How stiff the pavement structure is, layer by layer, and therefore how much traffic it can still carry. Unlike visual or surface tests it reports on the base, sub-base and subgrade as well as the surfacing. That makes it the basis for deciding whether a road needs a thin surface treatment, a structural overlay of a calculated thickness, or full reconstruction, and for estimating how many years of life remain at the current traffic loading.

A mass is dropped from a set height onto a loading plate on the road, applying a short impulse that approximates a passing wheel load. Sensors set at increasing distances from the plate record how far the surface deflects at each point, producing a deflection bowl. Because deflections close to the load are governed by the upper layers and those further out by the subgrade, the shape of the bowl encodes the stiffness of the whole structure.

The analysis that turns a measured deflection bowl into elastic moduli for each pavement layer. Layer thicknesses are supplied from cores or records, a trial set of moduli is used to compute a theoretical bowl, and the moduli are adjusted until the computed bowl matches the measured one. The resulting values feed overlay design and remaining-life calculation. It depends on knowing the layer thicknesses, so coring or radar usually accompanies deflection testing.

On earthworks, capping and unbound base layers during construction, where the question is whether a layer has been compacted adequately before the next is placed. A light weight deflectometer is portable, operated by one person and gives an immediate dynamic modulus, so it suits routine compaction control across a large area. It does not replace a falling weight deflectometer for assessing a completed pavement structure under traffic loading.

A plate bearing test applies a static load through a plate and measures settlement, giving a bearing value or modulus of subgrade reaction. It is slow, needs a substantial reaction load such as a loaded lorry, and tests one point at a time. Deflection testing applies a dynamic impulse and is far quicker, which is why it dominates network assessment. Plate bearing remains common for foundation and hardstanding acceptance where a static value is specified.

Yes, and it provides information specific to rigid pavements. As well as slab and subgrade stiffness, testing either side of a joint or crack gives load transfer efficiency, the proportion of load carried across to the adjacent slab. Poor load transfer indicates failed dowels or loss of aggregate interlock and predicts faulting and corner breaks, so it directly informs whether joints need retrofitting or slabs replacing.

Layer thicknesses, from cores or ground penetrating radar, because back-calculation cannot resolve moduli without them. Pavement temperature at the time of testing, since asphalt stiffness varies strongly with temperature and results are corrected to a standard temperature. Traffic loading data to convert structural capacity into remaining life. And an accurate location reference, so that weak sections can be tied back to the network and to any visible distress.