Increase Thickness of Concrete Pad (for BBQ Island)

Question

I poured a concrete pad a year ago in preparation for a BBQ island that I'm building. Unfortunately, I was slightly off on the elevations and as a result, the pad is level with the patio pavers (was meant to be 3" higher). While the patio has channel drains, the concrete pad gets wet very easily and will puddle in rain. As such, I'm concerned that water will collect in the BBQ island once built and cause stagnant water and all the issues that come with that.

As I see it, I have two options:

1. Reframe and pour additional concrete on top of the existing pad to add 3-4" What would be required for this? How do I ensure that it's bonded to the previous slab?
2. Start by framing the perimeter with 4x4 Pressure Treated wood and use waterproof adhesive between the wood and concrete pad (idea is to provide a water barrier while the water flows into the channel drains). Then frame the BBQ island on top of the 4x4s (using steel studs).

Anyone have experience with the above? I'd love your thoughts and ideas if you feel one of these options is cleary better or if there's an option I'm not considering. Thanks!!

Answer 1

I would drill a few/bunch of holes in the old concrete and fill with short steel rods/rebar/bolts as anchors. About two inches above the old level, and in about two inches in the old concrete.

Fill up with new concrete the height you want.

Might want a slight(1/4 inch) slope on top.

Answer 2

There's no need to raise it that much.

Put on ~1/2" of thinset, with proper slope for drainage (so, not 1/2" evenly, if it puddles now. Slope at 1/8" per foot towards the edges so water will run off, not puddle.)

You could also lay a single row of bricks around the perimeter, and dump 1/2-3/4" of brick-type mortar into the middle of those so the central area is higher than the surrounding surface. It does not need to be level to the top of the bricks. If the thing to be built here sheds water, not much need to fuss about slope, but you could slope it to weep holes in the mortar joints of the single row of bricks if you want to be sure.

If you want to pour an overkill slab on top of the slab you have, just do that. There's no particular need to stress about bonding it to the previous slab - gravity will do very nicely at 150 lbs/cubic foot, and a slab on soil or a slab on a slab has no real inclination to go elsewhere. Bonding or keying would be a concern if the original slab was structurally inadequate, but in this case you just need a stable material to adjust the level, not something suitable for driving a 747 over or mounting a punch press on.

Answer 3

Since the pad has been out in the weather for a year, it won't have the best surface for keying into your new layer of concrete.

You'll need to waterblast the surface to clean off all organic material. Spend extra time on cracks and joins.

Prepare formwork/boxing to hold your wet concrete. It may be that your top-up layer doesn't need additional internal reinforcing if its well-bonded to the under-layer. If you made tie-downs for your BBQ then they may need extending.

You might choose to lay concrete blocks/bricks as a perimeter, and not use formwork. The top corner could be done using windowstill bricks/nose bricks.

Once surface has dried, you can treat it with a bonding agent to help the next layer "bite" or mesh. Muriatic acid is also known as Hydrochloric acid, and creates this tooth. BE AWARE its a hazardous chemical, and follow all instructions and use PPE.

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A completely different option might be to lay a wooden deck over the concrete pad. Downside, wood is expensive and still needs periodic maintenance.