Hello.
I hope someone with experience in my issues can help me.
I am in the process of casting concrete pillars for our driveway. I believe I have built the molds according to all the rules of art, but time and time again I encounter the same problem. So what is the problem? Well, the corners and edges of the pillars always become discolored. They are always darker than the rest of the pillars. Often completely irregularly discolored. I.e., wider discoloration in some places, narrower in others.
I am now on my 8th attempt, and it turned out the same.
I use plywood boards when casting because I think the surface turns out best that way. I have taken a lot of measures to try to avoid the problem. From trying to seal the mold so no air can get in to lacquering the cut surfaces. I have vibrated the molds to different extents, and it turns out really nice, I don't get any bubbles at all. But the dark discoloration remains.
Does anyone have any tips for this as it truly baffles me?
Best regards, Jonas
I hope someone with experience in my issues can help me.
I am in the process of casting concrete pillars for our driveway. I believe I have built the molds according to all the rules of art, but time and time again I encounter the same problem. So what is the problem? Well, the corners and edges of the pillars always become discolored. They are always darker than the rest of the pillars. Often completely irregularly discolored. I.e., wider discoloration in some places, narrower in others.
I am now on my 8th attempt, and it turned out the same.
I use plywood boards when casting because I think the surface turns out best that way. I have taken a lot of measures to try to avoid the problem. From trying to seal the mold so no air can get in to lacquering the cut surfaces. I have vibrated the molds to different extents, and it turns out really nice, I don't get any bubbles at all. But the dark discoloration remains.
Does anyone have any tips for this as it truly baffles me?
Best regards, Jonas
This is how all newly cast constructions look - and I actually have no logical explanation, other than that it usually becomes dark areas where the mold has absorbed moisture from the concrete (end grain). For example, a private customer had bought cheap regular plywood and had filled and sanded the unevenness on the plywood - his gate pillars have very dark sections after the filler - even after several years.
I must give you credit for your commitment.....but 8 times? The little warning bell rings that your future fashion sense will consist of a shirt with too long sleeves and leather straps on the back.
To achieve a really nice concrete surface, you should order a concrete truck with a C32/40 0-8 air 4.5 - 5.5% S5. ("air 4.5 - 5.5%" only applies to outdoor constructions without cladding). If you use form plywood for this concrete mix, the cast surface will feel exactly like a smooth glass sheet.
Ensure that the mold is cross-secured with braces and that the mold is extremely durable and completely watertight = 100% tight. This concrete mix creates enormous form pressure and can penetrate a gap of 2 mm. If you seal the mold with silicon, it will be visible on the concrete surface - in that case, do an extremely neat silicon job.
When you then cast - fill up 30 cm of concrete in the mold and insert the vibrator for MAX 1 sec (!) in each corner. Pour in another 30 cm and max 1 sec in all corners again and continue until the mold is full.
One important thing to get an extra fine cast surface is to dampen the inside of the mold with a plant spray 5 minutes before you pour in the concrete, so the formwork does not absorb moisture from the concrete - this also applies to form plywood, even though form plywood absorbs 1000 times less moisture than a wooden form.
If you want some very discreet patterns, e.g., discreetly striped columns, you create that pattern with duct tape - the concrete surface will be exactly like the texture of the duct tape - very nice! Otherwise, you can line the inside of the mold with water-resistant textured wallpaper.
This concrete mix captures and reproduces everything exactly - if you tape a stripe with regular cheap and thin masking tape, the stripe will discreetly appear in the concrete surface.
Through my job, I've seen so many really cool concrete castings from completely insane concrete ideas that I am loaded with thousands of cool things you can do with concrete.
Here you can see and read about how to build durable wall molds that I very often pump concrete into.
http://www.betongbanken.com/index.aspx?s=3842
I must give you credit for your commitment.....but 8 times? The little warning bell rings that your future fashion sense will consist of a shirt with too long sleeves and leather straps on the back.
To achieve a really nice concrete surface, you should order a concrete truck with a C32/40 0-8 air 4.5 - 5.5% S5. ("air 4.5 - 5.5%" only applies to outdoor constructions without cladding). If you use form plywood for this concrete mix, the cast surface will feel exactly like a smooth glass sheet.
Ensure that the mold is cross-secured with braces and that the mold is extremely durable and completely watertight = 100% tight. This concrete mix creates enormous form pressure and can penetrate a gap of 2 mm. If you seal the mold with silicon, it will be visible on the concrete surface - in that case, do an extremely neat silicon job.
When you then cast - fill up 30 cm of concrete in the mold and insert the vibrator for MAX 1 sec (!) in each corner. Pour in another 30 cm and max 1 sec in all corners again and continue until the mold is full.
One important thing to get an extra fine cast surface is to dampen the inside of the mold with a plant spray 5 minutes before you pour in the concrete, so the formwork does not absorb moisture from the concrete - this also applies to form plywood, even though form plywood absorbs 1000 times less moisture than a wooden form.
If you want some very discreet patterns, e.g., discreetly striped columns, you create that pattern with duct tape - the concrete surface will be exactly like the texture of the duct tape - very nice! Otherwise, you can line the inside of the mold with water-resistant textured wallpaper.
This concrete mix captures and reproduces everything exactly - if you tape a stripe with regular cheap and thin masking tape, the stripe will discreetly appear in the concrete surface.
Through my job, I've seen so many really cool concrete castings from completely insane concrete ideas that I am loaded with thousands of cool things you can do with concrete.
Here you can see and read about how to build durable wall molds that I very often pump concrete into.
http://www.betongbanken.com/index.aspx?s=3842
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I would guess the reinforcement when it comes to small pillars. However, it is likely to disappear over time.
Legendarisk said:
In that case, it should be the same for all sizes, as the rebars should always be embedded with 5 cm from the surface - regardless of the size of the construction.
The only thing the rebars can do is brown spots due to rust penetration when the rebars have been embedded too close to the surface.
Tyresö said:
Great tip! Thank you and much appreciated!