Building An Earthship In Darfield, B.C.

We are a family of five living in Darfield, BC.
Our house is six hundred square feet in total and we are feeling cramped.

We have decided to build an earthship!

So starts the adventure ...

Wednesday, March 31, 2010

Pouring The Concrete Bond Beam

A few days ago was the day and man am I tired!  After months of waiting through winter, we picked yesterday to pour the concrete bond beam on top of the back wall.  We have been preparing for this pour since we started work on the house again in February; screening gravel and sand, building the bond beam form and hauling in the portland cement we needed for the job.

It was an ugly morning.  Rain the night before with heavy winds, and neither the rain nor the wind had completely died down.  We started setting up not really convinced that we would actually start pouring concrete.  Nobody wanted to be the one to cancel the pour, and after a couple of rounds back and forth discussing options the decision was left to me.  Not one to be hasty I deliberated and we got started right away ... Fortunately the rain cleared up and we had sunshine with really heavy winds for most of the day.

We decided to mix our own concrete for this job rather than have it delivered from a plant in Kamloops.  I estimated that the pour would require at most 4 cubic yards of concrete and the minimum delivery is approximately 5 yards (concrete is now sold by the cubic metre which is slightly larger than a cubic yard).  So, if we had ordered concrete I would have had to have planned ahead by having additional forms ready for concrete.  Also, due to delivery charges (we are about an hour away from the closest concrete plant), the delivered concrete is that much more expensive if you order a small volume.  Given our desire to save money and our limited ability to plan ahead it seemed a no-brainer to mix our own concrete.

We have mixed lots of concrete on this project and in the past, but never this much at one time.  Our little cement mixer can mix two cubic feet of concrete.  Given that there are 27 cubic feet in a yard, we would need to mix about 60 batches of concrete for this job.  Assuming we could mix a load every ten minutes we would be looking at about ten hours of non-stop work just mixing the concrete, not to mention pouring it onto the forms and leveling.  We quickly decided we needed more manpower on this job ...

 The first workers we lined up for the job were John (aka John Deere) and  Bob.  Bob actually insists on going by BobCat ... I find this really '70's, but he packed all of the concrete from the mixer to the forms so whatever name he wants to be called is fine by me!  The cement mixer on the back of the green John Deere tractor is driven by the PTO (power take off) of the tractor, and it can handle about a sixth of a yard per batch.  So, we only had to to do about 20 batches of concrete.  The tractor was parked by the sand/gravel pile all day, and the mixer ran for most of the day spinning the mixer.  When a batch of concrete was ready it was dumped into the bucket of the bobcat and taken to the building site.  Thank you Tom, Stephanie, Robin, Jody, Mike and Linda for the loan of all of this equipment!

The bobcat is small and mobile enough that it could be taken inside the building walls.  The operator then lifted the bucket to the height of the forms and dumped the concrete out.  Unfortunately, the bobcat had just enough lift to get the bucket over the edge of the form and the bucket could not be tipped far enough so that the concrete would simply slide out.  We had to shovel the concrete from the bucket into the forms.  Still, this was much easier than building scaffolding and carting the concrete around in wheelbarrows!

After the concrete was dumped in the forms it needed to be shoveled (or vibrated) so that it flowed everywhere and eliminated any air pockets.  Finally, the concrete was leveled and smoothed so that it was consistent throughout the entire form.  We were not very concerned about smoothness as this concrete will not be exposed when the building is finished.

The last step was to insert the anchor bolts.  These bolts are spaced 4' apart and will be used to attach a wooden sill to the top of the concrete bond beam.  The rafters of the roof will ultimately be attached to this sill plate.  You want to get the bolts in fairly soon after the concrete is poured and leveled, as otherwise it becomes unworkable.  We simply put the bolts in after we finished leveling another 10' or so of the concrete in the form.

We started this job at around 9:30 in the morning and finished right around 5:00 in the evening with about a half hour for lunch.  We worked all day and our neighbors, Robin and Jody (thanks guys!), were there for about half of the day.  It was hard work but all of the equipment made the day go that much easier and painlessly (We still hurt at the end of the day!).

Some notes about the concrete ...

The concrete for our beam is specified as 3000 PSI (pounds per square inch) with reinforcement.

The reinforcement consists of two horizontal runs of 10 mm rebar with vertical rebar every other tire driven into the top three tires and extending to within the top two inches of the form.  We also used engineering fibres in our concrete mix.

Our formula for the concrete was 1 part portland cement, 2 parts sand and 3 parts gravel.  The dry ingredients were added to the mixer with sufficient water to hydrate the mixed materials (you do not want to add too much water as this will weaken the concrete).  We then added engineering fibres and allowed the batch of concrete to mix for another couple of minutes.

I figured out that one 40kg bag of dry portland cement has a volume of about 28.6 litres.  We dumped one bag of cement (very heavy!) into the mixer for each batch of concrete.  This in turn meant that we needed 143 litres of aggregate (2 parts sand; 3 parts gravel) for each batch.  We filled 7 twenty litre pails and added their contents to the mix.  Generally, I slightly under filled the aggregate buckets to ensure that I was getting a slightly higher proportion of cement in each batch.  I did this to make slightly stronger concrete.

We ended up using 22-40 kg bags of portland cement which works out to slightly less than 4 cubic yards of concrete.  So, my initial estimate was pretty close!  I am glad that day is over.

We have already started prepping the roof beams, but I suspect we will start work on the front wall first.  The front wall is required to finish the roof, and I am not keen to have the roof beams installed and weathering in the sun while we work on the front wall.

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