Concrete Nomograph (Surface Moisture Evaporation Rate)

[Kelvin Lung]

Hi guys,

I've got a technical question related to a nomograph that can calculate the evaporation rate of the surface moisture on a concrete slab. I have attached a picture of the nomograph for a better visual understanding (can also be found on Google). Basically,there are three graphs representing air temperature, relative humidity, concrete temperature and wind speed. By providing the results of the four factors, the nomograph gives the rate of water evaporation.

In general, at cooler temperatures concrete takes longer to set as opposed to warmer temperatures. In other words, time increases for the surface moisture (bleed water) to evaporate. However, this does not seem to be the case according to the results I found using the nomograph.

For example, referring to the nomograph, I used the following figures to obtain the rate of water evaporation:

Air Temperature: 15°C
Humidity: 50%
Concrete Temperature: 25°C
Wind Speed: 40km/h

Water Evaporation Rate: 1.9kg/m^2/h
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Air Temperature: 35°C
Humidity: 50%
Concrete Temperature: 25°C
Wind Speed: 40km/h

Water Evaporation Rate: 0.5kg/m^2/h

Logically speaking, wouldn't the the air temperature at 35°C have a faster water evaporation rate than the cooler temperature (15°C)?

I am quite confused at this result.

I would really appreciate if someone could share their knowledge on what the explanation is to this?

Thanks so much guys!