What is the effect of high concrete temperature

What is the effect of high concrete temperature (above 77°F) on transporting, placing, and finishing concrete, and on hardened concrete properties?

A: When the temperature of freshly mixed concrete exceeds 77°F (25°C) there may be a number of effects on the fresh and hardened properties of concrete.One of the effects on fresh concrete properties is accelerated setting, which leads to a shorter time period for transporting the concrete to the job site, and a shorter window for placement, consolidation, and finishing of the material.
The higher temperature also leads to a higher water demand to maintain the concrete at the specified slump, which can tempt the contractor to add water to retemper the mixture leading to lower than expected compressive strength of the hardened material. In addition mixtures at high temperatures may require higher dosages of air-entraining admixtures to produce the required air content for durability in severe climates. The effect on hardened concrete properties are a high early strength but low ultimate strength compared to mixtures placed with a lower temperature. In addition, if no precautions are taken, there is an increased potential for plastic shrinkage cracking during the finishing operations, and increased potential for cracking due to volume changes caused by drying shrinkage and thermal effects.

Q: What precautions do I have to take during hot-weather concreting?

What precautions do I have to take during hot-weather concreting?

I am planning to have a large volume of concrete placed as part of an industrial development project that will be built during the summer of this year. I have been told that the quality of concrete can be affected not only by cold weather but also hot weather conditions. What are the effects of hot weather on concrete, and what precautions should be taken to assure that the concrete will provide good durable service?
A: It is true that hot weather conditions above approximately 25°C (77°F) can adversely impact the quality of concrete. The precautions that should be taken to assure a quality end product will vary depending on the actual conditions during concrete placement and the specific application for which the concrete will be used. In general if the temperature at the time of concrete placement will exceed 25°C (77°F) a plan should be developed to negate the effects of high temperatures. The precautions may include some or all of the following:
1) Moisten subgrade, steel reinforcement, and form work prior to concrete placement.
2) Erect temporary wind breaks to limit wind velocities and sunshades to reduce concrete surface temperatures.
3) Cool aggregates and mixing water added to the concrete mixture to reduce its initial temperature.
4) Use a concrete consistency that allows rapid placement and consolidation.
5) Protect the concrete surface during placement with plastic sheeting or evaporation retarders to maintain the initial moisture in the concrete mixture.
6) Provide sufficient labor to minimize the time required to place and finish the concrete, as hot weather conditions substantially shorted the times to initial and final set.
7) Consider fogging the area above the concrete placement to raise the relative humidity and satisfy moisture demand of the ambient air.
8) Provide appropriate curing methods as soon as possible after the concrete finishing processes have been completed.
9) In extreme conditions consider adjusting the time of concrete placement to take advantage of cooler temperatures, such as early morning or night time placement.With proper planning and execution concrete can be successfully placed and finished to produce high quality durable concrete at temperatures of 35°C (95°F) or more.