Chapter 8: Micro-Surfacing

5.0 Construction Process

The main components of the construction process include:

• Safety and Traffic Control
• Equipment Requirements
• Stockpile/Project Staging Area Requirements
• Surface Preparation
• Application Conditions
• Types of Applications
• Quality Issues
• Post Construction Conditions
• Post-Treatments

Checklists at the end of the chapter are intended to guide project personnel through the important aspects of applying a micro-surfacing.

5.1 Safety & Traffic Control

Traffic control is required both for the safety of the traveling public and the employees performing the work. Traffic control should be in place before work forces and equipment enter onto the roadway or into the work zone. Traffic control includes construction signs, construction cones and/or barricades, flag personnel, and pilot cars to direct traffic clear of the maintenance operation. For detailed Traffic Control requirements, please refer to your agency’s guidelines.

Traffic control is required to ensure that the micro-surfacing has had adequate time to cure prior to reopening to traffic. The curing time for the micro-surfacing material will vary depending on the pavement surface conditions and the weather conditions at the time of application. Additional traffic control considerations are listed in the Field Considerations section at the end of this chapter.

5.2 Equipment Requirements

Equipment requirements for micro-surfacing machines are typically covered in agency specifications. Modern equipment, as shown in Figure 7, can be used to place either slurry seal or micro-surfacing.

One difference between slurry and micro-surfacing equipment is in the spreader boxes used. A slurry seal spreader box is a drag box, as shown in Figure 8. The drag box is pulled behind the paver by means of chains. This box may or may not have augers, although for quick set systems, augers should be used. The slurry seal should be easy to work and spread, and not cause any blockages in the box.

A micro-surfacing spreader box, shown in Figure 9, has to move a much stiffer mixture than a slurry spreader box, do it quickly, and then spread it before the emulsion breaks. To accommodate this, two sets of augers are used and a texturing rubber is added at the rear of the box. The texturing rubber is usually spread using an outrigger. The outrigger creates the desired texture for the surface. Additionally, a micro-surfacing box is rigidly attached to the rear of the paver, allowing a preset thickness of material to be placed.

Special boxes are used for rut filling applications when filling ruts greater than 12 mm (0.5 in) deep.  When filling ruts less than 12 mm (0.5 in) deep, a steel strike-off box is used for the scratch courses (see Section 5.6.2 below).  Adjustable width edge boxes are used for shoulders and to create clean joints between shoulders and the traveled way.  Figures 10 and 11 illustrate a rut filling box and an adjustable edge box, respectively.

5.2.2. Calibration

The design mix is proportioned by weight while the micro-surfacing machines deliver materials by volume. Due to this different nature of the measurements, it is essential that calibration be done with the actual job materials. No machine should be allowed to work on a job without a proper calibration.

5.3 Stockpile/Project Staging Area Requirements

The stockpile and project staging area must meet basic requirements which include:

• Clean, well-drained pad for aggregate piles
• Front-end loader for loading machines or Flow Boy-type vehicles in continuous operation
• Salt-free water supply
• Emulsion tanker
• Additive tanker

The stockpile and staging area should be as close as possible to the job site. Figure 12 illustrates a typical stockpile and staging area.

Operations should be scheduled to run as smoothly as possible and provide good traffic flow through the work zone. Aggregates that are below optimum moisture content should be remixed using the front-end loader to avoid segregation. In some cases aggregates that are separating in the stockpile or during loading may need to be sprayed with water to avoid fines loss.

5.4 Surface Preparation

The main objective of surface preparation is to provide a clean and sound surface on which the micro-surfacing is applied. The first step of surface preparation is to restore the pavement’s structural integrity and functional performance characteristics through patching and crack sealing (see Chapters 3 and 4 of the Maintenance Technical Advisory Guide for more information on these procedures).

Immediately before the micro-surfacing is applied, the road must be swept clean. If clay or hard-to-remove materials (such as organic matter) are present, high power pressure washing may be required. If left on the road, these types of contaminants will cause delamination of the treatment in these areas. Thermoplastic road markings must also be removed prior to placing a micro-surface, or at least be abraded to produce a rough surface. Paint markings require no pretreatment. Rubber on the roadway should be removed prior to applying a micro-surface.

Utility inlets should be covered with heavy paper or roofing felt adhered to the surface of the inlet. The paper is removed once the micro-surfacing has sufficiently cured. In addition to covering the inlets, all starts, stops, and handwork on turnouts should be done on roofing felt to ensure sharp, uniform joints and edges. Figure 13 illustrates the various surface preparation steps along with illustrations of delamination resulting from poor surface preparation.

5.5 Application Conditions

The basic requirement for success is that the emulsion must be able to break and form continuous films, as it is the only way a micro-surfacing mixture can become cohesive. As a result, humidity, wind conditions, and temperature (both surface and air) are important and need to be considered. Modifications to additives should be made according to the changing environment during application. Whereas a slurry seal is not suitable for night work, micro-surfacing can be placed at night because it uses a chemical break.

For the application of micro-surfacing, air temperature should be a minimum of 10°C (50°F) and rising. Humidity should be 60% or less and a slight breeze is advantageous. Work should not be started if rain is imminent. Micro-surfacing seals will typically resist rain induced damage after as little as one hour but typically require at least a three-hour cure to form a fully waterproof state. Additionally, breaking time for a micro-surfacing seal is affected by temperature. Work should not be started if freezing temperatures are anticipated within 24 hours of construction. Figure 14 shows the effect of temperature on the breaking rate of emulsion.

