Concrete Advice

[sam8364]

Hi,

I have a boat plot on a shingle beach and need to sink a concrete base to mount a winch on.

The winch base will essentially be a 1m3 block with a 6'x8'x8" slab on top. The 1m3 block provides the counter weight to the boat being dragged up the beach. A winch and motor will be bolted on to the slab.

The hole in the beach will be formed by making a 1m3 box from chipboard/OSb/whatever is to hand and placing weight on the top edge and digging out from inside - undermining the edges of the box so that sinks into the beach and forming the hole for the concrete.

I think that I am going to have to pour the blockand the slab seperately. However, a neighbouring plot recently built the same thing and when the slab dried a parfect sqaure crack appeared on top exactly where the block was. - Their guess was that this was caused by the top of the timber former for the block had drawn moisture away from the slab too quickly.

My questions are these...

1. How is is best to construct this - one or two pours?

Working on the basis of two pours....

2. How to I prevent the slab cracking
3. How long should I leave the block to cure before pouring the slab
4. Will the slab adhere to the block sufficiently on its own or will I need to provide a physical key as well - e.g. reinforcing rods.

5. The concrete will not be sumbersed in water but will on the upper surface be subject to salt air - should I adjust the concrete mix to allow for this.

Any advice or suggesstions greatfully received

Sam

 

[noseall]

surely the two components will have re bar and mesh reinforcement, thus linking them together?

 

[sam8364]

You saw surely........that is why I asked for advice

I can mix cement and dig a hole but my technical knowledge is a bit limited

 

[noseall]

You saw surely........that is why I asked for advice

I can mix cement and dig a hole but my technical knowledge is a bit limited

this is an unusual post!

my thoughts are that both sections require reinforcement and that the reinforcement will be needed to link the two together.

i think the hole digging will be interesting. how do you intend weighting the well box, without it being prohibitive? you will only be able to dig in one place at a time - tipping?

how deep is the shingle?

the rebar or mesh will need at least 75mm cover to prevent saltwater or indeed any water or oxygen being a problem.

 

[noseall]

the shutter box will need to be strong too. the deeper you go the greater the stresses.

you will need a fair bit of weight to sink the box too as friction will be a problem.

 

[Thermo]

i can only speak from experiance of when weve dug holes on beaches for large posts. Youll end up digging out more than you think to do the hole and the box will just be a hinderance when digging. Youll be better off to dig the hole first, drop the former in and then backfill the shingle against it. It would be worth flaring the sides of the box outwards towards the bottom so that the shingle helps to hold it in place.

As noseall says i would be inclined to use reinforcement to link the two. It can be done quite simply. get the former in the hole and then drop some reinforcement into it that bends over into where the slab will be. This reinforcemnt can then be tied into the mesh reinforcemnt for the slab. You can either do it all in one go or the block first and then the slab as they will now be tied together.

I cant see the top edge of the timber will be able to draw enough water away from the slab to crack it, but you could always wet it first before you pour it. Personally id pour it all in one go

There are special mixes to deal with the salt problem if you are that worried about it, speak to the readymix company if that is who you are using

 

[sam8364]

Some useful thoughts here thanks.

A friend recently used the weighted box idea to achieve a similar hole and it seemed to work ok. It was weighted by a large beer swilling friend on each corner. Having dug a much smaller hole on the beach last year I cannot begin to face the prospect of digging the hole putting the box in and then backfilling. The digging will have to be by hand - a meter cubed hole would just be too much.

I like this idea of the flared box though.

I was planning to mix the concrete with a mixer and generator as there is no practicable access to the beach for delivery by vehicle and in concrete terms it would be small quantity.

As far as rebar and mesh are concenred - again I am going to want relatively small quantities - what is the easist way of buy this at a reasonable cost?

 

[Thermo]

any builders merchant will sell it. Fabric is about £20 a sheet and the bars are peanuts. Fairly easy to bend and can just be tied together with wire.

 

[PaulJR]

You should be ok digging the way you plan - we sink manholes in a similar manner, but using concrete rings and a 360° excavator.

