Understanding and Managing Evaporation in Hot Weather Concrete Pouring

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When pouring concrete in hot weather, it’s crucial to consider evaporation. High evaporation rates can lead to problems like workability issues and plastic shrinkage cracks on the concrete surface. These cracks not only compromise the concrete’s integrity but also invite water and chlorides, affecting both durability and appearance.

Importance of Estimating Evaporation

Estimating evaporation at the construction site is essential to determine if it poses a threat to the concrete. ACI 305.1 sets an upper limit for evaporation rates, beyond which protective measures must be taken.

Factors Affecting Evaporation Rate

The evaporation rate depends on concrete temperature, air temperature, relative humidity, and wind velocity during concrete placement. Higher concrete and air temperatures, along with increased wind speed, result in higher evaporation rates. Conversely, lower relative humidity contributes to faster evaporation.

Methods to Measure Evaporation Rate

Paul J. Uno’s Formula : Paul J. Uno’s formula, a simplified version of the Menzel formula, is a quick on-site tool. The formula, given in both SI and US customary units, calculates evaporation rate using concrete temperature, air temperature, relative humidity, and wind velocity.

Equation 1: E = 0.7(Tc + Ta)(1-r) + 4v

Menzel Formula : ACI 305.1 presents the Menzel formula, a more detailed equation. It considers water mass evaporated, saturation water vapor pressure, water vapor pressure in the air, and average wind speed. The formula is provided in both SI and US customary units.

Equation 3: W = e0 – ea + 0.26v

Using Tables for Reference

Tables 1 and 2 offer saturation water vapor pressure values over water at different temperatures, aiding in calculations for the Menzel formula. These tables provide essential data for accurate estimations.

Nomograph for Graphical Estimation

A nomograph, derived from Menzel’s formula, visually estimates evaporation rates based on concrete and air temperature, relative humidity, and wind speed. Figures 1 and 2 illustrate the nomograph for both SI and US customary units.

How to Use Nomograph

  • Enter air temperature and move up to relative humidity.
  • Move right to concrete temperature.
  • Move down to wind velocity.
  • Move left to read the rate of evaporation.

Online Tools for Convenience

Various online tools simplify the estimation process, allowing users to set parameters manually. These tools are user-friendly and provide quick insights into conditions for a desired evaporation rate.

Precautions for Accurate Measurements

  • Use average wind speed, not maximum gust speed, measured at 0.5 m (20 in.) above the concrete surface.
  • Avoid reported wind speeds from weather stations; measure in the shade, upwind of the pouring, 1.2 to 1.8 m (4 to 6 ft) above the surface.
  • Consider concrete bleeding rate, as it influences the likelihood of plastic shrinkage cracking. Low bleeding rates reduce the required evaporation rate.

In conclusion, understanding and managing evaporation during concrete pouring in hot weather is vital for ensuring the quality and durability of the structure. Utilizing formulas, tables, nomographs, and online tools, along with taking necessary precautions, helps achieve accurate estimations and effective preventive measures.