Difference between revisions of "Category:409 Seal Coat"

Seal Coating (previously called chip sealing) is the application of a bituminous binder immediately followed by the application of an aggregate cover. The aggregate is then rolled to embed it into the binder. The desired functions of a seal coat are:

1. Seal the road surface against the entrance of moisture and air.

2. Provide a new skid resistance wearing surface.

3. Rejuvenate or enliven a dry or weathered surface against abrasion and disintegration.

4. Reinforce and help restore design surface.

5. Provide a demarcation in texture between the road surface and the shoulders for traffic guidance.

6. Cover ragged patches and other discrepancies.

Seal coats cannot be used to correct the pavement profile or correct a rough ride. When selecting a roadway for seal coating, the existing pavement should exhibit a good cross section and a good base, and have a good ride quality. Working cracks should be sealed, and non-working cracks ≥ 3/8 in. wide should be filled prior to the placement of the seal coat (For more information on working and nonworking cracks see Crack Treatment in Bituminous Pavements).

409.1 Design

In contracts using Standard Specification Section 409, the type of bituminous material and grade of liquid asphalt is selected by the contractor. Differing aggregate qualities are available and specified according to the roadway traffic, as shown in the tables below:

Grades A1 and B1 are coarse (3/8”) aggregate seals and should be used on pavement surfaces with minor distresses.

Grades A2 and B2 are fine (1/4”) aggregate seals and should be used on pavement surfaces with little to no distresses.

To improve the perception of the driving public, Sec 409 requires the finished seal coat to have a dark appearance. See EPG 751.1.3.6 Deck Treatments for guidelines on aggregate grade selection for bridges.

A seal coat placed on a porous surface such as an aggregate base should receive a prime coat.

409.2 Construction Inspection for Sec 409

409.2.1 Description (Sec 409.1)

Seal coats are used to lengthen the service life of an existing pavement by waterproofing it and improving its surface texture.

409.2.2 Material (Sec 409.2)

See also 1003 Aggregate for Seal Coats and EPG 1015 Bituminous Material.

409.2.3 Job Mix Formula (Sec 409.3)

Proper design of a seal coat consists of determining the proper rate of application for the bituminous material and cover aggregate. The mix design procedure will be in accordance with Materials Inspection.

409.2.4 Equipment (Sec 409.4)

See Distributor in Paving Equipment. A rotary power broom or vacuum sweeper will be used to remove excess loose aggregate. Pneumatic tire rollers will be used to embed the cover aggregate. The contact pressure of the tires shall be at least 80 psi. In order to achieve the required contact pressure, the contractor may need to adjust the inflation pressure of the tires.

409.2.5 Construction Requirements (Sec 409.5)

Weather Limitations (Sec 409.5.2)

See Weather Conditions in Paving Operations.

Application of Bituminous Material (Sec 409.5.4)

See Distributor in Paving Equipment. Excess binder works upward to the surface of the pavement and creates a black, sticky surface condition (i.e., flushing, bleeding, etc.). In wet weather, the surface may become slippery. A black appearance in the surface can also be the result of insufficient cover aggregate or a loss of cover aggregate.

Applying too little binder to the surface leads to a loss of cover aggregate because insufficient binder is present to hold the aggregate in place. Generally, the use of too little binder occurs less frequently than the use of too much.

Application of Cover Aggregate (Sec 409.5.5)

Cover aggregate is applied with a self-propelled spreader. Aggregate spreading should progress rapidly as a continuous operation. Stopping and starting of the distributor and/or aggregate spreader should be held to a minimum. The operation shall proceed in such a manner that the binder will not be permitted to cool, set up, dry or otherwise impair retention of the cover aggregate. To achieve this, the maximum time interval between applying the binder and spreading the cover aggregate should be limited to 30 seconds.

Any deficiencies in application of cover aggregate resulting in uncovered areas or nonuniform application of aggregate should be immediately corrected behind the spreading equipment. Any excess aggregate should be removed immediately to avoid damage to the seal coat and possible build up of materials, which could adversely affect the ride quality. Pull brooms or drags should not be used to distribute cover aggregate until embedded aggregate is set. If dust is a nuisance or impairs visibility, the contractor will be required to pre-coat the aggregate.

The rolling operation should immediately follow spreading to embed the aggregate while the binder is still soft and tacky. Pneumatic tired rollers must be operated slow enough to prevent the tires from displacing or picking up aggregate. All rolling must be completed the same day that the binder and aggregate have been applied. After material has cured, the surface should be thoroughly swept.

