Seif-compacting Concrete Type Ⅰ Fracture Mechanical Properties And Theoretical Research

Self-compacting concrete（SCC）refers to concrete that can flow and compact in engineering pouring applications under the action of its own gravity,easily pass through the gap of dense steel bars to fill the corners of the formwork,and does not require additional manual vibrating.Self-compacting concrete has been widely used in construction,municipal administration,road and railway projects in recent years.For example,research and development of high-speed railway in our country CRTS Ⅲ type plate without a frantic jumble orbit the SCC materials is used in our country.However,in practical engineering applications,there are natural cracks in SCC,these cracks will gradually expand as SCC bears external loads,which will reduce the bearing capacity of SCC and shorten the service life of building components.Because SCC material is different from ductile materials such as steel,it is a quasi-brittle material,and its fracture behavior cannot be simply considered by linear elastic fracture mechanics,so it is significantly vital to review the nonlinear fracture mechanical properties of SCC.In order to study the SCC I fracture mechanics performance,15 sets in total of 45 roots of standard three-point bending beam specimens with prefabricated SCC initial cracks in the middle span were produced in this paper,and 4 sets of initial fracture height ratios were preset:0.2,0.3,0.4,0.5;Sand ratio changes in 3 sets of mix ratios:0.4,0.5,0.6;and 4 sets of structural geometrically similar variable-size specimens（LxH xT）:440×100×100 mm³,520×120×100 mm³,600×140×100 mm³,680×160×100 mm³,The influence of the above control variables is applied to the study of the self-compacting concrete type I fracture mechanical properties among.In this paper,the working performance of SCC mixture was measured by slump cylinder test,and the ft and fts of SCC standard test block were measured by conventional mechanical property test.According to the principle of EM,the elastic modulus E of SCC standard test block was measured experimentally.Three point bending loading（TPB）was applied to prefabricated crack specimens,and the crack initiation load P_ini and maximum load P_max were measured by strain gage method（SGM）.Digital image correlation method（DICM）was used to measure the fracture opening displacement（CMOD）and fracture process zone length（FPZ）of the specimen.Based on the point method（PM）and line method（LM）in the critical distance theory（TCD）,the nominal fracture toughness K_c and the length of the fracture process zone（FPZ）of the specimens were investigated.Based on the fracture extreme theory（FET）,the initiation fracture toughness of fracture initiation K_Iⁱⁿⁱ,instability fracture toughness KIun and tensile strength of the specimen ft were studied.The main study work and perorations are as follows:（1）Performance tests were conducted on three SCC mixes with different sand ratios.The test results showed that slump extension and the slump extension time(T₅₀₀)increased with the increase of sand ratio in the SCC mix ratio,and the measure test data were in line with the normative standards.The basic mechanical properties of three SCC mixtures with different sand ratio were tested.Through the compression test and splitting tensile test,it was concluded that the compressive strength and splitting tensile strength of the standard cube decreases gradually with the increase of the sand ratio in the SCC mix ratio.（2）According to the principle of electric measurement method,the relevant test was designed to measure the elastic modulus of SCC.The test results showed that the elastic modulus measured by electric measurement method decreased slightly with the increase of the sand ratio in the SCC mix ratio,and the elastic modulus measured by electric measurement method was close to the elastic modulus value obtained by using the compressive value of SCC converted by the standard formula.（3）The strain gauge method was used to measure the crack initiation load P_ini and the maximum load value P_max of the SCC beam specimen under pressure.The test results show that the P_ini value is approximately equal to 0.7～0.8 of the P_max value.Under the condition of controlling a single variable,the crack initiation load value and the maximum load value both show a decreasing trend as the initial crack height ratio of the specimen increases.The cracking initiation load and the maximum load increase almost linearly with the increase of the specimen height of three-point bending beam,and the growth rate of the P_ini is faster when the beam height is 100～140 mm than when the beam height is 140～160 mm,and the P_max basically follows this law.When the proportion of sand ratio in the SCC mix ratio is higher,both the initial crack load and the maximum load generally show a decreasing trend.It is more obvious when the initial crack height ratio is between 0.2～0.3.However,when the initial crack height ratio is further increased,the effect of the influence trend gradually slows down.（4）The CMOD and FPZ of the three-point bending beam specimen during loading were measured by DICM.The experimental results indicate that,get command of other variables,the CMOD value aggrandizes with the increasing of the inception crack height ratio,but the change trend of CMOD value is not obvious with the change of the SCC sand ratio.The value of FPZ decreases with the increase of the initial crack height ratio.For the specimens with geometrically similar structures and different sizes,FPZ is positively correlated with the height of the specimens to some extent,but the trend of change is not obvious.（5）Based on the critical distance theory in calculation,the nominal fracture toughness values K_c of the specimen is between calculated by the double K fracture criterion of crack inside fracture toughness KIcun and the fracture toughness of instability KIcini,and for the geometrically similar variable size specimens,calculated by the TCD,this nominal fracture toughness changes with the height of the specimen transform,but the values calculated by the double K fracture criterion is almost unaffected.（6）According to the comparison of calculation of TCD in PM and LM method,and the generalized critical distance value were measured by FPZ in DICM.It can be conclusion that the value of the FPZ as specimens decreases with the increasing of inception crack height ratio,and half of the FPZ value is measured by the DICM close to the values calculated by the PM and LM,income,proved by TCD forecast the FPZ length value in the process of actual test specimens.（7）The fracture toughness calculation formula of the specimen is derived based on the fracture extreme value theory and combined with the SCC theoretical knowledge of the softening curve.The maximum load value P_max measured in the test is used as the only unknown quantity,which greatly simplifies the integration of the traditional method to calculate the fracture toughness value.The calculated fracture toughness values representing the fracture initiation K_Iⁱⁿⁱ and fracture instability KIun of the specimen were compared with the double K fracture criterion values and by TCD calculated nominal fracture toughness K_c values.It is found that the method the calculate the K_Iⁱⁿⁱ and KIun values of partial less than double K value,indicating that the K_Iⁱⁿⁱ and KIun value has a good safety reserve space.At the same time,the calculated K_c values that from TCD is calculated by other laws,and its value is closer to the K_Iⁱⁿⁱ of the double K calculation.（8）According to the two parameters of crack initiation load P_ini and maximum load P_max required by FET,the calculation formula of tensile strength f’t of specimens was deduced.The fluctuation range of tensile strength value of specimens with different sand ratio by this method was 0.17 MPa.Compared with the tensile strength ft calculated from the compressive strength fcu of the specimen by the standard formula,it is found that the values are close to each other and have a high degree of agreement,indicating that the formula has a strong applicability.By analyzing the calculated data,it can be concluded that the tensile strength ft calculated by FET almost does not change with the increase of the initial crack height ratio of the specimen when the single variable is controlled.However,as the height of the specimen increases,its value has a certain positive correlation trend.