The influence of moulding moisture content on the engineering properties of aggregate-lime-natural pozzolan mixes

Abstract

The current trends in the road transport sector show a growth in axle loads as well as vehicle numbers on all types of roads in highly industrialised countries as well as in developing countries. This increase in axle loads and numbers has forced road agencies to amend their design standards adopting designs that provide roads with higher load bearing capacity. However, the rapid depletion of natural road construction gravel, as well as strict environmental conservation laws have resulted in many agencies in-charge of road construction and maintenance to resort to use of alternative materials that will be economically feasible and environmental friendly. The use of natural pozzolans for stabilising pavement layers fulfils this requirement. This research study was performed with the aim of evaluating the engineering properties of aggregate-lime-natural pozzolan (ALP) mixtures at varying compaction degrees of saturation and to compare them with conventional cement-stabilized aggregates. Two types of pozzolans found in Tanzania were used. The laboratory investigation was carried out in two parts, namely a pilot investigation where the strength behaviour with time, shrinkage and CBR were determined at three degrees of saturation for a washed river sand specimen followed by the main investigation using two different types of sands at four varying degrees of saturation. The study showed that the compaction degree of saturation for ALP mixes plays an important role in their tensile and compressive strengths development regardless of their optimum moisture contents. The ratio between tensile and compressive strengths for ALP mixes was also found to closely obey the relation given by Fulton (2001) for concrete. The ALP mixes were also observed to develop their strength similar to cement mixes with the formation of tobermorite crystals with the additional of water and appropriate activator. Both pozzolan mixes developed significant tensile and compressive strength after 28 days of curing similar to cement mixes. High CBR values for the two ALP mixes were obtained in mixes moulded at degrees of saturation lower than that corresponding to their optimum. Similarly, the shrinkage of the mixes was found to decrease with a decrease in the degrees of saturation. The CBR and shrinkage of the ALP mixes were found to show similar trend to that of the control cement mixes. The ALP mixes showed no significant strength loss with an increase in the fines content in unwashed sand mix in comparison with that of washed sand mixes. No significant strength loss was observed in the ALP mixes as in the control cement mixes at all moulding degrees of saturation. Finally the study concluded that the ALP mixes could be used in stabilization of pavement layers. However, care must be taken in deciding the compaction degrees of saturation as the specifications used in conventional cement stabilization does not necessarily yield desirable strength development in ALP mixes.