The Challenges of Using Shotcrete in Waterproofing Applications
The utilization of structural shotcrete for below grade foundation walls has benefits for building developers, above and beyond its performance capabilities. Time savings and material costs are so significant that using shotcrete often outweighs the potential drawbacks and risks to the building envelope – risks that are, at times, pawned off on the waterproofing manufacturer.
But manufacturers are not the ones who should shoulder this responsibility entirely. Waterproofing manufacturers need to learn how to better manage the risks shotcrete presents and simultaneously educate project teams on its less desirable traits, which tend to present challenges or create project delays. Proper consolidation of shotcrete at application can mitigate some of these issues, but there are still factors which can be problematic.
In blindside waterproofing, cast-in-place concrete walls are vibrated once they have been placed to remove air pockets and minimize large voids. In contrast, shotcrete is – literally - shot directly into place. Ideally, the shotcrete nozzleman should be highly skilled in its application, and be able to force the shotcrete onto the wall at such a high pressure that it is placed and compacted simultaneously.
According to the article Shotcrete Nozzleman Basics – Vertical Wall Construction appearing on Shotcrete.org, some material – or aggregate - will rebound off the surface and can accumulate in the joint during placement, leading to poor consolidation or voids. If consolidation of the shotcrete is poor at the perimeter columns, complications can occur, particularly in hydrostatic conditions as there is no concrete present to support the waterproofing system.
Along the perimeter wall, interior support columns are required for shotcrete walls to provide additional support. They are poured in place after waterproofing materials have been installed but before the shotcrete is applied. Consolidation challenges here are similar as those found at the slab-to-wall interface, except the joint is vertical and runs the length of the wall.
Poor consolidation can also create issues in back-lagged conditions and at building corners. It can also be present in areas where long joints are created and when block outs are created to allow for the removal of tiebacks.
Due to the nature of its application, and the unpreventable, loose aggregates which inevitably rebound off the foundation surface, shotcrete systems are often riddled with consistency issues that create voids in the foundation wall. This is most common along the slab-to-wall interface where rebar congestion, and the work of an unskilled shotcrete nozzleman may produce shotcrete that falls below the 90% compaction standard. As an additional challenge, the shotcrete application process lacks a mechanism for removing air, giving it the propensity to create large voids if not properly applied.
And this is not the only instance where voids can be an issue: They also create problems in a structural wall. Voids can put a lot of stress on the waterproofing system, something that cannot be predicted precisely in the field, or replicated in a laboratory setting. Testing of the waterproofing system can help predict its performance, but real-world conditions are different and nuanced. A test done in lab to analyze a system’s tensile strength will be very different than the organic, real-world stresses that may be created on the same system in the field.
Waterproofing materials can only preform when properly supported, and this means owners must be willing to spend some of their savings to help accommodate the challenges shotcrete presents. Prudent investments can be made in the following ways:
- Building time into the construction schedule to address complex transition areas and areas with poor access
- Taking time to encourage proper coordination with trades: Many of the challenges found with shotcrete can be overcome with proper sequencing and discussion.
- Conducting research to utilize more robust composite waterproofing systems that leverage redundancy
As an example, a multi-component, redundant system can help keep the building envelope dry and safe from groundwater infiltration or harmful contaminants. Such a method requires that systems be fabricated in the field – not in the factory – using multiple protective materials designed for a specific use. This redundancy is achieved by combining different types of waterproofing materials to leverage the positive attributes of each and to eliminate a continuous seam. EPRO Services Inc. are pioneers of this method, and tailor their systems to precisely and effectively match changing and sometimes unpredictable site conditions.
Many of the challenges presented by shotcrete and its application can be prevented when project teams, and specifically building owners, understand that the cost savings generated with shotcrete walls is not absolute. In fact, there are more effective methods and products that can be used to protect the building envelope – ones that will protect the building and its occupants well into the future.
By Peter Grant, Vice President of Sales and Marketing, EPRO, Inc.
Peter is the Vice President of Marketing & Sales at EPRO, Inc. and has 16 years of building envelope protection experience