It seems there are still some misconceptions regarding structural shotcrete and the issue of blindside honeycombing and voids that ultimately affect water tightness. There’s an erroneous opinion in some corners of our industry that shotcrete applications result in more honeycombing and voiding issues than form-and-pour applications. It’s just not true. Let’s set the record straight by taking a look at the facts.
The most crucial factor in preventing honeycombing and voids and therefore achieving a more watertight concrete wall, regardless of application technology, is compaction. Properly placed concrete will achieve a superior level of compaction that serves to eliminate honeycombing and voids on blind-side walls. With a form-and-pour application, the skill of the vibrator person is crucial to avoiding improperly placed concrete. In a shotcrete application, a blowpipe operator working in front of the nozzleman uses an air compressor lance to spray highly compressed air on the target area immediately prior to the shotcrete being placed to ensure that no contaminants and or rebound remain in the wall. In highly congested rebar areas, the assistant nozzleman will sometimes use a small vibrator, similar to that used in a form-and-pour application to tickle/vibrate the shotcrete to ensure proper compaction. The skilled combination of these application details makes all the difference in achieving a superior level of impaction. Simply put, the greater the expertise of the application team, the better the compaction results.
Another important component of quality blind-side walls is the proper design and installation of the reinforcing rebar. There is nothing additional required for a shotcrete application, except that reinforcing bars should be lapped in front of each other at splice points rather than side by side to allow for the concrete to flow with less interruption during the shooting process.
All of this applies to both form-and-pour and structural shotcrete placement; the needs for both processes are the same. The quality of concrete placement with blindside waterproofing depends on the experience of the concrete contractor, whether it’s a shotcrete or form-and-pour job. Both application methodologies offer identical opportunities for success or failure. It’s not the process that matters, it’s the prowess. Qualified concrete contractors use properly trained people, the correct placement equipment, and they plan ahead with the on-site team to make sure the concrete is placed correctly.
Far from being a problem, when all the advantages are considered, the utilization of structural shotcrete for blindside walls is rapidly expanding and is becoming a common solution on tight urban sites with fast-track schedules that require a water-tight below-grade structure. Once project teams fully understand the similarities and differences of form-and-pour and structural shotcrete and get past the misconceptions, they can benefit from the following shotcrete advantages to create a better blindside waterproofing assembly. To that end here are some of the positive differences that can result from a shotcrete choice:
- Shotcrete speeds up the concrete placement schedule because little or no formwork is needed, thus freeing up the workforce and crane time, and requiring less on-site formwork storage.
- A steel trowel final finish on the exposed surfaces, requiring far less touch-up than form-and-pour.
- The ability to place concrete for blindside walls around obstructions such as shoring struts and rakers while still addressing tall, complex wall configurations.
- The unique ability to shoot around properly spaced in-place control joint inducers, which helps to meet ACI requirements to reduce wall thickness by 20 to 25% at control joints.
- Shotcrete delivers concrete to the bottom of the wall without the segregation of concrete dropping through the reinforcement and with no formwork in the way, enabling complete consolidation at the bottom of the wall and creating a better construction joint for the blindside waterproofing.
- Shotcrete can achieve better concrete consolidation in walls with dense reinforcement because greater access is available with no formwork in the way, making it easier to create a denser surface for the blindside waterproofing integrity. Pre-shoot mock ups replicating the most heavily reinforced areas will confirm the success of this.
- With much lower concrete pressure and no impact dropping concrete from the top of a form, shotcrete inherently has far less risk of damaging the waterproofing system.
- Shotcrete is placed in short, visible lifts, with overspray blown off the blindside waterproofing continuously until the end of placement. There is no additional risk for both mechanical and chemically bonded waterproofing degradation due to a buildup of dry concrete paste, which typically happens in form-and-pour.
- Shotcrete reinforcing bar stabilization requires far fewer anchorage penetrations than most one-sided formwork systems, therefore requiring less detailing, and reduces the risk of leakage.
- Structural shotcrete now creates the opportunity to use a post-grouted hydrophilic curtain wall fabric, which accommodates shrinkage cracking, and other irregularities that can occur in any concrete placement This system cannot be used with form-and-pour because the fabric grid would be crushed by the form pressure, vibration would force cement paste into the fabric, and the injection tubes would be knocked off by the concrete drop placement and vibration. Also, no concrete contractor would approve several holes drilled through expensive formwork, let alone trying to fish the pipes through the holes in the form while closing the form panel.
Given the facts, it’s not hard to see why structural shotcrete is becoming THE solution to improving blindside waterproofing. When all the misconceptions and incorrect details regarding shotcrete blindside waterproofing projects have been addressed with facts, project teams can objectively consider the total performance of a completed concrete wall. It’s becoming abundantly clear that shotcrete is the smartest choice.
Torrent Shotcrete. More wall. Everyday.