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Ready Mixed Concrete FAQs (Frequently Asked Questions) Why does concrete crack? What does it mean to "cure" concrete? What is meant by "time of set" or "set time"? What is "compressive strength"? What is air entrained concrete and why is it important? Q: Why does concrete crack? There are many reasons that concrete cracks. 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. Drying shrinkage cracking is caused by the loss in volume that is accompanied by the concrete losing water after it has sufficiently hardened and gained strength. Most concrete will shrink approximately 500 millionths, or .05%. This can be more easily expressed as 1/16th of an inch in 10 feet. Plastic shrinkage cracking occurs before the concrete has hardened, and is caused by a rapid loss of moisture at the surface of the concrete. Concrete mixes that take very long to set and are subjected to low humidity and wind are very susceptible to this type of cracking. < back to top > Q: What does it mean to "cure" concrete? 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. Since the extent of the strength development of concrete is determined by the extent of the hydration of the portland cement and water, it is therefore necessary to maintain the water within the concrete so optimum hydration can occur. 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. Concrete curing should continue as long as it takes for the concrete to develop 75% of its’ design strength. In the winter, this may take two weeks, whereas in the summer, this could take only four days. < back to top > Q: What is meant by "time of set" or "set time"? In specific terms, the time of set is defined as the time it takes for mortar wet-sieved through a No. 4 screen to attain 500 psi. In general terms, it is the time it takes for concrete to harden to the point at which it can no longer be worked, consolidated, or finished. What is probably more important to the user is the time it takes for concrete to reach 20-40 psi, which is the strength necessary for concrete to support a finisher without his boots sinking in more than 1/4". For a wall contractor, he is interested in the point in which he can place the next lift of concrete without causing a form failure, while still being able to penetrate the previous lift with a concrete vibrator. < back to top > Q: How is cement made? Raw materials such as limestone, shale, clay, iron ore, and sand are heated in a rotary kiln 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. The raw materials that feed the kiln have to be carefully proportioned to contain the desired amount of calcium oxide, silica, alumina, and iron oxide. When water is added to cement, a chemical process called hydration occurs, the paste then hardens to a stonelike mass. This paste is used as an adhering binder, which glues’ together the stone and sand to form a solid mass called ‘concrete’. Concrete is the most versatile and commonly used building material in the world! Portland cement gets its name from the English mason Joseph Aspdin, who in 1824 patented this product. He said it resembled the natural limestone quarried on the Isle of Portland, a peninsula in the English Channel. < back to top > Q: What is "compressive strength"? Compressive strength is expressed as force per unit cross-sectional area. In other words, a load of 50265 lb. applied axially to a concrete test specimen whose cross sectional area is 12.5664 square inch (in2), then the compressive strength calculates to 4000 in2. Concrete is very strong in compression, but is comparatively weak in tension. For this reason, concrete is often reinforced with steel. < back to top > Q: What is air entrained concrete and why is it important? 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. 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. 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. The American Concrete Institute recommends 4.5-7.5% entrained air in concrete made with 1" maximum size coarse aggregate when such concrete is subjected to severe freeze thaw exposures. < back to top > Q: What is a water reducer? A water reducer is a chemical admixture that is added to concrete to achieve certain concrete properties that would otherwise be impractical or impossible. These concrete properties include one or all of the following:  Higher slump or more fluidityLower water / cement ratioHigher compressive strengthExtended time of set
Water reducers work by neutralizing the electrical charges that are present at the surface of cement particles. These electrical charges cause cement to flocculate, or stick together. By neutralizing these electric charges, the water reducer works by dispersing the cement particles, which results in greater workability and higher compressive strength. Water reducers can reduce the required mixing water by 3% all the way up to 35%, depending on the type and amount used. Typically, water reducers are used to attain 3-6% water reduction. < back to top > Q: What is a slump test? A slump test is performed to determine the consistency of the concrete, which relates to the workability of the concrete. Most specifications limit the slump to 4 inches, which is the vertical distance that concrete subsides after being deposited into a 12" tall cone shaped mold, which is subsequently removed by pulling off the deposited concrete in an upward motion. In theory, and for lack of a more practical means of measuring, the more water that is in the concrete, the greater distance it will subside after the mold is removed, and the higher the measured slump. The slump test does not determine how much water is in the concrete, but it does give a very good indication of the water in the concrete when compared to the designed slump produced by the proportions of the known mix design. < back to top > Prepared by Kevin Kane Aggregate Industries West Central Region Technical Development & Product Manager NRMCA Certified Concrete Technologist |
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