The Variation of Compressive Strength and Elastic Modulus of Fly Ash High Strength Concrete
Supervisor:wang yuan feng
High-strength concrete has been widely used in high-rise buildings,bridges,harbor marine engineering,underground engineering and other civil engineering fields.The high-strength concrete with certain admixtures(especially fly ash)is a kind of multi-phase composite material with large dispersion.If the mechanical properties and variability of high-strength concrete are not fully understood,it may lead to the quality accident of the new material structure or component.Moreover,the change of mechanical properties of concrete is the main factor that affects the bearing capacity of structure or component with time.Therefore,in order to improve the safety and reliability of structure or component design,it is of great theoretical and practical significance to study the variability of mechanical properties of high-strength concrete and its variation with time in design stage.In this paper,the variability of the mechanical properties of high-strength concrete in the following aspects are explored and studied:(1)The difference of the standard value and variation coefficient of concrete compressive strength in different codes is analyzed.It is found that the variation coefficient of high strength concrete(C60-C80)in Chinese code(GB50010-2010)is 0.10,which is not consistent with the actual project.Therefore,the test strength of C80 high-strength concrete is investigated statistically,and the statistical parameters of the uncertainty of compressive strength of C80 high-strength concrete are obtained by mathematical statistics.The normal distribution model is used to test the distribution of compressive strength of C80 high strength concrete.The standard deviation of compressive strength of C80 high strength concrete is 11.09 MPa and the variation coefficient is 0.12.(2)The prism compressive strength of C80 fly ash high strength concrete at different ages under standard curing condition is studied.The results show that the prism compressive strength mean,standard deviation and variation coefficient of C80 fly ash high strength concrete vary with time.Finally,considering the difference of prism compressive strength and cubic compressive strength variation coefficient of high strength concrete,the difference of the standard value of prism compressive strength with different fly ash content at different age is analyzed and calculated.Considering the difference between the coefficient of variation of the prism compressive strength and cubic compressive strength,the standard value of concrete prism compressive strength is less than the presumed value of the code.The design value of prism compressive strength of high strength and high performance concrete should be determined by test in structural calculation and analysis.(3)The time-varying law of the strength of C80 fly ash high strength concrete under standard curing condition are studied.It is found that the fly ash content has great influence on the mean,variance and variation coefficient of the cubic compressive strength of high strength concrete.Comparing and analyzing the predicted value and experimental value of the compressive strength of concrete in different specifications,the results show that the predicted formula of concrete compressive strength in current specifications is no longer applicable to the prediction of compressive strength of C80 fly ash high strength concrete.Based on the CPM model and considering the contribution of fly ash to compressive strength,the compressive strength calculation model of fly ash high strength concrete is established.Through the analysis of influencing factors,it is found that the maximum particle size of aggregate gradation has a great influence on compressive strength,and the influence of aggregate packing density on compressive strength calculation results is not obvious.(4)Because the aggregate distribution characteristics have great influence on strength,a random aggregate model of high strength concrete is established.Then,based on the random aggregate model,the maximum paste thickness is analyzed by using the program of matlab,and the maximum paste thickness obeys the normal distribution.The chord length density theory is used to verify the range of maximum paste thickness,and the result shows that the formula for calculating maximum paste thickness based on random aggregate model is reasonable.Finally,a computational model for compressive strength of fly ash high strength concrete considering the randomness of maximum paste thickness is established and compared with the experimental results,the results show that the prediction effect of the model is well.(5)The time-varying regularity of elastic modulus of C80 fly ash high strength concrete under standard curing condition is studied.The results show that fly ash content has a great influence on the change of mean,variance and coefficient of variation of elastic modulus of high-strength concrete with time.By comparing the predicted values of the formulas of concrete elastic modulus with the experimental values in different specifications,the results show that the predicted formulas of concrete elastic modulus in the specifications are no longer applicable to the prediction of the elastic modulus of C80 fly ash high strength concrete.A formula for calculating the elastic modulus of C80 fly ash high strength concrete based on CPM is proposed.Then the influence of aggregate elastic modulus,hardened cement paste elastic modulus and aggregate packing density on concrete elastic modulus is analyzed.(6)Based on Monte Carlo simulation,the stochastic process expressions of compressive strength and elastic modulus of fly ash high strength concrete were established,which can analyze the mean value and probability distribution of compressive strength and elastic modulus of fly ash high strength concrete at any time.The MIC value of influence factors at different ages and their changing regularity with time were studied,which provide a reference for further understanding the reaction mechanism of compressive strength and elastic modulus of high strength concrete and improving the calculation formula of compressive strength and elastic modulus of high strength concrete.