Ornamental concrete block

NT

I think you can improve the quality of your seeded exposed aggregate in one or both of two ways. First, make sure the substrate has had time to settle, and allow te cream to rise to the surface. At this point, the top surface will be quite moist, and any aggregate that is seeded in should be easily worked into the top surface. It is possible to seed too early, and this will result in the aggregate sinking out of sight. It is a good idea to use some method such a pressing, tapping, or rolling the seeded aggregate inot the surface, in order to work it inot the concreate fines, and produce a solid bond to the substrate. Second, you may find it better to pour you molds to about 2 or 3 cm from the top, using your regular concrete mix. Then, use a sand/cement 'mortar' mix to complete the fill, and seed the aggregate into that. Allow the normal mixture to begin to just harden before adding the mortar topping. This will allow the base to support the topping mixture, but still provide a good bond between the layers. I have used this method to create large stepping stones, as well as stair treads and landings. Normally, I use a roller to press the aggregate into the base substrate, but on these kinds of surfaces, it was not possible, so I came up with this method as an alternative. I have a couple of web pages that show some of these techniques:

--- rod.

Thanks NT, I'll use that mix as described on website.

Rod, the problem is I have to seed two opposite sides, the front and back vertical sides of the block, which I do by turning each side upwards in turn, but in doing this at least one of the sides tends to go wrong. Maybe I need to be more careful about putting a single layer of aggregate on what is effectively the bottom side?

I was thinking of trying it a different way, painting retarder on the outsides of the block as soon as it is stable enough to loosen the mold; then hosing it down after 24 hours to expose the surface aggregate that is already in the mix. would this be a better approach?

Tony

Rod, Is that your site? I read it already & found it v helpful. Having read it, I don't think I'd have a problem with a flat surface such as a patio or a driveway. It's the two sided aspect of my block.....

Tony

Rod Is that your site. I already read it recently. It's v helpful and I would have no probem I think following your advice on creating rough driveway. The big problem is how to deal with the two opposing sides of my block...

Tony

I believe the method that you need to use is to apply a retardent gell to the inside of the mold before you fill it. After the piece is de- molded, the retardent will have left the surfaces uncured, and the

Hope this helps. --- rod.

Yes that makes sense. I'll ask at the local builders yard. I think I'll try lining my wooden formwork with polythene too, so that it comes away from the block more easily. It's a darn slow process trying to get it right - 2 days per block during the trial-and error experimentation, so I hope to get it right before I lose heart. I'll keep you informed, and when I get there , I'll make a web page on how to do it.

Thanks again Tony

I'd try that too, brushing & hosing to expose aggregate. But I dont think you need any retarder for that, concrete has very little strength when first set, it takes a few days to have serious strength, so you could do it after 24 hours or sooner.

NT

I have created bricks by casting concrete in wooden forms/molds, and I found that 24 hours was too soon to de-mold them. The edges and corners were still too fragile, and ended up chipping off during the extraction from the mold. At that point, there is no way at all that I could have exposed any of the aggregate without extremely vigorous brushing and spraying. I think people use the retardent for a reason. I have never seen the stuff for sale at any home centre style of retailer. I think you would have to go to a wholesaler that specializes in serving the concrete contracting industry. --- rod.

I actually bought 5l of retarder in a local builder's yard (Beesley and Fildes). It is a combined plasticiser/retarder and is liquid rather than gell. I think it will help because I've found it tricky to judge the best time to spray without it. Too soon, and I wash holes into the surface. And too late is too late.

Another unavoidable problem is that granite chippings are different every time I buy a bag. I could do with adapting the technique so that I save the 'best finish' chippings for the outer coat, maybe add timber slips to the front and back of my mould so that I can make an undersize block, then add the last half inch either side as soon as it is strong enough to sustain its shape.

An idea that I thought of is to retard one side, just the back face. This means I can have the front face upwards, seed it etc as per Rod's patio, wait till it is setting, then flip it over and work on the retarded back face later.

I could do with a small manual cement mixer rather than mixing a small amount in a bucket for my trial blocks. Save my back! And an old pan for measuring quantities.

Sounds like the problems you get making a fruitcake i.e. getting the ingredients to stay where you put them. I'm always amazed how little plasticiser it takes to replace most of the water you would normally have to use but I don't know if that makes for a better end product in your case.

Interesting. Last time I did concrete work I did most of the brushing after 1 day, and a bit after 2 days, and it came away easily.

NT