Research On Composite Structure Performance Of FRP Bar-ultra High Performance Seawater Sea Sand Concrete

The contribution of marine resources to the global economic development is undoubtedly huge.Attributed to the advantages of geographical location near the sea,many countries and regions developed fast.The nearby use of sea sand and using FRP composite material to replace the steel in engineering construction can not only alleviate the lack of local river sand resources,but also fundamentally solve the problem of steel corrosion.Compared with ordinary concrete,ultra-high performance concrete(UHPC)has the characteristics of good compactness,high strength and excellent durability,which provides technical feasibility for the use of sea sand and FRP materials.Therefore,a UHPC was developed in the paper using non-desalinated sea sand.The influence of the amount of chopped basalt fiber,the ratio of length to diameter of basalt fiber,and the amount of coarse aggregate on the physical and mechanical properties of seawater sea sand UHPC was studied and optimized.The microscopic analyses were conducted to reveal the high performance mechanism and discuss the difference between seawater sea sand and river sand UHPCs using scanning electron microscopy(SEM),X-ray diffraction analysis(XRD)and Fourier infrared spectroscopy(FTIR).Combined with basalt fiber reinforcement(BFRP)with good corrosion resistance,6 seawater sea sand UHPC beams with different reinforcement ratios were designed to clarify the bending performance of the beams.The research conclusions are as follows:(1)The volume content of high-strength basalt fiber does not adversely affect the compressive strength of seawater UHPC.The flexural resistance of UHPC of sea water and sea sand increases with the increase of the volume of basalt fiber.In contrast,longer basalt fibers are easier to establish a spatial network structure inside UHPC,which limits the movement of internal aggregates,as a result seawater sea sand UHPC exhibits better mechanical properties.(2)The addition of proper amount of coarse aggregate can effectively improve the mechanical properties of seawater sea-sand UHPC,especially the compressive strength.Due to the synergistic effect of fiber,the addition of coarse aggregate also has a positive effect on the flexural strength of the specimen.However,the adverse effect appears while the coarse aggregate exceeds the optimal dosage value.(3)The early mechanical properties of seawater sea-sand UHPC are better than those of river sand UHPC,but the strength of river sand UHPC will be higher with the increase of curing age.From the microscopic view,the seawater sea sand UHPC is much denser in the early stage due to the reaction of chloride ions and cement hydration which produces a large number of insoluble substances to fill the concrete pores.After 28 d curing,calcium hydroxide is enriched in UHPC,and ettringite and Friedel’s salt generated by the hydration reaction of sulfate ions in seawater affect the overall density of UHPC.(4)The plane section assumption is always satisfied during the bending process of BFRP reinforced seawater sea sand UHPC beam.The reinforcing ratio has a great influence on the ultimate bearing capacity,deflection,maximum crack width,crack spacing,and crack height of the beam.The reinforcing ratio of BFRP bars has a little effect on the cracking load and the initial crack width.Insufficient thickness of the protective layer of concrete can easily lead to the splitting failure of the concrete in the tension zone,as a result the bearing capacity are not fully utilized.It is recommended that the thickness of the protective layer of the BFRP reinforced seawater sea sand UHPC beam should not be less than 25 mm.