Observatory Build – Filling The Hole

Today the hole was filled by an AnyMix concrete mixer. I spent a while looking into how to fill the hole, whether to manually mix the concrete myself and hire an electric cement mixer and order in the cement and bags of ballast.

But after using some online concrete calculators I discovered I actually needed 2x 1ton bags of sand and possibly something like 13 bags of cement. This price came to about £160 with the hire of the cement mixer, I thought it would originally cost me about £70 to do it myself, but a 1 cubic metre needs more aggregate than I thought.

The 1 ton bags of sand were not too bad price wise, as BuildBase sold them for about £45 each. Anyway I then decided to call around concrete companies and get some prices.

At the time I did not really know what mix type I wanted, as it all seems a bit confusing to me. James at ScopesNSkies recommended a 3 to 1 mix, so (1 cement, 1 gravel, 2 sand).  I was told by some companies that this may be too rich, as adding too much cement would mean the block would crack.

So in the end I got prices for a C35 mix type, although Hanson advised a fibre mix with hair in it.

Hanson quoted about £250, Cemex quoted £206, Tarmac would not deliver to my area, and finally AnyMix originally quoted £167, but after phoning back and saying I was originally quoted £150, they agreed on this price.

Hole with Wood FrameThat was great as this price, was less than what it would cost for me to do the mixing myself, so the delivery was booked just 24hrs before, for a 3.30pm delivery.

My father and I began by building a wooden frame so aid with levelling the concrete. I bought the 2×3.6m 1″x2″ wood from Travis Perkins for about £6.50. We then also used some blue plastic liner to contain the moisture of the concrete. We actually used 2, one inside the other.

Hole with Plastic LinerThe delivery was a bit late and arrived at about 4pm, but the driver was very accommodating and friendly, and we used 2 wheelbarrows to offload the concrete.

Even though the hole was nearly exactly a 1 metre cubic hole and I ordered 1 cubic metre, we still had about 5 wheelbarrow loads left over in the concrete lorry. Luckily the driver decided to get rid of it for us, although with hindsight we could have used it for the footings to go under the slabs. But we had not thought of this and had not pre-dug the space.

Hole Filled with ConcreteNext time, if there is a next time of doing this, then I will pre-dig the footings to be ready for any over supply of concrete.

The concrete went in well, and it was a bit of hard work barrowing the concrete. My Dad levelled off the concrete with a large length of wood and now it’s drying off. Apparently it will take about 1 week to fully set.

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