Study On The Boundary Effect Model For Determining The Initial Cracking Toughness And Strength Of Concrete

The accurate determination of the true parameters of materials and the reasonable evaluation of the true characteristics of the actual structure are always the direction of unremitting efforts in the engineering and scientific circles.Initiation strength and initiation toughness are key material data of concrete materials.However,in the laboratory environment,there is a significant size effect.When using small-size specimens to obtain the initiation strength and fracture toughness,in order to obtain fracture data that is not affected by the size,it is necessary to build a larger size sample;Wedge the height W of the samples such as split tensile,direct tensile and three-point bending.When the maximum aggregate particle size d_max=8mm～40mm,W must reach 500 mm or greater than 1000 mm.However,under the premise of an ordinary laboratory,it is difficult to complete the casting and testing tasks of large-size samples.If you use the material data directly obtained from the laboratory’s small-sized samples,such as the initial cracking toughness,the size effect will definitely occur.There is now a scientific problem that needs to be addressed urgently:when small-size specimens(such as relative size W/d_max<50,or W=100mm～400mm)under the condition of quasi-brittle fracture,How to get the actual material parameters of concrete without size effect（including cracking strength and cracking toughness,tensile strength and fracture toughness）together.This paper expounds the concept of material parameter initiation toughness and initiation strength of concrete.The research thinks that the concrete material is heterogeneous and has no size effect.The fracture theory of concrete material parameters and the determination of corresponding methods have been further developed.The parameters include initiation toughness and initiation strength,as well as fracture toughness and tensile strength.According to different types of concrete specimen fracture tests（including four-point bending（4-p-b）,wedge splitting（WS）,three-point bending（3-p-b）,etc.）,the corresponding cracking toughness and starting Cracking strength,fracture toughness and tensile strength,etc.,and compared them with the experimental strength values and the double-K fracture data obtained with the double-K fracture model.The practicability and rationality of the scheme and model proposed in this paper are further improved.Be proven.According to the determination of material parameters,the full curve of concrete cracking failure and the full curve of fracture failure are created;when the cracking toughness without size effect is known,the minimum theoretical size of the concrete sample is also given.Analytical formulas related to the crack initiation toughness and the crack load were created,and the crack initiation toughness of different concretes and the crack initiation load of different types of concrete specimens were successfully predicted（including four-point bending（4-p-b））,Wedge splitting（WS）,three-point bending（3-p-b）,etc.).