Nassar, Ahmed Izat Mohammed
(2016)
Enhancing the performance of cold bitumen emulsion mixture using supplementary cementitious materials.
PhD thesis, University of Nottingham.
Abstract
Several benefits are gained from using cold mix asphalt (CMA) instead of hot mix asphalt (HMA). The benefits include conservation of materials and reducing energy consumption, preservation of the environment and reduction in cost. One of the common types of CMA is cold bitumen emulsion mixture (CBEM) which is the mixture produced by mixing mineral aggregate with bitumen emulsion. Despite the efforts applied in the last few decades in order to improve and develop CBEM utilization, certain deficiencies remain that make it inferior to HMA, resulting in restricting or minimizing of its use. However, the development of CBEM for road construction, rehabilitation and maintenance is steadily gaining interest in both pavement engineering industrial and research sectors.
The present study was primarily aimed at evaluating the effect of using different cementitious materials on the performance of CBEM. The idea of the research is to provide a sustainable filler from supplementary cementitious materials (SCMs) to be used as fillers to provide enhanced properties of CBEMs. By achieving this aim it is expected that the utilization of CBEM would increase, allowing them to be used as structural pavement materials with some confidence.
Research was first undertaken to optimize the mix design of CBEM using a statistical approach known as response surface methodology (RSM), as an alternative approach to achieve acceptable engineering properties. The optimization of CBEM was investigated, to determine optimum proportions to gain suitable levels of both mechanical and volumetric properties. This optimization focussed on the mix design parameters, namely bitumen emulsion content (BEC), pre-wetting water content (PWC) and curing temperature (CT). This work also aimed to investigate the effect of the interaction between these parameters on the mechanical and volumetric properties of CBEMs. The results indicate that the interaction of BEC, PWC and CT influences the mechanical properties of CBEM. However, PWC tends to influence the volumetric properties more significantly than BEC. The individual effects of BEC and PWC are important, rather than simply the TFC which is used in conventional mix design of CBEM. Furthermore, the experimental results for the optimum mix design corresponded well with model predictions. It was concluded that optimization using RSM is an effective approach for mix design of CBEMs.
The study also investigated in-depth the performance characteristics of CBEMs using different filler treatments. The study was extended to understand the performance enhancement through mineralogical and microstructural investigations. The research was designed to use cement, binary and ternary blended fillers (BBF and TBF). Fly ash (FA) and ground granulated blast-furnace slag (GGBS) were used as BBF while silica fume (SF) was added to the BBF to obtain TBF. A significant improvement was achieved in mechanical and durability properties of CBEMs due to incorporation of both cement and blended fillers. Also, the results indicated that TBF was more suitable than BBF for the production of CBEMs. The microstructural assessment indicated that the effect of BBF on the internal microstructure of CBEMs was slightly negative and more noticeable in CBEMs containing FA. Mineralogical and microstructural assessments also suggested that the presence of bitumen emulsion might not affect the hydration of the silicates in treated CBEMs. The formation of additional CSH was observed due to the replacement of conventional limestone filler by cement, BBF and TBF. However, it seems that this can cause a delay in the formation of other hydration products (Ettringite) resulting from the hydration of aluminates in cement. Furthermore, it is proposed that the addition of SF to BBF mixtures can eliminate the delay in formation of hydration products caused by the bitumen emulsion.
The present work was also aimed at better understanding the curing mechanism of CBEMs and to bridge the gap between laboratory curing and field evolution of these mixtures. This was achieved by evaluating the effect of the curing process on CBEM performance and developing a prediction model to assess in-situ CBEM performance using maturity relationships. Different contributory factors affecting the curing process were investigated such as curing temperature and relative humidity (RH) in addition to the impact of curing time and the presence of cement/active fillers. The results indicated that high curing temperature is responsible for additional stiffness gain by increasing the binder stiffness due to ageing and by increasing the moisture loss by evaporation during the curing process. However, at high curing temperature the moisture loss by evaporation may hinder the hydration of cement/active fillers. Moreover, the results also indicated that the high RH level influences the stiffness modulus of CBEMs negatively. The laboratory results were then used to develop a tool to assess in-situ curing of CBEMs using the maturity approach, which is widely used to estimate in-situ concrete properties. A strong correlation was found between maturity and the stiffness values obtained from the laboratory tests, which resulted in development of maturity-stiffness relationship. The application of this relationship to assess the in-situ stiffness of CBEMs is presented using three hypothetical pavement sections in the United Kingdom, Italy and Qatar; to simulate different curing regimes.
A pavement analysis and design study was conducted to evaluate the incorporation of treated CBEMs into a pavement structure. CBEMs are suggested to be used in two scenarios: the first is as a surface course and the second is as a base course. The scope of the study is limited here to design based on the fatigue criterion only. Although, the structural design was based on practical hypothetical layer thicknesses, the results provided useful insight into the structural capabilities of CBEMs.
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