Study Of Properties And Mix Design Of Wet-mix Shotcrete With High Air Content

Wet-mix shotcrete is paid full attention for the great advantages in improving theenvironment on construction site, focusing on batching, controlling qualities and developpinghigh performance shotcrete. However, wet-mix shotcrete faces an inharmonious contradictionbetween pumpability and shootability and the mix design of it is ruleless. Bad pumpability,less build-up thickness, larger rebounds, higher amount of liquid accelerating agent and otherdrawbacks greatly limit the application of wet-mix shotcrete in infrastructure construction.Consequently, the workability, harden properties and mix design of common wet-mixshotcrete and wet-mix shotcrete with high air content are thoroughly studied in this paper. Themain research work of this paper is summarized as follows:(1) The test methods of the build-up thickness and rebound ratio are optimized. Both themethods that the consistence of fresh wet-mix shotcrete is characterized by penetration andthe cohesion of fresh wet-mix shotcrete is tested by direct shearing apparatus are put forward.(2) The slump is no less than of50mm and the relative pressure bleeding ratio ofconcrete is under40%before pumped are set as evaluating indicator of pumpability ofcommon wet-mix shotcrete. The effects of component materials on the pumpability arestudied. The results show that the stability of fresh common wet-mix shotcrete is easy to besatisfied and the controlling indicator of pumpability is fluidity.(3) For the wet-mix shotcrete with high air content, the methods of optimizing the type ofair entraining agent is put followed and the influence law of component materials on thepumpability is revealed. The results show that when0.02%AES air entraining agent matcheswith0.5%polycarboxylate superplasticizer, the air content of fresh concrete arrives at about18%and the fluidity and stability are perfect. The air content reduces to6%even below aftershotted and the build-up thickness is more than170mm without accelerator.It proves that thewet-mix shotcrete with high air content can solve the inharmonious contradiction betweenpumpability and shootability.(4) With the help of direct shear testing method and XRD patterns, the influence factorsof cohesion are analyzed by orthogonal testing method and the primary and secondary factorsand racking results affecting cohesion of fresh concrete are founded. Based on the transverse bending theory of material mechanics, the mechanical model of build-up thickness is set up.The stress characteristics are analysed and the relationship between the build-up thickness andthe cohesion is stablished. The effectiveness of the mechanical model of build-up thickness isverified.(5) The variations of rebounds with air flow, sand ratio, fluidity of concrete and shottedthickness are achieved. Based on the rebound criterion established by Armelin, themechanism of rebound is revealed. It shows that the wind pressure of work has an ideal value.The rebound ratio shows an increasing tendency with the reduction of fluidity of freshconcrete and an decreasing tendency with the increase of shotted thickness, while it tends tobe stable when the shotted thickness is up to50mm. The rebound ratio of wet-mix shotcretewith high air content is5%less than the common wet-mix shotcrete and the fly ash canimprove the shotability of wet-mix shotcrete.(6) The cubic compressive strengths and the frost resistance of shotcrete are contrastedbetween the common shotcrete and the wet-mix shotcrete with high air content. The hydratesof shotcretes are analysed with the help of scanning electron microscopy (SEM). It shows thatthe cubic strength on the28th day of wet-mix shotcrete with high air content is up to34.4MPa which satisfies for the strength grade of C25and the frost resistance degree of concretepasses F300.(7) According to the Calculation method and Simple Test method, the slurry that fillingthe granule-void is comprehensively determined. Based on the relationship between thediameter of the pumping hose and relative amount of paste for the lubricating layer which isset by Dennis, the lubricating layer is figured out. And then, the minimum slurry for pumpingis got by the granule-void and lubricating layer. At last, on the basis of the minimum slurry,the mix design method of wet-mix shotcrete with high air content is proposed, which isapplied in inclined shaft and front slope of a tunnel. Contrasted the economic cost of1m3shotcrete between common wet-mix shotcrete and wet-mix shotcrete with high air content, thelatter has obvious advantages.