Stop Criteria for Flexure for Proof Load Testing of Reinforced Concrete Structures

Abstract

Existing bridges with large uncertainties can be assessed with a proof load test. In a proof load test, a load representative of the factored live load is applied to the bridge at the critical position. If the bridge can carry this load without distress, the proof load test shows experimentally that the bridge fulfills the requirements of the code. Because large loads are applied during proof load tests, the structure or element that is tested needs to be carefully monitored during the test. The monitored structural responses are interpreted in terms of stop criteria. Existing stop criteria for flexure in reinforced concrete can be extended with theoretical considerations. These proposed stop criteria are then verified with experimental results: reinforced concrete beams failing in flexure and tested in the laboratory, a collapse test on an existing reinforced concrete slab bridge that reached flexural distress, and the pilot proof load tests that were carried out in the Netherlands and in which no distress was observed. The tests in which failure was obtained are used to evaluate the margin of safety provided by the proposed stop criteria. The available pilot proof load tests are analyzed to see if the proposed stop criteria are not overly conservative. The result of this comparison is that the stop criteria are never exceeded. Therefore, the proposed stop criteria can be used for proof load tests for the failure mode of bending moment in reinforced concrete structures.

Digital Image Correlation for Evaluation of Cracks in Reinforced Concrete Bridge Slabs

“…In proof-loadings, a load safety factor will be applied and it can therefore be argued that a larger crack width may be acceptable. Hence, it can also be argued that the values described by [30] (w max ≤ 0.11-0.19 mm) might be overly conservative. This open discussion seems to present an opportunity for "crack detection" to be a more governing criterion and perhaps that is the way to proceed.…”

“…Of the mentioned guidelines, the German guidelines offer stop criteria for crack widths in buildings, w max ≤ 0.5 mm for new cracks and w max ≤ 0.3 mm for existing cracks. A more recent theoretically based criteria is proposed by [30]. In the presented research, the calculated values for crack widths vary from w max ≤ 0.11 mm to w max ≤ 0.19 mm.…”

“…Research has recently been carried out in regards to gathering stop criteria from different national guidelines as well as combining these in a proposal for flexural stop criteria for concrete bridges [30]. Earlier work has also involved stop criteria related to shear failure mechanisms, but this is still under further development [31][32][33][34][35][36] due to the potentially brittle nature of such failure mechanisms.…”

“…A stop criterion should show sufficient margin of safety, but should also not be overly conservative [30]. It therefore seems important to investigate what level of precision is needed for a stop criterion to work as intended, but also what level of reliability it is possible to obtain in an in situ environment.…”

“…This means that the cracks were detected in time and can be monitored towards the stop criteria. However, had the chosen criteria been in the range described in [30] (w max ≤ 0.11-0.19 mm), there would not have been a timely warning. These results were achieved at a camera distance of 3.8 m with one single camera covering the entire slab surface.…”