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 (𝒜₂₈)’ 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 (Formula presented.) 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.