FAQs

Some manufacturers of synthetic fibers are promoting what they call structural fibers. They are saying these fibers will eliminate the need for conventional reinforcement and also replace the need for steel fibers.

The use of synthetic fibers as a secondary/temperature-shrinkage reinforcement is well documented. The use of synthetic fibers at elevated dosage levels has also proven beneficial, as the 3.0 pounds per cubic yard in Ultra-Thin Whitetopping has displayed. There are numerous other positive uses for synthetic fibers but when it comes down to structural benefits, which would include increased flexural strength, punching shear and point loading as well as dynamic loading synthetic fibers cannot replace steel fibers. Not even the so-called "structural" synthetic fibers. Only steel fibers can do what conventional steel does when it comes to structural responsibility. The Westergaard and Myerhof equations are only valid with steel fibers.

So there is no misunderstanding what is being discussed. There are two ways to design a slab-on-ground. The first method would use the principles of ACI 318 wherein structural steel is required. The second approach uses plain structural concrete, which is covered in ACI 302. This discussion is the focus on ACI 302 concrete design.

The engineer must look at the specific application and determine whether his need is as a pure secondary reinforcement or does he require a contribution to the structural properties of the plain structural concrete. If the engineer wants to enhance the structural properties of the plain structural concrete slab-on-ground, his only choice is steel fiber. The principle technical difference between the steel and synthetic fibers, including those with the 'structural' designation is Modulus of Elasticity. The Modulus of Elasticity of steel is 29 million psi whereas the Modulus of Elasticity of polypropylene fibers is less than 500,000 psi.

The 'structural synthetic fiber' promoters say their fibers will not rust or corrode and that they will not react with the alkalis in the concrete. The same can be said and has been documented for the steel fibers.

This is not a subject that should be treated lightly. This is not a matter of going with "manufacturer's recommendations". Real engineering design methods need to be used in evaluating the quantity of steel fiber required and said methods do not apply to synthetic so-called structural fibers.