Mechanistic Structural Analysis and Design of Recycled Aggregates in Road Construction Case Study: City of Saskatoon
The current manner of constructing roads with virgin aggregates is unsustainable for many urban centers as natural sources for quality aggregates are being or have been depleted. As well, there is little understanding or scientific data published as to the impacts on roadway design and life cycle performance with poorer quality aggregate material. To improve future sustainability of roadway utility, there is a need for better understanding of the mechanistic behavior of road aggregates and their respective role in road structural performance in the field. As well, there is a need to find more sustainable sources of quality aggregates to construct roadways. The goal of this research is to improve road utility sustainability through a better understanding of life cycle performance and incorporating field state mechanistic principles in the initial design of the roadway structure. The primary objective of this research was to investigate the application of recycle rubble materials using a mechanistic materials characterization and structural design process for urban roadways within typical City of Saskatoon roads and field state conditions. Specific technical objectives of this research were to characterize various recycled aggregate materials with regards to their road structural behavior as a high quality base coarse, quantify the cost comparison between various design cross sections, and evaluate the structural behavior of these alternate aggregate sources in typical structural designs and Saskatoon field state conditions. To validate the field behavior of recycled aggregates, various test sections were constructed with different recycled and virgin aggregate structural systems. These test sections were evaluated using non-destructive structural assessment to determine their structural quality in the field. This research studied the use of recycled portland cement concrete aggregates and recycled asphalt cement aggregates as structural granular layers of typical City of Saskatoon roadways. These materials were characterized using conventional and mechanistic lab characterization protocols. Field test sections were constructed to validate that recycled materials could be employed as quality replacements for virgin aggregates. Research was also conducted on how to incorporate mechanistic based materials testing and structural design into City of Saskatoon Design and Materials Selection Specifications and Processes. The resilient modulus of the various road materials was also compared to relate to other mechanistic-empirical design methodologies. The laboratory testing conducted in this research indicated that although conventional empirical testing shows recycled asphalt materials to be of lesser quality, when evaluated using mechanistic characterization protocols, recycled asphalt concrete material yielded superior structural behavior. To illustrate, the dynamic modulus of recycled asphalt concrete was 697 MPa under a fully reversed stress state and a frequency of 0.5 Hz compared to 264 MPa for a high quality conventional high fracture granular base under the same stress state and frequency. As well, the recycled asphalt material showed less moisture susceptibility than conventional granular aggregate. This research showed recycled portland cement concrete aggregate materials showed good drainage and capillary break qualities when tested against the standard granular base materials. Although the well graded recycled asphalt cement and well graded recycled portland cement concrete were shown to have slightly higher moisture intake values, the increased moisture did not increase the swell and therefore indicates adequate frost resistance due to moisture. This research showed conventional roadway design utilized by the City of Saskatoon does not have the means to evaluate recycled asphalt and portland cement aggregates from a materials selection and structural design perspective. Roadway designs using a mechanistic approach were able to accurately represent the field structural primary responses of test roadway structures considered in this study and were able to incorporated recycled aggregate in the design process. Designing roads using a mechanistic design process showed a significant improvement in roadway structural responses in designs using recycled aggregate material. From an economic perspective, this research showed road cross sections utilizing recycle aggregate materials proved to be the least expensive option when evaluated by the initial capital cost and the projected life cycle costing. When comparing primary structural responses to construction cost, up to 20 percent of costs to construct a road can be saved, and a properly designed road structure using recycled aggregates will reduce the strains in the structures by up to 90 percent. As well, using recycled aggregates to construct roadways will reduce the fuel consumption during construction by up to 20 percent due to a reduction in aggregate hauling distances. In summary, when evaluated with a mechanistic road structural design method that accounts for the material characteristics of various aggregates, recycled asphalt rubble processed as a black base and recycled portland cement concrete as a stress dissipating drainage layer within the construction of new roadways is a more sustainable approach to designing and constructing structurally sound roads than the conventional methods. Based on the findings of this research, proper stockpiling and recycling of asphalt and concrete rubble materials is recommended in the City of Saskatoon. In order to optimize and incorporate various recycled aggregate materials into road design it is recommended the City of Saskatoon employ mechanistic based road material characterization and structural design.
DegreeMaster of Science (M.Sc.)
DepartmentCivil and Geological Engineering
CommitteePark, Peter; Sparks, Gordon; Putz, Gord; Antunes, Jorge
Copyright DateJune 2013