413.1 Micro-Surfacing

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Revision as of 09:26, 7 May 2020 by Smithk (talk | contribs) (Per CM, guidance for single pass and double pass micro-surfacing was expanded; guidance for micro-surfacing grade selection and surface preparation also expanded. New guidance for post construction and the method of measurement and basis of payment now available.)
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413.1.1 Description

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. Micro-surfacing will not stop further rutting, does not contain rejuvenators and will not rejuvenate an oxidized surface, and does not fill or seal cracks. Existing cracks wider than ¼-inch should be crack filled or pre-treated with a scrub seal or seal coat prior to a micro-surface. Otherwise, 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.

413.1.2 Use in Contracts

The bituminous material in a micro-surface is a polymer modified asphalt emulsion that is in accordance with Sec 1015.20.5.2. Differing aggregate sizing (Type II, III, or IIIR) and aggregate types (Grade A, B, or C) are available. Also, selections of single pass or double pass of micro-surfacing may be selected depending on the conditions of the existing pavement and performance goals. The following guidelines help the appropriate selection process for micro-surfacing mixtures.

413.1.2.1 Micro-Surfacing Type Selection

Single Pass Micro-Surfacing

Type II micro-surface contains a very fine aggregate gradation that is most beneficial to be used strictly as preventative maintenance treatment. The existing pavement should be in good condition with minimal distress. “Good condition” is defined as an IRI of less than 100 in/mile and a Condition Index of 7 or greater. A single pass of Type II is not recommended for Portland cement concrete surfaces.

Type III micro-surface is relatively coarser aggregate gradation that is used on pavements that are in good to fair condition that have minor rutting issues and low quantity and severity of cracking. “Fair condition” is defined as an IRI of less than 120 in/mile and a Condition Index of greater than 5. Type III micro-surface has a rough texture that can be used to increase the frictional properties of a roadway. A single pass of Type III is not recommended for Portland cement concrete surfaces.

Type IIIR differs only in application compared to a Type III. Type IIIR is specified as a separate operation at isolated locations for treating moderate to severe rutting, filling existing rumble strips, or other depressed areas. Type IIIR is measured and paid for by the ton and placed in multiple lifts as necessary to level the profile prior to a separate surface treatment.

Double Pass Micro-Surfacing

A double pass micro-surface should also be utilized as a preventative maintenance treatment that follow the same pavement condition requirements as the single pass. However, a double pass of micro-surfacing can be viewed as a short-term pavement solution that is comparable to the performance of an Ultra-Thin Bonded Asphalt Wearing Surface (UBAWS). A double pass of micro-surfacing is also acceptable on Portland cement concrete pavement. There are multiple micro-surfacing combinations to consider, each having a distinct difference that can be selected by the team’s preference. The different combinations are described below:

  • Type II over Type III micro-surfacing is the preferred option for a double pass micro-surfacing treatment. The Type III bottom course will fill in any surface irregularities and provide more stability, while the Type II top course will be a smoother and quieter riding surface.
  • Type II over Type II micro-surfacing can be considered if the existing pavement is in good condition with minimal surface distresses. The Type II/Type II combination would be less expensive due to the ability to place less material and have the same gradation and mix design for the contractor.
  • Type III over Type III micro-surfacing can be selected to correct minor to moderate rutting with the bottom course and maximize frictional properties with the top surface course.

There is no benefit to select a Type III over Type II, and therefore not recommended.

When scoping a double-pass micro-surfacing, a Type B or C UBAWS should be shown as an optional preventative maintenance treatment, unless surface water flow is blocked within the open graded structure of the UBAWS (i.e. treatment is required to be milled into an adjacent lane, curb-n-gutter, or other profile constraints).

413.1.2.2 Micro-Surfacing Grade Selection

All micro-surfacing types are classified as either Grade A, B or C. The grade of micro-surfacing refers to the aggregate quality and is directly selected by the traffic volume. Grade A requires a high-quality aggregate material to be used on Interstate and high-volume roadways, while Class C uses a lower quality aggregate that should be specified on the low volume routes. The table below provides guidance on the micro-surfacing grade selection based upon the route’s Average Daily Traffic (ADT) count.

Traffic Recommendations
Micro-Surfacing Grade ADT
Grade A Greater than 14,000
Grade B 3,500 – 14,000
Grade C Less than 3,500

413.1.2.3 Surface Preparation

A tack coat is required to be placed on older existing pavement surfaces prior to a micro-surfacing treatment. Tack coat should be measured and added as a separate bid item in accordance with Sec 407. Tack coat is not required on new surface treatments less than one year old such as scrub seal pre-surface treatments, new HMA overlay, or in between the two-pass micro-surfacing lifts.

Existing cracks wider than ¼-inch should be crack filled prior to the application of the micro-surface. Crack filling should be measured and added as a separate bid item in accordance with Sec 413.70 for bituminous surfaces or Sec 413.80 for Portland cement concrete surfaces.

If numerous cracks exist and the pavement is in good condition, then it is more economical to place a surface treatment instead of crack filling prior to the micro-surface. A chip seal or a scrub seal treatment should be considered as a pre-treatment to the micro-surface to seal existing cracks and fill in minor surface irregularities prior to the micro-surface.

When a pre-treatment is conducted (e.g., crack sealing, chip seal, etc.) it is imperative that the pre-treatment be completely cured to allow for any water evaporation within the material. A minimum of two weeks is required by the standard specification; however, this curing period will depend on the product type and environmental conditions. The curing period may need to be extended during cooler weather for some pre-treatments with visible signs of the pre-treatment not setting up under traffic. Likewise, if a two pass micro-surface is being placed, the first pass needs a minimum of 24 hours of traffic for compaction and curing to allow excess water to evaporate before the second micro-surfacing lift is placed.

Generally, existing durable and painted pavement marking lines (4”, 6”, 8”), may remain in place and covered with a micro-surfacing treatment if the existing marking itself is well bonded to the pavement. However, existing durable intersection pavement markings (stop lines, arrows, words, symbols, etc.) shall be removed in accordance with Sec 620.50 and shall be paid separately.

Pavement failures and distresses caused by subgrade or underlying layers should be corrected by the appropriate pavement repair selection (Sec 613) prior to a micro-surfacing treatment.

413.1.2.4 Post Construction

The time frame to install permanent striping on micro-surfacing is relatively longer compared to hot mix asphalt due to the curing of the emulsion. The contractor shall be responsible for placing and maintaining temporary markings until the permanent marking can be installed.

New rumble strips may be re-cut over top of the old rumbles when the micro-surface material was used to completely fill in the existing rumbles.

413.1.2.5 Method of Measurement and Basis of Payment

Type II or Type III micro-surface are paid for by the square yard. Type IIIR is paid for by the ton. For estimating purposes of Type IIIR, 1.440 tons per cubic yard should be used for a conversion factor.

413.1.3 Materials Inspection

413.1.3.1 Scope

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.

413.1.3.2 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.

District Procedure

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. Also, when blast furnace slag sources are submitted, the sources should be verified that they have been previously approved with a history of satisfactory performance. 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.

413.1.3.3 Report

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
Resident Engineer 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.

413.1.4 Laboratory Procedure

Micro-surfacing mix properties shall be determined, when required, in accordance with the applicable International Slurry Surfacing Association (ISSA) Technical Bulletins.