| Islands and reefs in the South China Sea are important links to the Maritime Silk Road and have extremely high economic and military value.Construction of infrastructure on islands and reefs requires a large number of building materials.The long-distance ocean transportation of these building materials is not only economical but also affected by weather(such as typhoons),which will lead to a significant increase in construction costs.Some scholars have proposed that coral aggregate concrete prepared by using seawater and corals on islands and reefs as coarse and fine aggregates can solve the above problems.However,the harmful ions from the high temperature and high humidity marine environment of the South China Sea and the coral aggregate concrete will significantly increase the corrosion rate of steel bars and seriously affect the service life of the steel-reinforced coral aggregate concrete structures.In recent years,fiber reinforced polymer bar(FRP bar)has gradually become a potential force-bearing bar for replacing steel bars in concrete due to its good corrosion resistance and mechanical properties.However,since the short time from development to application of FRP bars,especially basalt fiber reinforced Polymer bar(BFRP bar),designers do not have enough understanding of the service performance and deterioration law in high temperature and high humidity marine environment.As a result,the long-term performance of FRP bars and coral aggregate concrete members is still uncertain,which limits their large-scale application.Furthermore,some problems of concrete members reinforced with FRP bars(such as large deflection deformation,rapid crack development,etc.)caused by the mechanical properties of FRP bars will be magnified in a high temperature and high humidity marine environment.Therefore,it is necessary to improve the durability of the CAC matrix;use ductile materials to improve the degradation of the normal service limit state of BFRP bar CAC members caused by the hot and humid marine environment;carry out the durability research of BFRP bar CAC members in the hot and humid marine environment.In view of this,the permeability test of coral aggregate concrete incorporating supplementary cementitious materials(SCAC),the interface bonding durability test of BFRP bars embedded in coral aggregate concrete and engineered cementitious composites(ECC),durability test of SCAC beams reinforced with BFRP bars and SCAC beams reinforced with BFRP bars and ECC(SCAC(E)beams reinforced with BFRP bars)were carried out.The specific research contents and results are as follows:(1)The mechanical properties of coral aggregate concrete were improved by adding different kinds of mineral admixtures,and on this basis,the water absorption and gas permeability tests of coral aggregate concrete were carried out.The relationship between water permeability and gas permeability was established by the slip factor,and thus the result of gas permeability was analogous.The microscopic changes of coral aggregate concrete mixed with different mineral admixtures were characterized from the aspects of microscopic morphology,phase composition,and thermal analysis.The coral aggregate concrete with different mineral admixtures was characterized by microscopic morphology,phase composition,and thermal analysis.The results show that mineral admixtures can reduce the permeability of coral aggregate concrete.When adding 20%slag and 10%metakaolin,the coefficient of water permeability of coral aggregate concrete is 2.65×10-12 m/s,which is close to that of ordinary concrete with a water-cement ratio of 0.4(2.14×10-12 m/s).The secondary hydration reaction of mineral admixtures generated C-S-H and C-A-S-H gels and formed Friedel salt with chloride ions in seawater,significantly increasing the bound water content of coral aggregate concrete and reducing the content of coral aggregate concrete.Absolute porosity is the reason for the decreased gas permeability of coral aggregate concrete.(2)According to the mechanical performance test of BFRP bars wrapped in coral concrete and ECC in high temperature and high humidity marine environment,and based on the degradation process of BFRP bars controlled by the degradation of the resin matrix,a BFRP bar degradation model considering the effects of temperature,humidity,and its fluctuations was proposed.On the basis of the measured temperature and humidity data,the retention rate of the tensile strength of the BFRP bars in the atmospheric,tidal,and immersed areas of the marine environment was predicted.The results show that after being exposed to the high temperature and high humidity marine environment at 35°C for 90 days,OAC had the best protective effect on BFRP bars.The internal strength retention rate of BFRP bars was 95.02%,the tensile strength retention rate of BFRP bars in ECC was 94.69%.CAC had the weakest protective effect on BFRP bars,and the strength retention rate of its internal BFRP bars was 89.54%.The 50-year environmental impact coefficients CE of coral concrete and ECC are 0.80and 0.81,0.74 and 0.76,and 0.65 and 0.67,respectively,for marine atmosphere,tidal,and immersion areas.The proposed time-varying degradation law of BFRP bars can provide a material degradation basis for the long-term design of SCAC(E)beams reinforced with BFRP bars.