Experimental Study On Durability Of Steel-polypropylene Hybrid Fiber Reinforced Concrete On Salt-frozen Environment

Hybrid fiber reinforced concrete was first proposed in the 1970 s,followed by vigorous development due to its excellent mechanical properties and a more reasonable economic effect.At present,hybrid fiber reinforced concrete has become a research hotspot because of its broad application prospects in the airport runway,large span structure,large tunnel lining and other engineering applications.At present,the research on the mechanical properties of hybrid fiber reinforced concrete is mainly focused on the ordinary concrete level while the research results are relatively to see for the hybrid fiber reinforced high strength concrete,and its durability research more focused on single factor,two factors and multi-factor research rarely reported.In this paper,the durability of steel-polypropylene hybrid fiber reinforced concrete under the influence of chloride salt and freeze-thaw cycles was studied.The effects of steel fiber content,steel fiber type and polypropylene fiber content on the basic mechanical strength,anti-freezing performance and chloride ion permeability of hybrid fiber reinforced concrete were analyzed and the main research content is as follows:(1)The effect of steel-polypropylene hybrid fiber on the mechanical strength of high strength concrete was studied and the characteristic parameters of hybrid fiber reinforced concrete were analyzed by Rapid Air457 instrument.The results show that the effect of hybrid fiber on the tensile strength of high-strength concrete is much higher than that of compressive strength,which is up to 96.9%;the effect of steel fiber type on the splitting tensile strength of hybrid fiber reinforced concrete is significant,but it has little effect on the compressive strength;the effect of hybrid fiber on the internal pore structure of concrete is better than that of single fiber which makes the number and the specific surface area of concrete bubbles increasing,the bubble spacing coefficient and the bubble average chord length decreasing,that is,the internal structure of the concrete is moving toward the small aperture direction.(2)Experimental study were carried out on the macro-performance degradation of hybrid fiber reinforced concrete under chloride salt and freeze-thaw cycles.The results show that the content of steel fiber 1% and polypropylene fiber 0.1% is the best dosage in this experiment and under which the mass loss rate of hybrid fiber concrete is obviously lower than that of ordinary one;The law of saturated surface dry water absorption of hybrid fiber concrete is consistent with the law of mass loss and the saturated surface dry water absorption can be used to evaluate the salt-frozen damage;the relative dynamic elastic modulus of hybrid fiber reinforced concrete is much lower than the mass loss rate and the relative dynamic elastic modulus is not suitable as the hybrid fiber concrete salt-frozen index;the residual mechanical strength of hybrid fiber concrete is much larger than that of the ordinary one after salt-freezing cycles.(3)The chloride ion diffusion law of hybrid fiber reinforced concrete was studied and the chloride ion diffusion coefficient was calculated according to Fick's second law,with which the correlation between the diffusion coefficient of chloride ion by RCM method was established.The results show that the change of free chlorine ion content of hybrid fiber concrete under salt-freezing environment is that the peak concentration first increase and then deep into concrete,and the chloride ion concentration in the deep layer is gradually increased and which is not affected by steel fiber content,steel fiber type,polypropylene fiber content and other factors;with the increase of the freeze-thaw cycles,the diffusion coefficient of chloride ion of hybrid fiber reinforced concrete is increasing and the existence of hybrid fiber reduces the chloride ion permeability of concrete;the diffusion coefficient of chloride ion in hybrid fiber concrete determined by RCM method is obviously higher than that of the actual situation and the reason may be that the hybrid fiber concrete contained steel fiber has a higher conductivity,that is to say,the RCM method is not suitable for the determination of chloride diffusion coefficient of steel fiber reinforced concrete.