The major causes of loss of cover aggregate are insufficient binder, dirty or dust covered aggregate, or opening the roadway to traffic before the seal coat has cured. If there is a long delay between the application of the binder and the cover aggregate, even during warm weather, the binder may cool and harden and good adhesion is seriously impaired.

Sec 409.5.5.1 If required by the contract, a seal coat may be applied to a bridge deck before a new hot-mix asphalt (HMA) surface is placed using the following guidelines. If an emulsion is used in the seal coat, the HMA surface may be placed in the same day. However, the haul trucks must not be allowed to make turning movements on the fresh seal coat application so that displacement of the cover aggregate does not occur. If a cutback is used, the seal coat must be allowed to cure for a minimum of seven days. Otherwise, the diesel in the cutback will soften the asphalt binder in the HMA overlay.

409.2.6 Traffic Control (Sec 409.6)

See Traffic Control in Paving Operations.

409.2.7 Basis of Acceptance (Sec 409.7)

The criteria outlined in Standard Specification Section 409.7 must be met prior to acceptance of the project. The following items are suggestions for an acceptable project:

(1) Route trucks onto the project in such a manner that they will not have to turn on the newly spread aggregate.

(2) Keep the distributor and the aggregate spreader within approximately 30 seconds of each another (220 ft. at 5 mph).

(3) Construct transverse joints with building paper that is held securely in place.

(4) Construct longitudinal joints with a slight overlap so that a gap between the applications is avoided. The joint should be located near the lane line.

(5) Operate the distributor at a slow enough speed to ensure uniform spread and to reduced the elapsed time between the application of binder and aggregate.

(6) Use a shield on the outside end of the spray bar on the first of two adjacent spreads, and on both ends on the final passes, to prevent tapering of binder spread where aggregate will not adhere.

(7) Operate the distributor and the aggregate spreader at approximately equal speeds with a minimum distance between them.

(8) If necessary, patching, brooming, and/or salvaging should be performed immediately behind the aggregate spreader.

(9) On superelevated sections, the length of the binder application may need to be reduced in order to minimize the transverse flow of the binder before the application of cover the aggregate.

409.2.8 Method of Measurement

The quantity of emulsion to be paid for will be determined in accordance with Sec 1015.6 and rounded to the nearest whole gallon. The volumetric determination, which requires adjustment to the volume at 60° F, should be made using the formula in EPG 407.1.5 Method of Measurement.

409.3 Materials Inspection for Sec 409

409.3.1 Procedure

409.3.1.1 Mix Design Procedure

In order for a seal coat mix design to be approved, the contractor’s proposed job mix formula (JMF) shall be submitted as required in Report. Trial mix samples will not be required unless requested by the Field Office. If requested, trial mix samples, along with samples of asphalt binder, must be obtained and submitted to the Central Laboratory in accordance with EPG 1001 General Requirements for Material. When possible, the JMF and correspondence should be transmitted electronically. The Materials Field Office e-mail address is "mfo".

409.3.1.2 District Procedure

When a proposed mix design has been received by the District, as required by the Standard Specifications, the District shall check the calculations thoroughly and ensure that the materials listed and sieve analyses shown are correct and that the proposed mixture components and proportions comply with Specifications. It may be necessary for the District to advise the contractor to make changes in the proposed mixture in order to fully comply with Department policies. When the District is satisfied that a proposed mixture is acceptable, a copy of the formula and the contractor's letter shall be submitted to the Materials Field Office, accompanied by a letter of transmittal with comments, any corrections made and recommendations. The letter shall contain the following information:

Project information – Job Number, Route, County, Contract Number.

Grade and Source of Asphalt Binder

Letting Date

Proposed Work – Type of Seal Coat, Job Location and Length

Annual Average Daily Traffic (AADT)

Mix Use – Mainline, Shoulders, etc.

Quantity of Mix

Included in the letter should be information regarding the approximate date on which the contractor intends to begin placing the mixture on the roadway.

409.3.1.3 Field Office Procedure

The Materials Field Office is charged with the responsibility for processing the mix design. General procedures for processing a seal coat mix design are as follows:

a. A letter from a District requesting a mix with a copy of the contractor's formula and letter attached is received.

b. Contract specifications for the project are checked for necessary items.

c. Grade of asphalt as well as the refinery to be used and percent asphalt recommended are reviewed.

d. Gradation of the aggregate is checked for specification compliance and compared with the gradation determined by the Laboratory.

e. All calculations on the proposed job mix are checked.

