Study On Compressive Behavior Of CFRP-confined Seawater Sea-sand Concrete Columns Under Different Wrapping Schemes And Environmental Conditions

Due to the increasing shortage of traditional natural resources(i.e.,river sand and fresh water)and the need for sustainable development,using abundant seawater and sea-sand for concrete mixing in marine and coastal constructions can not only alleviate resource shortage problems but also significantly save transportation costs and construction period.Considering the extraordinary mechanical properties and excellent durability of fiber-reinforced polymer(FRP),the idea of using seawater sea-sand concrete(SSC)with FRP can effectively eliminate steel corrosion problems,which facilitates the applications of the novel composite structures.Although some projects have been conducted on FRP wrapped SSC columns,no research has been concerned with the short-term and long-term behavior of FRP confined SSC with different wrapping schemes and exposed environments.Therefore,comprehensive studies related to the CFRP wrapped SSC columns were carried out in this project via experimental and theoretical methods to investigate several critical problems of the axial compressive performance of CFRP confined SSC columns with various FRP wrapping strategies under different environmental conditions.The main contents and conclusions are stated as follows:(1)Effects of various test parameters on the compressive behavior of partially CFRP confined seawater sea-sand concrete short columns were investigated by experimental study and theoretical analysis.The results reveal that the usage of seawater and sea-sand in concrete has a negligible effect on the mechanical performance of wrapped columns.However,due to the existence of unconfined regions,obvious transition segments and strain-softening behavior easily occur for axial stress-strain responses.The clear spacing and confinement stiffness of FRP strips are the two most important factors for the stress-strain relationships and dilation properties.Decreasing the clear spacing ratios can significantly enhance the slopes of post-peak branches of stress-strain curves,leading to the improvement of ultimate strengths and corresponding strains,especially for the specimens with higher FRP volumetric ratios.Besides,the dilation of concrete core decreases with the increment of FRP layers and the reductions are heavily related to the clear spacing ratio.Based on the test observations,a new lateral strain-axial strain relationship and an analysis-oriented stress-strain model are proposed with satisfactory accuracy.(2)An experimental investigation was conducted on the compressive performance of CFRP nonuniformly wrapped seawater sea-sand concrete short columns.According to the test observations,the failure process of nonuniformly wrapped specimens exhibits a less brittle manner compared with that of fully and partially wrapped concrete columns,which can provide some warning signs before the final failure.Moreover,nonuniformly wrapped specimens possess the highest strain efficiency coefficients among the specimens with different wrapping schemes under the same FRP volumetric ratio.With the decrease of FRP clear spacing ratios,although more conspicuous improvements of ultimate strength are observed,the nonuniformity of the lateral deformation of concrete along the height decreases,resulting in the reduction of superiority of nonuniform wrapping schemes.Based on the test results,a unified formula for confining pressure is developed by introducing a new nonuniform confinement effective coefficient to better depict the confinement mechanism in nonuniformly FRP confined concrete.Besides,a new design-oriented stress-strain model and predicted formulae for ultimate conditions are also proposed in this study based on the above-mentioned confinement effective coefficient.(3)The long-term behavior of CFRP confined SSC columns with various wrapping strategies was explored under simulated marine environments.The correlations among different variables were assessed based on the properties’ degradations of FRP,SSC and confined specimens.Test results show that no obvious material degradation is observed for CFRP,while the compressive strength of unconfined concrete monotonically increases with the increase of aging time,which leads to the significant distinctions between compressive strength retentions and compressive strength enhancement retentions of CFRP wrapped specimens.Compared with the other two wrapping schemes,the specimens wrapped with CFRP strips possess higher compressive strength retentions.Different strength degradation coefficients are proposed as per the wrapping schemes and a series of refined ultimate strength formulae is obtained by introducing the above coefficients into the existing predicted models.(4)The axial compressive behavior of medium-scaled CFRP partially wrapped SSC columns internally reinforced with epoxy-coated reinforcement(ECR)was studied.The results show that the hoop strains recorded for the CFRP strips near the column corner are smaller than those measured at the center of the column side during the loading process regardless of the wrapping strategies and clear spacing ratios.After the yield displacement,the influence of wrapping schemes becomes more obvious on the development of CFRP and stirrups strains.Higher confinement efficiency exerted by the CFRP strips can be achieved when the CFRP strips are placed in between the internal steel stirrups longitudinally,which leads to more gradual post-peak behaviors and larger ultimate axial deformations.However,the location of external FRP strips has little impact on the load capacity.Based on the test observations,new ultimate strength and corresponding strain models are developed and a simpler load capacity formula is also proposed.The predicted results provided by the above-mentioned theoretical models all match reasonably well with the test results.(5)The long-term behavior of ECR-SSC columns wrapped with CFRP strips was investigated under a simulated marine environment.It can be found from the experimental results that with the increase of aging time,the development of the compressive strength of seawater sea-sand concrete shows an ascending-descending trend,while the environmental condition has little influence on the mechanical properties of ECR and CFRP.The exposure has a significant impact on the load-displacement curves,especially for the peak load and post-peak behavior.But the external CFRP confinement can alleviate the negative effect of exposure.Compared with fully wrapped ECR-SSC columns,higher bearing capacity and the corresponding retentions can be achieved by the specimens wrapped with FRP strips.Thus,considering the ductility,energy absorption and material usage,it is more reasonable to adopt the partial wrapping scheme for strengthening to reduce the detrimental effect of environmental exposure.Finally,a new load capacity formula is proposed,which can consider the negative effect of conditioned exposure.