Commercial Foundation Waterproofing
The major cause of many common foundation problems is water. Waterproofing systems are a critical component for keeping foundations dry. Like air control above grade, it is important to consider water control below grade as a system solution, not as a single component.
A properly installed and performing waterproofing membrane is important, but without a proper drainage system to allow the water to flow away, water can damage virtually any construction project. An improperly conceived waterproofing system can allow hydrostatic pressure to build, making any crack in the foundation an access for water damage.
To properly protect a concrete foundation, the wall must be sealed against the water, and the water must be directed away from the wall. Selecting an appropriate below-grade solution for commercial buildings is one of the most important aspects of maintaining structural integrity, indoor air quality, and comfortable living or working conditions for all occupants.
Understanding Site Conditions
In accordance with the 2018 International Building Code (IBC) section 1805.3.2, concrete or masonry walls that retain earth and enclose interior spaces and floors below grade shall be waterproofed – designed and constructed to withstand the hydrostatic pressures and other lateral loads the walls will be exposed to. When selecting a waterproofing system, it is important to understand the overall water management system goals for the site conditions. Soil quality, water table location, and how the system will be installed are all important considerations.
Different soils drain differently. Some hold tons of water, and others will let water filter through. A geotechnical report will help outline soils, possible contaminants and chemicals that may need to be addressed before selecting a below-grade system.
The phenomenon of hydrostatic pressure is simply the weight of the accumulated water pushing against the structure. If the soil surrounding a building is saturated with water, this places intense hydrostatic pressure against the foundation, allowing moisture to push its way through cracks and pores in the concrete walls and up through the floor slab. Four feet of water can exert nearly 300 pounds of pressure per square foot of wall.
Rather than seal a foundation against this pressure, it is more effective and more cost efficient to relieve, control, and manage. The solution to hydrostatic pressure is to remove the water. Drainage systems redirect water away from the foundation, channeling it where it needs to go, but they are generally not enough protection. The inclusion of a footing drain (also known as perimeter drains) installed alongside a waterproofing membrane offers the highest degree of protection. It will carry away all the water that has been directed down to it, helping to prevent continual build-up.
Just as a dry sponge soaks up water, concrete walls have the capacity to wick moisture up from the footings. As Dr. Joe Lstiburek from Building Science Corporation states, the theoretical limit of capillary rise in concrete is 10 kilometers. So-called “capillary rise” is often a serious challenge in structures with concrete footings and concrete foundation walls. And according to Martin Holladay, capillary rise can contribute up to 15 gallons of water a day to a building’s interior moisture load.
Green Building Advisor explains that the phenomenon occurs when the forces of adhesion are stronger than the forces of cohesion. When the attraction between water molecules and molecules in the wall exceeds the attraction of water molecules to one another, capillary rise occurs. This capillary action causes moisture in damp soil to migrate first to the footings and then up into the foundation walls.
In many instances, moisture can travel several feet before the forces of cohesion and adhesion are in equilibrium. In a typical residence, capillary rise may add numerous gallons of water to the moisture load inside the home. The contrast in capillarity from one material to another can even be striking. Water may rise as much as 20 feet in certain clay soils, but only inches in crushed stone.
To avoid the adverse consequences of such moisture migration, a capillary break should be placed between the footing and the wall. Architects, designers, and contractors have two primary options when it comes to capillary breaks between basement walls and concrete footings: membranes and fluid-applied waterproofing.
Contractors should follow manufacturer recommendations and wait before using fluid-applied products on new concrete footings. Some products may require up to a 4-week wait as the concrete must fully cure before application. The concern is that such products will be prematurely applied, potentially compromising effectiveness through improper bonding and even cracking. Of course, scheduling pressures are all too common. Architects that stipulate the use of a membrane applied while the concrete is freshly poured as a capillary break can avoid this problem.
