We study history in an attempt to avoid making the same mistakes that the civilizations before us made. Even though history has a tendency of repeating itself, we do get some insight and can at least compare the same current mistakes to other times in history. The same context can be made for waterproofing design as past failures can direct us to proper design methodology. Therefore, it is beneficial for a designer to have knowledge of why waterproofing materials or systems fail to prevent duplication of these errors in the future.
Most waterproofing materials and systems have limitations to their use or specific application requirements that must be followed for long-term success. Use of these materials or systems in elements that contribute to their failure can be considered as designer error. Therefore, it is paramount that waterproofing designers understand the failure modes prevalent to specific waterproofing components prior to design.
The most expensive waterproofing failures occur at below-grade foundation walls. The major cost of these repairs – even if they are at localized points – is through the excavation and removal of surfacing, which may include concrete and expansive landscaping. There are five prevalent failure modes at foundation walls:
Improper backfilling
Defective joint details
Concrete wall defects
Defective flashing at penetrations
Vapor entrapment
It should be noted that all of these defects can be avoided with proper quality assurance of the waterproofing materials during application. However, there are design elements that can be employed to limit these problems.
Improper backfilling
Improper backfilling is often times directly related to contractor error. However, if the designer properly specifies backfill requirements in the design documents there is a greater probability that contractor error will be observed (and corrected) during quality assurance.
Proper backfill requirements include the type and size of aggregate applied and the compaction rates (90% per ASTM 1557) required for successful application. Any aggregate applied should be rounded with no sharp edges that could puncture the membrane; the backfill should also be free of tree-branches and roots.
Compaction of the backfill is required to eliminate the potential of membrane slippage that could occur through movement of the backfill materials. The backfill operation should occur in controlled lifts and not from simply dumping backfill into the excavation. Most industry experts require that the backfill material be applied in twelve (12) inch lifts while mechanically compacting the backfill throughout the application.
Defective joint details
One of the most common failure points at foundation walls is from moisture infiltration that occurs at open joints located at the top and bottom of the foundation wall. The top foundation wall joints are highly susceptible to moisture infiltration because there are multiple points where successful waterproofing is reliant on the proper construction of these joints. The two critical top joints are:
Where the structural slab supports plazas or planted sites
Where the horizontal construction joint where the structural slab attaches to the foundation wall
Waterproofing membranes applied at these locations should be reinforced to prevent splitting from possible stress concentration.
The bottom of the foundation wall is also vulnerable to moisture infiltration at joints. Proper waterproofing at these locations is paramount due to the cost of excavation that would be required to reach these points. A potential problem area is at a wall footing that is at the same elevation as the mud slab. In this application the waterproofing membrane is applied down from the wall and horizontally over the toe of the wall footing of the mud slab. The problems at this point typically arise due to a decrease in adhesion between the underlying mud slab waterproofing and overlapping wall membranes created by soil and contaminants that inevitably build up on the wall footing from excavation or storm water in the lapse of time between the waterproofing of the mud slab and the pour of the footing.
This is primarily an application error created by improper preparation of the membrane surface. However, the designer could make it clear at this location that proper membrane preparation (removal of all debris, residue and contaminants) is required for complete adhesion.
Concrete wall defects
Foundation walls differ from structural slabs or slabs-on-ground in the fact they are typically not finished to a continuous plane surface. Because of the less stringent finish requirements, the vertical surfaces of the concrete walls often exhibit deformations such as tie-rod holes, honeycombing voids, formwork kick outs or rock pockets. All of these irregularities should be properly repaired prior to the waterproofing application because hydrostatic pressure could produce flexural stresses that rupture the membrane and lead to moisture infiltration.
The waterproofing designer should provide language in the design documents that indicates proper finish requirements, how to determine what concrete defects require attention and the proper repair methods for effectively correcting these defects. A very good source for insertation into the design documents on this subject is ASTM D 5295. This standard provides the required language to properly identify and correct concrete defects. The designer should comply with the manufacturers concrete finish requirements.
Defective flashing at penetrations
Industry statistics indicate that approximately 75% of all moisture infiltration occurs at flashings and penetrations. It is the designer’s responsibility to ensure that the proper materials are specified at these critical points. The designer should also include design details for these locations.
Waterproofing flashing material should always be reinforced even in non-reinforced liquid applications. It is also critical that the flashing is set a minimum of eight (8) inches above the surface so the designer should ensure that penetrations are a minimum of twelve (12) inches above the slab. Another best practice would be to provide a minimum space of six (6) inches between penetrations to allow for sufficient room to complete the proper flashing application.
Vapor entrapment
Evidence of vapor entrapment at foundation walls is typically illustrated by blisters that occur between the substrate and the membrane. This occurs from an elementary design error of specifying a membrane on both sides of a foundation wall. In these cases, the water vapor is trapped in the wall because it has no opportunity to vent out in either direction.
An Ounce of Prevention
Waterproofing is something you’ve got to get right the first time. That’s the mantra of John D’Annunzio, president of Paragon Roofing Technology, who has been a building consultant for more than 25 years. “The cost of repairing a waterproofing issue after the building is completed is almost always extremely expensive and time consuming — especially when you compare it to the cost of initially applying waterproofing materials in a new construction project,” he said. “Once you bring in a backhoe and start ripping up landscaping and concrete, the cost of ensuring the waterproofing materials were applied correctly will seem nominal.”
D’Annunzio has investigated lot of waterproofing failures over the years, and he chalks most of them up to three factors:
1. Poor workmanship. “The vast majority of problems I’ve seen are due to application errors,” he said. “The most common trouble spots are at details and penetrations.”
2. Lack of oversight. “Often theses errors aren’t caught because the installations aren’t being adequately inspected,” he said. “Unlike other phases of construction — roofing, for example — we don’t see enough oversight when it comes to waterproofing applications.”
3. Inadequate design details. “Details are the most difficult spots to execute, but manufacturers often do not provide specific details for every type of application, leaving designers to rely on general language in the documents, which can be subject to misinterpretation.”
In many cases, D’Annunzio’s failure investigations determined the waterproofing materials were never even applied. “This is all too common in below-grade applications that are completed in stages over long periods of time,” he said. “The wall applicator, waterproofing applicator, and backfill applicator have to be in sync, or you might end up with a section of wall that is backfilled before the surface is even waterproofed.”
These types of application errors might not be the fault of the designer, but that won’t be apparent when the water begins pouring through the basement walls. The key is preventing theses types of problem in the first place — and that takes communication. “It’s in everyone’s interest to make sure all the parties are communicating and that the work is completed in accordance with the design requirements,” D’Annunzio said.