Research On Concrete Softening Relationship From Notched Three-point Bending Beam Based On Evolutionary Algorithms

Based on the load-crack opening mouth displacement (P-CMOD)curves of notched three-point bending beam tests from a national naturalscience fund (50409005), this dissertation established a simple concretesoftening curve determining method. Based on the fictitious crack model,finite element method and evolutionary algorithms, the method assumestri-linear concrete tension softening curve to determine the fractureparameters by fitting the tested P-CMOD curve. The correspondingcalculation software, with the advantage of efficient, accurate, and simplevisual interface, was developed. However the main defect alsoaccompanied: the specimen geometry and corresponding rigidityrelationship must be known before inverse analysis.Based on the obtained softening curves of dam and correspondingwet-screened concrete, the conclusions of this work are as follows:(1) The fracture energy determined by inverse analysis method has agood agreement with that by fracture work method.(2) Studying on the influence of specimen sizes on fracture parameters (G_F、w_c、f_t),the following coclusions were obtained:fractureparameters show size effect. The influences of beam depth on fractureparameters can be described as follows:①The obtained fracture energy G_F shows significant size effect，namely with the increase of beam depth D, the fracture energy G_Fincreases firstly and tends to be stable afterward.②For small aggregateconcrete specimen groups, the influence of beam depth on the maximumcrack opening width w_c are consistent with that of G_F， showssignificant size effect. However for large aggregate concrete specimengroups, there are difference between the influences of beam depth on W_cand G_F，which show a little size effect.③For small aggregate concretespecimen groups, the tensile strength f_t decreases with the increase ofbeam depth, while for large aggregate concrete specimen groups, f_ttends to be a constant. Practically，the obtained tensile strength f_t hasno size effect.(3) With the increase of the maximum aggregate size, both G_F andf_t increase firstly and decrease afterward, while w_c increasescontinuously. This phenomenon can be attributed to the inhibition ofaggregate on crack expansion.(4) The f_t slightly decreases with the increase of the compressivestrength f_c. The quantitative relationship between f_c and w_c is notobvious. The G_F decreases significantly when increases from32.1MPa to38.6MPa. The G_F tends to be stable when f_c increases from38.6MPato55.4MPa.(5) The f_t of fully-graded dam concrete is smaller than that ofcorresponding wet-screened concrete. With the increase of specimen size,the difference between the f_t of two types concrete increases firstly andtends to be stable afterward. The G_F of fully-graded dam concrete islarger than that of corresponding wet-screened concrete. With theincrease of specimen size, the difference between the G_F of two typesconcrete increases firstly and tends to be stable afterward.