Long-term pavement performance experiments : a personal experience

Abstract

The author shares his experience over the last 40 years of the design, monitoring and final evaluation resulting from 23 purpose-built road experiments consisting of over 200 individual sections plus many observation sections in Botswana, Namibia and South Africa. The purpose-built LTPP experiments and the lengths of existing road selected as observation sections are of four types. The first three are intended to show how to make the best use of local, low-cost materials for low to medium volume roads in the dry and moderate macroclimatic zones of southern Africa ? some two-thirds of the region: ? Bases and subbases of marginal to substandard (mostly G5-G7) calcrete, weathered granite, weathered basalt, neat (G7), cement- and bitumen-treated Kalahari sands, and sulphide- and acidic sulphate- contaminated gold and copper mine waste rock. ? Brack, saline and sea water for compaction of all layers including the base and in the slurry of a Cape seal. ? Selected calcrete as chippings in various types of seal coats in the Kalahari where conventional stone does not occur. The fourth type is intended to provide proven countermeasures to eliminate or at least mitigate the damage to roads on expansive clay roadbeds ? apart perhaps from drainage-related failures the most common form of roadbed-related distress in southern Africa. In all cases control sections of G3 crushed stone, G4 gravel, stabilized gravel, fresh water, or with no countermeasures as appropriate were included as part of the experiments. These experiments are proving ? by means of monitoring over periods in excess of 20 years ? that, for example, and depending upon traffic, substandard calcretes with PIs of 20, sulphidic and acid-saline mine rock, seawater and even unsound basalt can all be used successfully in the base course, and that the effects of expansive clays can be greatly reduced without incurring increased maintenance costs provided that certain precautions are taken. The design requirements and the problems of preserving and monitoring such long-term experiments are discussed. The advantages and disadvantages of LTPP vs HVS trafficking are discussed and it is concluded that a judicious mix of both is necessary.