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OCR for page 90
90 Light reflection/retro-reflection and glare--High levels of The fine aggregate to be used in the concrete mix is a blend macro-texture help break up possible water levels in the of natural and manufactured sand, and the coarse aggregate wheel tracks (Sandberg, 1998). is crushed limestone. Historical data on the polishing char- acteristics of the blended fine aggregate indicate a long-term DFT(20) of 60 is expected. Economics Grinding or grooving of the pavement post-construction The final assessment of feasible textures involves costs-- is not permitted; there is no local experience with porous both the initial cost of constructing the texture and its long- concrete and EAC. term or life-cycle cost. Rough estimates of the unit costs asso- Tire whine complaints were reported in the recent past and ciated with constructing the various texturing options on new should be avoided. concrete pavements and re-texturing options for existing pavements are provided in Chapter 2. Examination of the Steps 1 and 2: The information in Table 6-1 indicates that costs associated with new textures indicates a substantial dif- the project is best represented by friction design category B. ference in cost between traditional formed textures (drags The IFI F(60) minimum friction level for this category is 32, and/or tines) and the more labor- and technology-intensive but because of the wet environment, a minimum friction cut textures (ground or grooved), exposed aggregate textures, of 36 would be desired. For this value and the long-term and porous concrete. For re-texturing, there appears to be a DFT(20) value of 60, and considering the exclusion of cer- basic cost advantage to shot-abrading over grinding, grooving, tain texturing methods, Table 6-3 indicates that transverse and thin resurfacing with asphalt mixes. However, depending tining, transverse skewed tining, and longitudinal tining are on the depth of diamond grinding and the hardness of the the most feasible options. aggregate (which affects spacing), grinding costs could be Step 3: Because the project is in a noise-sensitive environ- equal to or less than alternatives. ment and considering the project's traffic characteristics, The initial costs provided in Chapter 2 can be used as part Table 6-5 indicates that the selected texture must reduce exte- of a pavement evaluation strategy that considers the design rior noise to the 100 to 102 dB(A) range (qualitative noise life and projected maintenance and rehabilitation activities. level A). Table 6-6 shows that, of the three friction-based fea- Life-cycle cost analysis (LCCA) techniques, such as net pres- sible textures, only longitudinal tining with certain features/ ent value (NPV) and equivalent uniform annual cost (EUAC), dimensions will meet the criteria--specifically, narrowly spaced may be used to identify the texture(s) with the lowest life- grooves of shallow or standard depth. cycle costs. Based on information provided earlier in Table 2-2, 0.75-in. (19-mm) spaced longitudinal tines with shallow groove depths can be expected to provide an MTD value of about 0.03 in. Example Application (0.8 mm). For this value and the long-term DFT(20) value of of Texture Selection Process 60, Figure 6-6 indicates that this texture just barely meets the This section provides an example to illustrate the application minimum IFI F(60 ) = 36 criterion. A standard-depth longitu- of the texture selection process. The example given is for a proj- dinal tine with slightly higher MTD value would better satisfy ect involving the reconstruction (using PCC) of a four-lane this criterion. Thus, both textures would be considered as the freeway with the following features: final feasible options and would be evaluated for other surface characteristics and economics in Step 4. The project is located in a suburb of a large city in a wet non- freeze climate (annual precipitation >55 in. [1,400 mm]). Texture Construction Specifications The land adjacent to the highway facility is mostly a mix of and Practices professional buildings and residential subdivisions. The current two-way ADT is approximately 35,000 veh/day Appendix F, available on line at the TRB website, contains and, although there are occasions of congested traffic flow, sample guide specifications for the following selected group of most of the time, traffic is in free flow condition at the concrete textures that provide good friction and noise charac- posted speed limit of 55 mi/hr (89 km/hr). teristics on high-speed pavements: The percentage of heavy commercial trucks that use the facility is estimated to be 12 percent. Heavy turf drag The freeway has partially controlled access, with inter- Transverse skewed variable tine changes every 1 to 1.5 miles (1.6 to 2.4 km). Longitudinal tine The terrain is mildly flat; there are no major horizontal Longitudinal diamond grind curves. Longitudinal groove.

