29/06/15 20:46 Filed in: Concrete
There is a great lorry on Skye, recently upgraded, that mixes the concrete on the lorry to a fairly exact mix so that you only use what you need with no wastage. The cost was just shy of £120 per cubic meter of concrete. The cost is also determined by the strength of the mix. We were told in the specification that we had to use a C25 strength. We decided on a stronger C30 mix that put up the cost to nearly £120 per m3.
The Lorry at over 40 tons was too heavy to use the track to get onto the site right next to the building. Sean therefore had got his dumper truck on site together with his digger which was ready on the Studio site. So the lorry poured the concrete into the dumper which then travelled over to the site where Sean used the bucket of his digger to very carefully place the initial 100mm of concrete into the trenches. Simon and I were in the trenches spreading the concrete with rakes so that it was level across the trench. We knew it was level as we had drilled vertical steel bar into the rock where the centre of each pier was. These were painted with yellow on their tips and they were level with each other across the trench. They were drilled to a height where just their tips would be showing when 200mm of concrete, at the shallowest point, had been poured. At 100mm from the top we placed the steel mesh carefully within each trench before filling up to the tip of each bar.
This work happened quickly and I am always amazed at how delicately Sean could use the digger and bucket to gently drop the concrete where we needed it. When we put the first steel mesh in we also placed each of the central pier cages over the middle of each of the vertical bars, already in position. Simon together with Sean built up the column around each of the piers so that there was a level area large enough for the blockwork to sit upon. This work was complete within a couple of hours. We used 6.3 m3 of C30 concrete. This was probably two or three cubic meters of concrete more if the ground had easily allowed us to dig 12 separate holes rather than four trenches with three piers in each trench.
We left the concrete to set and would begin the following day on building the smallest piers. Before leaving we distributed the blocks and stacked them around the site so they were ready to hand at each place a pier was to be built. To have mixed all this on site would have taken days of hard work. So in two hours and thanks to a good access track the pour for the main foundations was done in around two hours.