4.1 This test method may be used as a substitute for, or in conjunction with, coring to determine the thickness of slabs, pavements, decks, walls, or other plate structures. There is a certain level of systematic error in the calculated thickness due to the discrete nature of the digital records that are used. The absolute systematic error depends on the plate thickness, the sampling interval, and the sampling period.
4.2 Because the wave speed can vary from point-to-point in the structure due to differences in concrete age or batch-to-batch variability, the wave speed is measured (Procedure A) at each point where a thickness determination (Procedure B) is required.
4.3 This test method is a pplicable to plate-like structures with lateral dimensions at least six times the thickness. These minimum lateral dimensions are necessary to prevent other modes3 of vibration from interfering with the identification of the thickness mode frequency in the amplitude spectrum. As explained in Note 12, the minimum lateral dimensions and acceptable sampling period are related.
4.4 The maximum and minimum thickness that can be measured is limited by the details of the testing apparatus (transducer response characteristics and the specific impactor). The limits shall be specified by manufacturer of the apparatus, and the apparatus shall not be used beyond these limits. If test equipment is assembled by the user, thickness limitations shall be established and documented.
4.5 This test method is not applicable to plate structures with overlays, such as a concrete bridge deck with an asphalt or portland cement concrete overlay. The method is based on the assumption that the concrete plate has the same P-wave speed throughout its depth.
4.6 Procedure A is performed on concrete that is air dry as high surface moisture content may affect the results.
4.7 Procedure B is applicable to a concrete plate resting on a subgrade of soil, gravel, permeable asphalt concrete, or lean portland cement concrete provided there is sufficient difference in acoustic impedance3 between the concrete and subgrade or there are enough air voids at the interface to produce measurable reflections. If these conditions are not satisfied, the waveform will be of low amplitude and the amplitude spectrum will not include a dominant peak at the frequency corresponding to the thickness (Eq 2). If the interface between the concrete and subgrade is rough, the amplitude spectrum will have a rounded peak instead of a sharp peak associated with a flat surface.
4.8 The procedures described are not influenced by traffic noise or low frequency structural vibrations set up by normal movement of traffic across a structure.
4.9 The procedures are not applicable in the presence of mechanical noise created by equipment impacting (jack hammers, sounding wi......