sprinkled at short intervals to keep them moist. The surface should not be allowed to dry even for a short duration. This method can be employed for horizontal as well as vertical components.
3. Sprinkling Method:
This is an excellent method of curing, provided sprinkling of water could be done continuously. This method needs much more water for curing than any other method. This method is more suitable for floors. Intermittent sprinkling may cause formation of cracks, which is a very serious defect.
4. Ponding Method:
This method is useful for horizontal members only such as pavements, floors, slabs etc. In this case small dikes are formed of earth and water is filled in between the dikes. Water may be filled 2 or 3 times a day depending on the climatic conditions of the region. This method has been found more effective in neutralizing the heat of hydration than wet covering.
Ponding method has been found to give more strength than wet covering method at all ages. From Fig. 12.1, it will be seen that with water/cement ratio of 0.32 the increase in compressive strength at 28 days is about 16%, at 3 days strength about 6% and at one day about 4 to 5%. With water/cement of 0.36 only 28 day strength is found about 6% more and water/cement ratio more than 0.4 shows no difference in compressive strength.
5. Membrane Curing:
This method is employed at places where water is scares. Chemical compounds or polythene sheets or w/c Ratio water proof paper etc. are used for curing of concrete.
While using chemical membranes such as bitumen emulsion following precautions should be observed:
i. Such compounds should not be applied when there is free water on the concrete surface or after the concrete surface has dried out.
ii. It should be applied after curing for 24 hours, the freshly laid concrete with moist gunny bags.
For this purpose bitumen emulsion, asphaltic and coal tar cut back can be used with advantage.
However their use is restricted as the resulting surface is black, which absorbs excessive heat and the look also is not pleasing.
Membrane Curing Compounds:
To retard the loss of water from concrete during early period of setting and hardening liquid membrane forming curing compounds may be used. They are used not only for curing fresh concrete, but also for further curing after removal of form work. They can also be used for curing purposes after initial water curing for one or two days. The white pigmented curing compounds also reduce the temperature rise of concrete exposed to radiation from sun.
Curing compounds may be made with the following bases:
1. Synthetic resin
2. Wax
3. Acrylic
4. Chlorinated rubber
Resin and wax based curing compounds seal the concrete surface effectively. However their efficiency reduces with time and after about 28 days they get disintegrated and peel off. After 28 days surface of concrete may be plastered. If plastering is required to be done earlier, the surface of the concrete should be washed off with hot water before plastering. The average efficiency of resin and wax based membrane forming curing compounds may be taken as 80%, though it was found varying with time. In one study the typical curing efficiency was found 96% for 24 hours, 84% for 72 hours, 74% for 7 days and 65% for 14 days.
Acrylic based membrane forming curing compounds have been found to have better adhesion with subsequent plaster. This is their additional advantage. This membrane neither gets crumbled nor does it need washing of surface with hot water. Due to the inherent property of acrylic emulsion, the bonding with the plaster is better.
Chlorinated rubber curing compounds not only form a thin film on the surface of concrete but also fill the minute pores in the surface of the concrete. Eventually the surface film wears out.
Method of Application:
The curing compounds may be applied by brush or spraying while the concrete is wet. In case of beams or columns the curing compound may be applied after the removal of form work. On horizontal surfaces the curing compound should be applied upon the complete disappearance of all bleeding water. In horizontal surfaces as that of roads and air field pavements, where texturing is required, the curing compound should be applied after texturing.
In case, the concrete has dried, the surface should be sprayed with water and thoroughly wetted and made fully damp before applying the curing compound. Before use the container of curing compound should be well stirred.
At present there is no Indian standard specification and code of Practice for membrane forming curing compounds and ASTM standards are followed.
Water Proof Paper or Polythene Sheet:
For horizontal large surfaces polythene sheet or water proof paper can be used for curing concrete. The polythene sheet is spread over the surface as soon as possible without damage to the concrete surface and the lap joints between adjoining sheets lightly sealed. The polythene sheet or water proof paper serves dual purpose of preventing loss of water from the concrete and protecting the surface from damage.
