Concrete Mix ratios

[jlamplough]

Hi all,

I need to core fill a retaining wall. The engineer specified a strength of F'c=12MPa. What Gravel:Sand:Cement ratio is this?

Also, what does a slump of 230mm mean?

Cheers
Jeremy.

[Sir Stinkalot]

Ring up your mini mix company and get it delivered .... don't mix it yourself.

[powderpost]

A standard hand made concrete mix is four parts 3/4" crushed metal, two parts river sand and one part cement, measured by volume e.g. a bucket is prefered. Got no idea what mPa's that will produce. Slump is a measure of the fluidity of the concrete. A slump of 230mm would be called a thin soup. Slump is measured by filling a slump test cone with concrete and compacting it. The cone is slipped off and inverted beside the test. The distance measured from the top of the cone and the 'slumped' concrete is measured vertically and should not exceed 50mm. Too much water in the mix will lead to separation of the aggregates and will produce very weak concrete.
Jim

[johnc]

Stinky is right on the money, hand mixing even for the experienced is not going to give you 12mpa with 230mm of slump in any consistancy if at all. For the cost and the fact you are trying to match an engineered spec you would be well advised to contact the local batching company and get a truck in. Pouring it in a timely manner is going to sort you out, have you thought of engaging a boom and pumping it in?

JohnC

[ian]

Have to agree. Jim's mix will give you approx 20MPa and a slump of about 80mm.

For 12MPa and 230mm slump you'll need to either use a super plasticiser or add lots of fly ash to the mix, or both. Noway you're going to mix it by hand. You may even have problems getting the spec from your local mini mix supplier. Adding water to get the slump, which looks the easy way, will almost lead to segregation of the aggregate and cement and not achieve the strength you need.

Ian

[johnc]

I would also question aggregate size, the sloppy mix no doubt aids the slurry as it makes its way into the wall cavity, which would suggest something less than 3/4" stone.

[boban]

Call the concrete co. and ask for block fill. Then hire or borrow a vibrator. thats the easiest way.

The slump is specified in order to get the concrete everywhere. A vibrator will do that for you. I dont know what the design of the wall is but a vibrator might not be appropriate. If its a normal block wall - no problems.

I've filled hundreds of block walls with my father, a concretor, and weve done it like that almost every time. If its a small job then I have used a concrete mixer with smaller blue metal and a stick or vibrator. The stick is to get the concrete moving into place - hard yakka though.

12Mpa is rather weak though. From what I've seen and done, for a normal block wall weve always order the block fill which is basically a mix with smaller aggregate- 10mm I think and its certainly not 230 slump.

[Woodlice]

Use bricky sand and road base.

2 parts sand to 4 parts road base to 1 part cement to 1 and a bit parts water using the same size bucket. Add calair or dishwashing liquid for elasticity.

Core fill requires a "stickier" mix but one that is super fluid and consistant (but not overly wet/sloppy -ie water isnt floating on top of it.) Reduces likelyhood of airpockets and aggregate clumping in deep cores ensuring a consistant tensile strength on setting.

If you are using concrete besser / cinder blocks (the old style) then by all means use a compacter (but make the mix a bit drier - otherwise you'll end up with the "sticky" bit floating on top.) A compacter will f*ck the crushed lime type bessers.

Best way I find is using a good length of 1 or 2" wood purlin and aggitating the mix into the core... :D

[jlamplough]

Thanks for all who replied.

As usual, what I thought was straight forward is a little more complex. The blocks are Boral Besser blocks. I have 150 blocks to fill which according to the boral technical info is about 1.5m3 of concrete. The reason I was going to mix it with a mixer rather than get a truck is that it is very awkward to get to. And a pump isn't cheap.

The boral technical information on core filled blocks specifies:
Characteristic Compressive Strength 20MPa;
• Cement content not less than 300kg/m3;
• Round aggregate (if any) not bigger than 10mm;
• Clean sand (salts can leach through to the face); and
• A pouring consistency which ensures that the cores are
completely filled and the reinforcement completely
surrounded without segregation of the constituents.

Now, is it possible to do this with a mixer? If so, what sand:aggregate:cement: mix is this? Is Jim's receipt earlier good enough?


