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The Architect's Guide to Blindside Waterproofing

When your project requires blindside waterproofing, don’t jump into it blind. With numerous product technologies and types used in applications today, it is crucial to understand how their performance characteristics will impact your waterproofing system design. Read on to learn the key differentiators among blindside waterproofing options and best practices for managing risks and preventing moisture infiltration.

Basics of Blindside Waterproofing

below-grade construction site

Blindside waterproofing is used in construction situations when excavation outside of foundation walls is not feasible. This method is common in urban areas where a project has close proximity to bordering structures or there is little room for maneuvering heavy equipment. Here is the basic process for blindside application:

  1. Excavation is shored with the lagging wall using methods such as soldier pile-and-wood lagging (SPAWL), soil-nailing, or shotcrete.
  2. Drainage mat is installed (when applicable).
  3. The waterproofing is applied to the shoring wall and detailed.
  4. The structural walls are placed against the waterproofing.
  5. The foundation is poured.

When rebar is installed for preparation of the structural wall, or when shotcrete or concrete is placed against the waterproofing, there is a risk of damage to the waterproofing membrane. Several waterproofing technologies are designed to help overcome these challenges. Designers and specifiers should be aware of their features to identify which may be best for a particular installation.

Blindside waterproofing technology options

Spray-Applied Membranes

Spray-applied membranes provide for a monolithic coating with a chemical bond to the concrete.  This system is easier to detail, and using a co-spray accelerant, such as calcium chloride, allows the membrane to cure quicker. The material is typically sprayed on a carrier sheet. Some manufacturer systems spray material directly on the drainage mat, which in some cases may not be optimal due to the dimples on the drainage.  It is best to spray onto a flat surface to allow for the correct mil coverage and consistency across the entire surface.

Bentonite-Based Sheet Membranes

Bentonite sheet membranes have a long history of success due to their controlled mil thickness and multiple components for waterproofing, such as an HDPE material with bentonite. When the bentonite is hydrated, it swells to create an impermeable barrier. Some manufacturers also offer variations that are usable in salt water. These systems tend to be resistant to chemicals and are particularly robust for methane mitigation when heat welded, which is a common practice on the west coast. Bentonite sheet membranes can have some drawbacks, however. They require compaction to the concrete, and installation is typically a labor-intensive process that requires a specialized trade. For all applications, care needs to be taken to prevent the bentonite from prematurely hydrating, so it needs to be covered with a plastic protection sheet if wet weather is imminent in exposed conditions.

Asphaltic Sheet Membranes

Derived from the roofing industry, asphaltic sheets are a reliable blindside waterproofing option when you need a flexible material or when applying in cold temperatures. They are comprised of a modified asphalt-polymer blend with reinforced fiber and are adhered to concrete with a mechanical bond. Seams are heat-welded together, which can make the application process more labor intensive.

application of below-grade waterproofing

Plastic Sheet Membranes

For high strength, consider sheet waterproofing membranes made with synthetic materials such as polyethylene, polymers and resins. They are impact, chemical and low-temperature resistant, but they can be a more complicated system due to additional detailing involved with penetrations. Their fabric or adhesive layers can provide a mechanical or chemical bond to concrete.

Synthetic-Rubber Sheet Membranes

Synthetic-rubber sheets include flexible and durable materials such as EPDM. They have excellent water resistance and can withstand UV and extremely high temperatures. They bond mechanically to concrete, and some product options may have benefits such as self-healing and self-sealing capabilities for small penetrations.

Additional considerations

When deciding which route to go in product selection, consider these key project factors:

  • Water table height
  • Hydrostatic pressure
  • Construction method
  • Soil characteristics
  • Temperature restrictions
  • Code requirements

Also check the products’ adherence to ASTM testing to ensure it fits your project requirements. Some important performance characteristics related to physical properties include:

  • ASTM D5385 – Hydrostatic head resistance
  • ASTM D1970 – Low temperature flexibility
  • ASTM D903 – Peel strength of adhesion bond
  • ASTM D412 – Elongation / strength at break
  • ASTM D4833 – Puncture resistance
  • ASTM E96 – Permeance
  • ASTM D4068 – Resistance to micro-organisms

Mitigation of blindside application risks through proper sequence and detailing

After selecting the best product or system for your project, the installation itself has risks that can inhibit effectiveness as a barrier to moisture intrusion. But the design team can and should help minimize the likelihood of errors in the field by ensuring the following information is included and described in detail in the project specifications: 

Pre-construction meetings

The purpose of the pre-con meeting is to establish communication between all trades and parties involved prior to construction to ensure that everyone understands who is responsible for each task and when completion should occur.

The pre-construction meeting should include all of the following activities:

  • Review all project drawings to determine if the proposed details can be constructed as intended by the designer/specifier. Consider requiring a performance mock-up to validate the performance of the system. Read more about performance mock-ups here.  
  • Review the system specifications.
  • Discuss material selection with each trade and confirm product compatibility.
  • Review construction details, including tie-in areas.
  • Identify sequencing of all trades with a comprehensive construction schedule.
  • Discuss any other project-specific considerations.

Check out five additional ways the specifier can help prevent leaks.

Surface preparation

The surface to be covered must be smooth, firm, and free from mud, loose mortar, wires, fins, metal projections or any other substances that might prevent placement of membrane. With a wood lagging wall, no gaps greater than 1” are acceptable. Any protrusions or changes in plane greater than ¼” should be made even with mastic, sealant or plywood.

Installation of sheet below-grade waterproofing

Pay attention to vulnerable areas

The lack of proper detailing and installation of proper gaskets or waterstops in the structure are the most common causes of ground water infiltration. Several key areas below-grade are particularly prone to moisture infiltration:

  • Penetrations
  • Cold joints
  • Expansion joints
  • Foundation drains
  • Structural connections
  • Damaged membranes
  • Transitions and change of condition

To learn more about how to ensure proper detailing of these key penetrations and transitions, attend this on-demand AIA-accredited webinar: Building Enclosure Architectural Details – Conveying Construction Drawing Clarity

A warranty that protects

Lastly, it is important that your project is protected, as such a vulnerable stage cannot be easily remediated. When looking for a blindside waterproofing product warranty, consider a single source warranty that offers watertight leak coverage, including at seams, details and transitions, and that provides specific repair information.

Always check your blindside

Blindside waterproofing can be a challenging but necessary and increasingly common method of below-grade waterproofing. It is crucial to ensure your system is not compromised by properly detailing areas susceptible to moisture intrusion, as it can be very tricky to make any repairs after the fact.

For more on this topic and to earn AIA CEUs, watch this Architectural Record webinar: Minimizing Risk in Blindside Waterproofing Applications.

blind-side waterproofing application
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