Although calcrete has been regarded as a poor-quality gravel, it is used extensively in the Kalahari region of southern Africa, because it is too expensive to import high-quality material. A joint project was set up by the Transport Research Laboratory (TRL) and the Botswana Roads Department, to establish reliable guidelines for the use of calcrete. The first part of the research was to locate and map the local calcereous deposits, and investigate their engineering properties. Experimental sections of road were constructed on a highway with fairly heavy expected levels of traffic. Eight sections had calcrete roadbases and four were controls; all sections used sand as sub-base. The main mode of deterioration was deformation due to shear failures within the roadbase, which was apparently caused by a moisture-induced loss of strength. Simple performance models were developed, relating rut-depth statistics to traffic and/or age for each section. In order of improving performance, the materials used in the test sections were: (1) calcified sand (CS-4); (2) nodular calcrete (NC-2); (3) hardpan calcrete (HC-1); and (4) powder calcrete (PC-3); Suggested traffic limits for these types of calcrete are tabulated, together with recommended specifications for calcrete roadbases for different traffic flow levels.
Samenvatting