What is the benefit of using steel fibers over synthetic macro fibers?

Question

Accepted Answer

Mechanically anchored steel fibers have been proven as reinforcement, even for structural application. Steel fibers are made from a material with well known engineering properties; e modulus, Poisson’s ratio, tensile strength and creep. the e-modulus of steel is greater than that of concrete. thus, the steel fibers pick up the stresses quickly and affect the cracking process immediately. the long term load carrying capacity of the steel fiber reinforced concrete is significant. Steel fibers have a material specification of AStM A820. Macro synthetic fibers come in a large variety and have very different material properties. Macro synthetic fibers do not have a material specification in AStM. All macro synthetic fibers do have an e-modulus lower then that of concrete and relatively low tensile strengths. thus, macro synthetic fibers need a certain crack width to occur prior to being able to engage in the concrete and then only moderate post crack strength values can be achieved. Macro synthetic fibers are also subject to creep which makes the long term loading capacity of the fiber lower or non existent. the rate of creep can be increased with increased ambient temperatures.

There are at least four factors to review when considering reinforcement; Modulus of elasticity, Poisson’s ratio, tensile strength and creep.