Frequently Asked Questions Regarding Concrete Mixing, Installation, Maintenance and Safety
Some of the questions below also appear on our Ask the Experts page. If you have a question to send us, please complete the form on that page.
What is ready mix concrete?
Ready mix concrete refers to concrete that was mixed at a central location and then hauled to the project site. Prior to ready mix concrete, the concrete was mixed on-site until about 1920 when the idea of mixing at a central location and then hauling it to the project site became popular. This innovation improved quality and consistency and was so convenient for contractors that it became the preferred method of producing and using concrete. The term “Ready-Mixed Concrete” caught on because the concrete was already mixed when it arrived at the jobsite.
Is there a difference between cement and concrete?
Yes, there is a difference. Cement, or portland cement, is a finely ground powder made from limestone and other raw materials. The materials are blended together and fired in a kiln at extreme temperatures. The resulting stone sized pieces are then ground into a fine powder that we call portland cement. Portland cement is the active ingredient in concrete.
Concrete is the product used in buildings, bridges, road paving, sidewalks, patios, etc. Concrete is a mixture of portland cement, water, aggregate (sand and stone), and miscellaneous chemical admixtures. Concrete is mixed at a concrete plant facility and delivered to customers in revolving drum truck mixers, also referred to as concrete trucks or mixer trucks. Concrete is sometimes generically referred to as “ready mix” because it is ready to use or already mixed upon arrival at the project location.
Are there any safety concerns regarding concrete?
Yes. Fresh (wet) concrete is highly alkaline and can cause skin irritation, severe third degree chemical burns, and serious eye damage. Always wear protective gloves and glasses or goggles when working with wet concrete. Waterproof boots must also be worn if standing or walking in wet concrete. If clothing becomes contaminated with wet concrete, it should be removed at once and the affected body area(s) washed immediately. Flush eyes with clean water immediately after contact. Seek medical attention if you have persistent or severe discomfort.
Is concrete a “Do-It-Yourself” project?
Concrete can be a do-it-yourself project provided recommended concrete practices are followed. Some things to consider are subgrade preparation, finishing methods, jointing, and curing. Concrete is a perishable material so it is imperative that it be placed and finished in a timely manner. Additionally, most concrete producers assess “holding charges” if concrete trucks are held at the delivery location in excess of a predetermined amount of time. Concrete is very heavy so it is recommended that you have adequate assistance and the proper equipment and tools when placing and finishing. Please also refer to our Recommended Do's and Don'ts When Color Staining Concrete.
Should you decide to hire a concrete contractor, it is recommended that you verify your contractor’s references.
How is concrete quantity calculated?
Concrete is sold by the cubic yard, in one-quarter cubic yard increments. Concrete producers usually require a minimum purchase of one cubic yard as smaller batches are sometimes less consistent. All measurements should be converted to feet then multiplied, and finally, divided by 27. An example would be a patio measuring 10’ 6” long x 14’ 3” wide x 4” thick. The example would be calculated as: (10.5 x 14.25 x .33) ÷ 27 = 1.83 cubic yards.
Can concrete be placed in extremely hot or cold weather?
Concrete can be placed at various temperatures if precautionary measures are taken. In extremely cold weather, concrete should not be allowed to freeze until it has gained sufficient strength to withstand the damaging affects of freezing. Concrete will lose its workability and set up considerably faster in hot weather.
Can all concrete mixes be pumped?
Many concrete mixes can be placed successfully with a concrete pump. Some considerations that may affect pumpability are type of pump, cement content and aggregate size. It’s best to discuss mix selection with your concrete pumping contractor.
What causes concrete to crack?
There are different types of concrete cracks. The most common of these is related to the natural shrinkage of concrete as it cures and dries. Shrinkage cracking can be controlled or “hidden” by following proper jointing techniques. Different factors and conditions can contribute to each type of cracking. It is important to understand these potential causes and take appropriate action to prevent cracking.
Why does some concrete develop surface deterioration?
Surface deterioration is usually referred to as scaling. Scaling is a local flaking or peeling of the finished, hardened concrete surface, resulting primarily from multiple freeze-thaw cycles. The condition can be aggravated by the presence of de-icing materials such as salt. Any product that is intended to melt ice and snow can contribute to scaling. Even products labeled “Safe for Concrete” can contribute to scaling. The probability that scaling may occur can be reduced or eliminated by using a durable concrete mix and following recommended concrete placing, finishing and curing/sealing practices.
What causes discoloration of concrete?
There are different types and causes of concrete discoloration. Dark, shadowy areas may appear if the fresh concrete was placed over alternating wet and dry areas of the subgrade. Darker areas are typically located where the subgrade was too wet or actually muddy. Temporarily covering concrete with poly sheeting or construction blankets can also cause the hardened concrete to be darker in appearance.
Another possible cause of dark, mottled areas is the uneven application of curing/sealing products. Curing/sealing products should be applied as uniformly as possible with a nozzle that will produce a fine spray or mist. There can also be differences in coloration between concrete slabs placed on different days. This is usually due to slight variations in temperature and weather conditions from day to day. The potential for discoloration can be greatly reduced by following recommended placing, finishing and curing practices.
How soon can new concrete be put into use?
During periods of moderate to warm weather, new concrete could be opened for pedestrian traffic in twelve to twenty-four hours, or when the surface becomes “scuff resistant.” A general guideline is that new concrete should not be opened for vehicular traffic for a minimum of seven days. Using a faster setting, stronger concrete mix can significantly reduce this seven-day period. Please note that concrete gains strength more slowly in colder weather so more time may be needed before putting your new concrete into use.
What are the benefits of fiber reinforced concrete?
Fiber reinforcing will reduce the potential for cracking and improve the impact and abrasion resistance of your concrete slab.