Making A Concrete Slab 8ftx8ftx2in

[Daffy D]

I'm going to make a concrete slab for the mother-in-law the dimensions are 8ft by 8ft and 2in deep how many bags of cement , sand and aggregate will I need?

I've had a Google and the ratios seem to be 1 cement 2 sand and 3 aggregate but how much material will I need?

Thanks.

(No it's not FOR the mother-in law is for a project of hers)

[Wash]

I cannot advise how many bags you will need, but suspect that professional cement company is far better than doing it yourself. I suspect with a 2 inch 8X8 foot slab you will need special cement and rebar. It you only want it to last for a day or two have at it as a do it yourself project. But for sure it will crack to pieces in no time if you use conventional cement.

[SeanMoran]

I cannot advise how many bags you will need, but suspect that professional cement company is far better than doing it yourself. I suspect with a 2 inch 8X8 foot slab you will need special cement and rebar. It you only want it to last for a day or two have at it as a do it yourself project. But for sure it will crack to pieces in no time if you use conventional cement.

Yes, for 288,000,000 cubic mm of concrete, it might be wise to include a sheet of mesh to stop it from cracking under its own weight. Also, I can't claim to be an expert, but I'd go more towards the 1:1:1 ratio for concrete:sand:aggregate if you're forming a slab that can be lifted up and trucked to the site. It's going to need a pair of 3.5 tonne forklifts to lift it.

---o0o---

PS: To answer the question, I'd imagine that 6x50kg bags of concrete ought to do it, but grab 8 just so you have enough to complete the job.

Edited March 8, 2010 by SeanMoran

[Crossy]

You'll have to convert to metric but this should do the trick http://www.source4me.co.uk/calculate_concrete_mix.php

Note, it assumes a 1:2:4 ratio

If you're going to use ready-mixed bags try here http://www.source4me.co.uk/calculate_concrete_mix.php (only works properly with Internet Explorer).

You will definitely need a reinforcement mesh to stop it cracking.

Do consider however, that a slab only 2" thick won't keep the MiL at bay for long and she will soon escape and continue to make your life hel_l.

[Daffy D]

Thanks for the answers so far.

This "slab" of concrete is for a washing area at the back of the house. Washing machine buckets and the like. I have already laid and packed down a bed of about 6in of hardcore and intend to use bamboo lenghts for reinforcing.

As for the 2in thickness this is only an approximation as the hardcore is not absolutely flat and an average of 2in seems about right.

Round here you can buy loose sand and aggregate delivered from the back of a pick-up or in those plastic fiber stringy bags that are about the same size as a bag of cement. I don't want the loose stuff as it's always a job to get the leftover cleared up so it's stringy bags for me but how many to one bag of cement.

I realize it's a bit vague but I can't be the only one that's tackled a small concreting job there must be someone who's bought materials in bags that has an idea of the number of bags I would need.

[SeanMoran]

Thanks for the answers so far.

This "slab" of concrete is for a washing area at the back of the house. Washing machine buckets and the like. I have already laid and packed down a bed of about 6in of hardcore and intend to use bamboo lenghts for reinforcing.

As for the 2in thickness this is only an approximation as the hardcore is not absolutely flat and an average of 2in seems about right.

Round here you can buy loose sand and aggregate delivered from the back of a pick-up or in those plastic fiber stringy bags that are about the same size as a bag of cement. I don't want the loose stuff as it's always a job to get the leftover cleared up so it's stringy bags for me but how many to one bag of cement.

I realize it's a bit vague but I can't be the only one that's tackled a small concreting job there must be someone who's bought materials in bags that has an idea of the number of bags I would need.

If you're able to do the job onsite, and it won't be getting trucked there, then bamboo (however useless it is for strength in concrete) should suffice (to allow the greeblies to find a way through it) and the 1:2:4 ratio will handle it. 3x50kg bags of concrete should be enough, but get four, so that you might have one left over to give away for good Karma.

[chiang mai]

I wonder if the OP has priced delivery of redi mixed, I suspect that he could get delivery of a quarter load or similar from a local supplier plus lay a decent thickness bed at the same time, at a cost effective price, a tad easier also!

[jackr]

The problem with readymix is that I doubt they'll deliver for anything under a cubic metre.

Have a look at this recent thread on premix, op. Can get gravel bagged, too, and save the mess on such a small job. Best to go down at least 4" and use a grid of rebar even though you have the hardcore. Can knock it up in sections (perhaps 4' x 4') and batten off... if you find you haven't enough, just buy some more.

[patong]

you will need 24 x 25 kg = 600 kg total

1 cement 3 sand & ballast

Edited March 9, 2010 by patong

[longball53098]

The problem with readymix is that I doubt they'll deliver for anything under a cubic metre.

