Investigation On The Early-age Cracking Of Artificial Sand HPC With FS/FP Composite Mineral Admixture

High performance concrete(HPC)technology made a mighty advance after more than 10 years' research and application.It is well known that the durability of HPC is greatly influenced by its shrinkage,cracks and brittleness,caused by HPC's characteristics including low water binder ratio(W/B),high binder content,low coarse aggregate content,high powder value as well as many kinds of admixtures.Especially,early-age cracking is a serious problem of HPC,because cracks not only reduce the concrete strength but also lower the protection property of concrete against environmental deleterious medium,accelerating the degradation of concrete.In this paper,low grade mineral admixtures and artificial sand concrete in Yunnan province were investigated and the influence of low grade mineral admixtures on the early-age cracking of artificial sand concrete HPC was focused.In Kunming area,artificial sand and mountain gravel have been used in concrete industry for a long time.Compared with concrete using river sand,artificial sand concrete is more easily to crack,because the latter always has a higher sand ratio,high cement content and water content,induced by the shortcomings of artificial sand,such as angular shape,inner crack developed,high pore content,big specific area as well as changeful stone powder content.This paper presented some results of a joint research project,with a title of " Key technology of Green High Performance Concrete—Develop green HPC by domestic materials".The project was executed by Yunnan Jiangong Concrete Corporation and China Academy of Building Research. Following topics were studied:1)Reasons of early-age cracking of HPC;2)Analysis of usually used test methods of evaluating early-age cracking;3)By using slab knife-edge restraint method, studied the influence of W/B,kinds of mineral admixtures and kinds of water reducer;4)Study the influence of composite mineral admixtures on the early-age cracking of HPC,including fly ash-blast furnace slag(FS)and fly ash-phosphorous slag(FP).Following results are obtained:1)Slab knife-edge restraint method,which is sensitive to reflect the influence of W/B,or the kinds of mineral admixtures or different water content,is a reasonable way to evaluate the anti-crack property of concrete;2) Composite mineral admixtures,FS or FP,can suppress cracking effectively when used in artificial sand HPC.The total crack index can be reduced more than 70%of control specimen.