The American Concrete Institute defines hot weather as any
combination of high temperature, low relative humidity, and wind velocity tending to impair the quality of fresh or hardened concrete or otherwise resulting in abnormal properties.In our area of Utah, a combination
of low relative humidity and windy conditions can lead to hot weather concrete problems at most anytime of the year. High concrete temperature, high air temperature, wind and low relative humidity, or combinations
thereof cause rapid evaporation, which significantly increases the likelihood, that plastic shrinkage cracking or drying shrinkage cracking will occur.
These types of conditions may produce a rapid rate of moisture
evaporation from the surface of the concrete along with increased setting time. The high rate of evaporation can dry and remove surface water beneficial for proper hydration unless proper moist curing methods are
employed.
High temperatures accelerate slump loss, and can cause some loss of entrained-air, and increase water demand resulting in lower compressive strength. Temperature greatly affects the setting
time of concrete.
Proper planning
is essential to minimize the detrimental effects of hot weather. Effort should be made to keep the concrete temperature uniform. Proper scheduling in advance of the pour should be timed to avoid unnecessary delays in delivery and placement. Necessary precautions should be taken to see that concrete is
promptly
placed upon arrival at the job. Adequate, qualified personnel should be available to place, consolidate, and finish the concrete. Preparations for proper curing should be made in advance of the pour. Flatwork in particular should be protected from excessive drying during finishing operations, and each operation should be performed without delay as soon as the concrete is ready for it.
Under hot, dry, windy conditions exposed flatwork surfaces may require curing during the pour
to prevent stickiness, surface sponginess and surface crusting from rapid drying of the surface during the placement and finishing operations.
Curing during the pour
with coverings, fog spray or with evaporation retardants may be needed between screeding and bull floating, between bull floating and power floating and between subsequent floating and troweling passes to prevent excessive moisture loss. Evaporation retardants, readily available at concrete supply stores, are packaged as concentrates when mixed with water and applied to the surface of freshly placed concrete the chemicals develop a monomolecular film that reduces the rate of moisture evaporation. These products can be reapplied during the finishing operations as required to reduce the moisture evaporation rate, however, they should not be used as a final curing compound.
Final curing of concrete should commence immediately after final finishing operations are complete
and as soon as the surface will not be marred or damaged by the chosen method of moist curing in order for the concrete to reach it’s ultimate design strength and to minimize excess shrinkage and cracking. Types of curing methods include water or moist curing, covering material, i.e., burlap or sheeting or ASTM C 309 liquid-membrane forming compounds, readily available at concrete supply stores for application on horizontal and vertical concrete surfaces to reduce the loss of moisture during the early hardening period. The need for final curing is greatest during the first few hours, and throughout the first day after concrete is placed. Providing proper temperature and moisture conditions for curing of concrete is much more critical and important in hot weather than under normal conditions.
