Can a warm, breezy afternoon damage a fresh slab before it sets? The Federal Highway Administration warns that evaporation rates as low as 0.3 kg/m2/h may cause critical losses. Hot, windy pours need a plan before the first truck arrives.
Hot, windy weather pulls surface water out of fresh concrete faster than bleeding can replace it. The FHWA guide names wind velocity and concrete temperature as the two dominant variables. It asks for extra caution when wind is over about 2 m/s (about 4.5 mph). Protect the surface early, and follow your project specification for the exact steps.
When Evaporation Outruns Bleeding
Evaporation becomes a problem when water leaves the surface faster than bleeding replaces it. The FHWA guide calls that gap a dangerous condition. It links the gap to plastic shrinkage cracking.
The risk this article covers is that gap. Keep the focus on the surface while the concrete is still fresh, and treat the rate as the early warning.
Two terms carry the whole idea. Evaporation is water leaving the surface for the air. Bleeding is water that rises to the top of fresh concrete after placement. When evaporation outruns bleeding, the surface loses more water than the mix brings up.
The guide measures the problem as a rate in kg/m2/h. Judge the day by that rate, not by how hot or breezy it feels.
Timing matters too. Evaporation is a rate, so a short spell of harsh conditions can matter as much as a long, mild one. Check conditions during the hours you are actually placing and finishing.
This article does not add mechanism detail, because that detail was not verified in this batch. For cracks from other causes, read why concrete cracks and what helps.
Wind and Concrete Temperature Set the Pace
Wind velocity and concrete temperature are the two variables the FHWA guide calls dominant for evaporation. Both push the evaporation rate up, so a hot, breezy day deserves its own plan.
For the number itself, the Concrete Evaporation Rate Calculator does the arithmetic. Use its result as one planning input, not as a pass or fail mark.
The guide names the two drivers together, so watch both at once. A cool, calm morning and a hot, windy afternoon need different plans, even on the same site.
Conditions shift during a long pour. A cool start can become a hot, windy finish, so recheck the rate as the day moves.
Concrete temperature is not the same as air temperature. The guide lists concrete temperature as its own dominant variable, so measure the mix itself when you can. Air readings alone can miss the number that matters.
The sketch below shows how those two drivers connect to the cracking risk that FHWA describes.
Reading the FHWA Figures in US Units
FHWA states its evaporation figures in kg/m2/h. The table below adds a US value for each figure, so you can compare it with pounds-per-square-foot readings.
| FHWA rate (kg/m2/h) | Approximate rate (lb/ft2/h) | What the FHWA guide says |
|---|---|---|
| 0.3 | about 0.06 | Evaporation rates as low as this may cause critical losses. |
| 0.5 | about 0.10 | An evaporation condition at this rate warrants caution. |
| Above 1.0 | above about 0.20 | Definite action to retard evaporation is necessary. |
The third column summarizes the guide. It is not a verdict on your pour.
The conversion uses 1 lb/ft2/h equals 4.88243 kg/m2/h. Hand-check: 0.06 lb/ft2/h times 4.88 is about 0.29 kg/m2/h, which rounds to the 0.3 figure.
This conversion is arithmetic only. It does not confirm any band in the calculator. The FHWA figures come from the guide, while the calculator uses its own planning band.
Units cause more mistakes than the math does. A rate in lb/ft2/h and a rate in kg/m2/h differ by a factor of about 4.9. Confirm both numbers use the same unit before you compare them.
The guide reports only kg/m2/h. Convert its figures to US units for your own records rather than mixing them with calculator output.
What the Rate Means for a 100 Square Foot Slab
A rate per square meter is hard to picture, so convert it to a whole slab. One square foot equals 0.0929 square meters, so 100 square feet is about 9.29 square meters. Hand-check: 100 times 0.0929 is 9.29.
At 0.3 kg/m2/h, the guide’s critical-loss figure, the slab loses about 2.8 kg of water each hour. Hand-check: 9.29 times 0.3 is 2.787, which rounds to 2.8.
At 0.5 kg/m2/h, the caution figure, the same slab loses about 4.6 kg each hour. Hand-check: 9.29 times 0.5 is 4.645, which rounds to 4.6.
Above 1.0 kg/m2/h, where definite action is necessary, the loss exceeds about 9.3 kg each hour. Hand-check: 9.29 times 1.0 is 9.29, which rounds to 9.3.
Water loss grows with area, so a large pad holds more water to lose than a small patio. The rate sets the loss per square meter, and the area sets the total.
This example uses only the guide’s kg/m2/h figures and a unit conversion. It does not use the calculator’s formula, and it does not confirm any calculator band.
Wind Over About 4.5 mph Needs Extra Care
When wind runs over about 2 m/s, FHWA says an extra level of caution is required. That speed is about 4.5 mph.
Two meters per second times 3.6 gives 7.2 km/h. Divide 7.2 by 1.609 to get about 4.5 mph. Hand-check: 4.5 mph times 1.609 is about 7.24 km/h, close to the 7.2 km/h above.
