Chapter 7: Slurry Seals

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 this chapter help guide project personnel through the important aspects of applying a slurry surfacing.

5.1 Safety and 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 slurry surfacing has had adequate time to cure prior to reopening to traffic. The curing time for the slurry 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 (Checklists) at the end of this chapter.

5.2 Equipment Requirements

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

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; for quick set systems augers should be used. The slurry seal should be easy to work and spread, and not cause any hang-up in the box.

The design mix is proportioned by weight while the slurry 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 some basic requirements. These requirements include:

• A clean, well-drained pad for aggregate piles
• A front-end loader for loading machines or Flow Boy-type vehicles in continuous operation
• A salt-free water supply
• An emulsion tanker
• An additive tanker

The stockpile and staging area should be as close as possible to the job site. Figure 9 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 slurry surfacing is applied. The first step of surface preparation is to restore the pavement’s structural integrity and functional performance characteristics through crack sealing and patching (see Chapters 3 and 4 of this guide for more information on these procedures).

Immediately before the slurry surfacing is applied, the road must be swept clean of all debris including clay and if 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 slurry 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 slurry 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 slurry 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 10 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 slurry 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. In any case, application of a slurry seal is generally not suitable for night work. This is due to the lower evaporation rate at night, which results in longer breaking and curing times.

For a conventional slurry seal project, 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. Slurry seals will typically resist rain induced damage after as little as one hour but typically require at least three hours to cure to a fully waterproof state. Additionally, breaking time for a slurry is affected by temperature. Work should not be started if freezing temperatures are anticipated within 24 hours of construction. Figure 11 shows the effect of temperature on the breaking rate of emulsion.

5.6 Quality Issues

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

5.6.1 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 12 illustrates high quality and poor quality longitudinal joints.

5.6.2 Transverse Joints

Transverse joints are inevitable when working with 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 slurry machine starts up at the joint. Some contractors tend to add too much water to the mix at start-up, leading to poor texture and scarring at the joints. Starting transverse joints on roofing felt can eliminate these problems. Figure 13 illustrates high quality and low quality transverse joints.

5.6.3 Edges and Shoulders

Slurry sealed edges and shoulders can be rough and look poor. 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 14 illustrates high quality and poor quality edge and handwork.

5.6.4 Uneven Mixes and Segregation

Poorly designed slurry 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 a “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 15 illustrates segregation and delamination resulting from a false slurry.

5.6.5 Smoothness Problems

Slurry 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 16 illustrates the washboard effect.

5.6.6 Damage Caused by Premature Reopening to Traffic

The slurry seal 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 form 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 slurry seal is that it can be opened when it has turned black. Figure 17 illustrates raveling caused by premature opening to traffic.

5.7 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.

Although asphalt emulsion-based systems cannot re-emulsify, if not fully cured, 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 slurry seal is acceptable, heavy rain coupled with heavy traffic will likely lead to surface damage, especially in high shear (e.g., turning movement) areas.  Figure 18 illustrates damage caused by heavy rain in a high shear location.

5.8 Post Treatments

5.8.1 Rolling

Slurry seals will lose stone until the surface voids have been closed off, but it is acceptable for approximately 3% of surface stone to be lost.  Stone loss may be limited by rolling with pneumatic rollers.  For rut filling applications, rolling is almost always recommended.  The roller should be light (6-7 tons maximum) and non-ballasted.  One to two passes at a slow speed are recommended.  This allows the water to be pressed to the surface, promoting evaporation and curing.  Larger stones will be punched into the surface, reducing early raveling.  Figure 19 illustrates a typical roller operation.

5.8.2 Sweeping

On heavily trafficked roads or where opening has lead to excessive stone loss, sweeping is essential. A suction broom is the best type of sweeper to use. Sweeping should be done just prior to opening to traffic and at periods determined by the level of stone loss. Figure 20 shows a suction broom.

5.8.3 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 slurry surface. Sanding may also be used on wet spots. Sanding should not be done until the slurry can withstand walking traffic. Figure 21 illustrates the use of sanding at a cross street.