Hot weather can also create colour differences in the concrete due to the rates of hydration and cooling effects. Hot weather speeds up moisture loss in the concrete slab which can result in early surface drying that leads to shrinkage.
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.
It's advised that concrete be cured at a moderate temperature between 50 °F – 90 °F. In a perfect world, the temperature of fresh concrete should be above 50 °F, but cured and maintained at 50 °F.
If your concrete is exposed to the elements, you won't be surprised to learn that the concrete will dry faster in warmer, sunnier conditions. While this isn't always an option, choosing to start your project in summer is a surefire way to minimise curing time.
There are a few factors which lead toward soft concrete. Excess water is the biggest culprit but we also have to consider the following; poor or insufficient curing, too little cement, carbonation, incorrect proportion of trace ingredients such as pozzolans or shrinkage additives.
Water facilitates the curing and hardening processes. Without it, the chemical reactions needed to form the hard crystals that give the concrete its strength can't take place. Too little water leads to structurally weak concrete, and too much will disrupt effective curing and cause flaking, shrinking, divots or cracks.
However, it is not completely cured until after 28 days, which is when concrete achieves its full, effective strength—so be sure to avoid using heavy equipment and large vehicles on the slab until then. Waiting for concrete to dry can feel like a chore, especially if it looks dry and ready to go.
As long as the concrete is given 4 to 8 hours after mixing and finishing to set up, rainwater won't affect it nearly as badly as it could. Some concrete mixes will actually benefit from rainwater on the surface because it's beneficial in aiding the hydration and curing process.
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.
Concrete that is not moist-cured at all dries too rapidly, and reaches less than half its potential design strength. It will also have a greater number of shrinkage cracks.
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.
The most efficient way to cure concrete is to use a plastic cover. The covers are placed on freshly poured concrete until the concrete has a chance to dry. Plastic concrete curing covers seal off concrete and prevents water from evaporating.
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.
Add an accelerating compound, such as calcium chloride, to the concrete mix before pouring. This accelerant will help the concrete solidify much faster.
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.
If the rain continues for several days, water accumulation on a concrete surface can slow down the curing process. As a result, the internal bond between mixture particles will be influenced, and concrete strength will be weaker than the designated strength.
Concrete temperatures which exceed air or ground temperatures by more than 35ºF can cause thermal cracking.
At a minimum, concrete should be cured at temperatures above 7.2 degrees Celsius (45 degrees Fahrenheit). Below this temperature, concrete's chemical reaction completely stops – which can result in costly delays and rework.
The ideal curing temperature is 23°C. Cure concrete for at least 7 days. Note: 35°C is the maximum concrete temperature for field placing allowed under AS1379.
After 24 hours, you can safely walk on your new concrete.
If there has been a lot of rain or you've done decorative stamping in the concrete, you should wait longer. Normal foot traffic should be fine on standard concrete at this point, but you should still be cautious about damaging the surface of the new paths.
Check that the surfaces you are pouring onto are dry before you begin pouring concrete. Never pour concrete into water-filled cavities or wet surfaces from which the water cannot be displaced and escape.
The general rule of thumb is that concrete takes about 28 days to dry for every inch of slab thickness. Within 24 to 48 hours, the concrete will be ready for foot traffic.
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.
To put it simply, the goal is to keep the concrete saturated during the first 28 days. The first 7 days after installation you should spray the slab with water 5-10 times per day, or as often as possible. Once the concrete is poured the curing process begins immediately.
Does concrete take 100 years to cure? No, this is a bit of a myth with the concrete industry. While concrete does continue to harden indefinitely, pore moisture has to drop below a certain level at some point and this isn't typically 100 years.