Uneven trench footing

[mangosRus]

Hi guys

I had my trench pour a few weeks ago, and it went to plan, for the most part. I was checking how level the footing was with an optical level and there is an average variation of the footing by around ~4cm - but there is one section that dips by around 10cm.

I did level it on the day with my level, but it wasn't long enough, and the pour went over the lines I drew so I was having some problems gauging which bits were too high and vice versa, and I was alone, so yeh, it wasn't pleasant.

I will be using engineering bricks below DPC, so I would like to know how I can manage the fact the footing isn't fully level. I was thinking of pouring some concrete on the areas that dip quite a bit, and for the rest just use a thick bead (4-6cm) of 1:4 mortar. Thoughts?

 

[cjard]

Last time I faced that problem I used a few quarry tiles I had kicking around to build up. They have a similar enough compressive strength to engineering brick (30nm vs 50 for the brick, vs < 7nm required for design loads) but only 10mm thick so allowed for easy building up.
Feel free to chuck some more concrete in the 100mm low spot. I wouldn't bother for the 40mm one. Heck. I might not even bother for the 100mm one, just cut a trench block up using an old knackered wood saw and build it up. Only has to be level by the time you get out the ground, it can be any old Shiite up to that point

 

[mangosRus]

Last time I faced that problem I used a few quarry tiles I had kicking around to build up. They have a similar enough compressive strength to engineering brick (30nm vs 50 for the brick, vs < 7nm required for design loads) but only 10mm thick so allowed for easy building up.
Feel free to chuck some more concrete in the 100mm low spot. I wouldn't bother for the 40mm one. Heck. I might not even bother for the 100mm one, just cut a trench block up using an old knackered wood saw and build it up. Only has to be level by the time you get out the ground, it can be any old Shiite up to that point

Thanks for the reply.

Ah man, that sounds like a lot of work, and a huge PITA.

If compression strength is the issue, can I not use a stronger mortar mix ?

 

[^woody^]

can I not use a stronger mortar mix ?

You certainly can.

Or just lay some brick on edge in the low section

 

[mangosRus]

You certainly can.

Or just lay some brick on edge in the low section

Thanks for the reply.

What mortar mix would you recommend? I was thinking a 1:3, but I could also use lime, although I don't have much experience with it.
I was thinking of spreading the mortar out by around 75-100mm, on each side of the brick, and make the depth enough so that the brick work is level. Now, since the mortar will be spread out across a much wider area, it won't look too bad and will appear to be a part of the footing - good idea ? or a waste of mortar ?

 

[cjard]

In the time you've spent faffing around on the internet you could have dumped a mixer load of concrete in the low section and sorted it out

 

[John D v2.0]

That's why all diy jobs take 8 times as long as the pros would.

Double cos you don't know the shortcuts. Double that because there's only one of you doing the work. And double again because you spend the first half of the time on the internet working out where to get sensible advice!

 

[^woody^]

You would not even bother mixing concrete and trying to level the thing up

Build the corners (gauging down from DPC level), run a line along the first course and then just start laying the bricks to the line, using a big or tight bed, or brick on edge or split brick as necessary. You don't need to get to the line in one go (although it helps), so as long as the wall won't be seen you could have a few course of big or tight joints if that is easier.

 

[cjard]

Woody makes a good point, but you should also consider that as an inexperienced builder, it's better to get things as straight as you can at a particular stage before launching into the next thing you've never done.. If you're having to apply a new skill and simultaneously correct for a mistake made earlier theres a bigger risk that your subsequent labour efforts will be affected by the earlier mistake. It's not going to take much time or effort to chuck a mixer load of concrete in a hole to remove a gradual slope down to a 100mm low point, but it'd take quite a bit of skill to build 3 corners of e.g 3mm beds and one corner of e.g 23mm beds so that you reach level by the time you're above ground (if you're still bedding up to counteract by the time you reach DPC it's gonna look pretty bad..)

 

[mangosRus]

Thanks for the reply guys.

So, I took some proper measurements today, and basically, it's just one corner (~1.5m length on each side of problematic corner) that is quite elevated, and that is effecting the measurements for all the other sides. If I take out the problematic corner, then the average depth variation is around ~22mm, which is perfect.

What I am thinking of doing is cutting up the engineering bricks in half (horizontally) and use those for the problematic corner, that way, I have less work to do to build up the other corners and can probably get it level with some big beads.

Also, do I need to use bricks below dpc? or can I use blocks ? this will be a single skin garage and the footing is only 150mm below ground. Blocks will be much easier to cut and I won't need to do any build up either since they are much larger, I can cut a single block to size, as opposed to building up several bricks.

