6-48 Section 661
ULTRA- THIN BONDED WEARING COURSE
661-1 Description
Produce and place an Ultra- thin Bonded Wearing Course (UBWC), including an application of a warm Polymer -Modified Emulsion Membrane (PMEM) followed immediately with an UBW C hot mix asphalt overlay. Spray PMEM immediately before applying hot mix asphalt. The Contractor may elect to use an alternate method for the placement of the UBWC. As an alternate to spraying PMEM prior to placement of the asphalt mix with a spray paver , the Contractor may use a non -tracking hot -applied polymer asphalt tack coat material prior to placement of the asphalt mix. If the alternate method is selected, submit documentation and proposed plan to the Engineer prior to beginning any work. Provi de and conduct the QC and required testing for acceptance of the UBWC in accordance with the contract.
661-2 Materials
Refer to Division 10. Item Section Anti-strip Additives 1020 -8 Coarse Aggregate 1012 -1(B) Fine Aggregate 1012 -1(C) Mineral Filler 1012 -1(D) Polymer Modified Asphalt Binder 1020 -2 Polymer -Modified Emulsion Membrane (PMEM) 1020 -4 Use either PG 70 -28 or PG 76 -22 binder conforming to Section 620 in the asphalt mix design. Ensure that t he asphalt binder is compatible with the PMEM (or alternate) and the existing pavement. Use an approved non-tracking hot applied (NTHA) polymer asphalt tack coat found on NCDOT’s APL for Non-Tracking Asphalt Tack Coat - Hot Applied (NTHA) .
661-3 Composition of Mix
Do not use crystalline limestone, crystalline -dolomitic limestone or marble for aggregates and do not use reclaimed asphalt pavement ( RAP ). Use a mixture of coarse and fine aggregate, asphalt binder, mineral filler and other additives when required. Size, uniformly grade and combine in such proportions such that the resulting mixture meets the gradation and physical requirements of Table s 661-1 and 661- 2. At least 20 days before start of asphalt mix production, submit the mix design and proposed JMF targets for each required mix type and combination of aggregates to the Engineer for review and approval. Establish the JMF target values within the mix des ign criteria specified in Table 661 -2 for the particular type mixture. Determine and certify compatibility of all asphalt emulsion, asphalt binder and aggregate components.
6-49 Table 661-1
UBWC GRADATION CRITERIA Sieves (mm) % Passing by Weight 12.5 100 9.50 85 - 100 4.75 28 - 2.36 17 - 1.18 13 - 0.600 8 - 0.300 6 - 0.150 4 - 0.075 3.0 - 7.0 TABLE 661-2 UBWC MIX DESIGN CRITERIA Property Requirement Asphalt Content, % 5.0 (minimum) Draindown Test, AASHTO T 305 0.1% max Moisture Sensitivity, AASHTO T 283A 85% min Application Rate, lb/sy 70 lb/sy Approximate Application Depth, in. 5/8" Asphalt PG Grade, AASHTO M 320 PG 70 -28 or PG 76 -22
A.Specimens for AASHTO T 283 testing are to be compacted using the gyratory compactor. The mixtures shall be compacted using 100 gyrations to achieve specimens approximately 95 mm in height. Use mixture and compaction temperatures recommended by the binder supplier.
661-4 Construction Methods
A.Equipment Use asphalt mixing plants in accordance with Article 610 -5. F urnish paving machine with the following capabilities:
1.Self-priming paving machine capable of spraying the PMEM , applying the hot asphalt concrete overlay and screeding the surface of the mat to the required profi le and cross section in one pass at any rate between 30 and 92 feet per minute .
2.Receiving hopper, feed conveyor, storage tank for PMEM material, PMEM emulsion single variable -width spray bar and a variable width, heated, vibratory- tamping bar screed.
3.Screed with the ability to be crowned at the center both positively and negatively and have vertically and horizontally adjustable extensions to accommodate the desired pavement profile and widths.
4.Sprayer system capable of accurately and continuously monitoring the rate of spray and providing a uniform application across the entire width to be overlaid.
