United States
Hong KongCold and hot weather concreting
Cold weather provisions apply below 40 degrees Fahrenheit and hot weather precautions commonly at 90 degrees. Cold slows hydration and risks freezing; heat accelerates it and risks cracking.
Concrete is a chemical reaction, and temperature changes how fast it runs. Both directions cause problems.
The two thresholds
| Condition | Threshold |
|---|---|
| Cold weather provisions apply | Below 40 degrees Fahrenheit |
| Hot weather precautions commonly apply | At or above 90 degrees Fahrenheit |
Cold weather
Hydration slows, strength gain is delayed, and freezing before the concrete has gained enough strength causes permanent damage.
Water expands as it freezes. In fresh concrete that expansion disrupts the structure and the strength never recovers.
- Heat the mixing water or aggregates
- Insulate or enclose and heat the placement
- Extend the curing period
- Do not place on frozen ground
The ground thaws later and settles, and the slab cracks. It is not a curing problem, it is a subgrade problem, and it is why the rule exists separately.
Hot weather
Hydration accelerates, workability is lost faster, and rapid surface evaporation causes plastic shrinkage cracking.
- Cool the mix, sometimes with ice replacing part of the water
- Place early or at night
- Dampen subgrade and forms
- Cure immediately and continuously
- Use windbreaks and fog to slow evaporation
The temptation both ways
In heat, adding water on site to restore workability. It restores the slump and reduces the strength, which is the trade the exam wants you to recognize.
Common questions
What temperature triggers cold weather concreting?
Below 40 degrees Fahrenheit.
What is the hot weather threshold?
90 degrees Fahrenheit is a common threshold for precautions.
Why is freezing dangerous for fresh concrete?
Water expands as it freezes, disrupting the structure before strength has developed, and it never recovers.
Can you place concrete on frozen ground?
No. The ground thaws and settles later, cracking the slab.
What is the hot weather temptation?
Adding water to restore workability, which restores slump and reduces strength.