Mastercivilengineer

What is Hot Weather Concreting?

Hot weather concreting is described as one or a combination of the conditions such as high ambient temperature, high concrete temperature, low relative humidity, and high wind speed that tend to impair the quality of freshly mixed or hardened concrete by accelerating the rate of moisture loss & rate of cement hydration, or otherwise causing detrimental results.

Concrete needs to be prepared mixed, transported, placed, compacted, cured, and protected into consideration the ambient temperatures. Construction practices need to vary depending on the climatic conditions, often requiring a complete change, especially in countries that face very cold winters and hot summers.

Countries, which have moderate ambient conditions, are least problematic to concrete producers. The ambient temperature range between 10^oC to 16^oC is supposed to be the ideal condition. The Indian sub-continent is generally divided into three types of climatic conditions i.e. dry-tropical, wet-tropical, and desert.

The Hot Weather Conditions Create Following Difficulties in Fresh Concrete:-

• Increased water demand.
• Rapid loss of workability or slump of concrete.
• Rapid setting and hardening of concrete.
• Increased possibility of plastic cracking because of rapid surface drying conditions.
• Critical need for early curing.
• Difficulty in controlling entrained air content.
• Adding water at the job site is not advisable as it can adversely affect many important properties in the fresh and hardened concrete and reduce serviceability during its resulting in the following defects:-
  • Decreased strength.
  • Decreased durability and water tightness.
  • Non-uniform surface texture.
  • Increased likelihood of drying shrinkage.
  • Increased tendency of differential thermal cracking.

To avoid the above situation, the concrete mix must be designed considering the ambient conditions in the field.

Precautions Recommended For Hot Weather Concreting

Recommended practice for hot weather concreting published by the Bureau of Indian Standard IS 7861 (Part-1) defines hot weather concreting as any operations of concreting done at atmospheric temperatures beyond 40^oC or any operation of concreting where the temperature of concrete at the time of placing is expected to be beyond 40^oC. However, many international specifications recommended the temperature of concrete between 29^oC to 32^oC.

However, the concrete temperature may vary from site to site depending on materials and equipment used and practices followed. The wider range of concrete temperature control that will allow good results is between 24^oC to 38^oC. As there is nothing likes an ideal concrete temperature, it is important that realistic conditions are considered and then temperature limitations exercised. If problems such as cracking or rapid drying or loss of slump are observed, the temperature of concrete will need to be towered further.

Temperature Control

Concrete temperatures increase due to hydration of cement and hot ambient conditions. Due to this water is lost in the hydration process and evaporation. This loss of water thereby creates a loss of slump making the concrete stiff and unworkable. This in turn results in difficulties in placing and compacting concrete. Defects like porosity and cold joints can occur which have undesirable effects on concrete durability.

The below figure shows an increase in the water requirement of a typical concrete mix due to an increase in concrete temperature.

The figure shows the study of high-temperature effects on concrete compressive strength at various ages for a typical mix at different temperature conditions at the time of mixing, casting, and curing. The higher the concrete temperature higher is the early strength. At a later age concrete at 23^oC give strengths higher than concrete produced at 32^oC, 40^oC, and 49^oC. If the water content of the concrete mixes at higher temperatures is increased to maintain the same slump or workability, without an increase in cement content, the reduction in strength would have been much larger than those seen in the figure.

The setting time of concrete gets reduced at high temperatures and water in the mix is lost due to evaporation and cement hydration. This results in a faster rate of hydration thereby reducing the length of time within which the concrete can be transported, placed, compacted, and finished. Concrete should remain in the plastic state sufficiently long so that each layer can be placed without the development of cold joint or discontinuity. Retarding admixtures can be beneficial in offsetting the rapid hardening process due to high temperature as they chemically prevent cement hydration for some time.

Due to evaporation losses and faster cement hydration, concrete also loses its workability and becomes stiff and difficult to compact. This can result in voids and poorly compacted concrete will have lower strength and durability due to the porosity created by these voids.

There is also a tendency for concrete to crack in hot weather due to plastic shrinkage and drying shrinkage. Rapid early drying conditions due to hot weather are mainly responsible for plastic shrinkage. In the case of hardened concrete, drying shrinkage cracks can develop in hot weather due to the addition of a higher quantity of initial mixing water, poor curing, and volume change in concrete caused by cooling from high temperature at the time of placement to low temperature in service.  

