421 Microsurfacing

Description (421.01)

This item of work is used to provide a cold laid polymer modified emulsified asphalt pavement course to fill ruts, depressions, and to provide a leveling course.  It can also be used as a surface course over existing pavements.  The mixture includes the use of crushed aggregate, mineral filler, water, polymer modified emulsified asphalt binder, and other additives. 


Materials (421.02)

Materials used in the mixture and the completed mix are approved by the Laboratory prior to beginning work.  The Contractor must provide a certificate of analysis and statement of compliance from the manufacturer with each load of binder.


Proportioning (421.03)

Verify job mix formula (JMF) approval by the Laboratory.


Weather Limitations (421.04)

The mixture cannot be placed when it is raining.

Allow microsurfacing placement if:

  1. Surface temperature is 40 °F (5 °C) or above.
  2. Air temperature within 24 hours of placement is NOT forecasted to be below 32 °F (0 °C).
  3. Surface temperature is 50 °F (10 °C) or above between September 30 and May 1.


Mixing Equipment (421.05)

Mixing equipment must conform to the specifications.  Equipment must be self-propelled, front feed, with a continuous loading conveyor system.  A gear pump must accurately proportion aggregate and asphalt emulsion. 

Mineral filler must be dropped into the aggregate before reaching the pug mill.  A spray bar must pre-wet the aggregate with water and additive before the addition of asphalt emulsion and before it drops into the pub mill.

The pug mill must be continuous flow type and a minimum of 49 inches long. 

The equipment must have driving stations on both sides.  The back mixing station must have forward speed control to allow the mixing operator to control speed and the level of mixture in the paver.

A water pressure system with spray nozzles that can spray water ahead of the spreader box may be required.

The Contractor can use truck-mounted machines with a conveyor belt delivery system on project segments of less than 15,500 square yards or use it for spot repairs.

Description: Description: Description: HPIM0253Description: Description: Description: HPIM1342

Figure 421.A – Self-Propelled Front Feed, Continuous Loading and Mixing Machine (left) Truck-Mounted Machine (right)


Equipment Calibration (421.06)

Before mix production, calibrate the mixing equipment in the presence of the Engineer. Perform a new calibration if there is any change in the mix design. Obtain documentation of individual calibrations of each material from the equipment calibrator.  After the initial calibration, no further changes or adjustments to the mixing equipment can be made without the Engineer’s approval.


Spreading Equipment (421.07)

The spreading equipment must apply the mixture uniformly.  A standard spreader box is allowed when placing a leveling course or surface course.  For surface courses, a secondary strike-off must be attached to the spreader.

If a rut fill course is required, the Contractor must use a V-shaped rut filling spreader box equipped with a steel strike-off.

Either spreader box must be attached to the mixer and must be equipped with adjustable paddles that continuously agitate and mix the materials throughout the box.  Check for excessive buildup and settling in the spreader box and have the Contractor correct the issue.

Burlap or other drags are allowed for use to provide the desired surface finish.


Surface Preparation (421.08)

Be sure the surface is thoroughly cleaned before application.  Apply a tack coat consisting of 1 part asphalt emulsion and 3 parts water and apply at a rate of 0.06 to 0.12 gallons per square yard.  Apply tack in accordance with 407.

Remove raised pavement markers according to 621.08. Fill depressions with the same material being used for the microsurfacing.

Pavement markings and all other paint must be removed using an abrasion method.


Test Strip (421.09)

A 1,000-foot test strip must be constructed for evaluation by the Engineer.  The test must include all courses specified and must be constructed at the same time of day as the scheduled, full-scale production.  For example, if the Contractor plans night work, the test strip must also be done at night.

The Engineer will evaluate the test strip after 24 hours of traffic to determine if the mix design is acceptable.  Full-scale operations can only begin after the Engineer accepts the test strip.

If work is scheduled after May 1 and before September 30 and the Contractor has constructed a test strip with the same JMF and materials in the same construction season, the test strip may be waived. 


Application (421.10)

During application the following items should be closely checked and monitored:

  1. Uniform and complete coverage that fills cracks, potholes, and surface irregularities.
  2. No skips, lumps, or tears are allowed in the material placed.
  3. No unmixed aggregate is allowed.
  4. In a rut, fill material must fully fill ruts without excess overfilling that creates a crown of 1/8 inch after 24 hours of traffic compaction.
  5. Rut fill must be applied in 5- to 6-foot passes in each wheel path.
  6. Restore the design profile of the pavement cross-section.
  7. Leveling course must be applied at 14 ± 2 pounds per square yard (7.6 kg/m2).
  8. Surface course must be at 16 ±1 pounds per square yard (8.7 kg/m2) when placed on another microsurfacing course.
  9. Surface course must be placed at a minimum 18 pounds per square yard (9.8 kg/m2) when not placed on another microsurfacing course.
  10. Surface courses must be wide enough to cover rut fill and leveling courses.
  11. Straight lines must be maintained along curbs, shoulders, and intersections.
  12. Stop placement if excessive streaking or other problems develop.
  13. Contractor must use a rubber tire roller if the material is not exposed to traffic within 48 hours.
  14. The pneumatic roller must meet 401.13 and have a tire pressure of 40 to 60 psi (275 to 400 kPa).
  15. Squeegees are required where hand spreading and finishing is needed.


Acceptance (421.11), Method of Measurement (421.12), Payment (421.13)

Accept microsurfacing based on the Engineer’s summary of quantities for each day provided the proportion of binder to dry aggregate is maintained within 2 gallons per ton (8.5 L/metric ton). Ensure all other control requirements for proportioning and spread rate are in conformance with the specifications.

The final pavement surface must be free from excessive scratch marks, tears, rippling, streaks, and other surface discontinuities.  Longitudinal joints and transverse joints must be neat and straight.

Microsurfacing (surface and leveling course) is measured and paid by the square yards completed and accepted.  The pavement width is as shown on the plans, specified in these specifications, or as directed by the Engineer.  Length is measured along the centerline of the roadway.

Rut fill course will be measured and paid by the number of tons of dry aggregate used and accepted based on ticket weights.

The test strip will be paid based on the acceptance at the individual bid prices for each course placed.  For example, if the plans require a leveling and surface course, the test strip must be constructed using these courses and would be paid on the measured quantities for both of these items.

Tack coat is not paid separately and is considered incidental to the work.

Description: Description: Description: HPIM1229

Figure 421.B – Unacceptable Streaking (left); Unacceptable Surface Prep (right)


Description: Description: Description: HPIM1210

Figure 421.C – Unacceptable Transverse Joint and “Skip”

Description: Description: Description: HPIM0258

Figure 421.D – Uniform Coverage


Documentation Requirements - 421 Microsurfacing

1.      Check for certificate of analysis and compliance from the manufacturer of the binder and document for each load of binder.

2.      Check and document surface preparation, pavement and atmospheric temperatures, and forecast for next 24 hours.

3.      Verify and record conformance to specifications for the mixing equipment.

4.      Verify that an approved JMF is being used.

5.      Document equipment calibration.

6.      Document construction of test strip and the evaluation as well as any changes implemented.

7.      Document surface preparation and tack coat application.

8.      Document placement operation regarding uniformity, rut filling, leveling, pounds per square yard used, construction of straight lines, roller compaction (if required), issues, and corrections made.

9.      Document acceptance, method of measurement, and calculations for pay.

10.  Document on CA-D-3A or other approved forms.