Making paving slabs

[beer is good]

Hi All, I am new here and thought I would pick the collective brains. I am thinking about making some paving slabs about 400mm square and about 40mm thick for a garden makeover in our new house in Perth. Why make them? Partly to save some money and partly to get something a bit different. My question relates to aggregrate - I am reluctant to use bluemetal as it a bit big and might make it difficult to get a smooth finish on the top. Do I need aggregate, could I use bluemetal dust? Any tips from "slab makers" would be appreciated.
Thanks,

[Ekim]

Firstly, I suggest that you don't make your slabs "glassy" smooth as they will become slippery when wet. Secondly, you are probably looking at large aggregate (20 -25mm) - you can get smaller stuff (5 -10mm).

I suggest that, as an experiment, you mix a batch of concrete in a wheel barrow using bagged products. Some 18 months ago, Mitre 10 (Osborne Park) was selling bagged aggregate (5 -10mm). You can also buy bagged brickies sand (probably the cheapest source is Soils aint Soils (Balcatta) or Bunnings (Balcatta). Make sure that it's brickies sand. Confirm that you are getting the desired standard before spending too much money on bulk aggregate, sand etc.

To get a smooth finish, use a steel float. For a rougher finish, use a timber float. Some years ago, I made a few slabs (600mm x 300M) and used a heavy duty plastic float (Bunnings) to push the concrete around in the mold - manually vibrate the plastic float or use a light tapping motion to ensure that the concrete is pushed into the corners and against the sides of the mold. Vibrating the concrete pushes the aggregate below the surface. Screed off, wait say 20 minutes them float off (I used a timber float). Remove the mold in say 48 hours and allow the slab(s) 7 - 10 days to harden (it will take about a month to fully harden).

You will have to make a decision as to whether it is cheaper to make your own slabs. A decent steel or timber float is not cheap (get a good one i.e. it has an absolutely flat float surface and a comfortable handle). In addition, buying small amounts of aggregate and brickies sand can be expensive when you factor in delivery costs (unless you have a trailer). Finally, mixing concrete in a barrow is a PITA.

HTH

Mike

[silentC]

My uncle used to make his own paver slabs. He made them upside down. He'd sprinkle sugar in the mould first, then pour in the concrete. After it has set, you hose/scrub the crud off the top and you have a paver with exposed aggregate. He was using river stones rather than blue metal. He had about 10 moulds so he could do a batch of them. Buy a mixer. I bought one second hand from a dealer in Sydney for a couple of hundred. It has been well used.

[Bleedin Thumb]

Silents on the money. The top of your slab is actually the bottom of your mold.

One thing to be careful of -Always fill your molds up to the same point (overflowing) dont try to skimp and underfill them as you get slabs of differing thickness which are a PITA to lay.
If you look at commercial slabs you will note that they have beveled sides so that they release from the mold easily.

Also try rock salt instead of sugar.

[joe greiner]

Bevelled edges would be advisable for one-piece molds, with each mold flipped to release. But, a gang mold for 18 pieces could be made on a sheet of plywood or masonite or particle board or ... (Q&D conversion from Imperial) 1200 x 2400, with furring strips 19x38 at about 400mm (adjust for equal size pieces). Knock the edge strips off and dismantle as you pull the pieces. Clean the mold parts and repeat as often as needed.

BT, you might have your wires crossed. Salt accelerates, sugar retards. Leave the bottom 2-3mm soft until you brush it off to expose the aggregate (after flipping).

I don't know about the timing with sugar. When I worked in the precast architectural concrete game, we used commercial retarders, about 17 hours set, with a slower potion for Fridays (stripped on Monday). Also used high early strength concrete IIRC.

Joe

[Terrian]

local garden supplies should be able to sell you bags of concrete / washed sand (brick sand is too 'fatty') & stone, 10mm or 14mm stone, or look at using similar size pebbles,
3 - 2 - 1 mix does not mean using a shovel as the measure :-)

[Bleedin Thumb]

BT, you might have your wires crossed. Salt accelerates, sugar retards. Leave the bottom 2-3mm soft until you brush it off to expose the aggregate (after flipping).

IJoe

Joe I was thinking of course Rock salt, its used to trowel into concrete - sometimes to form designs. Once the concrete is set you dissolve the salt out to give an etched effect.
Must admit I don't know how it would go in a mold.

Personally I wouldn't worry too much about the concrete being slippery theres a plethora of products you can use to alleviate it after casting or you could roughen your mould for texture then use mold release etc.


My biggest concern would be getting uniformity of the thickness..

