The presence of fiber supports the homogeneous mix and functions as millions of bridges that evenly distribute the internal shrinkage stresses that try to separate the matrix.
These same fibers intercept the propagation of micro-cracks and paralyze their growth. The fibers interrupt the capillary action of moisture and thus allow for slower curing.
When the fiber is mixed to a concrete matrix, the load-deformation curve presents an increase in the load capacity, and post-cracking is improved, allowing to sustain loads up to high deformation values, resulting in greater ductility in the element. In a concrete without fibers, initially the curve is linear, then there is a sharp decrease in the load capacity, causing sudden failure of the material.
Increased fatigue resistance (which is directly related to durability), impact resistance and reduced permeability contribute to a longer life of the concrete, benefiting the associated maintenance costs in the long term.
The three-dimensional distribution of the mini-networks leads directly to a reduction in the cracking of the concrete and consequently to a modification in its behavior; in fact, in the plastic shrinkage phase, a large number of micro-cracks are produced.
When the concrete is subjected to stress, the micro cracks evolve, becoming larger and interlocking with each other. As the load continues to be exerted, some of these cracks begin to become unstable and to fail, passing through the concrete from one side to the other, and the fibers finally interrupt and stabilize the micro-cracks.
Impact strength (ability to absorb energy), as a measure of concrete degradation resulting from point loading, reflects the bonding characteristics of the fibers in holding together fractured segments of originally sound concrete. The modifications in the concrete ratio, the property of being chemically inert, (as well as anti-alkali and non-corrosive), and the reduction in setting, allow the application of polypropylene fibers in chloride ion environments.
FINISH
A fiber concrete is smoothed and finished like any normal concrete. It does not require any special tools. The action of the fiber is normally to reduce the slump of the concrete by 20% to 30%. This creates an illusion of additional water requirement but no water should be added.
For slump increase, a superplasticizer or water reducer should be used since the addition of water would result in a drop in strength.
The aesthetic finish with fibers will depend on how the concrete was finished. In stamped concrete the fiber will not be visible.
If the slab is smoothed by hand, some will be visible. If the slab is broom-finished, fibers will be visible, but you must get very close. As a general rule the presence of fibers on concrete surfaces is acceptable, however, if they are not allowed they can be removed with a light fire (blowtorch).
AVOID ITS USE FOR:
Crack control resulting from external stresses (structural)
Increase the number of concrete in pavement or slabs on slopes.
Development of a higher structural resistance.
The elimination or reduction of kinking and/or plastic runoff.
The justification for a reduction in the size of the supporting columns.
The replacement of any moment or structural steel reinforcement.
Unlimited elimination of control joints.
The thinning of bonded or unbonded sections of overlapping layers.
School of Construction Engineering - UCV
Source: Luis Santalla