Study On Uniaxial Tensile Properties And Constitutive Relation Of Steel-polypropylene Hybrid Fiber Reinforced Concrete

Fiber reinforced concrete has been widely used in the field of civil engineering at home and abroad for its excellent physical and mechanical properties. However, single fiber reinforced concrete generally have certain advantages in terms of some respects only, which restrict its application in some ways. Hybrid fiber reinforced concrete, which have been mixed with two or more kinds of fibers, may have excellent integrated performance such as strength, toughness and durability, has attract more and more attention of research community or engineering world. So far great progress has been acquired on the study of mechanical properties and theory of fiber reinforced concrete. However, the study of hybrid reinforced concrete is at an early stage, and there are no relevant provisions in the current specification. Many problems about the mechanical properties and constitutive relation are urgently need to be addressed. Given this, based on the previous research work of the research group and supporting of NSFC project (No.51078295), the author systematic study the tensile properties and tensile constitutive relation of steel-polypropylene hybrid fiber reinforced concrete (which will be hybrid fiber reinforced concrete for short) through uniaxial tensile test. The main contents and results are as follows:(1) In consideration of three major factors of fiber types, aspect ratio and volume ratio,35batches specimens were poured. Axial tensile test was conducted to study fiber content and aspect ratio effect on axial tensile strength. Computing formula for fiber characteristic parameters effect on on axial tensile strength of hybrid fiber reinforced concrete are presented by regression analysis. Results of the test manifest that tensile strength of hybrid fiber reinforced concrete are significantly improved compared to matrix concrete with the same mixing proportion, the average increasing amplitude are:initial cracking strength32.7%, peak strength47.5%. When other factors remain unchanged, the increasing of steel fiber content can improve initial cracking strength and sharply improve peak strength, the increasing of polypropylene fiber content can improve initial cracking strength and slightly improve peak strength, and the increasing of steel fiber aspect ratio can proportionally improve tensile strength.(2) Based on the experimental results, using a combination of statistical and factor analysis, a quantitative analysis is carried out for the fiber’s effect on the axial tensile strain of hybrid fiber reinforced concrete. Computing formula for fiber characteristic parameters effect on on axial tensile strain of hybrid fiber reinforced concrete are presented by regression analysis. When other factors remain unchanged, the increasing of steel fiber content can improve initial cracking strain and significantly improve peak strain, the increasing of polypropylene fiber content can significantly improve initial cracking strain and improve peak strength, and the increasing of steel fiber aspect ratio can slightly reduce initial cracking strain and significantly improve peak strain.(3) Analysis the tensile fracture failure mode of hybrid fiber concrete by means of experiments and theoretical research, reveal the mechanism of hybrid fiber, found the tensile failure criterion of fiber reinforced concrete. The results shows that:the tensile failure process of hybrid fiber concrete is slow and steady, showing obvious plastic characteristics; the whole process of axial tension can be divided into six stages: elastic stage, micro cracks extending stage, macro-cracks extending stage, fracture stage, sustained damage stage, convergence stage, steel fibers and polypropylene fibers play distinct roles of progressively crack resistance and reinforcement in various stages.(4) Based on the experimental measured axial tensile stress-strain curves, regarding the influence of the fiber content and aspect ratio, the axial tensile stress-strain curve equation of fiber reinforced concrete was proposed through theoretical analysis and derivation. The equation is corresponding to the uniaxial tensile stress-strain curve equation of common concrete in present specification (GB50010-2010). This research conclusion provided provide theoretical support and experimental basis for improving present specification, and provide some reference for engineering design and computational analysis.Finally, based on the full thesis, some recommendations for further research is proposed.