Category:409 Seal Coat

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409.1 Construction Inspection for Sec 409

409.1.1 Description (Sec 409.1)

Seal coats, also known as chip seals, are used to lengthen the service life of an existing pavement by waterproofing it and improving its surface texture.

409.1.2 Material (Sec 409.2)

See also Field Sections 1003 and 1015 of the Materials Manual.

409.1.3 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.

409.1.4 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.

409.1.5 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 and dangerous. 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. All loose aggregate must be removed before the roadway is opened to traffic.

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.

409.1.6 Traffic Control (Sec 409.6)

See Traffic Control in Paving Operations. Otherwise, the diesel used in the production of the cutback will degrade the binder in the subsequently placed HMA overlay.

409.1.7 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 Materials Inspection for Sec 409

409.2 Procedure

409.2.1 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 Field Section 1001. When possible, the JMF and correspondence should be transmitted electronically. The Materials Field Office e-mail address is "mfo".

409.2.2 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 insure 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
Average Daily Traffic (ADT)
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.

409.2.3 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.
f. For verification, a trial is prepared and submitted to the Laboratory.
g. When Central Laboratory tests are completed, the results are compared to the contractor’s and against the specifications. If the mixes tested cannot be used, the mixture will be rejected.

409.3 Report

A letter of transmittal will accompany the approved mixture to the district Operations Engineer with distribution as follows:

Title Copy of Transmittal Letter and Approved Mix
District Operations Engineer 1
Project Operations Clerk 1
Resident Engineer 1
Field Office File 1

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 Operations Engineer is to be forwarded to the contractor.

409.3 Laboratory Procedures for Sec 409

This guidance establishes procedures for Laboratory testing and reporting performance characteristic tests on seal coats.

409.3.1 Procedure

The aggregate and bituminous material shall be tested, when required, for performance characteristics in accordance with MoDOT Test Method T72. Test results shall be recorded through SiteManager.

409.3.2 Sample Record

The sample record shall be completed in SiteManager as described in Automation Section 3510, and shall indicate acceptance, qualified acceptance, or rejection. Appropriate remarks, as described in Reporting Test Results, are to be included in the remarks to clarify conditions of acceptance or rejection. Test results shall be reported on the appropriate templates under the Tests tab.