Study On Cracking Test System,Dynamic Strength Ratio Effect And Triaxial Shear Properties Of Hydraulic Concrete

Concrete used in hydraulic Engineering is of more special behaviors than those in other undertakings.So that the large dam concrete and plastic concrete are the typical representatives.The large dam concrete is used as the principal structural bearing force material,and the cracks caused by its contraction and the dynamic flexure-pulling destruction under the seismic action are the key problems confronted.The evaluation for the concrete cracking resistant behavior has been the hot-button and difficult points,which can not be well solved hitherto.With an aim at the cracking resistant evaluation difficult problem of concrete material,a new-type of concrete cracking testing device is invented in this paper.Through introducing temperature stress action,this device has adopted the steel ring of low-line expansion coefficient as the constrained ring so as to improve the cracking sensitivity of experiment equipment,whereby the concrete cracking resistance can be quantitatively evaluated.With an aim at the problem of improving dynamic strength of concrete materials under the seismic action,this paper suggests that improving concrete dynamic strength is the united action results of concrete inhomogeneity and inertia.Through theoretical analysis,and also,the specially-designed three-point bending experiment is adopted to prove the correctness of this theory.Finally,with an aim at the special plastic concrete shearing resistant behavior,such effect factors as the cement bentonite ratio and ambient pressure,etc upon the plastic concrete strength and deformation behavion influence laws are studied under the condition of triaxial stress.The principal achievements are described as follows:（1）Based on the thermal elastic mechanics theory,the analytic expression for the stress,strain and displacement of the steel ring and concrete ring combination system under the condition of temperature and dryness is deduced so as to provide the theoretical base for the improvement of test device,and ABAQUS finite element software is adopted to test the correctness of analytic calculation results.The analysis results indicate that the steel ring linear expansion coefficient,steel ring thickness,temperature magnitude and elastic modules of early stage concrete have had the remarkable effect upon steel ring and concrete ring stress,strain and displacement,of which the temperature decreasing magnitude and steel ring linear expansion coefficient have had the most remarkable effect upon then circular tensile stress of concrete ring.So that the analysis results can decide the direction of improvement of physical experiment device.（2）Based on the theoretical analysis results,the optimal selection of steel ring thickness is 6mm,8mm and 10mm,to carry out the contrast experiment of concrete cracking respectively.The experiment results indicate that the steel ring thickness can have the remarkable effect upon steel ring strain measured values.Under the condition of same temperature decrease when the steel ring thickness is 6mm,the corresponding strain measured values are even larger,and easier to have accurate measurement.Finally,the integrated analysis has decided the steel ring thickness,being 6mm in more optimum.The concrete cracking contrast test results under different curing temperatures and temperature decrease rates show that when the curing temperature is 30℃,and the temperature decrease rate is 1℃/h,the concrete round ring doesn’t occur to have cracking in the test,this means that the circular tensile stress of concrete ring cannot reach the concrete limit tensile stress.When the curing temperatures are 60 ℃ and 80 ℃ and the temperature decrease rate is 15℃/h,the concrete ring all produce the cracking damages.The test results of the curing temperatures being 60 ℃ and 80 ℃ indicate that when the curing temperature is 80 ℃ and the temperature decrease rate is 15℃/h,the test result stability is more superior so that the sizes and testing method of new type of concrete cracking test device have been determined.（3）The new type of concrete cracking test method is adopted to study the raw material and curing age factors（mineral admixtures,applied agent types,slump,aggregate types,and fibers）to affect the concrete resistance against cracking.The test results indicate that under the same conditions of cube compressive strength,the incorporation of flyash and silicon ash can make the concrete cracking resistance decrease 21%～47.6%.The water-reducing agents have had the most apparent effect upon the concrete cracking resistance.The cracking resistance of concrete incorporated with naphthalene water-reducing agents is more apparently reduced than that of concrete incorporated with polycarboxylic water-reducing agent,being with the decreased amplitude reaching 61.2%.The cracking resistance agent can have little effect upon the early cracking resistance of concrete.The contrast test results of the cracking resistance of concrete with different slumps indicate that the cracking resistance of large guss concrete（19cm）is reduced by 12%or so as compared with that normal guss concrete（6cm）,being with little influence.The contrast test results through different