Mechanical Properties of Residues as Unbound Road Materials - experimental tests on MSWI bottom ash, crushed concrete and blast furnace slag

Abstract: For recycled aggregates and industrial by-products to beused correctly in road construction, it is necessary to knowtheir properties. Existing material specifications and testmethods for aggregates used in Sweden and in many othercountries are indirect and are based on empiricism. Over theyears they have been adjusted to conventional aggregates, whichmakes the introduction of new materials difficult. Research oftheir properties is being conducted in many places althoughknowledge has been inadequately disseminated.The objective of this thesis is to increase knowledge of themechanical properties of certain selected residues for improveddesign of pavements using these residues.The study has concentrated on residues in unbound roadlayers. The materials selected were processed municipal solidwaste incinerator (MSWI) bottom ash, crushed concrete andair-cooled blast furnace slag (AcBFS). The deformation onloading, the possible strength development over time and theresistance to mechanical and climatic action were studied inthe laboratory and in the field. The results were compared withthose of the conventional aggregates they could possiblyreplace, such as sand, gravel and crushed rock. The methodsused in the laboratory were cyclic load triaxial tests, LosAngeles tests, micro-Deval tests and freeze-thaw tests. In thefield, test sections with residues and reference sections withconventional aggregates in the unbound layers were monitored bymeans of falling weight deflectometer (FWD) measurements.The laboratory results showed that a high content ofunburned material in MSWI bottom ash limits the resilientmodulus but not the permanent deformation to the same extent.Both laboratory and field results showed several years?growth in stiffness for unbound layers with crushed concreteand AcBFS, which is not present for unbound layers with naturalaggregates. This was thought to be caused by calciumdissolution and precipitation in the compacted material layer.A special investigation of the material in question, togetherwith knowledge of the planned construction, could permit ahigher value to be used in the design modulus than for crushedrock and thus benefit from the increased stiffness.The Los Angeles test and other tests developed forsingle-sized aggregates did not really justify the performanceof the materials studied. Recycled aggregates and otherresidues, as well as conventional unbound road materials,should be analysed using cyclic load triaxial tests in thelaboratory and FWD measurements in the field, both of whichtake into account the whole composite material or layer.Consequently, a new methodology for material assessment andcomparison is proposed, based on permanent deformations incyclic load triaxial tests.According to the laboratory and field tests, some bottom ashcould be used, not only in embankments and capping layers butalso to bear the stress levels expected in a sub-base. Recycledaggregates and other residues should be used near the source ofproduction and not necessarily in roads with low trafficvolumes. Their properties should be used to the greatestpossible extent although their limitations must be taken intoaccount.Key Words:residues; unbound materials; MSWI bottom ash;crushed concrete; blast furnace slag; mechanical properties