Frequently Asked Questions
In its simplest form, concrete is a mixture of paste and aggregates (sand and rock). The paste, composed of Portland cement and water, coats the surface of the fine (sand) and course (rock) aggregates. Through a chemical reaction called hydration, the cement paste hardens and binds the aggregates together into a rock-like mass that is known as concrete.
Concrete will take an initial set in generally 2-3 hours depending on the air temperature, amount and type of cement in the mix and several other environmental conditions. We recommend that you stay off freshly placed concrete, as much as possible, during the first 3 days. After 7 days, light vehicle traffic is possible. Concrete has its most rapid strength gain over the first 7 days and is considered to be cured and near full strength in 28 days although the process of strength gain can virtually go on for years.
The temperature is a very key part of the curing process. On hot days when curing, the heat causes excessive moisture from the concrete to evaporate. However on the other hand if the temperature drops too close to freezing, hydration will occur at a snails pace. If the concrete is cured below freezing, the water inside the concrete will freeze and completely ruin the internal structure of the concrete. For concrete to gain strength it is essential for the temperature to be moderate. It is extremely important that the temperature of new concrete should not be permitted to fall below 10 degrees Celsius (50 Fahrenheit) during the curing period.
Many people use the two terms cement and concrete interchangeably because they believe there is no difference between the two. However, cement is actually an ingredient in concrete. Concrete is essentially a blend of aggregates and paste. The aggregates consist of sand gravel and/or crushed stone; the paste is water and cement. Cement comprises from 10 to 15 percent of the concrete mix, by volume. Over many years the cement and water harden bringing the aggregates closer together, making the concrete stronger. This process is known as hydration. The process continues for a very long time, therefore concrete keeps getting stronger over many years.
Air-entrained concrete simply put, is concrete with air in the mix. The air creates many tiny air pockets. These air pockets ease internal strain and pressure on the concrete by giving water a place to expand when it freezes. Air-entrained concrete is made with air-entraining agents. The amount of air used in the mix varies but is usually between 4 percent and 8 percent of the total amount of the concrete.
Curing if done correctly greatly increases concrete strength and durability. Curing primarily means to give the hydration process a jumpstart. When concrete is in the curing process, water is available and the concrete’s temperature stays within a set range. This encourages hydration and makes the concrete stronger. The duration of curing usually lasts for five to seven days. New concrete may be wet with sprinklers, hoses, covered with wet burlap, or curing compounds which seal in moisture.
Concrete’s volume will slightly change after it has dried out. Typically concrete shrinks 4mm in 3 meters (1/16 of an inch in 10 feet). Contractors put joints in concrete pavements and floors to permit the concrete to crack in a uniform, straight line at the joint when the concrete shrinks and changes its volume.