In its simplest form, concrete is a mixture of paste and aggregate or rock. A paste consisting of Portland cement and water is applied to the surface of the fine (small) and coarse (larger) aggregates. Through a chemical reaction called hydration, the paste hardens and gains strength, forming a rocky substance called concrete.
The key to getting rugged concrete is to carefully mix and mix ingredients. A mixture that does not have enough paste to fill all the voids between the aggregates will be difficult to place and will produce a rough surface and porous concrete. Mixtures containing excess cement slurry are easy to place and produce a smooth surface; however, the resulting concrete is not cost effective and is more susceptible to cracking.
The quality of the slurry determines the properties of the concrete. In turn, the strength of the slurry depends on the ratio of water to cement. The water to cement ratio is the weight of the mixed water divided by the weight of the cement. High quality concrete is produced by minimizing the water to cement ratio without sacrificing the processability of fresh concrete, allowing it to be properly placed, consolidated and cured.
A properly designed mixture has the workability required for fresh concrete and the durability and strength required to harden the concrete. Typically, the mixture is from about 10% to 15% cement, from 60% to 75% aggregate and from 15% to 20% water. The air entrained in many concrete mixtures can also account for 5% to 8%.
Almost any natural water that is drinkable and has no obvious taste or odor can be used as a mixed water for concrete. Excessive impurities in the mixed water not only affect the setting time and concrete strength, but also cause weathering, pollution, enhanced corrosion, volume instability and reduced durability. Concrete mix specifications typically impose limits on chloride, sulfate, alkali and solids in the mixed water unless testing can be performed to determine the effect of impurities on the final concrete.
Although most drinking water is suitable for mixing concrete, carefully select the aggregate. Aggregates account for 60% to 75% of the total concrete. The type and size of the aggregate used depends on the thickness and use of the final concrete product.
Smaller aggregates are required for relatively thin building sections, although aggregates up to 6 inches in diameter have been used in large dams. In order to effectively use the paste, a continuous particle size grading is required. In addition, the aggregate should be clean and free of any substances that may affect the quality of the concrete.
Shortly after the aggregate, water and cement are combined, the mixture begins to harden. All Portland cements are hydraulic cements that solidify and harden by chemical reaction with water hydration. During the reaction, nodes are formed on the surface of each cement particle. The nodes grow and expand until it joins or adheres to nodes from other cement particles.
Once the concrete is thoroughly mixed and can be used, it should be placed in a form before the mixture becomes too hard.
During placement, the concrete is consolidated to compact it within the form and eliminate potential defects such as honeycombs and air pockets.
For the slab, the concrete is allowed to stand until the surface moisture film disappears, and then the concrete is smoothed using a wood or metal hand-mixed mixture. Floating produces a relatively uniform but slightly rough texture with good slip resistance and is often used as the final finish for exterior slabs. If a smooth, hard, dense surface is required, the wire is smoothed after the float.
The curing begins after the exposed surface of the concrete has sufficiently hardened to resist damage. Curing ensures that the cement continues to hydrate, allowing the concrete to continue to gain strength. The concrete surface is cured by spraying water mist or using a moisturizing fabric such as burlap or cotton pad. Other curing methods prevent evaporation of water by sealing the surface with a special spray of plastic or a curing compound.
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