In art criticism, concrete refers to things which are real, particular, tangible; as opposed to abstract. The more general use of the term refers to the concrete building material, which is extremely heavy and durable when set. First employed by the ancient Romans, it's made from a mixture of Portland cement, aggregate (typically sand and gravel), and water.

Concrete is typically poured into a form; very rarely modeled or carved. Slabs should be between four and eight inches thick, depending on their function. A basement floor: 4 inches; home garage floor or porch: 4-6; sidewalk: 5-6: driveway: 6-8. Concrete is sold by cubic volume. Calculate need as: slab thickness in feet x slab width in feet x slab length in feet = cubic feet of concrete. 27 cubic feet = 1 cubic yard. Large projects require steel reinforcing bars (called re-bar). The strength of concrete increases when the amount of cement in the mixture increases, the amount of water relative to cement decreases, the density of the concrete is higher, and the aggregate is coarser. (

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