Effect Of Alkalinity Of Seawater Sea Sand Concrete On Durability Of BFRP Bars

Basalt fiber reinforced composite(BFRP)bars have high specific strength and resistant to chloride attack,making them very suitable as substitution of rebars in marine concrete structures to solve the durability problem caused by corrosion of steel bars.However,BFRP bars can be corroded in the strong alkaline environment of concrete.The durability issues of BFRP bars can exist after long-term service in humid marine environments.In order to address this problem,this paper investigated the feasibility of reducing alkalinity of seawater sand concrete(SWSSC)through optimization of cementitious materials,expecting to slow down the degradation of BFRP bars and improve their service life.The accelerated aging tests of BFRP bars,BFRP bars and SWSSC bonding properties,and of BFRP bars reinforced SWSSC columns were performed,with the aims of assessing the influence of alkalinity on the durability of BFRP bars reinforced SWSSC structures under marine environment.Microstructural analysis techniques and related theoretical models are utilized to further analyze experimental results.The works of this study provides ideas for the design of seawater sand concrete materials and structures with high durability BFRP bars.The main research contents and results are as follows:(1)The mechanical degradation of BFRP bars immersed in simulated pore solution of SWSSC and wrapped by seawater and sea sand mortar with different alkalinities were evaluated by accelerated aging tests.The degradation degree and mechanism of BFRP bars were analyzed by Fourier transform infrared spectroscopy(FTIR),thermogravimetric analysis(TGA),scanning electron microscopy(SEM),inductivelycoupled plasma mass spectrometry(ICP-MS)and X-ray microcomputed tomography(X-CT).The results show that reducing the alkalinity of SWSSC can significantly reduce the deterioration of BFRP bars.When the p H value of the simulated pore solution decreased from 13.2 to 10.1,or the p H value of seawater sand mortar decreased from 12.5 to 11.1,the interlaminar shear strength retention and tensile strength retention of BFRP bars increased by more than 40%.The plasticization and hydrolysis of resin matrix,the debonding of fiber-resin interface,the alkali etching and elemental leaching of fibers also greatly mitigated(2)The effective p H threshold to depress alkaline corrosion of BFRP bars is proposed.The advantage of mitigating the degradation of BFRP bars gradually diminishes as alkalinity decreases.After accelerated aging at 55℃,the interlaminar shear retention and tensile strength retention of BFRP bars increased by more than 40%as the p H value of the simulated pore solution decreased from 13.2 to 12.3,while whereas lowering the p H from 12.3 to 10.1 enhances the tensile strength retention by less than3%.When the p H value of the seawater sea sand mortar decreases from 12.5to 11.6,the respective interlaminar shear retention and tensile strength retention of the embedded BFRP bars increase by 46.51% and 38.95%.As the p H further reduced from11.6 to 11.1,the retention differences are less than 5%.In general,speaking,the alkaline corrosion to BFRP bars can be basically eliminated by controlling the p H value of concrete below 12.(3)The low-alkalinity SWSSC was designed with ordinary Portland cement(OPC),silica fume(SF)and fly ash(FA)through the simplex centroid design method,and the basic physical and mechanical properties of the low-alkalinity SWSSC were studied.The evolutions of micro-morphology and hydration phase were characterized by isothermal calorimeter,SEM,X-ray Diffraction(XRD)and TGA.The experimental results indicated that the low-alkalinity SWSSC with p H less than 12 can be prepared by 35% cement,15% silica fume and 50% fly ash.This SWSSC can meet the multiple requirements of strength,workability and volume stability as required for ordinary high-performance concrete.Compared with normal SWSSC,the microstructure of lowalkalinity SWSSC is more compact,the hydration heat release is less,and the content of calcium hydroxide,AFm phase and Friedel’s salt in hydration products is reduced.(4)The bonding durability between BFRP bars and low-alkalinity SWSSC was evaluated by central pullout test.The results show that the bonding durability of BFRP bars embedded in low-alkalinity SWSSC significantly outperformed these embedded in ordinary SWSSC because of the less affected BFRP bars.Compared with the 28-days unaged specimens,the bond strength retention of the ordinary SWSSC specimens decreased to 73.48% after 270 days of accelerated aging at 55℃,while the bond strength retention of the low-alkalinity SWSSC specimens increased to 107.11%-110.63%.(5)The durability of sea sand concrete columns reinforced with BFRP bars was studied by axial compression tests.Compared with the unaged specimens,the failure process and failure mode of BFRP bar reinforced ordinary and low-alkalinity SWSSC columns remains unchanged after the accelerated aging in seawater,and the bearing capacity of the columns slightly improved(about 5%).However,the corrosion of BFRP reinforcements in the aged ordinary SWSSC column is serious.The corrosion of longitudinal reinforcement leads to uncoordinated deformation between SWSSC and the longitudinal BFRP bars.The corrosion of stirrup considerably weakened its lateral constraint on the core concrete,resulting in the loss of the load-holding process of the load-displacement curve and the 30% decrease in ductility of the column specimen.In contrast,the corrosion of BFRP bars inside the conditioned low-alkalinity SWSSCC column is slight.Consequently,the deformation longitudinal BFRP bars and SWSSC is coordinated and the binding capacity of stirrup are well maintained,which contributed to the persistence of the load-holding stage and the ductility of the lowalkalinity column specimen.(6)Referring to the existing FRP long-term performance prediction model,the effects of temperature and humidity on the degradation rate of BFRP bars under different alkalinity were analyzed and discussed.The prediction results show that the environmental impact factor of BFRP bars wrapped by ordinary SWSSC is 0.44,while that wrapped by low-alkalinity SWSSC is 0.85 in the environment of high humidity(100%RH,seawater submerged area and tidal zone).For BFRP reinforced concrete structures exposed to marine atmosphere,the prediction results based on indoor accelerated aging experiment should be refined with the change of monthly average atmospheric temperature and annual average humidity.According to the prediction results,the environmental impact factor of BFRP bars in ordinary SWSSC is 0.77 in atmospheric region,and that of BFRP bars in low-alkalinity SWSSC is 0.93.