Use of Pulverized Fuel Ash in Trench Backfill

A study was made of the use of pulverized fuel ash (PFA) mixtures to make a flowable material to backfill trenches. An optimum mix was designed to comply with current national specifications. The 4:8:1 (PFA:sand:cement) mix gave a compressive strength six times that of PFA:cement and 2.5 times that of sand:cement for the same (PFA + sand):cement ratio. Frost resistance was generally good. The material showed no expansion when tested in accordance with the freeze-thaw test procedure given in BS 812: Part 124. The use of calcium chloride to improve early strength was more effective than the two commercially available accelerators investigated. More work is needed to improve the early strength in cold weather and reduce cement content of the mix. A field trial showed that cementitious materials were better than conventional Type 1 granular subbase, both in terms of having less permanent deformation and elastic deflection. The cost of PFA:sand:cement trenchfill mixtures was within 10-15% of that of foamed concrete.