HS2 (short for High Speed 2) is Britain’s new high-speed rail line, and the country’s largest infrastructure project. Balfour Beatty VINCI (BBV)—a joint venture between Balfour Beatty and VINCI Construction—HS2’s construction partner in the Midlands, is responsible for delivering 90 km of the route, from Long Itchington in Warwickshire to the center of Birmingham and on to Handsacre in Staffordshire.
This section of the route includes 11 km of viaducts, 64 overbridges, 76 embankments, 44 cuttings, 28 underbridges, 2 twin-bore tunnels and 2 cut-and-cover tunnels.
Over 2 million m3 of concrete is required to deliver this section of the route. More than 1 million m3 of concrete has already been poured, using a CEM I GGBS-based composite cement.
In accordance with the United Kingdom’s norms and standards, the project uses 90% EXEGY-compliant low-carbon concrete solutions incorporating ground granulated blast-furnace slag (GGBS).
Three ready-mix concrete companies are supplying the concrete: Tarmac, Cemex and Holcim. Between them, they operate 11 concrete batching plants assigned to the HS2 operation, located within site compounds or existing facilities close to the project, to minimize transport distances.
So far, approximately 130 concrete mixes are being supplied to this section of the route, covering structural applications, ground engineering and ancillary works, with strength classes ranging from C2/3 to C70/85. The most common structural concretes are C35/45, C40/50 and C50/60 and meet the requirements for the XC3/4, XF4 and XD3 exposure classes. Structural mixes are also designed to meet DC3 or DC4 design chemical classes, dependent on environmental criteria, while piling mixes meet DC4 + 1APM due to sulphate-bearing ground conditions.
Two types of composite cement have been used for almost all mix designs; CEM III/A, used above ground, and CEM III/B, used in structures which come into contact with the ground. The CEM III/A mixes are typically 50% GGBS and 50% CEM I, although 40% GGBS blends have been used, while the CEM III/B mixes are typically 70% GGBS-based with blend levels of up to 85% GGBS being included in mix designs.
The concrete is being used across a number of different structures, including bridges, embankments, foundations, retained cuts and precast segments for tunnel lining and viaduct spans.
BBV collaborated with its suppliers to optimize these low-carbon mixes based on the required performance, which includes the maximization of GGBS. While the use of GGBS has been the norm in the United Kingdom for years, the mixes were tailored to achieve the required performance, while reducing carbon to a minimum.
BBV has also implemented the use of digital technology to monitor concrete mixes. This approach has reduced the need for physical testing, improved productivity and reduced waste. Digital sensors installed in the concrete mixer allow control of peak curing temperatures to avoid thermal-related concrete defects whilst tracking strength development to improve formwork striking times.
The range of mixes typically falls within the low carbon or very low carbon categories compared to the EXEGY bandings, based on benchmarks from European standards. By comparison, the mixes fall within Categories C or D compared to the Arup Embodied Carbon Classification Scheme for Concrete. The percentages of these mixes which fall into the EXEGY categories can be seen in figures 1 and 2.[1] One supplier, Holcim, has 96% of their concretes provided falling into the low carbon or very low carbon categories, while another, Tarmac, has 84% in the same categories.
1. Figures 1 & 2 | Pie charts showing the percentage of structural mixes falling into each EXEGY classification
