Fixing the supports for a safety fence

[Jackie]

A lot of people fitting their own 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

[Poolguy]

John

You do truly have a problem.

Its not just a problem of 'getting a grip'.

Its a problem of compliance, as I presume you have forked out good money on a compliant fence. In which case then you must also install the fence according to the manufacturers specification. This is the installation technique which has been tested and approved.

If you do not follow this to the letter then your fence does not comply.

Hence, if there is a problem and an inspector of the Region comes to inspect your fence he will naturally look at the fixings as well as the fence. If you have no solid concrete base to hold the fixing which were supplied with the fence than your security device does not comply.

I suggest you go back to the beginning and lift the slabs where the fence will run a pour 150mm strip footing, replace the tiles and then mount your fence using the supplied fixing bolts. I fear that any other 'making do' solution will cause you more problems than doing it properly.

Probably unwelcome advice.

Andrew

 

 

[Jackie]

Hi Andrew, this is more or less the reply I got from Chris this morning, my having sent an email to Ted earlier.

(Pour 150mm strip footing) Not too sure what this means but if I go this route I was going to lift the slab and gouge out a depth of concrete under and replace with say 3 to 1 mix and lay or replace (more likely if yesterday is anything to go by) the slab on top.

The way slabs are normally laid, I thought, was as I now have with soft mix under a thin adhesive coat of cement. This being so, most pools will surely have the same problem. Certainly Chris seemed to think that this was often the case. Perhaps it should be more widely known though, to be fair, Ted did make reference to this in our earlier conversation. Anyway thanks for the info, a route I may have to go , still hopeful for a simpler solution, just lazy I guess................John

 

[Poolguy]

John

A footing should consist of 150mm wide and 150mm deep, hard concrete using 1:3:2 (cement:sand:aggregate) and at least two if not three steel reinforcing bars say; 8mm the length and most importantly around the corners.

I have discovered on many occasions as you have that many slabs are laid on sand dusted with cement which is an easy way to lay tiles. However it must always be contained with a ring of concrete otherwise the slabs will 'walk'. If yours has not, its a must not just for the fence but the rest of the slabs.

Personally, I prefer at least 30 mm of mortar under slabs as its more permanent and resistant to slight movements which look really bad after a while. But the concrete ring is still an must if you want to keep the structure looking good for a long time.

As my dear old Dad used to say " Boy, do it right, do it once!"

bon courage

Andrew

 

[Stevehudson]

According to a friend of mine who is a registered French builder, this method of laying a terrace on a drymix screed no longer conforms to French building regs. If your terrace was laid recently it may be worth checking to see if this is in fact the case.

Steve

[Jackie]

Ok this is all good stuff. I have lifted another slab where the next post is to go, 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 under the slab instead of a washer 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