409.3.2 Report

A letter of transmittal will accompany the approved mixture to the District Construction and Materials Engineer with distribution as follows:

The letter of transmittal and the approved mixture will be sent by electronic mail to the individuals listed above.

A copy of the approved formula accompanied by a letter of transmittal from the District Construction and Materials Engineer is to be forwarded to the contractor.

409.4 Maintenance Seal Coat

Seal coats have traditionally been used on lower volume roads because of vehicle windshield damage caused by the loose aggregate that remained on the roadway surface. However, with improvements of the bituminous binder, proper design and construction procedures the loose aggregate can be minimized. The small details are very important with the application of a seal coat. Seal coats should be restricted to 2-lane, 2-way roads with less than 3,500 vehicles per day, and 2-lane, one-way roads with less than 14,000 vehicles per day (7,000 vpd 1-way).

Maintenance seal coating shall not be done on asphaltic concrete pavements unless approved by the Maintenance Division or unless the asphaltic concrete section has been downgraded to a service road, supplementary road, etc., carrying low volumes of traffic. If leveling of the surface or increased stability is needed, consideration should be given to the placement of a leveling course or mat. Continuous seal coats are recommended only for level, even, and stable surfaces, preferably with an appreciable base, as a seal coat in itself is not intended to increase the strength of the roadway. All seal coating should be done when weather and surface conditions are right and this definitely applies to continuous seal coats and seal coats on bridges. The roadway surface should be dry, humidity low, and it should not be done when precipitation is imminent. Temperatures should be warm but not extremely hot.

Seal coats require proper design, adequate supervision and inspection to obtain good results.

409.4.1 Seal Coat Considerations

The type and grade of asphalt, specification of cover aggregate and the rate of application of the asphalt and aggregate should be carefully determined. The condition of the surface to be treated and the type and specific weight of the aggregate should also be considered during the design of the seal coat.

Adjustments should be made in the field if necessary to obtain proper results. Recommendations as to equipment, materials, construction procedures and traffic control for continuous seal coats are as follows: Also listed are some types of failures and their causes.

1. Normally CRS-2 emulsified asphalt or penetration asphalt is used. Rates of application of the asphalt may vary from .20 to .40 gallon per square yard dependent on the condition of the surface to be treated, the type and grade of asphalt and the specification of the cover aggregate, 0.3 gallon per square yard is the normal rate. The rate for the asphalt should be so that the aggregate is embedded approximately 70% in the cured asphalt.

2. Bituminous distributors used for seal coat work should be operating properly and should be equipped with the necessary controls to ensure proper application of the asphalt. Pull type distributors are not to be used for major or continuous seal coat work. The application rate for a bituminous distributor with a positive displacement pump is a function of the spray bar length in feet, the pump discharge in gallons per minute, and the distributor speed in feet per minute. The normal spray bar "factor or pump discharge is 10 to 15 gallons per spray bar foot of length for 1/8 in. nozzles spaced 4 in. apart. Center to center spacing on the spray bar is now almost universally 4 in. apart although some older models still have a 6 in. spacing of nozzles. For distributors equipped with different size nozzles spacing consult the manufacturers recommendations. Spray bar nozzles are to be clean and adjusted so that the solid fan patterns do not interfere with or distort each other. Normally each slit opening of the nozzle is adjusted to a 15 to 30 degree angle with the spray bar centerline. A 30 degree setting will allow passage of air currents without undue distortion to the fan pattern.

3. The spray bar height setting is especially important for several reasons. It should be set at a minimum height to reduce blotches and voids caused by air turbulence. All distributors with four inch nozzle spacing must be set at a height which provides an exact triple coverage and all distributors with six inch nozzle spacing must be set for a height which provides an exact double coverage fan pattern. If the spray bar height is set higher than that needed to obtain the double or triple coverage, voids or ridges will result. The proper spray bar height for triple coverage can visually be checked before use by using the asphalt material specified at normal spray temperature and pressure by closing the second and third, fifth and sixth, eighth and ninth, etc., nozzles of the center section only of the spray bar, and adjusting the bar height until perfect single coverage is obtained. Double lap coverage involves the same procedure as above except every other nozzle is left open. If the distributor has already been checked for double lap coverage, increasing the spray height 50% will give triple coverage. These tests should be run on a clean, damp, dust free surface for good observation. The rear springs of the distributor are to be chained down to the rear axle under full load to prevent a change in spray bar height as the load is emptied.

