Research On Early-age Shrinkage And Crack Properties Of Continuous Rigid Frame Bridge Concrete Exposed To Various Environments

In recent years,as prestressed concrete continuous rigid frame bridge has the advantages of reducing weight,good rigidity,comfortable and smooth running,no expansion joint,simple construction,good seismic performance and good economy,it is widely used in highway construction in Mountainous Areas.But at the same time a large number of continuous rigid frame bridge were found to produce different degrees of cracks.The main forms of cracks were non structural cracks which were caused by early-age shrinkage.These cracks would indirectly accelerate the deterioration of internal reinforced concrete structure,affecting the safety and durability of bridge structure.Therefore,it is one of the most effective ways to solve the cracking problem of the continuous rigid frame bridge.The early-age shrinkage and cracking properties of box girder concrete were researched by means of measuring total shrinkage during upper surface drying,evaporation mass loss and plate cracking index of natural medium sand and mixed sand concrete in different environment during first 3 days.The effect of SAP(Super-absorbent Polymer)on workability,mechanical properties(compressive strength of 3d/7d/28d),early-age autogenous shrinkage,total shrinkage during upper surface drying,evaporation mass loss and plate cracking index of box girder concrete under various environments were discussed by changing SAP incorporation method and dosage.The mechanism analysis were carried by mercury injection test and microscopic morphology.The effect of mould paste curing ways on early-age shrinkage and impermeability properties of box girder concrete during first 7 days at different temperature and humidity were discussed by testing internal moisture distribution,total shrinkage during upper surface drying,seepage height,etc.The main conclusions are as follows:(1)Reducing humidity,Increasing wind speed and temperature significantly increased mass loss rate per unit area at 2h and mass loss per unit area at 24 h.And it could also increase total shrinkage of box girder concrete at 72 h and early-age cracking degree.The influence of that in mixed sand concrete was generally higher than natural medium sand concrete.Under 30?C/60%RH/5m/s,72 h shrinkage value of mixed sand concrete and natural medium sand concrete reached 2495um/m and 2270um/m,respectively.Differences in early shrinkage and cracking properties of mixed sand concrete and natural medium sand concrete decreased with the increase of humidity.Total shrinkage during upper surface drying at 72 h,total cracking area per unit area and the maximum crack width of mixed sand concrete was generally higher than that of natural sand concrete in the same environment at low humidity(60%RH).(2)The incorporation of SAP could make the stable time of autogenous shrinkage development of box girder concrete in advance,and significantly reduce autogenous shrinkage.Compared with no extra water,SAP with extra water could decrease the autogenous shrinkage more.SAP with no extra water mixed can reduce total shrinkage during upper surface drying at 72 h.However,the reduction ratio of shrinkage decreased rapidly with the increase of the dosage when SAP was over critical content.SAP with extra water did not reduce early-age total shrinkage during upper surface drying of box girder concrete in high temperature and humidity environment(30?C/60%RH).(3)SAP could effectively reduce the early shrinkage cracking trend of box girder concrete.Compared with no extra water,SAP with extra water could inhibit the early cracking better.However,the early cracking resistance of SAP concrete was very sensitive to the SAP content.The effect of inhibiting cracking was decreased when the critical content(0.1%~0.2%)of SAP was exceeded.In the environment of 30?C/60%RH/5m/s,adding 0.1% and 0.2% SAP to the concrete in the box girder,the total cracking area per unit area was reduced by 85.6% and 42.4% respectively,and the crack resistance was increased from III to V and IV respectively.Box girder concrete specimens with SAP with extra water cracked only in extremely harsh environment.Its early-age crack resistance to the environment was extremely strong.The mechanism of SAP box girder concrete with good crack resistance was the improvement of the degree of hydration in the vicinity of SAP large pores.(4)The mould paste curing could significantly reduce early-age total shrinkage rate and total shrinkage during upper surface drying.Mould paste curing could effectively reduce the water loss rate of concrete box girder and concrete surface relative humidity gradient to maintain the relative humidity of the concrete at a high level.The maintenance effect of mould paste curing in three side is slightly better than that in two side.There was an obvious relative humidity gradient in concrete off surface 5mm in high temperature and low humidity environment(30?C/60%RH).Therefore,model paste curing could not play the obvious effect of good curing in this environment.