Investigation On The Time-dependent Characteristics Of Concrete Fracture Process Zone Under Long-term Constant Load

Under the effect of long-term load,the cracks which inside the concrete structure will expand consistently.It will affect the safety of the structure seriously.The result which is obtained under quasi-static fracture load is different from the result which under the long-term constant load.Therefore,it is important for the safety evaluation of concrete structures to research the time-dependent fracture characteristics of concrete under long-term constant load.The thesis consists of the experimental study and theoretical analysis by wedge-splitting compact tension specimens:1.There twelve wedge-type compact tensile specimens in which three groups of specimens were cast,with the different sizes（2H×D×B=480 mm×400mm×200 mm,2H×D×B=720 mm×600 mm×200 mm,2H×D×B=960 mm×800 mm×200 mm）.Two specimens of each size are used to determine peak loads by means of the monotonic static loading test.The remaining specimens are then tested under 0.85P_max constant load levels.2.Time-dependent crack mouth opening displacement CMOD（t）and time-dependent crack tip opening displacement CTOD（t）in concrete under long-time constant loading are monitored by electronic extensometers.Analyzed the relationship of the curves between CMOD（t）,CTOD（t）and the crack mouth opening rate CMOR（t）.The critical crack mouth opening displacement of time-dependent fracture CMOD（t_c）and the critical crack tip opening displacement of time-dependent fracture CTOD（t_c）are obtained by extensometers and compared with which measured by static fracture test under monotonic loading.The curves of CMOD（t）-t and CTOD（t）-t both present three-stage characteristics.To map the CMOD（t）-CTOD（t）curve and analyze the linear correlation of the curve.The test results indicate that time-dependent fracture curves CMOD（t）-t and CTOD（t）-t of the wedged compact tensile specimens are similar.There are three-stage characteristics:in the first stage,CMOD（t）-t and CTOD（t）-t increase rapidly;in the second stage,CMOD（t）-t and CTOD（t）-t increase steadily;In the third stage,CMOD（t）-t and CTOD（t）-t increase rapidly until the specimen fractures.At the same time,CMOR（t）-t curves also presents three-stage characteristics.Compared with the monotonic load static fracture test,the time-dependent critical crack mouth opening displacement CMOD（t_c）and the time-dependent critical crack tip opening displacement CTOD（t_c）are both larger than the values CMOD_cand CTOD_cmeasured by the monotonic load static fracture test.And with the size of the specimen increases,the values of CMOD（t_c）and CTOD（t_c）both increase.3.Crack propagation was observed by means of strain gauges.The development rules of CMOD（t）-t、CTOD（t）-t、CMOD（t）-a（t）and CTOD（t）-a（t）were analyzed.The time-dependent fracture propagation is compared with the static fracture test under monotone loading.The test results indicate that because of the width of the resistance strain gauge and the distance between the strain gauges,the crack propagation length cannot be measured at any time.Therefore,the method of linear interpolation is used to obtain the crack propagation length at each time.Curves of CMOD（t）-Δa（t）and CTOD（t）-Δa（t）can be divided into deceleration,stable development,acceleration in three stages.Compared with the measured value,the calculated value of the two-parameter model and the calculated value of the hinge model,the result about CTOD（t）-Δa（t）shows that the hinge model based on the geometric relationship is closer to the measured value.Comparing the time-dependent fracture test with the monotonic loading static fracture test without considering the time dependence,when the crack propagation lengthΔa is the same,CMOD（t）and CTOD（t）are both larger.So,it indicated that the softening effect of time-dependent fracture is more than monotonic loading.The equivalent crack propagation length,crack mouth opening displacement,crack tip opening displacement,critical equivalent crack propagation length,critical crack opening displacement and critical crack tip opening displacement when the time-dependent fracture specimen finally break are greater than the critical value at the peak point of static fracture test.4.By analyzing the development law of time-dependent crack mouth opening angleCTOA（t）-Δa（t）and crack tip opening angleCMOA（t）-Δa（t）curves.The following conclusions are drawn:the time-dependent fracture crack tip opening angleCTOA（t）-Δa（t）curve and the time-dependent crack mouth opening opening angleCMOA（t）-Δa（t）curve both have an obvious decline stage in the initial stage of the time-dependent fracture test.And the crack opening angle has a certain difference in the initial stage.