Silica Fume in Shotcrete

Silica fume is a highly pozzolanic mineral admixture that has been used mainly to improve concrete durability and strength and as portland cement replacement. Silica fume has been used primarily in the United States, Canada, and the Scandinavian countries, but is now finding increasing use elsewhere in the world.

Development of a Centrifugal Sprayed System for Shotcrete Application

A new Japanese Ministry of Health document provides regulations and guidelines for the allowable dust concentration in tunnel works. The recommended maximum concen-tration should be less than 3.0 mg/m3 (2.3 mg/y3) at 50 m (164 ft) from the tunnel face. To observe this guideline, it is a serious problem as to how to reduce the generation of mineral particles of dust during shotcreting operations. In general, shotcrete is placed using pneumatic energy. This is a main factor causing dust generation during shotcrete operations. It may be easily imagined that if shotcrete application can be conducted by centrifugal force, dust concentration would be dramatically reduced. In this paper, we describe the development of a centrifugally sprayed shotcrete system named Dustless Shotcrete and also provide the results of a practical application of this technology at the Hishino Tunnel.

Steep Slope Stabilization with Fiber-Reinforced Shotcrete

For the past 150 years or so, roads have been built through the mountain passes in the western U.S. and Canada. Sometimes these roads led to mines; sometimes they started as logging roads. Some were built for access roads for the railroad. Many were built so that people could drive wagons, stagecoaches, and later, automobiles to their destinations. As time went on, some of these roads were abandoned, while others were turned into highways and scenic byways. These roads can be seen on maps, criss-crossing through mountain passes, valleys, and wherever passage was possible. Many of these roads were constructed along mountain slopes, carved out using the largest equipment available at the time, sometimes by hand work, and sometimes by blasting through rocky areas. Road conditions in some of these areas can change dramatically throughout the year. In the mountains, vast amounts of snow can accumulate during the course of just a few days. Sometimes there are heavy rain storms. There can be snow avalanches and mudslides from the rain and snow. In times of drought, vegetation might dry out and die, leaving slopes exposed to erosion, increasing the probability of avalanches and mudslides. Because of varying climactic conditions, freezing-and-thawing cycles, radical changes in the amount and nature of moisture, steepness of slopes, and other factors, slopes need to be stabilized so that rocks, trees, debris, and other factors not listed do not unearth them-selves and become hazards to all things below them. One of many ways to secure and stabilize highway slopes is by the use of fiber-reinforced shotcrete (FRS), usually along with either rock bolts or some other mechanical device drilled into the rock or slope. This paper provides an overview of why fiber-reinforced shotcrete is an excellent choice in lieu of plain shotcrete reinforced with either welded wire mesh or rebar mats for such slope stabilization work.

Specification of Shotcrete Toughness

Fiber-reinforced shotcrete has become an established material for ground support in tunnelling and mining applications as well as in new construction and infrastructure repair. Designers and specifiers frequently require such shotcrete to maintain some quantifiable postcrack strength or toughness. Until the newly published round panel test method (ASTM C 1550-03)1 becomes more widely used, North American designers and specifiers will likely continue to refer to toughness parameters as determined by the beam test method (ASTM C 1018).2 The following sections discuss various toughness parameters associated with this beam test and their significance.

Innovative Shotcrete Application Over Geofoam Structure Supporting Boston’s Central Artery Tunnel

The objective of Boston’s Central Artery/Tunnel (CA/T) Project is to ease the congestion of approximately 190,000 vehicles per day traveling through the city of Boston. This objective is being achieved through the total reconstruction of Interstate Highway I-93 as it passes through the heart of Boston, together with the extension of Interstate I-90 from its terminus at I-93 just south of downtown Boston to Logan International Airport in East Boston. The Artery replaces an elevated six-lane highway that opened in the late 1950s and originally carried approximately 75,000 vehicles per day. The focus of this article is on two transition structures and ramps in the C09C2 Construction Contract: the I-93/I-90 Interchange Ramps and Surface Restoration at Albany Street, where shot-crete facing has been implemented as a facing system for lightweight embankments.