If you can get hold of one a pre-cast manhole ring would be a perfect shutter for this project and would be heavy enough to sink under it's own weight as you dig. (Ø1200 or Ø1500 by 750 deep would be a similar volume)

Don't bother reinforcing the mass concrete blob under your slab, simply lay a couple of sheets of mesh in the slab. what your friend experienced is what we (civil engineers) call reflective cracking. Basically the weight of the slab settled slightly on the beach and whilst it cured it cracked around the hard spot of the counterweight. If you do the concrete in two pours then leave some dowel bars sticking up out of your counter weight after its concreted - just push them in the wet concrete when you finish..

If you are mixing the concrete by hand - go for a strong mix, more like 1-1-2 (2-2-4) cement-sand-water and throw in some mortar plasticiser - air entraining mix will help the concrete against the elements.

You are after a 1m³ box - 2.4 tonnes of concrete, why not make it 1.5x1.5 wide and 0.5m deep - 1.125m³ and 2.7Te. More material but a shallower and I think easier dig?

 

[PaulJR]

Missed a couple of your questions

My questions are these...

a1. How is is best to construct this - one or two pours?

All at once if you can mix enough concrete

Working on the basis of two pours....

3. How long should I leave the block to cure before pouring the slab
Sam

Pour them as soon as you can - but if you pour them separately you need to remove the top surface from the first pour to key them together (we call it scabbling) It's graft without air tools - so I would advise you to go for it in one.

 

[noseall]

If you can get hold of one a pre-cast manhole ring would be a perfect shutter for this project

good thinking Paul, like the idea.

be a bit of a maul down on the beach but it sounds more realistic to me.

 

[sam8364]

There are some great ideas and advice here. A few more questions from the answers given....

If you do the concrete in two pours then leave some dowel bars sticking up out of your counter weight after its concreted - just push them in the wet concrete when you finish..

Not quite sure what you mean by this - dowel bars would be reinforcing rods. Do you mean set some bars so they protrude from the couter weight and sit in the upper slab?

If you are mixing the concrete by hand - go for a strong mix, more like 1-1-2 (2-2-4) cement-sand-water and throw in some mortar plasticiser - air entraining mix will help the concrete against the elements.

Would you not put any aggregate other than sand in? Being a beach aggregate is what I have for free!

Don't bother reinforcing the mass concrete blob under your slab, simply lay a couple of sheets of mesh in the slab.

So if i go with one pour I don't need to tie the couterweight to the slab at all?

I love the idea of the concrete ring - might find it difficult to manourvre by hand over a single beach however. It would however, add its own considerable weight to the project - I am guessing the the poured concrete would adhere itself quite nicely to the ring?

 

[PaulJR]

dowel bars - yes these are steel reinforcing normally- but almost anything will do - coach bolts, threaded rod. Leave about 4" sticking out and tie the mesh to it.

Concrete - sorry mental slip - I mean cement/sand/gravel normal "builders mix" is 1-2-4, double the cement quantity it will help - especially if you are using beach aggregate

If you pour it all in one go the chances of it shearing off the blob underneath are pretty slim, if you want stand a couple of bits of mesh upright in the shutter to key it together.

I assume you can get a few mates together - flip the manhole ring on it's side and roll it (with scaffold poles through the lifting holes). Do you know anyone with a Defender or Toyota pickup? Once you fill it with concrete you will never separate the two - the Ogee grooves in the ring will help key in the slab as well

 

[sam8364]

Cheers PaulJR, fantastic advice, really helpful, thanks.

 

[sam8364]

Right, it is not 9 months down the line and I have dug the hole - not something I am going to be rushing to repeat.

I have been phoning around for a price for concrete. I am just a bit uncertain about the mix.

The base will only be exposed to the air on the top surface and will never come into contact with the sea (unless global warming runs wild - but then my house will also be knee deep in water and this base will be the least of my worries). Therefore there is no mechanical action again it - it just needs to be strong enough to take the concrete fixings to bolt down the winch and small engine to it.

I have had three suggestions for concrete mix

1. a standard 3:2:1 mix

2. C35

3. C50, 100 slump, 380 minimum cement content, 0.4 water cement ratio, CEM1 20mm gravel, super plasticiser.

As you can imagine this is in ascending order of price and as a DIY project I want to spend as little as possible. I don't want the base to crumble to nothing, on the other hand I am not planning to repel an invasion either.

Most other bases of the beach have been hand or DIY mixed and nothing seems to have failed yet - certainly the 40 year old base we have to remove too some smashing up.

What would people advise?

Cheers

Sam