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Concrete is the world’s most commonly used building material. In its simplest form, concrete is a mixture of paste and aggregates. The material (paste) used to manufacture concrete pipe is composed principally of Portland cement and water, and is used to coat the surface of the fine and coarse aggregates. The Portland cement is a closely controlled chemical combination of calcium, silicon, aluminum, iron, and small amounts of other compounds, to which gypsum is added in the final grinding process to regulate the setting time of the concrete.  Portland cement's chemistry comes to life in the presence of water. Soon after the cement and water are combined, a chemical reaction called hydration occurs and the paste hardens and gains strength to form the rock-like mass known as concrete.  During hydration, a node forms on the surface of each cement particle. The node grows and expands until it links up with nodes from other cement particles or adheres to adjacent aggregates. Within this process lies the key to the remarkable trait of concrete - it’s plastic and malleable when newly mixed and strong and durable when hardened.

The character of the concrete is determined by the quality of the paste. The strength of the paste, in turn, depends on the ratio of water to cement. The water-cement ratio is the weight of the mixing water divided by the weight of the cement. High-quality concrete is produced by lowering the water-cement ratio as much as possible without sacrificing the workability of fresh concrete. Generally, using less water produces a higher quality concrete provided the concrete is properly placed, consolidated, and cured.  Typically, a mix is about 10 to 15 percent cement, 60 to 75 percent aggregate and 15 to 20 percent water. Entrained air in many concrete mixes may also take up another 5 to 8 percent.

Almost any natural water that is drinkable and has no pronounced taste or odor may be used as mixing water for concrete. However, some waters that are not fit for drinking may be suitable for concrete. Specifications usually set limits on chlorides, sulfates, alkalis, and solids in mixing water unless tests can be performed to determine what effect the impurity has on various properties.

The type and size of the aggregate mixture depends on the thickness and purpose of the final concrete product. A continuous gradation of particle sizes is desirable for efficient use of the paste. In addition, aggregates should be clean and free from any matter that might affect the quality of the concrete.

Curing begins after the exposed surfaces of the concrete have hardened sufficiently to resist marring. Curing ensures the continued hydration of the cement and the strength gain of the concrete. Concrete surfaces are cured by steam or water. The longer the concrete is kept moist, the stronger and more durable it will become. The rate of hardening depends upon the composition and fineness of the cement, the mix proportions, and the moisture and temperature conditions. Most of the hydration and strength gain take place within the first month of concrete's life cycle, but hydration continues at a slower rate for many years. Concrete continues to get stronger as it gets older.

Precast concrete products are cast in a factory setting. These products benefit from tight quality control achieved at a production plant. Precast concrete pipe is produced in highly controlled plant environments under rigid production standards and testing specifications.

Canadian Concrete Pipe & Precast Association

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