505 - Chip Seals

505 Chip seal

GENERAL DESCRIPTION

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Definitions and Terms

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General Overview

Chip sealing is a common preventive maintenance activity across the State of Michigan. It involves spraying asphalt cement on the surface of an existing pavement followed by the application of a cover aggregate. The asphalt cement is emulsified (suspended in water) to allow for it to be applied without the addition of extreme heat. The cover aggregate is typically either naturally occurring gravel or crushed aggregate such as granite, quartzite, or trap rock (basalt), or limestone. Steel blast furnace slag is also used with great success.

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Contractor Prequalification

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Code of Federal Regulation (CFR) / FHWA Regulations

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MATERIALS

The two main materials used in chip seal construction are the asphalt binder and the cover aggregate. The asphalt binder is typically an asphalt emulsion. The cover aggregate can be either natural or crushed. Asphalt Binder The most common asphalt binder used for chip seals in Michigan is an asphalt emulsion. MDOT has two Chip Seal Emulsions, CRS2M and CSEA.

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Asphalt Emulsion

An asphalt emulsion consists of the following components:

• Asphalt Cement
• Water
• Emulsifying Agent (surfactant)
• Polymers

An asphalt emulsion is mostly asphalt cement, similar to that used in hot mix asphalt, with added latex or polymers for improved chip seal performance. Water is the second largest component and serves as the medium that suspends and transports asphalt particles. When the asphalt and water separate—changing the emulsion from brown to black—the emulsion is said to have “broken,” so aggregate chips must be applied and rolled quickly. A surfactant allows the asphalt to disperse into droplets by reducing surface tension and gives the emulsion its electrical charge. Because most aggregates in Michigan are negatively charged, cationic emulsions like CRS-2P and CSEA are used. After breaking, the asphalt settles to the pavement while the water rises and evaporates, leaving the asphalt behind.

CSEA vs CRS2M

CSEA is a more durable chip seal emulsion used on MDOT routes with higher volumes and in certain parts of the State. CSEA has higher residual asphalt content and higher toughness/tenacity with a higher elastic recovery which helps with repeated traffic loading. CSEA will contain higher polymer content than CRS-2M. CSEA will have a higher bond strength to the aggregate and will typically allow better results when opening to higher traffic.

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Aggregate

Cover Aggregate The cover aggregate must be clean and dust free (≤2% retained on no. 200), uniform in size and hard so that it provides a tight, durable surface able to withstand the abuse of snowplows and vehicles. Aggregates used on MDOT Chip Seals are classified as either 34CS or CS-T. 34CS is a large gradation with some aggregate in the 3/8 with the majority being ¼ inch. CS-T is intended for the top course of double chip seals and has some ¼ inch stones but is mainly retained on the no. 4.

Pictures of Materials

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EQUIPMENT

Asphalt Distributor

Asphalt Distributor - the role of the asphalt distributor is to apply the binder uniformly and in specified quantities. The distributor consists of a truck- or trailer-mounted insulated tank ranging in capacity from 800 to 5,500 gallons (3,000-20,800 liters).

Distributors must be equipped with a heating system that will maintain the binder at the proper spraying temperature and has a system of spray bars with nozzles that apply the binder to the roadway surface. These spray bars cover widths up to 30 feet (10 meters) in one pass.

The major components of a distributor are:

• Asphalt tank - a distributor can consist of a truck- or trailer-mounted insulated tank ranging in capacity from 800 to 5,500 gallons (3,000-20,800 liters). The tank should also have baffle or surge plates to stabilize the load while in motion.
• Heating system - the tank should have one or two propane or diesel fired burners that direct heat into flues that pass through the tank. This allows the contents of the tank to be heated to the correct application temperature. A thermometer must be mounted in the side of the tank so that the correct temperature can be observed.
• Circulating and pumping system - the asphalt pump, radar, control valve and strainer are important parts of the circulating and pumping system. These parts must be checked to ensure they are set properly and there are no leaks.
• Spray bar and nozzles - ensure that the spray bar is at the correct height and that the proper sized nozzles are installed and at the correct angle.
• Distributor controls and gauges - the distributor operator must be familar with the following controls and gauges to ensure that a uniform and precise amount of asphalt binder is applied onto the roadway. They include: thermometer, volume gauge, strap stick pump pressure; and the control panel.

