Research On The Effect Of Compensating Shrinkage On Crack Resistance Of Hydraulic Panel Concrete

Water conservancy and hydropower progects are one of the important measures to solve the goal of carbon peak and carbon neutral proposed in the 14 th Five-Year Plan of China.Among them,face rockfill dam is the mainstream dam type,and its demand and development will also usher in a period of rapid development.The concrete slab,as the impermeable structure of the face rockfill dam,belongs to the typical thin long strip structure,which is easy to generate cracks in the construction stage or the initial operation stage.The existence of these cracks not only leads to the seepage of concrete slab,but also significantly accelerates the migration of harmful ions to the concrete or the dissolution of calcium ions in the concrete under the erosion of soft water,which aggravates the deterioration of concrete slab and reduces the service life of concrete face rockfill dam.Therefore,it is more and more important and urgent to study the antishrinkage cracking measures of face concrete.In this paper,based on the theory and method of compensating shrinkage and reducing shrinkage and cracking,and combined with its hydration characteristics,pore structure and microstructure of AFt,the effects of different influencing factors on shrinkage compensation effect and restraint cracking resistance of hydraulic panel concrete are systematically studied,with the aim to provide technical support and lay a theoretical foundation to achieve hydraulic concrete crack resistance performance.The main research contents of this paper are as follows:(1)Based on the hydration characteristics,pore structure and AFt microstructure of two kinds of ettringite-type expansive agents,the effects of two kinds of expansive agents on autogenous shrinkage,drying shrinkage and restrained ring shrinkage cracking of matrix were analyzed.Experimental results showed that the microstructure and distribution of AFt significantly affected the shrinkage cracking properties of the matrix.Among them,the long and thin needle ettringite would overlap each other to form a threedimensional network structure,which provided constraints to the early expansion effect and formed the preloading stress at the same time.This generated preloading stress could compensate the drying shrinkage and reduce the risk of cracking.While the short columnar ettringite,mainly presented a one-dimensional directional distribution,which was conducive to the expression of early expansion effect generated by AFt,but not conducive to drying shrinkage and restrained ring shrinkage cracking.(2)Based on the optimized ettringite-type expansive agent,the influence of different dosages and curing mechanisms on the shrinkage compensation effect of the matrix was studied.The results showed that the shrinkage compensation effect of the matrix increased with the increase of the amount of expansive agent under sealed curing and prosoaking-dry curing conditions,but it was reduced under dry curing condition.This phenomenon was mainly related to the microstructure and distribution of AFt,and the compactness of the matrix under different curing conditions.Under dry curing condition,the rapid loss of water reduced the crystallization growth rate of needle ettringite and leaded to its small crystal size.In addition,AFt was mostly distributed in the pores of hydration products,and conld not continue to form a good network structure.At the same time,the loose structure of the matrix was adverse to the expression of expansion effect generated by AFt.(3)Based on the optimized ettringite-type expansive agent,the effects of the composite ettringite-type expansion agent and polypropylene fiber on autogenous shrinkage,drying shrinkage,plastic shrinkage cracking and restrained ring shrinkage cracking of matrix were studied.The test results showed that there was a good synergistic effect on the properties of anti-shrinkage and anti-cracking of matrix.This phenomenon was mainly related to the reinforcing effect generated by PP fiber and the threedimensional network structure of ettringite distribution.This reinforcing effect could make partial expansion energy during autougenous shrinkage stage to be transform into preloading stress of the matrix.Thus,the effective shrinkage stress in the later drying stage could be reduced and the anti-shrinkage cracking ability of the matrix could be improved.(4)Based on the optimized ettringite-type expansive agent,the effect of the combination of ettringite-type expansion agent and shrinkage reducing agent on the shrinkage compensation effect of the matrix under different curing mechanism was studied.The experimental results showed that CSA and SRA have good synergistic effect on shrinkage compensation effect under seal curing and dry curing,in which SRA mainly reduced the drying shrinkage strain by reducing the surface tension of pore solution,CSA could reduce the drying shrinkage strain mainly through the dual effects of compensating shrinkage effect of ettringite and decreasing pore volume content at 5-50 nm,and the combination of CSA and SRA reduced the drying shrinkage due to the combined effect of the above three effects.SRA reduced the content of ettringite generated by CSA expansive agent at early,and made ettringite crystal more elongated in the later stage,which is conducive to the formation of a good three-dimensional network structure.This reinforcing effect could make partial expansion energy during autougenous shrinkage stage to be transform into preloading stress of the matrix.Thus,the effective shrinkage stress in the later drying stage could be reduced.(5)The effects of fly ash and slag content on the resistivity,drying shrinkage and mass loss rate of the OPC-CSA cement pastes were studied.The experimental results showed that when compared with slag,the addition of fly ash could reduce the drying shrinkage,but increase the buck electrical resistivity and mass loss rate.At the same time,it is beneficial to reflect the early expansion effect and compensate the drying shrinkage.There is a natural logarithmic function relationship between the buck electrical resistivity and the drying shrinkage,which provided a preliminary research basis for predicting the drying shrinkage with the buck electrical resistivity in hydraulic face slab concrete in the future.