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When placing concrete and cementitious concrete repair mortars, especially in hot and dry weather, evaporation rate is one of the most critical factors to take into consideration. The evaporation rate is the rate at which moisture is being lost from the concrete or mortar surface, which affects a few elements of a concrete or repair mortar placement. If the evaporation rate is too high, water is leaving the surface too rapidly, and creates difficulty during finishing operations. Equally important is that a high evaporation rate during placement and initial curing greatly increases the likelihood of plastic shrinkage cracking in the concrete surface. Plastic shrinkage cracks leave a concrete surface vulnerable to the future intrusion of water and chlorides and leave the surface with an overall unpleasant appearance.

Evaporation rate is a function of relative humidity, concrete temperature, air temperature, and wind velocity at the time of placement. Here is a brief summary of how each of these properties affect the evaporation rate: Evaporation Rate Table | Preventing Rapid Evaporation When the Heat is On

  • The lower the relative humidity, the higher the evaporation rate
  • The higher the concrete temperature, the higher the evaporation rate
  • The higher the air temperature, the higher the evaporation rate
  • The higher the wind velocity, the higher the evaporation rate

These relationships are summarized in the adjacent “five point” chart from ACI 305, “Guide to Hot Weather Concreting”, which aids in calculating approximate evaporation rates.

Plastic shrinkage cracking becomes a strong possibility when the rate of evaporation exceeds 0.2 lb/ft2/hr (1.0 kg/m2/hr), and measures to control and lower the evaporation rate should generally be considered if it exceeds 0.1 lb/ft2/hr (0.5 kg/m2/hr). Euclid Chemical manufactures several products to assist in mitigating evaporation rate-related problems.

Article originally written by Euclid Chemical and published on www.euclidchemical.com. View the original article here »

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