US Patent 7267873 Fiber reinforced concrete

Fiber reinforced concrete has thin steel wire of diameter between 0.05 and 0.3 mm such as cut from recycled vehicle tires. To avoid the problem of balling when mixing, two alternatives are suggested. The first consists of strands of fiber, which demonstrate excellent bond characteristics. The second consists of a mixture of fiber lengths and thicknesses, giving a wide distribution of l/d ratios not exceeding 250, which has the effect of reducing balling tendency so that significant densities can be achieved.