Research On The Mechanical Performances And Strengthening Application Of Sprayed UHTCC

Ultra High Toughness Cementitious Composite(UHTCC)is a kind of short cut fiber reinforced cementitious composite characterized by psudo-strain hardening property and multiple fine crack performance.UHTCC has drawn a lot of interests of scientists and engineers due to its great capacity of deformation,tight crack control and durability,and it has significant potential of application in repair or retrofitting of concrete structures.The technologies of situ construction for conventional UHTCC are mainly involving pouring and plastering ones,which could not meet the trend and request of large-area and mechanized construction.Study on the UHTCC suitable for spraying and improving its mechanical performance is still in a great need and necessary.On the supports of the National Natural Science Foundation of China(No.51578495)and Zhejiang Communications Department Science and Technology Project(No.2014H13),the investigation on the approach to improve the mechanical performance of sprayed UHTCC and the application in the repair and strengthen of concrete structures were carried out,and the main content and conclusion of this paper consist of:1.A comparison of physical and chemical properties among several kinds of available fly ash and an investigation on the mechanical performances of sprayed UHTCC involving these kinds of fly ash were conducted,and a kind of fly ash with low water demand ratio,small loss on ignition and high content of glazed glass beads,which is well fitting the matrix of sprayed UHTCC,was elected among them.Then a uniform experiment design method was introduced to optimize the mix proportion and mechanical performance of sprayed UHTCC based on the elected fly ash.The influence of content of sand and air entraining agent on the mechanical performance of sprayed UHTCC was also studied.Efforts were made to explain the mechanism of the optimization of the mix proportion and mechanical performance of sprayed UHTCC with SEM technology.Significant improvements on mechanical performance of sprayed UHTCC were achieved by the optimization of the mix proportion.2.Tensile and flexural tests were carried out on the thin plates of carbon textile or steel wire mesh reinforced sprayed UHTCC,and the results show a significant advantage in the cooperative work ability between carbon textile or steel wire mesh and sprayed UHTCC,which significantly contributed to the far better mechanical performance of carbon textile or steel wire mesh reinforced sprayed UHTCC when compared with carbon textile or steel wire mesh reinforced mortars.A tensile strength more than 4MPa with an ultimate tensile strain about 4.5%,and a flexural strength more than 20MPa with ultimate mid span deflection larger than 25mm was achieved for steel wire mesh reinforced sprayed UHTCC.And A tensile strength more than 7MPa with a ultimate tensile strain about 3.3%,and a flexural strength more than 33MPa with ultimate mid span deflection larger than 15mm was achieved forcarbon textile reinforced sprayed UHTCC.3.Orthogonal experiments were performed to study the influence of the direction of spraying,the surface roughness and the use of interfacial mortar on the bonding performance between sprayed UHTCC and the concrete substrates,and SEM technology was introduced to investigate the bonding mechanism between them.The results revealed that the spraying direction has significant influence both on the split-bond strength and interfacial shear-bond strength of the sprayed UHTCC-concrete composite cubic specimens,and the interfacial shear-bond strength was also significantly influenced by the surface roughness of concrete substrates,while the split-bond strength and interfacial shear-bond strength between sprayed UHTCC and concrete substrates was rather slightly but actively affected by the use of interfacial mortar.4.Experimental study were carried out to investigate the flexural performance of plain concrete beams(notched plain concrete beams were included)and RC beams(notched RC beams with reinforcement ratio reduced were included)strengthened by thin-layer sprayed UHTCC.The results demonstrated that sprayed UHTCC has significant potential to improve the flexural load carrying capacity and flexural stiffness of concrete beams,meanwhile sprayed UHTCC improved the crack control ability of concrete beams.Sprayed UHTCC gained greater strengthen efficiency for notched plain concrete beams or notched RC beams than the ones in good condition,respectively.5.Mechanical performance of tunnel lining model strengthened by sprayed UHTCC was investigated by the means of model experiments.Model specimens of tunnel lining without strengthen technique,strengthened by sprayed UHTCC,strengthened by carbon fiber sheet and strengthened by the composite of sprayed UHTCC and carbon fiber sheet were tested respectively for contraction.The results revealed that all the strengthen technique could improve the radial deformation rigidities.And the crack initiation load,yielding load and ultimate load of sprayed UHTCC strengthened model specimen were improved by 13.6%,68.9%and 29.9%respectively,which shows a favorable strengthen effect.6.Theoretical analysis was conducted to study the flexural performance of RC beams and the load carrying capacity of tunnel lining model specimen strengthened by sprayed UHTCC,and the calculated results shows a good agreement with the experimental results,by which the strengthening mechanism of the sprayed UHTCC layer to concrete flexural members and arch structures can be clearly interpretated.