Study On Evaluation Method Of Brittleness And Toughening Measures Of High Strengthen Concrete

As makes the high strength concrete getting wide-ranging application, the strength of concrete has a series of merits. So it is a development direction for concrete. But it is thought that concrete is quasi-brittle materials. And it is thought that concrete is brittle materials when the size of its structure is very large. Concrete materials give expression to obvious brittle fracture when it rupture. With the development of strength of concrete, to be related brittle questions are more and more striking. For realizing entirely meaningimprovement of strength in concrete-the improvement of strength of structural bearingcapability in engineering, the cause of brittle fracture must be understood. On the other hand, brittleness and size effect of strength for concrete are related, Not to study size effect phenomenon, the correlation between the brittleness of small scale specimens using by laboratory and structure in sites could not be defined and furthermore improved brittleness proved by laboratory may not true for practical structure.(1) Based on the study for brittleness and size effects of strength of concrete as well as their correlation, the limitation of recognition on concrete brittleness for a long time is overcome, and referenced idea to improve strength of cementitious materials is given. Meanwhile, the study is also propitious to the development of fracture mechanics. In this paper, brittleness and brittleness index of concrete is thoroughly discussed, and existing theories for size effects of strength and fracture parameters are analyzed, and 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. And different effect factors for brittleness of concrete are studied by the experimental study of relation of brittleness and size effect of strength; Toughening measures of high strength concrete are studied, in which toughening mechanism of aggregate in HSC is emphatically studied. It is thought that wedge splitting (WS) specimen deserved to be popularized by compared experimental study on the measure methods of concrete fracture energy.(2) Through the analysis of brittleness index and evaluation method , it is come to conclusions that, 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 include size effect. Most existing brittleness index is not exact for its including parameters vary with size. Degree of brittleness can be qualitatively reflected by the curve of Bazant's size effect. Brittleness index can be quantitatively expressed by a point of intersection η that is a point of the curve of Bazant's size effect. Brittleness index η is in direct proportion to the dimension of plastic zone. The bigger η is, the smaller brittleness is. So as definite brittleness index quatitated, it is simple and reliable for η can be used any laboratory asevaluating actual effect of different toughening measures.(3) Through the systematic analysis and contrast with existing brittleness index and evaluation methods for concrete, it is of the opinion that the brittleness of concrete relates to size effect of strength in theory and different theories all have their limitation and can only apply to specific range. Fracture energy is a basic performance parameter of material brittle fracture and is inseparably interrelated fracture process zone (FPZ) . 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, either. The results have shown that it is still important that size effect of strength on the effect of brittleness is verified or reflected by experimental study.(4) Research based on high strength concrete and super high strength concrete, the relationship between size effect of mechanical parameters and brittleness for concrete is decided by test. The testing results and theoretical analysis have shown that as for as improving strength of concrete, it is necessary that brittleness of concrete relates to size effect of mechanical parameters, which is helpful to more thoroughly and consummately understand some problems due to improving strength of concrete. The size effect of flexural strength on the brittleness of concrete exists different influence. In the conditions of specimens notched, with the development of strength of concrete, the brittleness of concrete may be more outstanding generally and size effect of mechanical parameters may be more conspicuous; In the conditions of specimens non-notched, flexural strength of concrete finally tends to ultimate tensile strength and with the increasing of brittleness of concrete, the strength of concrete rapidly close to ultimate value. The creasing of concrete brittleness restricts to the improving of actual compressive strength due mainly to interior flaw sensitivity. In a word, as far as non-notched specimens of compressive strength, Both Weibull theory and fracture mechanics principles can not well explain the testing results. The study on size effect of compressive strength of notched specimens from indirect aspect reflects brittleness to effect compressive strength of concrete. In the general testing conditions, the size effect of splitting tensile strength is related to actual bearing area of specimens, that is related to fractal characteristics of fracture surface. With the creasing of concrete brittleness, the irregularity of material surface disappear more quickly. Namely, compressive strength of concrete tends to stabilize more quickly. With the change of specimen size, fracture toughness tends to relax more quickly, that is, it is more close to stable ultimate value. In practice, high strength concrete and super high strength concrete are more close to linear elastic material. Fracture toughness is stable in small scale specimens. It is reasonable and safe based on linear elastic fracture mechanics dealing with crack propagation. Fracture energy has same tendency to fracture toughness with thechange of size, as is shown that size effect between fracture energy and fracture toughness has inherent correlation.(5) The material composition of concrete affects the brittleness of HSC, as is reflected in the change of brittle index. In the same time, the brittleness of concrete is influenced strikingly by environments. The brittleness of HSC increase with the reduction of water-cementitious matrials ratios and the increasing of sand ratio in general extent. Using fly ash as well as silica fume composite mixture is helpful to improving toughness of HSC. With the graving of concrete age, the brittleness of concrete is creasing and fracture toughness and fracture energy are lower. Furthermore under the condition of natural curing in this paper(indoor curing, average temperature is 25 "C, relative humidity is 60%), the brittleness of concrete improving obviously.(6) In concrete toughening aspect, air entrain raise the brittleness of HSC and the relation of its size effect of strength reappear the change relation of strength with size on the influence of concrete brittleness; Fiber decreases the brittleness of concrete. As far as HSC, the quantity of fiber in general extent improves toughening function for concrete and the brittleness of HSC decreases obviously. As regards different kinds of aggregate, using high strength and hard basalt aggregate may be conspicuously decrease the brittleness of HSC and SHSC and improves fracture properties; With the increasing of their strength, both fracture toughness of scree and detritus concrete come to improve; The fracture energy of scree concrete shows increasing tendency with the improving of its strength; The fracture energy of detritus concrete does not show increasing tendency with the improving of its strength and tendency line is level and straight. The change of aggregate's anti-cracking capability may be obvious for the brittleness of concrete. Increasing strength and toughness for aggregate are the most effective measures of improving the brittleness of HSC and SHSC. Besides, changing composition of cementitious materials is also better way. For soft matrix materials, using mixture of high A^OySiC^ rate is important technical way.(7) In this paper, the methods of present measure fracture energy are compared. It is pointed out that direct tensile method is theoretically the most ideal measure method. But for the restrictions of various conditions, as far as present testing conditions as well as testing technology, it is not the most suitable method. Wedge splitting method compares with others its merit being outstanding. So it hopes to become standard measure method that tests fracture energy of concrete.