Study Of The Service Properties Of Copper-Plated Steel Fiber Reinforced Ultra-High Performance Concrete

Ultra High-Performance Concrete(UHPC)is an ultra-high strength,ultra-durable concrete matrix composite material and has been importantly used in modern building structures.UHPC is often mixed with steel fibers to increase the strength and toughness of the matrix.However,the long-term corrosion resistance,fiber structure and stressstrain behavior of the fiber-matrix interface of Brass Coated Steel Fiber(BCSF)reinforced UHPC have not been studied in depth.In this study,the effect pattern of the type and content of BCSF on the static mechanical properties of UHPC was studied to determine the optimal BCSF addition content based on the variation pattern of UHPC macroscopic porosity.Finite element analysis(FEA)was used to analyze the relationship between structure and properties during fiber/matrix pullout process.The dry and wet cycles,immersion and salt spray tests were used to study the erosive effects of sulfate and chloride salts on straight BCSF-reinforced UHPC,and the patterns of p H and chloride ion diffusion in UHPC in the pore fluid were discussed.The main research conclusions are as follows:(1)Increasing the content of BCSF in different shapes(straight,corrugated and hooked),the flexural and compressive strengths of BCSF-reinforced UHPC were improved.The flexural strength of straight BCSF-reinforced UHPC was higher than that of corrugated and hooked BCSF-reinforced UHPC,and the compressive strength of corrugated BCSF-reinforced UHPC was higher than that of straight and hooked UHPC.Compared with corrugated and hooked BCSF-reinforced UHPC,straight BCSF-reinforced UHPC has slightly higher resistance to crack expansion.The straightshaped BCSF content increased from 1 wt.% to 6 wt.%,the crack length decreased from34.2 mm to 12.1 mm,and the average crack width decreased from 0.78 mm to less than0.1 mm.(2)The fiber content increased from 1 wt.% to 6 wt.%,and the macroscopic pores in UHPC were reduced,and the porosity decreased from 18.4% to 8.3%.Most of the macroscopic pores were distributed in the upper part of the specimen,and the porosity decreased from 15% to 8% from top to bottom.With the BCSF content increasing from4 wt.% to 8 wt.%,the BCSF inside the specimen appeared significantly interwoven,and the BCSF connection rate increased sharply.The BCSF content increase leads to a large number of BCSF entanglement inside the specimen to form a mesh structure,and the volume resistance of UHPC decreases largely.(3)After wet and dry cycles of sulfate attack,the flexural strength of BCSFreinforced UHPC increased and then decreased in the age of leaching.Increasing the number of cycles,sulfate erosion could be divided into three stages.After the 5%Na Cl immersion test,the flexural strength of the copper-plated fiber specimens was higher compared to the non-copper-plated fiber specimens.Under salt spray conditions,the stable p H pore solution moved towards the interior of UHPC after stabilization by extending the erosion time.The chloride ion concentration in UHPC decreased with increasing depth of UHPC.(4)At 90° perpendicular to the matrix cross section,there was no stress concentration at the end of the steel fiber,and the resistance was the internal friction between the steel fiber and the matrix when pulling out.At 30° of inclination between steel fiber and matrix section,the steel fiber was embedded in the end of the matrix to produce stress concentration.Steel fibers with inclined angles are hindered from being pulled out due to compressive stress on the substrate.The steel fiber stress-strain transfer caused microcracks at the fiber-matrix connection to limit the fracture and crack expansion of the fragile matrix.