Concrete dual-track roads

Fully paved farm tracks which look like roads are coming in for a~ increasing amount of public eriticism, for they do not fit the ima~e of originally unpaved tracks consisting of two parallel wheel paths with a grassed strip in the middle. Trackways built of concrete do resemble the old unpaved tracks. The deciding reasons for choosing this form of construction in Southern Germany were not only aesthetic and ecological, but also the existence of old conc~ète trackways which, after more than 20 years of carrying traffic, are still in perfectly good condition.Thus, since 1984, a satisfactory synthesis between the essenti~l traffic function and the claims of suiting the structure to the landscape has been accomplished here. The standard cross-section comprises two unreinforced concrete strips, each 80 em wide and 14 cm thick, a 90-em-wide central strip and 25 - 50-em-wide edge strips with the necessary cross-fall; a non-bound sub-base extends across the full width of the crosssection. In general, only transverse contraction joints spaced 2.0 to 2.5 m apart are provided; expansion joints have been largely eliminated. For trackways built of precast concrete paving blocks, the preferred type are 8-cm-thick interlocking blocks forming wheel paths of 2 x IlO em width, for example, plus a central strip, 80 cm in width, and edge strips. For in-situ construction, the concrete is required to be of grade B 25, generally made with cement of grade 35, a favourable aggregate grading with 32 mm maximum particle size, a water content suited to the consistency for slip-forming and an air-entraining agent. In-situ concrete trackways are advantageously constructed with slipform pavers. The contraction joints can suitably be formed by vibration of a groove which is then provided with a filler strip. Less commonly, the joints are sawn. The construction work is completed by broom-finishing the surface and spraying a curing compound. Concrete vehicular trackways which are relatively heavi ly trafficked may be paved wi th concrete blocks, perforated "grass slabs", concreting the central strip with superplasticized concrete; or laying concrete slabs extending aeross the full width. Trials with methods of forming contraction J01nts without filler strips and trials with entirely jointless construction are in progress with a view to establishing ways and means of further rationalization.