Research On Regression Coefficients In Bowromi Formula In Hengyang

Concrete structure is one of the world's most widely used structure form,while the volume of coarse and fine aggregate accounted for about 70%~80%of total volume of concrete.A large number of experimental results show that the relationship between the 28d compressive strength of concrete and the actual strength of the cementitious materials and cement/water ratio conform to the Bowromi formula under the definite raw materials,that is the compressive strength of concrete and cement/water ratio show a linear relationship.The regression coefficient is about the production regions of aggregate and cementitious materials,therefore,THE DESIGN REGULATIONS OF MIX RATIO OF ORDINARY CONCRETE(JGJ55-2011)suggest using test to determine the regression coefficient according to the raw materials,using recommended value only when there is no statistical information of test.Actually,most areas in China adapt the value recommended by JGJ55-2011 for concrete that in its proportioning design.However,the raw materials of concrete differ greatly due to China's vast territory,especially the widely used of admixture,machine-made sand and admixture.Thus,the boundary conditions of Bowromi formula differ greatly.Also the regression coefficients have significant deviation with the value recommended by JGJ 55-2011.The research used fly ash,powdered coal gangue,waste glass powder and mineral powder as admixture together with the combined aggregate,gravel and broken pebble most frequently used in Henyang area as coarseaggregate and the sand rooted in broken pebble as fine aggregate.The regression coefficients in Bowromi formula was obtained by using the weighted least squares.The research also measured the geometric features of aggregate combined with the image measurement technology and the distribution characteristic of interstice of different admixtures under different water/cement ratio analyzed by mercury intrusion method to analysis the influence of different aggregate and admixtures to the regression coefficients in Bowromi formula.The results show that the strength is still show a linear relationship while the water/cement ratio is within 0.25-0.65.The compressive strength of concrete exists a phenomenon of different variance under different water/cement ratio.As a result,the regression coefficients in Bowromi formula was obtained by using the weighted least squares.The weight being the reciprocal of the square of the average value of compressive strength of concrete or the water/cement the square of ratio has a good elimination for heteroscedasticity.The regression coefficients in Bowromi formula are influenced by the types of coarse aggregate and admixtures,thus different types of coarse aggregate and combination of admixtures have great difference on the regression coefficients in Bowromi formula..The regression coefficients?_a in Bowromi formula decrease with the increase of age.The confidence interval is significantly greater than the middle of compressive strength of concrete under the same confidence level.The deviation of preparation is large for preparation of concrete with the low intensity(<20MPa)and the high intensity(60MPa).Thus it do not fit with the regression coefficients.