5.Use pavers equipped with an electronic screed control that will automatically control the longitudinal profile and cross slope of the pavement. Control the longitudinal profile through the use of either a mobile grade reference(s), including mechanical, sonic and laser grade sensing and avera ging devices, an erected string line(s) when specified, joint matching shoe(s), slope control devices or the approved methods or combination of methods. Unless otherwise specified, use a mobile grade reference system capable of averaging the existing grade or pavement profile over at least a 30 feet distance ; or by non-contacting laser or sonar type ski with at least three referencing stations mounted on the paver at a minimum length of 24
6-50 feet. Establish the position of the reference system such that the average profile
grade is established at the approximate midpoint of the system. The transverse cross slope shall be controll ed as directed by the Engineer. Use an erected fixed stringline for both and longitudinal profile and cross slope control when required by the contract. When an erected fixed string line is required, furnish and erect the necessary guide line for the equipment. Support the stringline with grade stakes placed at maximum intervals of 25 f eet for the finished pavement grade. Use the 3 0 feet minimum length mobile grade reference system or the non- contacting laser or sonar type ski with at least three referencing stations mounted on the paver at a minimum length of 24 f eet to control the longitudinal profile when placing the initial lanes and all adjacent lanes of all layers, including resurfacing and asphalt in- lays, unless other wise specified or approved by the Engineer . A joint matching device (short 6 inch shoes) may be used only when approved by the Engineer . Use the automatic slope control system unless otherwise approved by the Engineer . The Engineer may waive the use of automatic slope controls in areas where the existing surface (subgrade, base, asphalt layer, etc.) exhibits the desired cross slope of the final surface. The Engineer may also waive the use of automatic slope controls in areas where the use of such equipment is impractical due to irregular shape or cross section (such as resurfacing). When the use of the automatic slope controls is waived, the Engineer may require the use of mobile grade references on either or both sides of the paver. Manual screed operation will be permitted in the construction of irregularly shaped and minor areas, subject to approval. Waiver of the use of automatic screed controls does not relieve the Contractor of achieving plan profile grades and cross slopes. In the case of malfunction of the automatic screed control equipment, the paver may be manually operated for the remainder of the workday provided this method of o peration produces acceptable results. Do not resume work thereafter until the automatic system is functional. The Engineer will waive the requirement for use of pavers for spreading and finishing where irregularities or obstacles make their use impractical. Spread, rake and lute the mixture by hand methods or other approved methods in these areas. Operate the paver as continuously as possible. Pave intersections, auxiliary lanes and other irregular areas after the main line roadway has been paved, unless otherwise approved by the Engineer . Compact the wearing course with a steel double drum asphalt roller(s) with a minimum weight of 10 tons. Maintain rollers in reliable operating condition and equip with functioning water system and scrapers to prevent a dhesion of the fresh mix onto the roller drums. Supply adequate roller units and compact promptly following the placement of the material. Request approval of equipment before the start of any work. Maintain all equipment and tools in satisfactory workin g condition at all times.
B.Surface Preparation Perform the following items before the com mencement of paving operations.
1.Protect and cover manhole covers, drains, grates catch basins and other such utility structures with plastic or building felt before paving and reference for location and adjustment after paving.
2.Remove thermoplastic traffic markings symbols, characters or other markings greater than 1/4 inch in thic kness on the existing pavement.
6-51 (3) Clean and completely fill pavement cracks and joints greate r than 1/4 inch wide. Do
not overband the existing cracks and joints. Apply sealant per manufacturer’s recommendation.
4.Fill surface irregularities greater than 1 inch deep with a material approved by the Engineer.
5.Thoroughly clean the entire pavement surface, giving specific attention to accumulated mud and debris. Pressurized water and/or vacuum systems may be required to ensure a clean surface.
