Concrete:
Concrete is a synthetic construction material made by mixing CEMENT, FINE AGGREGATE (usually sand), COARSE AGGREGATE (usually gravel or crushed stone), and WATER together in proper proportions; the product is not concrete unless all four of these ingredients are present. A mixture of cement, sand, lime, and water, without coarse aggregate, is NOT concrete, but MORTAR or GROUT. Mortar is used mainly for bonding masonry units together. The term grout refers to a water-cement mixture (called neat-cement grout) or water-sand-cement mixture (called sand-cement grout) used to plug holes or cracks in concrete, to seal joints, and for similar plugging or sealing purposes.

The fine and coarse aggregates in a concrete mix are called the INERT ingredients; the cement and water are the ACTIVE ingredients. The inert ingredients and the cement are thoroughly mixed together first. As soon as the water is added, a chemical reaction between the water and the cement begins, and it is this reaction (which is called HYDRATION) that causes the concrete to harden.

Always remember that the hardening process is caused by hydration of the cement by the water, not by a DRYING OUT of the mix. Instead of being dried out, the concrete must be kept as moist as possible during the initial hydration process. Drying out would cause a drop in water content below the amount required for satisfactory hydration of the cement.

The fact that the hardening process has nothing whatever to do with a drying out of the concrete is clearly shown by the fact that concrete will harden just as well under water as it will in the air.

Concrete may be cast into bricks, blocks, and other relatively small building units that are used in concrete MASONRY construction. The proportion of concrete to other materials used in building construction has greatly increased in recent years to the point where large, multistory modern building are constructed entirely of concrete, with concrete footings, foundations, columns, walls, girders, beams, joists, floors, and roofs.

Strengths	Examples of use
15mpa or 2175psi	-
17mpa or 2500psi	-
20mpa or 3000psi	Best for Footings, etc..
25mpa or 3500psi	Footings, Barbeque, pads etc..
28mpa or 4000psi	Garage floors, sidewalks & stairs, etc..
30mpa or 4350psi	Garage floors or something with weight, etc..
32mpa or 4650psi	Garage floors & slabs with weight, etc..
35mpa or 5075psi	Curbs for driveways, etc..
30mpa or 5075psi {M.T.O.}	Curbs on roads & sidewalks, etc..

Other Ingredients:
Aggregates for concrete are chosen carefully. Aggregates comprise 60 to 75 percent of the total volume of concrete. The type and size of the aggregate mixture depends on the thickness and purpose of the final concrete product. Relatively thin buildings sections can require small coarse aggregates, though aggregates up to six inches (150 mm) in diameter have been used in large dams. 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.

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.

Excessive impurities in mixing water not only may affect time and concrete strength, but also may cause efflorescence, staining, corrosion of reinforcement, volume instability and reduced durability.

Specifications usually set limits on chlorides, sulfates, alkalis, and solids in mixing water unless tests indicate that the water will not negatively impact concrete properties.

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