The Australian Standard 1379: Specification and Supply of Concrete stipulates that concrete temperatures should be between 5°C and 35°C at the point of delivery. Precautions are recommended once the air temperature falls below 10°C.
As a rule of thumb, 90 degrees Fahrenheit is the cut-off point. If it's over 90 degrees, plan to pour another day. During summer, pour as early in the morning as possible. This tends to be the coolest part of the day and will therefore offer the best chance for moisture retention as the concrete begins to cure.
While most everyone enjoys the warm summer months, the temperature can make it difficult when pouring concrete in hot weather. Concrete cures best in a range between 70° to 80° F., but as your temperature approaches 90° F, you'll start to run into problems.
On a sunny day, asphalt can quickly heat up, even if the air temperature doesn't feel super hot to you. For example, if it's 85 degrees, then the blacktop temperature is 110. If the air temperature is over 100 degrees, then the blacktop can reach 160 degrees.
Problems That Occur When You Pour Concrete in Hot Weather
In conditions hotter than 77 °F, the water used to hydrate and cure the cement can evaporate, increasing the concrete's temperature and presenting problems such as weakness, shrinkage, and cracking.
Explosive spalling is of special concern to tunnel engineers. If a fire breaks out in a tunnel, heat has limited escape routes and can increase temperatures to over 1,000°C (1,832°F) in a short time, causing concrete to explode like popcorn.
The threshold of significant degradation of concrete is around 65-93°C (150-200°F). For this reason, current codes and industry standards dealing with reinforced concrete structures specify a maximum temperature limit of about 65-93°C (15O-2OO°F) to ensure predictable concrete behavior.
Concrete Gets Very Hot
Wood can reach 90°F, composite decking 100°F, but concrete can get as hot as 175°F. That's too hot for most people to walk barefoot on.
On a 95 degree day, concrete can get as hot as 125 degrees in minutes.
While concrete is known to be a very fire-resistant material, it is by no means impervious to it. In fact, prolonged exposure to high temperatures can cause permanent damage, affecting it mechanically and chemically. The most obvious damages are cracks, spalls, & discolorations.
In addition to the potential for shrinkage cracks, hot weather will tend to cause concrete to lose slump (workability) and set much more quickly. Concrete will typically set in about 4 hours in 80o F temperatures, a 10o F increase in ambient temperature will reduce the set time to about 2.5 hours.
Warm temperatures cause cement to set faster, and the faster cement sets, the faster it begins to cure. At 80 degrees, concrete usually sets in about four hours. At 90 degrees, you can expect it to set in 2.5 hours, and at 100 degrees, your concrete will set in roughly an hour and a half.
Plan to pour concrete before 10 a.m. or after 8 p.m. in order to avoid the hottest part of the day. If you are concerned about the temperature while working on a summer project, consider pouring concrete at night to give the mixture the best chance to harden and cure before the sun comes up.
Experts agree that the best temperature to pour concrete is between 50-60 °F. The necessary chemical reactions that set and strengthen concrete slow significantly below 50 °F and are almost non-existent below 40 °F.
Moist curing is a common method of concrete curing. It involves wetting the concrete slab often with water (5-7 times per day) for the first 7 days. This method ensures your concrete slab will be extremely strong and durable, because it allows the moisture to evaporate slowly, preventing cracks and shrinks.
Many construction specialists recommend watering it down five to ten times per day, for the first week, starting 2-4 hours after it has been poured. The first two to four hours lets the concrete “finish”, which refers to the setting process.
Add a concrete sealant.
As the surface expands, there is the potential for damage like cracks and divots. You can help prevent this by coating the concrete driveway or patio with a waterproof sealant. This will keep water from entering, and prevent expansion during high temperatures.
In direct summer sun a dark colored concrete surface can reach in excess of 130-140 degrees F while the air temperature is in the 90s.
Asphalt's dark color absorbs sunlight and releases it slowly, while concrete's lighter color reflects it. This makes asphalt up to 10 degrees hotter than concrete.
Bottom line: Cement is a powdered ingredient used to make concrete. Concrete is the material used to make sidewalks, driveways and buildings. Sakrete makes a number of different concrete formulations in varying strengths and set times to meet the needs of different job requirements.
A good slump for most flatwork placed by hand: five inches. For slabs struck off with mechanical equipment: two to four inches.
Salt eats concrete for breakfast, lunch, and dinner.
However, any chemical containing chlorides, including: sodium chloride, potassium chloride, calcium chloride etc.
Luckily, concrete can withstand heat up to 1,000 degrees. Concrete ingredients like cement and aggregate are virtually non-combustible, plus, concrete's slow rate of heat transfer means your home and possessions are protected from flames and intense heat.
Concrete fireplaces and fire pits should be constructed carefully. When exposed directly to high amounts of heat, the moisture trapped inside concrete can expand and cause the concrete to crack and in extreme situations, explode.
You may not know this, but concrete blocks can actually explode if they get too hot, making your fire pit a serious danger. It's quite possible that building a fire pit may be a lot more complicated than you thought.