Specific techniques and methods are described in this thesis in order to adapt Hot Mix Asphalt (HMA) so that desert sand can be used as a fine aggregate. In this way, the HMA can use a freely available local material. Such a locally available material is both accessible and low-cost but introduces the question of processing so that the final road construction is of comparable quality to that which is made using mechanically crushed aggregate. The challenge of this research is to meet the societal needs for low-cost HMA roads, made with readily available desert yet rounded sand, while finding an allowable ratio of natural sand to crushed sand and other fillers thereby ensuring a stable asphalt mix under hot temperatures.
Due to its viscoelastic properties, HMA is greatly affected by temperature variation, rate of loading and other climatic conditions. Hot and arid climates provide the challenge of extreme daytime temperatures, particularly for a black, road surface layer. Climate change is also considered to be worsening this effect, specifically because of hot wind currents further degrading desert sand, reducing its angularity. As such, changes in the climate may challenge conventional empirical methods used to predict pavement distress, resulting in an overestimation of projected pavement life.
Where road infrastructure is aging, pavement condition must be improved to maintain driver safety and comfort; this is a critical issue in hot and arid regions. Starting in the 1970s, it has become more common to rehabilitate road networks than to reconstruct them. For budgetary reasons, the growing costs of virgin aggregate, binders, and other materials have created a demand for more sustainable techniques that allow for the use of recycled or otherwise lower grade materials in HMA. Specifically, a sand-asphalt mix is a promising alternative to HMA, especially for Low Volume Roads (LVRs).
The laboratory investigations conducted in this research use round desert sand obtained from the desert region in the south of Libya. This region is part of the Sahara Desert where temperatures range from 30 to 50 degrees centigrade; there is an abundance of sand but there is no local source of what is typically the standard structural component for roadbuilding: an aggregate of gravel and sand. Currently, such aggregate must be transported from the north of Libya, hundreds of kilometers away. To reduce these costs, preserve good quality aggregates and reduce the transport associated pollution, this study develops techniques to use the sand readily available in the south. Typically, natural desert sand does not satisfy angular and abrasion requirements for use as a pavement material in its untreated state. Nonetheless, after processing, it has been used successfully in a number of road projects in Australia and elsewhere. In standard HMA construction, mechanically crushed sand provides an angular grain that resists compacting because the particles naturally interlock against each other and form a mesh. The problem with naturally occurring sand is that it has a rounded grain. Roads built from this rounded grain sand develop premature rutting because the particles do not interlock with each other. Although an HMA road using only sand as an aggregate will only ever be suitable for LVRs, a better understanding of how to integrate and process the naturally found sand will save a great deal of time and money. Beside the fact that desert sand is freely available, crushed sand is the by-product of the creation of larger aggregate; therefore it is the cheapest form of manufactured aggregate.
In this research, the HMA is mostly used as a surface layer. The tests used in this study are the Marshall, the gyratory compaction, the rutting analyzer, the creep, the Indirect Tensile Strength Modulus (ITSM), and the complex modulus. These tests have been performed on all samples in order to draw relevant statistical conclusions and recommendations.
The results demonstrate that the type of material affects the volumetric properties of the HMA that, in turn, significantly affects the rheological behaviour of the mix. This mix depends on three factors: ratio of desert sand to mechanically crushed sand; choice of bitumen; choice of mineral filler. In this research, the optimal percentage of desert sand has been found to be 33% for all mixes; this is complemented by 63% crushed sand and 4% mineral filler (either limestone or brick powder). These samples were mixed with two different types of bitumen, either Performance Grade 70-10 (PG70-10) or Penetration Binder 60/70 (B60/70). The effect of the bitumen type on the viscoelastic behaviour of the mix was investigated to improve the prediction of the service life of the mix and so was the type of filler (limestone or brick powder).
The most critical test results were the complex modulus analysis, the rutting test, the ITSM, and the creep tests. In the complex modulus test, it was found that the influence of high temperature conditions on the stiffness behaviour of the mix is higher as the number of cycles increases. Regarding the rutting test, the PG70-10 mixture was more resistant to rutting deformation than the B60/70 mix; in the same test, the brick powder mix was more resistant than the limestone mix. The ITSM results indicated that using brick powder as a mineral filler instead of limestone increases the stiffness modulus, performance, and durability of the asphalt surface mix. The creep test showed that the brick powder mixture has less creep than the mixture with limestone powder. This means that the brick powder mix increases the resistance to rutting deformation. The results from this research contribute to a better understanding of the sand-asphalt mix design method based on Superpave techniques.
| Date | 16 Apr 2020 |
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| Original language | American English |
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| Awarding Institution | - École de technologie supérieure
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| Supervisor | Gabriel J. Assaf (Supervisor) |
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