To facilitate the use of a membrane as a capillary break, a concrete wall should be keyed to a concrete footing through the use of a keyway. Vertical rebar can be employed for increased structural integrity when necessary and is typically required in earthquake zones. When vertical rebar is present, contractors often opt for fluid-applied products. However, membranes still remain a viable option.
Although claims have been made that the use of an appropriate concrete additive will reduce capillary rise, many manufacturers of such a product are unwilling to make such a broad promise. For a different solution, some builders choose polyethylene sheeting under footings; however, bonding issues can make it less reliable.
Specifying the Right Drainage Board
Drainage composites are broadly specified as critical components of a successful below-grade moisture management system. However, they are often seen only as adjuncts to the main waterproofing or dampproofing course, tagged onto specifications as mere accessories. Too often treated as a commodity, drainage products are frequently viewed as interchangeable for one another without regard of their ability to meet the performance standards actually required of them.
Geocomposite membranes (i.e. drainage boards) available on the market today consist of two main components: an extruded dimple plastic core with filter fabric attached. The dimple cores are manufactured from one of two types of plastic; high impact polystyrene (HIPS) or polypropylene (HDPE). The fabric is usually a non-woven needle-punched polypropylene geotextile.
All drainage composites are not created equal. Often, specifiers focus on compressive strength and geocomposite water flow rates as the main criteria.
These are very important criteria, but reliance on just those technical criteria can result in less than desirable performance over the life of the structure. In addition, the savvy specifier should consider the durability and strength of the membrane, 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: High Impact Polystyrene (HIPS), Polypropylene (PP), or High-Density Polyethylene (HDPE).
HIPS has the advantage delivering high compressive strength ratings (according to ASTM D6364-06). However, on the downside, 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 critical waterproofing vulnerable.
Many designers will specify the drainage board by the compressive strength numbers without regard to the actual requirements of the projects. HIPS drainage boards are specified with, for example, an 18,000 lb/sf compressive strength required. But this requirement is because that is what the HIPS tests at, not because that’s what the project actually requires. For instance, in specifying a drainage board, one might see a requirement for a drainage board with 15,000 lbs/sf for a roof that is engineered for only a few hundred lbs/sf.
PP and HDPE will show somewhat lower compressive strength numbers (according to ASTM D6364-06), but will provide more durability, having more resistance to stress cracking under long term load. Unlike HIPS, neither can be torn by hand. Flow rates through the composites, though remain similar to those of HIPS drainage boards. Both PP and PE can be more easily formed into other dimple shapes, patterns, and heights that will deliver appropriate performance at an appropriate cost.
This suggests specifications that focus on compressive strength as the primary performance criterion are not taking into account other important criteria.
Selecting a Dimpled Membrane
Dimple membranes can be an excellent choice for applications less than 12 feet (3.7 m) in depth with no 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 overkill.
Dimple membranes create an air gap, producing a way for water to drain to the footing tile and diffuse hydrostatic pressure. 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 virtually any foundation type: poured concrete, concrete block, insulated concrete form, or preserved wood foundations.
A dimpled membrane offers many benefits that sprays simply do not. 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 builders. 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, allowing it to flow freely to the perimeter footing drain.
You choose a product based on project requirements, decent specs, and apparent good quality, but how can you know if the product you’ve chosen is truly appropriate for the job? When selecting a drainage membrane, you should consider the following:
- What type of protection do you really need?
- What type of protection do you believe you’re getting?
- Do you know the best/recommended way to install or secure the drainage membrane to the foundation?
- What accessories are provided, and how do they affect performance?
- Might the product sag, tear, or collapse? How do you avoid this?
- Would you recognize the signs of a problem with your foundation protection?
- Do you know the damages that can result from a failed drainage membrane, or from an improper installation?
- Do you know the proper recourse should an issue/damages arise?
