Effect of Carbonation and Drying-Wetting Cycles on Chloride Diffusion Behavior of Coral Aggregate Seawater Concrete

Based on seawater immersion,drying-wetting cycles,carbonation and drying-wetting cycles for coral aggregate sea-water concrete(CASC)with different strength grades,the effect of carbonation and drying-wetting cycles on chloride diffusion be-havior of CASC is studied.The results show that the free surface chloride concentration(Cs),free chloride diffusion coefficient(Df)and time-dependent index(m)of CASC in the drying-wetting cycles is obviously higher than that in seawater immersion.The Df and m of CASC of carbonation and drying-wetting cycles is higher than that in the drying-wetting cycles.Carbonation increases the Df and m of CASC,which is against CASC to resist chloride corrosion.The corrosion possibility of CASC structures in different ex-posed areas is as follows:splash zone(carbonation and drying-wetting cycles)>tidal zone(drying-wetting cycles)>underwater zone(seawater immersion).Besides,the chloride diffusion rate of C65-CASC is 17.8%-63.4%higher than that of C65-ordinary aggre-gate concrete(OAC)in seawater immersion(underwater zone).Therefore,anti-corrosion measures should be adopted to improve the service life of CASC structure in the oceanic environment.

Effect of carbonation-drying-wetting on durability of coral aggregate seawater concrete

Based on the drying-wetting cycles experiment and the carbonation-drying-wetting cycles experiment for coral aggregate seawater concrete(CASC)with different strength grades,the effects of carbonation-drying-wetting on the durability of CASC are studied with the surface state,mass loss rate,relative dynamic elastic modulus,ultrasonic wave velocity and cube compressive strength as indices.Results show that the mass loss rate of CASC increases gradually with the increase in cycle times in the drying-wetting and carbonation-drying-wetting cycles.The mass loss rate increases relatively slowly at the initial stage but it increases remarkably after 10 cycles.The relative dynamic elastic modulus and ultrasonic wave velocity decrease gradually with the increase in cycle times.After 6 cycles,the decrease rate of the relative dynamic elastic modulus and ultrasonic wave velocity of CASC tends to be flat and the surface is slightly damaged.Compared with the initial 28 d cube compressive strength,the cube compressive strength of CASC decreases by 8.8%to 11.0%.Drying-wetting cycles and carbonation can accelerate seawater erosion on CASC,and drying-wetting cycles result in salting-out and accelerate the destruction of concrete.Therefore,the carbonation-drying-wetting accelerates the destruction of CASC.

Bond of Seawater Scoria Aggregate Concrete to Stainless Reinforcement

This study investigates the bond between seawater scoria aggregate concrete(SSAC)and stainless reinforcement(SR)through a series of pull-out tests.A total of 39 specimens,considering five experimental parameters—con-crete type(SSAC,ordinary concrete(OC)and seawater coral aggregate concrete(SCAC)),reinforcement type(SR,ordinary reinforcement(OR)),bond length(3,5 and 8 times bar diameter),concrete strength(C25 and C30)and concrete cover thickness(42 and 67 mm)—were prepared.The typical bond properties(failure pattern,bond strength,bond-slip curves and bond stress distribution,etc.)of seawater scoria aggregate concrete-stainless rein-forcement(SSAC-SR)specimen were systematically studied.Generally,the failure pattern changed with the con-crete type used,and the failure surface of SSAC specimen was different from that of OC specimen.SSAC enhanced the bond strength of specimen,while its effect on the deformation of SSAC-SR was negative.On aver-age,the peak slip of SSAC specimens was 20%lower while the bond strength was 6.7%higher compared to OC specimens under the similar conditions.The effects of variables on the bond strength of SSAC–SR in increasing order are concrete type,bond length,concrete strength and cover thickness.The bond-slip curve of SSAC-SR specimen consisted of micro-slipping,slipping and declining stages.It can be obtained that SSAC reduced the curve curvature of bond-slip,and the decline of curve became steep after adopting SR.The typical distribution of bond stress along bond length changed with the types of concrete and reinforcement used.Finally,a specific expression of the bond stress-slip curve considering the effects of various variables was established,which could provide a basis for the practical application of reinforced SSAC.