Note the wind at the start of placement, then check again later in the day, because conditions change.
Forecasts often list wind in mph, while the guide uses m/s. Convert the forecast before you compare it with the 2 m/s point.
The guide gives no stop point for wind. It asks for extra caution above the 2 m/s point. That makes the wind figure a trigger for more planning, not a simple yes or no test for pouring.
The guide asks for caution at this speed. It does not list specific actions in the passage this article used, so the next section covers common steps that need local confirmation.
Field Steps to Confirm Before Pouring
The FHWA guide says definite action to retard evaporation is necessary above 1.0 kg/m2/h. The specific actions were not verified in this batch, so the list below reflects common practice. Confirm each step with your project specification or the product guide.
- Plan placement for cooler hours. Lower concrete temperature addresses one of the two dominant variables. Not verified here as a scheduling rule.
- Reduce wind at the surface. Windbreaks or screens are a common option. Not verified in this batch.
- Consider an evaporation retarder. Apply it only as the manufacturer directs. Not verified in this batch.
- Plan curing before the pour starts. For the water side of curing, read Should You Water New Concrete? Wet Curing Explained.
Each option works on one of two things. Cooler placement and wind reduction act on the two dominant drivers. A retarder acts on evaporation at the surface itself. Sorting the options this way helps you ask the right questions of your spec writer.
Keep a short record of wind, concrete temperature, and the protection steps you used, with the times. A simple log gives your spec writer something concrete to review after the pour.
This list is not a full protocol. Your engineer or project specification sets the required steps for your pour.
Where the Calculator Stops
The calculator estimates a rate from the values you enter. It uses its own planning band, so its verdict is not an FHWA figure.
The calculator page says its formula came from one secondary source. This batch did not check that formula against a primary text. The FHWA page also refers to an ACI 308 nomograph but does not reproduce it.
When you compare the calculator with the guide, check three things: the units, the source of each figure, and whether the number is a limit or a planning value. The guide gives figures and cautions from a federal publication. The calculator gives a band from its own planning choice.
Use the calculator for the arithmetic, the FHWA figures for context, and your project specification for the decision. A calculator result is only as good as the values you enter, so measure those values on site.
Estimate the rate with the Concrete Evaporation Rate Calculator, then compare the result with the FHWA figures in this article before you decide on protection.
FAQs About Hot and Windy Concrete Pours
Can wind alone crack fresh concrete?
Wind is one of two variables FHWA calls dominant for evaporation, along with concrete temperature. The guide ties the danger to evaporation exceeding bleeding.
How fast is too fast for evaporation on a concrete pour?
FHWA says evaporation rates as low as 0.3 kg/m2/h may cause critical losses. It calls a 0.5 kg/m2/h condition a reason for caution, and says definite action is necessary above 1.0 kg/m2/h.
How much is 0.3 kg/m2/h in US units?
It is about 0.06 lb/ft2/h. One lb/ft2/h equals about 4.88 kg/m2/h, so divide 0.3 by 4.88. This conversion is arithmetic and does not confirm any calculator band.
Is the calculator’s band the same as an FHWA threshold?
No. The calculator uses its own planning band. FHWA figures come from its curing guide. The calculator band was not verified against a primary text in this batch.
How much wind triggers extra caution?
FHWA says an extra level of caution is required when windspeeds are over about 2 m/s, which is about 4.5 mph. Check wind at the start of placement and again as conditions change.
Does every hot day mean cracks?
No. A hot day can raise concrete temperature, one of two dominant drivers. The risk still depends on whether evaporation runs ahead of bleeding, so check the rate, the wind, and your spec.
What should I check before a hot, windy pour?
Check wind speed, concrete temperature, and the estimated evaporation rate. Then confirm protection steps in your project specification and the product guide, since those steps were not verified here.
Sources and Further Reading
References Used in This Article
- Federal Highway Administration, Guide for Curing Portland Cement Concrete Pavements (FHWA-HRT-05-038), II, Chapter 5, accessed 2026-11-09.
- NRMCA, Concrete in Practice CIP 12: Hot Weather Concreting, accessed 2026-11-09. Quoted: “high wind velocity, and low relative humidity can affect fresh concrete”.
- Concrete in Practice CIP 5, Plastic Shrinkage Cracking (unattributed copy; the copy shows no publisher line), accessed 2026-11-09. Quoted: “Consider using synthetic fibers (ASTM C 1116) to resist plastic shrinkage cracking.”
The evaporation thresholds come from a federal highway manual, not the calculator. Check the project spec and local practice before pouring. Reviewed for accuracy by Prof. Dr. Khalil Mudassar, PhD. Last updated 2026-11-09.
Shakeel Muzaffar is the Founder and Editor-in-Chief, with over 15 years of experience in digital publishing and online tool development. He oversees editorial quality and formula verification to ensure accurate, practical, and easy-to-use calculators.