413.3 Ultrathin Bonded Asphalt Wearing Surface
413.3.1 Material Inspection
Aggregate for use in surface treatments shall be inspected in accordance with 1001 General Requirements for Material. Asphalt Binder for use in surface treatments shall be inspected in accordance with 1015 Bituminous Material.
Mix Design Procedure
In order for an Ultrathin Bonded Asphalt Wearing Surface mix formula to be approved, the contractor’s proposed job mix formula (JMF) shall be submitted as required in Standard Specification Sec 418.104.22.168. The time for approval starts when the completed design is delivered to the District. This time restarts when the District receives information omitted from the original JMF or corrected by the contractor. Review/verification time includes District and Central Office processing, therefore, each mix should be processed as soon as possible. Trial mix samples shall be obtained and submitted to the Central Laboratory in accordance with Field Section 1001. When possible, the JMF and correspondence should be transmitted electronically. The Materials Field Office e-mail address is MFO.
Field Office Procedure
The Materials Field Office is charged with the responsibility of processing the mix formula. General procedures for processing an ultra thin bonded asphalt wearing surface mix formula are as follows:
- a. A letter from a District requesting a mix with a copy of the contractor's JMF and letter is received.
- b. Contract specifications for the project are checked for necessary items.
- c. Grade of asphalt as well as the refinery to be used and the percent asphalt recommended are reviewed.
- d. Gradations of the aggregates are checked for specification compliance and compared with the gradations determined by the Laboratory.
- e. All calculations on the proposed JMF are checked.
- f. For verification, a trial mix is prepared and submitted to the Laboratory.
- g. When Central Laboratory tests are completed, the results are compared to the contractor’s and against the specifications. If the mixes tested cannot be used, the mixture will be rejected.
413.3.2 Laboratory Procedure
Test results and calculations required for Ultrathin bonded asphalt wearing surface trial mixtures are as shown in paragraphs 413.2.1 through 413.2.11 below. Test results and calculations shall be recorded through SiteManager.
413.2.1 The Los Angeles Abrasion value of the aggregate shall be determined, when required, in accordance with AASHTO T 96. Los Angeles Abrasion shall be determined on an individual ledge basis.
413.2.2 Sodium sulfate soundness of the aggregate shall be determined, when required, in accordance with AASHTO T 104. Soundness shall be determined on an individual ledge basis.
413.2.3 Flat and Elongated Particles in Coarse Aggregate, 3:1, shall be determined in accordance with ASTM D 4791. Flat and Elongated Particles shall be determined on the aggregate blend.
413.2.4 Percentage of Fractured Particles in Coarse Aggregate shall be determined in accordance with ASTM D 5821. Percentage of Fractured Particles shall be determined on the aggregate blend.
413.2.5 Micro-Deval shall be determined in accordance with AASHTO TP 58. Micro-Deval shall be determined on an individual ledge basis.
413.2.6 Plastic Fines in Graded Aggregates and Soils by Use of the Sand Equivalent Test shall be determined in accordance with AASHTO T 176. The Sand Equivalent shall be determined on the aggregate blend.
413.2.7 Methylene Blue shall be determined in accordance with AASHTO TP 57. Methylene Blue shall be determined on the aggregate blend.
413.2.8 The Uncompacted Void Content shall be determined in accordance with AASHTO T 304. The Uncompacted Void Content shall be determined on the aggregate blend.
413.2.9 The Acid Insoluble Residue (AIR), when required, shall be determined in accordance with MoDOT Test Method TM 76. AIR is to be determined on the aggregate blend.
413.2.10 Drain Down of the loose mixture shall be determined in accordance with AASHTO T 305.
413.2.11 The Tensile Strength Ratio (TSR) shall be determined in accordance with AASHTO T 283, with the following modifications:
- (a) Section 6.2 - Specimens 4 inches (100 mm) in diameter and 2.5 inches (63.5 mm) thick shall be used.
- (b) Section 9.4 – Calculate the volumetric bulk specific gravity Gmb as follows:
- W = Weight – air dry (pounds)
- D = Diameter of specimen (inches)
- h = Height of specimen (inches)
- (c) Section 10.3.2 - Prior to placing the specimen in the vacuum vessel, a sheet of plastic film shall be placed on the balance to ensure any water draining from the specimen is not lost, and the balance shall be zeroed. The SSD mass of the specimen shall be determined within two minutes of removing the sample from the vacuum vessel.
- (d) Section 10.3.7 - If the degree of saturation is acceptable, the specimen shall be wrapped with the plastic film and placed in a plastic bag containing 0.003 gallon (10 mL) of water. Within two minutes of placing the specimen in the bag, the specimen shall be placed in the freezer. All moisture draining from the specimen during the process shall be collected by the plastic film and shall not be discarded.
413.3.3 Maintenance Surface Treatment
An ultra-thin (< 1”) hot mix asphalt (HMA) bonded wearing course is a hot mix asphalt overlay placed over a heavy asphalt emulsion layer or membrane. The thickness ranges from 1/2 to 3/4 in. The system is placed on a structurally sound flexible pavement that may exhibit minor amounts of surface distress. The layer thickness is determined by the maximum aggregate size in the HMA. The asphalt emulsion is usually polymer modified emulsion applied at 0.20 ± 0.07 gal/yd2. The HMA is a gap-graded mix using a crushed aggregate bound together with a mastic made of sand, filler, and asphalt binder. The maximum aggregate size ranges from 1/4 to 1/2 in. Generally a 3/8 in. mix is used. The grade of the asphalt binder is chosen based on the climate and the traffic conditions for the project. The binder content generally ranges from 5% to 6% depending on the traffic, climate and condition of the existing pavement.
The ultra-thin bonded wearing course is placed with a specially built machine that places the asphalt emulsion membrane and HMA in a single pass. The heavy application of the asphalt membrane seals small cracks in the existing pavement and helps to ensure the adhesion of the HMA to the underlying pavement.
The primary function of the ultra-thin bonded wearing course is to provide a durable, friction resistant surface on an existing flexible pavement. Specifications generally require one of three gradations. The gradation should be based on the traffic level, and the surface condition of the roadway. The thickness is approximately 1.5 times the maximum aggregate size.
For flexible pavements, the ultra-thin bonded HMA should not be used when the cracking exceeds the moderate severity levels. Any cracks greater than 1/4 in. should be cleaned and sealed. Patches and potholes should not exceed moderate severity levels. All potholes and any alligator cracked areas should be properly repaired. Rutting should not exceed 1/2 in. If the rutting does exceed this limit the surface should be milled or the ruts filled with a suitable materials prior to placing the ultra-thin overlay.
The ultra-thin bonded HMA overlay is applicable to all traffic levels, but because of its cost it is generally considered for high speed and high volume roadways.