Chapter 5: Chip Seals

4.0 Construction

4.1 Construction Process

The sequence of construction events is as follows:

1. Project Preparation
2. Surface Preparation
3. Binder Application
4. Aggregate Spreading
5. Rolling
6. Sweeping (Brooming)

Figure 6 illustrates the construction process from binder application through final sweeping. Details of the construction process are provided in the following sections.

4.2 Project Preparation

Preparing the surface is critical to chip seal performance.  Areas of the pavement exhibiting structural failures, such as potholes and deteriorated patches, should be addressed by removing or patching and sealing the failed area prior to placing a chip seal.  Avoid using cold mix for patching prior to applying the chip seal.  Finally, the prepared surface must be clean, dry, and free of any loose material before the binder is applied.  Preparing for a chip seal project typically includes:

• Milling the surface if there are extensive loose portions of asphalt concrete pavement or areas of bleeding that must be removed
• Crack sealing or filling cracks that are likely to reflect through the chip seal (see Chapter 3)
• Patching any deteriorated areas or dig outs where required (see Chapter 4)
• Cleaning or brooming any loose material from the pavement surface
• Removing pavement markers and delineators
• Placing temporary pavement markers for lane delineation until the road is re-striped

If the patched areas are generally more porous than the rest of the pavement, place a tack coat on the patched areas prior to chip sealing.  Never chip seal a location that does not have both the minimum ambient and pavement temperatures.

a) Binder Application	b) Spreading of Aggregate
c) Rolling	d) Sweeping

Figure 6: Construction Process for Chip Seals

4.2.1 Materials

A work site must have a facility for storing aggregate and binder.  Generally, binders are trucked directly from the manufacturer and off-loaded on site.  However, situations arise when distance and weather require off-site storage.  The site should be chosen well in advance of project start-up to ensure it has the proper requirements.  Ideally, the aggregate stockpile should be placed on a sloped and paved surface; at the very least it should be on a sloped surface that allows the stockpile to drain.    The stockpile also should be protected from contamination by foreign material.  Once stockpiled, the aggregate should not be moved until it is transported to the road being chip sealed.  Following completion of the project, any remaining aggregate must be removed from the stockpile site and the site restored to its original condition before being used again for stockpiling.  The methods for storing and handling binders and aggregates for chip seals is the same as those for terminal storage, as outlined in Chapter 2 (Materials).

4.2.2 Weather Conditions

When chip seals are constructed, the weather should be clear and warm.  In general, pavement surface temperatures should be 10°C (55°F) and rising, and the humidity should be 50% or lower.  A gentle breeze will assist in accelerating cure times, but wind may divert the emulsion spray and reduce the uniformity of the application rate.  Any rainfall immediately before, during, or after the construction of the PME chip seal will cause the treatment to fail, so chip seals should not be placed during such conditions.  Detailed requirements vary for different binder types and are included in the Caltrans specifications. should be included in the agency’s specifications.

4.2.3 Traffic Control

The Resident Engineer (RE) examines and approves the contractor’s traffic control plan prepared in accordance with the Manual of Uniform Traffic Control Devices (MUTCD) Part 6. Caltrans Safety Manual (8) and the Caltrans Code of Safe Operating Practices (9). .  Signs and devices used must match the traffic control plan.  The work zone must conform to requirements set forth in the MUTCD Manual. Caltrans practice and requirements set forth in the Caltrans Safety Manual and the Caltrans Code of Safe Operating Practices. All workers must have all required safety equipment and clothing.

During and after placement of the chip seal, pilot cars should escort traffic at a speed of 20 mph (30 kph) over the chip sealed surface for two to 24 hours.  Once all the loose aggregate is removed from the new chip seal surface, pilot cars are no longer needed.

