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
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.
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.
ˆ˙Ë
Fiber-reinforced shotcrete (FRS) has been used successfully for ground support for more than
In November 1999 I had the opportunity to attend the Third International Symposium on Sprayed Concrete for Underground Support.
An1amina copper mine is a Sl.2 btltion projcc1 in lhe Andes Mountailh of Peru. The projeci. owned by Canadian mining gi3111S Norandll. J’cck and Rio Afgom, i.s one of the. single largest mining inveso:nents in 1be Americ:iô€€£ at this moment and oomprises the cons1ruction of an open pi1, a co11veyor runnel, a tailings dam. a mill and pon facilities for ¢1’por1 of processed concenlrllle all over the globe.
Having discussed the basics for the successful application of SFR wctcrete, the issue of its quality maintenance needs to be addressed. There are a numÂber of critical indicators that the placed product has to meet.
In order to achjeve the objective. it was imporÂtant to develop a mix lhat would rapidly generate a high heac of hydration. The cold ambient air would cool down 1he shotcrete but the heat released durÂing the hydration process would allow 1he hydraÂtion of ccmem 10 proceed unril Lhe remperature of fresh shotcrete decreased lo the freezing tempera¢ ture of the brine solution (brine is a solution of calÂ
cium chloride). Al that 1empera1ure, brine freezes and the process of hydration remains suspended until thawing takes place. A laborntory test progi-am (not presented in 1his paper) was used to develop two dry-mix shotcrcte mixes for performance evaluation at a mine near the Arctic Circle under permafros1 conditions. The mixes evaluated had very shon initial and final set times. generated the most heat of hydration during the first 1rtinu1es, and developed the highest comÂpressive strcnglh during the first few hours.[2] It. was lherefore. expected I.hat these mixes could be apÂplied on frozen surfaces and would be able lo set and develop adequate mechanical properties.
Dry-mix shotcrete as a means of grouad support for lunnels and mining drifls is fasl, eflicient, and very safe when properly applied. lls effecliveness is unmatched: I can remember lining unstable clay mining tunnels (drifts) using a layer of shotcrete, wire mesh, and another layer of shotcrete. The arrival of steel fiber shotcrete made life even easier, taking care of severe ground conditions.
In 1969, when I bid farewell to work in field exploration, I knew I would miss the outdoors. I wasn’t sure if my love for geological mapping, and my penchant for using geophysical equipment to probe the earth for her riches, would be fulfilled. The truth was, I embarked on my new career in an underground mine with a major metal mining company seeking the opportunity to be at home with my family in the evenings and to have access to a university to complete my BSc in Geology.
Carl Akeley began the process of guniting, or what is now known as shotcreting, sometime around 1907. Since shotcreting subsequently found its way into underground 1uining operatio1t.s. ii has unc.lcrgone an evolution through the 20th CenÂtury Lo where we find the process being used today in North America and elsewhere. in mining around the world.As we look al 1be progress.< of sho1crete in Nonh AmedcM underground mines. it becon1es useful ro break down the pn)CCSS into five distinct parnmetcrs: mix design, equipment. logistics, applications, and operalors; for discussion (>f each.
While Some of our current practices have es1abÂlishcd a benchmark for others to emulate, we also stand to leam from the practices of the resl of lhe
mining world. However, what ever perspective one takes of the current state. of affairs, shotcre.tc is an important pan of ground support in modern mining applications. and its use and effec1iveness wW conÂtinue to grow as we. tackle the chaBenges that min¢ ing brings us in the 21¢ Century. Sbo1cre1e. or sprdyed concrete if one prefers, hos a bright future
