Use waterproofing to stimulate the use of shotcrete in underground construction

Photo © Brian Libby, Portland, via Wikimedia Commons

By Greg Austin and Xia Cao

Since the mid-2000s, the use of shotcrete in underground construction has skyrocketed. In some cases, contractors can install it faster and more efficiently than traditional pour-in-place concrete systems. Project teams supported the adoption of the technology based on what has been described as a significant acceleration in the time schedule typically required to complete a foundation. For large-scale projects, the shorter schedule can translate into millions of dollars due to accelerated occupancy and project completion on time. Advances in waterproofing systems designed specifically for the material allow this increase in the use of shotcrete.

Construction in the basement with poured-in-place concrete requires the installation of forms in the excavation, the pouring of the concrete, waiting for it to harden and, finally, the removal of all forms. The application of shotcrete is generally faster because it can be done quickly by skilled pitchers without the need for concrete formwork. However, this comes with its own set of risks.

For example, the use of shotcrete can result in voids in concrete walls and behind rebar. These problems, in turn, can allow water to migrate around and into a structure if the sealing system fails. Water migration can derail project completion on time at a high cost, while exposing the structure to leaks and flooding during its useful life. Additionally, the force of the shotcrete spray can damage waterproofing membranes, as can the impact of adjacent building construction and defects introduced by rebar, electrical or plumbing contractors.

In search of an efficient sealing system

When first working with shotcrete, contractors tried to adapt the pre-applied waterproofing solutions traditionally used with cast-in-place applications to shotcrete projects. After the shotcrete was placed on the waterproofing membrane, the contractors were forced to seal the leaks by drilling holes through the shotcrete wall in the hot spots and injecting grout. Since contractors could not see or follow the pattern or extent of the vulnerability, the only method to fill the gap or fault with grout was tedious trial and error. This blind approach has produced only limited success or outright failure.

One solution to these shortcomings is the use of an effective shotcrete waterproofing system consisting of a composite membrane, grout injection tubes and a specially formulated grout. The tough composite membrane can withstand the force and application of shotcrete. It is composed of a cavity reinforced by a polymer mesh, lined with a plastic film, and covered with a non-woven permeable geotextile acting as a shotcrete barrier. The plastic film is placed towards the soil retention system. The reinforced cavity between these two layers allows the injection of hydrophilic grout.

In the first step of the application process, the contractor attaches the membrane to the soil retention wall, usually a combination of piles and sheathing. Before applying the shotcrete, the contractor fixes a spaced array of injection tubes to the membrane geotextile and attaches them to the shotcrete reinforcement.

The grout is injected through the tubes after the shotcrete wall has hardened. This can happen one to two months after the wall is built and is a process that can be completed as the construction of the project moves past the foundation (helping to speed up the schedule).

During injection, the reactive grout fills the cavity created by the membrane and passes through the geotextile. As a result, it forms a monolithic curtain wall of grout, proactively sealing voids in the wall itself and creating a high performance waterproofing system bonded to shotcrete.

The system uses a series of confirmation ports to ensure that the grout flow is consistent and complete. Using a combination of these ports and known consumption rates, applicators can determine the appropriate grout volume and coverage.

This two-step method stops leaks before they happen, ensuring construction is on schedule and liability is minimized. The waterproofing solution protects the structure from water infiltration throughout the life of the project. The combination of pre-applied membrane technology and injection of sealants 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 large-scale, high-risk projects.

Shotcrete can be used almost anywhere, however, successfully waterproofing it has always been difficult. This type of two-step waterproofing can be used with shotcrete in any blind side waterproofing application.

Shotcrete is a good choice when it allows rapid acceleration of deadlines or on tightly configured projects where it is particularly beneficial to avoid formwork.

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Shawn G. Randall

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