For all its wonderful attributes and the undeniable impact it’s had on the building industry, concrete does have one inherent challenge that can’t be avoided; it’s not waterproof. All concrete, regardless of application methodology, is susceptible to leaking when proper preparation steps are not taken. It doesn’t matter if the application methodology is shotcrete or form and pour; the result, in either case, is concrete and concrete is not waterproof.
To combat this inherent issue, an entire industry has evolved to ensure that concrete applications avoid leaking problems. Whatever the method of application, the key to ensuring a water-tight concrete structure is preparation and more specifically, envelope preparation. Envelope engineers, consultants and material suppliers all work hand-in-hand with concrete providers to eliminate this challenge to concrete’s structural integrity.
When the envelope assembly is properly specified and installed, the inherent shrinkage cracking of concrete, regardless of application method, is not an issue because the impregnable envelope creates a barrier to water intrusion. With no water to get into any cracks, you have a waterproof installation. It follows then, that envelope integrity should always be the focal point of any discussion regarding waterproofing concrete.
Unfortunately, however, the focus is all too often mistakenly shifted to the concrete application methodology. Shotcrete applications have erroneously been described as more difficult to waterproof. This is simply not true. It’s time to set the record straight; time to get to the root of the problem by dispelling old myths and clarifying the facts.
If one application methodology can ensure a greater likelihood of uncompromised envelope integrity, it would only make sense that it would be considered the best choice to ensure a waterproof result.
Shotcrete applications work better than form in pour with regards to waterproofing in all scenarios. That’s a bold statement, but it’s the truth. Let’s look at why this is so:
- Shotcrete allows for better coverage and consolidation of concrete in tight reinforcing situations (dense rebar) where typical internal vibration required by form and pour would be impossible.
- Shotcrete minimizes penetrations in the envelope assembly compared to conventional formwork systems. These formwork penetrations can all be sealed properly as recommended by the manufacturer’s typical waterproofing specifications and details.
- When concrete is shot directly against the envelope assembly coated with the chosen membrane, it exerts less pressure and force on the membrane than form and pour. The drop pressure of concrete in form and pour is more likely to tear and/or fold or move the membrane than shotcrete.
- Shotcrete is less intrusive on the envelope than form and pour because it does not contaminate the drain mat surface area prior to consolidating the concrete against it.
- Shotcrete allows for a visual inspection during the application process to ensure that the integrity of the building envelope assembly is not violated. You can fix what you can see!Shotcrete eliminates the potential of cold joints as typically found in form and pour applications. With shotcrete, there’s no such thing as a cold joint. You can stop shooting and come back a year later and if the surface and surround are properly prepared you can resume shooting and still come away with an immaculate, seamless structure.
- Concrete that is shot into place is denser than form and pour. It all boils down to one word — velocity. When you shoot concrete onto a form or the earth at 300 to 400 feet per second, it compacts better and becomes denser than if it’s poured into place.High-velocity delivery produces excellent compaction. Velocity = compaction, compaction = density, density = strength, strength = water tightness.
New urban building regulations are also impacting concrete waterproofing efforts. The biggest regulatory change in downtown below-grade construction is the requirement to minimize groundwater to sewer discharge. New regulations are calling for what’s referred to as “bath-tubbing” of buildings – creating a watertight exterior barrier for the building to effectively sit in. Bath-tubbing creates more head pressure and puts a higher demand on the below-grade envelope system. This necessitates a more robust and watertight building envelope. Shotcrete application methodology has opened the door for materials suppliers to bring new, continuous, self-healing, bondable continuous membranes to the forefront. These membranes are far superior to existing typical building envelope solutions and are injected after all initial shrinkage cracking occurs. These new injectable membranes represent the future of waterproofing technology and they’re only possible with shotcrete applications. They can’t be used in form and pour applications.
It’s becoming more and more apparent that the answers to waterproofing challenges in concrete structures are more easily found when shotcrete is the method of application.