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.
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:
- For each Kg of OPC we did not use, we saved 3Kg of raw materials For each Kg of OPC we did not use, 1Kg of waste was diverted from landfill
- There was a 65-82% reduction in CO2e. As a result, we reduced the global warming potential of the concrete we used on the project There has been a 40% reduction in water used to produce the concrete for the piles
- We were able to use Cemfree without having to develop different procedures or use alternative equipment. As a result, there were no added extra costs.
- The strength gain performance during the trials of the higher binder content has proved positive and we aim to progress the use of Cemfree for a full secant piled shaft in the very near future.
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.
