The fibers do not mix well in the concrete, why?
Concrete fibers, like any additive, to work properly must be distributed and spread as evenly as possible throughout the conglomerate matrix. The lack or incorrect mixing leads to serious risks in terms of aesthetics or unsatisfactory mechanical performance, where an area with too high percentage of fibers is created. We see some typical causes of an incorrect mixing:
The fibers, especially the macro-fibers, if loaded in the mixer or in the empty auto-mixer, before having loaded at least the aggregates in the concrete mixer or in the mixer, could create real “tangles” which then no longer succeed in dissolve during normal mixing. A similar situation, although unlikely, could occur by adding the fibers at the end of the load, as the last element, too quickly. In this case, all the fibers would be incorporated into the cement paste in considerable quantities in a small volume, increasing the risk of creating tangles localized.
The correct way to load it, is distribute the fibers on the conveyor belt during loading of the aggregates in the concrete mixer or in the mixer. If this is not possible, add the fibers directly into the concrete mixer after loading but taking care to wait at least 30 seconds between the insertion of one bag and the next.
A concrete mixer truck operator may not have “mixed” long enough, or the construction site was too close to the loading point and the mixer turned too slowly. The correct maneuver requires, at the end of loading the fibers into the concrete mixer, is mix for at least 1 minute for each cubic meter of concrete present in the concrete mixer, at the maximum speed of rotation of the drum.
Too hard concrete (S1-S2) could cause problems in the fibers mixing. This case history is not conceivable when it comes to fibers for industrial floors, since the concrete produced for the construction of floors is definitely an S4 or even S5.
Another case is a too fluid concrete, in which even the fibers can tend to float. This is an extremely rare and unlikely case, given that the concrete in question should be so fluid that is similar to a grout.
If the propeller inside the mixer drum is worn and unable to mix the concrete properly, some fiber agglomerates may form and not be dissipated during mixing. It is a very rare case but these days, when entrepreneurs in the concrete world are plagued by a serious crisis and don’t desire to renovate the cocrete mixer truck fleet, it could become a slightly more probable case.
The floor is affected by superficial surfacing
The fibers are inside the concrete paste. The volume of concrete paste is generally sized with a certain amount of cement, water and additive in order to have a precise w/c ratio.
Introducing the fibers in the mixture changes the volume of the inert fraction at the expense of workability which, unfortunately, is often restored by adding water, worsening the w/c ratio. This activity negatively modifies the mechanical performance and, above all, penalizes the adhesive capacity of the same and, therefore, the ability to oppose the extraction of the fibers.
If the necessary rheological precautions are not implemented during the prequalification phase of the fibrous composite, the penalties of the w/c ratio manifest themselves with surfacings of fibers that tend to slip off above all on the surface where, due to sedimentation and bleeding, the ratio w/c is already physically pejorative compared to the remaining section. Furthermore, if the troweling and smoothing operations are carried out with additional water supply, the w/c ratio and the surfacings are further penalized.
It is therefore reasonable to state that the surfacings are generated by incorrect assessment of the correct volumes of mixture, of the w/c ratio, of a penalty applied during the smoothing operations or by the sum of these occurrences.
When the fibers emerge on the surface of the floor, especially along the perimeter, around pillars and inspection pit and generally in all those positions where the power trowel’s propellers have failed to flatten them into the thickness, the reason depends from the fact that in those positions the surface finish is made completely by hand with the addition of water to promote the formation of grout. This phenomenon is completely normal and unfortunately inherent in the nature of fiber-reinforced floors with structural fibers. The client must be adequately advised before the floor is laid giving him the right expectation. It is possible to remove 80% of the aesthetic problem, burning the leaking fibers with a sheath burner. In outdoor floors, in the same positions, the removal of the leaking fibers is aided by UV rays. In any case, vehicular and pedestrian traffic will quickly help to remove emerging fibers.
If the phenomenon of surfacing were to occur in a very massive measure and in areas distant from pillars, edges and corners, it could have happened that the paver had made a mistake in assessing the setting time of the concrete and floated the floor late, when the surface it was already too hard.
Can I use fibers in a "brushed" floor?
The “brushed” (or striped) effect is obtained by passing a normal broom on the surface of the concrete not yet completely hardened. This operation leaves small grooves on the surface, increasing friction. The action of the “broom” on a fiber-reinforced floor, however, is deleterious. The passage of the broom in fact has the same effect as a comb that combs the fibers and would make them emerge in great quantity, with very bad repercussions on the aesthetics quality of the floor.
Can more types of fibers be mixed in the same concrete?
The presence of different types of fiber does not normally cause problems. What we usually tend to do is to combine the anti-cracking properties of the auxiliary fibers with the structural properties of the macro-fibers, in fact, structural to have a combination of benefits.
In the case of industrial floors, for example, the combination of the READYMESH PF-540 with the READYMESH PM-180 is not only beneficial but it is also recommendable! In this case, given that the structural fibers have a beneficial effect also as regards the contrast to the phenomenon of shrinkage cracks, the advice is to use only 0.5 kg/m3 instead of the classic 0.75 or 1.0 kg/m3.