Behavior Of FRP Confined Simulated Sea Water Concrete Short Columns

With the development of global marine economy,a large number of coastal projects need to be built in coastal ports and reefs,and the amount of concrete used is quite large.Freshwater is in great demand in the preparation and maintenance of concrete,and many parts of the world,especially islands and reefs,are facing the problem of shortage of freshwater resources.Therefore,the development and utilization of seawater resources has gradually come into people’s vision.Generally,seawater needs desalination treatment to be used in the preparation and maintenance of concrete.But seawater desalination is not only tedious but also expensive.If sea water can be directly used to configure concrete,local materials can be obtained to save time and cost.The biggest obstacle to the direct utilization of seawater is that harmful ions(Cl^-and SO₄^2-)in seawater can adversely affect concrete and steel bars.Fiber reinforced polymer（FRP）,as a new type of composite material,has advantages of light weight,high strength and corrosion resistance.In recent years,FRP has been gradually applied in industrial and civil construction fields.In this study,the advantages of seawater and FRP are complementary,and FRP is proposed to restrain seawater concrete in order to achieve the purpose of direct utilization of seawater and avoid the adverse effects of harmful ions in seawater on reinforcing bars.Fifteen simulated seawater with five different concentrations and three different components were prepared.The simulated seawater ion concentration was 0 times（i.e.fresh water）,0.5 times,1 times,2 times and 5 times of the global average ion concentration.According to the composition,the simulated seawater containing only Cl^-and SO₄^2-were prepared,and the simulated seawater containing both Cl^-and SO₄^2-was also included.For 15 kinds of simulated seawater,12 concrete cylinders of 150 mm*300 mm were poured.Among them,three are plain concrete cylinders,three are wrapped with a layer of basalt fiber（BFRP）,three are wrapped with two layers of BFRP,and three are wrapped with a layer of carbon fiber（CFRP）.All specimens were maintained under standard conditions for 28 days,and then were subjected to axial compression tests to test their mechanical properties.A total of 180standard cylinders were tested under axial compression.The stress and failure modes of short column specimens under different confinement conditions are analyzed,and the effects of FRP type,number of wrapping layers and simulated seawater types on the bearing capacity and stress-strain curve of simulated seawater concrete short columns are analyzed.The results show that the ultimate bearing capacity of concrete short columns is not greatly improved by wrapping a layer of BFRP,but the ductility of specimens is improved to a certain extent.The ultimate load-carrying capacity and ductility of the specimens are improved to a certain extent by encapsulating two layers of BFRP and one layer of CFRP.The effects of different ion concentration and composition on the mechanical properties of short columns were analyzed.The results show that the strength of seawater concrete has little effect with the increase of mixed ion concentration,but compared with unconstrained traditional concrete,the strength of seawater concrete decreases in 28 days with the increase of mixed ion concentration,and the decrease range is about 9%-16%.Finally,the Teng et al’s（2009）design model and Jiang and Teng（2007）analysis model are used to predict all the tests,and the predicted results are compared with the experimental results,which shows that the mechanical properties of the short columns confined by FRP have a improvement.