The geocomposite membranes (i.e. drainage boards) available on the market today consist of an extruded dimple plastic core attached to a filter fabric. The dimple cores are manufactured from one of two types of plastic—high-impact polystyrene (HIPS) or polypropylene (PP). The fabric is usually a non-woven, needle-punched PP geotextile.
All drainage composites are not created equal. Often, specifiers focus on compressive strength and geocomposite water flow rates.
These are important factors, but reliance on just these technical details can result in less than desirable performance over the life of the structure. The specifier should also consider the membrane’s durability and strength, both during the backfilling process and over the life cycle of the building it is intended to protect.
The dimple portion of drainage boards are made from one of three main types of plastic—HIPS, PP, or high-density polyethylene (HDPE).
According to ASTM D6364-06, Standard Test Method for Determining Short-Term Compression Behavior of Geosynthetics, HIPS has the advantage of delivering high compressive strength ratings. However, HIPS is not as durable as other plastics. Despite giving high compressive strength numbers in laboratory testing, it is subject to stress cracking over time when under load, leaving the waterproofing system vulnerable.
Many designers specify the drainage board by compressive strength numbers without regard for the actual requirements of the project. HIPS drainage boards are specified with, for example, a compressive strength of 8165 kg (18,000 lb)/sf. However, this requirement exists, as that is what HIPS tests at, not because of actual project requirements. For instance, in specifying a drainage board, one might see a requirement for a drainage board with 6804 kg (15,000 lb)/sf for a roof engineered for only a few hundred lb/sf.
PP and HDPE show somewhat lower compressive strength numbers, per ASTM D6364-06, but provide more durability, having higher resistance to stress cracking under long-term load. Unlike HIPS, neither can be torn by hand. It is important to note flow rates through the composites remain similar to those of HIPS drainage boards. Both PP and HDPE can be more easily formed into other dimple shapes, patterns, and heights that will deliver suitable performance at appropriate cost.
This suggests specifications focusing on compressive strength as the primary performance criterion are not taking into account other important factors.
Selecting a dimpled membrane
Dimple membranes can be a suitable choice for applications less than 4 m (12 ft) in depth without hydrostatic pressure. These applications may be residential as well as multi-family or light commercial buildings where full waterproofing and drainage board systems would be overkills.
Deciding on a dimpled membrane for a foundation’s protection system is arguably the right choice for most buildings. Dimpled membranes have many advantages over other systems, including the ability to be installed over any foundation type: poured concrete, concrete block, insulated concrete form (ICF), or preserved wood foundations.
It is always best to protect a foundation at the point of installation before backfill is placed. A well-protected foundation should not require any maintenance. Digging out backfill for remediation can be quite expensive if leaks develop. Therefore, it is always best to approach a repair from the outside, since it is ideal to stop water before it enters the foundation wall. Understandably, this is not always possible when sites have zero lot lines or elaborate landscaping infrastructure. Repairing leaks from the inside is a more economical approach, but it could also be less effective. Water can move through unseen channels within the foundation once in the system. It may be stopped in one location only to arise later in another. Until water intrusion is stopped completely, any interior mold remediation will be a temporary fix since the source of moisture will remain.
Spray-applied asphalt emulsions are a common and code-compliant approach to dampproofing a foundation wall. These coatings are site-applied with effectiveness highly reliant on the installer. They are cost-effective, but do not have the ability to move with the building, leaving natural cracks in concrete exposed to potential water intrusion.
A dimpled membrane offers many benefits. Dimpled membranes provide an even application, factory-controlled quality, and the ability to bridge foundation cracks. This means no water intrusion, optimal comfort, and healthy living spaces for homeowners, and fewer warranty claims and call-backs for building owners. The dimples also create an air gap between the membrane and the foundation, which removes hydrostatic pressure from any incidental water getting behind the membrane, thus allowing the liquid to flow freely to the perimeter footing drain.
When selecting a drainage membrane, design professionals should consider the following:
- type of protection needed;
- best practices to install or secure the drainage membrane to the foundation;
- accessories, and how they affect performance;
- sag, tear, and product collapse risks;
- problem signs with the foundation protection;
- possible damages that can result from a failed drainage membrane or improper installation; and
- proper recourse if an issue arises.