Surface treatment of limestone concrete slab with the chipping technique

Since 1980 the technique of pavement construction with thick concrete slabs has been applied on 100 km of the A.26 motorway (Calais - Reims) in northern France. The concrete employed is made with regional limestone aggregates. In order to counteract lowered skidding resistance due to wear of these materials, a technique of "studding" the surface with suitable chippings, subsequently in conjunction with exposure of these stone particles in the freshly placed concrete, has been applied. The chippings are obtained from igneous rocks (diorite, porphyry), and, as finally adopted, are of 10 - 14 mm particle size. The quantity used is 7 - 8 kg per mZ of slab. The slip-forming machine for placing the concrete is equipped with a novel device for simultaneously spreading the chippings, driving them in, and finishing the slab to an even surface. This equipment for the partial exposure of the chippings additionally comprises a device for spreading a setting retarder and a curing compound. Exposure of the particles is achieved by mechanicAL brushing in conjunction with water jetting. With tbis whole set of equipment tbe surface finisbing treatment can proceed at the same rate as the pLacing of the concrete for tbe road pavement, i.e., averaging 1000 linear metres per day (for a slab width of 8 m). The results obtained in terms of evenness, especially with the technique of subsequent exposure of the ch.ipp i.ngs, fully comply with oresent;French standards, as verified by both the APL 25 and the APL 72 longitudinal profile ~alyser. The value obtained for the sideways force coefficient (European grading limits, A.I.P.C.R. smooth tyre) is 0.41 before the road is open to traffic. The roughness is good with a minimum texture depth of 1.6 mm as neasured by tbe sand patch method. Present costs are lower by about 40 7. for chippings/exposure -- about 60 7. for chippings only in comparison with the cost of a gritted bituminous surfacing applied to the concrete. Besides, with this technique of "studding" the fresh concrete with chippings it is possible to build a road pavement without using any bituminous material. It thus helps to build a pavement structure charact€ rized by an excellent degree of energy conservation.