The return of the most expensive cube of grout
Refresher
In my earlier blog on the most expensive cubic metre of grout ever poured I ended with a hope… and a contingency.
Taking core samples from suspect grout
The hope was to take core samples from the suspect grout on the chance that the in-situ strength was higher than the cube test results which had returned compressive strengths of <40MPa after 14 days (less than half the forecast strength at this age). The design consultants had worked around the clock (literally – Robert Bird Group enlisted one of their New Zealand based engineers to work the opposite hours to the UK team) to prove the scheme with a minimum grout strength of 50MPa.
Core Samples
100mm samples were taken where we could get at the face of the grout and clearly showed the separation that we feared.
As you can see in the photos above the grout has separated and the fine aggregates (<2mm sand) have settled to the bottom. The smooth grey upper section contains the majority of the cementitious content and the chalky white crust on top is the synthetic polymer additives. The granular layer was so weak several of the samples sheared in half during extraction revealing the cross-section above.
Testing
The specimens were sent for testing (which by the way takes a lot longer than cube testing due to the time required to prepare and cap the samples) and strength results were issued. We asked for testing of the lower sections of the specimens as this is where the bearing surface is and where we suspected the lowest results.
For the two samples in the left hand photo we received cube equivalent compressive strength results of 17 and 15.2 MPa. On one slightly longer sample we were able to test both the bottom granular section and the upper smooth section. This test returned 37.3MPa in the aggregate and 70.6MPa in the cement matrix. This again supported the theory of complete segregation due to over-watering of the mix.
For anyone coming across a similar situation I have uploaded a document summarising viable In Situ Testing Techniques.
Conclusion
Instruction was issued that all affected grout was to be removed by hydro-demolition and replaced with only a month long window in order to avoid knock-on critical path delays to the project as a whole – a certain contender for the title of world’s most expensive grout.
Holdfast Training Services Trial Blog 
Thanks Tom,
Always interesting to follow a topic as it develops. Interesting that the cubes didn’t continue to gain much from 7day to 14 day testing. I refer to my previous observation that “recognise as early as possible that there is an issue and get on with the necessary remediation as soon as possible instead of losing time to hope” is the recurring lesson that I think I see each year in one form or another. There is quite a lot of take home learning in this storyline and I value the CPD. I look forward to hearing some home truths from the design office.