4.3 Construction of Transverse Joints

Chip seal passes should begin and end on roofing felt paper or an approved equivalent.  This ensures that the transverse joints are clean and sharp.  Longitudinal joints may be made with an overlap.  In this process, a wet edge (i.e., one without an application of aggregate) of 75 to 100 mm (3 to 4 inches) is left (not in a wheel path), and the next run overlaps this wet edge.  The chip distributor then covers the whole run to the pavement’s edge.  Figure 7 illustrates the layout of felt paper at the end of a project lane.

4.4 Binder Application

4.4.1 Spraying Equipment

The spray distributor is the most important piece of equipment in the chip seal process.  Its function is to uniformly apply the binder over the surface at the designed rate.  Typically, spray distributors are truck mounted on a distributor truck, as shown in Figure 8.  Trailer units have also been used.  A distributor truck should have a heating, circulation, and pumping system, along with a spray bar (spraying equipment), and all necessary controls (manual or computer regulated) to guarantee proper application.

Figure 7: Start and Stop Passes on Roofing Felt (Transverse Joints)

Figure 8: Spray Distributor

4.4.2 Distributor Preparation

The steps associated with preparing the distributor include:

1. Calibrate the distributor first by spraying a pre-weighed area of carpet backed with a waterproof layer.  Subtract the initial weight from that of the sprayed carpet, then divide the difference by the area of the carpet.  Although this is the contractor’s responsibility, the inspector should verify that the distributor is spraying the binder at the correct application rate. See CT 339 for calibration procedures. 
   
2. Blow out the spray nozzles to ensure there are no blockages and check the nozzle angles (see Figure 9) to ensure they spray at a 15- to 30-degree angle from the spray bar axis.  Often, the outermost nozzles will be turned in (knifed) to give a sharp edge with no overspray.
   
3. Check the distributor bar’s height.  The height is usually set so that a double or triple overlap is obtained as illustrated in Figure 10.
   
4. Check the distributor bar’s transverse alignment to ensure it is closely perpendicular to the centerline of the pavement.
   
5. Check the binder temperature to ensure it is in the specified range for proper application.  Chip seal emulsion should be between 40°C and 85 °C (104°F and 185°F) (6).
   
6. Confirm that an adequate supply of binder is available for the day’s schedule.
   
7. Ensure that the amount of binder applied can be immediately covered by the aggregate on site.

Figure 9: Spray Bar with Nozzle Arrangement (6)

Figure 10: Spray Bar Height Arrangements (6)

Visual checks should be made throughout the spraying process to ensure that the spray bars are not clogged and are spraying even fans.  There should be no streaking of binder visible on the surface.  If streaking occurs, the operation should be stopped to recheck the spray bar’s functioning, the binder’s viscosity, and the binder’s temperature.  The contractor and inspector should check application rates frequently.  Monitor the application rate by using the calibration method mentioned above or by using the alternative method outlined in the Checklist section that follows this chapter.  The method above is recommended for equipment calibration; the alternative method in the Checklist is for quick spot-checking during construction.

4.5 Aggregate Application

4.5.1 Chip Spreader

Chip spreaders must be able to spread an even coating of aggregate one layer thick over the entire sprayed surface. Figure 11 shows a typical chip spreader.

Figure 11: Chip Spreader

Prior to applying aggregate on a project, the following steps should be taken:

1. Calibrate the spreader first by spreading chips over a pre-weighed area of carpet.  Subtract the initial weight from that of the carpet with chips spread onto it, then divide the difference by the area of the carpet.  Although this is the responsibility of the contractor, the inspector should verify that the spreader is applying the aggregate at the correct application rate.
   
2. Ensure all gates in the spreader open correctly.
   
3. Ensure the spreader applies the aggregate in an even, single-layer thickness.
   
4. Ensure that the spreader is not leaving piles of aggregate and is not spreading too thick a layer.  Too thick a layer of aggregate can result in improper embedment of the aggregate into the binder, thus compromising the finished chip seal.  Too thick a layer of aggregate can result in the lever and wedge effect illustrated in Figure 12, which also compromises the finished chip seal.
   
