Sanchez Melo, Diana B.
(2018)
Meso-scale rheological characteristics of foamed bitumen mixtures with high RAP content.
PhD thesis, University of Nottingham.
Abstract
The increasing concerns to reduce the production of harmful emissions, decrease energy consumption and preserve the natural resources have motivated the asphalt industry to lower the production temperatures of asphalt mixtures and incorporate alternative materials. One approach is to use Foamed Bitumen Mixtures (FBM) in combination with Reclaimed Asphalt Pavement (RAP) material. FBMs are composed of a combination of foamed bitumen and mineral aggregates, where unlike conventional HMA where the bitumen is in a liquid state during mixing, the bitumen present in these mixtures is in a foamed state during the mixture’s production. Foamed bitumen production has been mainly based on the techniques developed more than 50 years ago, where the bitumen is heated at high temperatures and then foamed by combining it with cold water, a process that is also known as mechanical foaming. In its foamed state, the bitumen has lower viscosity and expanded volume and can, therefore, be mixed with cold (and often damp) aggregates. These two conditions allow bitumen in its foamed state to coat the aggregates at lower temperatures in comparison to those required in regular HMA materials. More recently, new foaming technologies have been developed as part of Warm Mix Asphalt (WMA) technologies. WMA are asphalt mixtures that are produced at reduced temperatures (i.e. mixing temperatures around 120°C) compared to traditional HMA materials, obtaining mainly environmental and economic benefits. The possibility of reducing the mixing temperature for the asphalt mixture by using these types of foaming technologies is achieved through the use of a series of products and/or processes that act on the bitumen through different mechanisms to provide the adequate workability and coatability conditions when combined with the aggregates at these reduced temperatures.
On the other hand, RAP materials are essentially aged asphalt pavements that have reached the end of their service lives and are reclaimed to be used as part of the construction or rehabilitation of pavement structures. The incorporation of this material in Hot Mix Asphalt (HMA) reduces the amount of raw materials added to the final mixtures, which contributes to the conservation of natural resources. Thus, the use of foaming technologies together with the great potential shown by RAP materials to be successfully recycled in asphalt mixtures, are encouraging the use of these materials in road infrastructure projects, thereby requiring the study of their fundamental properties and providing an indication of their expected field performance. This evaluation is particularly important taking into account that a main concern related to the use of lower production temperatures of the mixtures is if the properties of the material are still comparable to those of the traditional HMA. Furthermore, in the case of RAP materials it is still unclear what role the old RAP bitumen plays in the presence of virgin materials, particularly because the blend of RAP-virgin bitumen has a great influence on the final performance of the produced mixtures. Thus, the use of foamed bitumen in mixtures that also include RAP constitutes an important subject of study.
This thesis is concerned with the use of the Dynamic Mechanical Analysis (DMA) testing technique as part of a comprehensive experimental methodology to evaluate the rheological response of foamed bitumen mixtures produced by the traditional mechanical foaming technique, and by the incorporation of a foaming additive based on zeolite minerals commonly used in warm mix technologies, and their behaviour in the production of mixtures containing high RAP content (i.e. 50% by total weight). The relevance and suitability of the testing methodology was evaluated at the meso-scale by testing specimens representing the fine aggregate matrix (FAM) or asphalt mortar existing in the full mixtures which has a relatively more uniform (or less heterogeneous) internal structure and it has been reported to accurately represent the mechanical behaviour of the whole asphalt mixture (Masad et al. 2006). In this study, FAM is defined as a combination of bitumen, with mineral aggregates of particles size smaller than 1 mm. Initially, the rheological characteristics of the FBMs produced with both foaming technologies were studied. This was followed by the evaluation of the rheological response of these mixtures with RAP material. In all cases, this evaluation included the effect of production temperature for half-warm (i.e. mixing temperatures of 90oC), warm (i.e. mixing temperatures of 120oC), and hot processes (i.e. mixing temperatures of 160oC).
Experimental results of applying DMA testing on FAM FBMs identified the characteristics of foamed bitumen mixtures manufactured with the two different foaming technologies. It was found that the foaming technology and production temperature have different impacts on the produced mixtures. In the case of the mechanical foamed mixtures, the rheological characteristics of these mixtures were found to be strongly dependant on the production process and the mixing temperature of the materials. Conversely, changes in the production temperature do not have a considerable effect on the rheological characteristics of the zeolite containing mixtures. When 50% RAP material was incorporated the properties of the mixtures were found to be influenced by the level of blending between RAP-virgin binders, which was found to be mainly a function of the foaming technology and the final temperature of the mixtures.
The present work was also intended to evaluate how the fundamental rheological properties of these FAM FBMs with RAP material are affected by several environmental conditions, such as the combined effects of water and oxidative ageing. This was achieved by applying two conditioning procedures including the BS EN 12697-12:2008 with further conditioning in a 60oC water bath at different times, and the Saturating Ageing Tensile Stiffness (SATS) protocols. It was found that in the presence of moisture and high temperature environments, the age-hardening related effects dominated the response of the FAM materials. These effects are believed to be linked to the fact that the FAM type mixtures contain more bitumen and smaller size voids compared to the full asphalt mixtures, which could impact the rate at which moisture can reach the inner portion of the testing specimens, whereas high temperatures certainly lead to some ageing and stiffening of the materials.
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