Corrosion Behavior of Epoxy-Coated Rebar with Pinhole Defect in Seawater Concrete

Experiments were carried out to investigate the corrosion behavior of epoxy-coated rebar （ECR） with pinhole defect（diameter in hundreds of microns） immersed in the uncarbonated/carbonated simulated pore solution （SPS） of seawaterconcrete. Corrosion behavior was analyzed by electrochemical impedance spectroscopy. The composition and morphologyof corrosion products were characterized by X-ray diffraction, energy-dispersive spectrometry and scanning electronmicroscopy. Meanwhile, oxide film produced by preheating before spray coating was investigated by X-ray photoelectronspectroscopy and Mott-Schottky technology. Results indicated that corrosion behavior of ECR with pinhole defectexhibited three stages when immersed in the uncarbonated/carbonated SPS. In the initial stage, steel in defect waspassivated when exposed in the uncarbonated SPS and corroded when exposed in the carbonated SPS, due to competitiveadsorption between chloride and hydroxyl ions. In the second stage, the oxide film under coating reconstituted （thethickness and defects density decreasing） in the uncarbonated SPS, which was caused by the synergy between highhydroxide and chloride activity, while in the carbonated SPS, crevice corrosion happened under the coating around pinhole,because of the different oxygen concentrations cell at the coating/steel interface. In the third stage, localized corrosionoccurred under the coating around the pinhole in the uncarbonated SPS, which was probably induced by ion diffusion at thenano-scale coating/steel interface. The corrosion products adjacent to the defects were re-oxidized from FeCIa.4HaO andFe2（OH）3Cl to Fe2O3.H2O, and the corrosion area was expanded outward in the carbonated SPS.

Experimental study on bond behavior between BFRP bars and seawater sea-sand concrete

Combining fiber reinforced polymer(FRP)with seawater sea-sand concrete(SSC)can solve the shortage of river sand that will be used for marine engineering construction.The bond performance of BFRP bars and SSC specimens is researched by pull-out test in this paper.The effects of the parameters,such as bar type,bar diameter,concrete type and stirrup restraint,are considered.It is beneficial to the bonding performance by the reduction of bar diameter.The utilization of seawater sea-sand has a low influence on the bond properties of concrete.The bond strength of BFRP is slightly lower than the steel rebar,but the difference is relatively small.The failure mode of the specimen can be changed and the interfacial bond stress can be improved by stirrups restraint.The bond-slip curves of BFRP ribbed rebar include micro slip stage,slip stage,descent stage and residual stage.The bond stress shows the cycle attenuation pattern of sine in the residual stage.In addition,the bond-slip model of BFRP and SSC is obtained according to the experimental results and related literature,while the predicted curve is also consistent well with the measured curve.

Compressive behavior and microstructure of concrete mixed with natural seawater and sea sand

Noncorrosive reinforcement materials facilitate producing structural concrete with seawater and sea sand.This study investigated the properties of seawater and sea sand concrete(SSC),considering the curing age(3,7,14,21,28,60,and 150 d)and strength grade(C30,C40,and C60).The compressive behavior of SSC was obtained by compressive tests and digital image correction(DIC)technique.Scanning electron microscope(SEM)and X-ray powder diffraction(XRD)methods were applied to understand the microstructure and hydration products of cement in SSC.Results revealed a 30%decrease in compressive strength for C30 and C40 SSC from 60 to 150 d,and a less than 5%decrease for C60 from 28 to 150 d.DIC results revealed significant cracking and crushing from 80%to 100%of compressive strength.SEM images showed a more compact microstructure in higher strength SSC.XRD patterns identified Friedel’s salt phase due to the chlorides brought by seawater and sea sand.The findings in this study can provide more insights into the microstructure of SSC along with its short-and long-term compressive behavior.

