Deterioration Of Decorative Ultra-High Performance Gradient Concrete Under Combined Erosion Of Acid Rain And Carbonization

Environmental factors such as acid rain and carbonization are the main causes of durability damage of urban concrete structures.To solve this problem,this paper proposed to use decorative ultra-high performance concrete（DUHPC）as the protective layer of ordinary concrete and design gradient concrete,which could significantly improve the aesthetic performance,mechanical properties and durability of building structures.Therefore,it was of great significance to study the deterioration rule of ultra-high performance concrete under acid rain and carbonization combined erosion and their mutual influence mechanism.The four-point bending test was conducted to explore the influence of the thickness of superhigh performance concrete with a veneer on the bending performance of the gradient concrete,and to study the deterioration law of macro performance（quality,erosion depth,strength,etc.）and the change of micro performance（micro-morphology,pore structure,etc.）of decorative ultra-high performance concrete under combined erosion acid rain and carbonization.The following conclusions were drawn:（1）The bond performance of the gradient concrete increased with the shortening of the pouring time interval and the decreasing of the strength difference.The more obvious the interface transition zone at the joint,the bond performance was better.When the pouring time interval was 0 h,the splitting tensile strength was greater than 1.5 MPa.When the thickness ratio（DUHPC:C40）were 1:4,1:3 and 1:1,the bending strength increased about89.79%,103.59%and 95.83%compared with NC,respectively.In the bending process,DUHPC is first stressed,and the cracks started from the bottom of the DUHPC span,but the overall development trend was relatively slow,and the high toughness of DUHPC would not cause the gradient concrete to break completely.When the thickness ratio of DUHPC to NC of the gradient concrete was 1:4,the bearing capacity of the structure could be significantly improved and the cost can be effectively reduced.（2）Acid rain had a great influence on the aesthetic performance of DUHPC,and a large amount of white crystal gypsum was generated on the surface of DUHPC,resulting in a 1.35%increase in mass and 20.8%increase in whiteness.Subsequently,the surface began to denude,and the quality decreased by 3.32%.The macropore content decreased at first and then increased significantly（68.48%）,which led to the compressive strength of DUHPC increasing at first and then decreasing.There was a positive correlation between acid rain erosion rate and erosion period.The erosion depth was only 1.85 mm after eight cycles,and the product size was gradually refined with the increase of soaking time.（3）The appearance of DUHPC did not change significantly under carbonization.At the initial stage of carbonization,a small amount of white crystal calcium carbonate was precipitated and attached to the concrete surface.The p H of the surface of DUHPC was reduced to dissolve the surface,resulted in a slight increase in mass and porosity,while the whiteness showed a trend of increasing first and then decreasing.With the increase of carbonization cycle,the strength of DUHPC first decreased（12.7%in four cycles）and then became stable.（4）The coupling of acid rain and carbonization could promote the influence of each other on the quality and erosion depth of DUHPC.Acid rain would dissolve the carbonized product Ca CO₃,resulting in a large number of holes on the surface of DUHPC,and the mass of DUHPC would decline instead of increasing,the mass reduction caused by acid rain would become more serious with the increase of carbonization cycle.The formation of holes in the surface would promote subsequent acid rain erosion.When acid rain followed by carbonization,acid rain would change the pore structure of DUHPC and promote carbonization,which led to the increase of mass caused by carbonization.For different acid rain and carbonization,the erosion rate was as follows:3S-T>T-3S>S-3T>3T-S.However,the compressive strength of DUHPC was mainly increased by capillary pores and decreased by macropores during four cycles of composite erosion,so the compressive strength remained stable,while the content of macropores increased significantly after eight cycles,which led to a rapid decline in the compressive strength of DUHPC.