aggregate concrete cracking resistance（including artificial granite aggregates,artificial limestone aggregates,natural quartz aggregates and natural gravel aggregates）indicate that with the artificial granite aggregates on the base,the cracking resistance of limestone aggregates can be raised by 12.5%as compared with that of granite aggregates and the cracking resistance of quartz aggregates and gravel aggregates is decreased by 23%and 33.8%respectly as compared with that of granite aggregates,that is,the cracking resistance of concrete prepared by the artificial aggregates is obviously superior to that of the gravel aggregate and quartz aggregate concretes,and the cracking resistance of limestone aggregate concrete is more superior to that of the granite aggregate concrete.The cracking resistance contrast test results between the polypropylene fibre and steel fibre concretes indicate that the resistance of the polypropylene fibre versus the concrete has had the unfaverable effect;the steel fibre can obviously raise the concrete cracking resistance.The steel fibre concrete can not occur to have the strain mutation in the process of temperature decrease,whose peak strain to constrain steel ring is 361με,corresponding to the concrete tensile stress is 5.13MPa,that is,the maximum tensile stress is provided by the equipment so as to satisfy the evaluation requirement for the majority of concrete cracking resistance.The results of concrete cracking resistance contrast tests in the same mixture ratio and different curing strengths show that the post-cracking resistance of concrete with the same mixture ratio has had the more remarkable decrease than that in the previous stage,with the decrease amplitude being between 16.0%～18.1%.With an increase in the curing age period,the tensile resistance strength can gradually increase,whose cracking resistance can have the increasing trend,but with a small increase amplitude.The new type of concrete cracking test method is to adopt the cracking temperature difference as the cracking resistance index,which can comprehensively reflect the cracking behaviors of concrete materials under the condition of temperature contraction,whereby the cracking resistance of concrete materials can be quantitatively evaluated.（4）with an aim at the improvement of concrete dynamic strength in the case of dynamic flexural tensile,this paper suggests that the difference of concrete material static and dynamic strengths is mainly caused by the material inhomogeneity and that the concrete material static and dynamic strengths are closely related with the crack development pathway after its damage.In static damage,cracks develop forward along the weak surface of materials,that is,the static cracks cracking follows the minimum energy consumption principle.while in dynamic damage,the strain energy within the inside of material needs to be released instantly,and cracks develop forward along the shortest pathway of the energy release.The cracks at this time pass through the part of high strength zone of materials（for instance,aggregates in concrete）,thus,making the material strength be higher than the static strength,so that,the dynamic crack cracking follows the relevant principle of energy release rate.Through special design of three-point bending test,when the diamond shaped test specimens simulate the bending aggregates,the universal testing machine and the freely falling body impact test are adopted to carry out loading on testing beam.In the case of loading at the slow speed（0.1mm/min）and at the fast speed（140mm/min）and the free impact with falling hammer with different heights（lm,3m,6m,and 9m）,it is necessary to analyze the damaged strength,damaged morphology and crack development strike laws of the testing specimens,whereby proving that this theory is correct and that the dynamic strength expression is given out under the condition of concrete dynamic flexural tensile.（5）with an aim at the anti-shearing behavior of plastic concrete under the action of tri-axial stress,the plastic concrete of 28-day cube with compressive strengths of 2MPa,4MPa,6MPa and 8MPa or so are prepared through changing the incorporated ratio of bentonite and cement in the mixture ratio of plastic concrete to carry out the conventional tri-axial compressive tests and to study the effect of the incorporated ratio of bentonite and cement,as well as ambient pressure,etc.upon plastic concrete strength and deformation behaviors.Testing results indicate that with an increase in bentonite incorporated quantities,plastic concrete single-axial compressive strength σc and cohsion force c have had the same changing laws,that is,after the bentonite is incorporated,the concrete single-axial compressive strength and cohsion force have had the apparent decrease,while there has been no much change in the inner friction angle(p of plastic concrete and the tensile pressure strength ratio a,that is,the bentonite incorporated quantities have had no much effect upon the hydrostatic stress sensitivity of plastic concrete and tensile pressure strength ratio.Finally,the relationship of the bentonite-cement incorporated ratio versus the cutting modulus E_0.5（axil strain ε₁=0.5%）,peak strain ε_if,volume strain ε_v and modulus strength ratio are obtained repectively.