HOW HOT IS YOUR CONCRETE & DOES IT REALLY MATTER? HEAT EVOLUTION & ITS EFFECTS ON CONCRETE CAST ON CONSTRUCTION SITES IN ASIA

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1 HOW HOT IS YOUR CONCRETE & DOES IT REALLY MATTER? HEAT EVOLUTION & ITS EFFECTS ON CONCRETE CAST ON CONSTRUCTION SITES IN ASIA Christopher Stanley*, Concrete Consultant, Hong Kong. 30th Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2005, Singapore Article Online Id: The online version of this article can be found at: This article is brought to you with the support of Singapore Concrete Institute All Rights reserved for CI Premier PTE LTD You are not Allowed to re distribute or re sale the article in any format without written approval of CI Premier PTE LTD Visit Our Website for more information

2 30 th Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2005, Singapore HOW HOT IS YOUR CONCRETE & DOES IT REALLY MATTER? HEAT EVOLUTION & ITS EFFECTS ON CONCRETE CAST ON CONSTRUCTION SITES IN ASIA Christopher Stanley*, Concrete Consultant, Hong Kong. Abstract The chemical reaction between cement & water, known as the hydration process, is an exothermic reaction. That is, heat is liberated during both the setting & hardening process of the concrete. The concrete expands as the temperature rises, & restraint against contraction on subsequent cooling, unless properly controlled, leads to cracking. The cracks that form usually pose a durability problem, because they often pass through the full section of the concrete. A lack of a proper understanding of the mechanism of thermal contraction cracking often results in the formation of such cracks being incorrectly attributed to other causes such as plastic or drying shrinkage. Thermal contraction cracking is a very common occurrence on construction sites throughout South East Asia. However it is little understood & the appearance of even the smallest cracks often leads to prolonged disputes on site. A number of factors combine to influence the magnitude & extent of thermal contraction cracking in Asian concrete & these are addressed together with case studies of examples of the temperature rise in hydrating pre-cast & in situ concrete on Asian construction sites. Introduction Many examples of concrete cracking are the direct result of concrete either cooling down too rapidly, or are due to the localised restraint against contraction afforded, for example, by bonding to previously cast concrete, or the presence of openings, or changes in section thickness. The cause of this type of crack formation is seldom understood & is often erroneously attributed to plastic or drying shrinkage, but is in fact a clear example of early age thermal contraction cracking. When water is added to cement, what chemists refer to as an exothermic reaction takes place. That is heat is liberated as the water reacts with the cement during the setting & hardening process. The amount of heat liberated is primarily a function of the amount of cement in the mix, the temperature of the concrete at the time of placing & the degree of insulation provided by the formwork in which the concrete is cast. A number of factors influence both the rate of temperature gain & the peak temperature attained & some of these are set out below.

3 To be able to predict the maximum temperature rise in concrete or to be able to control the peak temperature rise to within acceptable limits is therefore very important, particularly if the incidence of early-age thermal contraction cracks are to be avoided. Factors that influence temperature rise The main factors that influence the temperature rise in setting & hardening concrete, are as follows: (a) Initial temperature of materials. Obviously the use of warm materials will lead to a higher placing temperature. The initial temperature of the aggregates & the water has a more significant effect on the temperature of the mix than, for example, the use of hot cement. Generally the higher the placing temperature, the more rapid is the temperature rise & the higher is the peak temperature attained. This is because a high initial temperature acts as a catalyst, thereby increasing the rate of chemical reactivity, & consequently the rate of heat liberation. (b) Ambient temperature. During hot weather conditions the concrete temperature will have a tendency to rise more rapidly & the peak temperature may be higher. However the differential temperature between the concrete surface & the internal core will be lower & therefore the incidence of cracking will be reduced. In winter months there tend to be far more incidences of thermal contraction cracking because of greatly increased thermal gradients across the concrete from the surface to the core. (c) Cement content. The higher the cement content, the greater the amount of heat liberation. Also the rate, & total amount of heat liberation, increases with the fineness of the cement. Therefore RHPC has a faster rate of heat liberation than OPC. The current trend of partial replacement of cement with pozzolanic materials such as PFA or GGBFS helps to reduce the peak temperature. However these materials also contribute to the exothermic reaction when mixed with cement. The heat contribution depends on the placing temperature. The heat contribution of 25% replacement of OPC with PFA at a placing temperature of 30 may only result in a 3-5 reduction in the peak temperature. However the rate of heat gain is reduced. The same applies to GGBFS but because the % replacement is generally higher than with PFA, the peak temperature & rate of heat gain are generally significantly reduced. The reduction depends on the degree of reactivity of the slag, & some slags are more highly reactive than others. Because the reaction of cement replacements is pozzolanic there may also be a reduction in the rate of development of tensile strength & the corresponding strain capacity of concretes containing these materials, especially at early ages. This means that additional precautions have to be taken to prevent the concrete cracking within the first seven days after casting. Because the heat is liberated more slowly the thermal gradients across a concrete section are correspondingly reduced. (d) Use of admixtures. Accelerators will cause an increase in the rate of heat evolution, which is fairly obvious, but there is a widespread misconception that retarders will reduce the amount of heat generated they will not. They will only delay the onset of the main hydration process, which has the effect of moving the heat generation process forward by a day or so. Skillful use of retarders can cause the peak temperature to be reached at night-time when the ambient temperature is likely to be slightly lower. This helps to reduce the peak temperature especially in situations where a few extra may be critical, & cause the peak temperature to be beyond the specification limits. (e) Section size. Thick sections obviously develop more heat but this is also a function of the cement content & the degree of insulation provided by the formwork. (f) Type of formwork. Timber formwork provides some degree of insulation & hence will result in higher temperature development within the concrete than either steel or plastic formwork. The

