Evaluation of Various Treatment Methods for Enhancing the Properties of Recycled Concrete Aggregate for Hot Mix Asphalt
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Sustainability and awareness of the environment are becoming increasingly important for transportation agencies worldwide. Sustainable development has significance for obvious and relevant reasons in all areas of civil transportation infrastructure. In the asphalt industry, a large number of innovative materials and technologies are explored in order to evaluate their suitability in the design, construction and maintenance of pavements. Attaining a sustainable solution has become a critical and urgent priority in the asphalt industry to solve various problems including lowering the consumption of virgin materials, decreasing waste materials in landfills and reducing environmental problems. For the above-mentioned reasons, the utilization of Recycled Concrete Aggregate (RCA) is increasingly becoming a highly interesting issue in Canada, and worldwide. The use of RCA has been successfully applied in base aggregates and Portland Cement Concrete (PCC) aggregates. However, there have been very limited investigations for the usage of RCA in asphalt pavement mixtures due to its inferior properties such as higher water absorption and low density, but the main challenge is that RCA is a highly porous material. These characteristics have become strong restrictions for its usage in wide applications. The overall objective of this study is to evaluate the application of Coarse Recycled Concrete Aggregate (CRCA) as an alternative for coarse natural aggregate in asphalt mixtures within Ontario specifications. To achieve this, the research’ objective is divided into five major goals. The first goal is to enhance the physical and mechanical properties of different CRCA types by combining different types of treatment methods. The second main objective is the assessment of surface microstructure of different CRCA types. The effects of different treatment types on the morphological and mineralogical properties of the CRCA surface such as surface texture, roughness, surface morphology and chemical composition are examined. The influence of different treatment types on the interfacial transition zone (ITZ) is also extensively studied. Volumetric properties of hot mix asphalt (HMA) are necessary requirements to ensure good performance for asphalt mixtures. Therefore, the third major objective is the evaluation of volumetric properties of asphalt mixtures that include CRCA addition. The fourth main objective is the assessment of the application of treated and untreated CRCA of two different types in typical Ontario HMA mixtures, whereas the fifth main objective is to investigate a simple cost and statistical analysis for different mixtures that include various untreated and treated CRCA. Different types of treatments were performed under various conditions to evaluate each type of treatment separately, then the combinations of various treatments were tested. In order to achieve the objective, pre-soaking and heat treatment followed by different types of short mechanical treatment were conducted to evaluate the physical and mechanical properties of CRCA before and after the treatment method combination. Heat treatments were conducted at the following temperatures: 250 ºC, 350 ºC, 500 ºC and 750 ºC. The pre-soaking method involved the use of strong acid HCl and weak acid C2H4O2, whereas short mechanical treatment included the utilization of a Micro-Deval device with two different testing techniques with/without steel balls. To evaluate the surface characterization of the CRCA surface of both treated and untreated CRCA, various advanced techniques were performed: Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Microanalyzer (EDAX), X-Ray Diffraction Analysis (XRD). The performance of typical Ontario asphalt mixtures for usage in a Superpave (SP) SP19 with various percentages of CRCA (0%, 15%, 30%, and 60%) was evaluated using many laboratory tests such as indirect tensile test (ITS), dynamic modulus test, rutting and thermal stress restrained specimen test (TSRST) test. Compared to Natural Aggregate (NA), the obtained results indicated that RCA has inferior physical and mechanical properties. RCA has a lower Bulk Relative Density (BRD), higher water absorption, and higher porosity than NA. With respect to the mechanical properties, RCA has a higher abrasion loss and Aggregate Crushing Value (ACV) than NA. In these properties, a significant difference is observed between different CRCA types. The amount of adhered mortar and the original source of RCA have a considerable effect on these properties. The use of heat treatment method is highly successful in improving various physical and mechanical properties. For different CRCA types, the heat treatment has the maximum influence at 350 °C, which results in lower absorption with an approximate decrease of 9.5% and 11.23% for CRCA#1 and CRCA#2, respectively and decreased porosity with an approximate reduction of 8% and 10.3% for CRCA#1 and CRCA#2, respectively. However, it is recommended to use this method at temperatures between 300 °C and 350 °C because of the noticeable negative effects of higher temperatures on the CRCA properties. The acid treatment at low concentration is an effective technique to enhance the quality of CRCA depending on the acid type due to corrosive influence on the attached mortar. Nevertheless, it is concluded that using weak acid is more efficient than the strong acid to decrease the influence of acid attacks on the CRCA surface as demonstrated by the characterization images. In terms of water absorption, the combination of heat treatment at 300 °C and short mechanical treatment leads to a reduction of 30% and 23.0% for CRCA#1 and CRCA#2 respectively, whereas, the application of the pre-soaking with the weak acid solution and short mechanical method results in a reduction of 22.5% and 37% for CRCA#1 and CRCA#2, respectively. With respect to the porosity, a significant reduction, approximately 