How to resurface concrete slab?

HardYakka

Hello, 

I recently poured a concrete slab and it’s off level in the centre causing water to pool. It’s deteriorating the concrete and only going to cause more damage over time. There’s going to be a water tank (1000l) going on top. Does the slab need to be broken out and re-poured? Or can I use a screed to bring it up to the surrounding level? 

MitchellMc

Welcome to the Bunnings Workshop community @HardYakka. It's wonderful to have you join us, and many thanks for your question about concrete slabs.

You do not necessarily need to break out and re-pour the slab, but the best approach depends on how far out of level the centre is and how much material you would need to add to correct the fall. A small depression can usually be fixed with a cement-based screed, which can bond well to existing concrete as long as the surface is sound, clean, and properly prepared. 

If the depression is deeper and you need to build up more material, you would be looking at a structural grout and a primer made specifically for bonding to old concrete. The surface must be roughened, cleaned thoroughly, and free of any laitance or loose material. Once properly prepared, a bonded screed can form a durable topping that restores the correct fall.

The main question is whether the slab is strong enough overall to support the 1000 litre tank, since that will add about a tonne of weight. As long as the original slab is well compacted, not cracked, and thick enough, adding a screed topping to correct the level is acceptable. If the slab itself is poorly made, very thin, or already showing signs of deterioration beyond surface wear, then replacement would be the safer option.

Please let me know if you have any questions.

Mitchell