In tropical and sub-tropical countries, the performance of Hot Mix Asphalt (HMA) has often been disappointing, with road surfaces sometimes failing within a few months of construction. Under the high temperature conditions experienced in these countries, bitumen, which is a visco-elastic material, can become very soft. Bitumen can also undergo relatively rapid chemical changes that cause many of the desirable properties of HMA to be degraded or lost altogether. Overseas Road Note 19, which is based on the experience of TRL and collaborating organisations throughout the world, provides a guide to the appropriate design of HMA, and accounts for these deficiencies. This experience has been gained in carrying out a comprehensive, co-ordinated and long-term series of research projects as part of the DFID ‘Knowledge and Research’ programme. The research showed that the behaviour of asphalt surfaces in tropical and sub-tropical environments was frequently contrary to expectations and has given rise to a paradigm shift in our understanding of road behaviour. This Road Note is aimed at engineers responsible for roads and gives guidance on the design, manufacture and construction of HMA pavement materials in tropical and sub-tropical climates. The HMA requirements are described for different traffic loading categories, including severely loaded sites such as climbing lanes.

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