6-34 Section 650
OPEN -GRADED ASPHALT FRICTI ON COURSE
650-1 Description
Perform the work covered by this section including, but not limited to , construction of a plant mixed open -graded asphalt friction course (OGFC) properly laid upon a prepared surface in accordance with these s pecifications and in conformity with the lines, grades, thi ckness and typical sections shown on the plans; producing, weighing, transporting, placing and rolling the plant mix as specified in Section 610; furnishing the asphalt binder, anti -strip additive, fiber stabilizing additive , warm mix asphalt (WMA) additive and all other materials for the plant mix; furnishing and applying tack coat as specified; providing QC as specified in Section 609 as modified for OGFC; surface testing of the completed pavement; furnishing scales; making any repairs or correctio ns to the friction course that may become necessary and maintaining the friction course until final acceptance of the project.
650-2 Materials
Refer to Division 10. Item Secti on Anti-strip Additive s 1020 -8 Asphalt Binder, Performance Grade 1020 -2 Coarse Aggregate 1012 -1(B) Fiber Stabilizing Additives 1020 -10 Fine Aggregate 1012 -1(C) Mineral Filler 1012 -1(D) Reclaimed Asphalt Shingles (RAS) 1012 -1(E)
650-3 Composition of Mixture (Mix Design and Job Mix Formula)
A.General Design the open -graded asphalt friction course using a mixture of coarse and fine aggregate, asphalt binder, mineral filler, fiber stabilizing additive and other additives as required to produce a mix meeting Table 650 -1. 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. The mix design shall be prepared by a mix design technician approved by the Department in an approved mix design laboratory. Prepare the mix design in accordance with Article 610- 3 and the Department’s mix design procedures as found in Section 4 of the Asphalt QMS Manual . The mix design and JMF target values will be established within the mix design criteria specified in Table 650 -1 for the particular type of mixture to be produced.
B.Mix Design Criteria Design open- graded asphalt friction course (OGFC) mixtures conforming to the gradation requirements and other mix design criteria in Table 650 -1 for the mix type specified. Use the asphalt b inder grade shown in Table 650- 1 for the mix type specified. Reclaimed Asphalt Shingles (RAS ) may be used in accordance with Subarticle 610 -3(A). Use an anti -strip additive in all OGFC mixes. It may be hydrated lime or a chemical additive or both. Add chemical anti -strip additive at a rate of 0.5% by weight of asphalt binder. Add hydrated lime at a rate of 1.0% by weight of dry aggregate. Use an approved source and grade. Add the anti -strip additive to the asphalt binder in accordance with Articl e 620- 3.
6-35 If needed to prevent asphalt draindown, i ncorporate a stabilizing additive into all OGFC
mixes . Stabilizing additives may be fibers meeting Article 1020 -10, RAS, or an approved WMA additive. Add fiber or RAS at a dosage rate by weight of total mixture as approved by the Engineer . Add WMA additive at a dosage rate by weight of binder in the mix. If a WMA additive is used to prevent draindown, produce OGFC at a lower mix temperature as shown on the approved JMF. In addition to the required mix design submittal, the Contractor shall prepare and deliver gyratory compactor specimens to the Department’s Central Asphalt Laboratory for Cantabro durability testing. The Contractor shall prepare these specimens using lab produced mix in accordance with NCDOT procedures. Provide the samples at least 20 days before the anticipated beginning placement of OG FC mixture. TABLE 650-1 OGFC DESIGN CRITERIA Grading Requirements Total Percent Passing Sieve Size (mm) Type FC -1 Modified 19.0 - 12.5 100 9.50 75 - 100 4.75 25 - 2.36 5 - 0.075 1.0 - 3.0 Asphalt Binder Grade PG 76 -22 Binder Content, % 5.5 - 8.0 Mixing Temperature at the Asphalt PlantA 300 – 325°F Air Voids, % minimum 18.0 Cantabro Loss, % maximum 20.0 Draindown, % maximum 0.3
A.The JMF mix temperature shall be within the ranges shown unless otherwise approved by the Engineer .
650-4 Plant Equipment
Use plant equipment in accordance with Article 610 -5 and the requirements herein. When fiber stabilizing additives are used as an ingredient of the mixture, use a separate feed system capable of accurately proportioning the required quantity into the mixture and in such a manner that uniform distribution will be obtained. Interlock the proportioning device with the aggregate feed or weigh system so as to maintain the correct proportions for all rates of production and batch sizes. Accurately control the proportion of fibers to within ± 10% of the amount required. Provide flow indicators or sensing devices for the fiber system that are interlocked with plant controls such that mixture production will be interrupted if introduction of the fiber fails. When a batch type plant is used, add the fiber to the aggregate in the weigh hopper or as approved by the Engineer . Increase the batch dry mixing time by 8 to 12 seconds, or as directed by the Engineer , to assure the fibers are uniformly distributed before the injection of asphalt binder into the mixer. When a continuous mix or dryer -drum t ype plant is used, add the fiber to the aggregate and uniformly disperse at the point of injection of asphalt binder. Add the fiber in such a manner that it will not become entrained in the exhaust system of the drier or plant.
6-36 650-5 Construction Methods
Produce, transport to the site and place the OGFC in accordance with Section 610, except as otherwise provided below. Do not place OGFC between October 31 and April 1 of the next year, unless otherwise approved by the Engineer . The minimum air and road surface temperature for placing Type FC-1 Modified mix will be 60 °F. Before starting production of the mix, stockpile all aggregates for a sufficient period of time to facilitate the drainage of free moisture. Clean the existing surface in an acceptable manner before placement of any asphalt material. Remove all existing raised pavement markers as directed by the Engineer and repair any damaged areas caused by the removal. Use an approved dense graded mixture of similar type material for the repair. Apply tack coat in accordance with Section 605 and the following:
A.Use Asphalt Binder Grade PG 58 -28, Grade PG 64 -22 tack coa t material or an approved non-tracking hot applied (NTHA) asphalt tack coat material .
B.Uniformly apply the asphalt binder tack coat material at a n applied rate of0.06 to 0.08 gal/sy, or as directed by the Engineer . Spread and finish the friction course as s pecified in Article 610 -8. Roll the friction course as specified in Article 610 -9. Perform this work in accordance with and using equipment meeting Section 9.5 of the Asphalt QMS Manual. Use a Material Transfer Vehicle (MTV) when placing all types of OGFC. Use a MTV meeting Section 9.5.1 (E) of the Asphalt QMS Manual. Remove and replace any part of the finished friction course that shows non- uniform distribution of asphalt binder, aggregate or fiber at no additional cost to the Department. Coordinate plant production, transportation and paving operations such that uniform continuity of operation is maintained. If spreading operations are interrupted, the Engineer may require that a transverse joint be constructed any time the mixture immediately behind the p aver screed cools to less than 250 ºF. For end of project joints, provide a transition area consisting of one load of mixture per lane, or as directed by the Engineer. Taper the mixture in thickness from 3/8 inch at the end of the project to the typical thickness (approximately 3/4 inch) within the maximum distance of spread for one load of mixture. For ramps and gore areas, taper the mixture in thickness from that at the edge of the mainline, approximately 3/4 inch to 3/8 inch at the point of the ramp transverse joint. Construct the ramp transverse joint at a point specified by the plans or as directed by the Engineer .
650-6 Quality Management System
Produce the OGFC 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 Resul ts:
Source: North Carolina Standard Specifications for Roads and Structures, 2024 Edition. Pages 290–292 of 857.