Fixing fence supports

[Jackie]

I did post this in the swimming pool section but I guess it might be better here as it is really a structural problem I am having.

A lot of people fitting their own safety fence must have come across this problem.

The type of fence I have has a base for each fence support post having three large holes which are used to fasten the base to the slabs around the pool.

I have found that the concrete under the slabs around the pool is too soft for rock bolts (expanding metal bolts)to get a grip!
I did not lay these slabs but on inspection they sit on a thick bed of a very weak (sandy) concrete mix. The mix has been given a thin coat of what looks like pure cement and the slabs laid on top. The rock bolts just will not hold at all and the drillings come out mushy. I have been told by the fence supplier that levering up the slabs, where I am to fit a post base, and beefing up the concrete underneath is the best solution but my experiment yesterday evening showed that the slab is likely to break, this in fact happened so I have one slab to replace already. I don't fancy replacing 25 slabs!

My only thought so far is to drill the three holes for the base and then pour in a fine sand/cement stiff mix, say 1 to 1, and insert the rock bolts and give a couple of days to go off before trying to tighten them up. Someone else suggested a two part resin but I can find no information on this. Is there a better way, anyone got a solution?......John not Jackie

[Teamedup]

We've been having words about this chez nous recently. I want husband to bolt ours down and he insists that they need cimenting in, 'properly' was his exact word. Ours  aren't round a pool but at a boundary and would be bolted onto proper solid concrete.

So I shall be watching this thread and seeing what people have to say.

[Jackie]

As will others no doubt, but as I write this I hear the words of my old boss, when I was teaching, echoing around in my head. "Don't bring me problems, bring me solutions"!    Pretty please!..................John

[markn]

If the mix under the slabs is soft then the only way would be to lift the slab put new hard concrete underneath  lay the slab when wet and drill when gone off.  If you drill the holes for the bolts and put concrete in them, it won't spread far enough to get a grip and all you will do when drilling is either push what is nothing more than a slim concrete plug downwards or if you are lucky enough to drill into it then it will rotate or just break up - either way you have no grip.

Sorry I can't be more positive

Mark

[Jackie]

 

Ok this is all good stuff and thanks for the reply Mark. I have lifted another slab where the next post is to go having drilled the three holes, the rock bolts would not hold again, mix just too soft and laid in 2002.

I can knock out the concrete easily under the slab to the suggested depth. On putting in the new mix should I set the slab back on top with the rock bolts inserted in position and to the right depth so that the concrete sets around them, leave for say a week before tightening up?

Would it be better to use studding with a nut and washer bolting the base to the slab or better still a large metal plate instead of the three washers, bolted to the underside of the slab and leaving a length of studding sticking down into the wet concrete and would this be an approved method. My point is that the rock bolts are pretty smooth and might not key in too well into the wet concrete.

I could, as has been suggested, re-lay the slab on new concrete, wait for it to go off, say a week and then re-drill. Which do you think is best?...............John