It is impossible to place speed restrictions on a runway. It is impossible to detour a landing aircraft in the middle of the runway. Expensive new airplanes have less tolerance for pavement roughness than highway vehicles, and they impose heavier loads on the pavements. The high speeds of aircraft on runways result in the need for higher skid resistance to maintain directional control and provide for safe stopping. These are some of the reasons why there must be tighter control of materials and construction procedures for airport runway paving than in road construction. Runway smoothness is becoming a much more critical requirement for safe, efficient airline operations. Rough runways add to the stresses imposed on already heavily loaded landing gear. Additional stresses to aircraft parts due to rough runways are believed by some to result in the early retirement of many modern airplanes. Problems such as porpoising and skipping induced by undulating runway surfaces add to the control difficulties encountered by pilots during critical phases of the takeoff and landing operations. Rough runways also slow a plane's acceleration on take off. The construction of smooth pavements starts with the subgrade. The FAA requires that the cutting of the finished grade of the natural subgrade or the subbase on which concrete pavement is to be placed shall be controlled automatically by taut lines erected and maintained by the contractor. Special surface preparation is required for complete bonding of a concrete overlay to an existing concrete slab. The existing surface must be thoroughly cleaned and etched with acid before bonding grout can be applied. Improved directional control and stopping capability of runway pavements are important goals of the aviation industry. Runway grooving is currently the best method of decreasing hydroplaning, poor directional control, and unsafe breaking conditions. However, grooving is not a substitute for the use of high quality materials and good workmanship in pavement construction.