Based on the analysis and results for the three LCA and LCCA scenarios, a summary for the total Global Warming Potential (GWP) and Total Energy Demand (TED) for the three scenarios is presented in Figure 8.16 and Figure 8.17, respectively. The normalized impacts per 100,000 ESALs trafficked for Scenario #1 are also presented in these figures. All impacts were calculated for the construction of one 12 ft. (3.66 m) lane-mile, and considered impacts from materials acquisition, plant operations, transportation, and construction activities. Results from LCA analysis showed that due to the high environmental burdens associated with the production of cement, the materials acquisition stage for the cement-stabilized sections had the highest impacts. For fly ash-stabilized QB sections and the conventional test section, plant operations or construction stages resulted in the highest impacts. When the normalized impacts in Scenario #1 and the response benefits (based on FWD resilient deflections) reported for the pavement sections in the three scenarios are considered, it can be realized that cement-stabilized test sections, particularly those with QB blended with FRAP/FRCA can have relatively lower initial GWP and TED from materials and construction stages normalized over pavement life and anticipated traffic.
The results of the life cycle assessment of the pavement sections with stabilized QB applications provide guidelines for situations where the use of cement- or fly ash-stabilized QB layers can provide affordable, and sustainable pavement designs without compromising performance. Based on LCA results, it is evident that the use of stabilized QB layers can be suited for scenarios where higher traffic loads and volumes are expected. This is the result of the higher stiffness of the stabilized layers, which can deflect less and sustain more load repetitions in the use stage; thus possibly leading to environmental and economical savings. As such, sustainable applications of stabilized QB pavement layers can be especially attractive as an option for constructing local roads or higher volume roads such as county roads or collector/arterial roads.
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Figure 8.17 Summary of Total Energy Demand (TED) for all LCA scenarios [1 in. = 25.4 mm]
A summary of the total costs required to construct the pavement sections considered in the three scenarios are presented in Figure 8.18. For scenario #1, the cost was normalized and presented for each 100,000 ESALs trafficked. The total cost for constructing each pavement section was mostly driven by the pavement thickness, particularly the thickness of the HMA layer. The thicker the pavement structure, the higher the cost for excavation, site preparation, materials hauling, placement and compaction. When the normalized cost per traffic or the response benefits are taken into consideration, the largest savings in cost are expected when cement-stabilized base materials are constructed from QB blended with coarse recycled aggregates (FRAP or FRCA). Additionally, the cost of constructing similar thicknesses of cement-stabilized QB layers was lower than that of fly ash-stabilized layers, due to the larger percentage of fly ash used to achieve good performance (3% cement vs. 10% fly ash). Thus,
considering the measured field performance of the fly ash sections, their higher cost, and the variability in fly ash composition and performance; it is recommended that cement-stabilized QB applications are considered. In addition to their proven good field performance under heavy wheel loading, these applications can be sustainable and cheaper to construct.
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Figure 8.18 Summary of total costs for all LCCA scenarios. [1 in. = 25.4 mm]
8.10 SUMMARY
A comprehensive sustainability assessment of QB applications evaluated in Cell 2 and Cell 3 was conducted using Life Cycle Assessment (LCA) and a Life Cycle Cost Analysis (LCCA), and was presented in this chapter. The conducted LCA followed the structure recommended by ISO 14044:2006 and FHWA (Harvey et al., 2016). Three analysis scenarios were considered. The first scenario evaluated the cost and environmental impacts of the pavement test sections as constructed in Cells 2 and 3 by considering actual constructed layer thicknesses; measured at the end of performance monitoring from pavement test section trenching and HMA coring. The second scenario considered the as-designed thicknesses (4 in. or 102 mm of HMA and 12 in. or 305 mm of combined base and subbase) to eliminate the field variability in layer thicknesses during construction. The third scenario considered newly proposed thinner pavement structures (3 in. or 76 mm of HMA and 8 in. or 203 mm of combined base and subbase), to target applications for low volume roads.
Each of the three scenarios analyzed eight pavement sections with the QB material combinations studied in Cells 2 and 3. The first five test sections had a chemically stabilized QB or QB blended with recycled course aggregates base, the next two sections were inverted pavements, and section 8 had a conventional aggregate base. Actual field performance from the accelerated pavement testing were used to normalize sustainability impacts to present the results for each 100,000 ESALs trafficked in the first scenario. The response benefits, measured as the ratio of FWD maximum center deflection of the conventional pavement section to that of the section considered, were reported for each pavement section. The detailed results from the LCAs and LCCAs conducted for the materials and construction stages were discussed in this chapter. These results indicated that savings in costs and environmental impacts can be anticipated from constructing pavements with chemically stabilized QB base and subbase layers when the use stage benefits (i.e. higher traffic volumes and pavement lives) are accounted for.