| As a new type of green building material,high-performance recycled concrete can effectively recycle used concrete building materials.It is one of the most important measures to solve a series of problems related to resources,environment and ecology.Many scholars have done a great amount of research to study its performance.Due to limited time,space and technology,current researches are restricted to static or quasi-static mechanical properties,rarely being extended to dynamic mechanical properties.However,in practical engineering projects,structures may be subjected to earthquake,vehicle impact,incidents,terrorist attacks and other dynamic loads in their life cycle,further researches on mechanical properties of recycled concrete under dynamic loading bear make great significance.In this thesis,the dynamic load experimental system is SHPB.Five experimental control groups were designed as follows:?.The first one is experimental control group?high performance natural concrete?.?.The second one is Recycled coarse aggregate replacement group?including three different replacement rates?.?.The third one is Recycled fine aggregate replacement group?including three different replacement rates?.?.The forth one is Recycled coarse and fine aggregate replacement group?including three different replacement rates?.?.The fifth one is Fiber group?including natural concrete+fiber,100%replacement of recycled coarse aggregate+fiber,and 100%replacement of recycled coarse and fine aggregate+fiber?.The replacement rate of different recycled aggregate is 30%,50%and 100%respectively.Thirteen different components concrete specimens were made in total.The dosage of fiber by 1.5 Kg per cubic meter of concrete mixed for the fiber group.24 specimens were made respectively for each group which size is 75mm?dia?by50mm?height?cylinder specimen.Dynamic load impact speed range is 2 m/s7 m/s during which four speeds are averagely distributed for each group of specimens.Three times tests were carried out at least for each speed.The dynamic information about these different components specimens at different speed were obtained through the SHPB dynamic impact experiment.Based on the experiment data results and theoretical analysis,the following characteristics of high performance recycled concrete are explored:characteristics of stress-strain constitutive curve,peak stress,peak strain,ultimate strain,dynamic modulus of elasticity,energy transfer,damage performance,failure form and mechanism,with studying its constitutive equation and strain rate effect under dynamic loading as the main purpose.A series of results are obtained as follows:?1?The force properties change rule under static pressure effect of replacement rate of recycled aggregate or fiber on high performance recycled concrete?referred to as“the former”in following paragraphs?under static pressure,compared with that of high performance natural concrete?referred to as“the latter”in following paragraphs?as follow:Axial compression strength and Modulus of elasticity decrease with the increase of the replacement rate,but 50%is greater than 30%coarse replacement rate.Fiber can increase both for the former and latter and it increases more for the latter.Peak strain increases with the increase of the replacement rate but 50%is less than 30%coarse replacement rate.The former has stronger deformability than the latter.Fiber can increase the former's peak strain more.For the ultimate strain:only both fine and coarse aggregates are replaced,it shows the rule which is it increases with the increase of the replacement rate.Fiber increases the latter's ultimate strain more while it increases.The failure pattern is similar but the cracks of the latter develops faster than the former,rib edge exfoliation of the specimen is more serious.The toughness of the former is high than the latter.?2?Effect rule of strain rate on stress wave curveThe theoretical action time of stress wave almost equals the measured time,which proves that the test is reliable.The correlation between the amplitude of the reflected wave and the transmitted wave is positively correlated with the strain rate.The area between the reflected wave curve and the abscissa increases,which indicates that the damage degree of the specimen is aggravated,and the wave impedance decreases,resulting in more stress wave reflection.?3?Characteristics of typical dynamic constitutive relation curve of high-performance recycled concreteThe stress-strain constitutive relation is basically linear at the initial stage.With the increase of strain rate,the linear deformation segment becomes steeper and longer and yield strength increases.Stress-strain constitutive curves wavelike rise before the peak stress appears while the strain rate is low.The curves appear multiple?usually two?maximum values which are almost equal to the peak stress wave when the strain rate exceeds a certain threshold.The bearing capacity of the specimen decreases instantly and the specimen is damaged while the load reaches the peak stress.The threshold is different due to the different replacement rate of aggregate.When the strain rate increases,the incident wave amplitude become larger,the energy is higher and the extent of specimens failure is more serious,then the peak strength and residual strength appear.?4?Effect rule of strain rate on peak stress and dynamic elastic modulusPeak stress and dynamic elastic modulus demonstrates obvious rate sensitivity.Both represents exponential function correlation with strain rate.Within a certain range of strain rate,the dynamic compressive strength of high performance recycled concrete can be increased to 2.5 times that of under static axial compressing force.Compressive strength of high performance recycled concrete is more rate sensitive in the range of20s-1110s-1 strain rate than that of high performance natural concrete.The increase amounts are also different due to the different aggregate composition.?5?Effect rule of strain rate on peak strainPeak strain represents quadratic parabola function correlation with strain rate.The type and replacement rate of aggregates affect its critical strain rate and corresponding peak strain.Recycled coarse aggregate has greater influence.When the strain rate value is at the position of either end of the parabola,the dynamic peak strain is smaller than the static peak strain.When the strain rate value is between the two ends of the parabola,the dynamic peak strain is greater than the static peak strain,with up to 4times as much as the static peak strain.?6?Effect rule of strain rate on ultimate strainPeak strain has obvious rate sensitivity,similar to peak strain.Its correlation with strain rate is quadratic parabola function correlation.The critical strain rate?value of the strain rate at the apex of the parabola?and the corresponding ultimate strain are influenced by aggregate type and replacement rate,where coarse aggregate has greater influence.When the strain rate is less than a certain value or greater than a certain value,the dynamic limit strain is lower than the static limit strain.When it is in the middle,the dynamic limit stress is greater than that under static condition,and the maximum limit is 3.5 times that of the static limit strain.?7?Effect rule of strain rate on energy propagationThe four kinds of energy are positively correlated with the strain rate.The trend of incident energy and dissipation energy is power function correlated to strain rate.the increase amount of energy increases with the increase of the strain rate.Reflection energy and transmission energy are quadratic polynomial function correlated with the strain rate.With the increase of strain rate,the increase amount of energy declines and the reflection energy increase amount is least amount.The dissipation rate increases with the increase of the strain rate.The two are approximately quadratic polynomial or linear function correlated.The dissipation rates are different due to the composition of the aggregate.?8?Effect rule of strain rate on damage of the high-performance recycled concreteThe strain rate is exponentially related to the damage value of high performance recycled concrete material.The higher of replacement rate of recycled aggregate,the more serious of damage degree of specimens under the same other conditions.But the difference is not obvious under the condition of 30%and 50%coarse aggregate replacement.The three innovative results are summarized based on the experiment research as follows:?1?The effect rule of recycled aggregate replacement rate on compressive strength,elastic modulus,peak strain and ultimate strain of high-performance recycled concrete is gained based on the analysis on the static experiments.?2?Based on the analysis of dynamic experiments,high-performance recycled concrete presents strain rate sensitive property which is different due to the different recycled aggregate replacement rate.The equations are presented between peak stress and strain rate.The effect rule of strain rate on compressive strength,dynamic elastic modulus,peak strain and ultimate strain of high-performance recycled concrete is acquired.The characteristic of constitutive relationship curves of recycled concrete with strain rate increase is summarized.?3?Impact energy propagation and damage characteristics of high-performance recycled concrete under different strain rate are obtained by the analysis of experiment data.The equations are shown respectively which are energy dissipation and damage value with the strain rate due to the different aggregate component. |
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