Shear Wave Reflection Method for Slab-Base Interfaces

The contact between concrete slabs and supporting base layers influences the mechanical and durability performance of pavement systems. This paper presents a new nondestructive evaluation (NDE) methodology for quantifying slab-support contact conditions using shear wave reflections. While current stress-wave NDE methods, such as impact-echo and falling weight deflection, infer the interfacial contact condition given knowledge of layer properties and thickness, the proposed method quantifies the interface contact condition without these initial inputs. The shear wave reflection ratio ( SWR z ) is defined as the amplitude ratio between successive shear wave echoes reflected from a slab-base interface. To isolate the interface contact from other losses, the signal amplitudes are corrected for geometric spreading and material attenuation. Laboratory experiments were conducted using a phased-array shear wave system on 76-mm-thick plexiglass and 76- and 100-mm-thick mortar slabs, supported by several materials (e.g., air, marbles, sand, and cement) and under multiple vertical pressure levels. The results demonstrated that the SWR Z values showed statistical significance ( p < 0.01) between support material type and increasing applied vertical pressure to the slab surface. As expected, the slab-air interface specimens exhibited the highest SWR z of approximately 1.0. Additionally, increasing the vertical pressure at the slab-sand interface reduced the SWR z values, indicating improved slab-base contact. The proposed methodology provides a reliable, NDE tool for detecting changes in slab-base contact conditions (i.e., kissing bond to complete separations or air gaps), which can be effective in evaluating concrete slab-base and concrete overlay contact conditions over time.