Concrete base for shed/log cabin - urgent advice needed

Hello all,

Great forum - hoping someone can kindly provide some advice for me.

Basically, I’m wondering if it’s possible to tell if a concrete base is going to be good or not after just a few days? Below is the story so far:

I've just had a contractor lay a concrete base 4x5 m 100 mm deep for a shed/log cabin. I specified a completely level, flat and smooth base on a sub-bed of hardcore, and dpm inbetween concrete and hardcore. A bit of soil removal was needed as the ground wasn't level. I've laid bases before but on a much smaller scale, but this one was a bit too big for me to take on.

Hardcore sub-bed was broken slabs and house bricks and other various sized rubble. I was a bit concerned that the sub-base material was too big and not broken up – I was told not to worry as it needed to big. On the day of the job when the contractor arrived I raised concerns about the weather forecast – 1-3 degrees C and rain forecast with further cold days and snow to come (which it has done now two days later). Again, I was told not to worry. I wasn’t home that day to keep an eye but my son was around.

Got back home just as the contractor and his mate were clearing up – managed to have a quick look at the base which seemed ok, so I paid with a cheque. Yesterday, managed to have a good look at base and can see by eye that it’s not level in a few places. The dpm is also folded in a few places so the final edge finish is likley to be poor. It was wet with rain/sleet and I tested the surface and was able to scratch away some of the mix with my finger. Today, I swept away a few cm of snow from the base. The surface seems solid enough but still able to scratch surface with my shoe down to the aggregate which is also coming up. Also managed to push a steel rod 4 mm wide with a little pressure up to 2-3 cm in some places (I could probably have gone deeper with a bit more pressure). Not very scientific tests I know, but I would have thought the concrete would be hard enough after two days to resist this (despite the poor weather).

Spoke to my son (who has done construction at college) on what he saw on the day. A whacker was used to compact the hardcore but my son said it didn't seem to do much. He said no levelling was done shutter to parallel shutter but a 4 foot bit of wood to do the corners (as far as the contractor could reach) and the rest smoothed down with a square hand trowel. Although he can’t be sure on the exact numbers, my son also said they carried in about 12 bags of cement but only used about 6 bags. This is much too little for the job given the size of the base.

I’ve got a bad feeling the base will not be fit for purpose. The temperature is likely to dip below freezing again the next few days. The shed/cabin will be high spec and insulated, and will be very difficult to take down if the base starts to crack or crumble. I’m torn whether to bite the bullet and start again (with another more expensive contractor – yeah, I know pay cheap pay twice) or risk the base will hold up. That’s why I’m wondering if there is anything I can do now to check the suitability of the base.

I haven't spoke to the contractor yet, but wanted to make sure of my facts before I do, as I know he'll likley have an answer for everything I bring up.

So any advice or comments from you guys is welcome.

Cheers.

/tees

 

From what you have said does not sound good, really should not have laid it with the weather and temperatures we are having at the moment. I'm not an expert in this field but I can do my maths and 12 bags of cement is just not enough, you would need 26 bags of 25kg for that size

 

Clear any snow and get it covered up quick. I was supposed to be pouring a 220m2 slab on Monday but its off till after Easter due to temp.

 

Agree with the above poster regards cement quantity.

A 4m x 5m x 100mm thick base equates to 2m³ of concrete which, as said, would then require 26 x 25kg bags of dust.

Sounds like you have got a lean mix there boyo.

We abandoned a wall re-build having seen the weather forecast last Thursday. It would have been ok actually on the Thursday (mid-morning) to start laying but the cure time would have run into sub-zero temp's, so i called a halt.

 

Yes, I thought it was too little cement. Don't suppose there's any way of knowing after two days if the mix is lean or not? I asked if it should be covered before he left, but he said no it would only sweat. The contractor started job Thursday morning too, and as I said I did raise concerns about the weather but he probably wanted to get the job done and out of here.

Just been up to sweep away the snow and small cracks by the shuttering have started to show, I'll see how it is in the morning and call the contractor.

 

Did he give you any reasons as to why not to worry about how cold it was, ask for a break down of materials and labour cost aswell

 

He said it would be ok as there would be cloud cover in the evening. Good idea asking about a breakdown of labour and materials.

Cheers all for responding so far.

 

For a base that size, maybe they should have concrete delivered. Mixing on site for that quantity is not ideal

 

We could mix and lay 2 cube with our eyes closed, hands tied whilst hopping on one foot with a clog on.

However, we would not lay in low temp's and certainly would not be stupid enough to lay in low temps whilst laying lean muck.

 

As a matter of interest, noseall, how much would you charge for this job (without the clogs and hands tied)?

 

£1100 - £1500 ish.

 

And with the hands tied and clogs on?????

 

you suggesting he'll cheat and leave his eyes open?

 

Update:

Contractor came out. He said the concrete had not gone off properly. He suggested cutting out blocks of concrete where the cracks were and filling in with a new mix. Pic of one crack:

I declined and said I wanted it up and relaid by someone else. We came to an amicable arrangement and both parties were happyish. I asked how many bags of cement he used - enough for the job (there were some broken bags in black plastic sacks which I remember seeing but it didn't look a lot. Anyways, I didn't push it too much so left it at that.

So started busting up the concrete. A pile of which can be seen below:

I'm wondering if I could use this as a sub-base?

The current compacted sub-base under the dpm looks like this, which I take it is unsuitable?

Got half the base up this afternoon, and it now looks like this:

Will just get the site prepared for a firm to just lay the concrete. I think the shuttering is still good or should I get new?

Cheers.

/tees

 

It should have been covered and not left open to the elements.

The screed shutter looks banked behind with concrete. That will make it very difficult to remove.
All you need is a few pegs.

Were pouring 100C/m on Tuesday into a base with a concrete pump.
Steel fixing tomorrow. Then another 100m of retaining wall 2.4 high x 265mm thick. Quite slender this one.
Not our own baby this time as Architects and SE's involved.