Have a look at this recent thread on premix, op. Can get gravel bagged, too, and save the mess on such a small job. Best to go down at least 4" and use a grid of rebar even though you have the hardcore. Can knock it up in sections (perhaps 4' x 4') and batten off... if you find you haven't enough, just buy some more.

I know CPAC here in Banchang will deliver less than 3 cubic meters but they will add on a fuel and delivery charge. 3 or over and its included in the price for the batch.

The thinnest walks my builder would make were 4in or 10 cm with mesh in the middle.

Buy the premix in the bag. Has all the stuff and no measuring except the water.

[patong]

6 x 25 kg bags of cement

18 x 25 kg bags of sand and ballast mix

[Naam]

This "slab" of concrete is for a washing area at the back of the house. Washing machine buckets and the like. I have already laid and packed down a bed of about 6in of hardcore and intend to use bamboo lenghts for reinforcing.

As for the 2in thickness this is only an approximation as the hardcore is not absolutely flat and an average of 2in seems about right.

no, it's not right Daffy! the size you mentioned needs a thickness of 7.5cm (~3") and steel reinforcement. bamboo is useful for chasing away soi dogs.

[maccaroni man]

i wouls like to ad a question as i am building a similar project but mine is for 9k liters of water 4 holding tanks, i know i should use rebarb, and i am having footings added at each end but how deep should the footings be and how thick should the slab be?

[Beardog]

Daffy you need to get either WIRE mesh or a light gauge rebar . The light gauge steel mesh should do fine since it is not since it does not have a bearing load on it. Rebar better even a mix of metal bolts & wire snips would be better than bamboo as the bamboo is organic & will break down & leave pockets as it decomposes & eventually cracks in the slab. I laid down 5 cm of sand & wire mesh for our 2000 liter water tank & it is the only concrete in the property that isn't cracking. Go figure I did the job- not Somai or another Somchai. Use metal ....better . If not go with a heavy aggregate. Better than bamboo!

[Daffy D]

Thanks for the replies and concerns regarding thickness of the concrete.

This is a cheap job so any saving on materials and rebars, however slight, will be used.

The slab is not going to take a lot of weight, just people (person) and a washing machine so there will not be much stress on it and if it does crack at some time in the future it's no big deal. It's not going to fall anywhere and it's the Thai way so no one will be surprised

I read on here some time ago about using bamboo for rebars which gave me the idea.

Taking the advice from here and doing a bit more Googling have decided to go with 2 bags cement 2 bags sand and 4 bags aggregate. Will post how it works out.

Again thanks for all your input

[CM4Me]

Making a concrete slab - http://www.source4me.co.uk/calculate_concrete_mix.php - another piece of good advice/link from a Thaivisa member. Tks Crossy.

However a couple of questions that come to mind, specific to the LOS, are

* Sand & aggregate are usually sold in bags or in bulk by the Cube. How much is in a Cube, a tonne?

* It is common practice for concrete to be mixed by hand, on site, often in a large black plastic tub. Usually for this type of mix Thais use a bag of cement with quite a number of 'buckets' of sand & aggregate. Can someone advise what the mix ratio should be for this type of mixing.

Thanks, in advance Kevin

[trogers]

The slab is not going to take a lot of weight, just people (person) and a washing machine so there will not be much stress on it and if it does crack at some time in the future it's no big deal. It's not going to fall anywhere and it's the Thai way so no one will be surprised

If random cracking of the slab is acceptable, I wonder why you spend time and effort casting this slab in the first place. Would be easier and cheaper to place sand over the hardcore and then lay on 1ft sq 2-inch thick cement-sand concrete panels. At least the latter has straight joint lines...

Edited March 10, 2010 by trogers

[Cuban]

On the subject of laying concrete slabs like this, Crossy and I have a mutual friend that is very skilled in the area of construction and building design both within Thailand and beyond. At the end of last year he was supervising the completion of a house out here in the wilds while on a flying visit. Having directed the rural building team to put in a 6mx9m slab under the stilled area of the house, he told them they would first need to raise the basic level of earth and tap it down. (I don't think a vibrator was available to compact the earth.)

I can imagine him pointing to a nearby area of rough ground where the crew could take earth from to fill the space adding about 5-6 inches once compacted. At that point he left them to get on with it returning in the evening to find that they had indeed taken the earth from the rough ground. But knowing that farangs are strange and not fully understanding what they had been asked to do had laid out rough turfs of grass and the other wild vegetation so that he had a semi-indoor lawn under the house, no sign of preparing to lay the slab.

