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North Dakota Demonstration Project: Whitetopping on U.S. 2 West of Rugby
A key aspect of HfL demonstration projects is quantifying, as much as possible, the value of the innovations deployed. This entails comparing the benefits and costs associated with the innovative project delivery approach (as-built) adopted on an HfL project with those from a more traditional delivery approach (baseline) on a project of similar size and scope.
For the as-built scenario the actual construction time needed to mill, pave, and install turn lanes was only 34 days. For the baseline scenario, the construction time is based on a NDDOT time estimate for conventional pavement rehabilitation using North Dakota's mine and blend bituminous base-hot bituminous paving method or mine-and-blend method. An equivalent mine-and-blend design of the type constructed on US 2 (5.5 in of HMA on 18 in of granular base, assuming 5 in of the original base could be reused and 13 in of new base placed on top) would have resulted in 53 days. Using this information the total number of days saved during paving operations is 53 – 34 = 19 days. Table 2 details the mine-and-blend calculation.
A conventional mine-and-blend overlay NDDOT project (AC-HPP-NH-3-281(091)112 PCN#16385) had a similar scope (paving, earthwork, widening, etc.) and was constructed during the same year on U.S. 281 between Carrington and New Rockford. The conventional project serves as a suitable cost comparison, and the following information is a subjective analysis of the likely cost differential rather than a rigorous computation of the cost differential. The conventional project covered 13.07 mi (20.94 km) of two-lane highway and had a $6,764,162 contract value. Prorating the contract value to an 8.24-mi project length equates to $4,686,077, of which $3,213,381 was for items related to paving and traffic control. In comparison, $4,691,765 of the $7,670,203 as-built project was for paving and traffic control. The pavement construction cost for the whitetopping was $1,478,384 higher than the conventional overlay. Table 3 presents a breakdown of the baseline and as-built contract costs.
Generally, three categories of user costs are considered in an economic or life cycle cost analysis: vehicle operating costs, delay costs, and crash and safety-related costs. The user cost for this analysis focused on the delay cost to identify the differences between the as-built and baseline alternative. During construction the speed limit was reduced from 70 mi/h (112.6 km/h) to 60 mi/h (96.6 km/h) for the head-to-head detour on the eastbound lanes. NDDOT provided the delay time and cost information in table 4, which shows that the added time to travel the detour increased travel time by 1.1 minutes and the total user cost per day was $1,733. The benefit to the traveling public was $32,927. The benefit was essentially the cost saved per day for each day of construction reduced by the innovative approach.
Initial Cost Summary
Life Cycle Cost Analysis
A life cycle cost analysis (LCCA) based on a 4 percent discount rate and present worth method was performed to provide a detailed context to further compare the as-built and baseline projects. A deterministic approach was used to examine construction and future maintenance costs over the 20-year service life criteria chosen by NDDOT in the original design process. Both the agency costs and user costs are examined in this LCCA.
LCCA Agency Costs
Agency costs considered are those costs NDDOT is directly responsible for and include the cost of materials, labor, equipment, and traffic control necessary to construct and maintain the pavement over the service life.
The type, scheduling, and cost of maintenance and repair (M&R) of the baseline HMA pavement is based on NDDOT's experience with similar highways in its geographic region. After initial construction, NDDOT expects to seal cracks in 2 to 3 years and every other year thereafter, followed by a slurry seal in 3 years and again in 7 to 8 years. In 15 to 20 years NDDOT anticipates the need for a thin (3-in) HMA overlay. NDDOT's maintenance cost estimates for these activities are table 5.
In North Dakota, engineers have ample experience with maintaining HMA pavements, but only limited experience with maintaining whitetopping pavements over long time periods. Other agencies, such as CDOT, have a well-established and -documented history with highway whitetopping projects, which indicates that at year 20 rehabilitation may include diamond grinding to restore smoothness and repair of 1 percent of the pavement panels. This would place rehabilitation beyond the service period for this analysis. A growing consensus of DOTs suggests that whitetopping M&R recommendations are similar to conventional concrete pavements. NDDOT anticipates only minimal distresses and no additional maintenance costs to the district's budget.
LCCA User Costs
User costs during construction were considered in this LCCA and were a function of the vehicle operating cost from table 4 and the additional time needed to travel though the work zones associated with M&R activities. Free-flowing traffic conditions were assumed to be present through work zones, given that no observed backups occurred on the head-to-head detour during the as-built construction. Similar free-flowing conditions would likely exist while crews executed M&R activities and constructed the alternate baseline pavement when traffic would be maintained on the adjacent lane. During these periods, a work zone speed limit of 45 mi/h (72.4 km/h) was used in the analysis.
The slurry seal application was expected to take no longer than a day. Overlay operations would take longer because of the amount of HMA to be placed. To overlay the mainline lanes, the project would require about 19,317 tons of HMA, calculated as follows:
amount of HMA = project length X lane width X overlay thickness X unit weight of HMAwhere:
project length = 8.24 mi x 5,280 ft/mi = 43,507 ft
Assuming a typical paving crew can place 1,700 tons/day, the overlay operation would last 12 days (19,317 tons/1,700 tons/day).
The agency and user costs and the timing of these costs were combined to formulate a projected expenditure stream for the as-built and baseline pavement designs. The FHWA RealCost software was used to calculate the anticipated net present value (NPV) of future costs of the expenditure stream through the use of the discount rate, allowing for a direct dollar-for-dollar comparison. The salvage value of each alternative is included in the analysis and represents the value of the remaining useful service life of the initial construction and the remaining usefulness of the last M&R activity. Life cycle cost NPV was calculated as follows:
NPV = net present value, $
The LCCA shows the whitetopping pavement to be within $83,067 or 1.8 percent of the baseline option in combined agency costs and user costs during construction.
The initial summary of construction costs and user delay costs indicates that whitetopping costs more than traditional construction. However, a closer look at the agency costs and user costs during initial construction and M&R activities suggests these costs differ by 1.8 percent. The narrow LCCA differential is considered insignificant given the extent of variables in the analysis. The actual cost saving realized by the whitetopping resides in the user cost savings during initial construction of $32,927 by delivering the project in less time than traditional construction.
G. Lowery, "Life Cycle Cost Analysis of Thin Whitetopping," Proceedings of the International Conference on Best Practices for Ultrathin and Thin Whitetoppings, Denver, CO, 2005.R. Rasmussen and D. Rozycki, Thin and Ultrathin Whitetopping, National Cooperative Highway Research Program, Synthesis 338, Washington DC, 2004.