Durability And Life Prediction Of Reinforced Recycled Aggregate Concrete Beam Exposed To Seawater

The massive use of recycled aggregate concrete is an effective way to re-use waste concrete materials in a scientific manner.It is also a promising way to help the realization of sustainable development of construction materials,and reduce pollutions on environment.The use of recycled aggregate concrete has several advantages,such as protecting eco-environment,saving natural stone resources,andbringing economic benefits.Therefore,it would have great significance,as recycled aggregate concrete is widely used in a variety of engineering projects,such as roads,hydranlic engineering,harbors,and buliding constructions.However,due to the complexity of recycled aggregate concreteitself and the uncertainties of the recycled aggregate resources,there are large variations on mechanical properties.Besides,due to the corrosion effect from service environment,degradations of the concrete constituents and weakened mechanical performance are the important problems that have to face.Nowadays,research investigations of recycled aggregate concrete have mainly focused on mechanical performance;and many valuable results have already been obtained.However,the durability of recycled aggregate concrete has gained much less attention,as well as the performance of recycled aggregate concrete structures.Therefore,the theoretical and experimental investigations on durability of the recycled aggregate concrete structures are of significant importance in practice.The investigation contents of this paper include the degradation mechanism of recycled aggregate concrete,damage mechanism of the reinforced recycled aggregate concrete,and the chloride diffusivity.Meanwhile,time-depend reliability model of reinforced recycled aggregate concrete beam exposed to sea water service conditions,was proposed.In this model,performance and dimension parameters of recycled aggregate concrete were taken into consideration.This can be a valuable example for the later-on investigations on recycled aggregate concrete structures exposed to sea water.The main findings of this study are given as follows:(1)C30 and C40 recycled aggregate concretes incorporating 0%,10%,20%,30% and 40% fly ash were prepared and cast in moulds with the dimensions of 100mm×100 mm×100 mm.The concrete samples were cured for 56 days under standard curing conditions.Meanwhile,reinforced recycled aggregate concrete with the dimensions of 100mm×100 mm×900 mm was fabricated and cured in air for 56 days.Solution with the concentration five times higher than sea water was used as the corrosion media.Accelerated corrosion test that had a fixed wetting-drying cycle regime was carried out on plain recycled aggregate concrete samples for 140 days.Besides,this test was also conducted on reinforced recycled aggregate concrete beams for 240 days.(2)Damage evolution of recycled aggregate concrete exposed to seawater was studied.Properties that include compressive strength,relative dynamic modulus,mass loss rate,and modification sample morphology,were investigated,with the prolonged exposure period in sea water.Corrosion mechanism of the concrete exposed to sea water was investigated,which included both sulfate and chloride corrosion mechanisms.Based on this corrosion mechanism study,performance degradation mechanism of the recycled aggregate concrete under sea water wet-drying cycles was focused on.Meanwhile,damage evolution of the recycle aggregate concrete exposed to sea water was studied and the relationship between the damage level and relative strength was established.(3)Investigation on influence of the fly ash addition level on compressive strength,relative dynamic module,mass loss of the recycled aggregate concrete exposed to sea water was carried out.It shows that C30 and C40 recycled aggregate concretes have higher corrosion resistance than those with 0%,10%,20% and 30% fly ash.(4)Ultrasonic time values of reinforced recycled aggregate concrete beams at curing ages of 60 d,120 d,180 d,and 240 d were determined through the ultrasonic method using non-metallic ultrasonic instrument.Regression analysis was then carried out on the obtained data;and the thickness and damage level of the corroded concrete layer were estimated.Moreover,mathematic model regarding both corrosion damage level of reinforced recycled aggregate concrete beams and the exposure age was proposed.(5)Based on the bending test results,damage process and crack propagation of the corroded reinforced recycled aggregate concrete beamswere analyzed.Moreover,mathematic model regarding the relative loading that induced cracks,the relative ultimate load,steel corrosion level,and corrosion damage level of the steel reinforced recycled aggregate concrete was proposed.(6)Influence of steel corrosion level and thickness of the damaged concrete layer on loading capacity of the recycled aggregate concrete was investigated.Based on the available structural design standards,numerical model for calculating bending capacity of reinforced recycled aggregate concrete beams was proposed.The simulation results were further verified with the experimental results.It shows that the proposed numerical model has high accuracy of prediction.(7)Chloride diffusive characteristics of the recycled aggregate concrete exposed to sea water were studied.They included the relationship between free chloride concentration and diffusive depth,changes of chloride diffusive coefficients with corrosion time,relationship between surface chloride concentration and corrosion time.Based on the investigations,the chloride diffusive model of reinforced recycled aggregate concrete beams can be corrected by corrosion damage level.(8)Influence of fly ash addition level on chloride permeability resistance of the recycled aggregate concrete was investigated.It showed that C30 and C40 recycled aggregate concretes with 40% fly ash have higher the resistance of chloride permeability than those with 10%,20% and 30% fly ash.(9)Lastly,two life-span models were proposed for the elements of concrete.One was proposed based on the investigations of chloride invasion and steel corrosion.The other one was proposed based on the time-variety and reliability theory.