Release of New ASTM Round Panel Test

Following a 3-year development period, a new test for post-crack performance assessment of fiber-reinforced shotcrete (FRS) and fiber-reinforced concrete (FRC) based on round panels was passed by ASTM Committee C 09 in June 2002. The standard test method, known as C 1550-02, œStandard Test Method for Flexural Toughness of Fiber-Reinforced Concrete (Using Centrally-Loaded Round Panel), was published in the 2002 edition of the Annual Book of ASTM Standards V. 4.02.1 Publication of this standard test method is a major development in the fiber-reinforced shotcrete industry. It will, for the first time, permit a both reliable and economical estimation of post-cracking performance for this material.
The use of fibers in shotcrete has become an established form of reinforcement in many sectors of the underground construction industry over the last 20 years. The effective measurement of post-crack performance (toughness) in this material, however, is a problem that has plagued the industry and made the influence of parameters such as fiber type, mixture design, and spraying technique difficult to determine. Much of the difficulty is attributable to the high levels of within-batch variability obtained for even well-prepared sets of FRS samples when beams are used as the basis of toughness assessment. Typical levels of within-batch variability for toughness indices obtained using ASTM C 1018 beams range from 13 to 18%.2,3 More than18% is common for residual strength obtained using EFNARC beams.4 The imprecision associated with such high levels of variability has obscured trends in performance development and eroded confidence in the material. Some improvement occurred with the introduction of EFNARC panels in the 1990s,5,6 but this test suffered its own difficulties associated with seating problems and high costs. Other types of specimens have seen occasional use,7-10 but the size and expense of these tests have limited their use to special applications.
The first round panel test similar to the C 1550 configuration was undertaken in 1997 as part of an investigation of the influence of support conditions on structural behavior in FRC panels.11 The potential of this test was recognized by the Roads and Traffic Authority of New South Wales in Australia, which immediately sponsored a comparative study of FRS performance for several commonly available fibers.12 A specifi-cation based on this test13 was also introduced

Shotcreting in Australian Underground Mines: A Decade of Rapid Improvement

Over the last decade, dramatic improvements in spraying technology have allowed shot-crete to become the first-choice ground support in many underground mines in Australia. Before 1994, only a very small amount of dry spray shotcrete was used. Since then, the increased use of wet-mix fiber-reinforced shotcrete has been extremely rapid, spurred along by improvements in machinery, admixtures, fibers, and under-standing the way shotcrete behaves as a ground support element.
Today, nearly 100,000 m3 (130,000 yd3) of shotcrete is applied annually in some 20 under-ground mines. While volumes have leveled off during a recent period of depressed metal prices, it is almost certain to boom again as metal prices improve and new mines come online. Australian mines are characterized by reasonably shallow ore bodies hosted in hard rock. This made under-ground mining initially fairly simple with little ground support needed beyond a few rock bolts. As surface deposits have become depleted, however, mine owners are increasingly spending their exploration dollars drilling beneath existing deposits to find new resources. This has led to ever-deepening extraction depths and associated ground support difficulties.

Determination of Early-Age Ductility of Steel Fiber-Reinforced Shotcrete Lining System at INCO’s Stobie Mine

The state of technology in shotcrete materials has evolved steadily throughout the world and particularly in North America during the last 20 years. The use of supplementary cementing materials such as silica fume, fly ash and slag, the new generations of chemical admixtures, and the development of various types of fibers (steel and synthetic) significantly enhance the performance of shotcrete for a variety of applications.
These technological advancements have led the international mining industry to become a major user of shotcrete for underground support. Because the potential for instability in underground rock openings is a threat to the safety of miners, the support of permanent openings in underground mining is a critical area of shotcrete application. For over 20 years, mining companies have recog-nized the value of steel fiber reinforcement in shotcrete. It has been proven that the performance of steel fiber-reinforced shotcrete compares favorably with steel-welded wire mesh reinforced shotcrete in various ground support applications.1
The introduction of steel fibers in shotcrete increases its energy absorption or œtoughness, increases impact resistance, and provides increased ductility. Ductility is defined as the ability to continue to carry loads after the shotcrete micro-structure has cracked. These mechanical properties are considered extremely important parameters with respect to support linings designed for the underground environment.2 (The effects of addition rate, geometry, and property of fibers are beyond the scope of this article.)
Although the ability of steel fiber-reinforced shotcrete to carry loads in flexure beyond its flexural capacity can be assessed in laboratories using a variety of beam and panel test methods, the under-standing of how to relate it to ground support design guidelines for underground mine devel-opment is limited and subjective.3
Test methods evaluating the load-carrying capacity of steel fiber-reinforced shotcrete (SFRS) performed after 7 and 28 days of curing do not assess

Combining Shotcrete Mixes for Maximum Performance

Rebound is an essential element in the application of shotcrete. Rebound is defined as follows: œMainly large aggregate with some sand and cement that bounces or ricochets off the receiving surface and falls on to lower surfaces.1 There is a vital function that is achieved in the rebounding of shotcrete. The secret lies in knowing how much rebound is enough.
To paint a mental picture for the reader to understand rebound, consider a baseball. If you take a baseball and dip it into some fresh concrete and pull it out, it will be covered with mortar”a paste consisting of the cement and fine aggregate and water”that acts as the glue required to create an artificial rock called œconcrete. If you took this baseball covered with mortar and threw it at a high velocity against a solid block wall at a 90-degree angle to the wall, the ball would strike the surface and bounce off. Because the paste is also in motion at 95 miles per hour and the paste is not securely bonded to the ball, some paste will leave the surface of the baseball, contact the wall, and adhere to the surface. In layman™s terms, it would œsplat onto the wall. The harder the baseball is thrown, the more the paste would leave the surface of the