How is a structural reinforcement with fibers defined?

MACRO FIBERS are defined as fibers whose diameter is greater than 0.3 mm and whose length is between 20 and 60 mm.

It is known that in order to achieve structural reinforcement, long fibers (MACRO FIBERS) are more suitable because they allow to meet the requirement with much less dosage. This fact is one of the reasons that have encouraged their manufacturers to call them “structural fibers”. However a fiber should not be defined as STRUCTURAL simply because it is large. The word “structural” is not in fact a qualifier of the fiber, but of the type of reinforcement that has been given to the concrete after the addition of the fiber. (Structural reinforcement)

Many manufacturers of MACRO FIBERS (fibers over 20mm in length) have been calling their fibers “structural”, simply because they appear “strong” and are used to give the concrete a “residual” strength. This residual strength is what ends up reinforcing the concrete in a “structural” way. But not just any residual strength will do. There is a minimum residual strength that is required to be able to define that a concrete has been “structurally reinforced.”

The minimum amount of residual strength that a concrete must acquire after the addition of fibers is defined in standards.

What does the standard EN 14651 tell us structural reinforcement with macro fibers?

Minimum residual flexural strength value:

For fibers to be considered with structural function, the residual characteristic flexural tensile strength F1 shall not be less than 40 % of the proportionality limit FL and F3 shall not be less than 20 % of the proportionality limit FL .”

In other words, if we add fiber in a certain dosage to a concrete and we make a beam test (Standard EN 14651), we must measure the “Limit” resistance to first crack of the concrete, which we will call FL. We will continue to apply load on this specimen until the crack has reached an opening of 0.5mm. Right at this point we should measure the residual strength of the specimen which is called F₁. The F₁ value should be greater than 40% of FL.

Subsequently we must continue to apply load to that specimen until the crack has reached an opening of 2.5mm and measure the residual resistance that that specimen is capable of showing at that point. We will call this value F₃. F₃ must be greater than 20% of FL. Only if these two conditions are met together, we can say that the specimen has been structurally reinforced with the addition of a certain type of fiber at a certain dosage.

The answer to this question is the key. Depending on the type of fiber, the quantities that allow us to comply with the standard will vary.

Reviewing the tables of residual resistance of some types of fiber, we can see that this condition is reached with 10 kg of MACRO GLASS FIBERS (36mm), 20 kg of MACRO STEEL FIBER (50mm), 4 kg of MACRO SYNTHETIC CLASSIC FIBER (48mm), 2.5KG of BARCHIP MACRO FIBER… (Here we discover our first advantage of Barchip. Less fiber, more reinforcement)

Would it be acceptable to offer 5 kg of MACRO GLASS FIBER or steel fiber promising a structural reinforcement for a concrete slab? Obviously NOT, since the residual strength obtained in both cases would not comply with the standard. This dosage would provide “some” residual strength, but far from what the standard requires and the slab for sure needs.