Cadent Gas require to divert the existing high pressure gas pipeline at Austrey to Shrewley to provide mitigation from external loading from future HS2 traffic. As well as standard trench construction the diversion also compromises tunnelling at certain locations along the route. To launch and receive the tunnel boring machine (TBM) a secant piled shaft is constructed at the launch and reception sites.
Secant piled shafts are constructed with bored piles and typically compromise overlapping primary/soft and secondary/hard piles. Primary/soft piles are formed without reinforcement from a weak bentonite slurry to low strength concrete typically C8/10 with secondary/hard piles installed with reinforcement and using structural concrete typically at C30/37 or higher and cutting into the primary/soft piles to form an overlap which prevents water and fines entering an excavation. Secant pile shaft structures for TBM works are typically temporary works designs with less onerous full design life/durability criteria. The selection of the soft/primary pile concrete mix is critical to achieve the correct balance of structural performance and durability where required without impeding the installation of the secondary/hard piles. As very stringent installation tolerances are needed the strength gain profile of the primary/soft pile concrete mix is also required to be clearly understood to calculate the timings of the secondary/hard pile installation.
It is a priority for us to find the latest technology to minimise our impact on the environment. As a result, we were keen to approach DB Group to consider how we could use cement-free concrete on one of our projects. This is where Cemfree comes in. It is an ultra-low carbon alternative to standard concrete whereby OPC is replaced with novel alkali activated cementitious material (AACM). As it does not contain any OPC, it can save up to 88% of CO2 emissions on a project.
Cemfree has been used in the UK since 2010. However, so far it has only been used on small scale projects. Therefore, we have partnered with DB Group, Hanson and BRE to trial Cemfree concrete on the primary/soft piles for the secant shaft piling at our HP11 gas main diversion project and better understand and develop its characteristics for future use in piled structures.
Our aim was to trial Cemfree to assess its installation characteristics (slump, flow, and placement), its strength gain profile and its ultimate strength gain performance, in the process measuring how much CO2 we could save overusing a traditional concrete mix. The main and initial process was to trial the various binder content mix designs to assess which binder content would be most suited to replace a typical C8/10 primary/soft pile concrete and also observe its in-situ behaviour prior to use in the main shaft piling. In addition we also trialled higher strength binder contents to gather data to consider whether higher strength concretes could feasibly be replaced with Cemfree in the future.
With the temporary works design status of the TBM shafts there are less onerous design life criteria making this setting ideal for the trial as currently AACM’s in general are not appropriately covered by existing concrete standards making their use in permanent structures more complicated. The scheme engineers COWI were positive in the adoption and use of Cemfree for the primary/soft piles based on the strength gains achieved during the trial process.
The main works were completed successfully with the selected binder mix achieving a consistent strength gain profile and between 17N to 20N at 28-day strength. We have seen a number of benefits through trialling Cemfree on the HP11 gas main diversion project and have generated interest across the group and HS2, including:
By demonstrating the sustainable and economic benefits of Cemfree we hope to encourage other ground engineering companies to consider using more sustainable materials and practises. There are further characteristics of Cemfree which may have real benefits in other areas of pile design and construction, and we are exploring these as part of the LOCOWAG Innovate UK Smart Grant team over a 2-year period assisting with the development of data to support progression of relevant design codes addressing the current lack of inclusion of AACMs and supporting the wider commercialisation of Cemfree within the construction industry.