Setting fence posts next to slab

Hi,
I am building a fence around a concrete slab. I am using 4 x 4 fence posts, and do not want to set them in the existing concrete slab. I would like to set them next to the slab. I am using Quikrete fast setting fence post mix. my question is whether or not it is OK to set the fence posts directly abutting the existing concrete slab so that the new Quikrete mix would basically be on three sides of the post and the existing slab would be on the fourth side. Or do I need to make sure the new mix completely surrounds the fence post?
thanks for your help!
Randy

 

From my experience, the main purpose of the concrete is to fill up any gaps between the hole and the ground. That is why using the tamping the dirt method, backfilling with crushed rock or foam also works for posts (although each has their pros/cons). So as long as you can get the concrete to fill up all the voids (and touch virgin earth) it will work. The problem you will encounter will be if the hole you dig has a straight side on the side of the slab, it will be very difficult to fully fill up any void you might have. Any amount of void space will severely weaken the posts ability to remain solid/vertical.

I just replaced a portion of my fence in a similar situation as you. In my situation it was a retaining wall vs concrete slab but it presented the same problem. What I wound up doing is undercutting the one side about 4" away from where the post would reside and then filling the hole in "lifts" of about 4" each. After each fill I took a 2x2 and "vibrated" the concrete to ensure it settled around the side of the hole completely filling the void.

BTW, unless you are planning on completing the fence in the same day as you set the posts, standard quikrete concrete if fine and cheaper.

 

Thanks for the advice!