4 insulation is however beneficial in reducing the temperature gradient between the surface & subsurface concrete. (g) Formwork striking time. Early striking reduces the peak temperature but has the disadvantage of increasing the temperature differential between the surface & the core of the concrete. Even if insulation is placed on the concrete as soon as the formwork is removed, the rapid heat loss may already cause the concrete to crack. Delaying the striking time or insulating the formwork minimises the damage caused by large differences between the concrete surface & subsurface temperature. Just keeping the formwork in position for an extra hours may result in a significant reduction in early-age thermal cracking. (h) Curing. If the concrete temperature is still above that of the ambient at the time the formwork is removed it should be kept warm & allowed to cool down slowly. It is essential that the concrete is NOT sprayed with cold water on removal of the formwork as this will increase the temperature differential at the surface & may cause cracking due to thermal shock. A few days after the concrete has been cast, the exothermic reaction will begin to slow down. As the rate of heat gain falls behind the rate of heat loss the concrete will begin to cool. On cooling the concrete will contract. If the contraction is unrestrained the concrete will not crack. However, in practice, there is bound to be some restraint. Where the tensile stresses, resulting from any restraint, cause the concrete to exceed its current tensile strain capacity, it will crack. However if the concrete cools down slowly, as the tensile strength increases, it will then be able to develop more resistance to cracking. This is another benefit that can be obtained from practicing good early-age thermal insulation practice on site. Restraint of concrete may be the result of either external or internal influences. A common problem is external restraint, where the concrete is cast against a concrete base which has previously hardened, or adjacent to or between similar elements. The concrete is prevented from contracting as it cools down & a series of fan like cracks radiating from the joint between the old & new concretes is a common occurrence. Limiting the time interval between pours can reduce such cracking. The traditional practice of casting alternate bays in an attempt to allow for the effect of the onset of drying shrinkage is totally incorrect. Drying shrinkage takes place many months after the concrete has been cast. Controlling the maximum temperature rise, & also by adding more crack control reinforcement if necessary is the correct approach. Internal restraint is usually the result of conditions where the temperature of the surface of the concrete cools down faster than that of the internal core. This type of restraint cracking is very common but can be minimised by delaying the formwork removal & providing external insulation. The use of additional crack control steel reinforcement is also helpful in minimising the widths of any cracks that might form. Thermally induced contraction cracking in concrete is easy to identify. This is because such cracks normally form between one & two weeks after casting, as the concrete cools down. They occur well after any plastic settlement or plastic shrinkage cracks form, and well before any drying shrinkage cracks begin to occur. Apart from minimising the effects of thermal contraction cracks in concrete, there is also a need to control the maximum temperature rise of the concrete within the structure. This is because if the peak temperature is in excess of about 90 there may be some regression in the concrete strength. This is most noticeable in concretes made using limestone aggregates, where even lower temperatures than this can lead to regression in strength. There is also concern that concretes that reach high early age temperatures may suffer from the longer term effects of delayed ettringite formation & cracking at a later stage in their life. There is currently no evidence that this has yet become a problem in any Asian concrete. Concern is also expressed about the differential movement between aggregates & cement paste

