Issues Of Concreting In Hot Weather Construction Essay

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It is important to take hot weather into consideration while building concrete projects because of its effects on the fresh and recently placed concrete. As of hot weather, the demand for water alone rises. This leads to rises water-cement ratio, and hence further leading to low potential strength. Also to be noted that higher temperatures gives rise to higher slump loss. Concrete cured at higher temperatures when compared with the concrete cured at ambient temperature, would not be as strong at the end of 28 days.

It should be kept in mind that high temperatures, high velocity and low humidity can affect the quality of fresh concrete. The high rate of evaporation introduces early plastic shrinkage or drying shrinkage. Also the evaporation can remove the surface water which is necessary for hydration of the concrete unless proper curing methods are used. Rapid drops in the temperature introduce thermal cracks in the concrete structure. These occur when there are hot days followed by cool nights. High temperatures are largely responsible for cracking in massive concrete structures.

Statement of purpose

The main purpose for this report is to throw light on the problems arising during concreting in hot weather. And also to enumerate various methods via which the problems can be avoided to yield good throughput and quality for the concrete developed. Concreting in hot weather can produce adverse effects on the quality of concrete and structure of building for which it is to be used. So it is important to take into consideration hot weather when planning concrete projects. As the temperature of the freshly mixed concrete reaches 25°C which is approximately 77 °F, it has adverse effects on the quality of concrete. Also to be kept in mind, that temperature above 32°C i.e. 90°F with lack of protected environment for concrete finishing and placement could lead to difficulty in producing quality concrete required. Among other problems, hot weather conditions can lead to rapid rate of evaporation.

Problems and effects of concreting in hot weather

From ACI Materials Journal :" Impact of Extremely Hot Weather and Mixing Method on Changes in Properties of Ready Mixed Concrete during Delivery" by Abdulaziz I. Al-Negheimish and Abdulrahman M. Alhozaimy

Figure Nomograph

The successful hot weather concretion can be done by assessing the factors which affect the quality and stability of concrete and then planning to minimize their affects. Hydration of concrete can be seen as an exothermic reaction, which means it generates heat and also the reaction rate is faster when the concrete is hot. The main concern we identify here is not the air temperature but the concrete temperature. When concrete settles, it sucks up water, which causes crystallization of particles around the aggregate particles. When the concrete is hot, the reaction rate is faster and hence crystals are formed rapidly, but they don't have time to grow strong. Initial strength might be high but the strength seen after 28 days suffers highly. It is seen that if the concrete's temperature is higher by about 18°C the compressive strength decreases by 10%.The other problem which arises while concreting during hot weathers is surface drying. There occurs more and more drying and surface linkage if we have hot concrete, sun shining very hot and hot dry winds blowing. Also we should take into consideration the plastic shrinkage cracking caused in the hot weather. If outside temperature is very high i.e. the concrete is colder than the atmospheric temperature, the chances are that the water will be condensing on the surface rather than the surface drying out. There arises a major problem when the air is cooler than concrete. Contractors mainly aim to go for cooler concrete in hot weather. In this case too, hot weather can cause plastic shrinkage cracking, this case occurs then the concrete temperature is high and the humidity is low. In order to know,whether evaporation could pose a problem or not we use nomograph. By knowing air temperature, concrete temperature, wind velocity and relative humidity, we arrive at the rate of evaporation. Accordingly, if the rate is greater than 0.1 pounds per square foot per hour, shrinkage cracking is possible.

Hot dry sub grades and frameworks may also lead to cracks by absorbing water from the mix. Heat and concrete are also challenged by thermal differentials. This means that one part of the concrete is warmer while the other part is colder. It is observed that if the differential is greater than somewhere around 20°F, then cracks are likely to be formed.

Effects on the characteristics of the concrete formed

Setting time

The setting time of the concrete decreases with the increase in the concrete temperature. This in turn gives us very less time to place compact and finish the concrete.

Workability and slump

Higher temperatures of the concrete reduce the workability of the concrete more rapidly with time. Addition of water to improve workability of the mixture decreases the strength and increases the permeability of the concrete.

