Application And Experimental Study On Bitumen-Based Grouting Technology For Post-Tensioned Prestressed Structures

In post-tensioned systems, the grout is typically the last line of defense against contaminants attack of the post-tensioning strands. Cementitious grout injected into Post-Tensioning tendons has the important dual role of providing transfer of the prestressing force to the structural member, as well as providing a passive environment around the post-tensioned bars or strands. The prerequisites are complete filling of the void space within duct. Bleed water of grout makes this more difficult. The corrosion of voided tendons has been a long-standing issue in Post-Tensioned systems because it can lead to the failure of tendons and deteriorate bridge performance.Two measures of managing grout bleed are put forward in paper. The emphasis is on grouting technology using bitumen-based replacing cementitious grout. Based on detailed investigation of the material mixture ratio, groutability, the influence rule of the prestressing loss and the bond properties between prestressing steel and bitumen-based materials, the application on bitumen-based grouting technology for post-tensioned prestressed structures is demonstrated experimentally. The main research contents and research achievements are summarized as follows:(1) Grouting test results of current grouting technology and materials show that void space appears within all prestressing duct tested. The development and countermeasures of bleed water were discussed. To improve the bleed performance of aperture, a kind of combined rubber rod is proposed to form the compound cross-section prestressed aperture.The aperture presents a unique approach to addressing bleed water and its mechanism against the hazard of bleed water was studied. A full-scale grouting comparative test is conducted based on the prototype of China high-speed railway 32m simply-supported box girder. According to the test results, the characteristics of the generation, transmission and distribution of bleed water in aperture are investigated and the relationship between bend-up height of aperture and grouting compactness percentage is analyzed. Test results show that the introduction of compound cross-section aperture can separate the voids induced by the bleed water from prestressing steel to protect the tendon from corrosion. For the horizontal and bend-up aperture in post-tensioned structures, the compound cross-section prestressed apertures have much better bleed performance than the circular apertures.(2) Design target and experimental scheme of bitumen-based grouting material are proposed. Rock bitumen is used as modified bitumen for matrix bitumen and its adulteration proportion is 12.5% according to the preliminary experiment. Furthermore, according to the viscosity experiment, viscosity-temperature variation rule and shear strength test for the different mixture ratio, a suitable mixture ratio of bitumen-based grouting material that is:the matrix bitumen+rock bitumen (12.5%)+(slaked lime: cement=1:9) (40%). The shear strength is 697.21 kPa at room temperature and its grouting temperature should be higher than 115℃. With increasing slaked lime content and using fiber, a improved mixture ratio of bitumen-based grouting material is obtained: matrix bitumen+rock bitumen (12.5%)+(slaked lime:cement= 3:7) (40%)+fiber (1%). The shear strength of the improved bitumen grouting material is about 2 times of the normal and its grouting temperature should be higher than 140℃.(3) A calculation formula is deduced for convective heat transfer coefficient between bitumen-based material and prestressing duct. The finite element model of grouting test beam is established. Based on the simulation of grouting process, we obtain the key parameters of grouting technology as follows:the suitable grouting flow is 501/min, and the reasonable grouting temperature is 160℃ for inlet grout and 60℃ for the wall of duct, as well as the grouting pressure is 0.8 MPa. The correctness of the grouting process parameters and advancement of grouting effect are demonstrated by grouting test of concrete beam and external prestressing duct.(4) The duct-forming material of prestressing duct is better to choose HDPE material through the calculation of concrete beam’s temperature field and stress field, and grouting time is the most important parameter to influence them. Both maximum and minimum tensile stress less than the standard specified value under various conditions given in paper. Compared with the measured data of concrete beam’s temperature field and stress field, the calculation error of temperature field is less than 10%, and the calculation error of stress field is less than 15%. The calculation results given in this paper are accurate and effective. Equations on applicability of bitumen-based grouting material is established, with which the secure grouting time can be obtained for different strength grade concrete under different grouting temperature condition, and then it provides the basis for grouting construction of post-tensioned prestressed structure using bitumened-based material.(5) The experimental results show that the mechanical properties of the prestressing strand can reach 0.9 times of the original index value at grouting temperature and 0.965 times after grouting temperature, while the maximum stress relaxation can reach 6.36% at grouting temperature and 1.71% after grouting temperature. Furthermore parameter models are established which can describe the mechanical properties degradation and stress relaxation of the prestressing strand. Short-term and long-term prestressing loss models are estabilshed and calibrated due to the maximum calculation error is 14.1% comparing with the measured data. According to the calculation model, the long-term prestressing loss is 2.73% under the most unfavorable conditions for post-tensioned prestressed structure grouted with bitumened-based material(6) Formulas for bond properties between prestressing steel and bitumen-based materials are deduced based on the viscoelasticity constitutive relation. The stress distribution characteristics for prestressing strands, strands/matrix, and matrix of bitumen-based materials can be obtained by these formulas. The influence of factors including load, strands/matrix radius ratio, temperature and loading time on the bond properties are discussed. The formulas are calibrated by use of the current theory. The calculation results show that the post-tensioned prestressed structure grouted with bitumen-based materials is designed as bonded prestressed structure under short-term load effect as well as unbonded prestressed structure under long-term load effect.