The precautions required to ensure 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 77 degrees Fahrenheit a plan should be developed to negate the effects of high temperatures.
The precautions may include some or all of the following:
Moisten subgrade, steel reinforcement, and form work prior to concrete placement.
Erect temporary wind breaks to limit wind velocities and sunshades to reduce concrete surface temperatures.
Cool aggregates and mixing water added to the concrete mixture to reduce its initial temperature. The effect of hot cement on concrete temperature is only minimal.
Use a concrete consistency that allows rapid placement and consolidation.
Protect the concrete surface during placement with plastic sheeting or evaporation retarders to maintain the initial moisture in the concrete mixture.
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.
Consider fogging the area above the concrete placement to raise the relative humidity and satisfy moisture demand of the ambient air.
Provide appropriate curing methods as soon as possible after the concrete finishing processes have been completed.
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 95 degrees Fahrenheit or more.
Why Use Fly Ash?
The Benefits of Using Fly Ash
Concrete in its hardened state — with fly ash — shows improved performance with:
Greater strength. Fly ash increases in strength over time, continuing to combine with free lime.
Decreased permeability. Increased density and long-term pozzolanic action of fly ash, which ties up free lime, results in fewer bleed channels and decreases permeability.
Increased durability. The lower permeability of concrete with fly ash also helps keep aggressive compounds on the surface, where destructive action is lessened. Fly ash concrete is also more resistant to attack by sulfate, mild acid, and soft (lime hungry) water.
Reduced alkali silica reactivity. Fly ash combines with alkalis from cement that might otherwise combine with silica from aggregates, thereby preventing destructive expansion.
Reduced heat of hydration. The pozzolanic reaction between fly ash and lime generates less heat, resulting in reduced thermal cracking when fly ash is used to replace a percentage of Portland Cement.
Reduced efflorescence. Fly ash chemically binds free lime and salts that can create efflorescence. The lower permeability of concrete with fly ash can help to hold efflorescence-producing compounds inside the concrete.
The ball-bearing effect of fly ash in concrete creates a lubricating action when concrete is in its plastic state. This means:
Increased workability. Concrete is easier to place with less effort, responding better to vibration to fill forms more completely.
Increased ease of pumping. Pumping requires less energy; longer pumping distances are possible.
Improved finishing. Sharp, clear architectural definition is easier to achieve, with less worry about in-place integrity.
Reduced bleeding. Fewer bleed channels decreases porosity and chemical attack. Bleed streaking is reduced for architectural finishes. Improved paste to aggregate contact results in
Reduced slump loss. More dependable concrete allows for greater working time, especially in hot weather.enhanced bond strengths.