Effects of "cutting" into a slab to insert electrical?

[walkerlrcs]

I have a newly poured (about three weeks old) slab for a one-story home. The builder forgot to put in a 115 volt receptical in the main room floor. Now he tells me he will have to "chisel" out a line in the slab to do so. He will refill it with some sort of "concrete bonding material." My question is - will this pose potential slab problems in the future? I should tell you that this is a post tensioned slab and that it already has several "shrinkage cracks." Thanks for your help.

 

[ishvaaag]

Normally once filled there should be more likely no problem for there are fantastically good mortars for repairs (I assume no main wire or strand will be cut). Then from some reduced depth and the standing strength you may ascertain if the temporary situation is bearable or if it can be so with some shoring etc. Make an study of this kind and you will feel better about proceeding whatever the way. However, examine if the line can be brought under the floor unseen over a ceiling, then just a hole.

 

[Ingenuity]

walkerlrcs,

I presume this is a Slab-On-Grade and UNbonded post-tensioning was used.

Your contractor needs to be mindful of the tendons and their anchorages - do NOT chip too deep within the slab depth, NOR too close to any PT anchorages.

As "ishvaaag" indicated, there are repair materials available today that will restore the concrete to near what it was be before.

I would chase the slab parallel-to, but offset-from, the preferred conduit location and the actual tendon/s location/s. You should have a good knowledge of the tendon locations since this is a new pour. And if the slab edge is exposed you can check the tendon locations via the grout plug. Where the receptical box is going make sure that this is not at a tendon or tendon intersection location.

Make sure that the person who does the chipping is not the lowest paid laborer - if he/she chips too deep or too close to a tendon that may produce further problems. Good supervision is recommended. I would minimize the use of water during the chipping - and at tendon intersection locations that the laborer does a bit of "needle work" on the chipping to ensure that the cross-over tendon is not hit.

Re long-term effects - provided that caution is taken with the actual chasing (the extruded sheath is not exposed, nor water is introduced into the chase) and the repair material is selected and installed correctly i would see no long term detrimental effects.

I would recommend a prebagged product for the repair and not a site mixed or mason's mix material. Pre-bagged materials are easier to site batch.

I would keep the chase as shallow as possible, and it may be worth considering a 2 component epoxy as the repair material - probably mixed with a dry silica sand. For very thin and long patches cementitious materials can be problematic.

I would be documenting all aspects of the chipping, chasing and install - with photographs - i would also consider that this aspect of work should carry a longer warranty period - the contractor made a mistake and an increased warranty period is not unreasonable.


HTH

 

[jheidt2543]

Another thing to keep in mind is the type of conduit being inserted into the slab. Some conduit materials are not compatible with concrete (aluminum in particular). Not only will the conduit chemically react and be damaged but, it could also damage the concrete in the area of the conduit. Just something to check.

Good luck.

 

[walkerlrcs]

Thanks to all for the good advice. I have located the locations of the PT tendons and marked them on the concrete slab. Now to get the builder to use the appropriate cautions mentioned in your replies -- there doesn't appear much apparent concern on his part. They plan to use a jackhammer to cut the trench. Is a jackhammer the appropriate method in this case? Isn't there a more cautious, less drastic way to cut the trench? thanks again for all your help thus far.

 

[Ingenuity]

walkerlrcs,

I would pre-cut the concrete on each side for the entire length of the proposed conduit location - for a depth not exceeding 1/2" to 3/4" - this will make the patch easier to detail and avoid "feather edging".

I would use a chipping/jack hammer but preferably about a 15 lb'er NOT a 90 lb'er!
You do not want to bash the concrete to death . The contractor will probably be using an electric-type which are hand held and fairly standard for chipping work.

HTH

 

[Jim6758]

Ingenuity is so right about precutting the slab. Failure to do so will likely spread micro-cracks throughout the area being chipped. Micro-cracks have a nasty habit of growing into sizeable cracks, especially if the slab moves due to clay swelling.

 

[walkerlrcs]

Thanks once again for all the input. I just had a meeting with the builder and the company who will do the work. As a result of the input above I was able to talk (semi)-intelligently about the issues/concerns associated with this operation. Now I am satisfied that they won't just attack the concrete without regard for the location of the PT cables - they will use a small hand-held power chipping tool - and they will use PVC conduit. Thanks for the help. Walkerlrcs