(3)Interface bonding tests of BFRP bar and coral concrete and ECC in high temperature and high humidity marine environment were carried out to study the influence of environmental time,temperature,and concrete matrix on bond failure mode,bond stress,and bond-slip curve.The corresponding bond-slip model was selected to fit the bond-slip curve.A bond-differential-slip model considering local effects were established.Furthermore,a calculation method for the anchorage length of BFRP bars considering environmental effects was proposed.The results showed that the swelling effect of BFRP bars weakened the bond-slip effect.As time increased,the bond slips gradually increased.High temperature further damaged the ribs of the BFRP bars.Before and after the environmental conditions,during the interfacial bonding test,the bond stress,free end slippage and deformation coordination capacity of the BFRP bars in the ECC specimens and OAC specimens were greater than those of the SCAC specimens.Among them,the ECC specimens BFRP bars had the best interfacial cohesion.The PVA fibers in the ECC had a good ability to limit crack development and made the residual stress retention rate of the bonded samples exceed 45%,which was much higher than that of coral aggregate concrete and ordinary aggregate concrete.After the relatively large slip between the BFRP bar and ECC,the bond strength can still meet the anchoring requirements of the codes and standards.The proposed anchorage length settlement method can provide a theoretical basis for the structural design of ECC-coral aggregate concrete beams reinforced with BFRP bars.(4)For the problems of large deflection and deformation rapid development of cracks,excessive crack width,and performance degradation after the action of high temperature and high humidity marine environment of coral aggregate concrete beam reinforced with BFRP bars,a kind of ECC-coral aggregate concrete beam reinforced with BFRP bars was proposed.Based on the results of the four-point bending test of the beam before and after the environmental conditions,the failure mode,cracking moment,limit flexural moment,mid-span deflection,crack distribution,crack width,and ductility index were analyzed.The changes in the mechanical properties of coral aggregate concrete beams reinforced with BFRP bars before and after the environmental action were compared,and the improvement effect of ECC on the mechanical properties of coral aggregate concrete beams reinforced with BFRP bars was analyzed.The results showed that ECC significantly improved the shear resistance of coral aggregate concrete beams reinforced with BFRP bars.The flexural capacity of the ECC-coral aggregate concrete beam reinforced with BFRP bars in the normal service limit state was about 2-3 times that of the coral aggregate concrete beams reinforced with BFRP bars.ECC can limit the development of cracks and then give full play to the strength of BFRP bars.After environmental effects,the failure mode of the coral aggregate concrete beams reinforced with BFRP bars changed from concrete crushing failure in the upper compression zone of the beam to shear failure from the loading point towards the support,while the failure mode of the ECC-coral aggregate concrete beams reinforced with BFRP bars did not change.Since the short-term environmental effects,the concrete strength was increased,the flexural capacity of the beam was improved,and the cracking load was significantly improved.After environmental effects,the mid-span deflection and crack width of the coral aggregate concrete beams reinforced with BFRP bars first decreased and then gradually increased,while the mid-span deflection and crack width of the ECC-coral aggregate concrete beams reinforced with BFRP bars did not increase significantly.(5)Based on the results of the flexural test of SCAC(E)beams reinforced with BFRP bars,a calculation model was derived for the maximum flexural capacity of the beams,considering the over-,balanced-and under-beam reinforcement.At the same time,a simplified calculation method of flexural capacity was proposed,which was verified by experimental data.Based on parameters from the existing literature,the calculation formulas for the mid-span deflection of coral aggregate concrete and ECC-coral aggregate concrete beams reinforced with BFRP bars were deduced.In the absence of a large amount of experimental data,combined with the bonding characteristic coefficient method and the test results,the bonding characteristic coefficient was calculated and the crack width calculation formula was obtained.The calculated models for flexural capacity,mid-span deflection and crack width provide a theoretical basis for the design of coral aggregate concrete and ECC-coral aggregate concrete beams reinforced with BFRP bars in high temperature and high humidity marine environment.(6)According to the results of the flexural test of coral aggregate concrete and ECC-coral aggregate concrete beams reinforced with BFRP bars in high temperature and high humidity marine environment,combined with the degradation mechanism of flexural capacity,long-term design methods of coral aggregate concrete and ECC-coral aggregate concrete beams reinforced with BFRP bars in high temperature and high humidity marine environment were proposed.According to the material deterioration data,the flexural capacity,mid-span deflection and crack width of the test beam were verified.The long-term flexural performance of coral aggregate concrete and ECC-coral aggregate concrete beams reinforced with BFRP bars in high temperature and high humidity marine environment was predicted.The calculation process of cross-section design and cross-section check was shown in the form of calculation examples,which provided a theoretical calculation method for the application of this type of component in engineering. |
PDF Full Text Request