Proportion Optimization And Application Of Concrete Containing Admixtures Based On The Early-age Anti-cracking Perfermance

The applications of mineral admixtures in concrete are increasing.The studies on the early age crack resistance and the applications of the concrete with mineral admixtures are the basis and premise of the scientific and effective crack control technology.In this paper,the compressive strength,splitting tensile strength,tensile strength,tensile elastic modulus,ultimate tensile strain,quasi-adiabatic temperature rise and free shrinkage of concrete containing mineral admixtures are experimentally studied.Based on these properties,the optimum proportion was selected by the analytic hierarchy process,and the cracking resistance of the concrete with the optimum proportion was evaluated by early age plate test and the scale model experiment.The concrete with the optimized proportion was applied in the real project,which could provide the data and guidance's for the further theoretical research and engineering applications in the future.The following conclusions were drew in this study:(1)When fly ash content was 40%,the concrete exhibited the higher compressive strength,flexural strength and splitting tensile strength than that of concrete with 35% or 50% fly ash.The addition of 20% slag reduced the compressive strength,flexural strength and splitting tensile strength of fly ash concrete.(2)The early age tensile strength and tensile elastic modulus of concrete reduced when the fly ash content increased.The addition of 20% slag reduced the early age tensile strength of fly ash concrete,and improved the tensile elastic modulus of fly ash concrete.Generally,the ultimate tensile strain of the concrete containing mineral admixtures reduced firstly at very early age,and then tended to be stable.(3)The addition of fly ash reduced and postponed the temperature rise of the concrete,and the addition of slag also reduce the temperature rise of fly ash concrete.The addition of fly ash inhabited the free shrinkage of the concrete,while the slag enhanced the free shrinkage of fly ash concrete.(4)Based on the analytic hierarchy process(AHP),six cracking resistance indexes were selected,including compressive strength,tensile elastic modulus,tensile strength,ultimate tensile strain,free shrinkage strain and pseudo-adiabatic temperature rise.By comparing the comprehensive cracking resistance index,it was found that the concrete with the water-binder ratio of 0.42 and fly ash content of 40% exhibited the better cracking resistance.(5)The early-age plate test showed that the cracks appeared at 1 d,1.9 d and 2 d in the unsealed concrete specimen.The concrete specimens sealed for 1d and 3d did not crack,and a small crack occurred in the concrete specimen sealed for 2 days soon after the exposure.Generally,the concrete specimens sealed for more than 1 day did not crack severely.The monitoring results of scale experiments showed that the maximum temperature rise in the center of the sewage tank bottom model was 14.0 ?,the highest temperature was 43.4 ?,and the maximum temperature difference between the surface and the interior was 12.7 ?,which were in accordance with the requirements of < Code for Construction of Mass Concrete>;The maximum strain of steel bars in the sewage tank wall model was 116.2 ??,and the maximum strain of concrete was 137.4 ??;Harmful cracks were not found in the sewage tank bottom and wall models.(6)In practical projects,the highest temperature rise at the bottom of the sewage tank was 30 ?,the highest temperature was 46.5 ?,and the maximum temperature difference between the surface and the interior was 7.9 ?,which met the requirements of <Code for Construction of Mass Concrete>.The number of cracks in the bottom and the wall of the sewage tank was few and the widths of cracks were small,and all of the cracks were harmless cracks;the cracks at the bottom of the sewage tank were randomly distributed,and the cracks on the wall tended to be distributed near the frame columns.