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Quantification of digital image correlation applicability related to in-situ proof load testing of bridges

I’ve been working closely together with my colleagues at the Danish University of Technology for quite a while, and am cosupervisor of Christian Christensen. The first paper of this collaboration and the PhD research of Christian was presented at SMAR 2019, the 5th international Conference on Smart Monitoring, Assessment and Rehabilitation of Civil Structures. The title of the paper is: “Quantification of digital image correlation applicability related to in-situ proof load testing of bridges”.

The abstract of the paper is:
Advanced crack monitoring is crucial for high precision response- and threshold evaluation when performing proof- and diagnostic load tests on existing concrete structures. Mostly, crack monitoring techniques involve one monitoring method, which provide thresholds with regard to stop criteria and characterization information. Such thresholds and related precision uncertainties are expected to be of significant importance in identifying stop criteria as well as deliver input for probabilistic models. In the ongoing Danish bridge testing research program, it is hypothesized that several independent monitoring techniques are needed to reduce uncertainties related to crack detection and categorization. A number of novel monitoring methods are used in the research project. A special focus is however dedicated to two promising crack-monitoring techniques suited for combined use: a) Two-dimensional digital image correlation (2D-DIC) and b) Acoustic emission (AE). The output is expected to provide a unique crack evaluation, for which limitations and uncertainties of the techniques should be quantified individually as well as in combination. This paper presents initial research concerning evaluations related to digital image correlation based on sub-component beam tests performed in the DTU CasMat laboratory facility. The tested beams were prefabricated as TT-elements with a length of 6.4 m and cut into two T-beam elements. The test matrix consisted of ten beams strengthened with carbon fiber reinforced polymer (CFRP) in different configurations with and without post-tensioning of the CFRP, thus resulting in different crack initiation behavior. The investigations in this paper include: (1) time of crack detection compared to visual detection, (2) time of crack detection compared to time of crack width threshold values, and (3) crack width evaluation using 2D-DIC strain correction for out-of-plane deflection. The results show that cracks can be detected prior to both visual detection and significant stiffness change. After detection, crack development can be monitored for crack width stop criteria. Crack widths can also be successfully monitored for surfaces subjected to out-of-plane movement using a geometric correction method. The methodology is hypothesized to be of significant importance in future testing of full-scale concrete slab bridges in the Danish bridge testing project.

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