Finally, I didn't just take this task up without any real experience. I've been building up to it for the past 5 years - I've done loads of plastering, fitting (bathroom/kitchen etc), wood work, plumbing and so on. It's just my first time getting a ready-mix in to pour concrete; I wasn't expecting it to be so intense hence I was alone and the oversight was due to exhaustion, which is why there is going to be 3/4 of us when we pour the slab - never again.

 

[cjard]

Yes.. Shame you didn't ask here first! My rule of thumb is one man for every cube of concrete (given that a cube weighs 3 tons) if you don't have mechanical assistance. For what it costs it's definitely worth hiring a dump truck rather than doing a barrow delivery, if you can. My local independent charges 40 quid a day for a little 1 ton dumper, and it'll shift more concrete with fewer complaints than 2 80quid labourers
If you don't have the luxury of using hydraulics or concrete pumps (like a recent 6 cube conservatory pad pour I was roped into by being in the wrong place at the right time; walls already built and underfloor heating pipes laid, wagon couldn't get enough height for a barrow so we just had to dish it up runny and push it. Should have hired a concrete pump) then you're gonna be pretty tired after 3 tons..
For flattening the last pad off we used a roller striker; interesting and very easy to use device.. A massive metal tube, 6 metres long with a handle at each end. A portable hydraulic power pack drives a motor in the tube and you spin it in the opposite direction to what you pull it. At the right speed (not too fast so it isn't flinging concrete everywhere) the surface it leaves behind is smooth enough to be classed as finished without further polishing (unless you're planning on Karndean glued to the concrete). They come in various lengths, and the ends bear onto your footing walls. If your house dimensions are larger than the length of the striker, you need to set a batten into a hand levelled mound of concrete to create multiple bays to work in, but other than that, it beats tamping hands down (if you'll pardon the pun)
One other option for you re the slab; poured liquid screed.

- coincidentally, this video is produced by the guys I last used, and I do recommend them: well priced, punctual, did exactly what they said they would and left without making a mess of the place. I paid £13/sqm which is about concrete pad price, but it was for 400sqm. Smaller pours may be up to double.
I've no affiliation with them, i just picked on them because they've packed a lot of useful info into the video. If opting for this route (and I always will in future) do make sure your floor installation guy knows what kind of screed it is and prepares accordingly. These floors can't just be tiled/karndeaned with standard adhesive unless specific primers are used. Carpet or wood doesn't care.

You can use concrete blocks, sure. You can also use aerated concreted blocks, but They shouldn't be laid anywhere where they may be exposed to a cycle of wetting followed by freezing, as this can cause the block to erode. You can cut ACBs with a knackered wood saw or dedicated block saw. You can cheaply hire a stihl saw and cut concrete blocks up into half brick heights (easier than cutting engineering brick). For the sake of your lungs and your neighbours, hire a saw wih a hosepipe connection and use it. Do not dry cut, even outdoors

 

[tomfe]

[QUOTE="cjard, post: 3890404, member: 106701"
You can use concrete blocks, sure. You can also use aerated concreted blocks, but They shouldn't be laid anywhere where they may be exposed to a cycle of wetting followed by freezing, as this can cause the block to erode. [/QUOTE]
You can in fact use aac and 7n blocks bellow dpc as they are frost resistant.

You don't want a screed for a garage get a self leveling concrete if you have access to dump in right into the slab. Don't over wet concrete you will get a dusty week floor.

If you have enough people you'll be fine the next time with a normal mix.

 

[mangosRus]

cjard, yep - I've learn't my lesson. Thanks for the info, will consider it next time The Screed-it guys look good, although I will need a concrete mix rather than screed.

You can in fact use aac and 7n blocks bellow dpc as they are frost resistant.

You don't want a screed for a garage get a self leveling concrete if you have access to dump in right into the slab. Don't over wet concrete you will get a dusty week floor.

If you have enough people you'll be fine the next time with a normal mix.

Aha, I didn't know about self leveling concrete, a quick youtube, and oh my, that looks like a breeze. Presumably they are using admixtures to get such a runny consistency ? I would imagine that much water would weaken the concrete.
Does it cost more than your standard "ready-mix" ? This would have been a breeze for the footing pour, and I wouldn't be cutting blocks to get everything level!

 

[cjard]

Which YT video did you watch?

 

[tomfe]

QUOTE="mangosRus, post: 3891025, member: 244870"] they are using admixtures to get such a runny consistency ? I would imagine that much water would weaken the concrete.
Does it cost more than your standard "ready-mix" ? [/QUOTE]
Yes also it's a mix of different aggregate sizes and a filler. If extra water is needed in the design as the strength is set they add more cement.
This does come at an extra cost ~£150/m³ for an foundation mix from lafarge. So not cheap!
Over 30m I had a drop of 20mm and did not touch it.