The field of proof-loading has expanded over the last decade with excellent examples of successful collaborations and multidisciplinary approaches. Advanced testing has, as one of the essential subjects in an interdisciplinary assessment, brought significant value to further understanding of structural responses and failure mechanisms related to concrete bridges. This paper presents laboratory and field collapse testing examples in the Netherlands and Denmark, and related investigations of the response of non-shear reinforced slabs until failure. A special focus is dedicated to load application, examples of test approaches, some practical insights and test result comparison. Considerations of the laboratory and field test planning in synergy will be discussed based on the results and experiences obtained. A substantial margin to ultimate failure was found from crack identification or other measurable warnings in all tests. These observations suggests that it is possible to get sufficient warning even for non-shear reinforced concrete slabs structures.

Safe proof loading of concrete bridges requires reliable stop criteria. Such criteria must ensure sufficient margin to the ultimate resistance and may be based on observations from advanced testing. In particular, the identification of thresholds related to pre-stressed and non-shearreinforced slab structures is currently ongoing with an addition of specific focus on potentially brittle failure modes of such structures. This paper presents representative examples of identifying stop criteria and target load thresholds using a combination of laboratory- and in-situ testing. Responses from recently tested structures and structural elements will be presented to enable discussion and perspectivation. The presented test results show measurable warning with sufficient margin from crack initiation to stop criterion. It was additionally seen that the target load often may be the governing threshold when proof load testing.

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.

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. 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-dimensional digital image correlation (2D-DIC) and acoustic emission (AE). 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.