Though used interchangeably, cement and concrete are two different materials. Cement is the basic ingredient that goes into concrete, and is a powder that mixes calcium, aluminum, iron and gypsum. About 85 percent of its mass comes from lime and silica, made from limestone, shells and chalk, combined with shale clay or silica sand. Concrete mixes a paste of cement and water, with aggregates such as gravel or crushed stone, and sand. When soft, it is malleable and soft. When hard, it develops the strength of a rock.
Depending on the area it takes up, concrete can suffer from cracking, unless strengthened with steel bars. One type of bar is called rebar, which is short for reinforcing bar. Because these bars have strong compression, but weak tension, they carry tensile loads by going into the concrete vertically – with the long side perpendicular to the ground. Steel bars receive number designations that correspond to pounds per foot of bar. For example, bar 0.3 weights 0.376 pounds per foot, 0.4 weighs 0.668 pounds and 0.5 weights 1.043 pounds. Bar 0.9 weighs 3.4 pounds, 10 weighs 4.3 pounds, 14 weighs 7.65 pounds and 18 weighs 13.6 pounds.
Welded wire fabric adds tensile strength to concrete, and its weight depends on its designation. The first part of the name, such as 6x6, shows the horizontal and vertical spacing of strands in English. The number W number is the size of the longitudinal and transverse wire measure in one-one-hundredths of an inch. For example, mesh 6x6 W1.4/1.4 weighs 21 pounds per 100 feet; 6x6 W2.1/2.1 weighs 30 pounds; and 6x6 W2.9/W2.9 weighs 42 pounds. Mesh 4x4 W1.4xW1.4 weights 31 pounds; 4x4 W2.1/2.1 weights 44 pounds; and 4x4 W2.9/W2.9 weighs 62 pounds.
The average weights of reinforcing bars per cubic measurement of concrete depend on the application, according to Estimators’ Reference. For paving, weights range from 2 to 4.5 pounds per square foot. For exterior steps, weights run from 40 to 80 pounds per cubic yard. For non-cantilevered retaining walls, rebar weighs from 35 to 250 pounds per cubic yard. For supported slabs, weights run from 85 to155 pounds per cubic yard.