5 resulting in the formation of internal microcracks within concrete. Again there is no reported evidence of this taking place in Asian concrete where the peak temperature is below 100. In order to minimise the problems resulting from thermal contraction cracking in Asian concrete, there are a few options for consideration: Limiting the peak temperature. This can be accomplished by the use of chilled or iced water in the mix. However it should be remembered that this is of little use if the aggregates have been baking all day under the summer sun! Stockpiles need to be shaded, but in reality, just like curing, it seldom happens. More expensive solutions include injecting liquid nitrogen at a temperature of -196 into the mix. This has been tried in Hong Kong & Singapore. This currently adds a cost of over S$250 to a cubic metre of concrete so it is not really a practical proposition. Other expensive options include passing cooling water pipes through the concrete structure. The use of such pipes may contribute to long term durability problems, especially in aggressive exposure or subsoil conditions. Often the most practical solution is to start with the concrete temperature as low as is practically possible, & to programme the casting so that the peak temperature is reached at night, when the ambient temperature is at its lowest. The other practical option is to insulate the concrete & leave the formwork in position as long as possible. This helps to reduce the temperature differential. The coefficient of thermal expansion of a typical Asian granite aggregate concrete is about 10 x 10-6 / & the strain capacity is 80 x so 80 x 10-6 > 1/3 (t c t s ) x 10 x 10-6 t c t s < 80x3 = Where tc = concrete internal temperature. ts = concrete surface temperature. Therefore if the temperature differential between the core & the surface of the concrete does not exceed 24 the concrete is unlikely to crack due to a temperature differential effect. In Limestone aggregate concrete & lightweight concrete even greater differential temperatures up to about 40 are unlikely to crack the concrete. In concrete cast using Granite aggregates, practical experience shows that visible cracking only occurs when the differential exceeds 28, therefore setting a value of 24 in specifications where Granite aggregate is used is a reasonable precaution. The 20 figure that is currently being used in many Asian specifications is a little over conservative. Approximate guide to concrete temperature development. There are many complex computer programs available & guides to temperature development in concrete. Sometimes on site personnel need a rapid rough guide to help them. Below are two simple approaches sometimes adopted by the writer. They may upset the purists amongst you but they provide a useful guide: Take the concrete mix used in the graph of figure 7 for example. Cement content = 400 kg

6 Water = 182 kg Admixture = 8 kg Aggregate = 1800 kg Total = 2390kg Cement proportion of mix = 400/2390 = Temperature rise = cement fraction of mix x heat of hydration (about 275KJ/kg), divided by the specific heat of concrete (assume 1KJ/kg). = 275 x = 45.9 [actual achieved = 42 ]. Because it is only a semi-adiabatic situation the calculated temperature rise will be slightly higher than the actual figure. Another approach is to take a typical value of temperature gain per 100kg/cement/m3 which in this case = per 100kg. The temperature rise due to the 400 kg cement above ambient = 4 x = [actual temperature rise = 42 above ambient]. Temperature monitoring. In order to obtain practical information on the typical temperature rises that could be expected to occur in pre-cast concrete, & concrete cast in situ on site using ready mixed concrete, the concrete temperature was monitored on a number of projects. Type K thermocouples were embedded in the concrete & connected to data loggers. The thermocouples were positioned in concrete of differing thickness both near the surface of the concrete & in the centre of the section. In addition the ambient temperature was monitored. The data was recorded on a time basis & plots were prepared such as those illustrated in figures 1-7. For the purpose of this paper the results obtained on one large pre-cast element & two large in situ concrete structures will be illustrated. The overall ambient temperature influences the peak temperature reached in the centre of the section. The near to surface temperature fluctuates but the core temperature in all the projects is more stable. The temperature of the concrete is also affected by humidity, wind speed, solar gain, absorptivity, emissivity & thermal insulation. From the work carried out, the following points have been observed: (a) (b) For a 500mm thickness of concrete a 1 rise in ambient temperature generates an average of 1.02 rise in the concrete core temperature. The temperature rise relative to thickness is: Cement Content kg/m3 Mean Ambient Temp Thickness Temperature Rise Relative to Site A Site B Site C Peak Temp. Relative to Thickness

