Considerations for Selecting a Below-Grade Waterproofing System
When selecting a below-grade waterproofing system, conservatism is the watchword. Remember, you only have the opportunity to do this once! Proper research and due diligence is required to ensure that all below-grade waterproofing issues are properly addressed. Site-specific issues and building requirements also necessitate proper consideration. There are several considerations that an architect or waterproofing designer must examine prior to the selection of a below-grade waterproofing system. Some of the considerations are:
• The water table.
• Soil characteristics.
• Substrate stability.
• The construction sequence.
• The track record of the products to be used.
• Risk vs. cost.
• Ease of application.
Important design factors to consider include leak risk tolerance and sensitivity to humidity of an occupied space. Leaks are an obvious detriment in most occupied spaces, but they are intolerable in occupancies with book storage, art storage, computer equipment, electrical switchgear and medical facilities. These types of facilities also require tight humidity control. Intrusion of air can be as detrimental as water in sensitive facilities. Medical facilities, research facilities and testing laboratories fall within this classification.
Proper membrane design for the aforementioned facilities would require positive side waterproofing with a low vapor permeable membrane.
The Water Table
The water table level is an important consideration that not only determines the type of waterproofing required, it determines if waterproofing is required by code. The International Building Code requires that waterproofing or dampproofing be applied on all below-grade structures where the ground water table is maintained a minimum of 6 inches below the ground slab.
Accurate soil bearings - completed by a competent civil engineer - are required prior to waterproofing design. In the Northern Hemisphere, the water table is typically highest after the spring thaws have saturated the ground. The water table is typically lowest following the summer evaporation of the surface moisture. Proper waterproofing design should be based on the maximum water table level for the site.
Waterproofing materials are unique because they are exposed to much harsher conditions than any of the other building exterior components. Most of the exposure elements are continually present at the waterproofing surface and do not dissipate as they do at the other exterior components. For example, water can be present in below-grade surfaces for weeks, whereas water on roof systems is usually removed within 48 hours.
One element of consideration involves soil characteristics. Chemicals in the soil can have an adverse effect on some materials, and knowledge of the potential chemicals present is required for design. Chemical properties in soils can adversely affect waterproofing in various ways. Acids and alkaline substances in ground water can accelerate the deterioration of concrete and steel reinforcing bars. Salt in water corrodes reinforcing bars in concrete. Sulfates can have a negative reaction with Portland cement, resulting in internal shearing stress that causes spalling. Other chemicals that affect waterproofing include calcium hydroxides, oils and chemicals from fertilizer.
The physical properties of the soil can also affect the waterproofing and must be considered. Clay soils of low permeability limit underground hydrostatic pressure. The intensity and nature of hydrostatic pressure can force water into tie-rod holes, cold joints and rock pockets. Hydrostatic pressure can also turn minor imperfections into probable sources of leaks.
Waterproofing is applied on substrate surfaces to protect the substrate from structural deterioration caused by water, chemicals and soil characteristics. The applied waterproofing material must also be capable of performance if the substrate becomes unstable or minor imperfections occur. Some substrates are inherently prone to imperfection and this should be considered prior to waterproofing design.
With substrates that are vulnerable to cracking from any source, the waterproofing membrane used must be elastic and capable of resealing. Cracks can occur in masonry or other waterproofed components that have multiple construction joints. Dampproofing should not be considered in these conditions.
The types of soils at the site can also have an impact on substrate stability. Expansive soils and peaty soils can produce rising and settling footings that induce cracks in footings and foundation walls. All substrate openings can become potential points of moisture infiltration.
Waterproofing applications are typically completed in phases as construction of walls and plaza decks are completed. There may be extended periods of time between certain waterproofing applications and final completion. For instance, waterproofing of below-grade walls is typically completed in 6-foot to 8-foot increments from the bottom to the top. The initial section is completed, the site is backfilled and the waterproofing applicator uses the backfill as a scaffolding to complete the next increment. This process is completed until the full wall is waterproofed. This process could be completed over an extended time period because the waterproofing applicator’s work is subject to the pour schedule and backfilling operations.
Completed waterproofing materials are rarely exposed to exterior elements after final application procedures are completed. The membranes are covered by soil, concrete or another type of top surfacing. However, due to the initial construction sequence, these materials may be exposed to these elements for an extended period of time. Consideration must be given to this fact in the material selection process.
In the last decade, the waterproofing market has become very competitive and manufacturers that provide other exterior building component materials - primarily roofing - have entered the market. Some of these products are well suited for these types of applications. Some are not! The fact that as a designer you only get one chance to do this right should be a key basis of consideration when evaluating these products. One simple form of evaluation is to check the track record of the product in similar waterproofing applications. Sources of product performance history could include other designers, building owners, contractors or specialty consultants.
It could also be beneficial to ask specific questions of the manufacturers. The first question should be whether the product is manufactured by the seller or if the seller is merely a distributor (under a private label agreement). This is very common in the construction materials industry. Manufacturers typically private label some - or all - of the system components. Determining the identity actual manufacturer may be an important consideration in material selection. This type of information is available through Dun and Bradstreet.
If you need help determining a manufacturer’s track record, contact other designers or waterproofing contractors in your area or throughout the country to find out their history with these materials. If you talk to other industry professionals or manufacturers, it would also be important to find out if the product has a history of successful performance under comparable conditions for at least 15 to 20 years. Obviously, if the product has not performed well in the past, it is safe to assume it will have the same difficulties on your project. Designers should also be skeptical of other building component materials being marketed for waterproofing applications without a track record in similar conditions. No other building component is exposed to the harsh conditions that waterproofing materials must face.
Another important consideration is whether the material has maintained a consistent formulation for the past 10 years. This is a significant concern at this time. Environmental regulations and depletion of natural resources have had a large impact on current material formulations. Products that we have relied on for hundreds of years are not as readily available. In some cases, this situation has left the industry with unstable materials and new formulations with no verifiable track record.
Risk vs. Cost
The designer should always minimize risk despite any reasonable - or unreasonable - costs. If a building owner or general contractor wants to cut costs, the waterproofing system is not the place to do so. The cost of excavation for repairs - even minor repairs - far exceeds the initial cost of the waterproofing. Remember, as a designer, you only have one chance to do it right.
Ease of Application
The relative ease of difficulty of applying the product is a minor factor compared to the other considerations. However, it may result in better workmanship, so it should be considered. There will also be projects where access or space constraints are a consideration. Application methods, particularly material adhesion methods, may be determined by the site constraints. On projects where other factors are in balance, ease of application of a material or a system can be a determining factor in material selection.