Polypropylene fiber VS polyacrylonitrile fiber in concrete
Writer： admin Time：2023-11-01 16:28 Browse：
Polypropylene fiber: polypropylene fiber (PP) is a synthetic fiber made from isotactic polypropylene obtained by polymerization of propylene monomer. The fiber reinforced concrete has the advantages of light weight, high tensile strength, impact resistance and crack resistance. The density range of polypropylene fiber is 0.87-0.95g/cm3, and the melting temperature is 107-141 ℃. The ratio of length to diameter of concrete fiber is generally above 400. The repulsive force of interlaced fibrillated fiber bundles due to surface modification can be easily dispersed in the matrix. Although the chemical bond connection is limited, the mechanical bond is good, so that the fiber will not be pulled out when stressed. Although the tensile strength of polypropylene fiber is higher than that of ordinary concrete, its elastic modulus is lower.
Polyacrylonitrile fiber: PAN fiber, also known as polyacrylonitrile fiber, usually refers to the synthetic fiber prepared by wet spinning or dry spinning with the copolymer of more than 85% acrylonitrile and the second and third monomers. Because of the hydrophilic group, the fiber has good hydrophilicity and good dispersion in cement matrix. Compared with polypropylene fiber, it has higher elastic modulus, fracture strength and aspect ratio. The elastic modulus increases with the decrease of temperature in low temperature environment, which greatly improves the frost resistance of concrete. Although the fiber has the same acid resistance as polypropylene fiber, it has poor alkali resistance. Due to the high softening temperature, it can not be melt spun, but is produced by solution spinning.
Compressive strength: due to its molecular chain structure, the elastic modulus of polypropylene fiber is only 1 / 10 ~ 1 / 8 of that of ordinary concrete, which is difficult to bear stress directly, but it has good stress transfer effect, so it has no obvious effect on the compressive strength of concrete. Polyacrylonitrile fiber can generally improve the compressive strength of concrete. Compared with polypropylene fiber, the diameter of polyacrylonitrile fiber is smaller (about 2 times of the former) and the amount of polyacrylonitrile fiber is more, which affects the bonding condition of adjacent fibers and reduces the tensile strength of monofilament; On the other hand, the polar groups on the fiber endow the fiber with good hydrophilicity, which promotes the bond between the fiber and the matrix. The higher specific surface area makes the fiber distribution more uniform and better transfer the stress. The higher fiber strength and elastic modulus also enhance the crack resistance effect. Flexural property: due to the hydrophilicity of polyacrylonitrile fiber, the interfacial hydration is not sufficient at 28 days. On the other hand, the bonding property of polyacrylonitrile fiber is good, and it is mostly broken in the test. The less and longer the number of fibers in the cross section, the more the fiber tends to break. However, this does not mean that the fiber has no slip, Therefore, the effect mechanism of polyacrylonitrile fiber on flexural strength is more complex than that of polypropylene fiber. Due to the poor adhesion of polypropylene fiber, there is slippage phenomenon in each group of specimens, and the failure mechanism is relatively simple. The key factors that affect the flexural strength of polyacrylonitrile fiber are strength and hydrophilicity, and the key of polypropylene fiber is the bonding condition.
Impact resistance: both fibers can effectively improve the impact resistance of concrete. Although both of them belong to flexible fibers, they have different energy consumption mechanisms in the failure process. From the degree of complete failure, polyacrylonitrile fiber is mostly broken, and part of polypropylene fiber is broken. Polyacrylonitrile fiber is well bonded, and it will break if it can not be fully deformed during the impact process. The main reason is that the fracture of the fiber offsets the impact energy; The diameter of polypropylene fiber is large, the tensile strength of monofilament is strong, and the elongation can be very large in crack propagation. In the impact process, the impact of fiber on the impact resistance of concrete can be attributed to two aspects: the first is to bridge the matrix on both sides of the crack to prevent the occurrence of cracks; the second is the process of fiber pulling out or breaking to resist the energy generated by the impact to prevent crack expansion.
Freeze thaw resistance: the influence of fiber on the freeze thaw resistance of concrete has two sides, on the one hand, the increase of the number reduces the bond strength, on the other hand, increases the probability of blocking cracks. For polyacrylonitrile fiber, if the length of monofilament is short (such as 6 mm), the number of fibers will become the primary factor, followed by the bond strength; If the length of monofilament fiber is long and the continuity is good (such as 19mm fiber), the monofilament adhesion will become the primary factor, followed by the number. The more prominent feature of polyacrylonitrile fiber is that its elastic modulus increases with the decrease of temperature. When it freezes, the elastic modulus increases significantly, which counteracts the expansion force due to water freezing to a greater extent; When the ice melts, the volume of water decreases, which has a positive effect on releasing some expansion energy due to the decrease of elastic modulus of the fiber. The adhesion of polypropylene fiber is poor, but the breaking force of monofilament is high. When the content is less than a certain amount, the length will become the primary factor, but the experiment shows that the content is very small, only when the content is 0.5kg/m3, the longer the polypropylene fiber group, the better the freeze-thaw resistance, but the further extension of the fiber length can also improve the freeze-thaw resistance.
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