Testing Initial Leakage Resistance of Bentonite Waterproofing: An Exploratory Study
Bentonite is a naturally occurring material with a well established property—expansion in the presence of water. When the expansion is sufficiently confined, hydrated bentonite forms a dense layer that stops the passage of liquid water.
Traditionally, bentonite waterproofing panels have consisted of granular bentonite laminated between layers of kraft paper, in a product with the appearance of corrugated cardboard. In newer products, bentonite is incorporated in a composite sheet that is shipped in rolls and can be cut as required and fastened in place. These composite sheets are often faced with a layer of impermeable plastic sheeting such as high-density polyethylene (HDPE) or a geomembrane. In some products there is a protective layer added for materials that are intended to have concrete placed against them.
The laps of bentonite sheets are typically not sealed during construction; they are intended to be self-sealing. The confined bentonite is presumed to expand and seal against the impermeable plastic sheet facing at the lap. Depending on the product and its application, manufacturers required lap widths that vary from 38 mm (1-1/2 in.) to 15 mm (6 in.). One of the aspects of bentonite that this study explores is the effect of varying the lap dimension.
If bentonite is not confined, leakage can occur. This study explores the degree to which variations in the confinement of hydrating bentonite affects its performance. For example, it is reasonable to expect excellent confinement when bentonite is placed over a relatively smooth structural slab and is then covered with a concrete topping slab, which is typically at least 76 mm (3 in.) or 102 mm (4 in.) thick. The placement and finishing of the topping slab, in addition to its weight, assures that the bentonite is uniformly confined where it is covered by the topping slab.
Applications on walls present more of a challenge to achieving uniform and lasting confinement. In positive side applications, bentonite is fastened to a below-grade wall and then back-fill is installed. Bentonite manufacturers specify a minimum degree of compaction for backfill, but possible future settlement of soil may alter the bentonite confinement resulting in possible leakage.
Even more challenging is the use of bentonite in positive-side, blind applications. The bentonite is attached to a retaining wall, which may have an irregular surface. For example, with walls consisting of steel piers and wood lagging boards, there is an offset in the plane of the wall between piers and boards, and there may be an offset between boards depending on the presence of cupping or bowing of the lumber. During the life of a building, wood lagging boards may decay and allow some change from the original confinement of the bentonite. In some cases, retaining walls are covered with drainage composite boards which ought to be interlocked at edges but are sometimes installed overlapped at the edges. The practice of overlapping drainage boards results in an offset equal to the thickness of the drainage composite board, which may be in the range of 6.4 mm (1/4 in.) to 25 mm (1 in.) thick. The concrete wall is expected to provide confinement on its side of the bentonite waterproofing; however, placement of concrete can result in undetected rock-pockets or voids that result in localized lack of confinement.