Abstract In this paper, the current effect on the crack resistance of polypropylene fibers in concrete sense, on the mechanism of polypropylene fiber concrete strength and impact resistance, such as blurred or outside of a few comments along to talk about controversial issues exist, and described. Concluded that the performance of polypropylene fiber concrete significant improvement, but not yet effective test evaluation methods to show the effect of polypropylene fibers.
Key words polypropylene fiber reinforced concrete crack resistance effect intensity

1 Introduction

Polypropylene fiber is a new type of concrete reinforcing fibers, known as concrete "secondary ribs" for the road bridge, lining Wall, floor and other parts of the project in recent years in our municipal, roads and buildings the project has been used more often. Since 1997, only polypropylene road and bridge projects in Tianjin in the amount of fiber reinforced concrete has reached more than ten million square meters, the total amount of nearly 20 tons of polypropylene fibers, most engineering application results are better.

Due to the incorporation of polypropylene fibers to improve the quality of the concrete, so that the integrated performance of concrete is improved, the Federal Highway strategic plan (SHRP) through a lot of trials after research and engineering experience can be concluded that organic fibers such as polypropylene fiber reinforced concrete when as a high performance concrete pavement. But author and construction, on-site exchange designers, engineers and technicians found that many of the effects of lack of polypropylene fibers in concrete understanding that the polypropylene fiber function is only to prevent the occurrence of plastic concrete cracking, while the non-performance of hardened concrete active role, or polypropylene fiber and steel fiber reinforced effect compared to incorporate fiber increase the degree of flexural (tensile) strength as evaluation criteria. Analysis that the persons concerned to know the function of the one-sidedness of polypropylene fibers primarily due to the limitations and static load strength of hardened concrete target of thinking long-existing concrete test evaluation method in the present situation. Discuss this article on the issue, and describes the author's views.

Effect 2 crack resistance of polypropylene fibers

Compared with conventional steel fibers, polypropylene fibers is characteristic fineness (equivalent diameter 0.02-0.1mm), multiple number (0.9kg / m3) 700-3000 dosage sufficiently dispersed obtain single fibers Wangen) In concrete fiber spacing is small. These characteristics make the polypropylene fiber can effectively limit the early (plastic and hardened initial period) due to the occurrence and development of concrete segregation, bleeding, and other factors in the formation of shrinkage cracks native, reduce the number of existing fractures and scales. The existing fractures are usually the origin of concrete damage or performance degradation. From this point of understanding, can be considered the significance of these effects of crack resistance of polypropylene fiber is not only effective in preventing the occurrence and development of cracks in concrete plasticity early, but also because of its significance to improve the continuity of the material through the media, to make the performance of hardened concrete significantly improved. For pavement and bridge deck concrete, due to the tension force to withstand bending and repeated impact load sensitivity to native cracks in concrete quantity and scale of high, native cracks decreases in the number and scale to improve its performance is very favorable a.

The problem is: (1) a necessary condition for crack resistance effect of polypropylene fiber is able to play at the same time early in the hardening concrete deformation is limited and dehydration contraction, while such conditions indoors or on-site production of small specimens are not have. This difference reflects the fact assessed using small specimens in the actual structure of concrete strength or resistance to external sexual inadequacies, polypropylene fiber reinforced concrete, this tendency becomes more apparent. Which resulted in the underestimation of the role of polypropylene fibers in the concrete. (2) effects of crack resistance of polypropylene fiber is still difficult to pass the test to evaluate Xuan. Although the staff simulated field conditions produced a variety of test equipment to study the effect of crack resistance of polypropylene fibers, but by the role of various causal factors, poor reproducibility of test results. These factors caused the blur effect of the use of polypropylene fibers undervalued and understanding.

The original purpose of the incorporation of the fibrous material in the concrete is easy to crack for the problems caused by lack of concrete tensile strength caused by the effect of fiber crack resistance in concrete actually improves the tensile strength of concrete is the performance, but because of the different varieties of elasticity modulus, tensile strength, aspect ratio, content and so different, the effect of crack resistance in concrete mechanism of action and the effect is not the same. Crack resistance of steel fiber effect is reflected in the crack propagation to prevent damage when hardened concrete is hardened by concrete after cracking still maintain a certain degree of tensile strength to achieve, crack resistance effect is to increase the role of the result of deformation of hardened concrete capacity, so that after the destruction of the concrete base remains some ductility (false ductility), therefore, the ability and crack resistance of steel fiber elastic modulus fiber interfacial bond strength and tensile strength itself relevant. The effects of crack resistance of polypropylene fibers mainly in eliminating or reducing the occurrence and development of early concrete cracks natively, and simple to understand can be considered to improve the tensile strength of concrete early implementation, further analysis of the results by polypropylene fiber is poly acrylic fibers passivation native crack tip stress concentration, the stress field within the media more continuous and uniform actions. Since the early tensile strength of the concrete itself is very low, therefore, the tensile strength of polypropylene fibers and fiber capacity crack resistance, modulus of elasticity is not significantly related to other indicators, but with the increase of fiber fineness, fiber concrete pitch is decreased and increased.

Through the above analysis shows that the effect of polypropylene fibers and crack resistance of steel fiber can not be substituted for one another, due to improved performance in different periods were concrete. For bridge deck concrete pavement, no practical significance crack resistance of steel fiber reinforced concrete after effects crack can only play, but prone to performance deficiencies plastic concrete cracking in early but it can be resolved or improved by the incorporation of polypropylene fibers.