Compressive Strength

High concrete temperature and high water requirement can lead to lower 28 day strength. In case more water is added so as to maintain workability, it leads us to higher water-cement ratio which results in loss of strength and durability. This increases the drying shrinkage of the hardened concrete. In the opposite case, if the water is not added the reduced setting time and lower workability increase the potential inappropriate compaction, this results in formation of cold joints and poor finishing.

Concrete temperature

From Concrete Journal (2001) "Properties of Hot Weather Concrete and Countermeasures at Practice".

During hot weather conditions, temperature rise in the concrete may happen due to heat of hydration. The development of thermal gradients may lead to thermal cracking in the concrete slab. Although at higher concrete temperature, the early strength is higher, its long term affects a low strength of the concrete, where as the concrete cured at lower temperature has higher strength. Its can be seen in the laboratory tests that the increased exposure to higher temperature directly affects the strength of the concrete after it has hardened.

Poor surface appearance

Increased rate of evaporation leads to drying and stiffening of the surface. Among the cases of flatwork, this can lead to premature finishing of the surface, trapping some amount of water within the mix. The compacter layer (from finishing) causes rise in the water to be trapped below the surface and hence deboning of the surface. There may occur colour differences on the surface due to different rates of hydration and cooling.

Plastic Shrinkage Cracking

From American Concrete Institute: Journal titled "Plastic Shrinkage Cracking and Evaporation Formulae" by Paul J. Uno.

As discussed widely above, hot weather accelerates evaporation and loss in moisture from the surface. Plastic Shrinkage cracking occurs when the shrinkage stress exceeds tensile strength of the concrete. Plastic Shrinkage cracks formed may be quite deep because concrete has little capacity to resist shrinkage stress. Also the cracks continue to widen and form more and more cracks until the stress is relieved. It is also to be noted that a few times the Plastic Shrinkage Cracks reach to the free edges where unrestrained stress can occur.

Thermal cracking

From American Concrete Institute: Journal titled" Retempering of Prolonged-Mixed Concrete with Admixtures in Hot Weather" by Dan Ravina.

Concrete is exposed to thermal cracking when the concrete is initially placed and the interior of the concrete is exposed to heat rising from the settlement of the concrete and the heat of hydration. Rapid changes in the temperature on the surface of the concrete such as hot days followed by cool night leads to formation of temperature gradient. The warmer interior restrains the colder exterior, which wants to contract. If the differential of the temperature is large then thermal cracking may occur. The insulating effect is more in massive thick concrete, and hence they are more at risk towards the thermal cracking effect.

Precautions or Rules to be followed for hot weather concretion

From Science Direct : Journal titled" Water evaporation from freshly placed concrete surfaces in hot weather " by G.S. Hasanain, T.A. Khallaf and K. Mahmood.

and

From Concrete Journal (2001) "Properties of Hot Weather Concrete and Countermeasures at Practice".

Precautions should be taken to reduce the effects from high temperature against concreting. Use of water reducing, set retarding mixtures can help cutting the effects of high temperature concreting. In the cases where extreme heat conditions are unavoidable, chilled water or ice can be used as part of mixing water. Other methods such as sprinkling and shading the aggregate before mixing can also help lower the temperature of the concrete. Condition where there's low humidity and high winds, windbreaks and sunscreens or mist fogging can be used to avoid plastic shrinkage cracks in the slabs.

Many of the following suggestions can be used depending on the atmospheric conditions and requirement type of the concrete to be formed:

Concrete mixture designs can include set retarders, water reducers in order to gain lowest cement factor. Modification in the mixtures as appropriate i.e. changes in the composition percentages of retarders, moderate heat hydration cements, and Pozzolanic mixtures can be done to shield against hot weather concretion.

Enough manpower can be employed so that as soon as the concrete is delivered, it is quickly placed, finished and cured.

Mixing water and cool aggregates can be added to concrete mixture to reduce its initial temperature.

Using a concrete consistency can allow rapid placement and consolidation.

Sometimes in extreme conditions, we can adjust the time of concrete placement to take advantage of the time of the day when it is cooler i.e. early morning or night placement.

Fogging the area above the concrete placement so as to raise the level of relative humidity can help satisfy the demand for moisture from the air.

