Durability Study Of Alkali-activated Seawater Coral Aggregate Concrete And Its Structures Reinforced With BFRP Bars

With the advancement of‘‘the Belt and Road”initiative and the implementation of the marine development strategy,the construction of infrastructure on reefs and islands has become imperative for China,which significantly increases the demand for building materials in marine engineering construction.The utilization of locally available marine resources,i.e.,seawater,sea sand,and wasted coral aggregate,are used to produce coral aggregate concrete（CAC）that uses in the construction of ports,airports,and other island projects,which contributes to important economic benefits and engineering value.However,CAC also suffers from low compressive strength,low elastic modulus,and poor anti-permeability resistance due to the low strength and high porosity of coral aggregates.These weak characteristics of CAC combined with the harsh marine environments seriously affect the serviceability and durability of CAC structures,which significantly impedes the application of CAC in marine environments.Alkali-activated materials（AAMs）have prominent features,including excellent thermal stability,dense microstructures,and superior resistance to chemical attack,which can provide a hopeful approach for the elimination of bottleneck problems related to cement-based CAC.Furthermore,raw materials of AAMs or geopolymer binders originate from industrial by-products,such as fly ash,silica fume,slag,coal gangue,and metakaolin,which effectively alleviates the energy consumption and CO₂ emissions,thereby achieving the requirements for cost-effectiveness and eco-friendliness.Meanwhile,fiber-reinforced polymer（FRP）possesses a high strength-to-weight ratio and superior durability performance,which can be introduced to eliminate the corrosion issue associated with traditional steel bars.Therefore,the research on the durability of alkali-activated seawater coral aggregate concrete（AACAC）and its structures reinforced with FRP bars will be of great significance to the construction and development of marine engineering,and has a broad application prospect.The main research contents and conclusions are as follows.（1）The Taguchi orthogonal experimental design method was employed to investigate the effects of various parameters on the workability,setting time,compressive strength,flexural strength,and drying shrinkage of alkali-activated mortars prepared with seawater and coral sand（SC-AAMs）;the modulus of sodium silicate（M_s）,Na₂O-to-binder ratio（N/B）,water-to-binder ratio（W/B）,and replacement ratio of coral sand for sea sand（R_s）were also considered.Based on the orthogonal experimental results analysis,the optimized mix proportion for the SC-AAMs was M_s=1.2,N/B=4%,W/B=0.45,and R_s=100%.Then,the microstructure and crystalline phases in the SC-AAMs prepared with this optimum mix proportion were analyzed by scanning electron microscopy（SEM）,X-ray diffraction（XRD）,and Fourier transform infrared（FTIR）spectrometry.The experimental results indicated that the utilization of seawater and coral or sea sand can accelerate the hydration reaction and promote the formation of C-S-H gel phases.The main reaction products for the AAMs were C-（A）-S-H and N-A-S-H gels.The portlandite,AFt,and AFm phases in the hydration production of the AAM samples were undetectable but existed in the cement mortars.In addition,the utilization of coral aggregates improved the flexural strength of the mortars and lowered its drying shrinkage due to the compacted interfacial transition zone and natural internal curing effect of the coral aggregate.（2）The differences between cement-based CAC and slag-based AAC in terms of failure modes,compressive strength,splitting tensile strength,stress-strain curves,elastic modulus,drying shrinkage,and fracture properties were compared.Then,the effects of various corrosion ages and corrosion temperatures on the compressive strength,stress-strain curves,and elastic modulus of CAC and AACAC under seawater immersion and wet-dry cycle environments were analyzed,and the deterioration mechanism in mechanical properties of the concrete was analyzed by SEM and XRD techniques.The experimental results indicated that the failure modes for all coral aggregate concrete were characterized by the broken coral aggregates originating from their low strength and high brittleness.Compared with the cement-based CAC,slag-based AACAC contained a higher splitting tensile strength,axial compressive strength as well as elastic modulus,and had a denser microstructure at the binder-aggregate interface,but there was no significant difference in fracture properties of these two types of concrete.Meanwhile,AACAC exhibited better seawater erosion resistance than CAC after being subjected to a seawater dry-wet cycle environment.The dense microstructure,excellent pore structure,and stable hydration products of AAMs are the reasons for their superior performance against seawater corrosion to cement-based materials.