Measuring the Performances of Fiber Reinforced Concrete

As in any structural design procedure, it is essential to have models that describe the performances of the materials to be used. Conventional concrete is usually modelled by its strength class (C25/30, C30/37, etc.), and standard reinforcement steel is described by its yields stress. But how about fibers in concrete? How should one model their performances and behaviour?

Unlike concrete with conventional reinforcement, Fiber Reinforced Concrete (FRC) is a tricky composite material. With rebars/mesh, one knows where they are placed in the structural member and, consequently, where the tensile forces will be located. So, based on the class of concrete and the yield stress of steel, the structural engineer can easily design conventional reinforced concrete.

Fibers, however—responsible for resisting internal tensile forces—are randomly distributed within the concrete, making it difficult to describe the composite’s mechanical behavior analytically from separate information about the fibers and the concrete. One should therefore treat FRC as a single composite material for which a practical model is needed.

Even though fibers suppliers state on their TDS's information like the dimensions of the fiber, the material of which they are made, etc., these information are of very little use to structural engineers.

In this regard, fib Model Code 2010 suggests a test: Three-point bending on a notched beam per (EN 14651). It measures nominal properties that allow the structural engineer to model FRC in a structural calculation. The video below from Controls illustrates how the test is conducted.

The output of this test is the force-crack opening curve that you could see in the video. Four nominal "residual flexural strengths" are then extracted from said curve: \(f_{R1}, f_{R2}, f_{R3}, f_{R4}\). These stresses correspond respectively to the following crack mouth openings (CMOD's): \(0.5mm, 1.5mm, 2.5mm, 3.5mm\).

When the structural engineer have these nominal stregths, they can start designing FRC members, like slabs-on-ground. Fibers suppliers should provide them for their products, for each combination of concrete class and fiber dosage.

If these are not readily available, the structural engineer should request them to be measured according to EN 14651.

Residual strengths can also be set as project specifications to be achieved by the project contractor. For conventionnal fibers widely available from big suppliers, the residual strengths range from ~1 MPa to ~2 MPa. Some special fibers may go beyound these values.