Thin and Ultra-Thin Whitetopping (2004)

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10 The proper selection of candidate projects for UTW and TWT overlays is of paramount importance to their continued use as a viable rehabilitation alternative. This chapter discusses some of the issues related to proper project selection. Meth- ods to optimize the design and construction of the project will also be discussed. PROJECT SELECTION CRITERIA Traditionally, PCC has been perceived as a material for new pavement construction, in particular for heavy-duty pave- ments. However, with respect to pavement rehabilitation, agencies sometimes view HMA overlays as the first option, regardless of the existing pavement structure. It is within this environment that UTW and TWT overlays are gaining popularity. A number of agencies are beginning to use the same operational and economic criteria to evaluate both HMA and PCC overlay options. The barriers posed by traditional concepts of concrete as only a heavy-duty, long- term solution are coming down, and highway departments are recognizing that to best use their available resources, they must program a variety of projects meeting a range of service lives—a so-called “mix of fixes” (56). From the results of the survey, it was found that both the initial cost and longevity of the rehabilitation are the most important drivers in the decision-making process, with traffic control a close third. In order of decreasing importance, other factors included LCC, improvement to smoothness, improve- ment to texture (safety), geometric design, agency experience, contractor experience, and improvement to noise. For an existing HMA pavement nearing the end of its structural or functional life, an agency’s selection of the most appropriate rehabilitation alternative can be based on those criteria and a number of others. In many cases, the selection of the most appropriate strategy will be made by balancing several competing factors. Common factors include the fol- lowing (1,13,15,57): • Projected traffic loading • Existing pavement – condition – layer thicknesses – drainage • Costs – overlay construction cost – total LCC – user delay costs – vehicle operating costs • Time factors – number of construction operations – total construction time – repair and maintenance time – frequency of repair and maintenance – initial performance period • Corridor impact – noise level – excess pollution level – accident rate (vis-à-vis skid resistance) – ride quality (smoothness) • Material availability – cement – asphalt binder – aggregates • Contractors – availability (capacity) – experience – competition (number of bidders). Quite often, the design and constructability of the various overlay alternatives will contribute to many of the factors. Therefore, a need exists to assess some degree of engineer- ing design in the planning and selection stages. For example, one potential pitfall in selecting a UTW or TWT alternative is the result of the oversimplified charac- terization of the properties of the existing HMA pavement. Depending on these properties, the UTW or TWT can have a widely varied performance. It has been shown that all else being equal, the stiffness of the HMA layers can have a sig- nificant impact on the performance of these types of over- lays (2,48,49). Conversely, an HMA overlay is sometimes prone to the redevelopment of certain distresses, such as shov- ing at intersections. This consideration has been specifically cited in the survey responses, as well as in the literature, as a reason to select a whitetopping alternative (58). Another consideration that is commonly reported in the literature pertains to the benefits of a more reflective sur- face as a result of the lighter color of concrete. The increased CHAPTER THREE PROJECT SELECTION

11 value of money. Other considerations in the LCC analysis include maintenance costs and salvage value. Additional guid- ance on the particulars of LCC is available (62,66). Although LCC is most commonly used in planning to select the overlay type, Figure 4 illustrates how this technique can also assist in the strategy refinement process. In this exam- ple, the whitetopping thickness is the only variable different from alternative to alternative. As a result of the LCC pre- diction, a thickness of 150 mm (6 in.) is found to be optimum. A thinner section, although with a lower initial cost, has a higher NPV owing to the higher degree of maintenance asso- ciated with it. Conversely, a thicker section has a high initial cost that makes it prohibitive, even though its maintenance schedule is small. Although this example demonstrated that LCC can be used to optimize whitetopping thickness, it should be noted that it can also optimize more difficult and unconventional variables such as the joint spacing and fiber content, as shown in Fig- ures 5 and 6, respectively. In these examples, the LCCs of a number of discrete strategies are determined. As illustrated, a short joint spacing or high fiber content will have a high ini- tial cost. Conversely, a long joint spacing or low fiber con- tent may rapidly fail, increasing maintenance and decreasing user satisfaction. In both extremes, a high LCC will result. reflectivity has been reported to have a number of benefits, including • Increased reflection of headlights and aircraft landing lights, improving safety (59,60); • A lower demand for external lighting, reducing opera- tional costs (61); and • A cooling effect owing to lower absorption of solar energy, with environmental benefits (61). Finally, another reported benefit is the resistance to fuel spillage, which is a possible consideration in the construction of parking lots, fueling stations, and aircraft aprons (53). COST ANALYSIS The results of the user surveys collected during this synthe- sis have concurred with the literature that cost is the most sig- nificant decision-making criterion (56). LCC techniques are used by some agencies (27% of those surveyed) to compare alternative rehabilitation strategies on an equivalent basis (62). Because different overlay strategies can have a range of anticipated lives, two indicators of LCC are commonly used when comparing alternatives: • Net present value (NPV)—a total cost of the cash flow with all costs discounted to present-day equivalents in dollars. • Uniform Series Capital Recovery—an equivalent annual outlay of the alternative that, when discounted, is equal to the NPV of the alternative. This technique allows for a better comparison of unequal analysis periods. In many cases, LCC includes the prediction of both agency and user costs associated with the various analysis strategies (62). Although agency costs are straightforward to calculate, determining the costs to the users can be more involved. These costs can include delay, excess fuel and oil expenses, and even driver frustration. Potential societal costs such as noise, air quality, local business access, and others could also be considered. Methods of estimating the impacts include computer pro- grams such as MicroBENCOST, QUEWZ, TOMCAT, and others (63–65). These computer programs require knowl- edge of the lane closure configuration, estimated traffic flow, and other inputs to estimate the user costs in regard to delay, fuel, and oil. QUEWZ can also estimate excess emissions, as well as vehicle maintenance and depreciation costs (64). A discount rate (calculated from inflation and interest rates) is also needed in an LCC analysis to account for the time 75 100 125 150 175 200 225 Whitetopping Thickness (mm) N et P re se nt V al ue ($ /m 2 ) Maintenance User Cost Maintenance Agency Cost Initial User Cost Initial Agency Cost Joint Spacing (m) LC C ($/ m2 ) "Optimum" Joint Spacing (Lowest LCC) Long Joint Spacing (High LCC) Short Joint Spacing (High LCC) FIGURE 4 Example of LCC comparison of various whitetopping thickness strategies. FIGURE 5 Optimizing joint spacing using LCC.

12 The optimum of these factors can be found between the two extremes. Finally, it is worth noting that whereas consideration of LCCs continues to be encouraged, initial (construction) costs appear to be the dominant decision-making factor. However, by recognizing that UTW and TWT overlays do not neces- sarily require the same design and construction elements that a 30- to 50-year concrete pavement would require (e.g., stain- less steel dowels), one can institute cost-saving measures to improve the overall initial costs of this rehabilitation alterna- tive (67). Fiber Content (kg/m3) LC C ($/ m2 ) "Optimum"Fiber Dosage (Lowest LCC) High Fiber Dosage (High LCC) Low Fiber Dosage (High LCC) FIGURE 6 Optimizing fiber content using LCC.