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I have laid a concrete slab for the first time. I bought a general purpose, ready mix tub of concrete and followed the instructions (add 400ml water to 5kg of concrete mix)

The end result is disappointing enter image description here

As you can see, it's patchy and not smooth.

I read and watched countless videos and articles and so I am confused if this the result of my screeding effort

Arrogance would make me think the mix was too stoney.

Ignorance would make me think it wasn't wet enough (or wasn't mixed but I made sure there was no dry concrete)

Common sense would suggest it's my fault and a lack of technique

What would cause concrete to be patchy like this

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    Have you tried tamping? Basically tap the concrete, it causes the aggregate to sink and the 'juice' to rise.
    – UmH
    Nov 8, 2020 at 8:27
  • Let me research this. Thank you for the comment Nov 8, 2020 at 8:53
  • I did do that thought out. It has been down for 12 hours now so I'm guessing no point in continuing Nov 8, 2020 at 10:56

1 Answer 1

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The back and forth motion of the screed or float shakes the aggregates and makes the fine ones rise and the big ones sink, which gives a smooth surface. But the concrete has to be more liquid than what you did for that to happen. Then after screeding, before it hardens completely, spray some water on it and finish it with a float.

However if you put too much water in it, you'll have problems, like cracks. So you have to either get the water content "just right" or add some plasticizer which makes it fluid without extra water. You can also vibrate it, but that requires a special tool.

If you intend to tile this, then the rough surface is not a problem. It looks pretty good actually, since if the concrete will be covered, no need to bother with the extra work of making it smooth, all you need is to not have bumps. Tiles will cover roughness and holes just fine. If some aggregates stick out and the tiles stand on them, just smash the aggregates out of the concrete with a hammer.

If you intend to leave it visible, then you can spread a bit of mortar made with fine sand, or self-leveling, to make it smooth if you want.

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