7 (c) Temperature rise per 100kg/m 3 cement: Site A Site B Site C (d) Temperature rise per 100mm thickness of concrete is between 3 & 5. Conclusions Thermally induced cracking is very widespread on Asian construction sites. In almost all cases the problem is never correctly identified & the occurrence of cracking is often attributed to other causes. Thermal contraction cracking can often pose a future durability problem because in the majority of cases the cracks generated by the contraction process will pass through the entire section thickness thereby exposing the embedded steel reinforcement to probable future corrosion. A considerable amount of time & effort has been spent on some sites in order to minimise the effects of thermal contraction. This can end up becoming a very costly process. The attitude of many contractors is that it is cheaper & easier to let the concrete crack & subsequently inject the cracks with an appropriate sealant. This may end up being a false economy. In the end most cases of thermal cracking are the result of poor site practice combined with a lack of appreciation of simple basic concrete technology. Notes on figures 1 7. Figures 1 to 7 are temperature records of heat development over time for a series of concrete mixes from different sites in Asia. They show the temperature rise near the concrete surface in & in the centre of the concrete plotted against the time in days. For ease of comparison all the concrete mixes contained a cement content of 400kg/m3. No cement replacements were used in the mixes.

8 The mix in figure 1 contained a workability retention admixture, which also retarded the onset of the hydration & heat development. Once the hydration process commenced the peak temperature was reached within 1.5 days. However the concrete had not returned to ambient temperature ten days later. At 2.5 days when the formwork could, in theory, have been struck from the concrete it was 30 above ambient & the core temperature was a further 20 degrees higher. Rapid cooling of the surface could easily have produced a differential of 40 degrees, which would have been enough to crack the concrete. It can also be seen that although the near to surface temperature fluctuates due to changes in ambient temperature the core temperature is only affected in so much as the peak was reached during the night when the ambient was lower. If the peak had been reached at 3 days during the middle of the day it would have been higher because at that time the ambient stood at 40 degrees. The maximum temperature gradient across the section was 25 & the concrete did not show any signs of cracking because it was allowed to cool down slowly & not subjected to thermal shock at an early age. The formwork thickness was only 16mm hence why changes in the ambient temperature are reflected in changes in the near to surface concrete temperature. In figure 2 the mix contained a plasticising retarder. This retarded the onset of the main hydration process to 1.3 days after adding the mixing water. The peak temperature (74 ) was reached at 3.2 days but the temperature almost reached this level at 2.6 days. (72.5 ). Insulated steel formwork was used & left in position for 14 days & the concrete was allowed to cool down slowly. This minimised the presence of thermal cracking & the cracks that did form were only 0.1mm in width. The concrete in figure 3 used rapid hardening portland cement. The increased rate of reaction & heat evolution can be observed in that the peak temperature was reached at 1.6 days after casting, with most of the heat development taking place over 0.6 of a day. At nine days after casting the concrete core temperature was still 20 above the ambient.

9 In figure 4 the concrete also used RHPC cement but the concrete also had a workability retention admixture. It can be seen that despite larger fluctuations in ambient temperature than the concrete in figure 3, the peak temperature attained was one degree lower. That is because, although the peak ambient temperature was higher, the temperature range was larger between the day & night temperatures. Timber formwork was used & this is reflected in only minor changes in the concrete surface temperature because of the insulation effect of the 20mm thick formwork. Figure 5 shows a temperature plot of concrete cast in insulated plastic formwork. There are fluctuations in the near to surface temperature but the core temperature is fairly stable. The concrete was cast in the late afternoon. The concrete temperature at the time of delivery was 38. However the temperature fell, with falling ambient temperature to 21 after 6 hours. After the temperature began to rise the peak was reached in only 0.6days. The initial temperature drop had very little effect on the peak temperature attained.

10 The concrete in figure 6 used RHPC cement & the concrete had an overdose of lignosulphonate retarding admixture. This delayed the onset of the main hydration process to 1.3days. However it is interesting to note that once the hydration process was under way the retarder did not slow down the rate of heat evolution, & it also resulted in a higher peak temperature being attained. In OPC concretes the rate of heat gain tends to be slightly reduced but the peak temperature often exceeds that of non or only slightly retarded concrete.

11 The graph in figure 7, is that for a more traditional concrete, where a non retarding plasticiser was used. The concrete had a 75mm slump at the time of placing & water/cement ratio of The peak temperature of 67 was reached after 1.5 days.