Design And Engineering Application Of Highly Seawater Corrosion Resistant Geopolymer Concrete Reinforced By BFRP Bar

Under the guidelines of“21^st Century Maritime Silk Road”and"Development of South China Sea”,the constructions of marine infrastructures are blooming in recent years.Traditional Portland cement concrete structures in marine environment presented poor durability due to degradation of hydration products,steel bar corrosion,and etc.,resulting in undesirable long term performances and much shorter service life.Unlike the hydration products of Portland cement,the reaction products of geopolymer,mainly C-A-S-H and N-A-S-H gels,showed superior stability in marine environment,and Basalt-fiber reinforced polymer?BFRP?bar is also stable in chlorine-rich environment.Therefore,BFRP reinforced geopolymer concrete is expected to be used in marine structures.On the other hand,geopolymer presented lower cracking resistance due to high shrinkage during rapid reaction,and on available mixture proportion design method and procedures are found in literatures.With high tensile strength and low compressive strength,BFRP bar is only preferred in bending members,how about compression member?The time-dependent behavior and service life of BFRP reinforced concretes in marine environment have not been clarified yet.In this study,the volume stability of geopolymer was improved by adjusting the addition of MgO mixture with different hydraulic activity,mixture proportion design method for geopolymer concrete was established.BFRP reinforced concrete beam and column members were prepared by introducing spiral BFRP bars,then their mechanical properties and time-dependent behavior in marine environment focused,and a service life prediction model for BFRP reinforced geopolymer concrete components was proposed.Finally,BFRP reinforced geopolymer concrete was applied in seawalls constructions and their engineering properties was followed.The main conclusions are summarized as follows.According to the autogenous shrinkage process of the geopolymer pastes,high reactive MgO?Reaction discoloration time of 60 s?and low reactive MgO?Reaction discoloration time of 220 s?were,respectively,used to compensate the early and later shrinkage,and modified geopolymer?GII?with superior volume stability and desirable engineering properties was obtained.The mechanism of shrinkage compensation laid in that:optimized matching of shrinkage of geopolymer paste and volume expansion of MgO was achieved by adjusting the formation of Mg?OH?2 M-S-H gels and other products using MgO with different activities.Based on characteristics of geopolymer concrete,the mixture proportion design method and procedures of geopolymer concrete were established,and then fundamental properties of geopolymer concrete were investigated.Geopolymer concrete hardened rapidly and had higher early strength,the 3d-compressive strength can be as high as 50-70MPa.Relationship between 3d-compressive strength and the water-cement ratio?W/C?can be expressed by fcu,o=27.87×W/C-34.10,that between the slump?T?and the unit water content?W?can be described by T=0.36×W-53.3.For same workability,the sand ratio of geopolymer concrete was 3%lower than that of Portland cement concrete.No obvious differences in workability,mechanical properties and drying shrinkage of geopolymer concrete were observed when replacing river sand by sea sand.Bond-slip behaviors between BFRP bars and geopolymer concrete,as well as the mechanical properties of BFRP reinforced geopolymer concrete beam and column,were studied.With decrease of the diameter of BFRP bar and increase of concrete strength,the bonding between BFRP bars and geopolymer concrete was enhanced,and the bonding-slip was reduced.The yielding stage,a typical stage for steel bar reinforced Portland cement concrete,was not observed for BFRP bar reinforced geopolymer concrete,thus its stress-strain curve,therefore,contains elastic stage?I?and cracking stage?II?.Geopolymer beam with BFRP bar had higher ultimate bearing capacity and lower corresponding deflection than geopolymer beam with steel bar.A higher reinforcement ratio results in higher ultimate bearing capacity and deflection.To avoid low compressive strength and take advantage of high tensile strength,spirally reinforced BFRP bar was introduced and then the vertical force can be transformed into spiral stirrups force.As a results,BFRP reinforced geopolymer concrete column with superior mechanical properties were prepared,as creep and strain in both longitudinal and lateral directions were restrained by spirally BFRP bar.After addition of MgO,the ultimate displacement reduced and the stiffness increased,resulting in smaller longitudinal deformation.Based on stress-crack analysis of geopolymer concrete,safety coefficients of BFRP reinforced geopolymer concrete beam and column were suggested as1.52 and 1.82 respectively during bearing capacity calculation.The variation of mechanical properties and bonding-slip of BFRP reinforced geopolymer concrete beam exposed to seawater environment were characterized,then a mathematical model based on bearing capacity of was established to predict the service life of BFRP reinforced geopolymer concrete beams.The tensile strength of BFRP bar immersed in seawater declined slightly,the corrosive medium migrated from the outside to the inside,and the corrosion decreased using geopolymer mortar wrapped.Nearly no change in reaction products and microstructure of geopolymer pastes was found,indicating excellent seawater corrosion resistance.Both maximum bonding strength between BFRP and geopolymer concrete and the corresponding slip reduced after seawater corrosion,and consequently resulting in decreased ultimate bearing capacity and deflection?but still higher than those of steel bar reinforced Portland cement beam?.Based on the bearing capacity loss of both geopolymer concrete and BFRP bar,a service life prediction model of BFRP reinforced geopolymer concrete beams was established.When 0.66 was taken as a critical failure point for bearing capacity loss,the service life of geopolymer concrete beam with BFRP bar of?6mm and 8 mm in seawater was 148 years and 92 years,respectively.Finally,BFRP reinforced geopolymer concrete was applied in 1.2 km-long seawall construction project of Huilai Xigang,Guangdong province,the engineering properties were followed by in-situ tests.Compared with Portland cement concrete,the geopolymer concrete was more suitable to be applied in seawalls construction in tidal region due to rapid hardening and high early strength.The temperature of geopolymer concrete decreased 4⁷?C compared to that of Portland cement concrete,which is very help for to reduce the temperature shrinkage of the seawall panel and eventually increase the durability and service life.Therefore,this study provides theoretical foundation and technical support for civil engineering materials with superior seawater corrosion resistance,the BFRP reinforced geopolymer concrete can be used in long-life marine concrete structures,island-reefs construction far away from mainland,and other marine infrastructures.By this method better economic,social and environmental benefits future will be resulted in.