Peter Kenyon, TunnelTalk
- A rapid curing spray-on concrete offering compressive strength in just 15 minutes has obvious applications within the tunneling industry, but as Peter Kenyon of TunnelTalk discovers, this latest innovation in structural and ground support has its development and funding roots in the global war on terrorism.
- Imagine a quick-curing concrete that can be sprayed to reinforce a structure, almost immediately, before it fails catastrophically, providing safety for rescue workers who risk their lives minutes after disaster hits.
- A technology developed at the University of Kentucky's Center for Applied Energy Research (CAER) and Minova North America, headquartered in Georgetown, Kentucky, does just that.
- The products, Tekcrete Fast and Tekcrete Fast M, have been licensed by Minova, and can be used by engineers to stabilize damaged structures immediately. It is a high-strength, quick-setting concrete that achieves set and structural strength in minutes. Minova, a specialist in the chemical and concrete field, has also licensed the technology for application in the tunneling industry as well as in mining and civil construction.
- The new product is the outcome of three years of research and testing carried out jointly by the CAER in Kentucky, USA; the CAER based at the University of Dundee in Scotland; and their commercial partner Minova, which in addition to manufacturing concrete-based products is also the world's largest producer of polymer resin cartridges for anchoring rock bolts and manufactures injection grouts, sealants and explosion proof bulkheads.
Minova factory in Kentucky
- The project however owes its origin to the US Government's Department of Homeland Security, which wanted to develop a product that would protect both infrastructure and rescue workers in the event of a tunnel, dam or other structural attack by terrorists.
- "This project takes advantage of expertise developed in non-traditional cement systems at the University of Kentucky CAER," explained Tom Robl, the University's Associate Director and Principal Investigator. "Also critical has been a long-standing collaboration between researchers at Minova and CAER. These two factors allowed us to work together to develop solutions for these problems."
- "These are the only products of their type, not only in America, but I believe in the world," said Peter Mills, Technology Leader for Minova North America. "Wherever there is a need for effective and immediate structural support, these products fit the bill."
- "Obviously there is not a huge market for repairing infrastructure damaged by terrorist attacks," said Mills, "but there are several benefits of this product to the mining and tunneling industries and especially for drill+blast and NATM excavation methods."
- Tekcrete Fast can be used anywhere in a tunnel where conventional shotcrete is used and, as with any coating or sealant, it performs best when applied as soon as possible after the ground is exposed. Including rebound, Tekcrete Fast has a surface coverage of approximately 8.2ft2 per 60lb bag at a thickness of 0.5in (0.76m2 per 27.3kg bag at 12.6mm thickness).
- "There are plenty of other accelerated shotcrete products on the market but they offer only a few hundred psi strength," said Mills. "This special technology offers structural strength in an hour and, arguably, within 15 minutes."
- By way of comparison, traditional shotcrete offers an unconfined compressive strength of 400-700psi (0.28-0.49kg/mm2) within 15min of application. Tekcrete Fast more than triples this figure to 2,500psi (1.76kg/mm2) in 15 min, and 4,500psi (3.16kg/mm2) in an hour (Fig 1).
- It is the chemical bond of polyfiber with cement that gives the formula its hardening strength, while a patented chemistry gives the product its rapid-curing property.
- "With Tekcrete contractors could pretty much advance on their heading as they are spraying this product. With conventional shotcrete there are delays waiting for shotcrete to harden before the heading can advance. There are also advantages in using this product to stabilize exposed rock faces."
- As with any new product, the improved performance comes at a price. In this case, admitted Mills, the new product is up to three times more expensive than traditional shotcrete. "You would probably only use this product where you need superior rapid strength development. Tekcrete offers a strength of 11,000psi (7.73kg/mm2) after 28 days compared with the 7-8,000psi (4.92-6.62kg/mm2) at 28 days of traditional shotcrete mixes."
- "As far as tunneling is concerned," continued Mills, "I expect this to be a niche product although it will be up to us to convince potential customers to look not simply at the cost per bag but rather to factor in the construction time that will be saved by the potential for advancing headings more rapidly using Tekcrete. That is the message we have to get across."
Fig 1. Tekcrete Fast compressive strength
Fig 2. Tekcrete Fast flexural strength
Fire spalling of self-compacting concrete - TunnelTalk, May 2012
PP fibres to resist fire-induced concrete spalling - TunnelTalk, November 2010
Concrete contribution to Gotthard undertaking - TunnelTalk, October 2012
We at Natural Cement Distribution in the UK have been producing fast setting, fast curing, high early strength, waterproof shotcretes since 1996 that will out-preform the product described in the article. We have shotcretes that are not a mixture of many additives; will not poison future generation; and are better for the environment than the product describe. May I suggest that you look at Shotcrete 513 in the Shotcrete section of our website to know more.
Phil Richardson, Natural Cement Distribution Ltd, UK
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