4. The cover aggregate shall meet Standard Specifications for seal coat aggregate. Expanded shale, Iron Mountain chat or similar hard angular aggregate may be used on medium and heavy traffic asphaltic concrete surfaces. The rate of application of cover aggregate will depend on the specific roadway application. The amount of material may vary between 25 to 30 pounds per square yard and the specific weights of the material used for cover should be considered in determining this rate and also the type and grade of asphalt used. There are certain aggregates that are so ionized that they will not adhere to the asphalt District Materials personnel should be consulted regarding the aggregate and the asphalt that is to be used. The aggregate should be as free of moisture as possible, but normally this cannot be accomplished without excessive costs and usually the moisture present will not affect the adhesion.

5. The cover aggregate is to be applied with an approved mechanical spreader, at an even rate. Mechanical spreader types are wheeled box type spreaders and self-propelled spreaders. The wheeled box type and self-propelled spreader require special truck hitches. The self-propelled spreader provides for very close control over the application rate of the aggregate and also has a sloped screen attachment that allows the larger aggregate to be deposited on the fresh film of asphalt first. Operators of spreaders and supply trucks must be familiar with and efficient in the operation of their units prior to the start of operations. Self-propelled 5 to 8 ton steel wheel rollers and pneumatic tired rollers are used for rolling on continuous seal coats. Light rollers are used since only one layer of aggregate is involved and heavier rollers will crush softer aggregates. Pneumatic tired rollers are necessary due to the unevenness of most roadway surfaces. On spot seal coat work, rolling is accomplished with truck tires and pull type steel wheel rollers.

6. The following construction procedures are ideal and should be followed as closely as possible on major seal coat jobs especially on bridge floors, high type pavements and where traffic volumes are heavy. The preparation of all surfaces to be seal coated is of utmost importance especially on bridge floors. Surfaces should be clean and free of loose aggregate. All necessary patching, removal of corrugations, etc. is to be accomplished prior to sealing. Old seal coats or patches which may cause bleeding or failure on the new seal coats should be removed. On shoulder work any necessary wedging and edge joint pouring should be done prior to sealing.

7. The recommended spray temperature of the asphalt is to be maintained by reheating if necessary to provide for a uniform application. This is more critical with penetration asphalt than with cut-backs. The standard specifications should be consulted for the correct spraying temperatures. The asphalt application should be so that it may be covered with aggregate immediately. This is also more critical with penetration asphalt. With self-propelled spreaders, it is usually easy to accomplish. With other spreaders, the amount of asphalt applied should be controlled to that which can be covered immediately. This applies to spot seal coating as well as continuous seal coats. Building paper is to be used at transverse joints to prevent unsightly and bumpy joints. Care should also be exercised by distributor operators to prevent overlapping at the longitudinal joints. The design of the spray bar allows for overlap at the longitudinal joint.

8. Dry aggregate will provide best results under all conditions except when an emulsion asphalt is used but is not always possible. A slight amount of moisture on a dry, warm day will usually not cause any trouble. On bridge floors or high type pavement where traffic volumes are heavy, it may be practical to dry the material prior to application. Drying aggregate on a mixing board has proved to be practical in some areas. The cover aggregate is to be spread uniformly and as close to the design quantity as possible. Insufficient cover will lead to bleeding while excess aggregate will have a tendency to pull embedded aggregate loose under traffic conditions. A self-propelled spreader will provide best results. If a wheeled box type spreader is used the supply trucks should be operated by drivers who are proficient on this operation.

9. Any brooming with a pull broom after the embedded aggregate is set must be supervised closely and confined to the cooler hours of the day. Only enough pressure should be applied to remove excess aggregate. Any objectionable, loose aggregate which develops after the section is opened to traffic should be broomed off to prevent damage to vehicles. Any excess aggregate left along the edge of bridge floors is to be removed as soon as possible.

10. Rolling should begin immediately after the cover aggregate is applied. Rolling with a steel wheel roller is to be confined to one pass. At least two complete passes with pneumatic tired roller should be accomplished prior to opening to traffic. The recommended sequence of rolling is two complete passes with the pneumatic tired roller and then one complete pass with a steel wheel roller. On spot seal coating, rolling is done with a steel wheel roller and truck tires.

409.4.2 Traffic Control Considerations

Proper traffic control is of utmost importance in obtaining good results on any seal coat work and cannot be stressed too strongly. The degree of controls is dependent to some extent on the type and grade of asphalt, weather conditions, traffic density and traffic speed. After rolling is completed traffic will not normally damage a seal coat if restricted to low speeds. The controls listed below are considered a minimum and must be followed as closely as possible.

