Some projects in the Middle East are using ultra-high strength sprayed concrete which reaches strengths of 80 to 90 MPa after 28 days, according to Benoit de Rivaz, global tunnelling manager at Bekaert Maccaferri Underground Solutions. The high-strength concrete, selected for its durability and early demoulding performance, requires higher and higher strength fibres.
De Rivaz was speaking at Bekaert Maccaferri’s third UK Underground Solutions seminar, which was held at the Institute of Mechanical Engineering in London’s Westminster. Ross Dimmock, technical director – Global GCCT at Normet and an attendee at the seminar, challenged the idea that higher strength was better, pointing out that harder concrete creates more rebound, more wastage and potentially more problems with banding and lamination.
Very high early strength concrete is being specified in a bid to reduce the risk of concrete falls which is leading to high 28-day strengths. But there was general agreement that there are many reasons for concrete falls and a suggestion that the requirement for very high early strengths may be counter-productive.
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The day-long seminar included an explanation of the American Concrete Institute’s latest guidance on fibre-reinforced concrete segments, ACI 544.7R-16 Report on Design and Construction of Fiber-Reiforced Precast Concrete Tunnel Segments from Mehdi Bakhshi, senior tunnel engineer at Aecom. Attendees also heard the latest on SMUTI (strength measurement using thermal imaging) from Inbye MD Benoit Jones, and learnt about the use of TIM (tunnel information management) on London Underground Station upgrades from Ken Spiby and Matthew Breeze of London Bridge Associates.
Richard Foord, project manager for underground construction UK & Ireland at BASF, took the audience through lessons learned about sprayed waterproofing and Rene Wollmuth, sales manager at VMT explained his company’s semi-automated lasertracker system for checking mould and segment dimensions