I need to fix the uprights for my veranda into the ground. Rather than concrete posts in directly, I plan to dig a hole, fill with concrete and bolt the uprights to the resulting concrete using a post base with four bolts.

What mix of concrete is ideal for this? What do I want to put in it?

Ballast and cement at 6:1

Do I want to reinforce it with some bars?

There are some suggestions it could take a week to dry out fully? Is that how long I should wait before bolting things to it?

Thread hijack! Same for the base for an Asgard bike shed?

Takes a while to dry? A week is probably about right

421 4 agg 2 sand 1 cement

If your not hanging off it I’d not worry about rebar

I think its usual to set the bolts into the concrete while its wet(or when initially filling) rather than drilling holes into it when set. And probably better to add some reinforcing bars of some sort.
I know little, but were I doing such I’d look to YT for ideas or methods

@tobymc

You mixing by machine or by hand. If by hand I’d just set it on flags

@duncancallum no, I have access to a mixer. Going in a front garden, so thinking proper slab to bolt it to

@tobymc

5 to 1 using ballast is a good strong mix.

I’d also reinforce and maybe cast a couple of anchor points in

I’d be setting threaded bar in the concrete, with some large repair washers fixed in place on them with nuts, for good anchorage in the concrete. Make a jig in a bit of wood to hold the 4 pieces of bar in place while the concrete sets. If you want to speed up the setting and some postcrete.

Does a bit of plaster added to cement mixes do the same as a bit of cement added to plaster? If it does then a few handfuls of plaster will make it go off very quickly.

I made some footings for my wobbly garden gate (the gravel boards the frame sits on kept subsiding).

I made a dry mix in a barrow using what I had hiding around the garden; 1 bag of cement, 2 x 25kg bags of pea gravel, 2 x 25kg bags of sand and a few bits of old rubble. Wet the trench first and filled it with the dry mix. left it a couple of days no more wobbly gate.

I think its usual to set the bolts into the concrete while its wet

I was going to say that I can’t fix the post holders in place when wet, that’s just too hard – BUT I think I might be able to stick threaded bar in it. That would really hugely reinforce it against leverage applied by wind.

Of course I want the post to be vertical, so maybe I can get a piece of wood, drill holes in it to match the post holders I want, then put the piece of wood on top of the wet concrete – I can use the piece of wood to both level the top AND hold the threaded bars in place.

I find these concrete tips quite funny, plaster chucked in, wetting the sides of the hole, how long it takes to go hard etc and trying to accurately hang threaded bar into wet concrete if your not used to the stuff is very tricky
Drill it then resin from Toolstation
If the mixer is a Belle then 12 and 1/2 kg exactly a half bag of cement to each batch to 3 buckets or maybe 4 of concrete mix/ballast and remember to podge it, look the word podge up but not the urban dictionary version about the Mrs
We need Wrightyson back although he’d have wanted a ready mix lorry outside your door

Where to actually buy concrete is probably a more suitable question…

C10/C15, or higher strength concrete.

was going to say that I can’t fix the post holders in place when wet, that’s just too hard – BUT I think I might be able to stick threaded bar in it. That would really hugely reinforce it against leverage applied by wind.

You want something along this line

https://aylesbury-gates.co.uk/shop/fencing-posts/concrete-in-u-post-support-bracket/

If your concrete mix isn’t too slack, the saddle stops it from sinking. Level top of concrete, put the rod in, check it for level, done. No reinforcement needed for the concrete and long as it is in the ground. Rebar is only really needed when the concrete is unconstrained and can have sideways forces.

After coconut I would suggest anything from C10 to C25; anything beyond that is overkill – in fact, C25 would be over-specified.
Rebar would be helpful but getting a small quantity will be unattractive to suppliers.
Don’t forget that concrete takes time to cure; unless you use a special mix or additives assume 28 days for full curing and strength.

Any concrete fans watching the Hinkley Point C documentary on BBC / iPlayer? Hoooleeefooooooooq!

Complex concrete pours are fascinating but cranes are where it’s at.
Mammoet have the biggest and best.
Commissioned one of theirs to erect a tower crane in central Leeds to overfly two 15 storey tower buildings immediately adjacent to the city station.
Inspector turns up; I’m going up the tower and out along the boom to check everything; if you’ve got the necessary PPE and certification, come with me; you’ll be clipped in/out at every step.
Don’t like heights and didn’t have all the required kit so…didn’t do it.
Regret it.

How much do you need? You can buy bags of ready-mixed concrete for fence posts, if you just need a small amount get a few of those, takes the guesswork out and saves you having half a bag of cement left at the end.

For basic DIY, use the mix that’s written on the back of the cement bag.

You can buy bags of ready-mixed concrete for fence posts,

Been to the shops recently??

Have a look at what is available at your local stores before deciding what to buy, cement and cementitious products are in very short supply.

Complex concrete pours are fascinating but cranes are where it’s at.

I thought all the cranes were in China but actually most of them appear to be at Hinkley Point C including this…

Have a look at what is available at your local stores before deciding what to buy, cement and cementitious products are in very short supply

Not only that, but the price of bagged cement and Postmix-type materials (to say nothing of truck-delivered readymix) is about to rise, probably in early July, as a result of new costs being incurred by the manufacturers. It boils down to them having to purchase Carbon Certificates to allow CO2 emissions. So if you can find bagged concrete/cement now, and it suits your purpose, which it likely will, snap it up.