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Aggregate Spreader

The role of the aggregate spreader is to apply a uniform aggregate cover at a specified rate over the freshly sprayed binder. Aggregate spreaders must be self propelled and have a continuous feed feature. The major components of the aggregate spreader are:

• Truck Hitch - visual check for anything broken or bent. Ensure the locking mechanism is working properly.
• Receiving Hopper - the role of the receiving hopper is to receive the aggregate from the haul truck. The receiving hopper must be visually checked for overall condition.
  • There should not be any holes or significant gaps that would allow aggregate to fall through to the road surface;
  • The conveyor belt system should have rubber, neoprene, or fabric cowling around it to prevent aggregate loss; and
  • There must be a flap on top of the receiving hopper to ensure tight fit against haul trucks as aggregate is being dumped into the receiving hopper.
  • A rubber shield prevents aggregate from piling up on the belt, being carried ahead of the receiving hopper and being spilled onto the ground. The rubber shield must be inspected to ensure it is not torn or missing.
• Conveyor Belts - check to ensure they are in good condition and not frayed enough to allow aggregate to spill over the sides onto the roadway. Check to ensure belts are tight. Excessive amount of slack will cause sagging and loss of aggregate off the sides.
• Spreading Hopper - ensure the spreading hopper has no cracks or holes.
• Discharge Gates and Roller - ensure that the discharge gates open and close correctly and that the roller is not warped.

Aggregate Spreader

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Rollers

The functions of the roller are to:

• Orient the aggregate to their flattest side
• Embed the aggregate into the binder
• Achieve mechanical interlock between the pieces of aggregate.

MDOT requires the use of three rollers on chip seal projects.

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Sweeping Equipment

The functions of the sweeping equipment are to:

• Clean the existing roadway of dust and/or foreign materials before chip seal construction
• Removing excess aggregate from constructed chip seals

There are three different types of sweeping equipment typically used in chip seal construction:

1. Rotary Brooms - the commonly used type of broom on chip seal construction projects. The downward pressure must be kept to a minimum to not dislodge the embedded aggregate. The brush must be inspected to see that it can be raised/lowered and rotated horizontally. Rotary brooms do generate dust, which can affect visibility for traffic.
2. Pickup sweepers - generally used wherever dust must be minimized and it is desirable to remove all excess aggregate from the project limits. A pickup sweeper features a broom that sweeps the aggregate to a suction head that deposits the material in a storage tank. Pickup sweepers are particularly useful in urban areas where aggregate accumulation in gutters or along the edge of the roadway is undesirable.
3. Vacuum sweepers - equipment that removes the excess aggregate through suction only. The lack of contact with the chip seal’s surface minimizes damage.

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Equipment Specifications

Calibration

Calibrating and maintaining the equipment used in seal coating is an important step in achieving good results. Poorly calibrated or mis-calibrated equipment can negate what would otherwise be a quality seal coat project. See Form 0557 for instructions on calibrating on chip seals. Calibrating the chip spreader and asphalt distributor ensures that:

• All of the chip spreader gates are applying the same amount of aggregate across its entire width.
• The chip spreader is applying the desired amount of aggregate per unit area.
• The distributor spray bar nozzles are the correct size and all adjusted to the same angle, resulting in triple overlap coverage of the binder across the entire lane.
• None of the spray bar nozzles are plugged with debris.
• The spray bar height is correct.

Spray Bar Nozzle Adjustments

Tripple Lap Coverage

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PRECONSTRUCTION

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CONSTRUCTION

Application Sequence

The following is the general construction sequence of chip sealing:

1. Calibrate the chip spreader and binder distributor.
2. If required, a test strip must be placed in conditions similar to those that are expected to occur during the project.
3. Conduct surface preparation using the guidelines listed in the preparation section.
4. Apply the binder at the designed rate
5. Apply the aggregate at the designed rate
6. Roll the aggregate to embed it into the binder
7. Sweep off any loose aggregate
8. Open to traffic

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Spraying Operations

Distributor Pre-spray Checklist:

• Determine the asphalt distributor velocity and pump speed
• Delineate the distributor shot or limits
• Construct paper joints
• Clean and open nozzles
• Ensure binder is at the correct application temperature
• Check spray bar height

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Aggregate Operations

Stockpile Operation Checklist:

• Proper gradation - the front-end loader should penetrate the stockpile deep enough to have a full range of the aggregate gradation. Segregation could take place while the aggregate is in the stockpile, where the finer particles tend to shift between the coarse particles making the stockpile show a greater content of coarse particles near the top and outside.
• Contamination - each load must be free of contamination. The loader operator should not scrape too close the bottom of the stockpile allowing grass or clay balls to be picked up with the aggregate.
• Degradation - the loader should not allow the front wheels to roll over any of the stockpile. This could lead to larger pieces being crushed into minorer particles thus changing the aggregate gradation.
• Full trucks
• Excess Dust - if dust becomes a problem, it may be reduced by lightly sprinkling the stockpile with water. Only enough water must be used to reduce the dust.

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Rolling

Once the aggregate is placed, rolling should start immediately. The first rollers are the haul trucks that transport the aggregate and will be backing up to connect to the chip spreader. Speeds must be kept under 25 mph.

Rolling a chip seal is different than rolling asphalt. Hot mix asphalt rolling is rearranging the aggregate particles in the mix to create density or compaction. Chip seals are not compacted, rather the aggregate is simply repositioned to place the largest side of the aggregate in the binder.

MDOT requires a minimum of three rollers. They must be in a staggered pattern making a minimum of five passes for asphalt cement and three passes for emulsions. If rollers are unable to keep up with operations, the distributor should stop until rollers catch up. The rolling pattern will depend on the number and types of rollers. The rolling pattern must be checked to verify that aggregate orientation is correct and if it is not correct, then additional rolling will be required.

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Fog Seals

Fog seals help to retain the chips after the chip seal is placed and provide a black surface finish. Fog seals are high encouraged on all single chip seals and are optional on double chip seals but still encouraged. The fog seal operation will be similar to the application of chip seal emulsion.

Chip Seals with Fog Seal - Delay times

As per spec, the contractor must begin the application of the fog seal within 48 hours of the completion of chip seal, but not on the same day as the application of chip seal.

Because the fog seal prevents stone loss and also the temporary raised markings have taken a beating (especially on a double chip seal) the first application of pavement markings after a fog seal may be placed the next day.

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INSPECTION & TESTING

Troubleshooting

The following are some common construction problems with potential solutions. It may take an investigative approach to determine the actual problem.

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STREAKING OR DRILLING OF EMULSION

Streaking or Drilling of Emulsion

Potential Solutions

• Verify that the binder is at the correct application temperature
• Ensure that the correct nozzle size is installed
• Verify the nozzle angle is correct
• Make sure all nozzles are clear
• Verify triple overlap coverage

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WASHBOARDING

Potential Solutions:

• Reduce aggregate spreader speed. If the spreader’s forward travel speed is too fast, the spreader will begin to bounce or jump and the aggregate will fall out of the hopper. When this happens, the aggregate will be heavy for a few inches and then light for a few inches then heavy again. This cycle will continue repeating itself until the spreader speed is adjusted.
• Follow correct patching procedures. Poor patching can also cause this problem.

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LOSS OF AGGREGATE

Loss of Aggregate

Potential Solutions:

• Increase the binder application rate
• Removing the dust off the aggregate or switching to a cleaner aggregate
• Verifying that the aggregate gradation meets specification and is consistent with the mix design results
• Decrease the aggregate application rate to ensure it is not too heavy
• Pre-treat all patches before binder application
• Applying the treatment only when weather conditions are suitable
• Ensure that the aggregate spreaders and rollers were operating closely behind the asphalt distributor
• Control traffic speeds until emulsion is properly set
• Ensure that brooming does not occur before the emulsion is properly set

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BLEEDING OR FLUSHING

Bleeding or Flushing

Potential Solutions:

• Reduce the binder application rate
• Increase the aggregate application rate
• Verify that the distributor and aggregate spreader calibrations are correct
• Delay opening to traffic or use a pilot car for longer

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STREAKED APPEARANCE

Streaked Appearance

Potential Solutions:

• Verifying that the spray bar height is set at 12 inches
• Verifying that all nozzles are set at a 30 degree angle
• Making sure that all nozzles on the spray bar are clean and operating properly

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MEASUREMENT, DOCMENTATION & PAYMENT

Measurement

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Documentation Requirements

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Basis of Payment

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LOCAL AGENCY PROJECTS

Only if applicable

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