Dealing with sulphate attack in floor slabs

Remedial Works for Sulphate Attack in Solid Concrete Floor Slabs

New Kitchen Strip out Required to Facilitate Floor Slab Replacement

New Kitchen Strip out Required to Facilitate Floor Slab Replacement

Following on from our recent blog on investigating sulphate attack in solid concrete floor slabs; what we didn’t mention in that blog was that we got the lab results back, which proved what we suspected, that there were incredibly high levels of sulphates in the hardcore material and the failed concrete floor slab. What subsequently transpired was that the client asked us to tender and project manage the remedial works. Works were specified and put out for quotes to local building contractors; interestingly the spread in pricing was quite staggering, ranging from £15k to £36k. We got four quotes back before the client made a decision on who to award the work to.

Removing floor slab and other consequential works

Works are still ongoing and due for completion on August 19th but things are progressing incredibly well and the most complicated aspect is possibly the careful removal and refitting of a very expensive kitchen with Granite worktops. There is also a Bathroom to strip out and refit, a shower room and built in wardrobes in the bedroom.

Digging out sulphate contaminated hardcore material.

Digging out sulphate contaminated hardcore material.

We mentioned in the previous blog on this subject that we believed that leaking subfloor heating pipes had contributed towards accelerating the sulphate attack and indeed as the floors were excavated we noted a number of heavily corroded and leaking copper central heating pipes. These will all be cut out with new pipework runs being installed above the finished floor level.

Since the hardcore material was heavily contaminated with sulphates then it was critical that this material was excavated back to ground level and removed from site. We’d envisaged a slightly easier excavation process for the concrete since the original sampling area showed the concrete to be incredibly thin, however, the concrete proved to be circa three times thicker in some areas and showed a massive variance in thickness throughout the property.

Jablite insulation and upstands

Jablite insulation and upstands

The property is having to be done on a room by room basis since we have no external storage space for the kitchen and bathroom fitments, so the lounge was completed  first and  this has then provided the storage area for stripping out the kitchen and bathrooms.

The local authority inspector had his first inspection last week before the first concrete pour and was very pleased with what he found. You can see the Jablite polystyrene insulation and Jablite perimeter upstands, which of course are only installed to the external perimeter walls. The builders are fairly old school and are mixing the concrete on site as they go. The final image shows works well under way to complete the large section of flooring to the lounge.  In fact the lounge is now complete and is currently being used as a storage area for the kitchen and bathroom fitments and furniture. Works to this property are being completed under a CIOB Mini form of contract (general Use), which to my mind is far more suited to a project of this size than a JCT minor works.

Concrete being poured over 1200 gauge DPM

Concrete being poured over 1200 gauge DPM

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  • Freddie Cameron

    Great article regarding in floor designs and tips. I just want to say thank you so much for sharing your post.

  • Chris

    Hi Joe.

    I’m looking to buy a property whereby the sulphate levels in the hardcore infill are high (3x normal safe levels). This was found from a test being conducted on the back of it being in an area notorious for using unsuitable hardcore infill.

    The sellers of the property have had a damp proof membrane laid down classifying that as remedial work, however they’ve left the high sulphate infill in place. Would the damp proof membrane be enough to prevent any issues in the future or would the recommended action be to take the contaminated infill out as per this article? I’m not sure whether they’ve done what they can on the cheap in the hope it allows them to sell the house.

    Many thanks,


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