How is a structural reinforcement with fibers defined?
ANNEX 14 OF THE EHE08 CODE
MACRO FIBERS are defined in ANNEX 14 of our code EHE08, those fibers whose diameter is greater than 0.3 mm and their length between 20 and 60 mm.
You cannot define a fiber 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 a concrete after having added the fiber.
Many manufacturers of MACRO FIBERS (fibers over 20mm in length) have been calling their fibers structural, for the simple fact of having a strong appearance and being used to give concrete a residual resistance. This residual resistance is what ends up reinforcing the concrete in a “structural” way. But not just any residual resistance is worth it. There is a minimum residual strength that is required to be able to say that a concrete has been “structurally reinforced.”
The amount of residual resistance that a concrete must acquire after the addition of fibers is defined in annex 14 of the INSTRUCTION OF STRUCTURAL CONCRETE (EHE0-08)
How many kilos of fiber should we add to the concrete to meet the EHE08 requirement?
The answer to this question is the key. Depending on the type of fiber, the amounts that allow us to comply with the conditions set out in Annex 14 will vary.
It is known that to achieve structural type reinforcements, long fibers (MACRO FIBERS) are more suitable because they allow meeting the requirement with much less dosage. This fact is one of the reasons that have encouraged their manufacturers to call them “structural fibers” very lightly.
To meet the requirements of Annex 14 using 36mm macro glass fibers, we should use at least 10 kg of dosage per cubic meter.
Would it be reasonable to offer 5 kg of MACRO FIBER glass stating that the reinforcement is of the structural type because these fibers are structural? Obviously NOT, since the residual resistance offered by 5 kg would not comply with Annex 14. This dosage would provide “some” of residual resistance, but far from what the standard requires and the slab probably needs.
Reviewing the tables of residual strengths of some types of fiber, we can see that this condition is reached with 10 kg of MACRO GLASS FIBER, 20 kg of MACRO STEEL FIBER, 4 kg of MACRO SYNTHETIC CLASSIC FIBER, 2.5KG of MACRO FIBER BARCHIP …
What does Annex 14 of EHE08 say about structural reinforcement with fibers?
“31.4 Minimum value of resistance:
For the fibers to be considered as having a structural function, the residual characteristic resistance to bending traction fR, 1, k shall not be less than 40% of the proportionality limit and fR, 3, k shall not be less than 20% of the proportionality limit. . “
In other words, if we add fiber in a certain dosage to a concrete and we break a bending specimen (Standard UNE-EN 14651), we must measure the “Limit” resistance to the first crack of the concrete, which we will call FL. We will continue to exert pressure on this specimen until the crack has reached an opening of 0.5mm. Right at this point we must measure the residual resistance of the specimen, which is called F₁. The F₁ value must be greater than 40% of FL. Subsequently, we must continue applying tension 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 moment. We will call this value F₃. F₃ must be greater than 20% of FL. Only if these two conditions are met together, we can affirm that the specimen has been structurally reinforced with the addition of a certain type of fiber in a certain dosage.