Please use this identifier to cite or link to this item: http://hdl.handle.net/10266/6460
Title: Health Monitoring of RC Beam-Column Joints Using Acoustic Emission Technique
Authors: Singh, Shamsher
Supervisor: Kwatra, Naveen
Sharma, Shruti
Keywords: Acoustic Emission;Beam Column Joint;Cyclic Load;Damage Index;b-Value
Issue Date: 7-Apr-2022
Abstract: Structural Health Monitoring (SHM) is an important field to investigate the current state of structures and check the damages caused by devastating forces such as an earthquake. Various non-destructive techniques (NDT) have been employed to monitor the health of the structural system. The existing SHM techniques are capable of detecting the damage with limited precision. Apart from damage detection, quantification of damage is also very important. Load and deformation responses are essential parameters used in damage detection but these parameters are extremely difficult to measure especially in structures damaged due to seismic forces. The various techniques do suffer from certain deficiencies in estimating these parameters after the occurrence of an earthquake. To fill these gaps in research, the current study aims to identify and quantify damage in the reinforced concrete (RC) structural elements by exploring Acoustic Emission (AE) technique. The use of the AE technique is gaining popularity in monitoring various structural applications. It is a passive monitoring technique that is often used to obtain a qualitative and qualitative estimation of damage by studying the variation of AE parameters. The acoustic Emission Technique (AET) is susceptible to crack growth and can locate the source of the AE waveform initiated from the point of damage. This technique can perform real-time monitoring by detecting cracks as it occurs or grows. Despite these advantages, challenges still exist in using the AET, especially in analysing large volumes of AE data recorded during AE monitoring. Primarily this technique has three objectives. Firstly, it helps to locate the source of damage accurately. Secondly, it helps identify and differentiate signals from different sources of AE; lastly, it is capable of quantifying the extent of damage to structures. The study has given inspiring results for analysing test data, thereby opening an opportunity for its use in real-life structures. This research effort has focused on quantifying damage by developing a novel damage index using the AE technique for Beam-Column Joints (BCJ) subjected to cyclic loading with and without ductile detailing. The deformation and load responses have been studied to get energy dissipated during cyclic loading. The variation in trends of energy dissipated vis-a-vis acoustic energy has been observed, and a close relationship has been observed between these two energy parameters. A novel damage index has been developed to quantify damage in the BCJ subjected to cyclic loads based on the Acoustic Energy parameter. Further, BCJ was repaired using Engineering Cementitious Composites (ECC). This retrofitting composite has excellent behaviour under tensile and shear loading and helps improve the structures' ductility by distributing cracks uniformly. MATLAB mathematical models have been developed to find the relationship between AE energy and dissipated energy. MATLAB programmes have also been developed for the post-processing of AE data and drawing graphs to predict the damage in BCJ before and after the retrofit. The study has yielded significant results for health monitoring of BCJ designed with and without ductile detailing subjected to cyclic loads and further repaired with ECC as summarised below: From the load-deformation characteristics, the Energy Dissipation in ductile BCJ samples is about 3 to 4 times more than in the non-ductile BCJ when subjected to cyclic load. The ductility factor is a valuable parameter for checking the ductile behaviour of BCJ samples. The ductility factor of the ductile BCJ sample is about three times more than the non-ductile BCJ sample. The variation of the energy dissipation curve closely matches the AE energy curve at various stages of damage of BCJ subjected to cyclic loading. A relationship has been developed between these two parameters using MATLAB. Thus, the energy dissipation can be estimated using AE energy obtained from AE testing. The Improved b-value analysis, primarily based on Gutenberg-Richter law, helps recognise different stages of damage in the structures and evaluate the presence of macro cracks. Ib-value <1 indicates the occurrence of severe damage, and a value from 1-2 shows intermediate damage. Ib-value > 2 is an indication of minor damage. BCJ members designed for ductility formed about two times more additional AE hits corresponding to severe damage than a non-ductile sample and about three times higher hits corresponding to moderate damage. The Felicity Ratio (FR), which is defined as the ratio of load at the start of AE activities to the previous cycle's maximum load, is a powerful tool for observing damage in beam-column joints. A Felicity ratio greater than or equal to one indicates no damage. A Felicity Ratio of 0.75 to 1 is indicative of minor damage. FR lying between 0.5 and 0.75 is indicative of major damage. But FR doesn't provide a true damage state when BCJ is deformed beyond displacement corresponding to peak load. Based on AE energy, a novel damage index has been developed in this study for quantifying damage in BCJ. This damage index is a function of two non-dimensional variables, i.e. the δ_i⁄δ_u and E_i⁄E_u where δ_i is the displacement at the end of i^th load cycle; δ_u is the ultimate displacement capacity of BCJ; E_i is the total AE energy at the end of i^th load cycle; and E_u is the ultimate acoustic energy capacity of BCJ. New damage states have been proposed. A damage index (DI_AE) value up to 0.5 is an indication of minor damage, DI_AE value from 0.5 to 0.75 points towards moderate damage and DI_AE value of more than 0.75 represents major damage. The DI_AE value of more than unity shows the collapse stage of the BCJ.  The ultimate load-carrying capacity of ECC-strengthened non-ductile BCJ is higher than control BCJ samples. In comparison, strengthened ductile BCJ is less than the control sample. In both cases, the load-carrying capacity is higher than the design load-carrying capacity of BCJ. Thus, it can be concluded that ECC material can be successfully used to restore the design load capacity of the BCJ. Rise-Angle (RA) Vs Average-Frequency (AF) plots help distinguish between different types of cracks, i.e., tensile or shear cracks. The number of tensile hits in an ECC retrofitted non-ductile BCJ is about 3 times more than in a control BCJ. The number of tensile hits in an ECC-repaired ductile BCJ is about 0.3 times that of the control BCJ. From these results, it can be concluded that there is a reduction in tensile cracks in ductile BCJ samples and an increase in tensile cracks in non-ductile samples. Thus, ECC helps to improve the tensile capacity of ductile BCJ. The number of shear hits in an ECC retrofitted non-ductile BCJ up to the unity damage index was three times more than that of a controlled BCJ. The number of shear hits in an ECC retrofitted ductile BCJ up to the unity damage index was reduced by about 0.2 times that of the controlled BCJ. From these results, it can be concluded that there is a reduction in shear cracks in ductile BCJ samples and an increase in tensile cracks in non-ductile samples. Thus, ECC helps to improve the shear capacity of ductile BCJ. The total AE Energy of the ECC retrofitted non-ductile BCJ was about four times higher than the corresponding control BCJ. The total AE Energy of the ECC retrofitted ductile BCJ was about 3.2 times higher than the corresponding control beam-column joints. Thus, it can be concluded that ECC material can be successfully used to retrofit the BCJ to achieve Acoustic Energy capacity.
URI: http://hdl.handle.net/10266/6460
Appears in Collections:Doctoral Theses@CED

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