For the OP. If this is an area where there will be much water running around consider special attention to the edges, running waste water may erode the supporting soil and allow the edge to crumble under pressure from above. If you don't want to lay rebar consider a metal sheet of open mesh instead of bamboo, I too have read about bamboo being used for non load baring jobs but it does rot over time and just leave big holes in the slab.

When we had a slab put in here for a new kitchen I wish I had instructed them to use a brush to put a drainage and non-slip pattern onto the semi-cured surface.

HTH

[Daffy D]

The bamboo rebars are out.

After spending the morning, under the blazing sun, splitting bamboo lengths laying them out and tying them in neat little squares on the hardcore, realized they were too thick and there would hardly be any concrete thickness over some of the places. So I'll have to see what is the cheapest alternative available at the builders yard, chicken wire springs to mind.

"Cuban" The sides of the slab are cement blocks and the water from the machine goes straight into a drain so that should not create a problem.

'trogers' This project just sort of happened with left over materials at hand, so there was not much planning

[katabeachbum]

The bamboo rebars are out.

After spending the morning, under the blazing sun, splitting bamboo lengths laying them out and tying them in neat little squares on the hardcore, realized they were too thick and there would hardly be any concrete thickness over some of the places. So I'll have to see what is the cheapest alternative available at the builders yard, chicken wire springs to mind.

"Cuban" The sides of the slab are cement blocks and the water from the machine goes straight into a drain so that should not create a problem.

'trogers' This project just sort of happened with left over materials at hand, so there was not much planning

3mm wired mesh and one m3 readymix 280 would cost you 2.000 baht. 17 cm thick, can load with 25 ton.

[canopy]

Yes, your slab is too thin for bamboo rebar. But in the off chance someone else is considering this it is noteworthy that bamboo rebar does work *IF* used correctly which like most things doesn't happen 99.9% of the time. Used incorrectly it is a disaster--the bamboo slowly swells as it gets wet which cracks up the cement, then the bamboo shrinks back down when it dries leaving loose joints and avenues for bugs to eat it. The net result is much worse than no rebar at all.

The US Navy has a published document on bamboo rebar for construction with good guidelines. It covers selection, curing, reinforcement design, strength, and just everything you need to get it right in a fairly compact document.

[Beardog]

Daffy

This is what we used to build the slab for our house- The klong & the slab for the 2000 liter water tank. For your job you can get most likely get away with just a smaller gauge mesh. In the U.S. the hicks in the sticks would commonly throw copious amounts of dead used nuts & bolts & bits of steel to make a NON bearing slab & that works good as well. Here it is pretty cheap for wire mesh. They do make a mesh about 3 times heavier than chicken wire that would work well. As long as you stay away from organic materials instead of metal it should be alright. The mesh will insure that you won't have to hear about it cracking to death & give you the satisfaction of doing the job right once. You can probably secure a small portion of some type of mesh at a mom & pops shop for next to nothing. Sometimes a neighbor has extra laying around & will give you a small portion. I have helped 2 people out with small amounts as the klong only required 1/2 of the roll.

Good luck

Edited March 12, 2010 by Beardog

[katabeachbum]

Daffy

This is what we used to build the slab for our house- The klong & the slab for the 2000 liter water tank. For your job you can get most likely get away with just a smaller gauge mesh. In the U.S. the hicks in the sticks would commonly throw copious amounts of dead used nuts & bolts & bits of steel to make a NON bearing slab & that works good as well. Here it is pretty cheap for wire mesh. They do make a mesh about 3 times heavier than chicken wire that would work well. As long as you stay away from organic materials instead of metal it should be alright. The mesh will insure that you won't have to hear about it cracking to death & give you the satisfaction of doing the job right once. You can probably secure a small portion of some type of mesh at a mom & pops shop for next to nothing. Sometimes a neighbor has extra laying around & will give you a small portion. I have helped 2 people out with small amounts as the klong only required 1/2 of the roll.

Good luck

2,5m x 2,5m is approx 650 baht. smallest amount of readymixed concrete to have delivered is 1 m3 at 1400-1700 baht depending of strength/quality. would make this slab approx 17 cm thick and able to carry 25.000 kg. the weight of the slab would decrese vibration noise from washingmachine

[Beardog]

Daffy

This is what we used to build the slab for our house- The klong & the slab for the 2000 liter water tank. For your job you can get most likely get away with just a smaller gauge mesh. In the U.S. the hicks in the sticks would commonly throw copious amounts of dead used nuts & bolts & bits of steel to make a NON bearing slab & that works good as well. Here it is pretty cheap for wire mesh. They do make a mesh about 3 times heavier than chicken wire that would work well. As long as you stay away from organic materials instead of metal it should be alright. The mesh will insure that you won't have to hear about it cracking to death & give you the satisfaction of doing the job right once. You can probably secure a small portion of some type of mesh at a mom & pops shop for next to nothing. Sometimes a neighbor has extra laying around & will give you a small portion. I have helped 2 people out with small amounts as the klong only required 1/2 of the roll.