Care should be taken that the polythene or water proof paper is not torn. The surface should be inspected occasionally to ascertain that the surface is wet, if not, it should be rewetted and then resealed. This method has been found very economical and effective in regions where water is scarce.
6. Application of Heat to Concrete:
The strength development of concrete is not only a function of time, but also that of temperature. When concrete is subjected to higher temperatures, the hydration process of cement accelerates, resulting in faster development of strength. To accelerate the hydration process of cement only dry heat cannot be applied as the presence of moisture is also essential. Thus for applying higher temperature and maintaining required wetness to accelerate the hydration process, steam curing is the best solution.
Steam Curing at Atmospheric Pressure:
The main object of steam curing is to obtain a sufficiently high early strength of the concrete products so that they may be handled soon after their casting.
Thus the advantages of steam curing are as follows:
1. Concrete members can be handled very soon after their casting.
2. Less space will be sufficient in the casting yard.
3. Higher output is possible for a given capital out lay.
4. A less curing space will be sufficient.
5. The construction work can be started much earlier
6. The completed work can be put to use at an early date.
7. The form work may be struck off at the early age and can be used elsewhere.
8. Pre-stressing bed can be released early for further casting.
Thus the steam curing will not only give economical advantage but-also technical advantage in the matter of prefabrication of concrete members.
The main disadvantage of steam curing is the lower strength at the later age, but for many applications long term strength of concrete is not of much importance.
Steam Curing at Ordinary Pressure:
When steam curing is carried out at one atmospheric pressure i.e. the temperature is below 100°C, the process can be regarded as a special case of moist curing. Steam curing can be used successfully with different types of Portland cements, but it should not be adopted with high alumina cement as it will have adverse effect on the strength of high alumina cement. Steam cured concrete of a low water/cement ratio has shown better results than a weaker mix.
Due to the nature of the operations involved in steam curing, the process usually is applied to pre-cast units. Low pressure steam curing is applied in special built chambers or in tunnels through which the concrete members are transported on a conveyer belt. The chamber should be big enough to hold a day’s production. The door is closed and the steam is applied. Alternatively plastic covers or portable boxes are placed over the pre-cast units and steam is applied through flexible connections. Due to the accelerated hydration at high temperature of steam, the normal 28 days strength is developed in the concrete in about 3 days.
Due to the influence of temperature during the early stages of hardening on the later strength, a compromise between the temperatures giving a high early strength and a high late strength has to be made. Fig. 12.6 shows the typical values of strength of concrete cured with steam at different temperatures immediately after casting. From the Fig. 12.6, it will be seen that above 100°F curing temperature, lower the curing temperature higher the later strength. Raising the curing temperature from 130°F, the strength after 72 hours reduces by about 33%.
Thus the rate of increase in temperature at the commencement of steam curing is very important. It has been observed that if 120°F is reached in a period less than about 2 to 3 hours or 210°F in less than 6 to 7 hours i.e. if the rate of increase of temperature is between 40°F to 60°F per hour or 30°F to 35°F from the time of mixing, the gain in strength beyond the first few hours is affected adversely. Hence such a rapid rise in temperature should not be allowed.
The effect of rapid rise in temperature is more pronounced in concrete with higher water/cement ratio and concrete with rapid hardening cement than ordinary Portland cement. It has been observed that excessively rapid heating can cause a loss of strength at later ages about one-third compared with wet curing at room temperature. The retrogression of the strength of steam cured concrete is due to the pressure development in the air pores during heating as air has higher coefficient of thermal expansion than the surrounding solid material.
As the temperature at the time of setting has the greatest influence on the strength at later ages, delay in the application of steam curing has been found advantageous. From experiments it has been found that a delay of 2, 3 5 and 6 hours for 100, 130, 165 and 185°F temperature respectively will not have any adverse effect on the strength of concrete i.e. after sufficient delay, rapid heating has no adverse effect on the strength. If concrete is exposed to higher temperature with a smaller delay, the strength will be affected adversely.
