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Have 2 100x750mm post spikes that I'd planned to set in concrete to make it easy to replace the post. Dug a 12 wide and deep hole with the spike driven in the middle, then was going to pour concrete round it for added strength. It's to support decking.
Only just realised there are specific spikes for concrete That are shorter and not tapered. Will the normal tapered spikes be OK to set in concrete? It's a 90min round trip to exchange them so hoping they will be ok.
Personally if using concrete I would just set pads and used fixed or adjustable brackets mounted to them.
Key thing is getting the wood out of the ground.
They will be fine for supporting decking, as they won't pull out (eg a fence in high winds).
Yeah there's no Lateral load. Just a fair bit of weight.
You could always drill big holes in the spikes and then the concrete would flow into the holes making it much harder to pull out?
And stick some 11" long M12 bar through other holes to reinforce it even more.
Those long spikes are usually drive ins to save needing concrete. Quicker and potentially more environmentally friendly than using excess concrete (smaller CO2 footprint per post and steel is easy to recycle of safely biodegrades / rusts away).
Just bang the rest in with a block amd sledgehammer to save digging holes and backfilling with concrete would be my recommendation. YMMV