Effect Of Silica Fume On Mechanical Properties And Damage Mechanism Of Recycled Aggregate Concrete

In recent years,with the construction of new buildings and the demolition of abandoned buildings,a large number of construction waste has been produced in urban and rural areas,and the amount of construction waste has a growing trend.This phenomenon is more common in China,India and other developing countries.With the vigorous development of the domestic construction industry,people start quarrying,refining cement,the collection of natural resources is more frequent,resulting in the shortage of natural aggregate and limestone materials.The use of recycled aggregate concrete and silica fume can be regarded as a sustainable development method to alleviate this situation.In this paper,the aggregate from Zhengzhou urban housing demolition is used as recycled aggregate,adding different content of silica fume admixtures and admixtures,and the uniaxial compression tests are carried out considering different silica fume content,dynamic strain rate,curing age and freeze-thaw cycles,the influence of silica fume on the damage evolution mechanism of recycled aggregate concrete under different conditions is discussed.In order to improve the practicability of recycled aggregate concrete and provide theoretical support for its application in practical engineering,the following research work is carried out in this study.（1）Uniaxial compression tests of recycled aggregate concrete are carried out with different silica fume content,dynamic strain rate,curing age and freeze-thaw cycles as dependent variables.During the test,the damage and failure characteristics of the specimens are observed.Combined with the macro stress-strain curve,the influence of the dependent variables on the macro mechanical properties（peak stress,elastic modulus and peak strain）of the specimens is summarized.The test results show that:with the increase of silica fume content,the overall mechanical properties of recycled aggregate concrete show a trend of first increasing and then decreasing,and reach the peak at 9%silica fume content;the dynamic strain rate and curing age have a positive correlation with the mechanical properties of recycled aggregate concrete,while the freeze-thaw cycles are on the contrary.（2）Acoustic emission test is carried out for recycled aggregate concrete with 0%and9%silica fume respectively,which is carried out at the same time with uniaxial compression test.When the acoustic emission parameters reached the peak value,the macro stress-strain curve behavior and damage characteristics of the specimens are mainly observed;Nuclear magnetic resonance（NMR）tests are carried out on recycled aggregate concrete with 0%and9%silica fume content in different curing ages and freeze-thaw times to explore the variation of internal porosity and pore size distribution with the increase of curing ages and freeze-thaw times;Based on the mix proportion of recycled aggregate concrete,two kinds of pure mortar cubes with the size of 40mm and the content of silica fume of 0%and 9%are made,and scanning electron microscope tests are carried out considering different curing ages and freeze-thaw times.The experimental results show that the peak value of AE parameters is at the position lagging behind the peak stress rather than the peak stress;With the increase of curing age,the internal porosity of concrete specimen decreases gradually,and the internal hydration effect of concrete specimen is better;with the increase of freeze-thaw times,the internal porosity of concrete specimen increases significantly,and the internal micro cracks of concrete specimen are longer and wider.（3）Based on statistical damage mechanics and statistical damage constitutive model,the effects of silica fume content,dynamic strain rate,freeze-thaw cycles and curing age on the damage evolution mechanism of recycled concrete are discussed at meso scale.The peak nominal stress state is distinguished from the critical state,which is defined as the precursor of local failure.Two meso damage modes,fracture and yield,are considered.It is assumed that their evolution forms obey the triangular probability distribution and are characterized by five damage parameters E₀、ε_a、ε_h、ε_b and H.The effects of silica fume content,dynamic strain rate,curing age and freeze-thaw cycles on the damage evolution of concrete are discussed.The results show that the meso damage evolution process of concrete specimens presents obvious regularity,which is consistent with the internal physical and chemical mechanism and macro nonlinear stress-strain behavior of concrete specimens.