Reinforced concrete can be described as a "macroscopic" composite made up of a concrete matrix and steel bar reinforcement. Because the bars are large and widely distributed, they effectively carry load only after the concrete develops a macro crack. Therefore, conventional reinforcing bars are reactive reinforcement. A microscopic composite can be created by combining concrete with Helix twisted steel micro reinforcement (micro reinforcement). Because this type of reinforcement is distributed throughout the matrix and is continuously deformed like reinforcing bars, it carries load both before and after the concrete develops a macro crack. Thus, micro reinforcement is proactive reinforcement that also acts as reactive reinforcement at higher strain levels.
Micro reinforcement is produced with a twisted profile that allows each piece to bond to the matrix over its full length. In addition, the reinforcement must untwist as it pulls out of the concrete. This makes this product significantly different than traditional steel fibers because pullout is governed by twisting resistance rather than friction. Two different products are being provided for tunnelling applications Helix 5-25 for shotcrete and Helix 8-48 for cast concrete.
Steel fibers have been used to reinforce shotcrete and replace traditional steel mesh for more than 20 years. They are added to concrete and shotcrete to improve energy absorption, crack resistance and to provide ductility. The latter property is the ability to carry load after the matrix has cracked and to redistribute loads. It is obvious that all three properties are of great importance for support systems designed for tunnel and mine conditions.
Twisted steel micro rebar, (TSMR), takes shotcrete reinforcement one step further. Due to the twisted anchorage and high tensile strength of the wire it is able to provide all these benefits and more at much lower dosages than had previously thought possible. Signi_ cant improvements in precrack properties such as compressive strength, splitting tensile strength and flexural strength have been documented. Compressive strength has been shown to improve from 10 to 25 per cent at volume fractions of 0.08 to 0.52 per cent (dosage rate between 10 to 66 lb/yd3 or 6 to 40kg/m3) and splitting tensile strength (Figure 1) has been shown to improve from 48 to 111 per cent at the same volume fractions of 0.08 to 0.52 per cent [ref 1].
Post crack properties are characterised using direct tension tests and conventional flexural beam tests according to ASTM C1609 [ref. 2]. Figures 2 is a laboratory test report of a set of shotcrete prepared beams using Helix 5-25 and Figure 3 is a laboratory test report of a set of cast beams using Helix 8-48.
Helix 5-25 has been used in a variety of shotcrete applications on tunnel projects including slope stabilisation, initial tunnel lining and portal support. As an example it was used in four tunnels on Mexico’s Durango-Mazatlan Highway project: El Cantil, El Sinaloense and El Varal. This project is one of Mexico’s greatest engineering feats. It includes 115 bridges and 61 tunnels and is designed to bring people, cargo and commerce safely across the Sierra Madre mountain range.
The tunnels that used Helix are El Cantil I and El Cantil II tunnels, which are approximately (245m) long, El Baral tunnel is 777m long and the Sinaloense tunnel. The Sinaloense tunnel, constructed by Grupo Hermes and FCC construction, is the longest of the 61 tunnels in this project and is the second longest tunnel in Mexico at 2.78 miles (4.5km). Its construction took nearly four years; a testament to the difficulty of constructing a road project in such rugged mountains. The tunnels were constructed by drill and blast technique using SEM. During the construction a geotechnical engineer at the face directed the ground support for every advance.
These professionals performed rigorous mapping of the geology including, definition of the materials, structural discontinuities (fractures, strata, contacts, failures, etc.) and determination of its physical strength properties (spacing, roughness, filling, humidity, persistence, etc.), geomechanic classifications; estimation of parameters of resistance; water input capacity to excavations; qualitative assessment of the excavation stability; photographic records; etc.
Based on these hard geologicalgeotechnical data, drawings were made in stations for at least 5m of excavation progress or when changes in lithology, the presence of faults or materials of poor quality were observed. The geology of the Sierra Madre Occidental, which crosses the Mexican states of Durango and Sinaloa is formed from volcanic rock overlying a basement of metamorphic rock (ref 3).
Helix 5-25 was used at a dosage rate of 9 to 11kg/m3 for initial ground support on all four tunnels in addition to rock bolts and steel arches when needed. Robotic spray equipment was used with wet shotcrete.
The Helix 8-48 product is designed for use in final tunnel linings and is able to provide flexural hardening properties at a volume fraction 60lb/yd3 (35kg/m3). It has a 50 per cent increase in splitting tensile strength.
Since 2003, when micro reinforcement came to market, it has been used on various concrete projects including tunnel linings, structural foundations, structural footings, slabs (slab-on-ground, slab on metal deck, elevated), walls (cast-in-place, tilt-up, precast), pavements/toppings, bridges, precast applications, and storm and blast-resistant structures