Experimental Study On Mechanical Properties And Frost Resistance Of High Performance Pumping Concrete With After-mixing Coarse Aggregate

At present,the process of urbanization is still accelerating in our country.More than 1.7billion cubic meters concrete have been used each year,of which more than 50% was consumed by the construction.works.A number of key projects are under construction and being planned,which require good durability of concrete to ensure their safety and longevity.The high-performance concrete was proposed in the 1990 s based on the durability requirements of concrete.Its construction mostly utilizes the pumping method.High-performance concrete should have many advantages,However,in order to ensure the pumping requirements,high performance is mistaken for high liquidity by many commercial concrete premixing stations.Therefore,the amount of coarse aggregate is usually controlled at a lowe level in the mix design.This not only has a negative impact on the service performance of hardened concrete,but also the excessive amount of slurry makes the early cracking phenomenon very common,which brings great hidden dangers to the durability of concrete.Although fly ash and other industrial wastes have been used in the production of high-performance concrete and alleviate the pollution to a certain extent,it has not fundamentally solved the problems of much pollution and energy consumption caused by cement production due to the fact that the amount of cement has not been reduced.At the same time,the higher cost has also limited its popularization and application.In order to solve the above problems,the research group has put forward a new construction technology which is suitable for construction works,can save resources and costs and reduce the environmental burden by applying the scientific thought of holism,drawing on the advantages of batch feeding mixing technology and considering the importance of coarse aggregate components—— after-mixing coarse aggregate technology,that is to say,add a certain volume fraction of coarse aggregate to the concrete which has been pumped to specified working face before pouring it into the mold and get a new type of green high-performance concrete after the secondary spiral mixing evenly—— after-mixing coarse aggregate concrete.This paper mainly studies the effect of after-mixing coarse aggregate proportion(10%,15%,20%,25%)on the working performance,mechanical properties,long-term performance and frost resistance of C30 high-performance pumping concrete(the reference concrete)which is the most commonly used in construction engineering and comparesafter-mixing coarse aggregate concrete with traditional mixing concrete which has the same coarse aggregate volume fraction.The results show that the working performance of after-mixing coarse aggregate concrete decreases with the increase of after-mixing proportion and the slump loss is faster than traditional mixing concrete,but it still can meet the requirement of construction pouring;The mechanical properties of after-mixing coarse aggregate concrete have different degrees of improvement compared with the reference concrete and are all higher than traditional mixing concrete;The autogenous shrinkage and early cracking resistance of after-mixing coarse aggregate concrete are weaker than traditional mixing concrete,but are significantly stronger than the reference concrete;And its improvement on the frost resistance of the reference concrete is also very significant.However,it is not the greater of after-mixing coarse aggregate proportion,the performance better.There is an optimal amount of admixture.According to the analysis,the best after-mixing coarse aggregate proportion of this C30 high-performance pumping concrete is20%.In addition,the results also confirm the difference in the formation process between after-mixing coarse aggregate concrete and traditional mixing concrete.Through the analysis of its improvement mechanism,it is concluded that after-mixing coarse aggregate technology realizes coarse aggregate interlocking effect,forms ITZ water-cement ratio gradient,thus increases the density and finally manifests as the cement reduction and macro-performance improvement of high-performance pumping concrete by increasing the volume fraction of coarse aggregate based on the advantage of batch feeding mixing technology.Through the quantitative analysis of economic and environmental benefits,it is found that after-mixing coarse aggregate technology is fully in line with the greenization and sustainable development requirements of building materials.More important,the invention of the relevant construction equipment has also laid the foundation for its broad engineering application prospect in the future.