This effect is more serious for higher curing temperatures. A practical curing cycle may be adopted as, a delay of 3 to 5 hours, heating at the rate of 40°F to 60°F per hour upto a maximum temperature of 150°F to I80°F, then storage at the maximum temperature, followed by a period of “soaking” when no heat is added, but the concrete takes in the residual heat and moisture and finally cooling period. The total time should be about 18 hours.
It may be stated here that a longer period of curing at a lower temperature gives a higher optimum strength than a higher temperature applied for a shorter period. It has also been found that for any one period of curing there is a temperature which gives an optimum strength.
High Pressure Steam Curing (Auto Claving):
The process is quite different both in the method of execution and in the nature of the resulting product from curing in steam at one atmospheric pressure.
As the pressure in this case is more than one atmosphere, the curing chamber used is of the pressure vessel type with a supply of wet steam. In these chamber arrangements for supply of excessive water is essential as super-heated steam should not be allowed to come in contact with concrete. Such a vessel is known as an autoclave. High pressure steam curing can be applied to pre-cast products of ordinary as well as light weight concrete. The optimum curing temperature has been found experimentally as 350°F (177°C), which corresponds to a steam pressure of 8 kg/cm2 above atmospheric pressure.
The high pressure steam curing has been found most effective when finely ground silica is added to the cement. The silica added reacts with the Ca (OH)2 released on hydration of C3S of the cement. Thus cements having higher percentage of C3S have greater capacity for developing high strength when cured at high pressure than those having high content of C2S.
The fineness of silica should be of the same order as that of the cement. The silica and cement should be intimately mixed before they are added to the mixer. Though the optimum amount of silica to be added depends on the mix proportions, but generally varies from 0.4 to 0.7 of the weight of cement. This amount makes the lime/silica ratio of the mixture very approximately 1. The high temperature during curing also affects the reactions of hydration of the cement itself.
The high temperature curing produces products of hydration as coarse and micro crystalline in character. The specific surface of high pressure steam cured paste is only about 5% that of cement paste cured at ordinary temperature. Hence not more than 5%, high pressure steam cured paste can be classified as gel.
Followings are the advantages and disadvantages of high pressure steam curing:
1. With high pressure steam curing, compressive strength within 24 hours can be obtained equal to that of 28 days normal cured concrete.
2. The high pressure steam curing improves the resistance of concrete to sulphates attack. The main reason for this is the formation of aluminates more stable in the presence of sulphates than those formed at lower temperatures. For this reason, cements with a high proportion of C3A have shown greater resistance to sulphate attack than those having lower C3A contents. Another reason for the improvement of sulphate resistance is due to the reduction in lime in the cement paste as a result of lime silica reaction. Further improvement in the sulphate resistance is due to the increased strength and impermeability of the steam cured concrete.
3. High pressure steam curing reduces drying shrinkage to about 1/2 to 1/6 of the concrete cured at normal temperature.
4. Moisture movement is also reduced.
5. Creep is also reduced significantly by high pressure steam curing.
6. High pressure steam curing reduces efflorescence, as no lime is left to be leached out.
7. High pressure steam curing also reduces the effect of freezing and thawing.
8. High pressure steam curing produces stable products and stabilizes unsound materials.
Disadvantages:
1. High pressure steam curing reduces the bond strength of concrete with reinforcement by about 50% in comparison with ordinary curing.
2. High pressure steam curing makes the concrete brittle.
3. High pressure steam curing produces drier and light colour products than moist cured units.
The rate of heating during high pressure steam curing should not be too high as it will interfere with the setting and hardening process of concrete. A typical steaming cycle may consist of gradual increase in temperature of 360°F (182°C) which corresponds to a pressure of 10 kg/cm2 over a period of 3 hours. The concrete is maintained at this temperature for 5 to 8 hours, and then the pressure is released within 20 to 30 minutes. Actual details of the steaming cycle depend on the plant used and the size of concrete members to be cured.