In massive concrete structures, of large dimensions, the difference between internal and external temperatures causes stresses due to high thermal gradients. This may cause stresses high enough to crack the concrete. However, this does not apply to normal building structures consisting of slabs, beams, and columns.

Air entrainment is also affected in hot weather. At elevated temperatures concrete requires an increase in the amount of air-entraining admixture to obtain specified total air content.

To counter the detrimental effects of high concrete temperatures, in hot weather efforts are required towards reducing the concrete temperatures at the time of placement. Cooler the concrete temperatures at the time of placement, lesser are the detrimental effects on concrete in hot weather.

Temperature Reduction of Concrete

The usual method to maintain low concrete temperatures is to control the temperature of concrete materials in the mix. The general practice is to lower the temperature of mixing water to as low as possible. However, the concrete temperature is greatly influenced by the temperature of aggregates. Aggregates being the major ingredient in the concrete mix, therefore, need to be cooled either by spraying chilled water or by covering them thereby preventing exposure to hot ambient conditions.

The contribution, to the overall concrete temperature, of each material in a concrete mix will be dependent on their individual temperatures, specific heat, and quality. Lowering the temperature of aggregates and mixing water is most effective. As aggregates are present in large quantities at the construction site cooling them can greatly influence the overall temperature of concrete.

There are several methods of keeping aggregates cool. Stockpiles can be shaded & protected from the sun & kept moist by sprinkling water. Since evaporation is a cooling process, sprinkling gives impressive cooling especially when humidity is low. However, sprinkling can cause excessive variation of water on the aggregate surface resulting in large variations of a slump. Refrigeration is another method of cooling aggregates. Aggregates can be immersed in cold water tanks or cool air can be circulated through storage bins or insulated tunnels.

The temperature of cement has just a minor effect on the temperature of freshly mixed concrete due to low specific heat & relatively small quantity present in the mix as compared to aggregates. Cement may be hot when delivered at the site and it loses heat gradually during storage. Some specifications limit the cement temperature at the time of use between 66^oC to 82^oC.

As mixing water comes in touch with all other concrete constituents, lowering its temperature can effectively reduce the overall concrete temperature. It is often necessary to use ice flakes in a certain proportion as a concrete constituent and the quality of water is reduced in equivalent proportion. Ice cubes and large chunks are not recommended for use as they may not melt fast enough before the mixing process of concrete ingredients is over. Ice flakes being much smaller dissolve during the mixing operation itself. When ice flakes melt they impart a greater cooling effect on the concrete mass than if only chilled water is used. Lowering the concrete temperatures results in slowing down the hydration reaction thereby making more water available for workability. Slower the hydration lesser is the heat generated and hence concrete even at a later stage will not gain alarmingly high temperatures which would cause drying shrinkage cracks and other defects.

Careful Selection of Materials in Hot Weather Concreting

To a certain extent materials used in concrete can pose problems in hot weather due to their physical or chemical proportions. Careful selection of materials may help in addition to the other precautions, suggested above in hot weather concreting.

Cement

Fine cement will react fast resulting in rapid loss of slump or workability due to rapid hydration reaction which results in a reduction of free water in the concrete mix. Besides this, it will also liberate heat much faster resulting in further loss of slump due to evaporation. However, this can be offset by the cooling of water and other ingredients.

The chemical composition of the cement can also influence rapid hydration reactions. Cement having high C₃A (Tricalcium Aluminate) and to a certain extent, high C₃S (Tricalcium Silicate) content will react faster and cause similar problems as cement having high fineness. It is preferable to design concrete mixes with minimum possible cement content in hot weather conditions. However, care must be taken to satisfy both strength and durability requirements. Concrete mix with less cement will evolve less heat and thereby evaporation and hydration losses of water will reduce considerably. Less cement will also reduce to some extent drying shrinkage cracks in concrete.

Aggregates

It is often observed that not much care is taken in the selection of aggregates as it is taken in the case of the selection of cement and chemical admixtures. The shape, impurities like silt/crusher dust, grading, and specific gravity of aggregates influence the cement content of the mix. If these properties of aggregates are not proper then it will lead to high cement consumption which is not advisable in hot weather concreting.

