Concrete Surface Evaporation Rate Calculator

Concrete Evaporation Rate Calculator

Based on Menzel Formula (NRMCA Nomograph - SI Units)

Air Temperature (°C): Relative Humidity (%): Concrete Temperature (°C): Wind Speed (km/h):

Formula Used (SI Units)

W = 0.315 × (e_o - e_a) × (0.253 + 0.060 × V)

Where:

• W = Evaporation rate (kg/m²/h)
• e_o = Saturation vapor pressure at concrete temperature (kPa)
• e_a = Vapor pressure in air = (saturation at air temperature) × (RH / 100)
• V = Wind speed (km/h)

Saturation vapor pressure is calculated using the Magnus approximation for accuracy matching the table.

Risk Assessment

• W < 0.5 kg/m²/h: Generally safe.
• 0.5 ≤ W < 1.0 kg/m²/h: Caution – take measures to reduce evaporation.
• W ≥ 1.0 kg/m²/h: High risk of plastic shrinkage cracking – precautions necessary.

NRMCA Nomograph Examples

The classic graphical method for estimating evaporation rate:

Evaporation Rate Control in Concrete – Why It Matters

In hot and windy site conditions, plastic shrinkage cracking is one of the most common early-age concrete defects. The key reason?

High evaporation rate from the concrete surface.

To assess this risk, we rely on Menzel's formula, which estimates the rate of evaporation based on:

1. Air temperature
2. Concrete temperature
3. Relative humidity
4. Wind velocity

Why evaporation rate control is critical:

When the evaporation rate exceeds the bleeding rate, the concrete surface dries rapidly.

This leads to plastic shrinkage cracks, surface crazing, and reduced durability.

Loss of surface moisture also affects strength development and finish quality.

As per industry practice, an evaporation rate greater than 1.0 kg/m²/hr is considered critical and demands immediate control measures.

Typical Appearance of Plastic Shrinkage Cracks

Effective Control Measures Include:

1. Scheduling concrete pours during cooler hours
2. Use of windbreaks and sunshades
3. Fog spraying before and after finishing
4. Application of curing compounds or immediate curing
5. Reducing concrete temperature (chilled water, ice)

Windbreaks and Sunshades

Fog Misting / Spraying

Applying Curing Compound

Controlling evaporation rate is not optional — it's essential for durability, performance, and long-term quality of concrete.

Understanding and applying Menzel's formula at site helps engineers take proactive decisions, preventing defects rather than repairing them later.

NRMCA Nomograph Examples (Based on Menzel Formula)

The classic graphical method for estimating evaporation rate:

Prepared based on standard concrete technology practices and NRMCA guidelines.
Original document by Balkeshwar Rai (3-page PDF on Menzel Formula).