Below we present some basic principles and suggestions for the correct use of concrete fibers.

For a correct use of reinforcing fibers in concrete, the basic principles for quality concrete must be respected according to the current national or international legislation. The READYMESH fibers are made with high quality materials and are compatible with most of the additives on the market, as well as obviously resistant in an alkaline environment.

In the addition of primary reinforcement fibers, whose dosage is normally higher than the auxiliary fibers, it is recommended to check that some mixture parameters (typically the grain size curve, the cement content, the water content, and in general the value w/c) are those typical of a good concrete class C25/30 or higher. In particular, recipes that are too thin from cement or with low contents of fine aggregates and fillers may have a matrix that does not match the presence of fibers, compromising the quality and final performance of the conglomerate. Consult our technical-commercial manager closest to you for a preliminary assessment of the concrete mix-design.

Extremely fluid concretes, with a high water/cement ratio and excessively ‘vibrated’ ones during the installation could present sedimentation defects for the steel fibers and floating fibers for the polymeric fibers. In the first case, in fact, steel has a specific weight about 3 times higher than concrete and therefore will tend to be placed on the bottom, on the contrary for polymeric fibers the specific weight is about 2.5 times lower than concrete and therefore will tend to float. It must be said that in common practice and with normal concretes typically used in construction practice, even in the presence of fluid concretes (S5), the phenomenon is marginal and almost always negligible.

We do not recommend the use of fibers with non-fluid concretes (S1 and in some cases also S2) because the excessive ‘hardness’ of the mixture could compromise the homogeneity of their dispersion during mixing, especially if the bags have been inserted directly into the truck concrete mixer (see below).

The addition of the fibers could alter the fluidity and viscosity of the cement matrix during the ‘plastic’ phase. It is advisable to buy only fibers with CE marking, for which in the DoP document it will be possible to verify the parameter called “effect on the consistency of concrete” or “class of Vebè” executed according to the EN 12350-3 standard. This parameter, expressed in seconds or in Vebè class (from V0 to V4) indicates the time difference (in seconds precisely) between the controlled spreading of a fiber-reinforced concrete and the same recipe without adding fibers. For a value expressed in seconds, a small value will be preferable, while for a value expressed as Class of Vebè, a high class will be preferable.

Explanation of classes:

  • V0 – Vebè time: ≥ 31 s
  • V1 – Vebè time: from 30 to 21 s
  • V2 – Vebè time: from 20 to 11 s
  • V3 – Vebè time: from 10 to 6 s
  • V4 – Vebè time: from 5 to 3 s

The evaluation of Vebè time for a concrete fiber model is essential to evaluate potential implications in the installation phase, especially in the construction of industrial floors, where the correct workability of the matrix is ​​of fundamental importance. A fiber with a low Vebè time (high class) can be used without particular problems since it will not significantly affect the rheology of the fresh cement mix. On the contrary, a fiber with a high Vebè time (low class) will make it necessary to modify the formula to compensate for the loss of workability.

The dosage of steel fibers can variate from 15 to 75 kg/m³.  the most common structural applications consume in average 25-30 kg/m³ if we are talking about floors or up to 40-50 kg/m³ if we have  spritz-beton or application with high static load. Short steel fibers, spefic for non structural applications or with low loads usually have a dosage between 15-20 kg/m³.

As far as synthetic/polymeric fibers are concerned, the dosage is normally in a value between 0.5 and 1.5 kg/m³ of concrete for anti-cracking micro-fibers, and between 1.5 and 6.0 kg/m³ for structural macro-fibers. In this last group, in highly hyperstatic applications such as industrial flooring, the typical dosage is 1.5-3.0 kg/m³, for prefabrication it ranges from 2.0 to 4.0 kg/m³ and for the spritz-beton it typically ranges from 3.0 to 6.0 kg/m³. All the values ​​presented here are purely indicative and represent a rough estimate based on our experience; each application has its own specificities and it is therefore necessary to evaluate the dosage case by case.

The ideal time for the introduction of the fibers into the mixture is in concomitance with the introduction of the aggregates (sand and gravel) in the mixer or in the drum of the truck concrete mixer. In this case, open the bags and empty them a few seconds apart on the moving belt.

If the aggregate loading belt is not present (for example in vertical batching plants) or if operators cannot access the loading belt in safety, the addition of the fibers can be carried out directly in the concrete mixer, through the appropriate doors, and only after the loading of the aggregates has already begun.

There is also the possibility of adding the fibers directly into the truck concrete mixer: in this case it will be possible, after the load has been completed, to add the fibers directly into the loading door of the drum while the same is at its maximum rotation speed, having the foresight to wait a few tens of seconds between the introduction of one bag and the next. Some types of fibers are packed in a special bag that dissolves in an alkaline environment: if this is the case, it will be possible to add the fibers by throwing the entire bag, also closed, into the loading door.

An empirical rule (but very effective and used by many operators) to determine the mixing time is the following: at the end of loading the fibers into the concrete mixer (or the conglomerate itself) the concrete must be mixed at the maximum rotation speed of the drum for at least one minute every cubic meter loaded (so a concrete mixer loaded with 8 m3 must mix for 8 minutes).

The case of mixing at the mixing plant is different: the final mixing in the truck concrete mixer can be reduced by half due to the mixing already occurred during the cycle in the pre-mixer.

It is good practice, especially in the case of special mixes or low-flow recipes, to visually inspect the concrete before installation. Or unload a small amount of product and check the actual homogeneity of the fiber + concrete mixture. If defects are found, proceed to further mixing for the time necessary.

The addition of the fibers to the concrete, especially the anti-cracking ones, allows an effective contrast to the onset of cracking phenomena during the first hours of life starting from the installation. Despite this, as a further protection, it is strongly recommended to provide for a correct curing to avoid unpleasant aesthetic or performance defects of the final work, using the appropriate anti-evaporation treatments or the use of polyethylene sheets and similar systems.