Prediction of residual tensile strength of glass fiber reinforced polymer bars in harsh alkaline concrete environment using fuzzy metaheuristic models

The long-term durability of glass fiber reinforced polymer(GFRP)bars in harsh alkaline environments is of great importance in engineering,which is reflected by the environmental reduction factor in vari-ous structural codes.The calculation of this factor requires robust models to predict the residual tensile strength of GFRP bars.Therefore,three robust metaheuristic algorithms,namely particle swarm optimiza-tion(PSO),genetic algorithm(GA),and support vector machine(SVM),were deployed in this study for achieving the best hyperparameters in the adaptive neuro-fuzzy inference system(ANFIS)in order to obtain more accurate prediction model.Various optimized models were developed to predict the tensile strength retention(TSR)of degraded GFRP rebars in typical alkaline environments(e.g.,seawater sea sand concrete(SWSSC)environment in this study).The study also proposed more reliable model to predict the TSR of GFRP bars exposed to alkaline environmental conditions under accelerating laboratory aging.A to-tal number of 715 experimental laboratory samples were collected in a form of extensive database to be trained.K-fold cross-validation was used to assess the reliability of the developed models by dividing the dataset into five equal folds.In order to analyze the efficiency of the metaheuristic algorithms,multiple statistical tests were performed.It was concluded that the ANFIS-SVM-based model is robust and accu-rate in predicting the TSR of conditioned GFRP bars.In the meantime,the ANFIS-PSO model also yielded reasonable results concerning the prediction of the tensile strength of GFRP bars in alkaline concrete en-vironment.The sensitivity analysis revealed GFRP bar size,volume fraction of fibers,and pH of solution were the most influential parameters of TSR.

Modifying effects and mechanisms of superfine stainless wires on microstructures and mechanical properties of ultra-high performance seawater sea-sand concrete

Ultra-high-performance seawater sea-sand concrete(UHPSSC)presents a prospective solution to address the natural resource shortage in marine infrastructure construction.To eliminate the corrosion risk of steel fibers and broaden the applicability of UHPSSC,this study investigates the mechanical properties and free chloride ion content as well as microstructures of UHPSSC reinforced with superfine stainless wires(SSWs)under natural curing.The results indicate that 1.5%SSWs can remarkably improve the flexural strength and toughness of UHPSSC by 127%and 1724%,respectively,and mitigate the long-term strength degradation of UHPSSC.The strong interfacial bond between SSW and UHPSSC improves the compactness of UHPSSC,thus reducing the growth space for Ca(OH)_(2) crystals and swelling hydration products generated by sulfate and magnesium ions.This can be supported by the observed reduction in the Ca/Si ratio of C–S–H gels,CH crystal orientation index,and porosity.Moreover,through mechanisms such as pull-out,rupture,overlapping network,and internal anchor interface,SSWs effectively prevent microcrack growth and propagation,transforming single long-connected microcracks into multiple-emission microcracks centered on SSW.Additionally,the free chloride ion content of the composites at 28 and 180 d meets the ACI 318-19 standard requirements for concrete exposed to seawater.This compliance is attributed to the chloride immobilization facilitated by Friedel’s salt and C–S–H gels within the interfaces around SSWs and sea-sand.Consequently,SSWs-reinforced UHPSSC exhibits considerable potential for applications in sustainable marine infrastructures,demanding long-term mechanical properties and high durability.

Durability evaluation of GFRP rebars in harsh alkaline environment using optimized tree-based random forest model

GFRP bars reinforced in submerged or moist seawater and ocean concrete is subjected to highly alkaline conditions.While investigating the durability of GFRP bars in alkaline environment,the effect of surrounding temperature and conditioning duration on tensile strength retention(TSR)of GFRP bars is well investigated with laboratory aging of GFRP bars.However,the role of variable bar size and volume fraction of fiber have been poorly investigated.Additionally,various structural codes recommend the use of an additional environmental reduction factor to accurately reflect the long-term performance of GFRP bars in harsh environments.This study presents the development of Random Forest(RF)regression model to predict the TSR of laboratory conditioned bars in alkaline environment based on a reliable database comprising 772 tested specimens.RF model was optimized,trained,and validated using variety of statistical checks available in the literature.The developed RF model was used for the sensitivity and parametric analysis.Moreover,the formulated RF model was used for studying the long-term performance of GFRP rebars in the alkaline concrete environment.The sensitivity analysis exhibited that temperature and pH are among the most influential attributes in TSR,followed by volume fraction of fibers,duration of conditioning,and diameter of the bars,respectively.The bars with larger diameter and high-volume fraction of fibers are less susceptible to degradation in contrast to the small diameter bars and relatively low fiber’s volume fraction.Also,the long-term performance revealed that the existing recommendations by various codes regarding environmental reduction factors are conservative and therefore needs revision accordingly.