OCR for page 90
91 Successfully constructing these textures requires great quartzite) requires more time and effort than projects with attention to detail to both the materials production and con- softer aggregate, such as limestone (Correa and Wong, 2001). struction processes. Good QC procedures combined with a statistically based QA program will help ensure that the Curing and Protection--For drag and tine textures, imme- as-built texture provides the friction and noise characteristics diate application of curing compound or membrane following for which it was designed. Therefore, when specifying the the texturing operation is essential to achieve good pavement depth of grooves and/or texture depth (as measured by the surface durability. If the pavement cures too quickly, the mor- sand patch method, CT Meter, or other texture devices), it is tar forming the texture ridges will not set properly, its durabil- important to account for the expected loss of macro-texture ity will be reduced, and its friction (and noise) properties will over time/traffic. Important considerations in constructing be diminished more quickly (FAA, 2004). Generally, curing the selected textures successfully follow: compounds can be applied earlier for longitudinal dragging and Mix Workability--For drag and tine textures, uniform con- tining operations than for transverse tining operations. crete slump that is not too dry (workable mix) must be main- Quality Control (QC)--Continuous evaluation and mea- tained throughout the paving process. Slight adjustments to surement of groove dimensions created by tining will help the mix (within the limits of specified concrete mix), such as identify and correct deviations from the design profile. Random increasing the slump, adjusting the sand content, or adding a checks of depth may be made using a tire tread depth gauge retarder, may be required to achieve the desired workability. or similar tool together with visual checks of the amount of Texturing Operations-- mortar deposited on the surface by the tining operation and the straightness and width of the grooves; deeper tine pene- Drag and tine equipment (preferably a tine and cure trations generally result in more ragged and widened (at the machine) should allow the operator to maintain a consis- top) grooves. tent distance behind the paving and finishing operations, Quality Assurance (QA)--Groove and/or texture depth apply the proper amount of pressure (uniformly over the measurements on hardened concrete should be made to deter- width of the paving) on the drag and/or tine assemblies, mine compliance with texture specifications. The measure- hinge the tine rake to optimize the angle of tine insertion, ments should be made at random locations throughout a and have the capability to water-mist the surface. paving run (or lot) at the earliest possible time following the tex- Drag and tine operators should be capable of monitoring turing operation. The surface at the locations of testing should texturing characteristics closely and making proper adjust- be wire brushed or lightly scraped with a steel straightedge to ments in response to site conditions (e.g., changes in mix remove all mortar deposits that could affect the measurements. consistency, rapid drying of the mix due to high winds Structural Design Considerations--Because diamond grind- and/or temperatures, delays in the paving and finishing ing generally reduces slab thickness by 0.19 to 0.25 in. (4 to operations, and buildup of mortar on the drag and/or tines). 6 mm), it can influence the cracking potential of a concrete Timing of the texturing operation is critical: texturing too pavement. This is particularly true if the grinding is performed early may result in grooves filling up with mortar or surface shortly after construction to serve as the initial surface texture. tearing, and texturing too late may result in reduced groove Research has indicated that a 0.25-in. (6-mm) reduction in slab depth (Iowa DOT, 2007). thickness can result in roughly a 30% reduction in fatigue life For heavy turf drags, the potential for significant mortar (Rao et al., 1998). Thus, where diamond grinding is to be used build-up and release should be considered because this can as the initial texture, measures should be considered to offset influence the surface profile and increase roughness. this effect (e.g., increased thickness or strength requirements). The speed of diamond grinding operations will be influ- Diamond grinding of an older pavement has less effect on enced by the hardness of the aggregate and the depth of cut. fatigue life because of the strength gain with time (typically Grinding of pavements with extremely hard aggregate (e.g., 20% higher than the design strength).