| As an important material in civil engineering,concrete is of great importance,however its inherent defects can not be avoided.As a result,many researchers are devoted to the research of concrete fracture mechanics,which has achieved fruitful achievements today.However,most of these research achievements are devoted to the study of monotonous static load fracture while few studies on time-dependent fracture are done.However,many concrete structures in service are undoubtedly under long-term load.In a short time,many structures are reliable,but under long-term loads,concrete may fracture.In this paper,a long-term load test is carried out on the three-point bending beam specimens recommended by the relevant standards,and the rule and mechanism of the time-dependent fracture are explored.1.The size of the specimens in this paper is S×DxB=400mm×100mm×100mm,and the initial ratio of seam to height is a0/D=0.4.2.The loading levels of P(t)=0.7Pmax,0.8Pmax,0.85Pmax and 0.9Pmax are carried out respectively.The time-dependent crack mouth opening displacement CMOD(t)and mid-span deflection are monitored by electronic extensometers.The crack length is monitored by strain gages.3.According to the monitored data of CMOD(t),it's derived function CMOR(t)curves and CMOR'(t)curves were plotted with origin9.0 drawing software.The physical significance of monotonous static load fracture unstability is that CMOD starts constantlyaccelerated expansion.Based on this feature,the critical time of time-dependent fracture is determined.Time-dependent critical crack mouth opening displacement CMODc(t)is determined then combined with the monitored time of CMOD(t).4.The determination of the modulus of elasticity.The pressure testing machine is used to press the cube specimen made in the same batch with the test specimens,and the cube compressive strength fcu is measured.The value offcu is taken into the empirical formula of China,CEB-FIP MC90,ACI 318-77,Russia,then the modulus of elasticity is calculated and the average value is regarded as representative value of the modulus of elasticity.5.By substitution of the known CMODc(t),long term load P(t)and modulus of elasticity into the existing formula,the time-dependent critical equivalent crack length ac(t)is calculated,and the calculated value is recorded as ac(t)?;by substitution of the known ac(t)?,long term load P(t)into the existing formula,the time-dependent unstable toughness Klcun(t)is calculated,and the calculated value is recorded as Klcun(t)?.6.The time-dependent critical equivalent crack length ac(t)is monitored with strain gauges,and this monitoring value is recorded as ac(t)?.As there is a certain distance between the sensitive grids of each two row of strain gages,the strain gauges can only monitor a range of the length of the crack.Based on the known data,regression analysis is carried out to get the relation between the equivalent time-dependent crack length ac(t)and the monitoring time t.The critical time tc is substituted into the relational expression of a(t)-t,and the value of ac(t)is accurately calculated.By substitution of the known ac(t)?,long term load P(t)into the existing formula,the time-dependent unstable toughness Klcun(t) is calculated,and the calculated value is recorded as Klcunt)?.7.The time-dependent fracture parameters ac(t)?,and ac(t)?are compared with the monotonous static load fracture parameters ac.The time-dependent fracture parameters Klcun(t)? and Klcun(t)?are compared with monotonous static load fracture parameters Klcun,and the relation between the time-dependent parameter and the load P(t)is obtained.8.The application of monotonous static load fracture formula to the problem of time-dependent fracture is analyzed.The related parameters of the formula which can't be directly applied to time-dependent fracture are corrected and the formula form remains unchanged,so that the main monitored parameters of time-dependent and monotonous static load fracture test are unchanged,and monitoring technology and means are simple and feasible. |
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