Stiffness Improvement of Expansive Soil-Rubber Mixtures with Off-Specification Fly Ash

The potential use of off-specification fly ash to increase the stiffness of expansive soil-rubber (ESR) mixtures was investigated systematically in this study. Both high-sulfur content and high-carbon content off-specification fly ashes were tested. A standard Class C fly ash was also used as a control fly ash. The ESR mixture consisted of high-plasticity clay blended with 20% 6.7-mm granulated rubber (by weight). The off-specification fly ash content required to develop pozzolanic reactions was determined based on the lime fixation point concept and kept constant for all ESR-fly ash mixtures. At this selected off-specification fly ash content, ESR-fly ash mixtures were tested at a single relative compaction and curing time of seven days. Unconfined compression testing was performed on compacted specimens to validate the selected fly ash content and evaluate the effect of curing time on the development of pozzolanic reactions. Stiffness changes due to fly ash addition were evaluated in undrained axi-symmetric compression with mean effective stress of 100 kPa. Large- and very small-strain stiffness was evaluated using external displacement transducer and bender elements, respectively. Results suggest the stiffness improvement imparted by off-specification fly ashes is equal to or better than the improvement imparted by standard Class C fly ash.