What we did:
- Excavations (part 2)
- Installed steel reinforcement and Claymaster
- Site prep for pour
- Concrete pour
A huge week, entering a new chapter.
The excavator, its operator and his chain of Marlboros were getting through the ground at great speed, so much so that we couldn’t get the excavated earth out fast enough to ensure a clean and tidy site. To put this into context, one of the big challenges of this project is the relatively small size of the site and the restricted access into it. As you may have seen from the plans and the intro page, the site is behind a retail parade and accessed through a narrow alley, which can just about squeeze in lorries, although this requires careful organisation, safe traffic management and a lot of sweet-talking the neighbours, who no doubt still (un)reasonably still get annoyed.
Once the trenches had been dug out, we had to install steel mesh into the foundations.
Here’s the GCSE Physics lesson...
Concrete has high compressive strength, meaning that it can take (uniform) loads pushing against it; this is fine in 90% of cases, especially in the UK. Where the concrete may experience non-uniform forces which may bend it e.g. if there are tree roots pushing against it or if there is an earthquake(!), concrete doesn’t handle theses tensile forces well. The tensile strength of concrete foundations can be improved using steel reinforcing bars (rebar), which is laid as a mesh and concrete set around it. Given we also have clay in the soil here, ‘Claymaster’ sheets were used to protect the concrete from fatigue caused by the soil shrinking and swelling around it.
Now to the fun stuff.
Getting concrete trucks into the site was always going to be a huge pain, but thankfully following a moment of genius (and more sweet-talking), the school which the site backs on to allowed us to use bring our trucks through their access road. All we then had to do was feed the long elephant-trunk-like-hoses through the treeline and start pumping! For me this was the biggest moment of the project so far and everyone was quite jovial and abnormally excited about concrete!
"Start of a new life Jitin!"
As we needed so much of the stuff, approx. 50m3 (120 tonnes!), we first had a series of 4 trucks lined up, all mixing concrete. This was all fed into the front 2, which pumped the mix using very high pressure along 20m long pipes into the trenches. A nice bragging point was that this was the first time the concrete company had done this, giving rise to some sort of concrete paparazzi taking promotional pictures. The fifth lorry then topped the trenches up separately.
Within my pictures you’ll also notice a long metal rod being used, this creates powerful vibrations within the wet concrete, bringing out any air bubbles whilst also covering the unfortunate user in splashes.
After a few hours we had reached a uniform, smooth level across all the trenches.
Time for the champagne and whiskey!
Next week however saw the first major issue…