1. Signing and traffic control devices are to be in accordance with the Traffic Control for Field Operations, and are to be in place as long as any hazards, such as loose aggregate on the traveled surface exists. On extremely heavy traffic sections, it may be necessary to request some assistance from the State Highway Patrol during peak hours of traffic. Adequate warning signs and flagging must be provided on any section where lanes are barricaded overnight.

2. On continuous seal coats, no traffic is to be allowed on the seal until rolling is completed; then traffic is to be controlled by pilot vehicle at a maximum speed of 20 miles per hour for at least two hours before being opened fully to traffic. On sections with less than 250 AADT, this control by pilot vehicle may be reduced to one hour. On multi-lane pavements, where traffic is normally very high, it is recommended that no traffic be allowed on the seal coat for a period of 4 to 8 hours or possibly overnight and then allowed to pass at 20 miles per hour for one hour before full opening. This control at restricted speeds will not only ensure a better seal coat but will also reduce the hazards to traffic from loose aggregate. On shorter sections of continuous seal coats, such as on bridge floors, where a pilot vehicle would not be feasible, the traffic is to be restricted to the 20 miles per hour by flagging and it will probably be necessary to stop traffic from both directions to hold their speed to 20 miles per hour.

3. Traffic control on spot seal work is usually not as critical as on continuous seal, but the proper flagging and signing to assure safety and to minimize the whipping off of aggregate by traffic must be used.

409.4.3 Seal Coat Failures

It would be extremely misleading to give the impression that all seal coat jobs are successful even though proper materials are used. Failures do occur and the important thing is to know why they occur and how to prevent them.

1. Streaking results when alternate longitudinal strips of the seal coat contain different quantities of asphalt binder. This is due to lack of uniformity of application. Some of the more common causes of streaking are mechanical faults, improper spray bar adjustment, and careless operation of distributors. Another frequent cause is applying the asphalt at a temperature so low that it is not fluid enough to fan out properly from the nozzles on the spray bars. Streaking can generally be avoided by using a clean and properly adjusted distributor and applying the asphalt at the correct temperature. Careful spray bar adjustment is important to prevent streaking when high viscosity materials are used.

2. The use of too much asphalt in seal coat work is a common fault as well as a serious one because of the safety hazard it presents. Excess asphalt works upward onto the pavement surface and is the origin of the black and sticky surface condition referred to as flushing, bleeding, fattening up, etc., which becomes slippery to traffic in wet weather. It should be pointed out that a black appearance can also be caused by insufficient cover aggregate or may in part, be due to loss of a portion of cover aggregate.

3. The application of insufficient asphalt binder leads to loss of cover aggregate because not enough binder has been applied to cement the aggregate particles into place. Generally, the use of too little asphalt occurs less frequently than the application of too much. This emphasizes the need for very careful attention to proper application quantities.

4. The major causes for loss of cover aggregate are insufficient asphalt, dirty or dust covered aggregate, or permitting fast traffic on the road before the seal coat has cured. If there is a long delay between spraying the asphalt binder and spreading the cover aggregate, even during warm weather, the asphalt may become so cooled or hardened that good adhesion between the two is seriously impaired. This is especially true when cutback asphalts and asphalt cements are used for the binder. Field studies show that non-uniform asphalt application occurs most frequently during cool weather. The asphalt temperature drops to that of the pavement within approximately two minutes. For this reason weather and temperature limitations should be observed.

5. The strength of a road section is analogous to the strength of a chain and its weakest link. Obviously, any seal coat will fail if the supporting road structure cannot bear the traffic. Although a seal coat must be tough and durable along with its other desirable characteristics, it is not generally considered a load bearing factor in the overall design. The subbase, base and surface courses are designed for this purpose. Seal coats are not feasible for a road which has a soft or poor base or uneven surface. Also roadway surfaces which will require excessive spot sealing, patching or wedging should not be considered for seal coats as these conditions will penetrate the seal coat and cause bleeding and tracking of asphalt and possibly failure. Leveling courses or mats should be considered for surfaces such as the above. When a level, stable surface has been obtained a seal coat/chip seal may then be considered.

409.4.4 Maintenance Seal Coat Design

Refer to Chapter 4 of the Minnesota Seal Coat Design Procedure regarding the proper procedures for designing a seal coat for a specific roadway application.

409.5 Maintenance Planning Guide

Printable Maintenance Planning Guide for Chip Sealing.

Index of all Maintenance Planning Guidelines.