"No fines" concrete

Ron0z · 22 Oct 2025

Hi there, I'm new here.

I'm about to build a timber deck next to an old hut. It's a very run down hut and when finished I suspect the deck will be the best part of the setup. (One project at a time.)

It'll be a low deck (less than a meter off the ground at the high end), and I was planning on placing the posts directly into the ground, and I was also considering using no fines concrete to secure them. I have to say I have only recently heard of this type of concrete. Though, I see it gets a mention on this site for house construction. Just to be clear, this is what I understand by the term. I'll be mixing it by hand, probably in a wheelbarrow.

Has anyone actually used this type of concrete mix for small jobs (eg. post holes)? If so, are there any tips or tricks you can pass on?

stevie888 · 22 Oct 2025

No fines is used for a specific purpose:

(continuiing after computer hissy fit!!...)

....because it allows water flow it is sometimes found behind retaining wall though there are more modern an better methods. There was a housebuilding ststem back in the day that used it (Wimpey?)

Since it has a lot of air in it, it's comparitively light but is lacking in strength. Where we want to reduce weight in concrete structures we tend to form voids or use Lytag.

Ron0z · 22 Oct 2025

It has a variety of uses, apparently. In my case, I want to use it for concreting timber posts in the ground.

The open nature of the concrete allows for good drainage. It is also used in retaining walls for the same reason (ie. water drainage). I believe it's also used in certain bridge components due to its light weight (ie. no sand), and as part of road construction and footpaths. I doubt you'd ever see it in a multi-storey buildings because it must be weaker than standard concrete. And because reinforcement steel cannot be used with it. Horses for courses.

Ron0z · 24 Oct 2025

So, I take it that no one has actually had any experience in using No Fines concrete.

stevie888 · 24 Oct 2025

We used a load of it way back in the 70s to form a drain behind retaining walls. We cast the walls with weep holes at about 3m crs iirc then cast a 200 mm layer of normal but weak concrete to form troughs and valleys between the weeps and then finally topped off the 200 layer with no fines.It was 20mm single sized aggregate and cement no sand no small aggregate and not a lot of strength.

Deluks · 24 Oct 2025

For a post in the ground, no problem, it doesn't need to be strong. The posts will be joined to the other posts anyway, so they won't be subject to much lateral load.
You can get away with tamped gravel to hold a post in.

Ron0z · 25 Oct 2025

Thanks for that. I came across a couple of videos that show No Fines being used in a footpath. The first provides some hints on how to judge the correct amount of water in the mix. The second goes into detail on pouring for a footpath or driveway. It makes for interesting viewing. My application is for posts. Therefore, a lot of the detail was irrelevant, but it may be of interest to others. I actually think it has a very nice look for a footpath, particularly with the smaller aggregate.

The video notes a mix of 3:1 (aggregate:cement). I quite like that ratio. Other websites suggested 8:1 or 10:1 which seems in my mind too weak a mix to have much strength. And while this video suggests mixing the aggregate, cement, and water together, I came across one that suggested mixing the cement and water into a paste first, before adding to the aggregate. Perhaps when all is said and done it doesn't make any difference.

stevie888 · 25 Oct 2025

I'm still struggling to see why you want to use the stuff at all. Most peeps will just go for postcrete for this job though proper concrete is much better.

At 3:1 no fines you'd be using at least double the cement you need for an uncertain product.

Ron0z · 25 Oct 2025

Never heard of "postcrete." What's that?

3:1 seems reasonable to me, because a pretty common mix is 3:2:1 (aggregate:sand: cement). With the sand gone, it'll have (mostly) the same bulk. Just a lot of air gaps.

It's a new concept to me. I was sceptical due to its lack of strength, but in a hole (and as noted above) its reduced strength is not a problem. Timber posts supported using standard concrete tend to promote rot. As the post shrinks, moisture will gather in the gap between the post and the concrete, facilitating wood rot. With no-fines, drainage is assured and rot is minimised. One article I read suggests a sleeve around the post toward the top of the hole, and another suggested sealing the upper portion of the hole with clay. Both in addition to the no-fines concrete.

All of this extra fuss is an argument for stirrups, but in this particular project, I'd prefer not to use them.

stevie888 · 25 Oct 2025

Postcrete is lazy fencer's go to. It's premixed and you just put water in your hole and tip the stuff in or pour it in dry and water it. It goes off very quickly (10mins sort of scale). What you don't get is proper mixing and therefore you don't get proper hydration which means weak concrete. There was a good post on utube a few months ago where a guy runs trials on how it performs: eye opening.

3:2:1 is way too strong for posts: 8 or 9:1 with ballast and kept good and stiff would be my approach. A little gravel in the bottom of the hole so the end grain doesn't wick water up is often advocated.

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"No fines" concrete

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