FABM are used to capping, sub-bases and bases of all classes of road and airfield pavements and footways.
FABM are construction materials for pavements that are mixtures of pulverised fuel ash (PFA) or fly ash, as it is known in many countries, and one or more other components whose performance relies on the pozzolanic properties of PFA.
PFA is a pozzolanic material, which in the presence of lime [Ca0 (Quicklime) or Ca(OH)₂ (Hydrated Lime)] hardens under water. Compared to ordinary Portland cement the rate of hardening of the PFA/lime combination is more protracted. This has advantages in pavement construction as follows:
- In the short term, FABM have extended handling times and thus the flexibility of unbound granular pavement materials.
- In the long term, FABM develop significant stiffness and strength with the performance and durability of bituminous and cement bound materials.
Where quicker hardening is required, say in cold weather, partial or complete replacement of lime with cement or the addition of gypsum or other suitable material can be employed. Dry or conditioned PFA can be utilised for FABM. The PFA need not be freshly produced, with the utilisation of old PFA perfectly acceptable and in certain cases advantageous.
With respect to the quality of the finished product FABM are preferably and generally produced in central batching plants utilising pug-mill continuous mixes, although the use of other stationary mixers and the mix-in-place method of construction can be employed in certain situations.
Placement and compaction of FABM is by conventional plant such as drot, grader, paver and vibrating roller. Pneumatic-tyred rollers are usually specified for finishing purpose and, for some FABM, as the only means of compaction.