Bentonite comes in premanufactured sheets, such as CCW’s MiraClay, a specially constructed below-grade foundation waterproofing system. (Photos courtesy of Carlisle Coatings and Waterproofing.)

Bentonite waterproofing systems have been on the United States market for nearly 80 years. A primary reason for their longevity has been technological advances that have made them economical and easy to apply. The traditional form of bentonite was extremely labor intensive and required skilled workmen for precise applications. The material was sprayed or troweled onto the substrate, which sometimes led to inconsistent application thickness.

Trowel and sprayed bentonite applications are rarely completed anymore.

Bentonite is granulated smectite clay that provides waterproofing capabilities by swelling to nearly 15 times its dry volume when it absorbs water. The swelling is caused by the material’s molecular structural form, which is comprised of expansive sheets that can expand based on the grading and clay compositions. Bentonite sheets typically contain 85 percent to 90 percent montmorillonite clay and a maximum of 15 percent natural sediments such as volcanic ash.

In recent times advanced technology of polymer chemistry has increased the use of bentonite by making it a more versatile material. It is currently offered in four forms:

1. Prefabricated panels.

2. Prefabricated geotextile sheets.

3. High density polyethylene sheets.

4. Trowelable mixtures used for detailing.

Advantages of Bentonite

Bentonite waterproofing systems offer the following advantages:

• Easy installation.

• No VOC restrictions.

• Safe application - even at extreme temperatures.

• Easy leak detection.

• Can bridge cracks up to 1/4 inch.

• Adapts to complex geographic shapes.

Disadvantages of Bentonite

Bentonite waterproofing systems have the following disadvantages:

• Need for constant (high) hydrostatic pressure.

• Lack of dependable resistance to vapor migration.

• Limited options for future repairs or replacement.

Application of Bentonite

The increased use of bentonite as a waterproofing system stems from the versatility of the material and the fact that it can be used in difficult conditions. Bentonite is not suited for all applications; however, in it is well matched for intricate conditions such as blindside applications in deep excavations. The material is also commonly used in tunnels and subways and has a long history of waterproofing New York City basements. Application is not recommended in areas sensitive to moisture infiltration.

The primary form of application is with prefabricated panels or sheets. Installation for slabs-onground are completed by simply laying the panels or sheets on the substrate and lapping the panel joints. On vertical surfaces, the panels or sheets are nailed to the substrate. Application over concrete substrates requires a smooth surface with no deformations or shark fins. All honeycombs, indentations or rock pockets should be repaired with trowelable bentonite prior to panel or sheet application. Trowelable bentonite is also applied at all penetrations and bottoms of walls. Panel or sheet laps must be taped if precipitation is likely or if the material will not be backfilled the same day.

Backfill should be completed immediately after installation. Installation is typically completed in 8-foot to 10-foot lifts with backfill after every course. The backfill should be compacted to a minimum of 85 percent protector density and applied within 2 inches of the top of the panels or sheets. Backfill should be placed directly against the panels or sheets or a protection course (film or insulation) is required to prevent moisture from reaching the bentonite. Once the backfill is complete, hydration of the benonite is required.

For blindside applications, benonite can be applied over concrete, steel, lagging walls, slurry, retaining walls, and diaphragm walls retaining earth. Most applications simply require nailing of the sheet or panel to the substrate. The one difference with these applications is the requirement of block outs in walls for tieback plates or rakers. The details for these block outs should be in accordance with the manufacturer’s specifications.