Since the mid-2000S, shotcrete usage in underground construction has soared because contractors can install it more quickly and efficiently than traditional cast-in-place concrete systems. Project teams have supported adoption of the technology based on what has been described as a meaningful schedule acceleration of the time typically required to complete a foundation. For large-scale projects, that can translate into millions of dollars through accelerated occupancy and timely project completion. Enabling this surge in shotcrete use are advancements in waterproofing systems designed specifically for the material.
Below-grade construction with cast-in-place concrete requires installing forms in the excavation, pouring concrete, waiting for it to cure, and finally removing all the forms. Shotcrete application is typically faster because it can be done quickly by qualified nozzle operators without the need for concrete formwork. However, it does come with its own set of risks.
For example, shotcrete use can lead to voids in concrete walls and behind rebar. These problems, in turn, allow water to migrate around and into a structure should the waterproofing system fail.
Water migration can derail timely project completion at great cost, while exposing the structure to leaks and flooding over its useful life. Also, the force of shotcrete spray can damage waterproofing membranes, as can the impact of adjacent building construction and flaws introduced by rebar, electrical, or plumbing contractors.
Searching for a Waterproofing System
When first working with shotcrete, contractors tried adapting preapplied waterproofing solutions traditionally used with cast-in-place applications to shotcrete projects. After shotcrete placement onto the waterproofing membrane, contractors were forced to seal leaks by drilling holes through the shotcrete wall in trouble spots and injecting grout. As contractors could not see or track the pattern or extent of the vulnerability, the only method of filling the void or defect with grout was time-consuming trial and error. This blindside approach produced only limited success or outright failure.
One solution to these deficiencies is use of an effective shotcrete waterproofing system, comprised of a membrane, grout-injection tubes, and grout. A tough composite membrane can stand up to the force and application of shotcrete, while a polymer-meshreinforced cavity sits between a layer of plastic film and a nonwoven, semipermeable geotextile.
In step one of the application process, the contractor attaches the membrane to the soil retention wall—typically a combination of piles and lagging. Before applying the shotcrete, the contractor fastens a spaced matrix of injection tubes onto the geotextile of the membrane. In step two, the grout is injected through the tubes after the shotcrete wall has cured. This can be one to two months after the construction of the wall and is a process that can be completed as construction on the project moves beyond the foundation (allowing for schedule acceleration).
During injection, the reactive grout fills the cavity created by the membrane and permeates through the geotextile. As a result, it forms a monolithic curtain wall of grout, proactively sealing voids in the wall itself and creating a highperformance waterproofing system bonded to the shotcrete.
This two-step method stops leaks before they happen, ensuring construction stays on schedule, liabilities are minimized, and the waterproofing solution shields the structure from water infiltration throughout the design life of the project. Combining preapplied membrane technology with injected leak-sealing materials results in a fully integrated waterproofing system. With this type of technology at their disposal, engineers and general contractors are increasingly turning to shotcrete to accelerate their high-profile, high-risk projects.
Conclusion
Advancements in waterproofing have enabled the rapid growth of shotcrete usage in traditional structural applications. With this type of technology available, contractors, designers, installers, and engineers will continue to expand the use of shotcrete, reaping the associated schedule benefits.
