The development of specifications for the use of low-grade calcretes in lightly trafficked roads in Botswana.
A full-scale road experiment that incorporated four representative types of calcrete was constructed in Botswana in 1979 to determine more appropriate criteria for sealed calcrete bases in lightly trafficked roads. The suitability of calcrete for stabilization with Portland cement or hydrated lime and its mechanical stabilization with local Kalahari sand were examined. A test
section was also constructed in which a calcrete was compacted at a moisture content substantially below the normally required level as a means of saving water in arid areas. Calcretes were also used in the experiment as unsurfaced shoulder materials. Standard laboratory tests were made on the calcrete materials before, during, and after the construction of the experiment. The condition of the experiment was monitored at regular
intervals by means of measurements of surface deformation, rutting, cracking, longitudinal roughness, and surface deflection. Panel inspections were also conducted. In addition, traffic counts and axle-load surveys were performed, climatic data were collected, and the density, moisture content, and strength of the pavement layers and subgrade were measured on a
number of occasions. Satisfactory performance was obtained from all untreated calcrete base sections, although it is apparent that more strict selection of unsurfaced shoulder material is required. The behavior of cement- and lime-stabilized calcretes and a mechanically stabilized calcrete was unsatisfactory. In the case of the stabilized calcretes, this was attributed to the lack of a stabilization reaction (only modification occurred) and the
instability of the bases under traffic, during which time they behaved similarly to single-sized sands. The results of the experiment have led to specifications being recommended that permit a wider range of calcretes to be used as road bases for design traffic levels up to 160,000 equivalent standard axle
loads. This, combined with existing specifications for higher traffic categories, now enables limits for the use of calcretes to be clearly defined. Limitations on the use of calcretes as unsurfaced shoulder materials are also given.
Fourth International Conference on Low-Volume Roads, Transportation Research Record 1106, Volume 1. pp. 26