Concrete is a low compressive strength and tensile strength of brittle materials, with the continuous development of modern building technology of concrete put forward higher requirements, it is moving in high-strength, high toughness, high resistance to crack, high durability and excellent work of direction. In general, small cement matrix fibers are randomly distributed some to improve the toughness of concrete, impact resistance, impermeability and resistance to fracture of an effective way to shrink. Various available fibers such as steel fibers, synthetic fibers, natural fibers have been used in concrete, the paper outlines the cracking mechanism of polypropylene fiber reinforced concrete.

Working mechanism of a fiber reinforced concrete
1.1 fiber concrete application status
Fiber concrete fiber is a polypropylene as raw material, special process of high strength, low elongation fiber, can effectively control the concrete plasticity, shrinkage, temperature shrinkage cracks and improve the impermeability of concrete performance, impact the ability to extend the life of concrete products. Polypropylene fiber is manufactured by a special process of high strength, low elongation fiber, acid, alkali environment has a high resilience. The product by the 120 days of acid (pH value 2-3), alkaline (pH value 14.0) test, almost no aging or damaged. The fibers through a special UV treatment, with anti-ultraviolet aging ability, to ensure the effectiveness of fiber in concrete long-term play, maintaining the excellent performance of concrete structures.
Polypropylene fiber is chemically stable, just by changing the physical structure of concrete and improve the performance of concrete, the fiber itself does not have any chemical reaction. With concrete aggregate, admixtures, after admixture and cement mix to stabilize their chemical and physical properties, and therefore have a good affinity concrete material.
For a variety of strength grade of concrete cracking, the special needs of the concrete cracking and abrasion resistance requirements have concrete; concrete panel of hydraulic seepage, cracking a significant effect; on the pavement, bridges crack, impact grinding capacity and prevent skidding road has a good effect; for industrial, civil roof, walls, floors and basement crack, waterproof construction to improve the seismic capacity of a great advantage; impermeability of concrete cracking on the port terminal is also very good.
1.2 The working mechanism of fiber reinforced concrete
General theory is that adding fiber cement concrete matrix are the following effects:

① improve the ability of concrete to resist tensile deformation, prevention and protection against all kinds of cracks in the concrete reason.

② improve resistance to penetration of the concrete.

③ improve the toughness and impact resistance of concrete.

Uniform random distribution of the chaos of the short fiber reinforced concrete mechanism, there are two explanations theoretically concrete.
① US Rmualoli's "fiber spacing mechanism" (also known as fiber crack resistance mechanism), according to the linear elastic fracture mechanics to explain the role of fiber constraints for the occurrence and development of cracks. Think inside the concrete originally flawed, want to improve the strength of this material, must minimize the extent of the defect improve toughness and reduce stress concentration factor of internal crack tip. Theoretical analysis and experiments show that when the average distance between the center of the fiber is less than 7.6mm, bending tensile strength fiber reinforced concrete crack are beginning to improve.

② UK Swangat et al "composite mechanism," the starting point of the theory is mixing theory of composite material, the fiber is fiber reinforced concrete looking system, and application to estimate mixing principle tensile and flexural strength of fiber reinforced concrete, proposed relationship between the amount of doped fiber reinforced concrete and fiber strength, orientation, aspect ratio and adhesive force between.
Above two theories explain the fiber used in the concrete / mortar does have a significant role.
Formed in the hardening process of the concrete strength, since the hydration of cement and water to form a new cement crystal, causing volume shrinkage of concrete, the concrete at the same time may be due to free water through the evaporation under reduced pressure and to cause shrinkage in the early . These stresses exceed the tensile strength in a certain period of the cement matrix, thus causing micro-cracks in the concrete, these micro-cracks present in localized contact surface of the aggregate and gel inside the gel and the cement within concrete, this phase particles with little energy and can easily be absorbed by the fiber, the fiber per unit volume in a larger number of uniform distribution of the chaos inside the concrete, so the micro-cracks have suffered fiber barrier in the process of development, the consumption of energy, thus blocking the development of cracks reach cracking effect. Concrete shrinkage process will produce shrinkage stress, like the addition of fiber doped fiber reinforcement in concrete. These fiber reinforced concrete cracking inhibit the process of improving the fracture toughness of concrete, but also improves the tensile strength of concrete.
Performance improvement of concrete fiber, mainly through physical and mechanical effects, does not change the chemical properties of various materials in concrete chemistry itself, it will not affect the durability of concrete.

Function and role of two polypropylene fibers
2.1 effectively improve the crack resistance of concrete
Polypropylene fiber is a very effective to improve the capacity of fiber concrete cracking, through a special fiber production process and fiber surface treatment process, and cement-based materials have a strong binding force, the bond fibers more aggregate, more closely meshed degree, it can play a more effective role in the concrete cracking. Because the fibers can be quickly and easily mixed with concrete materials, extremely uniform distribution, it can constitute a uniform internal chaos concrete support system to produce an effective secondary strengthen results.
A pilot study institutions show that the distribution of fibers in concrete chaos to help undermine the formation of plastic shrinkage of concrete and tension during freezing and thawing; the ability to be dispersed to the contraction of the tens of millions per cubic root has a high tensile strength, elastic relatively low modulus of single fibers, which is extremely effective in enhancing the toughness of concrete, inhibit the emergence and development of micro-cracks; while numerous single fibers in the concrete to form a chaotic system to support care, can effectively prevent fine aggregate segregation, separation of fine aggregate have some effect; the elimination of early and significant cuts bleeding of concrete, thus preventing the formation of subsidence cracks.