Is Computerized Shotcreting a Possibility? It’s a Reality!

A nozzleman who has made a transition from hand-nozzling to using a robotic arm knows the value that mechanical technology has brought to our industry. Far beyond just saving human backs and arms during a shift of spraying concrete, robotic arms (booms, lances, etc.) have taken the application of shotcrete to the next level, facilitating rapid deployment of material to significant thicknesses, in hard-to-reach areas, particularly in underground construction environments (tunnels or mines).
This quantum leap in technology, while requiring training and skill to operate, makes shotcreting easier.
But what if those human factors that detract from the quality of in-place shotcrete could be controlled or eliminated? With the use of emerging technology, human error can be reduced or even eliminated.

Shotcrete for Ground Support: Current Practices in Western Canada

Historically, in Western Canada, the stabilization of rock slopes and construction of excavations have been achieved using methods such as soldier piles and lagging or construction of cast-in-place concrete retaining walls. In the case of reinforced cast-in-place concrete, there is a requirement for erection of formwork, fixing of reinforcement, pouring the concrete mixture, and vibration to ensure good concrete consolidation and steel encapsulation. These methods have proven to be relatively ineffi-cient and costly in many cases. In recent decades, however, the use of shotcrete for ground support has seen increased use, as shotcrete has allowed

The Art of Tunnel Rehabilitation with Shotcrete

The art of rehabilitation of tunnels has flourished and developed significantly over the last couple of decades. Several hundred railroad, highway, and conveyance tunnels have been successfully rehabilitated, converted, and/or enlarged. Much of this development can be attributed to the successful use of steel fiber-reinforced shotcrete. The flexibility and adaptable nature of steel-fiber microsilica shotcrete is ideal for rehabilitation of tunnels. Thanks to shotcrete, enlargement and rehabilitation of tunnels without fully taking the tunnel out of service is not only technically but also economically feasible consid-ering the cost of other alternatives including the œdo nothing alternative. Enlargement was usually accomplished by raising the crown but some have been enlarged by lowering the invert, which is much more difficult and time-consuming.

Shotcrete in Fires: Effects of Fibers on Explosive Spalling

In those of us in the concrete industry, the events over the last year have caused many of us to focus on the effects of blasts and fire on concrete structures. Information on the effects of fire on normal strength concrete (NSC) less than 7000 psi (50 MPa) has been available since the 1950s.1 On the other hand, little information on the behavior of high-strength or high-performance concrete (HPC) in fires has been developed until recent times, particularly with reference to thermal shock (high temperature rise rates) and sustained high temperatures.2

Welcome to Underground Atlanta

We are standing in the very bowels of the City of Atlanta. This isn™t the trendy œUnderground Atlanta of rock-till-you-drop nightclub fame. The millions of busy Atlanta citizens above us today don™t have any idea that we are down here. All they care about is that when they flip the little silver lever on the water tank, everything that is disagreeable goes somewhere else.
Where it goes, how it gets there, and what happens to it is our job. This 12-ft-diameter, man-made cavern known as the Highland Creek Trunk Sewer Interceptor stretches seemingly forever before and behind us. The professional shotcrete applicators of the Coastal Gunite Construction Company are here placing a new, biologically active gunite in this sewer interceptor. The Department of Public Works is here to inspect the rehabilitation work as Coastal Gunite recoats the interior of the Highland Creek Interceptor with this new bacteria-fighting gunite material.

Shotcrete for Ground Support: Current Practices in Western Canada

Each ground-support project is unique and will likely have its own design and performance re-quirements. Performance requirements for shotcrete can generally be divided into two groups: fresh state parameters and hardened state parameters. The following briefly elaborates on the variables included in each of these groups.

The water/cementitious materials ratio is one of the most important parameters controlling shotcrete quality and performance under long-term conditions. In cases where the shotcrete may be exposed to severe environments, the water/cementitious materials ratio would be limited to a specified maximum value, e.g., 0.40. Also, limiting the water/cementitious materials ratio helps reduce shotcrete shrinkage.

Having an adequate air content and air-void spacing factor in the as-placed shotcrete mixture has long been recognized as critical for frost resistance of wet-mix shotcrete. Usually, in wet-mix shotcrete, an as-batched air content of approxi-mately 8 to 10% is used to achieve an as-shot air content of 3 to 5%. However, even if frost resistance is not a concern, there is a definite advantage in using a high air content during batching of shotcrete in that the air entrainment enhances the workability (slump) of the shotcrete. Upon impact on the receiving surface, the air content is reduced, resulting in a reduction in slump in the in-place shotcrete. In projects where the use of accelerators is specified, this slump-killing effect helps reduce the amount of accelerator required to provide slump reduction and shotcrete adhesion.14

The required slump of wet-mix shotcrete for a particular application depends on the specific

Determination of Early-Age Compressive Strength of Shotcrete

Tere has long been a need for a reliable, simple-to-use means of determining the early-age rate of strength gain in shotcrete.During approximately the first 24 hours after shotcrete has been placed, its compressive strength is typically too low to measure using standard core-extraction and testing procedures. Monitoring the rate of early-strength development in shotcrete is important in tunneling, mining, and other applications such as the underpinning of structures. Recent studies by the authors have demonstrated that there is a simple, direct method for determining the early-age compressive strength development of shotcrete. It involves the shooting of a set of beams in a standard steel mold and testing the beams after stripping, using an adaptation of ASTM C 116, œStandard Test Method for Compressive Strength of Concrete using Portions of Beams Broken in Flexure. This œTechnical Tip describes a procedure for deter-mining the compressive strength of shotcrete beams and presents results of tests conducted with plain and accelerated shotcretes produced by both the wet- and dry-mix shotcrete processes.

Design Guidelines for the Use of FIber-Reinforced Shotcrete in Ground Support

Developments in Shotcrete in Hobart, Tasmania, Australia in April 2001. One of the outstanding papers presented at the Conference was a paper by Grant, Ratcliffe and Papworth on œDesign Guidelines for the use of SFRS in Ground Support. Frank Papworth was asked to submit an updated paper on the subject for publication in the ASA Shotcrete Magazine and so here it is. It is more technical than most of the papers published in the ASA Shotcrete Magazine, but was selected because it was considered that it would be of considerable value to designers of fiber-reinforced shotcrete linings for ground support in civil and mining applications.
Abstract: There are presently no design guide-lines based on toughness for the use of fiber-reinforced shotcrete (FRS) in ground support for underground mine development. Typically, in the Australian mining environment, the approach to the use of FRS has been one of borrowing experiences from other mines and a œtrial-and-error method of design, installation, and assessment. There is a need for a ground support design guide that can be simply applied by œfront-line personnel.
This paper provides an overview of the performance characteristics of FRS and how the various shotcrete guides specify its use. Practical experiences with the use of FRS in Australia and Canada in various applications and ground conditions are combined with existing empirically based ground support-design methods to develop a ground support guideline that incorporates the concept of toughness. An assessment of structural synthetic fibers shows that their low modulus makes their performance characteristics different from those of steel fibers, and that they are not likely to be economical in linings where crack widths are limited, but that they are preferable where large deflections are permissible.
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Fiber-reinforced shotcrete (FRS) has been used successfully for ground support for more than