Good luck

2,5m x 2,5m is approx 650 baht. smallest amount of readymixed concrete to have delivered is 1 m3 at 1400-1700 baht depending of strength/quality. would make this slab approx 17 cm thick and able to carry 25.000 kg. the weight of the slab would decrese vibration noise from washingmachine

That small of a pour you can get a laborer for 300 baht to hand mix the cement & pour it. I usually have my girl mix it up & I do the pour & screeting,

[katabeachbum]

Daffy

This is what we used to build the slab for our house- The klong & the slab for the 2000 liter water tank. For your job you can get most likely get away with just a smaller gauge mesh. In the U.S. the hicks in the sticks would commonly throw copious amounts of dead used nuts & bolts & bits of steel to make a NON bearing slab & that works good as well. Here it is pretty cheap for wire mesh. They do make a mesh about 3 times heavier than chicken wire that would work well. As long as you stay away from organic materials instead of metal it should be alright. The mesh will insure that you won't have to hear about it cracking to death & give you the satisfaction of doing the job right once. You can probably secure a small portion of some type of mesh at a mom & pops shop for next to nothing. Sometimes a neighbor has extra laying around & will give you a small portion. I have helped 2 people out with small amounts as the klong only required 1/2 of the roll.

Good luck

2,5m x 2,5m is approx 650 baht. smallest amount of readymixed concrete to have delivered is 1 m3 at 1400-1700 baht depending of strength/quality. would make this slab approx 17 cm thick and able to carry 25.000 kg. the weight of the slab would decrese vibration noise from washingmachine

That small of a pour you can get a laborer for 300 baht to hand mix the cement & pour it. I usually have my girl mix it up & I do the pour & screeting,

just wanted to show how inexpensive it is to make a proper slab in one hour.

[Naam]

Daffy

This is what we used to build the slab for our house- The klong & the slab for the 2000 liter water tank. For your job you can get most likely get away with just a smaller gauge mesh. In the U.S. the hicks in the sticks would commonly throw copious amounts of dead used nuts & bolts & bits of steel to make a NON bearing slab & that works good as well.

no it does not work Beardog. only "connected" steel over the full length gives concrete a certain flexibility thereby avoiding cracks. even a zillion single nuts and bolts won't help

[Beardog]

Daffy

This is what we used to build the slab for our house- The klong & the slab for the 2000 liter water tank. For your job you can get most likely get away with just a smaller gauge mesh. In the U.S. the hicks in the sticks would commonly throw copious amounts of dead used nuts & bolts & bits of steel to make a NON bearing slab & that works good as well.

no it does not work Beardog. only "connected" steel over the full length gives concrete a certain flexibility thereby avoiding cracks. even a zillion single nuts and bolts won't help

Yea but the oakies don't know no better. Still it would be far better then the bamboo. I never used nuts & bolts method & prefer rebar as it is much stronger & does not flex. mesh seems to work pretty good on small projects. But I wouldn't even consider less than some type of connected steel (And larger diameter than chicken wire.)

[Daffy D]

Again thanks everyone for their input

Thanks to "canopy" for the "Bamboo" manual. Interesting reading and just goes to show that a little knowledge is is not always a good thing.

As I posted earlier, have scraped the bamboo and am looking into metal of some sort.

[canopy]

Yea but the oakies don't know no better. Still it would be far better then the bamboo.

Nope. Nuts and bolts are essentially a coarse aggregate that increases the compressive strength of concrete. But cement is already strong in compression to begin with. Reinforcement such as steel or bamboo rebar on the other hand increases the tensile strength of concrete which resists cracking. Cement has a weak tensile strength so this is critical in the soundness of a structure. How rebar is placed and joined is also critical as has been mentioned.

[trogers]

Yea but the oakies don't know no better. Still it would be far better then the bamboo.

Nope. Nuts and bolts are essentially a coarse aggregate that increases the compressive strength of concrete. But cement is already strong in compression to begin with. Reinforcement such as steel or bamboo rebar on the other hand increases the tensile strength of concrete which resists cracking. Cement has a weak tensile strength so this is critical in the soundness of a structure. How rebar is placed and joined is also critical as has been mentioned.

Make sure any type of reinforcement to increase tensile strength is not placed along the neutral axis, but rather near the bottom or top of the slab, depending on how it will bend under loads.