The type of protection clients can expect depends a great deal on the quality of the drainage membrane you select. The dimple height, sheet thickness, and compressive strength vary between manufacturers. Understanding the true requirements for the project will allow the selection of the appropriate drainage board by calling for the appropriate performance criteria
But, none of these moisture-resistive steps addresses the problem of moisture migrating from the footings into the foundation wall. Only proper capillary breaks on footings can stop this undesirable movement of water.
Waterproofing the World’s Longest, Deepest Tunnel
The Gotthard Base Tunnel runs beneath the Alps as part of a new north-south railway in Switzerland. At 56.8 km, it is the longest tunnel in the world, surpassing the Seikan Tunnel in Japan. The total is 152 km, when service tunnels and other shafts are included. With an expected service life of 100 years, and no major repairs being necessary for at least 50 years, this can be a very challenging environment for drainboards.
The Gotthard Base Tunnel is a double-shell tunnel with a combination of waterproofing system and drainage layer between the shotcrete outer shell and the concrete inner shell. This design continuously drains seepage water away to protect the concrete shell against hydrostatic pressure, and to transfer high loads onto the concrete support structure.
Dealing with Water Seepage
Tunnel engineers faced prodigious challenges on this landmark project, including dealing with hydrostatic pressure.
The base tunnel features double-shell construction with a shotcrete outer shell and a concrete inner shell. Between those shells, a crucial drainage layer and waterproofing system continuously drains seepage water and reduces hydrostatic pressure on the concrete shell.
At points where the tunnel burrows under 7,500 ft of mountain cover, geothermal processes send the both alkaline and acidic seepage water soaring to temperatures of up to 45˚C.
To handle these intense conditions, the world’s longest tunnel uses High-density Polyethylene (HDPE) drainboards. These drainboards improve waterproofing by relieving hydrostatic pressure that often builds up against subterranean surfaces.
Although installed in a relatively harsh environment, HDPE drainboards retain their integrity for many decades despite unprecedented physical, mechanical, chemical, and biological demands. The new Gotthard tunnel is engineered to require no major maintenance for 50 years, so the aging processes of polymeric products must be minimal.
Higher Performance Demands New Testing
The anticipated challenges for the tunnel’s drainage systems were such that new testing methods had to be developed to address the unique qualities of the Gotthard base tunnel.
Traditional testing methods were reinforced by newly developed procedures that evaluated:
- Aging characteristics in oxygen-enriched, high-temperature water
- The simultaneous impact of horizontal shear and lateral loads
- Compression creep involving rough surfaces
- Installation issues related to concrete outer shell construction
The testing was conducted by a third party using standardized ISO International tests and was subject to the approvals process of the tunnel authority.
The selected drainboard was subjected to a variety of extreme conditions, including water heated to temperatures up to 70˚C. The drainboards were also tested using 50˚C acidic water containing 0.5 percent sulfuric acid and 50˚C alkaline water saturated with calcium hydroxide. Oxygen-enriched water circulated at 70˚C further tested the durability of the drainboards.
The Gotthard Base Tunnel was a very demanding project with extreme conditions. A special polyethylene-based drainboard was manufactured specifically for the Gotthard Base Tunnel project. Using stringent test procedures, samples were aged over a period of 24 months. During the aging period, the specimens were submerged in acidic and alkaline solutions at 50°C and in oxygen-enriched water at 70°C and then tested again. All required product specifications needed to confirm the aging properties could be met.
Selecting an appropriate below-grade solution for commercial buildings is one of the most important aspects of maintaining structural integrity, indoor air quality, and comfortable living or working conditions for all occupants. A water-managed foundation system prevents water build-up at the foundation wall. No hydrostatic pressure, no force. And without pressure, there is no need to worry about water seeping through concrete cracks and pores that will lead to mold and other moisture-related problems.
Selecting a proper waterproofing system is a critical component for keeping foundations dry. A properly installed and performing waterproofing membrane coupled with a drainage system will correctly allow the water to flow away without damaging your construction project.
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