Research On Impact Mechanical Properties Of Concrete Under Freeze-Thaw Cycles

Concrete structures in the northwest and northeast regions of China are subject to freezethaw cycles caused by low temperatures,which requires consideration of the influence of temperature changes on the characteristics of concrete.At the same time,wind and snow loads,hydrodynamic pressure,traffic accidents,blasting,earthquakes,etc.also often occur.The dynamic loads caused by the above incidents will pose a serious threat to the safety of building structures.As concrete is an indispensable material in geotechnical engineering construction,the study of its dynamic performance has attracted researcher’s attention.At present,there have been many experiments and theories on the dynamic characteristics of concrete,however,the studies on the mechanical properties and fractal characteristics of concrete specimens after freezing-thawing cycles at low and medium strain rates are few.In this paper,by used of the Split Hopkinson Pressure Bar impact device,impact tests were carried out on C35,C40 and C45 concrete that have undergone different freezing-thawing cycles.This article mainly completed the following tasks:1.This paper carried out freeze-thaw cycle(FTC)tests with different times on concrete,and then use the SHPB test device to perform dynamic mechanical tests on the concrete samples,the stress-strain curves and dynamic peak strengths of the samples after being impacted by three kinds of air pressure are obtained,at the same time,the influence of freeze-thaw cycles and strain rate on the dynamic stress-strain curve,peak stress and dynamic strength growth factor(DIF)of concrete was analyzed.It is found that the dynamic strength and DIF value of the concrete sample were reduced by the action of freeze-thaw cycles,while that of increased by impact speed.2.The impact toughness of concrete samples after different freeze-thaw cycles under different impact pressures was studied and calculated.The area enclosed by the stress-strain curve of concrete be the representative of the ultimate toughness,and the variation of the ultimate toughness of concrete specimens with the number of FTC and loading rate was analyzed.It is concluded that as the freezing-thawing cycle intensifies,the brittleness of the sample itself increases,resulting in a decrease in the ability to absorb external energy,and its ultimate toughness continues to decrease.As the loading rate increases,the aggregate of the sample is penetrated,which improves the ultimate toughness of concrete.3.The fractal characteristics of broken concrete samples under impact loading after freezethaw cycles were studied.Collect the concrete fragments after the SHPB impact test,use standard vibrating machine to screen the impacted concrete fragments and measure the quality of the fragments in different particle sizes after sieving,the fractal dimension of the sample was calculated according to the distribution equation of the concrete impact fragments,and the influence of different freeze-thaw cycles and impact air pressure on the fractal characteristics of the sample was analyzed.It is found that with the increase of the number of FTC,the cohesive force between the phases in the concrete reduces,resulting in increasing the concrete fragmentation at a higher loading rate,which ultimately leads to an increase in its fractal dimension.4.Microscopic observation of concrete impact fragments after FTC test was carried out by SEM test,and mechanism of concrete subjected to the freeze-thaw cycles was discussed based on the development of micro-cracks,hydration products and the particle size distribution of the fragments.It is found that there are more filling C-S-H gels in the sample at the beginning of freezing-thawing cycles,and no obvious cracks are observed.With the increase of the number of FTC,especially after reaches 75 times,the integrity and compactness of the concrete are severely reduced,many cracks appear inside the sample,and the impact fragments tend to be powdered.