Conclusions

All conclusions discussed below are predicated on the various assumptions and model parameters used to create the life-cycle cost profile for each infrastructure system. Most notably, the ECC system is modeled as having a service life double that of the CC

system. Therefore, all conclusions should be considered within the context of this study. 1. ECC outperforms CC across all life cycle cost stages and in overall life-cycle cost For each of the life cycle stages, the ECC system had lower costs than the CC system. While the ECC system has substantially greater unit volume material costs, it had lower costs in the material production and distribution stages compared to the CC system over the 60-year bridge service life. Thus, upfront costs are a poor indicator of long-term cost performance.

While the amount of construction time needed for a given construction activity is longer for ECC than for CC, construction stage costs were lower for the ECC system. This demonstrates the cost advantage of the ECC system because it requires fewer

construction activities and fewer total days of construction over the life of the bridge. The ECC system resulted in lower overall life-cycle costs than the CC system even after altering the annual average daily traffic flow rates (AADT) and including detours. 2. ECC has lower LCC regardless of discount rate tested, but cost advantage varies All sensitivity analyses on the agency and social discount rates showed that the ECC system had a lower life-cycle cost compared to the CC system. The ECC system showed a cost advantage ranging from about 10% to over 50% for each discount rate tested, except when a 7% discount rate was used on life-cycle costs from the base case; in this case, ECC had only a 2% cost advantage over ECC. Only when the discount rate for agency costs exceeded approximately eight percent did the CC system have lower costs. A discount rate of eight percent is more than double that of the traditional four percent

that is used for infrastructure projects and thus an extreme discount rate assumption is needed before the CC system becomes attractive from a cost standpoint.

3. Environmental and user costs were lower with ECC system

The emissions damage costs associated with an individual ECC link slab replacement are almost twice as great as those for a CC joint replacement. Over the course of the 60-year analysis period, however, there is less construction for the ECC system; as a result, using the ECC link slab enabled the ECC system to have lower agency activity and traffic emissions damage costs vis-à-vis the CC system.

Fewer days of construction for the ECC system allowed it to have lower user costs than the CC system. User delay, vehicle operating, and traffic crashes costs were all lower with the ECC system.

4. Use phase and social costs dominate life-cycle cost results

The majority of costs are accounted for by the use phase for both systems. The largest component of use phase costs is user costs, rather than costs resulting from emissions. These results reiterate the importance of including all social costs when conducting a life- cycle cost analysis. Among the user costs, vehicle operating costs related to traffic congestion dominated, highlighting the importance of minimizing construction time. 5. Emissions damage costs were driven by CO2, but had little impact on final results

Emissions damage costs had a small impact on final results, representing only 1-3% of total life-cycle costs across the five scenarios. Aside from CO2, the per-ton emissions damage cost for each pollutant would need to be increased greatly to have even a one percent impact on final results. The emissions damage cost for CO2, however, needed only to be raised by $3, from $6, to have a one percent impact on final results; this indicates that among the pollutants, CO2 played the largest role in driving emissions damage costs.

6. Agency costs are a rough indicator for construction activity emissions While the deck replacement construction activity required the most materials and construction time, absorbing approximately 60% of the agency costs for both systems, it accounted for approximately 70% of the agency activity emissions damage costs for both systems. Conversely, the deck resurfacing accounted for about 30% of the agency costs for both systems and was responsible for 17% of the agency activity emissions damage costs. Emissions costs due to a construction activity can be estimated only very roughly by using agency activity costs as a proxy; a life-cycle costs analysis is required.

7. AADT assumptions can have major effects on life-cycle costs, while detours play a smaller role

When the AADT traffic flow figure was varied, life-cycle costs showed significant changes. With each percentage increase, life-cycle costs grew exponentially. These results assumed that no action was taken to alleviate the traffic, which does not reflect how decision-makers would respond in reality. Nonetheless, this analysis highlights the impact that AADT growth can have over the course of the life cycle.

When detours were factored into the model, life cycle costs fell, although not by a substantial amount. It is interesting to note, however, that, as the percentage of cars assumed to pursue detours increased, user costs decreased because user delay was lowered. In addition, emissions damage costs decreased because detour cars traveled at speeds that were more efficient, with respect to CO2 emissions, than the vehicles

traveling over the bridge deck. Parties only focusing on one of these two types of costs could come to different conclusions about whether detours are a desirable result. Using a comprehensive life-cycle cost model accounts for all costs associated with detours and can help provide information on the overall cost-effectiveness of infrastructure

alternatives.