Investigation of the Foamability of Tar and the Characterisation and Optimisation of Foamed Tar

ABSTRACT In southern Africa the scarcity of good road construction materials and long haulage distances have necessitated the use of methods to improve substandard materials or re-use by means of recycling. Tar is available in large quantities as a by-product of the fuel-from-coal process. The good adhesive properties of tar to aggregate are well known, but suitable construction processes limited the use of tar. Foamed bitumen (asphalt in the US) has been used extensively, and because of the low temperature application has major construction benefits. Due to the similarities in properties between bitumen and tar it was anticipated that tar could be used as a stabilising agent in the same manner as bitumen. The aim of the paper is to investigate the potential of foaming tar, and to present the key design parameters. The paper first presents the properties of tar, and specifically addresses the health considerations of low temperature carbo-tar which was the focus of this study. Thereafter the characterisation of foamed tar based upon the principles developed for foamed bitumen is presented. Foamed tar is investigated with respect to expansion ratio and half-life, and a new parameter known as the Foam Index. Optimisation methods for the foamability of tar are formulated with respect to the Foam Index, foaming temperature and foamant water content. Conclusions are drawn with regards to the tar foaming process and tentative limits are set with respect to the expansion ratio, half life and Foam Index necessary to achieve a satisfactory foamed tar stabilised material.