Limestone Filler Implementation in Geotechnical Concrete Mixes

Bachy Soletanche Balfour Beatty Ground Engineering Joint Venture (SB3) is the geotechnical subcontractor for Balfour Beatty Vinci Joint Venture (BBV), responsible for the geotechnical works on the 90 km high-speed railway HS2 in the West Midlands. The scope of the works includes 4 500 linear meter of diaphragm walls, 19 000 piles for deep foundations and retaining walls, and 85 000 rigid inclusions. The total amount of concrete is expected to exceed 600 000 m3 for the geotechnical works. The concrete for the project is supplied by three different concrete suppliers. Each supplier sources cement, aggregates, and admixtures.  In May 2023, the limestone filler, prompted by SB3, was successfully implemented into the concrete mix design. Following a series of trials in the laboratory and at the batching plants, the new concrete mix, incorporating limestone filler as a substitution for cementitious materials, was integrated into SB3's supply chain. The new mix design suggests 3-6% reduction in embodied CO2. Although the reduction may appear modest, the extensive scope of the project means that 225 tonnes of CO2 emissions were avoided through the implementation of limestone filler for the period from May 2023 to May 2024.  Furthermore, limestone filler is widely recognized in the concrete industry for its ability to mitigate common issues with fresh concrete in deep foundations, such as segregation and bleeding. The physical properties of limestone filler, including its particle size and non-reactive nature, effectively block bleeding channels and enhance the density of the concrete paste, thereby preventing segregation. Since its implementation, no bleeding or segregation events have been recorded in freshly poured piles or routine production control tests, when it was not the case before.