Research On The Frost Resistance Of Recycled Concrete

With the rapid urbanisation in China,about 2 billion t of construction waste is produced each year,of which waste concrete,masonry and other materials account for 30%-40%.These materials are regenerated to produce recycled aggregates,and the process produces fine particles with a particle size of less than 75μm,called recycled micronized powder,which accounts for 15%-20%.The application of recycled aggregates and micronized powder is the key to achieving zero construction waste emissions.Most studies have been carried out around the macroscopic mechanical properties of recycled concrete,with less research on the longterm durability of recycled concrete,and the microscopic mechanism is not yet clear.This paper firstly characterises the material properties of recycled micronized powder and recycled aggregates(recycled coarse aggregate and recycled fine aggregate);secondly,the workability,mechanical properties and resistance to chloride ion penetration of recycled concrete are studied by preparing recycled concrete with water-cement ratio of 0.56 and 0.41,and its frost resistance is studied by a combination of fast-freezing and slow-freezing methods,focusing on the effects of single and multiple blending of recycled micronized powder,substitution rate of recycled aggregates,water-cement ratio and other factors on the performance of recycled concrete;then the effect of recycled concrete is studied.The effect of single and multiple blending of recycled micronized powder,the replacement rate of recycled aggregate and water-cement ratio on the performance of recycled concrete was investigated;then the typical mix ratio in recycled concrete was improved by adding antifreeze waterreducing agent,compounding air-entraining agent and compounding antifreeze agent,and the improvement effect of the three methods was compared and analysed;finally,microscopic analysis was carried out using scanning electron microscopy,ultra-deep field microscopy and mercury compression.Research results show that:(1)recycled micronized powder has large internal surface area,internal micropore pore size is concentrated in 2-5nm,rough surface,large water absorption in concrete;recycled micronized powder has larger particle size,lower potential activity,microaggregate effect and activity effect after mixing into concrete;with the increase of recycled micronized powder,concrete workability,mechanical properties,resistance to chloride ion permeation performance decreases,but within 20% of mixing amount,concrete compressive strength,resistance to chloride ion permeation performance increases.Concrete compressive strength ratio of not less than 0.7,and fly ash or mineral powder compounded into concrete after the compressive strength,resistance to chloride ion permeation performance,mineral powder: recycled micronized powder = 2:1 when the best;compounded with mineral powder of recycled micronized concrete frost resistance performance is generally better.(2)The surface of recycled aggregate is rough and angular,and the water absorption rate is high,with the increase of recycled aggregate substitution rate,the amount of concrete water reducing agent increases;with the increase of recycled coarse aggregate substitution rate,the compressive strength,frost resistance and chloride ion penetration resistance of concrete increases first and then decreases,and the performance is better when the substitution rate is50% and 75%;with the increase of recycled fine aggregate substitution rate,the compressive strength of concrete does not fluctuate greatly,and the overall performance is Frost resistance and chloride ion permeation resistance are reduced;in the frost resistance test,the results of the fast freezing method and the slow freezing method are similar,but the slow freezing method has a small mass loss rate and the surface of the specimen is less damaged;the water-cement ratio has a greater influence on the frost resistance.(3)All three methods can play a role in improving the frost resistance of recycled concrete.After mixing with antifreeze water reducing agent,the compressive strength and chloride ion penetration resistance of recycled concrete are slightly reduced;after compounding with different amounts of antifreeze agent,the early compressive strength of concrete is increased,but the long-term strength of recycled aggregate concrete is lower than that of natural concrete,and the antifreeze performance is improved significantly when the antifreeze agent is mixed with 1%,but the chloride ion penetration resistance is reduced The compressive strength of concrete decreases after compounding with air-entraining agent,and the anti-freezing performance and anti-chlorine ion permeation performance are better at 5% air content.(4)Microscopic analysis by scanning electron microscope,ultra-deep field microscope and mercury compression,after improvement,the total porosity,total pore volume,most available pore size,average pore size and critical pore size of recycled concrete decreased,the proportion of harmless pores and less harmful pores increased,and the number of harmful pores decreased;the number of hydration products increased,the interface transition zone was tightly connected,and the overall structure was more dense.