Study On Under Anchor And Longitudinal Distribution Testing Technique For Prestressing Of PC Beams

The diseases of prestressed concrete girder bridges in China is relatively common.Several of studies have shown that in the process of bridge construction,the lack of prestress tension is the main cause of beam cracking and deflection.Therefore,the detection of prestress is particularly important.At present,the lift-off method is the most promising test technology for effective prestress testing under anchor during construction.In addition to the test under anchor,the prestressed longitudinal tensile force distribution feature is a technical difficulty in the engineering field,and there are very few related studies.Currently,only optical fiber Bragg grating(OFBG)sensors are capable of data acquisition for multiple points,but it is expensive.Therefore,it is of great theoretical significance and engineering application value to evaluate the longitudinal tensile force distribution of the prestressed steel strand considering the friction effect combined with the effective prestressing under anchor of the lift-off test.Based on the Shaanxi Transportation Science and Technology Project "lift-off test criterion and effective prestress distribution detection technology(17-17K)",this paper through accurately identifies and corrects of effective prestressing results under anchor,combined with the indoor research of OFBG-smart steel strand,the anchorage and longitudinal distribution techniques of PC beam prestressing are studied.The main research work and results are as follows:(1)Research:The research status of prestressed concrete girder bridge detection technology at home and abroad was expounded,and the development of the lift-off and OFBG detection technology was briefly summarized.(2)Criteria:The research on the detection technology of the lift-off method was conducted,the detection mechanism of lift-off method was expounded,and the typical test curve of the lift-off method was obtained.The indoor and outdoor tests were designed.Combined with the finite element analysis and analytical solution derivation,the bite force between the taper hole and the clip was studied.The bite force test of the inner and outer taper holes of the porous anchor was carried out,and the difference in bite force between the taper holes of the anchor is obtained.The results show that the lift-off test does not change the effective prestress of the steel strand under anchor.By Comparing bite force and the internal force transient redistribution of the exposed and internal section of prestressed steel strand when the clip was voided,it can be concluded that the clip bite force is the sum of the redistribution of the internal forces outside and under anchor.The outer taper hole bite force of the porous anchor is 1.125 times of the inner.Based on the above research,the formula for calculating the bite force of the whole bundle of steel strands is proposed,and the effective prestressing criterion of the steel strand under anchor is obtained.(3)Correction method:Long-term real-time dynamic monitoring based on on-site prefabricated beams was carried out,and the law of effective prestressing under anchor was obtained.The results show that the prestressing force changes periodically with the temperature besides the overall attenuation trend after the strand is tensioned.The formula for calculating the effect of temperature on the effective prestress under anchor and the formula for calculating the time correction of the effective prestress under anchor within 48 hours are obtained,which can be used to correct the temperature and time of the field lift-off test results of actual bridge engineering.(4)Prestress longitudinal distribution:The OFBG-smart steel strand water bath cycle tester has been designed and processed.The test device has the functions of temperature change and steel strand tensioning The force and temperature sensing characteristics of OFBG-smart strand were studied.Based on the prefabricated box girder on the engineering site,the prestress and temperature data of the pressure sensor measuring point under anchor and the smart steel strand are analyzed.considering the influence of anti-friction friction,the longitudinal distribution characteristics of prestress are studied.The results show that the test results of OFBG-smart steel strand and vibrating wire pressure sensor are in agreement,which verifies the force performance of fiber grating technology.The temperature sensing coefficient of OFBG-smart steel strand is 0.0226nm/?.The external temperature difference of the beam reaches 25.3 ?,and in the beam is only 7.9 ?,which is 3.2 times.The temperature inside the box girder is about 4 hours behind the external temperature.Therefore,when performing temperature correction,the at:mospheric temperature after four hours should be divided by 3.2 as the current temperature of the fiber measuring point.The prestress of the three measuring points of the OFBG intelligent steel strand inside the actual bridge was tested.The prestresse measured values of the three measuring points are compared with the calculated values of the"Specifications for Design of Highway Reinforced Concrete and Prestressed Concrete Bridges and Culverts"and"Code for Design on Reinforced Concrete and Prestressed Concrete Structure of Railway Bridge and Culvert" The comparison results show that the average error of the calculated and the railway code measured values is 2.2%,the average error of the calculated and the railway code measured values is 6.2%.The steel strand prestressed longitudinal distribution test data is more consistent with railway specifications.