From Engineering Policy Guide
Micro-surfacing is a slurry seal that uses a polymer-modified emulsion binder, a high quality dense graded aggregate, mineral filler, water and other additives, properly proportioned, mixed and spread on a paved surface. The filler can be Portland cement, hydrated lime or other approved materials. A self-propelled continuous loading machine or a truck-mounted machine is used to proportion and mix the material and apply the mixture to the pavement surface.
Micro-surfacing is used to retard raveling and oxidation of the pavement, fill non-plastic ruts, reduce the intrusion of water, improve surface friction, and remove minor surface irregularities. After placement, the water "breaks" and evaporates, leaving a hard asphalt/cement/aggregate mixture that is resistant to further compaction or movement. Because the aggregates required to make such a mixture are hard and angular, the mixture has good friction properties, and can also be used to improve surface friction conditions. It should not be used primarily for sealing surfaces, due to the rigidity of the mixture.
Micro-surfacing is not a structural layer and will not bridge any distress. It only fills depressions and will not stop further rutting. It does not contain rejuvenators, but hard asphalt, which will not rejuvenate an oxidized surface. It does not fill or seal cracks. Existing cracks will reflect through the micro-surfacing within a few months.
Micro-surfacing is applied using a slurry/screed operation. It may be used to fill ruts, one at a time, or, for shallower rutting, it may be used with a "scratch" coat, just filling the low or shallow points to restore the cross slope. There is some consolidation as the water leaves, thus deeper applications should be done in multiple layers with a time interval in between applications.
413.2 Use in Contracts
A single course micro-surfacing can be used to retard oxidation and light to moderate raveling, improve skid resistance to a structurally sound pavement. A multiple course micro-surfacing is used to correct other minor pavement surface deficiencies, including moderate rutting due to consolidation rather than plastic flow of the mix. Due to the brittle nature of the micro-surfacing is a poor crack sealer. Surface preparation for a micro-surfacing should include the sealing or filling of the cracks. Refer to Sec 413.10 in the Standard Specification.
Type IIIR should be specified for filling deeper ruts (>1/2 in.) or for raising shoulders. Type IIIR is applied with a narrow screed box, with no limit on the quantity used per square yard. Type II is a light, one-pass application and should be used to correct minor surface irregularities. For this application, it is only recommended for light traffic (<3500 AADT) routes with no rutting. Type III is a thicker application and should be specified for heavier traffic routes. Two passes are required to place Type III. The first pass is used to fill shallower ruts (<1/2 in.), and the final pass completes the coverage. Type II or III might be used in conjunction with Type IIIR applications.
When micro-surfacing is applied over any type of existing striping, the material can release from the old pavement marking in a few months. This can cause confusion as well as make for an unsightly appearance. Contracts for micro-surfacing should include removal of the existing striping before micro-surfacing.
413.3 Materials Inspection
To establish procedures for mix design, inspection and acceptance of materials used in Micro-Surfacing. Aggregate for use in surface treatments shall be inspected in accordance with EPG 1001 General Requirements for Material. Asphalt Binder for use in surface treatments shall be inspected in accordance with EPG 1015 Bituminous Material.
413.3.2 Mix Design Procedure
In order for a micro-surfacing mix formula to be approved, the contractor’s proposed job mix formula (JMF) shall be submitted as required in Standard Specification Sec 413.10.3. Trial mix samples will not be required unless requested by the Field Office. If requested, the samples are to be obtained and submitted to the Central Laboratory in accordance with EPG 1001 General Requirements for Material. When possible, the JMF and correspondence should be transmitted electronically. The Materials Field Office e-mail address is MFO.
When the district receives a proposed trial mix formula, as required by the Standard Specifications, the mixture properties, components and proportions should be checked to ensure compliance with Specifications and that they are approved for the intended use. It may be necessary for the district to advise the contractor to make changes in the proposed mixture in order to comply with Department policies. A QC plan in accordance with EPG 1001 General Requirements for Material covering each aggregate fraction should be on file in the district office or received with the JMF. The target gradations shown on the QC plan and JMF must match. When the district is satisfied that the proposed mixture is acceptable, a copy of the JMF and the contractor's letter shall be submitted to the Materials Field Office, accompanied by a letter of transmittal with comments, any corrections made and recommendations. The transmittal letter shall contain the following information:
- Project information – Job Number, Route, County, Contract Number.
- Mixture Type
- Grade and Source of Asphalt Binder
- Letting Date
- Proposed Work – Job Location and Length
- Annual Average Daily Traffic (AADT)
- Mix Use – Mainline, Shoulders, Outer Roads, etc.
- Quantity of Mix
Field Office Procedure
The Materials Field Office is charged with the responsibility of processing the mix formula. General procedures for processing a micro-surfacing 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.
- e. All calculations on the proposed JMF are checked.
- f. For verification, a trial may be prepared and submitted to the Laboratory.
A letter of transmittal will accompany the approved mixture to the District Construction and Materials Engineer with distribution as follows:
|Title||Copy of Transmittal Letter and Approved Mix|
|District Construction and Materials Engineer||1|
|Project Operations Clerk||1|
|Field Office File||1|
The letter of transmittal and the approved mixture will be sent by electronic mail to the individuals listed above.
A copy of the approved formula accompanied by a letter of transmittal from the District Construction and Materials Engineer is to be forwarded to the contractor.