The need for major maintenance or reconstruction of a pavement is described in 5 4 in engineering and operational terms. In addition a runway, or other Operational pavement, may become unsatisfactory because it is below strength for aircraft which are operating or about to operate. If strengthening is delayed, the maintenance problem can increase quite rapidly. Further delay will cause a pavement to deteriorate to a point where economical strengthening is impossible. It is, therefore, important that the Engineer should decide, as early as possible, on any major strengthening measures.48. Fortunately a rigid (concrete surfaced) pavement, regularly inspected, will give early warning of trouble in the form of corner and edge cracks of greater than casual occurrence. It is less fortunate, however, that arrangements for large-scale surfacing take some time, whether it be the preparation of a contract, stockpiling of materials or purely financial difficulties. In the event of severe overloading the interim deterioration may be quite rapid.49. Asphalt surfacing of 100 mm or more on an undamaged rigid pavement, that is one with few loading cracks or areas weakened by fatigue, strengthens the structure considerably. The superimposed layer spreads the load, producing less pressure on the pavement, and, more important, moving the centroids of critical wheel loads further from edges and corners of slabs. In addition, in the UK, the daily variations in temperature at the top of the concrete slab will be almost negligible, so that temperature stresses are greatly reduced and the slabs wiIl probably exhibit no measurable warping either as upward curling in late summer and early autumn or arching earlier in the year. The same applies, with seasonal variations. in most parts of the world.50. Whether a rigid pavement has been resurfaced with asphalt to reduce maintenance, to improve operational performance or for strengthening, there is a problem in evaluation of the strength of the resulting composite construction. If the concrete construction is in good condition and a reasonably cautious evaluation is made of the concrete strength, the problem is fairly easy to solve, for surfacing thicknesses of 100 mm to 150 mm, provided the surface is unbroken or exhibits no more than hairline reflective cracks. Thicker layers of asphalt and underlying layers which are more or less damaged, as well as open reflective cracks (e.g. above expansion joints), make the problem much more difficult. Surface load tests, short of loading nearly to destruction over large areas, will lead to no reliable conclusion. These diffculties have been recognized by some authorities on pavements, who have from time to time given somewhat arbitrary evaluations of the various materials in multi-layer pavements for comparison with their own conceptions of the design and evaluation of flexible construction. Eventually, however, the evaluation will depend on the judgement of the Engineer who will need,