Experimental Study On Mechanical Properties And Microstructure Of Concrete After Carbonization At High Temperature

The reduction of concrete alkalinity caused by carbonization is an important prerequisite for steel corrosion.In recent years,frequent fires have become one of the main factors causing the damage of concrete structures.It is necessary to evaluate the residual mechanical properties of concrete structures in CO₂ environment all the year round after fire.Therefore,it is of great significance to study the mechanical properties of concrete after carbonization at high temperature.The mass loss rate,compressive strength and flexural strength of concrete specimens were studied after rapid carbonation test and high-temperature test.The results showed that the mass loss rate increased with the increase of temperature.The compressive strength decreases with the increase of temperature and reaches the peak at 400℃.The flexural strength decreases with the increase of temperature overall,and decreases at first and then increases with the increase of carbonization age.The carbonation has a little effect on the flexural strength of concrete at 600℃⁸00℃.The variation of calcium hydroxide and calcium carbonate contents in concrete after carbonization at high temperature were studied by XRD phase analysis and TG-DSC thermal analysis method.The results showed that the calcium hydroxide have been completely decomposed at 800℃;There are obvious calcium carbonate absorption peaks in concrete of different carbonation ages;The decomposition temperature of calcium hydroxide and the calcium carbonate is 420℃and 710℃,respectively;The mass loss before 200℃,400℃⁶00℃,600℃⁸00℃were due to the free water evaporated,the decomposition of calcium hydroxide dehydrated and the dehydration decomposition of calcium carbonate,respectively.The pore structure characteristics and pore size distribution evolution of concrete after carbonation at high temperature are analyzed by mercury porosimetry method.The results showed that the total pore volume,average pore size,total porosity and most probable pore diameter increased obviously with the increase of the temperature.The total pore volume,average pore size and total porosity decreased significantly with the increase of carbonization age.The porosity of harmless pores,multi-harmful pores and harmful pores increased at first and then decreased with the increase of carbonization age.However,the porosity of less harmful pores decreased with the increase of carbonization age.The grey entropy method is used to analyze the correlation degree between pore structure parameters and compressive strength,flexural strength and mass loss rate of concrete after carbonization at high temperature.The pore parameters and fractal dimension based on thermodynamic relationship are obtained by grey entropy analysis.The mathematical models of pore parameters,fractal dimension and compressive strength are established.The results show that the biggest factors affecting compressive strength,flexural strength and mass loss rate are harmless pore,total pore area and porosity,respectively.The mathematical model of multi-factor compressive strength is in good agreement with the experimental results,and can be used to accurately describe the quantitative relationship between compressive strength and pore structure parameters.Through digital image correlation technology,the horizontal displacement and horizontal strain of concrete flexural specimens after carbonation at high temperature were tested and the crack propagation characteristics were analyzed.The results showed that the flexural failure process of concrete can be divided into three stages:micro crack diffusion stage,macro crack cracking stage and macro crack diffusion stage.The crack height increased with the increase of load.When the crack height is the same,the load of carbonization at 28 days and 20℃reached the maximum respectively,which indicated that the high carbonization age could prevent the crack propagation and the high temperature accelerate the crack propagation.