limecrete pad foundations for post and beam frame

Hi All,

First off hello I am a new member on this forum, though i have enjoyed reading it for some time.

I have started a building project recently and have been scouring the internet for guidance. Having already built a small shack using roundwood posts partially buried (with some shifting over time but not major) I want this project (which is bigger) to seem a bit more solid and am thinking about foundations. The design is roughly a 20 x 36 ft rectangle, with loadbearing posts at 4ft intervals which will then be infilled with straw bales. At this stage I am thinking about pier/ pad foundations to fix each post to. Ideally I would use limecrete. I'm also thinking about how and whether to brace the posts at the bottom, as well as to lift the first layer of bales

Has anyone got an opinion on this plan or could even point me in the direction of some guidance about how to form pads with limecrete please? I have seen ready made concrete pads for sale and they don't look complicated. The plot is excavated down to shale and i have drained the whole terrace, so this should be a good bearing soil.

Any advice much appreciated.

cheers M

Limecrete is not typically used in load bearing applications. What compression strength are you getting out of it?

Thanks for the reply, Terry.

I can't say for sure what the compression strength of limecrete will be, though I have looked at different guidance from limecrete companies (either supplying limecrete for you to mix or pouring it for you) that say a slab would be as strong as a concrete slab so I didn't think there would be that much in it- interested to know more, though?

Also, I'm not looking to pour a whole slab if possible. What I'm hoping to do is to closely mimic something like a "swift plinth" this is a ready made concrete plinth- (I haven't linked out to it's webpage as i'm not trying to promote them!) The main issue I have is that I want to lift the structure so I can use straw bales, and I also want to use breathable materials again to protect the bales from moisture and allow moisture out of the bales over time.

In my area there is a presumption in favour of low or zero carbon building which incentivises using products with lower embodied energy where possible, hence my thinking to avoid concrete.

cheers. M

You are welcome. Lets get a couple of terms straight so we can better communicate. Concrete is a generic term. Cement strength is what we are referring to, Organic Portland Cement (OPC) due it's higher kiln temperatures (~1000F/ 530C) being higher than lime production has a MUCH higher strength. Some concrete mixes using OPC range from International Code minimum @ 2500 PSI to 15,000 PSI needed for load bearing piers too. You also want to look at flexure strength that causes cracks and some other properties compared to OPC. Lime is or crete (aggregates) is typically used in low load plaster and insulation. Not that they can't get to higher strengths with admixes but I would ask to see the specs, testing to a standard like ASTM or ACI we use in the USA. Geopolymer cements like MGO with pozzolans can far exceed OPC. Iron rebar is also an issue, there is ALOT to cements and concrete designing. If you are not a pro follow code.

As far as embody energy of lime vs OPC, I would guess lime is higher here in the USA due to the locations, quantity, shipping, etc.. of the manufacturing plants. I'd ask for proof on that too before you get too carried away and design to lower untested strengths, or lack of data/standards I have not seen posted yet.

As far as how to anchor to piers or plinths, we'd need alot more info like what wind speeds and seismic activity, dead and live design loads.

Thanks for another detailed response, Terry, much appreciated.

OK to answer the easy questions first, if you can guess I'm in the UK so codes are different though I expect many of the principles are the same. My particular region is starting to incorporate eco building techniques into building regs and planning so while a few things are still grey areas there are a few materials that are being approved more readily, including lime. Lime is abundant and easily available in my area, cheap and rated very low carbon.

My anchoring plan was to use 3/4" steel threaded bar rather than iron rebar for durability.

My plot is not on unstable ground according to the relevant geological surveys, wind speed is 22 m/s (think that is metres/ second) and regs don't require seismic activity to be taken into account, probably because we have so very little.

I was thinking to use NHL5 as the lime component to replace cement in the concrete and use leca or similar as the aggregate. As I said before it's mainly a case of maximising breathability for the straw bale infill which I don't think cement based concrete will do. I've looked at a few lime-based foundation designs that have been approved by regs in the UK, but none so far that specifically form piers/ plinths. Also I'm finding it hard to find any tables outlining limecrete compression strength with any specificity. I found some for hempcrete, though I don't have much access to hemp!

I'll go back to the regs and take another look, and also I'll take a look at Geoployner cements as I had completely overlooked that.

cheers, M

Yes Milo, most of the bigger countries are on the same boat with lowering C02 emissions which can be completely opposite of a good natural healthy sustainable building. I call it a state of confusion with all the air sealing.

If my memory serves me well NHL 5 has a compression strength in the 100's, International Code (IRC) is a catch all with testing across the globe. The International Code council is the gate keeper and most jurisdictions start with it, modify, re-write, etc.....I mention it since if you look at 2012 or 2015 IRC you will not find lime based foundations since their compression strength is so low as in the 100's PSI vs IRC min @ 2500 PSI. It can be found in the stucco tables, NHL 5 or Type S. Lime/Hemp around 80-100 PSI. Add rock and lime I'd say 300-500 PSI at best. If you did the math the average one story build never sees close to 2500 PSI, we use high compression and rebar (steel is a iron alloy btw) to resist flex and tension. Bassalt or fiberglass rebar is better, chopped and continuous.

What I am going to guess is the UK foundation specs are going to have to use much more lime and use bigger foundations than OPC to get the same results which means the embodied energy is not better. Or they are getting their strength by some other means. If they are using a spread like rubble trench with some lime in it that would make more sense. They could be using MGO or another pozzalan to get the content OPC down. There are some pozzolans like ashes that make OPC stronger needing less.

Geopolymers take some experimenting with MGO (which BTW) is combined with Calcium(lime) in different quantities to get more strength...NHL 5 and Type S has the most MGO) and the least breathability (perm rating of ~ 1-3 perm) due to more cement and higher density for strength. There is much more to the word "breathability" than perm rating.

I'm not sure if you are planning on setting bales directly on lime or OPC concrete but not a good idea on NHL 5 low perm due to capillary uptake (another form or "breathbility") If anything NHL 2.5 would be better against bales or most set them on stick framing or a MEP chase.

The best place to get properties of the lime would be the manufacture. Here in the US we have "The American Lime Association" with alot of publications maybe UK has some thing like that. If you are not finding any data in your local code that usually means they want you to hire an engineer and there is no code path to comply to safety regs.