| Concrete in its service process is usually subject to a combination of various environmental factors,such as dissolution,freeze-thaw cycles,etc.In order to improve the durability of concrete materials against freeze-thaw damage,domestic and foreign scholars have conducted various research,such as the use of Feeding,early strength agent,antifreeze,air entraining agent,fiber and other measures.Internal curing is the use of pre-wet water-absorbing inner curing material instead of aggregate as concrete raw material.It is important to provide internal moisture compensation to concrete to relieve self-drying,reduce self-shrinkage,enhance concrete compactness and improve structural durability.In this paper,through indoor test,theoretical analysis and other means,100-250?m particle size super absorbent resin(SAP)and light fine aggregate(LWA)are selected as internal curing materials,and the amount of different internal curing materials and the amount of water introduced are set for rapid freezing.In the experimental study of melting and salt freezing,the mechanism of freeze-thaw damage of internal curing concrete was analyzed by observing macroscopic mass loss,SEM micro-morphology structure and phase composition,and the optimal frost-resistance scheme of internal curing concrete was proposed.The mechanism of durability damage degradation of high-strength concrete in salt-free environment is designed to provide important theoretical basis and application value for the comprehensive performance design of high-strength concrete in service under special circumstances.The specific research is as follows:(1)It is found through experiments that the incorporation of internal curing materials will reduce the compressive strength of high-strength concrete,but the reduction is small,mainly due to the ettringite AFt and AFm caused by the hydration caused by internal curing water.The compactness is weakened,the strength caused by the formation of large pores after SAP water release is reduced;the floating phenomenon occurs during the vibrating process of light and fine aggregates,and the water-cement ratio and surface concrete properties of the surface of the test block are changed,which has a great influence on the strength of concrete.However,the strength of the internal curing concrete still meets the requirements of the normative design;(2)Through the rapid freeze-thaw test of clear water and 3% NaCl salt solution,the mass loss rate and relative dynamic elastic modulus of macroscopic freeze resistance index are obtained.For the convenience of analysis,the test block containing the maintenance material in SAP is marked as HSC+ number.The figures are the amount of water introduced,the test block containing the LWA internal curing material is marked as LWA+ number,and the reference concrete with the matching ratio is labeled as HSC.The analysis is as follows:In 150 freeze-thaw cycles in a frozen environment,the SAP content is 0.24%,and the pre-wet water volume is 10%.The HSC10 concrete has the lowest mass loss rate of only 0.2%;the mass loss rate is from small to large in order of HSC10> HSC0>LWA5>LWA10> HSC5>HSC>LWA0,it can be seen that the internal curing technology can improve the frost resistance of concrete,and the internal curing material SAP improves the frost resistance better than LWA.After 25,50,75,100 and 150 cycles of freezing and thawing,the relative dynamic elastic modulus of the test block is known.Compared with the benchmark concrete HSC,the relative dynamic elastic modulus of the concrete of LWA0 group and HSC10 group is larger,and the frost resistance is better than the reference concrete.The dynamic modulus of the other test blocks decreased slightly,but the decrease was not large.In general,the addition of LWA is more conducive to the experimental group with SAP,which has a more positive impact on improving the frost resistance of high-strength concrete.The mass loss rate of the test piece fast frozen by 3% NaCl solution is about 50% higher than that of the standard freeze-thaw test.The surface peeling is serious,but the relative dynamic elastic modulus is slightly lower than that in water.slow.(3)After salt freezing(unilateral freezing and thawing),the mass loss rate decreases with the increase of the amount of water introduced.HSC10-<HSC10< HSC5-< HSC5< HSC0<HSC,the main reason is: concrete surface caused by water freezing Slurry flaking is caused by a decrease in mass.The relative dynamic elastic modulus is expressed as HSC10->HSC5->HSC10>HSC5>HSC0>HSC.The inside of the concrete produces tiny and uniform bubbles,which are not easily filled with water and the water saturation is significantly reduced.At the same time,timely absorption of the cold water in the frozen area,relieve the freezing pressure generated by the salt solution,can improve the salt-freezing performance of the concrete;chloride ions react with the hydration product to form hydrated calcium chloroaluminate,which can relieve the erosion and damage..Electron microscopy analysis was carried out to observe the microscopic morphology and image composition of SAP pores,aggregates and cement paste joints and micro-cracks.According to the growth morphology of ettringite,it can be proved that the SAP water release forms pores.The air forms a vacuum due to the inaccessibility,and there is a pressure difference between the inside and the outside to form a "micro-pump",which provides a basis for the "micro-pump" hypothesis.It provides a theoretical basis for the reduction of strength caused by the addition of SAP,the reduction of self-shrinkage and the improvement of antifreeze performance.(4)According to the experimental research results,the optimal internal curing material dosage scheme was selected.It is considered that the HSC10 test group(0.24% SAP content and 25 times internal SAP water quality)has the best freeze resistance and self-shrinkage effect.It provides a theoretical basis for the comprehensive performance design and engineering utilization of internal curing high-strength concrete. |
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