Crack Analysis Of Precast Box Girder In Hoisting Stage Based On Acoustic Emission Technology

With the development of national economy and society,China’s high-speed railway construction is developing continuously.Precast box girder is widely used in high-speed railway due to its advantages of fast construction schedule,but the micro-cracks in the hoisting process of precast box girder have a negative impact on its safety and long-term durability.The hoisting crack is difficult to be found because of its special location and some of the cracks close by themselves after hoisting.Therefore,this paper makes a systematic study on the crack monitoring problem in the hoisting process of precast box girder based on acoustic emission technology.The main work and conclusions are as follows:(1)The cracking of precast box girder before and after hoisting was investigated.The cracking position and cracking degree were analyzed statistically.The main cracking positions and cracking rules of precast box girder during hoisting were found.(2)A finite element calculation model for the hoisting process of precast box girder was established.The stress state of box girder under different working conditions was analyzed numerically.The stress and damage distribution near the hoisting hole under different working conditions were studied in order to predict in advance which areas were prone to cracks during hoisting.(3)The layout scheme of acoustic emission sensor was determined based on the finite element calculation results.The damage development of precast box girder during hoisting stage were monitored based on acoustic emission.AE and strain signals generated by beam damage during the test were recorded in detail.According to the experimental phenomena,the cracking process of the member was divided into four stages,and the strain development of the concrete was corresponded with the acoustic emission signals of the four stages in real time.(4)The characteristic parameters of AE signal were analyzed.The qualitative and quantitative relationship between AE parameters and damage stages of box girder was established.The acoustic emission signal source was located in space.Compared with the cracks actually produced in the test,the positioning results were found to be accurate.(5)The time domain waveform of AE signal was programmed by fast Fourier transform and wavelet packet transform.The amplitude-frequency characteristics of the signal and the energy distribution of the frequency band were obtained by analyzing the transformation results.(6)The correlation dimension method was used to judge whether the AE time series had chaotic characteristics.The analysis results showed that the AE signals in the damage process of large reinforced concrete structures had chaotic characteristics,and the change trend of correlation dimension had a clear correlation with the stages of the damage process.