Aggregates have to be clean, properly shaped, well-graded, and dense. Natural rounded aggregates will be preferred as the concrete mix will require less water and therefore less cement to achieve the required strength and workability. Porous aggregates have a tendency to absorb more water from the concrete mix thereby resulting in a rapid drop of a slump which is not desirable in hot weather. The maximum size of aggregate, if possible, should be increased in hot weather. This results in less cement requirement which is beneficial for concrete mixes in hot weather.

Chemical Admixtures     

Slump retention admixtures are most suitable for hot weather concreting than water-reducing admixtures or superplasticizers. In hot weather, concrete loses its workability fast due to various factors. If slump retention admixtures are used they retain the workability of concrete for a longer period. These chemical admixtures slow down the hydration reaction between cement and water thereby more water available for workability or slump.

Retarders are often used in hot weather more specifically in large concrete pours or massive sections where concrete is placed in layers. Retarders increase the setting time of concrete thereby giving more time for consumption. In massive structures concrete is poured in layers. To avoid cold joints between layers retarders are useful as they delay the initial set of concrete in the lower layer for an additional period than normal so that when the fresh layer above is completed the needle vibrator also pierces the lower layer and vibrates it thereby bonding the two layers together.

Often retarders cum plasticizers are also used to improve both plasticity and retardation of concrete.

Preparation before Hot Weather Concreting  

There are certain precautions necessary before concrete is mixed, transported, placed, compacted, and cured. Mixers, chutes, belts, hoppers, pumps, pump lines, walkways, and other equipment handling concrete should be shaded, painted white, or covered with wet burlap or hessian to reduce the effect of high ambient temperature.

Steel reinforcement, formwork & subgrade must be fogged with cool water just before concrete being placed on the surface. Fogging the area during placing & finishing of concrete operations cools the contact surfaces & surrounding air and increases its relative humidity.

Fogging lowers the temperature rise in the concrete and reduces the rate of evaporation of water from concrete after the placement of concrete. For slabs on grade, it is recommended to moisten the subgrade a day before concreting. However, care should be taken to see that water or puddles are not present when concrete is placed.

In hot weather, it is also recommended to place concrete either in the early morning or in the evening, or at night. As ambient temperatures during these periods are lower, it results in less thermal shrinkage and cracking of thick slabs and pavements as well as reduces the slump loss during transportation and placement.

Transporting, Placing and Finishing During Hot Weather Concreting

In hot weather it is a must, to plan to transport, place, and finishing activities so that all operations are performed as quickly as possible. Delays contribute to loss of slump or workability and an increase in concrete temperature. An adequate number of workers and equipment must be available at the job site, to handle and place concrete. Standby equipment generally does not function when the main equipment has a sudden breakdown. It is therefore advisable to also use standby equipment continuously and increases the capacity of concrete handling equipment. In such cases even if failure of one of the equipment occurs, adequate capacity is always available without loss of precious time.

Prolonged mixing, even at agitating speed must be avoided. If delays happen, the heat generated by mixing can be reduced by stopping the mixer and then agitating intermittently. Specifications for Ready Mixed Concrete require the discharge of concrete to be completed within 1½ hrs or before the drum has revolved 300 times, whichever occurs first. During hot weather, the above period can be reduced to 1 hr. or even 45 minutes.

In hot weather, concrete tends to lose slump and hardened more rapidly. It is therefore necessary that concrete is placed as quickly as possible so that proper compaction is achieved. In hot weather, care has to be taken in placing techniques also to avoid cold joints. When a fresh layer of concrete is placed over the earlier layer care must be taken to see that the time lapse between the two layers is such that the lower layer is still good enough to be compacted along with the concrete placed on the top layer.

Floating and finishing should be done promptly after the water sheen disappears or when concrete is hard enough to support the weight of a human being. Finishing on a dirty, hot, and windy day requires extra care as the rapid drying of the concrete at the surface may cause plastic racking.

Plastic Cracking in Hot Weather Concreting

Plastic cracking is generally associated with hot weather concreting. It occurs when atmospheric conditions produce rapid drying conditions due to high ambient temperature, low humidity, or high winds. Cracking is possible if the evaporation rate exceeds 0.5 kg/m²/hr.

Plastic cracks appear when water evaporates from the surface faster than it can bleed to the surface. This creates tensile stresses on the surface due to rapid drying conditions and results, in short, irregular plastic shrinkage cracks. Such cracks are often right through the slab or pavements. Simple precautions are therefore necessary to minimize the possibility of plastic cracking and should be seriously considered while planning hot weather concreting. These precautions are listed below in order of sequence of construction.