What are these bases supporting – you say veranda so is that a deck and a roof? Is there a cantilever action off the house? What are the ground conditions like – how far do you need to dig to get a suitable bearing? Do you know what a suitable bearing would be for the loads being applied?

These need to be considered in the design of your bases – mainly depth and size. Unlikely you’ll need reinforcement unless you end up with particularly wide shallow pads. But this will affect mix too.

As Redmex says, resin anchors are a lot simpler than casting in bolts.

To be honest if its a lightweight structure you can’t go too far wrong, biggest risk is if there is any future settlement and you have cantilevered off the house, you could pull out the fixings (or pull the wall down).

Rider – I am a construction professional, and tend to see all the complications!

You’re making pad foundations that will be hidden.
unless you are anticipating uplift from the roof, Anything solid and grey will do*, and reinforcement isn’t going to be doing anything useful.

The depth and width are far more important than the mix.

Check your soil here: http://mapapps.bgs.ac.uk/geologyofbritain3d/ if it is clay based then ground movement can do funny things to individual pads.

*technically, if you were ordering it, and building control would be checking it, what you want is Gen3 or C20 concrete.

@40mpg

It’s a veranda that’s about 4.5m wide and 2.5m deep, and will be made of fairly chunky green oak timbers. There’ll be three upright posts and they’ll be within the patio area, which is the difficult part. The ground is glacial till, which is enough to drive anyone attempting to dig in it to insanity. But we’ve already shifted about 30 tonnes of it so far so we’re fairly familiar 🙂

The current plan is to dig a hole deeper than wide beneath where a patio slab would be, fill it with concrete, get the top as flat as possible but level with the top of the sub base. The slabs will be fitted with wet mortar on top of the sub base. I plan to bolt a post base of some kind to the concrete (or set it in etc) then cut a hole in the middle of a slab and put it over the top. Not sure if I want to conceal the bolts or not.

The ground is very solid indeed, but my concern is the side load on the veranda caused by wind. This would act to lever the post off the top of the concrete, and if the concrete is not strong enough or the bolts aren’t long, it could snap the top part of the concrete off. I mean it might not, but I worry enough every windy night as it is. Long threaded rods set into the concrete would guarantee strength. I don’t want to set the posts directly into the concrete because I may screw it up, but also there’s no room for adjustment. If the post holder is slightly wonky I could shave some off the post to give it some wiggle room then bolt it in straight.

I’m going to put braces in between the uprights and the cross beams to help prevent sideways flex (subject to my carpentry skills obvs) and the fixing to the house will be designed to flex a bit and give way if needed.

I am a construction professional, and tend to see all the complications!

I’m in IT not a construction professional but my job is to think of everything and all the things that could go wrong!

Is it connected to the house or freestanding?

this but in timber?
https://www.glazingsystems.co.uk/product/5-0m-wide-6mm-glass-patio-cover-veranda-select-projection/

I thought it would involve a floor or deck too. If not, ignore my comment about anything solid and grey will do, it sounds like you are needing to restrain some uplift.

I could set threaded rod in by making a plywood form the same size as a slab sitting on surrounding slabs*, drill the holes in the right places and suspend the rod. Even bolt the post footing to it to ensure correct spacing and angles. Then trim the rods when it’s all set.

* that assumes I’ll be able to lay the surrounding slabs around a hole. But I will definitely lay some slabs so I can get the alignment right.

@ayjaydoubleyou yes – basically like this but smaller and consequently using a bit lighter timber

And yes, wind could get under it. As above I get anxious when it’s windy 🙂

I was picturing something more like this:

What you’re proposing will be fine for a simple timber frame.

Buy bags of ready mix concrete from wherever has stock, use resin anchors for the bolts. Keep it simple, what could go wrong!

anything from C10 to C25;

or C20 concrete.

Those designations haven’t been valid since last century, it’s either a concrete strength class e.g C8/10 or a designated concrete RC32/40. Unfortunately it takes the building industry a long time to catch up…………….

Oh and I have some plain cement – when everything was gone a month or so ago I grabbed 6 bags from the dwindling pile just in case. Although recently B&Q have had stuff in.

Bagged C40 is the easiest way to do it, then drill after a week or so and sink in hilti bolts

Those designations haven’t been valid since last century, it’s either a concrete strength class e.g C8/10 or a designated concrete RC32/40. Unfortunately it takes the building industry a long time to catch up…………….

Luckily everyone has the conversion charts.

More usefully for this application, what I’m still calling C20 corresponds to 1:3:6 cement:sand:aggregate

I’m doing a similar project, albeit pergola not veranda so the top isn’t filled in so there should be less “sail” effect pulling it out of the ground.

I’ve bought four Simpson strong tie post mounts that cast into wet concrete and keep the post clear of the surface. They spec a pull strength.

As an aside, wicked and B&q locally had no cement or postcrete but Travis Perkins had pallets and pallets of it. Picked some up this morning myself, to mix up with ready mixed ballast.

(Neither builder nor IT person)

It’s a bit of timer stuck in a back garden. It’s not the forth rd bridge.

Talk about over complicating things. We’ll be recommending drilling rigs, concrete pumps and sticking 15m deep caisson foundations in shortly.

Well given I’ve never done it before I should err on the side of caution, don’t you think that would be wise?

I’d go 40/20 PQ adding air entrainment agent to 5% if any concrete is going to remain exposed. I mean, you don’t want to risk it do you? Take some test cubes to crush at 7 and 28 days to confirm strength.