I am aware that the question has been raised in several threads before - and I have studied these before I started my project to cast kitchen countertops in concrete. The first attempt had to be discarded, the second attempt was okay but not perfect, so now I have some thoughts before I tackle the next slab!
I cast with Finja expanding concrete (fine) in form plywood which I placed on a fairly sturdy kitchen table. Watered well in the beginning and let the slab sit under plastic for four days (normal indoor temperature, around 20 degrees). The first thing that struck me was that there were a lot of bubbles on what would be the top side - we're talking almost Swiss cheese here! This despite the fact that, as mentioned, it was expanding concrete and despite some tapping on the mold. Could the large number of bubbles be due to too loose a mix of concrete? Someone suggested that one should lay a fairly thin layer at the bottom and press it before continuing to fill with concrete. Could that be something? I just poured directly into the mold, filled it halfway, inserted the reinforcement and then the other half. Does anyone have any other good ideas? Now the holes could be "patched," but it won't be a perfect surface as the cement in the holes tends to be lighter than the concrete as a whole when it's sanded.
An even worse problem was that the slabs somehow bent, which I believe happened AFTER I removed them from the mold and placed them on foam boards on a not completely flat wooden floor. The first one I took out of the mold after four-five days, the second after maybe eight-nine days. I let them lie on the floor for a few days while I sanded them. When I then put the level over them, they had become somewhat banana-shaped. Is this possible - that they are still not cured enough after just over a week? What else could be the problem?
Finally, I could state that sanding concrete is not an easy thing. Damn, it's tough! Despite endless wet sanding with various grits, both with an orbital sander and by hand, I didn't remove more than the outermost layer. The waviness that resulted from the form plywood is visible in the raking light now that it's polished. I had the thought of diamond grinding with an angle grinder but didn't dare attempt it. Could it be a better idea to cast against, for example, an MDF board that is first painted with oil-based varnish to protect the board against water penetration?
Grateful for opinions and ideas!
I cast with Finja expanding concrete (fine) in form plywood which I placed on a fairly sturdy kitchen table. Watered well in the beginning and let the slab sit under plastic for four days (normal indoor temperature, around 20 degrees). The first thing that struck me was that there were a lot of bubbles on what would be the top side - we're talking almost Swiss cheese here! This despite the fact that, as mentioned, it was expanding concrete and despite some tapping on the mold. Could the large number of bubbles be due to too loose a mix of concrete? Someone suggested that one should lay a fairly thin layer at the bottom and press it before continuing to fill with concrete. Could that be something? I just poured directly into the mold, filled it halfway, inserted the reinforcement and then the other half. Does anyone have any other good ideas? Now the holes could be "patched," but it won't be a perfect surface as the cement in the holes tends to be lighter than the concrete as a whole when it's sanded.
An even worse problem was that the slabs somehow bent, which I believe happened AFTER I removed them from the mold and placed them on foam boards on a not completely flat wooden floor. The first one I took out of the mold after four-five days, the second after maybe eight-nine days. I let them lie on the floor for a few days while I sanded them. When I then put the level over them, they had become somewhat banana-shaped. Is this possible - that they are still not cured enough after just over a week? What else could be the problem?
Finally, I could state that sanding concrete is not an easy thing. Damn, it's tough! Despite endless wet sanding with various grits, both with an orbital sander and by hand, I didn't remove more than the outermost layer. The waviness that resulted from the form plywood is visible in the raking light now that it's polished. I had the thought of diamond grinding with an angle grinder but didn't dare attempt it. Could it be a better idea to cast against, for example, an MDF board that is first painted with oil-based varnish to protect the board against water penetration?
Grateful for opinions and ideas!
I had planned to make a vibro to remove air from the concrete.
Take two bolts, type M8 100 mm
Saw off the head of one
Place 30 mm of the threads next to each other
Heads outward
Weld them together
Put the bolt without a head in a drill and work around with your homemade vibro in the mold after casting.
Haven't tested this myself, but it should work
Take two bolts, type M8 100 mm
Saw off the head of one
Place 30 mm of the threads next to each other
Heads outward
Weld them together
Put the bolt without a head in a drill and work around with your homemade vibro in the mold after casting.
Haven't tested this myself, but it should work
A smooth surface saves a lot of sanding work and you need to remove the air, I made window sills and used carriage bolts and a hammer drill around the form. Many people recommend Melamine forms, I used shelf boards wrapped in floor protection plastic.
Next time, I will use a machine like this for processing without http://www.aliexpress.com/w/wholesale-electric-wet-grinder-polisher.html
Next time, I will use a machine like this for processing without http://www.aliexpress.com/w/wholesale-electric-wet-grinder-polisher.html
Mm, the vibrating is always emphasized as an important element. I chose to cast with expanding concrete because I got the impression that it would only require a moderate amount of vibrating of the mold, even that you should be careful not to vibrate too much due to the risk of separation of aggregate and cement (this is what I've read, not what I claim to know). In any case, this seems to be inconsistent unless the reason is that I've mixed too wet concrete.
Melamine was my thought too, but I haven't managed to find wide enough melamine boards for casting a 60 cm wide countertop. However, I think MDF works just as well as long as you protect the surface from moisture in some way.
A concrete grinder would certainly ease the grinding significantly. I'm considering renting one, but you still can't avoid having to acquire a diamond grinding disc, which alone costs quite a bit for something you might only use once in a lifetime...
Melamine was my thought too, but I haven't managed to find wide enough melamine boards for casting a 60 cm wide countertop. However, I think MDF works just as well as long as you protect the surface from moisture in some way.
A concrete grinder would certainly ease the grinding significantly. I'm considering renting one, but you still can't avoid having to acquire a diamond grinding disc, which alone costs quite a bit for something you might only use once in a lifetime...