Thanks,
jeremy

[ian]

A 4:2:1 mix should be OK
10mm round aggregate is sometimes known as "pea gravel" and can be a lot more expensive than normal crushed stone.
look to substitute about 20-30% of the sand with fly ash - the fly ash having a very small particle size will help make the mix flow - which will help with the pouring constistency.
to get a mix sloppy enough to pour without being too wet you almost certainly will need to use a super plasticiser
BTW 1.5m3 of concrete weighs in at about 3.5 tonnes - price the raw materials (you will need at least 12 x 40kg bags of cement) including delivery add in your time and cost and time involved in building the ramps you'll need to each barrow load to the top of the wall.
Then put a price on saving your back, by hand involves lifting and moving about 7 tonnes (3.5t of raw materials into the mixer and then 3.5t of wet concrete to the wall).
You may be surprised how little extra a a truck and pump is!

Ian

[Forest Gump]

As it has been said before, ring a concrete supplier like Pioneer/Hanson. They have a product called Liquidfill which is used to fill tanks, pipes, trenches etc. Its made from stone dust, sand, cement and aditives and comes in at the slump you want.

Daniel

[jlamplough]

OK, thanks all for the advice, you've convinced me....

I'll drop the idea of mixing it myself.....

Cheers,
Jeremy.

[Sir Stinkalot]

If you were to do the mix yourself there will be no way that I know of that you can test to see if your mix complies with the Engineers specs ..... then there is the issue of who will certify the works .... If you were to do it yourself the finger (and cost) will be pointed at you if it went wrong.

Think of it on the brighter side .... you don't have to mix the concrete yourself and it will be over in a few hours by having it delivered.

[Metung]

Boban,
I have no problems about where he can hire a vibrator but have you got any suggestions as to where he might be able to borrow one.

[outback]

BTW 1.5m3 of concrete weighs in at about 3.5 tonnes
Sure?

[boban]

Boban,
I have no problems about where he can hire a vibrator but have you got any suggestions as to where he might be able to borrow one.
Every concretor I know has one, so I guess thats the answer. If he dont know one, then hire is the only option. That said a stick will also do the same job, its just a hell of a lot harder. You know when its been vibrated sufficiently when the top of the concrete gets a bit of slurry (water and fines).

BTW all the walls we have done have had reo from the footings into the blocks and across the blocks. Has this been done? If not I hope the walls not too high.

[DavidG]

See
http://www.reade.com/Particle_Briefings/spec_gra2.html

Concrete, Gravel 2.4 tonne/cubic metre = 3.6 tonne

[journeyman Mick]

Jeremy,
if you really want to do this I don't think you should let the concrete specs put you off (but you might want to think twice about the hard work :p ) It would be almost impossible to not make the resultant mix stronger than specified if mixing by hand. the last bit of core filling I did was the retaining wall of my shed, only 1M high (5 courses) so no running up and down a ladder with buckets of sloppy concrete. There were about 220 blocks to fill and it took us about an hour and a half and a carton of beer. There was my next door neighbour and his son (both blocklayers) my cousin and myself (a landscaper and a carpenter) so there was four of us, all fairly used to working with concrete, plus two mixers and a swag of buckets. Working by yourself, with one mixer and not too much climbing up and down ladders you might knock it over in a long day. Make the mix reasonably sloppy, but with enough cement that it doesn't seperate and use 1/2" gravel. Use a length of reo rod to get the air pockets out by rodding each core thoroughly. Years ago before pumps were common and cheap we did hundreds of houses like this. Never seen a vibrator used in core filling though, I'd be a bit worried that the vibrations and pressure on a high wall might blow out the mortar joints at the bottom.

Mick

[jlamplough]

Thanks all,

I've decided to get a few friends around with wheelbarrows and get the mix delivered. The charge for waiting while we unload is much less than a pump. Since it is only 150 blocks and I can access the top of the wall without ramps etc, I don't think it'll take too long.

Thanks for the advice,
Jeremy.

[journeyman Mick]

.............I've decided to get a few friends around with wheelbarrows and get the mix delivered...................

Your friends know about this do they? :rolleyes: Well it's one way of finding out who your friends really are. ;)

Mick

[dallas]

ahhh.. the power of the forum search button!

i'm just abuot to do this very task myself (and what would you know, 150 odd blocks also!)... silly question, so here goes..

i'm also assuming that you need to build the wall up to it's finished height, let said wall set, then core fill the wall??

D

[jlamplough]

ahhh.. the power of the forum search button!

i'm just abuot to do this very task myself (and what would you know, 150 odd blocks also!)... silly question, so here goes..

i'm also assuming that you need to build the wall up to it's finished height, let said wall set, then core fill the wall??

D

Yes. Put all the required steel in the wall and then when any engineering inspections have occured, fill it with concrete. Getting the truck in with wheelbarrows worked fine.

jeremy.