Experimental Study On Hybrid Fiber Reinforced Concrete Under The Coupling Action Of Mixed Salt Freeze Thaw Cycles

Concrete in cold saline soil areas has suffered from long-term salt freezing,causing a serious decline in its safety and durability,so the problem of salt damage to concrete has been the focus of research in the field of civil engineering at home and abroad.It has been shown that the incorporation of steel fibres or polypropylene fibres in concrete can effectively improve the crack resistance and toughening ability of concrete and improve the salt freeze damage of concrete.At present,scholars at home and abroad on a single fiber concrete in the chloride salt or sulfate under the action of freeze-thaw cycles of salt freezing performance is more concentrated,while the mixed salt solution and freeze-thaw environment of mixed fiber concrete damage law and model research is less.Therefore,in this paper,the basic mechanical properties tests and nitrogen adsorption tests on mixed fibre concrete of C40 under the coupled action of mixed salt（3.5%Na Cl+3.5%Na₂SO₄）and freeze-thaw cycles are carried out with the number of salt freezes,fibre types and admixtures as the test variables,and the main research contents are as follows:（1）Study on salt frost resistance of hybrid fiber reinforced concrete.Every 25salt freezing cycles to the end of 200 cycles,five groups of hybrid fiber reinforced concrete（steel fiber SF:0.8%and 1.2%;polypropylene PPF:0.11%）cubes and trabeculas with different fiber ratios（steel fiber SF:0.8%and 1.2%）were tested to study the salt freezing durability.The evaluation indexes of salt freezing resistance were analyzed from three aspects:apparent morphology,mass loss rate and relative dynamic elastic modulus The results show that the salt content has an effect on the salt frost resistance of HFRC.（2）Study on the basic mechanical properties of hybrid fibre concrete under the action of salt freezing cycles.Five groups of hybrid fibre concrete were tested every50 salt freezing cycles.The damage pattern and strength loss of the specimens were analysed,and the mechanism of the effect of the number of salt freezing cycles and the type and admixture of fibres on the compressive,tensile and flexural strength of the concrete was investigated.At the same time,the Qiana model was improved by taking into account the boundary condition of the number of freeze-thaw cycles n=0,and the strength decay model of fibre concrete under the action of salt-freezing cycles was established.（3）Study on the microscopic pore structure of hybrid fiber concrete under the action of salt-freezing cycle.Nitrogen adsorption experiments were carried out for hybrid fiber reinforced concrete（HFRC）.The pore size distribution,BET surface area,cumulative pore volume and the most pore size before and after the salt-freezing cycle were analyzed,and the effects of salt-freezing cycle and fiber inclusion on the microstructure of HFRC were discussed.Moreover,the damage mechanism of salt solution and freeze-thaw action was analyzed and studied.（4）Damage analysis of mixed fibre concrete under the action of salt freezing cycles.The damage degree D（n）is introduced to describe the damage degree inside the concrete specimen caused by salt freezing,and the evolution model of salt freezing damage of hybrid fibre concrete based on Weibull distribution is proposed,and combined with the mechanical decay model,the evolution equation of salt freezing damage degree of hybrid fibre concrete expressed by relative strength is derived.