Asphaltic Plug Joints (APJ) for Bridges

Asphaltic Plug Joints (APJ) are a widely used expansion joint system for bridges, specifically designed to accommodate bridge deck movements. These joints are primarily used in bridges with a total motion of less than 50 mm (2 inches), making them a versatile and cost-effective solution for general-purpose applications. APJs are essential in sealing expansion joints on asphalt concrete overlay and Portland cement concrete decks. However, they are not suitable for applications where the expansion or contraction exceeds ±50 mm from the installation width.

This article explores the materials, construction process, physical requirements, testing methods, and procedures involved in the installation and use of Asphaltic Plug Joints in bridge construction.

Materials Required for Asphaltic Plug Joints

The key materials that go into the construction of an Asphaltic Plug Joint are carefully chosen to ensure durability, stability, and resistance to environmental factors. These materials include:

1. Asphaltic Binder: The asphaltic binder used in APJs is a thermoplastic polymeric-modified asphalt. This special binder is designed to be flexible and capable of withstanding the extreme conditions encountered in bridge expansion joints. The physical properties of the binder must meet specific standards to ensure the joint functions correctly over time.
2. Aggregate: The aggregate used in APJs must be crushed, washed, and dried to remove impurities. The aggregate is mixed with the binder at a specified ratio to achieve the right consistency and performance. The materials should be pre-weighed and packaged on-site to ensure accurate mixing.
3. Closed-Cell Foam Expansion Joint Filler: A closed-cell foam filler is used in the expansion gap to ensure the joint remains watertight and flexible. This filler is non-gassing and resistant to high temperatures during the installation process (up to 199°C), ensuring it can withstand the high temperatures of the asphaltic binder during application.

Physical Requirements of Asphaltic Plug Joints

APJs must adhere to strict dimensional and tolerance specifications to ensure their effective performance. The design of the joint, including its shape and size, must meet the necessary requirements outlined in the field construction specifications. The standard minimum blockout dimensions are 50 mm x 500 mm; however, these values can vary depending on the specific field conditions and the unique requirements of the bridge being constructed.

Construction Procedure of Asphaltic Plug Joints

The construction of Asphaltic Plug Joints involves several precise steps to ensure a durable and effective joint:

1. Heating the Materials:
   • The asphaltic binder is heated to the specified temperature using a melter equipped with a continuous agitation system. This system ensures that the binder remains homogeneous and free from impurities.
   • The aggregate is also heated in a manufacturer’s recommended mixer, with the temperature controlled by a digital sensor to ensure uniformity.
2. Blending and Coating:
   • Once both the binder and aggregate reach the required temperatures, the binder is blended with the heated aggregate in the prescribed ratio. It is essential that the aggregate is completely coated with the binder to ensure the joint’s strength and durability.
3. Placing Closed-Cell Foam Filler:
   • The closed-cell foam expansion joint filler is placed in the expansion gap at a depth that is no greater than the width of the gap. This filler plays a crucial role in maintaining the joint’s flexibility and preventing water infiltration.
4. Bridging Plate Installation:
   • The bridging plate, typically made from aluminum or mild steel, is installed over the entire length of the expansion gap. The plate should be a minimum of 6 mm thick and 200 mm wide, and it must be centered along the joint gap. In some cases, the plate will feature pre-drilled holes for securing the plate using locating pins.
5. Compaction Process:
   • The blended, heated asphaltic binder and aggregate mixture are then placed into the blockout and compacted using a roller or plate compactor. This ensures that the joint is tightly sealed and can withstand the forces exerted on the bridge over time.
6. Antiskid Surface (if required):
   • If an antiskid surface is needed, the surface of the APJ is heated before broadcasting antiskid material on it, following the manufacturer’s instructions. This surface ensures that vehicles do not experience excessive friction or slipping over the joint.

Test Material Sampling

For quality control, random samples of the materials used in the construction of APJs must be taken and tested to ensure compliance with standards:

1. Asphaltic Binder Sampling:
   • A batch of thermoplastic polymeric-modified asphalt is typically no more than 19,100 kg, and samples from each batch must be obtained for testing. Each sample for testing should weigh a minimum of 1.4 kg.
2. Aggregate Sampling:
   • A batch of aggregate may include up to 20,000 kg, and at least 23 kg of aggregate is needed for size and gradation analysis.
3. Closed-Cell Foam Filler Sampling:
   • For the closed-cell foam filler, a sample of at least 300 mm is required for testing.

Test Methods for Asphaltic Plug Joints

A key part of the quality control process for Asphaltic Plug Joints is testing the materials to ensure they meet required performance standards. One important test is the Low-Temperature Cone Penetration Test, which measures the binder’s ability to perform in cold temperatures:

1. Apparatus and Specimen Preparation:
   • The penetrometer cone should weigh 150.0 ± 0.1 g. The cold chamber must be capable of maintaining a cold test temperature of -61.1°C. Test specimens are prepared by pouring asphaltic binder into tins, which are then allowed to cure according to the manufacturer’s specifications.
2. Procedure for Testing:
   • The test specimens are placed in the cold chamber for at least 4 hours at -18°C to -6°C. Afterward, the cone is applied to the specimen, and penetration is measured to determine how well the material performs under cold conditions.
3. Test Report:
   • The average of three test results is recorded and reported as the penetration value in millimeters. This value helps assess the suitability of the asphaltic binder for use in cold environments.

Conclusion

Asphaltic Plug Joints (APJ) play a vital role in bridge construction, providing a reliable and flexible solution for accommodating the natural movements of bridges. With the proper selection of materials, precise construction methods, and rigorous testing procedures, APJs can effectively seal expansion joints, ensuring the safety and longevity of bridges.