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stic shrinkage cracks form during the first hours after the concrete casting and they are called plastic because they happen during the plastic phase of the concrete.
Plastic shrinkage cracks will harm both the overall durability and the aesthetics of the concrete.
The main cause of the plastic shrinkage cracks formation has to do with the water transfer dynamics of the concrete mixture.
On one side we have the natural phenomenon of water evaporation from the concrete surface.
On the other side, bleed water is supplied from the inside to the surface.
When the rate of evaporation exceeds the rate of water supply via bleeding, plastic shrinkage cracks will form.
At that moment the fresh concrete doesnt possess the necessary tensile strength to counteract the stresses due to volume shrinkage.
It is well acceptable that the use of polypropylene fibers is mostly advantageous to control plastic shrinkage cracks formation if all other parameters e.g. wind, temperature [link widoczny dla zalogowanych], curing regime remain the same.
Polypropylene fibers when added in the mix [link widoczny dla zalogowanych], provide a tridimensional reinforcement distributed homogenously. They will assume the role of counteracting the shrinkage tensions, defending thus the integrity of the powerless at that very time - concrete.
* The finer the polypropylene fibers, the better the shrinkage control.
* The higher the dosage of polypropylene fibers, the better the shrinkage control.
Lastly, fibrillated polypropylene fibers seem to have a distinct advantage over the monofilament type polypropylene fibers with regard to shrinkage control of concrete.
Monofilament or fibrillated ?
Monofilament and fibrillated are the two main kinds of polypropylene fibers. Their fields of application dont differ a lot.
Both monofilament and fibrillated polypropylene fibers are used:
- for reduced plastic shrinkage and settlement cracking (reduced early cracking)
- to improve impact and shatter resistance
- to increase the abrasion resistance of concrete floors
- to improve resistance to freeze/thaw cycles
- to increase overall durability of concrete structures.
THE DIFFERENCES
* Monofilament polypropylene fibers are much more preferable for architectural concrete. Barely visible, they are idoneous for finishing operations.
* The structure of fibrillated polypropylene fibers gives them better bond characteristics in comparison with the monofilaments which have a small specific surface area.
Fibrillated fibers are slit and expanded to form an open network which will increase the specific surface area and improve considerably the adhesion to the cementitious matrix.
This improved adhesion:
- will give concrete a higher residual strength
- will permit the replacement of the classic welded wire fabric used as secondary reinforcement for slabs on grade to control cracks from shrinkage and temperature variations (never use these fibers to replace any kind of structural reinforcement)!!
WHERE BOTH FIBERS SHOULDNT BE USED<br