Polypropylene fiber reinforced concrete

Introduction 

Micro-cracks are developed in concrete due to the drying shrinkage, thermal stresses, or other effects at the early stage, which will convert these micro-cracks into the macro-cracks after the loading is applied. 
Due to these cracks, we can see the large deformation in the concrete. To prevent this, fiber could be the solution.
Fiber-reinforced concrete improves the durability, ductility, fatigue strength, toughness impact resistance, and tensile strength of the concrete. 
There are so many types of fibers that we can use with the concrete, like bamboo, jute, sawdust, rice husk, coconut, polypropylene, steel, glass, nylon, coir or carbon, asbestos, polyvinyl alcohol, polyamides, polyethylene, etc,.
The improvement in the concrete properties depends upon the fiber type, strength, size the amount, etc,. 
These fibers can be used with both plain concrete and rebar reinforced concrete.
If you are looking for the thesis topic than Polypropylene fiber reinforced concrete could be an excellent topic. 

What is polypropylene?

Polypropylene fibers are synthetic fibers that can be purchased as a product from the textile industries.
Polypropylene is transformed from 85 percent propylene (monomer for polypropylene). it's a by-product of petroleum.  
Polypropylene fibers can be either mono-filament (processed for increasing the pull-out load) or fibrillated (processed for developing high surface area).
The melting point of pp fiber is 165℃. These fibers have both macro-fiber and microfiber.
Mono-filament fibers are available in the length size of 1/2, 3/4, 1-1/2 inch. Fibrillated fibers are available in the length size of 1/2, 3/4, 1-1/2, or 2 inches.
Polypropylene fiber
Polypropylene fibers Imagecreadit- bautech via http://www.bautech.eu/


Polypropylene in concrete

These fibers have some unique characteristics to make them versatile material to use in the concrete.
These fibers are mostly used among the various fibers, due to its low cost, low weight, resistance against acid and corrosion, high surface area, resistance toward shrinkage cracking, and also offers good toughness.
Polypropylene fiber reinforces concrete can be defined as composite material because it consists of fibers and concrete. It is essential to check the bonding between concrete and fibers as the bonding of these materials plays a vital role in composite behavior.
The applicable fiber fraction is about 0.06 to 0.5 % for polypropylene fibers.
While mixing polypropylene in the concrete, make sure that the mixing is long enough to disperse this material in the concrete very well, because it does not absorb the water.
The length of these fibers should be greater than twice the diameter of the used aggregate in the mix for bonding in concrete and dispersion in a concrete mix.
If we increase the number of fibers, it can lead to a decrease in the workability of concrete. in that can we can go for some admixture. also, pp fibers increase the air-content in concrete. 
pp fiber has a low modulus of elasticity under normal loading but increases the modulus in impact loading.
The density of the pp fibers is very low as compared to steel which makes concrete a lightweight concrete.
Polypropylene fiber reinforced concrete can be useful in highways, airport pavement, piles, piers, facing panels, bridge decks, coating for underwater pipes, making thin concrete, watersides, and others.
The typical properties of polypropylene fibers are,


  • Tensile strength ( Mpa )   - 550-780 
  • Young's modulus ( Gpa )  - 3-15
  • Ultimate elongation ( % ) - 8
  • Specific gravity                 - 0.9 


Affecting factors

 Here are some of the factors that will affect the properties of the pp fiber reinforced concrete.

  • The volume of the fibers 

The volume of the fibers plays a vital role in the concrete mix as the volume increases, the improvement in the mechanical properties can be observed.

  • The aspect ratio of the fibers, 
The aspect ratio can be defined by fiber length divided by the diameter of the fiber. As the aspect ratio increases, the strength and the toughness can be enhanced.

  • Size of aggregate

Studied shown that, to avoid the strength reduction in the concrete, the maximum size of the coarse aggregate should not exceed 10 mm. 

  • Fibers orientation 

Studies found that more tensile strength can be achieved by aligning the fibers in the direction parallel to the direction of the applied load as compared to the randomly oriented fibers.

  • Mixing

These fibers do not absorb the water, hence proper mixing should be done for better bonding and to avoid segregation and balling of fibers. 

  • Length of fibers 

To increase the bond between concrete paste and fibers, longer fibers can be useful. but, longer fibers requires the larger aggregate.

  • Curing condition

Curing conditions also affect the properties of the pp fiber reinforced concrete.


Effect of pp fibers on fresh concrete

The slump value will decrease in pp reinforced concrete as compared to normal concrete. also, increase the cohesiveness and adhesiveness in the concrete mix.
In the fresh concrete, the cracks are developed due to volume change. having a high tensile strength of these fibers help to resist the plastic shrinkage cracking in the fresh concrete. it will also increase the initial and final setting time of the concrete.
These fibers trying to make a bond with the concrete ingredients which will slower the aggregate settlement.
The large surface area in these fibers results in more segregation and bleeding as compared to normal concrete.

Effect of pp fibers on hardened concrete

The mechanical properties of the concrete are essential in many perspectives. 
Many researchers have found that the addition of polypropylene fibers in the concrete does not increase much of the compressive strength of the concrete. but, variation with the fiber's length may found the movement in the compressive strength of the concrete. 
The split tensile strength is much higher than that compressive strength in the pp reinforced concrete. more improvement if also found with the variation of the fiber length, aspect ratio, etc. for better results in strength, different curing condition also plays an important role.

Conclusion

From the above discussion, it is clear that using polypropylene fibers in concrete, is a good approach to overcome early stage cracking and shrinkage, loss of moisture in concrete, it reduces the weight of the concrete, up to a certain limit the mechanical properties of the concrete can be improved. 
It has low density, no corrosion risk, cost-benefit, abrasion and impact resistance, fatigue resistance, highest fracture strain, quit low modulus, and reasonable tensile strength.
The construction industries always demand the new construction material for improvement in the traditional concrete, These fibers could be a better alternative.
More developments and researches are required to improve the limitation of this material and to make this material a versatile material.
If you have any doubts about this article feel free to comment below.




 






   






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