Identification of Stop Criteria for Large-Scale Laboratory Slab Tests Using Digital Image Correlation and Acoustic Emission

Christian Overgaard Christensen,Eva Olivia Leontien Lantsoght,Per Goltermann,Jacob Wittrup Schmidt,Gabriela Zarate Garnica,Fengqiao Zhang

doi:10.3390/infrastructures7030036

Abstract

Advanced monitoring methods are required to identify stop criteria in proof-load tests. In this study, the combined methodology of two-dimensional digital image correlation and acoustic emission is investigated for its applicability for future implementation in field tests. The two monitoring systems are deemed to provide valuable insight with external measurements from digital image correlation and internal measurements from acoustic emission. Two overturned T-section reinforced concrete slabs (0.37 × 1.7 × 8.4 m) tested under laboratory conditions are used for the assessment. The first slab test served as a preliminary test to enable sensor placement and creation of a relevant loading protocol. The main scientific results lead to a proposal for a test procedure using the combined methodology based on results, observations, and experiences from an individual stop criteria assessment for the two methods. The results include full-field plots, an investigation of the time of crack detection and monitoring of crack widths with digital image correlation, and a qualitative assessment of activity vs. load followed by a quantitative evaluation of calm ratios using acoustic emission. The individual results show that both digital image correlation and acoustic emission can identify damage occurrence earlier than other secondary methods. At crack detection (415 kN), crack widths were measured at widths between 0.078 mm to 0.125 mm and can be monitored until reaching the stop criterion at 463 kN (Eurocode SLS threshold of wmax = 0.2 mm). The acoustic emission results were limited by the pre-defined loading protocol and thus, only indicated that damage occurred sometime between 300 kN and 500 kN (pre-defined load levels). Therefore, the proposal for test procedure involves a methodology, where the loading protocol may be updated during testing based on monitoring results and thus provide even more valuable data.

Highlights

Load-carrying capacity assessment of existing concrete slab bridges through load testing is a topic of growing interest among bridge owners
Following Overturned T-section (OT) test 1 as a preliminary test, OT test 2 was performed according to the designed loading protocol for evaluation of the combined use of digital image correlation (DIC) and acoustic emission (AE)
Cracking was detected through DIC at 415 kN, and the chosen crack width stop criterion was met at 468 kN; the results show damage before the 500 kN load peak

Summary

Introduction

Load-carrying capacity assessment of existing concrete slab bridges through load testing is a topic of growing interest among bridge owners. Many existing concrete slab bridges were built in the 1960s and 1970s, and these are not designed for the actual traffic loads of today. Several studies involving capacity evaluations have been conducted during the last decades to evaluate the actual in-situ capacity compared to theoretical predictions [1–4]. Many of these studies have shown that a capacity reserve may be present for a high number of bridge structures [5–9]. Based on the gained insight, it seems that load testing may be a good option for more accurate evaluations [10]. Three types of in-situ bridge load testing are typically used to evaluate the structural response; (1) Diagnostic load testing, (2) Proof-load testing, and (3) Failure load testing [11], see [12] for flowcharts

Results

Discussion

Conclusion