Abstract
A benchmark is carried out in order to compare how 13 experts define and can carry out an NDT investigation program and derive strength values from NDT measurements. The benchmark is based on simulations, which reproduces a synthetic data set corresponding to a grid of twenty 3m-high columns defining the structure of a building made up of beams and columns. The experts must assess the mean and the standard deviation of compressive strength. Three levels of assessment are considered corresponding to different quantities of test results (destructive or non destructive) available for the experts. The comparison of the various strategies used by the experts and the analysis of results enables the identification of the most influential parameters that define an investigation approach and influence its efficiency and accuracy. A special emphasis is placed on the magnitude of the measurement error. A model of the investigation strategy is also proposed.
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Notes
- 1.
The interested reader will find relevant information in other chapters, and we have chosen not to repeat this information here, for the sake of clarity.
- 2.
The three “levels of investigation” can be compared with the three “knowledge levels” defined by Eurocode-8. They were a first version of the Estimation Quality Level (EQL) concept as it is now defined in the TC 249-ISC RILEM Recommendations (see Sect. 1.1 and Table 1.1). However, the EQL concept is attached to the assessment results whereas the “levels of investigation” are attached to the resources that enable to reach the objective.
- 3.
This limitation is not inherent to synthetic simulations and the same limitation exists with real on-site studies. If two strategies had to be compared, because the effect of chance, they would have to be repeated several times in similar situations and their statistical fluctuations would have to be analyzed (for instance, as an indicator of robustness of the methodology).
- 4.
This fact is not inherent to synthetic simulations and can be found identically with real on-site studies. It has been identified as a main source of uncertainty at the conversion model identification stage, as it induces the trade-off between model parameters (Sect. 12.4).
- 5.
If one considers real situations, the choice of test precision is usually not explicit. However, it is implicit if one considers that a better precision comes, for instance, to follow standards or guidelines, or to work with expert practitioners … In both cases, a precision increase induces a higher cost and a smaller number of tests. In all cases (real structure or synthetic benchmark), the precision is negatively correlated with the test result uncertainty.
- 6.
In real practice, many other factors play a role, like the prior information already available or collected through visual inspection, time constraints, technical constraints… Our purpose here is not to model the process in its overall complexity.
- 7.
This method is very specific and it cannot be described into details here. The reader is invited to refer to [4] for more details.
- 8.
It should be noted that current structural codes (e.g. Eurocode 8 - 3 on the seismic assessment of existing structures) prescribes that in-situ concrete strength must be estimated by means of core tests, possibly complemented by ND tests.
- 9.
Some approaches may have a high potential but not enough robustness (i.e. sensitivity to some random fluctuations). It is therefore dangerous to conclude from a unique comparison (between all experts on a unique case-study), whereas a relevant comparison must be based on a large number of possible situations.
References
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Breysse, D. et al. (2021). How Investigators Can Assess Concrete Strength with On-site Non-destructive Tests and Lessons to Draw from a Benchmark . In: Breysse, D., Balayssac, JP. (eds) Non-Destructive In Situ Strength Assessment of Concrete. RILEM State-of-the-Art Reports, vol 32. Springer, Cham. https://doi.org/10.1007/978-3-030-64900-5_6
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