T-shaped arrangement of geophones for rapid quantification of asymmetric behaviour of concrete slabs in central FWD tests

ABSTRACT

The assessment of asymmetric slab behaviour is out of reach in standard Falling Weight Deflectometer (FWD) tests, because deflections are measured along the driving direction only. Herein, a new T-shaped arrangement of the geophones is proposed. It allows for rapid quantification of asymmetric slab behaviour in central FWD testing of concrete slabs. One geophone is positioned at the centre of impact (= centre of the slab), six along the driving direction, one right and one left of the centre. The ‘Lateral Asymmetry Index (LASIX)’ is introduced as a corresponding dimensionless deflection basin parameter. Its value increases with increasing asymmetric behaviour of the slab. The main research challenge tackled herein is to optimise the radial distance of the lateral geophones from the centre of the slab, such as to maximise the expressiveness of LASIX for the quantification of asymmetric slab behaviour. In this context, FWD tests with measurement of deflections in eight different directions are carried out, and the ‘effective asymmetry index (${\mathcal{A}}_{28}$)’ is introduced as another new dimensionless deflection basin parameter. It summarises the asymmetric behaviour based on deflection differences quantified for all 28 pairs of directions which can be combined out of the eight available measurement directions. The optimal radial distance of the lateral geophones from the centre of the slab is found as 1.20 m. Corresponding values of LASIX larger than $8\text{\%}$ refer to coefficients of directional variation of the AREA7 parameter larger than 5%. This indicates directional degradation of the pavement structure resulting from eccentric traffic loads. T-shaped FWD testing requires in situ efforts equivalent to those of standard testing, while allowing for a rapid and reliable quantification of asymmetric behaviour. It allows for the assessment of whether the standard evaluation of uniform moduli of subgrade reaction is realistic or questionable.

KEYWORDS:

• Concrete slabs
• falling weight deflectometer
• FWD
• multi-directional testing
• T-shaped testing
• deflection basin parameters
• asymmetric slab behaviour

Acknowledgments

The help of Pia Mandahus (TU Wien, Vienna, Austria), Marek Milcevic, Roman Oblak and Harald Aigner (Nievelt Labor GmbH, Höbersdorf, Austria) as well as interesting discussions with Wolfgang Kluger-Eigl (TU Wien), Martin Peyerl and Gerald Maier (Smart Minerals GmbH, Vienna, Austria) as well as Reinhard Lohmann-Pichler and Karl Gragger (ASFINAG Bau Management GmbH) are gratefully acknowledged.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This research received financial support by the Austrian Research Promotion Agency (FFG), the Austrian Ministry for Transport and Technology (bmvit), ÖBB-Infrastruktur AG (Vienna, Austria), and ASFINAG Bau Management GmbH (Vienna, Austria), within VIF-project 2015 ‘STRUKTurelle Zustandserhebung und -bewertung von Betondecke (CONcrete) auf Projektebene - ConSTRUKT’ and the Bridge Project 2021 ‘Grundlegende Analyse von FWD-Versuchen: innovative Experimente, moderne Struktursimulationen, statistische Datenanalyse – FALLINGweight’. The authors also acknowledge the TU Wien University Library for financial support through its Open Access Funding Programme.