26%, and 20.9%, is recorded for CRCA#1 and CRCA#2 respectively after applying the combination of heat treatment at 300 °C and short mechanical treatment, whereas the combination of pre-soaking in acetic acid method and the same mechanical treatment leads to a substantial decrease with an approximate reduction of 19.4% and 33.5% for CRCA#1 and CRCA#2 respectively. The observations of SEM and EDAX tests indicated that the surface morphology of untreated CRCA is a rough and heterogeneous surface and a highly porous structure, whereas the surface of treated CRCA is more homogeneous and exhibits less adhered mortar depending on treatment type. A significant enhancement of CRCA microstructure was obtained under influence various treatment types. However, improved microstructure mainly includes increased density, increased surface homogeneity, reduced pore size, reduced Ca/Si ratio, and improved properties of ITZ microcracks including width and length of microcracks. There was a significant negative impact of heat treatment within a temperature range of (350-500 °C) due to a considerable increase of pore size compared with untreated CRCA. The obtained results indicated that the optimum asphalt content (OAC), voids in mineral aggregates (VMA), and voids filled with asphalt (VFA) decrease as the filler content is increased. For control mix, the addition of filler with 2.5% percentage is very successful due to satisfying all Ministry of Transportation Ontario (MTO) requirements for volumetric properties of HMA. The NA replacement by CRCA leads to increasing the OAC for the mixtures, whereas the VMA and VFA are decreased. Compared to the mixture with 30% untreated CRCA, a small improvement and a slight increase in VMA and VFA properties, respectively, were seen for mixtures with 30% treated CRCA with different treatment techniques. The CRCA addition with different proportions is very successful for both untreated and treated CRCA due to achieving all MTO requirements for volumetric properties of HMA. However, CRCA treated with various treatment methods appears to be more successful than untreated CRCA application. The addition of different types of untreated CRCA in various proportions produces higher rutting resistance and higher stiffness modulus than the control mix. The CRCA type has an effect on the rutting characteristics of asphalt mixtures. The application of treated CRCA with heat treatment and short mechanical treatment leads to an increase in the rutting resistance, a decrease in the total rut depth, a slight increase in the stiffness modulus, and an increase in the rutting factor of asphalt mixtures depending on the type of CRCA. The application of treated CRCA with the pre-soaking method and short mechanical treatment results in an increase in the stiffness and rutting factor of mixtures, depending on the type of CRCA. The findings demonstrated that the mixtures that included untreated CRCA have much higher ITS values than the control mix, registering a significant increase of 68%, 70%, 85.6% and 86.7%, for the mixtures that included 30% untreated CRCA#1 and CRCA#2 in both unconditioned and conditioned samples, respectively. This is followed by the ITS values of the mixtures that included 60% untreated CRCA#1 and CRCA#2 for both unconditioned and conditioned samples with an increase of 41.4%, 49.0%, 71.5%, and 56.8%, respectively. Additionally, a reasonable improvement in the ITS values in both conditioned and unconditioned samples were recorded for the mixtures that included 30% treated CRCA with different treatment techniques compared to the mixture that included 30% untreated CRCA. Furthermore, all tensile strength ratio (TSR) values for mixtures that included untreated CRCA with different types and percentages are higher than the minimum required value of MTO specifications. This indicates a successful application for CRCA in these mixtures. The use of the combination technique of pre-soaking method with weak acid followed by a short mechanical treatment method was a highly successful method for enhancing moisture resistance of asphalt mixtures as compared to other combination methods. From TSRST test, the findings indicated that the fracture temperature is reduced due to the CRCA addition compared to the control mix. However, there is no significant influence for the RCA type on the thermal cracks at low temperatures. Additionally, the fracture stress of the mixtures that included different untreated CRCA types with various proportions is generally higher than the fracture stress of the control mix Furthermore, the application of the combination of various treatments leads to a significant reduction in the fracture temperature, indicating a successful application of treated CRCA in HMA mixtures in cold regions. However, the combination of heat at 300 °C and short mechanical treatment has a considerable impact on the fracture temperature of asphalt mixtures as compared to other combination techniques. The results of ANOVA statistical analysis showed that the type of CRCA had a significant effect on the stiffness and rutting, ITS, and the fracture temperature of asphalt mixtures. Additionally, the type of treatment method had a considerable effect on the stiffness, rutting, and fracture temperature of HMA mixtures. In contrast, both the type of treatment method and the type of CRCA have an insignificant effect on the fracture stress of asphalt mixtures. The results of cost analysis revealed that both heat and pre-soaking treatment is quite reasonable, referring to many economic benefits and indicating that these treatment methods could be applicable. In conclusion, the results of this study indicate that the application of different CRCA types in various forms: treated and untreated is very successful and can contribute greatly towards more RCA applications in the asphalt pavements.
Cite this version of the work
Hanaa Al-Bayati (2019). Evaluation of Various Treatment Methods for Enhancing the Properties of Recycled Concrete Aggregate for Hot Mix Asphalt. UWSpace. http://hdl.handle.net/10012/14579