Research On The Size Effect Of Three-Graded Aggregate Concrete Behavior In Mechanics Performance

Size effect is a common phenomenon and more prominent for heterogeneous materials,such as concrete. Test results of intensity of specimens produced from the same material in different sizes are different. Now hydraulic dams, bridges and other components usually use full grade or three-graded concrete that are large and difficult to carry out full-scale model tests to measure the strength. So in the project people generally choose wet sieving to form small specimens for dam design and safety identification. Researchers continue to study if measures which using a wet sieve concrete to reflect the full grade concrete are feasible and how to improve. This requires us to look for an economical、safe and feasible reduced scale simulation methods in the lab and build relationships between concrete specimens with large size and scale model, which may reflect the real strength of the specimen be in the large size by a scale model.The paper systematically studied the different specimen sizes, different intensity levels and different slag content of the wet sieve two- graded and three- graded concrete and its size effect on mechanical properties, derived the formula about its size effect rate and determined the three-graded slag concrete compressive strength and splitting tensile strength size effect on critical dimensions and the critical strength by using size effect theory. The main contents are as follows:(1) Research on influence of slag addition on wet sieve two and three-graded concrete compressive strength and splitting tensile strength showed that the optimal dosage of slag should not exceed 40%, the strength of concrete with the slag addition of 30% was higher than other concrete, when the slag content greater than 40%, with the increase of slag content,the concrete strength decreased linearly.(2) The compressive strength and splitting tensile strength of wet sieve two and three graded concrete were present size effect phenomenon that intensity values of specimens in smaller size is higher than the large size specimens, and the strength classes have greater impact than slag addition. Size effect of three- graded concrete is greater than the wet sieve two-graded concrete, which the former has more significant size effect phenomenon. For the compressive strength of C15 concrete with 30% slag content, the ratios about size effect250??and300?? of the wet sieve two-graded and three-graded concrete were 87% and 91%. Splitting tensile strength of concrete size effect is more significant than the compressive strength. Forthe compressive strength of C15 three-graded concrete with 0% slag content, the ratios about size effect250? and300? of the compressive strength and splitting tensile strength were 79% and95%.(3) The compressive strength and splitting tensile strength of three-graded slag concrete specimens were both less than the wet sieve two-graded slag concrete. The compressive strength ratio ranged from 0.78 to 0.92 with an average value of 0.86; the splitting tensile strength ratio ranged from 0.58 to 0.78 with an average value of 0.66. It showed that the difference between the two with the crack size inside the concrete and the strength of aggregate and cement paste interface had a great relationship from the microscopic mechanism analysis. Since the intensity ratio between the two was relatively discrete, and therefore it cannot directly use the test values of the wet sieve concrete as the engineering design and testing basis of the three-graded concrete directly,this method has many limitations,so we suggest expanding test on graded concrete if possible.(4) Through this strength test measured values fitting with Bazant Size Effect Law and multifractal size effect formulas to obtain parameters in two calculation formulas,and then obtained the size effect rate formula of the wet sieve with two and three- graded concrete in different intensity levels, different slag content of compressive strength, splitting tensile strength by using the parameters. From fitting case of size effect rate formula, the two size effect rate formula mentioned can better predict the compressive strength, splitting tensile strength size effect of the wet sieve two and three- graded concrete.But the strength fitting case of cubic concrete specimens which side length was 100 mm is poor, it cannot use the strength of cubic concrete specimens in that size to predict mechanical properties of the wet sieve two-graded concrete; and the fitting is good when cubic concrete with 200 mm long side,it can use the strength of cubic concrete specimens in that size to predict strength of the three-graded concrete.(5) By Bazant size effect rate and multi-dimension fractal effect rate calculation formula,different intensity levels and different dosage of slag three-graded concrete specimens’ critical dimensionscrD and the critical strengthcrf were obtained, which could be equipped with reference for mechanical performance experimental study and engineering application of large slag addition’ three-graded concrete.