Concrete can be used for a wide variety of outdoor projects including driveways, sidewalks and steps. These projects usually require pouring a concrete slab between four and six inches in depth, depending on local codes and individual needs. Properly poured concrete can last for decades, or even centuries. Concrete needs the proper time to cure to reach maximum strength. The concrete curing process is the result of a chemical reaction caused as water evaporates from the materials used in concrete. Generally concrete used in sidewalks driveways and similar outdoor projects needs 28 days at the proper temperatures to fully cure. The weather temperature is a major factor affecting concrete curing times. Pouring concrete in weather that is too hot or too cold can make it impossible for the concrete to cure properly.
Hot weather increases the evaporation rate of the water in the concrete. This is a problem because water rises to the top surface of the concrete. The heat coming down on the top surface of the concrete will cause the top surface of slab to dry faster than the lower part of the concrete. Because concrete shrinks as it dries this will create friction between the two layers that can cause a split or fracture between them. Pouring concrete in weather that is too hot will produce a weak and substandard concrete project. Generally, temperatures above 90 degrees Fahrenheit are considered to hot to pour concrete without taking precautionary measures. Other factors such as the wind speed and humidity level may make it inadvisable to pour concrete at temperatures well below 90 degrees.
Pouring concrete in cold weather is especially challenging. Cold weather causes the concrete curing process to slow down. This slow down in the concrete curing process can cause the water inside the slab to freeze before it evaporates causing an improper cure. Finishing the concrete is another concern in cold weather. The water that rises to the surface of the concrete can freeze before the water can cause a chemical reaction with the other materials in the concrete mix. The concrete finisher must trowel the concrete before the surface water freezes. However, if the water is toweled too soon the water is forced down and sealed beneath the surface of the slab. This can cause a division between the top layer and those below causing the concrete to spall later. Spalling is when the top layer of concrete separates and scales off the lower levels. Generally, it is not recommended to pour concrete when the temperature has been below 40 degrees Fahrenheit for three consecutive days. Concrete should never be poured on top of frozen or icy ground. Concrete can be poured in colder temperatures by taking precautions such as adding accelerants to speed up the cure and placing blankets over the slab to keep it warm and slow the curing process.
The concrete curing process is determined by a variety of environmental factors including the ground and air temperatures and weather the concrete is exposed to direct sunlight. Generally, concrete will cure properly when the temperature is between 55 and 90 degrees Fahrenheit. Many concrete contractors recommend pouring concrete when the temperature remains between 55 and 70 degrees Fahrenheit for the entire 28-day curing period for the best results. Following proper concrete curing procedures will make your driveway, sidewalk or other concrete project stronger and longer lasting.