Nearly all geomembrane installations are fixed at the perimeter of the containment by burying the liner in an anchor trench or by attaching it to concrete (i.e., a collar wall, foundation wall, etc.). The standard approach for connecting the geomembrane liner to concrete is to use a batten. In this technique, concrete lags are set (i.e., drilled) into the concrete. These are placed on 6" or 12" centers at an engineer’s discretion. Neoprene or butyl tape is then placed between the liner and concrete, and a stainless steel, like 316 SS, or aluminum batten strip is placed over the liner and the lag bolts are driven through the batten, liner, and taped into the lag. The top of the detail is then sealed with a waterproof caulk (see Figure 1).
This approach is tried and true and the only option when using geomembranes on sites with existing concrete. However, when working with new concrete, geomembrane manufacturers have had an alternative approach available for some time now: using the embedment strip. Embedment strips are “beams” made of the same polymer as the containment liner. They’re embedded into concrete in order to provide a surface on which to weld the liner. Embedment strips are available in all geomembrane polymer types; while their dimensions vary from manufacturer to manufacturer, dimensions are typically around 1" D x 4" W x 10' long (see Figure 2).
Figure 2: Embedment Strip
After an embedment strip is installed by screwing its face to the inside of the concrete form (see Figure 3), concrete is poured (complete distribution of the concrete into the embedment strip is crucial and vibration of the concrete is important). When the form is removed, the surface of the strip is revealed. Cleaning the embedment provides a surface on which to weld the geomembrane (see Figures 4a and 4b). The geomembrane is then typically “tack” welded with hot hair, the excess liner is removed, and the exposed edge of the liner is finished with an extrusion weld (see Figure 5).
Embedment strips offer a couple of advantages:
- Speed of installation: Welding to an embedment strip can reduce the time it takes to attach a liner to concrete—it can save you anywhere from minutes to hours.
- Cost reduction: Stainless steel batten and anchors are expensive. Those, along with the additional hours of labor, can add up to a greater cost for the geomembrane installation. Using an embedment strip can be a less expensive alternative.
- Use in horizontal or vertical applications: Embedment strips can be used in both. It’s a current best practice, however, not to use them below the water line in tanks and lagoons.
In addition to choosing the right concrete contractor, who will be pivotal in the installation of your embedment system, there are a number of other highly important considerations to keep top of mind when using embedment strips:
- Installation quality: It’s important the strip be affixed to the forms so that it remains level and has a good end-to-end connection—the point at which the ends of the strips touch should be sealed with an extrusion weld. Gaps out of tolerance can be corrected with this detail, but minimizing them is good practice. It’s strongly recommended someone oversees the concrete contractor’s embedment strip installation.
- Cost: Most concrete contractors are unfamiliar with this system. As the prudent among us do, they’ll often meet uncertainty with higher price. This is why providing clear specifications and a moment of instruction at a pre-bid meeting can help to reduce uncertainty and subsequent cost.
- Thermal expansion/contraction considerations: When deciding to use embedment by polymer type, note that there are two categories of geomembranes: reinforced and unreinforced (i.e., films). In general, reinforced geomembranes have lower coefficients of thermal expansion (in addition to greater mechanical properties) and films have a lower cost.
- The types of reinforced geomembranes: There are a number of reinforced geomembranes to choose from:
- Ethylene Interpolymer Alloys (rEIAs)
- Polypropylenes (rPPs)
- Chorosulfonated Polyethylenes (rCSPEs)
- The types of films: There are also a number of films from which to choose:
- High-Density Polyethylene (HDPE)
- Linear Low-Density Polyethylene (LLDPE)
- Polyvinyl Chloride (PVC)
Thermal expansion can be a problem during construction or after the project is complete. If the embedment expands (or contracts) after the concrete is poured, it can distort the shape of the embedment strip in the uncured concrete (on any axis) creating an uneven welding surface. After the concrete is cured, expansion/contraction of the embedment strip can cause cracking or spalling and separation of the strip from the concrete.
Thermal expansion/contraction is most likely to occur in polyethylene (PE) embedment. At the other end of the spectrum, EIA liners and embedments have a coefficient of thermal expansion that is roughly two orders of magnitude less than that of PE. Thermal expansion has not been observed in EIA embedments. The embedment strip must be made of the same polymer that the geomembrane is made of (with the exception that PVCs can be welded to EIAs).
Finally, installation guidelines vary by manufacturer. It’s important to comply with each manufacturer’s unique guidelines when using their form of embedment strip. When properly installed, using embedment strips can reduce cost and speed up geomembrane installation.
What is your experience with using embedment strips as an alternative approach for attaching geomembranes to concrete? Tells us about it in the comments!