C.Application of Ultra -thin Bonded Wearing Course Produce, transport to the site and place the UBWC in accordance with Section 610, except as otherwise provided below. Use only one asphalt binder PG grade for the entire project, unless the Engineer gives written approval. Do not place UBWC between October 31 and April 1 and when the air and surface temperature is less than 60°F. Apply the UBWC mixture at the rate per square yard as shown in Table 661- 2 for the mix type shown in the plans. Spray the PMEM at a temperature of 140 °F to 180°F. Provide a uniform application across the entire width. Use a target application rate of 0.20 gal/sy and adjust according to the mix design, existing pavement type and condition for the specified project , and the manufacturer’s r ecommendations . Ensure the rate of application is approved by the Engineer before beginning work. Do not allow wheels or other parts of the paving machine to touch the PMEM before the hot mix asphalt concrete wearin g course is applied. Place the hot asphal t concrete wearing course over the full width of the PMEM . Apply the hot mix asphalt concrete at a temperature of 300 °F to 330°F and within a maximum of 3 seconds immediately after the application of the membrane. Before opening to traffic, allow the pavement to sufficiently cool after the rolling operation to resist damage to the pavement. For the alternate method, use distributor equipment to uniformly place the non- tracking hot applied polymer asphalt tack coat i n accordance with Section 605 and shall be applied at a temperature in accordance with the manufacturer’s recommendations and at a target residual application rate of 0.12 ± 0.02 gal /sy. For placing the asphalt mix, use of a spray paver is not required.
D.Compaction Compact the wearing course with at least two passes of a steel double drum asphalt roller before the material temperature has fallen below 185°F. Do not allow the rollers to remain stationary on the freshly placed asphalt concrete. Compact immed iately following the placement of UBWC. A release agent (added to the water system) may be required to prevent adhesion of the fresh mix to the roller drum and wheels. Compact in the static mode.
6-52 661-5 Quality Management System for Asphalt Pavements
Produce the ultra -thin hot mix asphalt in accordance with Section 609 with the following exceptions . Sample and test the completed mixture from each mix design per plant per year at the following minimum frequency during mix production: Accumulative Production Increment Number of Samples per Increment 500 tons Record the following data on the standardized control charts and in accordance with the requirements of Section 7.4 of the Asphalt QMS Manual :
a.Aggregate Gradation Test Results:
1.2.36 mm
2.0.075 mm Sieves
b.Binder Content, %, P b
661-6 Measurement and Payment
Ultra -thin Bonded Wearing Course will be measured and paid by the actual number of tons of mixture incorporated into the completed and accepted work. The hot mix asphalt pav ement will be measured by being weighed in trucks on certified platform scales or other certified weighing devices. Application of Ultra -thin Hot Mix Asphalt shall be included in the per ton pay item for Ultra -thin Bonded Wearing Course . Polymer Modified Asphalt Binder f or Plant Mix will be paid in accordance with Article 620-4. Asphalt binder price adjustments when applicable will be based on Grade PG 64- 22, regardless of the grade used. Where PG 76 -22 is being used in the production of ultra -thin, the grade of asphalt binder to be paid will be PG 70 -28, unless otherwise approved by the Engineer . For the alternate method, Ultra -thin Bonded Wearing Course will be measured and paid by the actual number of tons of mixture incorporated into the completed and accepted work. The hot mix asphalt pavement will be measured by being weighed in trucks on certified platform scales or other certified weighing devices . Non -tracking hot applied polymer asphalt tack coat shall be included in the per ton pay item for Ultra -thin Bonded Wearing Course . No other pay item shall be associated with this alternate method. The above prices and payments will be full compensation for all work covered by this section 29 including, but not limited to, furnishing all materials, producing, weighing, transporting, placing and compacting the polymer modified asphalt emulsion; maintaining the ultra -thin bonded wearing course until final ac ceptance of the project; performing QC as specified in the contract; and making any repairs or corrections to the surface of the pavement or adjacent landscape that may become necessary. Payment will be made under: Pay Item Pay Unit Ultra -thin Bonded Wea ring Course Ton
Source: North Carolina Standard Specifications for Roads and Structures, 2024 Edition. Pages 304–308 of 857.