Acoustic emission techniques and cyclic load testing for integrity evaluation of self-compacting normal and self-compacting lightweight prestressed concrete girders.

摘要

In recent years structural evaluation of existing infrastructure has become a critical subject in civil engineering. Unfortunately the existing load testing methodologies for integrity assessment of civil structures such as the 24 hour load test method (24h LT) and the cyclic load test method (CLT) have been recently questioned about not providing an accurate diagnosis of the deterioration in the system, and also inducing new damage during the testing procedure.;In light of these circumstances significant efforts have been placed on developing nondestructive techniques such as acoustic emission monitoring (AE) that can effectively assess the integrity of a structure without causing unnecessary deterioration. However, AE methods still face several challenges regarding the subjectivity of their criteria and the lack of quantifiable parameters, which can be directly related to the mechanical response of the system.;Some authors have stated that an integrated approach of the CLT with AE techniques will overcome these difficulties and constitute an effective and true nondestructive evaluation methodology. At this time, various attempts to combine both approaches into a single method have shown promising results yet most of the drawbacks fore mentioned still remain unsolved.;This dissertation analyzes experimental data gathered from the four point load testing of six full scale prestressed girder specimens, the first set consists of two self-compacting concrete beams and one high early strength normal concrete girder, while the second set contains two self-compacting lightweight concrete girder specimens and one high early strength lightweight concrete girder. The specimens were subjected to the CLT and AE monitoring, in order to develop an alternative integrated approach for the structural evaluation of prestressed concrete girders.;A novel and practical methodology that accounts for the specific mechanical features of a prestressed concrete girder (elastic stiffness, cracking moment, fully cracked inertia, and ultimate capacity) and allows the objective delimitation of its damage levels is presented. Results from this procedure show excellent correlation when compared to the experimentally defined damage thresholds.;Experimental damage levels were identified by careful observation of the nonlinear behavior of the specimens during the CLT procedure and up to failure. Next, a detailed finite element model for the SCLC girders were developed and calibrated with the experimental data in order to further study the main phenomena (cracking, yielding, concrete crushing, etc) within each damage zone, as well as to evaluate the effect of changes in the prestressing force over these mechanisms. Also, the global integrity parameter is proposed as a new criterion for damage diagnosis and performance evaluation of prestressed concrete girders within the intermediate and heavy damage zones, showing excellent correlation with the experimental information obtained during testing.;Results from the application of current AE criteria to the specimens are presented along with important recommendations, regarding the monitoring and processing of the AE data, towards a successful standardization of AE monitoring for load testing of prestressed concrete girders.;Finally, the arch of damage is proposed as a new AE evaluation parameter from the CR vs. LR plots, and an alternative version of the GIP based on this parameter is developed. This evaluation criterion is shown to provide superior sensitivity for damage detection at significantly lower levels of load than all of the evaluation criteria presented here.