Q) What are the differences between concrete and cement?
A) Cement is a substance made by burning a mixture of clay and limestone and it is used as an ingredient in mortar and concrete. Cement is also defined as a substance, which by hardening between two surfaces to which it adheres, binds them together.
Concrete is a composite material which consists essentially of a mixture of Portland cement, aggregate, and water. When a concrete truck is seen driving down the road, it contains ready mixed concrete, meaning a mixture of materials forming concrete.
Q) How is cement made?
A) Materials that contain appropriate amounts of calcium compounds, silica, alumina and iron oxide are crushed and screened and placed in a rotating cement kiln and are heated to approximately 2700°F to form a partially molten mass. This partially molten mass is called clinker. The clinker is cooled and ground into a fine powder called Portland cement. When water is added to cement, a chemical process called hydration occurs, the paste then hardens to a stone like mass.
Q) Are there different types of concrete?
A) Concrete can be made thousands of different ways. Numerous types and sizes of sand, rock, gravel, cements, and chemicals can make concrete do almost anything. Concrete can be made of varying strengths, densities, colors, and characteristics, each serving varying purposes.
Q) Why does concrete crack?
A) Concrete can crack from a super-imposed load that is greater than the tensile strength or the compressive strength of the concrete. Concrete can crack from rapid thermal changes. But the most common type of cracking, shrinkage cracking, is due to volume changes within the concrete. As concrete changes volume, usually shrinking, the tensile strength of the concrete is exceeded by the strain of the volume change, and the concrete cracks. Shrinkage cracks can occur long after the concrete has hardened, shortly after the concrete has hardened, or even before the concrete has hardened.
Q) What is compressive strength?
A) Compressive strength is expressed as force per unit cross-sectional area. Concrete is very strong in compression, but is comparatively weak in tension. For this reason, concrete is often reinforced with steel. The compressive strength of concrete ranges from 500 psi for flowable fill to as high as 15,000 psi for high performance concrete.
Q) What is air entrained concrete and why is it important?
A) Air entrained concrete has an intentionally added air entraining admixture that puts microscopic air bubbles into the concrete. The purpose of the air bubbles is to provide durability for concrete exposed to freezing and thawing. Water that is absorbed by the concrete expands when it freezes. The air bubbles give the water room to expand, which greatly reduces the internal pressure created when water freezes.
If the concrete were not air entrained, this internal pressure would rupture the surrounding concrete, causing severe deterioration of the concrete. Concrete that is wet in service, such as a sidewalk or a driveway, and exposed to freezing and thawing, must be air entrained to be durable. The American Concrete Institute recommends 4.5-7.5% entrained air in concrete when such concrete is subjected to severe freeze thaw exposures.
Q) How do I determine how much concrete I'll need?
A) Click on the easy-to-use Concrete Calculator located on our main page to determine how much concrete you'll need for your project. If you woould like to speak to someone about delivery information, click here for information on contacting our friendly dispatch department.
Q) What goes underneath the concrete?
A) Concrete needs to sit on a stable, compacted base. Organic materials and clay should be avoided. The subgrade must be made reasonably uniform, with no abrupt changes in degree of support and with subgrade soils that are of uniform material and density. Attention to this aspect of pavement construction is often neglected.
Soft spots that show up during construction should be excavated and recompacted with the same type of soil as in the adjacent subgrade. Uniform support cannot be obtained merely by replacing the soft spot with granular material.
Q) Can fibers replace wire mesh or welded wire fabric in a concrete slab on ground?
A) One of the main reasons why wire mesh or fibers would be used in a concrete slab is for crack control. The Portland Cement Association, however, states that cracks in slabs on ground can be controlled with uniform subgrade support and proper joint spacing.
Although short joint spacings alleviate the need for reinforcement, wire mesh will allow for increased distance between joints if correctly placed in the upper portion of the slab, at least 2 in. below the surface.
Plastic fibers should not be expected to replace wire mesh in a slab on ground. However, plastic fibers are used to reduce plastic shrinkage cracking. Plastic shrinkage cracks are those that occur immediately following concrete placement, before the concrete has hardened. Polypropylene fibers are among the most common for controlling plastic shrinkage cracking. For more information regarding fibers in concrete, please visit our Concrete Reinforcement Page.
Q) What if I need concrete someplace that’s located away from the driveway or street?
A) Use a pump to place the concrete. AVR, Inc. & Affiliates features a full line of the latest pumping equipment to place concrete in hard to reach locations. For more information regarding concrete pumping, click here.
Q) What precautions do I have to take during cold-weather concreting?
A) The current American Concrete Institute definition of cold-weather concreting, as stated in ACI 306 is, “a period when for more than 3 successive days the average daily air temperature drops below 5°C (40°F) and stays below 10°C (50°F) for more than one-half of any 24 hour period.”
Rule number one is that ALL concrete must be protected from freezing until it has reached a minimum strength of 500 psi. In addition, whenever air temperature at the time of concrete placement is below 5°C (40°F) and freezing temperatures within the first 24 hours after placement are expected, the following general issues should be considered:
- Adjustment of construction schedule regarding loads imposed on the new concrete structure.
- Placing and curing temperatures to produce quality concrete.
Q) How long does it take for concrete to set up?
A) Concrete under normal temperatures will set in approximately four hours. If set time is an issue, it can be sped up with an accelerating admixture or slowed down with a retarding admixture. Our friendly dispatchers can help you with your questions on accelerator or retarder.
Q) What should I do to the concrete when the placing and finishing are completed?
A) The concrete must be provided with moisture to complete the hydration reaction of cement.
Q) What does it mean to "cure" concrete?
A) Curing is the maintenance of moisture and temperature within the concrete for a period of time after it has been placed. The extent of the curing has a great influence on the hardened properties of the concrete, such as compressive strength and durability. The length of the curing period depends largely on the maintained temperature of the concrete. Hydration occurs much slower at colder temperatures than it does at warmer temperature.
Q) How do I protect my new concrete?
A) To protect your new concrete do not drive on it for 3-4 days.
You should cure your concrete immediately after the final finishing has been completed. You should seal your concrete every 2-3 years.
Sealing concrete is done at least 30 days after the concrete has been placed. This process provides a barrier to prevent water and other chemicals from penetrating the concrete. Sealing is critical to exterior concrete to ensure durability and resistance to freeze-thaw cycles. The preferred method to apply these products is with a low-pressure sprayer.
AVR, Inc. & Affiliates recommends two methods to cure and seal concrete:
- Method #1 is to cure with “Apple Cure and Seal” immediately after final finishing is completed. Seal the concrete 30 days later with “Apple Cure and Seal”. Apply the “Apple Cure and Seal” at a rate of about 300 square feet per gallon.
- Method #2 is to cure the concrete with “Apple WB Dissipating Cure” immediately after final finishing is completed. Seal the concrete 28 days later after the cure has gone away (an aggressive wash may be necessary) with “Apple Siloxane”.