Shotcrete best practice recommendations 06 Feb 2020

Axel Nitschke, Chair, ASA Underground Committee

Two critical topics of underground shotcrete application – the application of shotcrete overhead; and shotcreting on synthetic sheet waterproofing membranes – are explored in two American Shotcrete Association (ASA) position statements, aimed at providing guidance to the underground and shotcrete industry on the best practices and a common technical understanding of the topics covered.

Axel Nitschke, Tunnel Practice Leader (NATM) for WSP USA and Chair of the ASA Underground Committee, gives background on the two papers from the association.

Best practice recommendations for overhead shotcreting
Best practice recommendations for overhead shotcreting

Spraying shotcrete overhead in underground applications

In underground applications, spraying shotcrete overhead is a standard and daily operation but successful application is challenging. Shotcrete placement propels concrete at high velocity against the overhead surface and the dynamic energy compacts the concrete in place. However, plastic shotcrete has no immediate strength. Depending on its thickness, the weight of the shotcrete layer can be significant. The weight of a single layer or multiple layers of shotcrete can be enough to pull the shotcrete down and cause local or large scale fallouts, which may pose a significant safety hazard. In addition, improper application of overhead shotcrete and other circumstances can lead to delamination and voids in the installed shotcrete.

Understanding the adhesion and cohesion effects is a key element of overhead shotcrete. During the plastic shotcrete phase, the adhesion of the plastic material to the ground surface in combination with the cohesion of the plastic shotcrete material to itself are the forces acting against the self-weight of the plastic shotcrete. Because the chemical reaction of the cement in the mixture is still in progress and has not yet created significant strength, the plastic shotcrete sticks overhead only if the conditions for adhesion and cohesion are right.

Overhead shotcreting for pipeline rehabilitation
Overhead shotcreting for pipeline rehabilitation

Another important factor is the interlocking effect during the shotcrete installation on irregular, blocky, or porous ground surfaces. The shotcrete interlocks mechanically, allowing the shotcrete material to fill and bridge over these irregularities. The use of this interlocking effect is one of the major aspects controlling the shotcrete bond when spraying in mining or tunnel applications and experienced underground nozzlemen take advantage of ‘reading the rock’.

The position statement provides guidance for the proper application of shotcrete overhead by covering mixture proportioning, surface preparation, the thickness of each pass, utilization of anchors, lattice girders, and reinforcement, as well as challenges with waterproofing membranes. In addition to discussing shotcreting procedures and techniques, it also highlights the importance of overhead shotcrete only being applied by qualified shotcrete nozzlemen.

One of the key questions for overhead shotcrete operations regarding safety is the definition of a re-entry criteria, allowing the crew movement beneath the newly installed overhead shotcrete.

The position statement provides some general guidance and notes that it is generally preferred to develop re-entry criteria based on actual early-age strength development, rather than a predetermined time period. However, it is emphasized that neither a strength- nor a time-based re-entry criterion provides a guarantee against fallouts as these are influenced by a multitude of factors.

ASA recommends qualified nozzlemen for overhead application
ASA recommends qualified nozzlemen for overhead application

It goes on to emphasize the need for a project-specific definition and evaluation of the re-entry criteria. Typical values for an early compressive strength-based re-entry criteria range between 150 to 500psi (1 to 3.5MPa) and are typically measured with a penetrometer or end beam tester. However, a re-entry criteria for the general public, for example, in a rehabilitation project, should be significantly higher.

Recommending that the contractor follows the guidelines provided by the American Concrete Institute Committee for Shotcreting (ACI 506), the position statement notes that preconstruction testing and mockups are essential to test the mixture design, strength development, equipment operation, and setup, in particular the accelerator dosing pump calibration, and project conditions. The tests are seen as a beneficial tool to avoid safety hazards and rework during execution and not simply a contractual obligation.

In summary, the installation of overhead shotcrete in underground applications is a challenge and requires special experience and skills to be successful. Overhead shotcrete can be executed safely and at high quality, if the process parameters defined in the position paper are consistently and correctly executed.

Spraying shotcrete on synthetic sheet waterproofing membranes

Applying shotcrete for final linings against synthetic sheet waterproofing membranes has become more common in underground projects. The sprayed shotcrete is propelled at high velocity against the membrane and the dynamic energy consolidates the plastic shotcrete in place. However, the smooth surface and potential pillowing or vibrating of the waterproofing membrane poses a challenge compared to a shotcrete application against a stiff and rough surface.

Wet shotcrete robot spraying
Wet shotcrete robot spraying

The use of shotcrete final linings in underground applications is a standard placement technique. However, improper application of shotcrete against waterproofing membrane in a double shell system, with a separate temporary lining at the extrados and the final lining at the intrados of the membrane, can lead to poor-quality shotcrete due to delamination and voids, and produce local or large-scale fallouts, and can be the subject of technical disputes about cause and effect responsibilities.

The ASA position paper about spraying shotcrete on sheet waterproofing membranes discusses the basic elements of the adhesion of overhead and vertical shotcrete on a waterproofing membrane, the proper application techniques, and emphasizes the use of mockups. The position paper also provides recommendations for contractors and owners from the perspective of the ASA, on how to properly apply, specify, and inspect shotcrete applied against waterproofing membranes.

The key technical challenge is the weight of the plastic shotcrete before it gains sufficient strength to support itself. Adhesive forces of the plastic shotcrete can only be transferred into the initial lining at the fixation points of the membrane or where the membrane is supported by a self-supporting reinforcing bar cage.

Reinforcement and lattice girders provide an important element during the installation of shotcrete. It is important to understand this concept and ensure that either the self-supporting rebar and/or the anchoring system is strong enough. However, the weight to be carried strongly depends on the shotcreting procedures and patterns as well as the strength development of the shotcrete over time.

Initial shotcrete support and lining
Initial shotcrete support and lining

When using fiber-reinforced shotcrete on top of the waterproofing membrane, the potential for damaging the membrane by fibers is a frequent question. There are two different scenarios. The first scenario deals with fiber-reinforced shotcrete in the substrate. To avoid damages to the membrane, typically a non-fiber-reinforced smoothing layer is installed. The second scenario is applying fiber-reinforced shotcrete as a final lining on top of the membrane. Spraying steel fiber-reinforced shotcrete does not damage or puncture the membrane because the forces acting on the fiber are not strong enough to push the fiber into the membrane, and typically the fibers tend to orient parallel to the membrane on impact.

Coordination between the membrane installer, the reinforcement installer and the shotcrete contractor is another key element. An improperly installed membrane can lead to performance issues of the shotcrete lining.

The ASA position papers on applying shotcrete overhead; and synthetic sheet waterproofing membranes address two technical applications of shotcrete that are frequent subjects of discussion on underground projects. They provide guidance in areas that are currently not well covered by other guidelines or generally accepted literature, and it is a goal of the ASA to support the industry by filling these gaps. Both position papers are available for free download.


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