Study On Static Performance And Impact Response Of Alkali Activated Concrete And Related Flexural Members

Concrete using ordinary Portland cement as the main raw material accounts for a large proportion of the large-scale infrastructure construction,which has triggered off increasingly severe environmental problems.Meanwhile,ecological issues resulted from the by-products generated during industrial activities cannot be ignored.Therefore,the cement free construction material,alkali-activated GGBFS/FA concrete(AAC),expected to solve the dilemma caused by construction and industrial development and environmental protection.The AAC performance of quickly being hardened is noteworthy,and the engineering projects of urgent repair caused by suffering from impact loads bring AAC’s superiority into full play.The utilization of AAC is limited at the range of precast concrete products because of the quick setting and hardening process of AAC.In order to explore the prospect of applying AAC in more widern practical constructions,a targeted study strats from solving one of the existing tricky problems of using AAC is conducted.Combination of optimization choice and retarder mechanism,effective compound retarders that can prolong the setting process of AAC are found.Based on the effective retarders,the following studies on AAC properties are carried out,inclusing the material properties,static mechanical performance,response under impact loads,bond performance of reinforced AAC and the performance of AAC beams under static and drop weight loads.According to the series of experimental studies,some analytical and desigen methods are proposed.Initially,various chemical agents are applied as potential retarders and the traditional method of penetration resistance is used to find out effective retarders on delaying setting times of AAC.Low-field nuclear magnetic resonance tests are performed to analyse the effect of retarders on setting time postponing of fresh AAC,and the penetration resistance and temperature changes are combined to dissect the setting and hardening process.Based on the analysis of the test results,efficient retarders for AAC are obtained and a definition of the deadline for casting as well as four steps within the early hardening process are proposed,i.e.dissolution period,setting period,acceleration period and hardening period.Furthermore,the experimental study on alkali-activated GGBFS/FA mortars(AAMs)using water-quenched slag(WQS)to replace river sand in same volume is performed to assess the effect of dosage of WQS on the properties of AAMs.The evaluation focuses on the setting times,workability,rheological property,mechanical strength,autogenous shrinkage,drying shrinkage and microstructures.It demonstrates that AAMs show the similar properties to the OPC-based mortars and WQS has an effect on the performance of AAMs to some extent.Workability of AAMs deteriorates and setting times are getting shorter because of the increasing dosage of WQS.Even though much more WQS shows an influence on the early strength of AAMs,the mechanical strengths of AAMs tend to be in a similar level as the curing time goes.The change of pore structures in hardened AAMs resulted from using WQS is one of the predominant factors to influence the drying shrinkage.The comparison results verify the feasibility of the fabrication of AAC using WQS as an alternative to river sand and lay a foundation for the subsequent study on the other properties of AAC.Then,AACs incorporating different percentages of WQS in fine aggregates are fabricated according to a substitution of WQS for river sand with same volume in the basic mix ratio of AAC,and the corresponding study on the static mechanical properties of AACs are carried out.From the static uniaxial compression tests and tension tests of AACs,some critical indexes are obtained,including peak strength,peak strain,ultimate strain,modulus of elasticity,Poisson’s ratio etc.The corresponding constitutive models are proposed for AACs incorporating WQS.Moreover,dynamic response of AACs caused by various strain rates is recorded and analysed through conducting split Hopkinson pressure bar(SHPB)tests,i.e.dynamic peak strength,ultimate dynamic strain,dynamic increase factor(DIF),stress-strain relationship and energy dissipation,and corresponding models are proposed to predict the DIF of AACs.Subsequently,based on the basic mechanical properties of AAC,experimental study on the bond performance of reinforced-AAC is performed.There are 48 pull-out specimens and 16 beam-end specimens being fabricated for the tests and some relevant parameters are considered in design period,including the types of concrete,rebar diameters,the thickness of concrete covering,the length of bond and space of two adjacent stirrups.The effects of these parameters on the bond performance of reinforcedAAC are analysed and discussed.The results reveal that the bond performance between steel rebar and AAC is better than that of conventional reinforced OPC-based concrete.Compared with the existing studies on the bond performance between steel rebar and geopolymer concrete,it is found that high Ca O content in the precursors decreases the ultimate bond strength.In order to reliably estimate the bond performance of reinforcedAAC,a calculation model related to the splitting tensile strength of AAC is proposed to predict the ultimate bond strength.After that,the study on the performance of reinforced-AAC flexural members is conducted.There are six simply supported beams with different reinforcement ratios being fabricated and four-point flexural loading is preformed to test the following properties: failure modes,crack propagations,load-deformation relationships,energy dissipation,etc.According to the test results,two AAC stress-strain models are proposed,which are used to conduct stress analysis in a cross-section of beams at the cracking moment and in the ultimate state,separately.According to the experimental study,the recommended calculation methods are proposed for reinforced AAC beams.Eventually,additional ten simply supported beams are tested under one-point loading at the mid-span of beams,where five beams are tested under static loads and the other five beams are tested under drop weight impact loads.The five beams subjected impact loads have the same configurations with the counterparts in static tests.From the drop weight impact loading tests,impact force vs.time relationships,mid-span displacement vs.time relationships and beam horizontal displacement vs.time relationships are explored quantificationally,and dynamic response of reinforced AAC beams is studied.