Steam curing should only be applied to concrete products made with Portland cement. High alumina and super sulphated cement would be adversely affected by the high temperature.
7. Electrical Curing of Concrete:
By this method the concrete can be cured by passing an alternating current through the concrete itself between two electrodes either embedded in or applied to the surface of concrete. While using this method care should be taken to prevent the moisture from going out leaving the concrete completely dry. This method is applicable mostly in very cold climatic regions and is not likely to find much of its use in ordinary climatic conditions due to economic considerations.
8. Infra-Red Radiation Curing Method:
This method of concrete curing is practised in very cold climatic regions in Russia. It is claimed that by this method much more rapid gain of strength can be obtained than with steam curing and that rapid initial temperature does not cause any decrease in the ultimate strength as in the case of steam curing at ordinary pressure. Usually this method is adopted for curing the hollow concrete products. The normal operative temperature is kept at about 194°F (90°C).
Maturity Rule:
The temperature effect on concrete is cumulative and can be expressed as the product of temperature and time, during which it prevails, known as maturity M. Thus maturity can be expressed as ―
M = ∑T x Δt
where,
M = Maturity
T = Temperature measured from a datum of – 11°C. Thus for temp, of 30°, T = 20 – (-11) = 41°C.
Δt = Time interval (usually in days).
The maturity rule can be of practical use in estimating the strength of concrete. In SI units the relation between strength and maturity of concrete is as follows ―
fc = -33 + 21 log M.
However this rule depends on the following factors:
(a) Cement actually used in the concrete work.
(b) W/c ratio.
(c) Whether any loss of water takes place during the curing period.
The harmful effect of an early high temperature vitiates the maturity rule. Hence for these reasons the maturity rule has not been widely accepted.
Period of Concrete Curing:
As per IS 456-1978, exposed surfaces of concrete should be kept continuously in a damp or wet condition by any method as discussed above at least for 7 days from the date of placing concrete as during this period about 66% strength of the 28 days strength of concrete is developed. As explained earlier 92% hydration takes place during 28 days period if concrete is cured at 70°F and atleast 95% humidity is maintained.
The strength at 28 days of such concrete is taken as standard. 70% strength is developed in about 10 days and 75% strength in 14 days. Hence the period of curing in the laboratory is recommended as 28 days and in the field at least 7 days. Further, looking to the conditions and importance of work engineer in-charge may fix the period of curing.
The increase in strength of concrete with the period of curing has been found as follows:
Further it has been observed that the strength of moist cured concrete increases by 50% over air exposed concrete for the entire period.
Maturity of Cured Concrete:
The temperature during curing period also controls the rate of progress of the reactions of hydration and consequently affects the development of strength of concrete. The results of tests carried out on specimens cast, sealed and cured at a given temperature are shown in Fig. 12.2.
This Fig. shows the ratio of strength of concrete cured at different temperatures to the 28 days strength of concrete cured at 70°F (21°C). The temperatures at which the specimens cast, sealed and cured are shown on the Fig. itself. From the Fig. 12.2, it will be seen that higher the curing temperature, higher the early strength, say upto 28 days. 
As the strength of concrete depends on both age and temperature, it can be said that the strength is a function of Σ (temperature x time interval). This summation is called maturity, and is measured in °C-hours (°F-hours) or °C-days (°F-day). The temperature can be reckoned between-12°C and – 10°C (11°F and 14°F). This is because at temperatures below the freezing point of water and down to about – 12°C (11°F) concrete shows a small increase in strength with time, but low temperature must not be applied until the concrete has set and gained sufficient strength to resist damage due to the action of frost.
Experiments have shown that the strength and maturity relation depends on the properties of cement used and on the general qualities of concrete and is valid for a limited range of temperature. The maturity rule applies fairly well when the initial temperature of concrete is between 16°C and 27°C (60 and 80°F) and no loss of moisture takes place due to drying during the period considered. Due to these limitations this rule could not be popular in actual practice.