Tarmac: Ready Mix Concrete

FREQENTLY ASKED QUESTIONS (FAQ'S)

There are many frequently asked questions pertaining to concrete and it's applications. Listed below are our expert's answers to some of these questions.

QUESTION: What's the difference between cement and concrete?

ANSWER: Cement (sometime referred to as Portland cement or hydraulic cement) is one component of concrete. Cement is to concrete what flour is to a loaf of bread. Concrete is basically a mixture of 2 components: aggregates and paste. The paste, comprised of cement and water, binds the aggregates (sand and stone or gravel) into a rocklike mass as the paste hardens because of the chemical reaction of the cement and water. This reaction is called hydration.

QUESTION: What makes concrete crack?

ANSWER: Concrete "shrinks" slightly as it hardens. A normal shrinkage rate is approximately 1/8" per 100 linear feet. This shrinkage is caused by loss of excess water from the mix. Obviously, the "wetter" the mix, the higher the shrinkage rate. Control joints should be placed in the concrete at intervals equal to 2.5 times (in feet) the thickness of the slab. For example, a slab 4" thick should have control joints every 10 feet.

QUESTION: Why should fresh concrete be properly cured?

ANSWER: The surface of freshly placed concrete should be kept moist for at least 7 days. Contrary to popular belief, concrete should not be allowed to "dry out." If the concrete is allowed to "dry out," the ultimate strength gain will be considerably less than it's designed strength. Proper curing will also minimize the potential of cracking. The simplest method of moist curing is wetting the surface of the concrete, then covering it with polyethylene.

QUESTION: How is concrete strength determined?

ANSWER: Concrete strength is determined by it's water/cement ratio. Water/cement ratio is defined as pounds of water per pound of cement, or gallons of water per bag of cement in the mix. The higher the water content, the lower the strength of the mix.

QUESTION: How can I make my concrete driveway less prone to damage from freezing weather?

ANSWER: When ordering concrete for outdoor applications (driveways, patios, etc.), always request air entrained concrete. An air entraining admixture can be added to the mix which produces microscopic interconnected voids. The voids permit the water in moist concrete to expand without damaging the integrity of the structure.

QUESTION: Why is concrete a more desirable driveway material than asphalt?

ANSWER: The reasons are many. First, a properly placed concrete driveway has a much longer life expectancy than an asphalt driveway. A concrete driveway is much cooler in the summer months. Residue from an asphalt driveway can potentially be tracked into your home. This problem does not occur with a concrete driveway. There are many architectural applications for concrete. This allows for different colors, textures, etc.

QUESTION: What causes scaling and surface shrinkage cracks?

ANSWER: These defects are generally a result of improper finishing of the concrete. As discussed earlier, the prime factor affecting concrete strength is water/cement ratio. If excess water is added to the surface of the concrete during placement and finishing, the water/cement ratio on the surface may be drastically increased. This condition greatly reduces the strength of the concrete on the surface. Unfortunately, this is where the wear takes place.

QUESTION: What are fibers? Can fibers be used to replace welded wire fabric as a secondary reinforcement?

ANSWER: Fibers are typically polypropylene (plastic) fibers introduced into the mix during the batching process. These fibers serve as a secondary reinforcement in the concrete. In most cases, fibers can replace welded wire fabric as a means of secondary reinforcement.

QUESTION: I've heard the term slump pertaining to concrete. What is slump?

ANSWER: The term slump simply refers to the consistency of the concrete in a plastic state (prior to hardening). Slump is a measure of how wet or stiff the concrete is. Obviously, the more water used in producing the concrete, the wetter (or higher) the slump will be. Again, excessive water causes a dramatic loss of strength. Applications exist where higher slump (wetter) concrete is necessary because of difficulty in placement or specification requirements. These higher slumps can be attained through the use of water reducing admixtures (chemical additives).

QUESTION: How long does concrete continue to gain strength after it is placed?

ANSWER: Concrete's most rapid period of strength gain occurs in the first 7 days. The accepted time standard for measuring concrete strength is 28 days. However, concrete will continue to gain strength for long periods of time (months, even years!) As long as moisture is present to continue the chemical process of hydration.

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