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WHat is the Difference between cement and concrete? Although the terms cement and concrete often are used interchangeably, cement is actually an ingredient of concrete. Concrete is basically a mixture of aggregates and paste. THe aggregates are sand and gravel or crushed stone. portland cement is not a brand name, but the generic term for type of cement used in virtually all concrete. cement comprises 1o to 15 percent of the concrete mix, by volume.
what does it mean to "cure" concrete? curing is one of the most important steps in concrete construction, because proper curing greatly increases concrete strength and durability. concrete hardens as a result of hydration: the chemicals reaction between cement and water. however, the hydration occurs only if the water is available and if the concretes temperature stays within suitable range. during the curing period from five to seven days after placement, the surface needs to be kept moist to permit the hydration process. new concrete can be wet with soaking hoses, sprinklers, or covered with wet burlap or can be coated with commercially available curing compounds, which seal in the moisture.
can it be too hot or too cold to place new concrete? temperature extremes make it difficult to properly cure concrete. on hot days, too much water is lost by evaporation from newly placed concrete. if the temperature drops too close to freezing, hydration slows to nearly a standstill. under these conditions, concrete ceases to gain strength and other desirable properties. in general, the temperature of new concrete should not be allowed to fall below 50 fahrenheit (10 celsius) during the curing period.
what is air-entrained concrete? air-entrained concrete contains billions of microscopic air cells per cubic foot. these air pockets relieve internal pressure on the concrete by providing tiny chambers for the water to expand into when it freezes. air-entrained concrete is produced through the use of air-entrained portland cement, or by the introduction of air-entrained agents, under careful engineering supervision as the concrete is mixed on the job site. the amount of entrained air is usually between 4 and 7 percent of the volume of the concrete, but may be varied as required by special conditions.
why does concrete crack? concrete, like all other materials, will slightly change in volume when it dries out. in typical concrete this change amounts to about 500 millionths. translated into dimensions- this is about 1/16 of an inch in 10 feet(.4 cm in 3 meters) The reason that contractors put joints in concrete pavements and floors is to allow the concrete to crack in a neat, straight line at the joint when the volume of the concrete changes due to shrinkage.
why do concrete surfaces flake and spall? concrete surfaces can flake or spall for one or more of the following reasons. 1. in areas of the country that are subjected to freezing and thawing, the concrete should be air-entrained to resist flaking and scaling of the surface. if air-entrained concrete is not used, there will be subsequent damage to the surface.
2. the water/cement ratio should be as low as possible to improve the durability of the surface. too much water in the mix will produce a weaker, less durable concrete that will contribute to early flaking and spalling of the surface. 3. the finishing operations should not begin until the water sheen on the surface is gone and excess bleed water on the surface has had a chance to evaporate. if the excess water is worked into the concrete because the finishing operations are begun too soon, the concrete on the surface will have too high a water content and will be weaker and less durable.
how do you remove stains from concrete? stains can be removed from concrete with dry or mechanical methods, or by wet methods using chemicals or water. Common dry methods include sandblasting, flame cleaning, and shotblasting, grinding, scabbing, planing and scouring. steel-wire brushes should be used with care because they leave metal particles on the surface that later may rust and stain the concrete. wet methods involve the application of water or specific chemicals according to the nature of the stain. the chemical treatment either dissolves the staining substance so it can be blotted up from the surface or it bleaches the staining substance so it will not show.
what are the decorative finishes that can be applied to concrete surfaces? color may be added to the concrete by adding pigments- before or after concrete is placed- and using white cement rather than conventional gray cement. by using chemical stains, or by exposing colorful aggregates at the surface. textured finishes can vary from a smooth polish to the roughness of gravel. geometric patterns can be scored, stamped, rolled, or inlaid into the concrete to resemble stone, brick or tile paving. other interesting patterns are obtained by using divider strips(usually redwood) to form panels of various sizes and shapes- special techniques are available to make concrete slip resistant and sparkling.