Research On Corrugated Tube Method For Testing Early-age Autogenous Shrinkage Deformation Of Concrete

With the development of concrete materials towards high strength and high performance,the water binder ratio(the mass ratio of water to cementitious materials)of fresh concrete becomes lower and lower,which leads to more severe shrinkage deformation of concrete materials in the hardening process,and causes cracking risk of concrete structure.The service life of concrete structure will be reduced when cracks appear in concrete.To promote the research progress of improving the early volume stability of concrete materials,it is necessary to test and evaluate the early shrinkage deformation of concrete scientifically and accurately.At present,the methods of measuring the early shrinkage deformation of concrete mainly include volume method and length method,both of which have their own advantages and disadvantages.As a new method with the advantages of both volume method and length method,corrugated tube method(CTM)has been widely recognized and applied.However,the CTM in the existing specifications is limited to testing cement paste and mortar and is not suitable for concrete containing coarse aggregate or steel fiber,so it is necessary to develop CTM for testing early shrinkage deformation of concrete.In this paper,through experiments and comparative analysis,the parameters,influencing factors and applicability of CTM for concrete are studied,and the results and conclusions could be drawn as follows:(1)The of CTM equipment is mainly composed of cylindrical plastic corrugated tube,sealing end plug,test support frame and data acquisition instrument.The inner diameter of the corrugated tube is 60 mm,the outer diameter is 80 mm,and the total length is 60 mm.After the concrete was filled to the corrugated tube,the end plug was used for sealing and placed on the test support frame coated with lubricating oil.The ambient temperature was controlled to 23 ± 2 ℃ by the gas temperature control method.One end of the corrugated tube specimen was fixed on the test support frame by the magnet adsorption method,and the other end was contacted by the digital dial indicator to realize automatic data acquisition.The deformation data were recorded immediately after the installation.During the test,no external load or moving operation was applied to the specimen.After the test,the initial setting time of concrete was taken as the starting time of autogenous shrinkage deformation.(2)The fresh state of concrete sample had an important influence on the difficulty of filling operation of the corrugated tube specimen.During the forming process,it needs to vibrate and tamp in layers to discharge the bubbles inside the fresh concrete as much as possible.The length,diameter and stiffness of plastic corrugated tube all had an impact on the test results of CTM.Too high stiffness would lead to the inconsistency between the test process and the test principle,and too low stiffness would increase the forming difficulty and friction constraint.When the filling degree of corrugated tube specimen was insufficient,the test result would be reduced;when the specimen was placed vertically,the shrinkage test result would be larger than that of the specimen placed horizontally,and the shrinkage deformation obtained by bidirectional test method was larger than that by unidirectional test method.Compared with the noncontact method,the test result of CTM is slightly larger.(3)In order to verify the applicability of CTM,ordinary concrete and ultra-high performance concrete(UHPC)with different mix proportions were prepared.It was found that the CTM can effectively distinguish the early-age autogenous shrinkage deformation of ordinary concrete with different water-to-cement ratio,cement type,maximum particle size of coarse aggregate and UHPC concrete with different water-tobinder-ratio,mineral admixture type,steel fiber content and expansive agent content,which had a wide application prospect in the universal system of concrete.(4)CTM has a clear test principle,the friction constraint was low in the test process,the testing process could be started immediately after the specimen was formed,and multiple concrete specimens could be tested automatically at the same time.It is suitable for testing the concrete(water-to-binder ratio was lower than 0.35)with large early shrinkage deformation.