Evaluation of optimum use of pozzolan aggregates in road bituminous mixtures using the marshall-mix design method

Abstract

This research documents an extensive study on the optimization of pozzolan and granitic aggregates in road bituminous mixes using the Marshall Mix design method. The Marshall parameters formed the basis for evaluation of the various resultant mixtures using both individual aggregates and combinations. The main objective of this study was to establish an optimum way of utilizing a mix of pozzolan and granitic aggregates in road bituminous mixtures using the Marshall method. The two aggregate types (pozzolan and conventional granitic type) were mixed with one bitumen type (80/100 pen). The materials were characterized to determine their properties. The tests conducted on aggregates included toughness, water absorption, specific gravity and gradation. Those conducted on the bitumen were; penetration, softening point and flash and fire point tests. The Marshall mix design was then carried out on the test specimens in accordance with ASTM 1559-89. Results obtained for bituminous mixtures comprising of conventional aggregates, namely stability, flow, and VFA all conformed to the specification ranges for suitability of use in HMA construction. Those obtained for mixtures comprising of only the pozzolanic type of aggregates (with mean stability of 1360lbf and VFA value of 85%) did not conform to the specification requirements of minimum stability 1500lbf and VFA range of 70-80%). However, mixes having a combination of pozzolan aggregates with conventional aggregates in the filler produced acceptable results for stability (1617lbf), though the voids filled with asphalt (VFA) value (86%) still fell outside the specification range. The fineness of the pozzolanic powder (filler) probably makes it unsuitable for use in bituminous mixtures as they rendered the mixtures unsuitable. On the contrary, the granitic dust (filler) was suitable for use in preparing mixtures and it improved the stability and other Marshall properties of pozzolan aggregates used in the study. High optimum asphalt contents were obtained for mixes that comprised of pozzolans especially in the coarse portion unlike the conventional granitic types.