There are a number of nondestructive test methods to evaluate the sub-surface structure of floors. Ground penetrating radar, using cart-mounted multi-frequency equipment can combine high resolution data from near the surface with penetration to greater depths. Thicknesses may be measured to an accuracy of 10%, or better. Sub-surface information on items such as tensioning tendons, and reinforcement or joint details may be obtained. Whilst at greater depths, the location of services, foundations and other structures may be determined. The technique is not suitable for steel fibre-reinforced concrete, and under these circumstances low-frequency ultrasonics may be used to obtain information to depths of approximately 500 mm. Greater penetration may be achieved by impact echo testing, in which the surface is tapped using a stainless steel ball. Load transfer at joints may be determined using the falling weight deflectometer test, in which a controlled load is dropped on the surface and deflections measured at different points at either side of the joint.