Rate of evaporation can be monitored using a nemograph which charts temperature, relative humidity and wind velocity.

Covering of the placed slabs should be done with the damp sand beds rather than polythene sheets if barrier is required.

When forms are removed, curing should be done to the newly exposed surfaces of the concrete.

It should be noted that aliphatic alcohols are not a substitute for curing compounds and it is advised to not to used them. Aliphatic alcohols are used for initial screeding and finishing operations. Although aliphatic alcohols are compatible with most of the curing compounds, it is a good practise to check their compatibility in case the two products are purchased from different manufacturers.

Another cut down can be done at the addition of water while at job site. Addition of water should only be done at the arrival only to adjust the slump. Later additions should be avoided. In no circumstances should the addition of water exceed 2 or 2.5 gallons per cubic yard. Water should never be added to concrete which is older than 1.5 hours.

Finishing should be done as soon as the sheen has left the surface, this should be followed by immediate curing. Curing should be continued for 3 days and covering should be done to prevent evaporation or a liquid curing compound should be used. A very useful idea is to add white pigment to the covering liquid curing compound so that it reflects the heat away from the concrete surface.

Moistening of sub grades, forms and reinforcement before it is placed. However it should be noted that standing water in such cases should be avoided.

Field test cylinder should be protected by shading and preventing evaporation. Field curing boxes with ice may be used to maintain temperature of 60°F -80°F for the cylinders.

Minimizing the effects of hot weather conditions

Controlling the concrete temperature

From Concrete Journal (2001) "Properties of Hot Weather Concrete and Countermeasures at Practice".

AS 1379 states requirement that temperature of the concrete at the time of delivery should be from 5 to 35°C. In cases of higher ambient temperature, the supplier needs to take precautions to ensure that the concrete temperature is delivered is within the allowable range. There are a number of methods available to control the temperature of the concrete. This includes adjusting the temperature of the ingredients of the concrete mixture. It should be taken into consideration that since the aggregates constitute the bulk of the concrete and have the highest heat capacity, the effect the temperature of the freshly mixed concrete greatly. But the temperature of the aggregates is the most difficult task to control. The temperature of the cement doesn't affect much in this case as it has low specific heat and relatively small amount in the mixture. Liquid nitrogen can be injected into the concrete while mixing to lower the temperature. The latent thermal energy of the gas cools the mixture drastically. Although this process is economical only on major projects involving vast amounts of concrete for construction.

Admixtures

From Science Direct : Journal titled" Water evaporation from freshly placed concrete surfaces in hot weather " by G.S. Hasanain, T.A. Khallaf and K. Mahmood.

Various types of admixtures can help lower the temperature and also reduce the evaporation from the surface. Water reducers can be used to decrease the amount of water required to bring concrete to a workable condition. Set retarders delay the amount of time required for setting of the concrete and hence increase the strength of the material. Although set retarders should be used with caution as rapid dry of surface is occurring at the same time. This can lead to finishing of the surface while below it is still spongy due to the set retarders. Thos can lead to non-uniformity on the surface.

Cement type

From Concrete Journal (2001) "Properties of Hot Weather Concrete and Countermeasures at Practice".

There can also occur additional benefits while choosing the type of cement. For example using slower hydration cements such as type LH with lower heat development rate can give extra time for placing and finishing. This also reduces concrete temperature and risk of thermal cracking upon cooling of the concrete.

Cement content

From Concrete Journal (2001) "Properties of Hot Weather Concrete and Countermeasures at Practice".

The temperature increase due to hydration of cement in a given concrete is directly proportional to its cement content. Therefore we should choose cement content depending upon our requirement of strength and durability.

Conclusion

Since hot weather concreting involves various challenges to be faced by the project supervisor as well as the workers. Proper pre-planning and organisation of the project can lead to good formation of concrete with good durability and strength. Successful placing and finishing producing high quality concrete can be done at 35°C. A must use of nemograph should be done to monitor the rate of evaporation so that appropriate resources and measure could be taken depending on the rate of evaporation. No doubt Hot weather poses great difficulties for concreting but development of new technologies, techniques and compounds can further reduce the adverse effects.

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