（3）The bond characteristics between the FRP bars and cement-based CAC or slag-based AACAC were analyzed,and the effects of parameters,including the FRP bar type,rib depth of the bars,bond length,and stirrup constraint on the bond performance were considered.Subsequently,the deterioration laws in the bond performance of BFRP bars with CAC and AACAC were investigated under different corrosive environments,corrosion ages,and corrosion temperatures.The tested results showed that AACAC specimens exhibited higher bond strength,bond stiffness,and residual bond stress compared to cement-based CAC specimens.Compared with CAC,the bond strength of FRP bars with AACAC increased by approximately 23%,while their bond stiffnesses at a slip value of0.02 mm increased by approximately 38%.In addition,the bond performance of AACAC specimens was less affected by the seawater corrosion environment than CAC specimens,and even AACAC specimens had an improvement in the bond strength at the early stage of seawater corrosion.After exposure to 60°C seawater conditions for 12 months,the bond strength of CAC specimens degraded by 12%,while that of AACAC specimens degraded by only 4.9%.This degradation in bond strength is associated with the decreases in mechanical properties of FRP bars and concrete,and the degradation at the FRP-concrete interface.（4）The effects of the reinforcement ratio,the diameter of the FRP bars,and the FRP type on the flexural performance of FRP bars reinforced CAC or AACAC beams were investigated,and the differences in load capacity,flexural stiffness,failure pattern,crack development,and deflection between CAC beams and AACAC beams were compared and analyzed.The results showed that two characteristic failure modes occurred in the CAC and AACAC beams,i.e.,concrete crushing failure in the compression zone（i.e.,bending failure）and shear-compression coupled failure.A large number of coral aggregates was observed to be crushed or pulled off at the concrete damage interface,while partial beams had FRP bars fracture or cracking phenomenon for the longitudinal tensile or compressive bars.With the increase in reinforcement ratio,the number of cracks of AACAC beams increased,and the crack spacing,crack width,and crack depth gradually decreased,while the cracking load,ultimate load,and flexural stiffness also showed a significant increase,but the deflection corresponding to the ultimate load and the strength utilization of internal FRP bars decreased.Based on the formulae of FRP reinforced concrete codes,the expressions of flexural stiffness and crack width for FRP bars reinforced CAC/AACAC beams were established,and the applicability of the formula was verified by the measured data available in the literature regarding the flexural stiffness and crack width of FRP bars reinforced CAC beams.（5）The deterioration law in flexural performance for BFRP bars reinforced CAC or AACAC beams under seawater immersion and wet-dry cycles were investigated,and the differences in failure modes,crack development,ultimate load,and flexural stiffness between BFRP bars reinforced CAC beams and BFRP bars reinforced AACAC beams under seawater corrosive environments were compared and analyzed.The results manifested that the failure mode of CAC beams after seawater corrosive environments was still dominated by the concrete crushing in the compression zone,while that of most AACAC beams changed from the concrete crushed to the shear-crushing failure.Meanwhile,the flexural stiffness of the CAC and AACAC beams increased after seawater corrosive environments,but this increased stiffness did not lead to an increase in load capacity.On the contrary,the ultimate load and deflection values for each CAC and AACAC beam decreased to different degrees as the corrosion age and corrosion temperature increased.Furthermore,this load capacity degradation was more severely affected by the seawater dry-wet cycle environment than the seawater immersion environment.Moreover,compared with CAC beams,AACAC beams exhibited better seawater erosion resistance.After being subjected to 60°C seawater environments for 12 months,the ultimate load capacity of the CAC and AACAC beams was reduced by 25%and 16%,respectively.