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Posted By Howard
Hi all
We are sometmes required to drill or break concrete floor slabs to a depth of @ 100- 120 mm
We have been critised by the client for the type of detector we use when scanning concrete prior to drilling. It would be used to identify the position of Rebar & services embeded in the concrete where there may not be any service drawings.
The detector we currently use was obtained by my predecessor and I have to agree with what the client is saying as the manufacturers literature advises it will detect to a max of 100mm in ideal conditions. I do not feel that it would get anywhere near 100 mm in concrete.
So the question is, do we need to go the whole hog for a CAT & Genny or has anyone come across anything else that would do the job.
thanks
H
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Posted By Alan Hoskins Howard, The usual means of detection is using a concrete covermeter, but even with deep penetration heads they only detect up to about 100 mm. Have you checked out the manufacturer's websites? Such as www.elcometer.com Alan
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Posted By Raymond Rapp
I am no expert but for most uses a CAT & Genny is the best form of detection for buried services that I know. The latest type from our supplier has a depth detector and visual/sound strike alert warning for shallow services. We also use the RD4000 where more accuracy is required.
Regards
Ray
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Posted By Peter Ash I had the same thing come up recentley with a company who were drilling through concrete, a CAT & Genny is definetley advisable, especially the Genny, not enough people use the genny and especially when your dealing with service connections it is essential. It is difficult to loacte utilities through reinforcing, but not totally impossible if you use the right technique. Hope its ok to post this www.sygma-solutions.com for any advice on both covermeters and Cable avoidance issues.
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Posted By Howard
Thanks for the replies.
Looks like the CAT & Genny is the way to go then. I shall book a course for the supervisors.
Howard
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Posted By TomP
I would be cautious about the capability of a signal generator to assist in detecting underground services under Re-bar. The problem is not the CATs capability to see through the concrete, but the fact that the re-bars will just shield any signals.
A CAT will pick up an AC electromagnetic signal radiated from a buried service due to electrical currents flowing in it. These can be direct (i.e. power and radio) or induced (through the use of a Genny).
In power mode, the Re-bar shields the signals form the services. In radio mode, the Re-bar is energised and that is what the CAT picks up.
Most Gennys have two modes – ‘Connection’ or ‘Induction’.
In ‘Connection’ you may have some luck but you need to be able to connect directly to the service for it to work and then, you may still find that the Re-bar is shielding the signal. In induction mode, all you will do is induce signals in the Re-bar – which the CAT will then pick up.
Sorry but a CAT and Genny are not really the best option for Reinforced concrete and there are no magic solutions that I know of. There is no substitute for good service plans, a safe system of work and a lot of hand digging. Refer to HSG 47 for better guidance.
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Posted By Peter Ash
If there is nothing else that will do it, then of course it is the best option.
It can be difficut as I said before, but certainly is possible using the right method.
I have done it on various sites numerous times, including A training centre I work work with has a concrete reinforced slab in the car park, and we regulary trace 2 Electric cables underneath it.
Lifting the Reciever up by 0.5m is one technique, but of course this reduces the depth to which you can locate.
Sometimes where it is impossible to locate through the rebar, you can still use a CAT to mark the service where it enters and leaves the slab, not ideal as it could bend and turn in the slab, but better than working completley blind.
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Posted By Ron Hunter
i recall some years ago attending a demo by an ex-defence industry person who had developed what was effectively a close-detail ground mapping microwave transmitter/receiver/decoder, which was claimed (and as I recall, successfully demonstrated)to be able to locate u/ground services whether cable, pipe, plastic etc at depths below 100mm.
Not much help I know, but a bit of 'Googling' might turn something up as an alternative to the CAT/Genny approach?
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Posted By Peter Ash
I have worked quite a bit with ground radar, and although it has its uses it is very limited.
Generally it still wont penetrate past the rebar, it will pick it up but not usually anything beneath it.
The skill is with the interprotation of the data, it needs quite a degree of skill, experience and training.
And also cost is usually the limiting factor, I have never come across one cheaper than about £9500, and usually average about £15,000 with logging features up to around £30k quite an expense.
Concrete aside,They are also quite restricted on ground conditions, and dont work well in clay type soils.
If I remeber right it was "Kinetik (not sure of spelling)"or something similar which developed one, initially for locating mines I think and kinetic was a division of the MOD who got sold off but retained the technology research contracts.
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Posted By Stuart Nagle
So far as I am aware you are able to induce a signal via a Genny that can be
located with a Cable Avoiding Tool (CAT), provided you have access to the cable
above ground or a box/lamp column etc (of conducting material e.g. metalic) so
the induced signal can be traced under the concrete.
As this signal is emitted as a 'bleep' it is easier to trace than using an
electromagnetic resinance signal that will be interupted or diffused through steel
reinforcing in concrete, giving a poor hit/miss rate.
The only other alternatives are:
1) ground penetrating radar, and
2) careful hand excavation through the concrete slab, or
3) complete isolation/disconnection/purging of the services before excavation
Stuart
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