5.6 Types of Applications

5.6.1. Full Width Micro-Surfacing

When applying a full width seal a standard spreader box is used. The edge of each pass should align with the longitudinal joints or paint lines on the roadway. Three passes are typically used for a two-lane roadway. This allows clean edges and minimizes overlaps (usually 75 mm (3 in)). Overlapped seals should only be used when the pavement being sealed is level and in sound condition.

5.6.2. Scratch Coat

Scratch coats are used to level pavements with minor transverse irregularities that are narrower than the width of the spreader box, or on pavements with longitudinal ruts less than 12.5 mm (0.5 in) deep. When applying a scratch coat, a steel strike-off is substituted for the secondary strike-off in the standard micro-surfacing drag box. The steel strike-off drags over the high spots of the pavement, filling in the irregularities. Such materials are highly friable and stone loss is often high. Scratch coats should always be covered with a surface seal. The scratch coat principle along with a photo of a finished section is illustrated in Figure 15.

5.6.3. Rut Filling

Ruts may be filled with a high stability micro-surfacing mix. A rut box is essential for this application; it channels mix into the ruts and leaves a crowned finish to compensate for post compaction due to trafficking. Generally, ruts filled in this manner are covered with a surface seal, but this is not essential. Rolling is often incorporated to ensure compaction of the mix placed in rutted surfaces. Figure 16 illustrates the principle behind rut filling and provides a cross sectional diagram of a filled rut. Rut filling should only be used on stable ruts that have resulted from long-term traffic compaction rather than failures in the base or sub-base. If rutting is ongoing, the micro-surfacing will not prevent its continued development. Figure 17 illustrates both suitable and unsuitable candidates for rut filling.

5.7 Quality Issues

Quality control is critical during construction to achieve a uniform surface finish. The main areas of concern are discussed below.

5.7.1. Longitudinal Joints

Longitudinal joints may be overlapped or butt jointed. They should be straight or curve with the traffic lane. Overlaps should not be in the wheel paths and should not exceed 75 mm (3 in) in width. Figure 18 illustrates high quality and poor quality longitudinal joints.

5.7.2. Transverse Joints

Transverse joints are inevitable when working with truck mounted batch systems; every time a truck is emptied a transverse joint is required. Transitions at these joints must be smooth to avoid creating a bump in the surface. The joints must be butted to avoid these bumps and handwork should be kept to a minimum. The main difficulty in obtaining a smooth joint occurs as the micro-surfacing machine starts up at the joint, particularly when working with micro-surfacing that is difficult to work by hand and breaks quickly. Some contractors tend to over wet (add too much water) the mix at start-ups, leading to poor texture and scarring at the joints. Starting transverse joints on roofing felt can eliminate these problems. Figure 19 illustrates high quality and low quality transverse joints.

5.7.3. Edges and Shoulders

Sealed edges and shoulders can be rough and look poor. This occurs more often with micro-surfacing applications, which break quickly, making them harder to work by hand than slurry seals. For micro-surfacing, handwork should be kept to a minimum. The edge of the spreader box should be outside the line of the pavement and edge boxes should be used when shoulders are covered. Figure 20 illustrates high quality and poor quality edge and handwork.

5.7.4. Uneven Mixes and Segregation

Poorly designed micro-surfacing mixtures or mixtures with low cement content or too high a water content may separate once mixing in the box has ceased. This leads to a black and flush looking surface with poor texture. Separated mixes may lead to “false slurry” where the emulsion breaks onto the fine material. In such instances delamination may occur, resulting in premature failure. These types of mixes can be recognized as non-uniform and appear to set very slowly. Figure 12 illustrates segregation and delamination resulting from a false slurry.

5.7.5. Smoothness Problems

Micro-surfacing mixtures follow the existing road surface profile and thus do not have the ability to significantly change the pavement’s smoothness. However, when using stiffer mixes, the spreader box may, if incorrectly set up, chatter or bump as the material is spread and produce a washboard effect. The chattering may be reduced by making the mixture slower to set, adjusting the rubbers on the box, or adding weight to the back of the spreader box. Figure 22 illustrates the washboard effect.

5.7.6. Damage Caused by Premature Reopening to Traffic

The micro-surfacing must build sufficient cohesion to resist abrasion due to traffic. Early stone shedding is normal, but should not exceed 3%. If a mixture is reopened to traffic too early it will ravel off quickly, particularly in high stress areas. It is important that the mixture develops adequate cohesion before it is opened. Choosing the right time to reopen a surface to traffic is based largely on experience. However, a general rule of thumb for a micro-surfacing is that it can carry traffic when it is expelling clear water. Figure 23 illustrates raveling caused by premature opening to traffic.

5.8 Post Construction Conditions

Although emulsion systems are water-resistant, they may retain some water for several weeks, during which time freezing temperatures could cause the binder film to rupture with subsequent raveling. For this reason, projects should not be started without a 2-week window when freezing weather will not occur.

Asphalt emulsion-based systems cannot re-emulsify if not fully cured, but they can be tender enough to re-disperse under the effects of traffic loading and excessive water, especially ponded water. In this process, broken aggregates or asphalt particles that have not fully coalesced into films are dispersed in water, which disintegrates the emulsion. Thus, while light rain 3 hours after placing a micro-surfacing is acceptable, heavy rain coupled with heavy traffic will likely lead to surface damage, especially in high shear (e.g., turning movement) areas. Figure 24 illustrates damage caused by heavy rain in a high shear location.

5.9 Post-Treatments

5.9.1 Sanding

Sanding may be used to reduce the times that cross streets or intersections are closed. Sanding is the application of a fine layer of dry, washed sand that is broadcast over the micro-surface. Sanding may also be used on wet spots. Sanding should not be done until the micro-surface can withstand walking traffic. Figure 28 illustrates the use of sanding at a cross street.