Workshop Footings - Is this gravel?

[paulc]

Hello forum,

I am beginning to make the concrete footings for my new workshop and am not quite sure if the 'gravel' I got from the builders providers is the right stuff?
It was in 40kg bags beside bags of mortar sand and simply said 'gravel' so I thought I couldn't go wrong. On opening however it seems to be about 2/3 sand and 1/3 pebbles rather than all small stones or gravel. Is this normal? I've made one footing using this where the measurements have stated gravel. And am worried that the footing contains too high a percentage of sand and may braek down over time . Any advice really appreciated. Thanks , Paulc.

 

[motownmartin]

Sounds like balast for concrete, you just need the cement to mix it with :?

It take you want some hardcore first.

 

[Lord Nibbo]

Sounds like gravel as dug, not washed. which is what you want for concrete. can you do a pic?

 

[aldel]

What you have is a concrete mix sometimes called "3/4 to dust". Bagged in the correct proportions for general work.Try a 6 to 1 of cement and mix well.
Saves the trouble of purchasing sand and chippings seperately.
For a super strong mix add a little more cement. but don't make too wet.

Aldel

 

[Gaz_XB9R]

By the way you can get 1tonne bags from B&Q for about £33.

Dunno what the delivery would be.

Might be a cheaper option if you are to be pouring a concrete pad and I found that any extra can be used round the footings as hardcore to level the ground.

 

[paulc]

Cheers for the advice , the mix I used was 3.5 of the gravel described , heres pic below :

to 2 of mortar sand to 1 of cement . And heres a pick of a completed footing after 5 days drying:

The edges have chipped off and its darker in colour to most concrete bases I've seen , I'm afraid theres too much sand and not enough stone. I don't want to have to dig it up , but don't want the workshop to tilt over in a years time either. What do you all think ?

 

[PowerTool]

By "mortar sand" do you mean soft sand ?
If so,it's the wrong stuff for concrete - you need sharp sand (as there is in your ballast mix).Sharp sand has pointy,angular shaped grains which lock together;soft sand has round,fairly smooth grains that slip over each other.So sharp sand for strength,soft sand for pliability.
If using ballast (combined sand and aggregate),normal mix is 6:1 with cement.
Concrete gains 98% of its strength in two days - I've laid concrete in a barn one day,and stacked pallets of fertilizer on it the next.Although the edges may normally chip slightly if knocked,I would be concerned if it is crumbly.

Andrew

 

[Gaz_XB9R]

Not so sure about 98% in 2 days but here's a link to a site full of FAQ's

http://www.tlblearningpoint.com/default.aspx?pid=faqlist&catid=7

 

[Shultzy]

The design strength of concrete is usually attained at 28 days.

As a rule of thumb the 7 day strength is usually two thirds of the 28 day strength.

 

[Lord Nibbo]

The design strength of concrete is usually attained at 28 days.

As a rule of thumb the 7 day strength is usually two thirds of the 28 day strength.

Spot on Shultzy, The company I sold manufactured Reconstituted Cotswold stone and architectural dressings every month we would send sample blocks to be tested for water absorbtion, shrinkage, thermal conductivity and strength. The strength test was two tests one to destruction with a compression test, the other a load test (think lintels). All of these tests were all ways on or about 30 days old. It still gets stronger as years go by (100 years) but the design strength must be reached by 28 days.

 

[Jake]

I'd make it a bit stronger than 1:6 for a footing, I'd go with 1:5 as a minimum. I always end up profligately mixing at 1:4 but that's over the top for most things.

 

[paulc]

Spot on Power tool, it is ballast, and yes I also got the wrong bag of sand , ie: mortar sand , but I can use that for replacing some blocks that came off a wall in a storm so not a complete waste.

Now to get the rest of the footings right after I dig up this one :

1 - Compact hardcore at bottom of footing
2 - Mix 5/6 Ballast to 1 Cement
3 - Pour in cement
4 - Level with batten across formwork
5 - Smooth with Float

Is this correct ? I don't want to have to break up any more footings gone wrong, there'll be no ground left

Cheers , Paul

 

[ianmelb]

Have you had a look at the Paving Expert Website?

It has a huge amount of information about sub grades, sub bases, concrete, etc.

There's also a friendly forum there but it is best to have a real good read of the main site to make sure that any questions haven't already been answered.

Ian

 

[Shultzy]

Paulc, I would try and dome the concrete slightly as this will allow water to drain away.

 

[beech1948]

paulc,
Have a look at the following.
http://www.pavingexpert.com/concrete.htm

 

[paulc]

Thanks for the advice and links everyone, all I need now is some free time to get down to work, Cheers

 

[paulc]

Paving Expert.com recommends a dpm in between Hardcore and concrete- is this necessary for footings for a workshop , or a little over the top? Cheers, Paulc

 

[ianmelb]

Probably not necessary for an outbuilding but it can help to prevent the concrete going off too quickly and cracking. Obviously it'll also help in the long run to prevent water ingress.

If you don't want to go to all the expense of a proper DPM, you could maybe get away with a sheet of polythene dust sheet - not a recommendation, just a a suggestion