Study On Brittleness And Size Effect Of Mehanical Parameters Of Concrete And Their Correlation

Size effect of mechanical parameters is the intrinsic character of concrete. The correlation between properties of small scale specimens using by laboratory and structure in sites could not be established and furthermore improved properties proved by laboratory may not true for practical structure. On the other hand, with the increases of strength of cementitious material, the brittleness come to more and more prominent, because size effect of mechanical parameters relates to concrete brittleness, so the phenomenon will be more distinct.Based on the study for size effects of strength and fracture parameters of concrete as well as their correlation, the limitation of opinions on concrete brittleness for a long time is conquered, and referenced idea to improve strength of cementitious materials is given. At the same time, the study is also propitious to the developing of fracture mechanics. In this paper, brittleness and brittleness index of concrete is discussed, and existing theories for size effects of strength and fracture parameters are analysis. Size effects of strength and its relations with brittleness for different type concretes are established by experimental study. Size effect phenomenon for fracture parameters is studied too. Also, a simple method for study on size effect of strength and brittleness index of concrete is established by using small size specimens. Furthermore, different effect factors for brittleness of concrete are studied by the new view on the grounds of relations of brittleness and mechanical parameters.Through the analysis of existing theories for size effects of strength and fracture parameters, it is come to conclusions that size effects of strength relate to brittleness of material in theory and different theories all have their limitation and can only apply to specific range. The length and width of fracture process zone of concrete vary with size of specimens and may be result in size effect of mechanical parameters. The other important reason for size effect of fracture energy is non-linear distribution of deformation nearby fracture surface and fracture energy distribution on fracture surface is not even also.According to the discuss of brittleness and brittleness index for concrete, as a true brittle material from microstructure point, magnitude of brittleness of concrete is relate to the dimension of plastic zone. Because brittleness behavior of concrete is influenced by size of specimen, brittleness index must not including size effect. Most existing brittleness index is not exact for its including parameters vary with size. Degree of brittleness can bequalitative reflect by curve of size effect proposed by Bazant and quantitative analysis by point of intersection do. The point do is direct ratio with dimension of plastic zone, so larger the do, smaller the brittleness. As an explicit quantitative brittleness index, do can be used by any laboratory to assess actual effect for improving toughness by different measures and is simple and credibility.In order to obtain the relation between brittleness and mechanic parameters, high and super-high strength concrete as well as air-entrained and steel fiber reinforce concrete are selected to test. Variation of flexural strength of specimens with notch and unnotch and compressive strength and splitting tensile strength of concrete with size of specimens are tested. At the same time, curves of load(P)-deformation( 8 ) and load(P)-crack mouth opening displacement(CMOD) as well as fracture toughness and fracture energy changing with size of specimens are given also. So relations between brittleness and mechanical parameters are based, and apply range for different size effect theory is demonstrated. Further experimental results are explained by these theories. It is point out that with strength increase and brittleness increase also, so size effect of flexural strength for specimens with notch is more distinctness and the trend for unnotched specimen is reverse. Compressive strength and splitting tensile strength tend...