5. Confirm that an adequate supply of aggregate is available prior to applying the binder.
   
6. Ensure proper moisture content of aggregate for PME chip seals.

Figure 12: Lever and Wedge Effect (5)

4.5.2  Haul Trucks

Haul trucks are vital in providing a continuous supply of aggregate from the stockpile to the chip spreader, so they must be in good mechanical condition.  Single-axle trucks carry between 4,500 and 6,350 kg (5 and 7 tons); trucks with tandem axles carry between 9,000 and 12,700 kg (11 and 14 tons).  The greater capacity of trucks with tandem axles makes them preferable; they require fewer hook-ups and result in less chance of spills and a more efficient operation.

4.5.3 Chip Spreading Process

The application of aggregate should follow the binder application by no more than 90 seconds for the best possible aggregate retention.  A good visual check is that the spreader should be no more than 30 m (100 ft) behind the distributor truck.  Depending on the type of chip spreader used, the first chip-spreading pass can be done with or against the flow of traffic to allow good centerline match up; this is normally decided by the chip spreader operator.  The direction for spreading aggregate is chosen mostly to minimize truck movements on the fresh chip seal; however, other factors can impact the chip spreader operator’s decision.

4.6 Rolling (Embedding Aggregate)

The function of the roller is to embed the aggregate into the binder and orient it into an interlocking mosaic.  This is initially accomplished with pneumatic rollers as shown in Figure 13.  Steel rollers are not normally recommended because they can crush the aggregate.  Hot weather can accelerate the curing process of the emulsion, so rolling should be expedited in hot weather to ensure proper embedment of the aggregate into the binder.

Figure 13: Pneumatic (Rubber Tired) Roller

When rolling (embedding) aggregate, the important variables for pneumatic rollers are:

• Contact pressure
• Number of passes and pattern
• Speed
• Smoothness of tires
• Adequate number of rollers to cover the width of the chip seal in one pass

Roller contact pressure depends on the roller’s weight, the number of tires, and tire size, pressure and rating.  Rollers that can be ballasted are very useful in assuring sufficient weight of the roller.  A minimum ballasted weight should be 4500 to 5400 kg (4 to 6 tons), but heavier weights are preferred,  with a corresponding tire pressure of 600 kPa (87 psi).  Tires must have a smooth tread and should not wobble.  Tire inflation pressure should not vary more than 50 kPa (7 psi).

Rollers should follow aggregate spreading by no more than 150 m (500 ft) and should not be operated at more than 10 kph (6 mph).  The rolling pattern will depend on the number of rollers used.  A minimum of two rollers should be used to cover the full width of the chip spreader.  However, more rollers may be needed to insure they cover the full width of the newly placed chip seal in each pass.  Three passes of the rollers are sufficient: one forward, one in reverse, and the final pass extending into the next section.

4.7 Brooming

Brooming is required before, after, and sometimes during the chip seal operation.  The pavement must be swept clean of dust and debris.  During multi-coat sealing, excess aggregate must be broomed after the first layer of aggregate is rolled and before the second layer of emulsion is placed.  After the final layer of chip seal has been constructed, excess aggregate must be broomed off to minimize whip-off by traffic.

Brooming is done using mobile pickup brooms (vacuum or rotary brooms with nylon or steel bristles).  The rotary brooms should not be worn, and should remove only the excess loose aggregate.  Figure 14 illustrates a typical brooming operation.

Mobile pickup brooms are usually capable of picking up and storing aggregate.  So-called “kick brooms” are sometimes used to form aggregate windrows that can be collected.  These brooms can be problematic because they often generate dust and may sweep aggregate into watercourses or gutters.  Dust can cause a visibility issue for the work zone or a compliance issue with local air pollution control districts.  Figure 15 illustrates a typical kick broom.

Hot applied chip seals can be swept within 30 minutes; emulsion chip seals can be swept in 2 to 4 hours.  A flush coat can be applied after brooming to eliminate further chip loss and improve durability prior to opening the pavement to uncontrolled traffic.

Figure 14: Brooming Process, Shown on a Shoulder Seal

Figure 15: Kick Broom