Testing times

This weekend has been all about the numbers. 


After the collapse we sent off a sample of our earth to ESG a company that do materials testing amongst other things. They have run tests for other REW projects so knew what we were looking for. The results came back on Friday and I was able to talk these through with Rowland Keable (builder of many unstabilised REW). It's all been very educational and interesting.


ESG did 3 tests for us. The results are as follows:

Sample 1: Tested at As Recieved Moisture, without Drying (therefore no Shrinkage)

Compressive Strength:  0.18 N/mm2

Sample 2: Tested as Oven Dried

Compressive Strength: 1.8 N/mm2

Shrinkage: 1.3%

Sample 3: Oven Dried, with 7% cement.  6% water added to achieve drop test criteria

Compressive Strength: 0.48 N/mm2

N = Newton (wikipedia def) 

Sample 1: The earth cannot dry in shuttering so when the shuttering comes off the wet rammed earth has to have the strength to support the wall. This strength is what let us down. Interestingly the 7m unstabilised rammed earth walls at the CAT centre in Wales have the same width and were built in the same way as ours (all in one go). Their rammed earth had a wet compressive strength of 0.5 N/mm2. So we would have been ok up to about 3 metres with ours. 

Sample 2: The dry rammed earth number looks better and would be absolutely fine for our wall.

Sample 3: This number was unexpected and sent us into a right quandary on Friday. Unfortunately ESG had left this test til last so it only had 3 days to dry before testing leaving it weaker than the unstabilised dried sample. 

The poor stabilised number didn't make sense and even though we know cement should make things stronger we wanted proper numbers to compare and contrast and know exactly where we stand. So, Saturday morning we drove down to ESG in Uxbridge and dropped in a stabilised block we'd made 11 days ago. They tested it for us first thing this morning. 

We had decided over the weekend that if the number was still less than the dry unstabilised we'd build unstabilised and figure out how/if to do the top half of the wall later, if the number was good we could stick to our (revised) time plan building the first half stabilised this week and the top half unstabilised in a couple of weeks. 

It was a tense wait ... to cement, or not to cement ... but the number came back good: 2.8 N/mm2. And so, we will have cement at the base of our wall. 

We're relieved. We could have gone for a 2 stage unstabilised build but ... we didn't know how effective 2 weeks drying out at this time of year would really be. We probably could have figured out a way to build the top half later - once the house was up - but the house design means there wouldn't have been much headspace to work in at the top which would have made things really tricky. And in the end there isn't a huge margin of error to play with here so we didn't have the stomach for it.