• Subgrade and formwork is required to be moistened.
• Aggregates if dry & absorptive must be moistened.
• Keep the temperature of concrete as low as possible by cooling aggregates and mixing water or by using crushed/flaked ice.
• In case of high winds, install wind breakers to prevent wind blowing over freshly placed concrete surface.
• Effect temporary shades to protect concrete surface from the hot sun and to protect concrete while transporting from losing workability or slump due to evaporate of water.
• Protect concrete by placing temporary polyethylene (white or transparent) sheets if there is any delay between placing and finishing.
• Reduce time between all operations especially between placing and start of curing.
• Protect concrete immediately after finishing minimizing evaporation. This can be done by spreading a white or transparent polyethylene sheet firmly placed on the concrete surface or by application of moisture to the surface by continuous fog spray. The above protective measures should continue till such time wet curing or curing using a curing compound is commenced. Wet burlap or hessian or continuous use of polyethylene sheets over the concrete surface will assist wet curing.

It cracks appear in freshly placed concrete they can be sealed by striking afloat on either side of the cracks. However, cracks can reoccur if the causes are not corrected.

Curing and Protection of Concrete at an Early Age in Hot Weather

Curing and protection of concrete at an early age is of utmost importance and needs to be over-emphasized in hot weather as defects developed during the early age due to poor curing and inadequate protection which seriously affect concrete durability.

Curing and protection are more critical in hot and cold weather than in temperature climates. Retaining forms in place cannot be considered a satisfactory substitute for curing in hot weather; they should be loosened as soon as possible without damage to the concrete. Water should then be applied at the top exposed concrete surfaces and allowed to run down inside the forms. On hardened concrete and flat concrete surfaces, in particular, curing water should not be excessively cooler than the concrete. This will minimize cracking caused by stress due to the temperature differential between the concrete and curing water.

The need for adequate moist curing is greatest during the first few hours after finishing. To prevent the drying of exposed concrete surfaces, moist curing should start as soon as the surfaces are finished & continued for at least 24 hours. In hot weather, continuous moist curing for the entire curing period is preferred or better. If moist curing is not continued beyond 24 hours, the concrete surfaces must be protected from a free circulation of drying air with curing paper, heat-reflecting plastic sheets, or membrane-forming curing materials while the surfaces are still damp. Moist cured surfaces should be allowed to dry out slowly after the curing period to reduce the possibility of surface crazing and cracking.

White pigmented curing compounds can be used on horizontal slabs. Utilization of a curing compound during hot weather should be preceded by 24 hours of moist curing. If this is not practical, the compound should be applied immediately after the final finishing of the concrete; the concrete surfaces should be moist.

Preventive Measured Required to be Taken in Hot Weather Concreting

The summary is given below of preventive measures required to be taken in hot weather concreting.

• Prevent aggregate materials from getting heated up at site due to high ambient temperatures by providing shades.
• Prevent water from getting heated up at site by storing in tanks having white or light color and by keeping the delivery pipe line buried and not exposed to direct sun. Shade the water tank.
• For massive structures select cement having low heat of hydration.
• Select slump retention admixtures compatible with cement if mix is to be transported through long distance.
• Select retarders compatible with cement if large pours are to be attempted.
• Cool aggregates by nay one of the several methods available.
• Chill water by any one of the several methods available.
• Use ice flakes or crushed ice to reduce the temperature of concrete. Heat of fusion of ice gives a considerable drop in concrete temperatures.
• Never add additional water to compensate loss of slump.
• Paint all equipment white or in light color and if possible position them under a shade.
• Transit mixers, concrete delivery lines must be covered with damp hessian or burlap.
• Do all concreting operations as fast as possible.
• To speed up work, and to reduce the possibility of stoppages, due to equipment failure, fully utilize your standby equipments.
• Plan the concrete pour properly to avoid cold joints.
• Cool the subgrade, formwork, reinforcement work and equipment by fogging or spraying water prior to commencing concreting.
• Provide wind breakers in areas of excessive wind.
• Floating and finishing of concrete surfaces should be done promptly as soon as the water sheen disappears.
• Ensure adequate and continuous moist curing/fogging for first hour after finishing.
• Ensure continuous moist curing for entire curing period.
• Application of curing compound during hot weather should be preferably done after first 24 hours of moist curing.
• At the end of the curing period surfaces should be allowed to dry our slowly.