Many years ago I did some paving to a new courtyard to a Sydney high school. Being a young turk contractor I sourced my product from this mad frenchmen over at Kurnell, he was heaps cheaper mate......

The job was a couple of hundred sq.m of 600 x 600's.

We got them delivered, the bloke took months to furfill the order, so the builder was screaming, then we find each paver was a different thickness so they had to be individually bedded like stone! and he also decided to put steel in them to make them stonger! did wonders for the blades.

Simple job turns into a nighmare....once again:-

[woodcutta]

Hi B.I.G

I made a couple of hundred meters of pavers ( after a mate made over 4000 sq mtrs)

The molds were strips of 16ga steel cut into strips 1200 long X 50mm high. When he made the molds he used a cold saw to cut slots half way through (25mm) the strips at 200mm spacing on some and 250mm spacings which were then assembled in squares to make a block of pavers 5 X 4 to the square meter.

I then layed a sheet of plastic on the concrete driveway and put the molds on top.

Make the mix with 6mm aggregate and make it a very dry mix. It should be like moist sand) Tamp down well and leave for about an hour. Lift off the molds and brush the tops with a small broom. I also had a triangular scraper that I would run along all the mold line that gave a bit of a cobblestone look.

woodcutta

[Bleedin Thumb]

Hi B.I.G

I made a couple of hundred meters of pavers ( after a mate made over 4000 sq mtrs)

woodcutta

Your mate wasn't French was he?:)

[joe greiner]

Roger about the thickness, BT. And aside from difficulty cutting, in material this thin the reo does more harm than good; inadequate concrete cover enables corrosion and the slabs crack and crumble in short order.

Joe

[ian]

Do you really need to make pavers?

would a tooled concrete slab (using a colour additative) achieve the same result? Commercially you can get a slab stamped to look like uniform or random paving and it shouldn't be too difficult to achieve the same effect yourself


ian

[Ironwood]

We had our pool surrounds done in exposed aggregate a few years ago.

To use up the stuff left over at the end of the job,
the guy that did the job, had a heap of pine formwork already cut up for the purpose, which he screwed together like a wooden ladder, with 400 x 400mm x 60mm gaps for the concrete to go into.

He just laid this down on the ground and filled up each one till the concrete was used up, and treated these with the same process as the pool slab.

We ended up with about 20 of these stepping stone/pavers to match the rest of the pool surrounds, they look great and cost us very little.

I thought this was a very good idea, a lot of other concreters that we've used over the years wouldnt have gone to the trouble.

[beer is good]

Thanks for all the speedy replies. I am thinking of making a mould or form like a french window to make about 10-12 slabs in one hit. I envisage placing it on a sheet of chipboard on the ground and covering the board with black plastic then putting the mould on top. The wooden mould would be screwed together to make it easy to pull apart when the concrete has set. For those who have done it, I presume that the top of the slab - the bit you walk on - faces up when you are pouring?

I have a cement mixer so that saves the strain of mixing in a barrow.

[joe greiner]

The technique discussed by silentC, BleedinThumb, and me relates to face-down casting, with an exposed aggregate finish obtained using sugar as a retarder; the pieces are flipped over after about 16 hours, and the retarded surface can be brushed off.

For face-up casting, exposed aggregate can be achieved by seeding the top of the casting with aggregate after it's taken its initial set. This takes some experience to get it right, and may not be best for you. A simpler finish would be to broom the top after the initial set.

Consider, though, whether you need to precast the pieces at all, in contrast to pouring in place with divider strips (removable or not). One way or another, the concrete must arrive at its final location. Precasting can be done at a single location if it's more convenient that way. But the pieces must then be carried to their final location. If cast in place, the wet concrete must be barrowed to the final location; or, you could move the mixer and the dry mix to each final location; the water can be moved by hose. If mixer or barrow access is awkward, precasting would probably be preferred.

Joe

[Blocklayer]

I bought this plastic paver mould thing years ago for about $20.
Its about 600 mm across.
Place it on the (prepared base) ground, fill it with concrete, tap, remove and rotate it as you go to so it all fits together.
I trowel some black and red oxide over the top for a better finish also.
Its pretty quick and the finished result looks good.
Dunno if thy're still available though.

[Terrian]

I bought this plastic paver mould thing years ago for about $20.
Its about 600 mm across.
Place it on the (prepared base) ground, fill it with concrete, tap, remove and rotate it as you go to so it all fits together.
I trowel some black and red oxide over the top for a better finish also.
Its pretty quick and the finished result looks good.
Dunno if thy're still available though.

Can't recall if it was Melcan or Blue Circle that had those, but you bought them and bags of pre coloured concrete to make your own crazy paving walways, we sold hundreds of them :-)