Replies

1. 
   Rich_ | Mar 02, 2002 09:55am | #1

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   I suppose this is in a concrete slab on grade for a residence? I further suppose that you are a layman. With that as a premise, I ask how you are able to determine that the concrete is a "weak mix"? IOW, how are able to make such a determination?

   You need to define what constitutes a weak mix and what exactly is dusting?

   If you have a defective slab you should not expect to proceed with tile installation. What constitutes a defect is defined by many varied criteria. You should provide much more information here--and identify the source thereof--before a useful answer can be given.

   For starters, provide geographical region, temp on day of pour and following month, age of concrete since installation, other 'defects' noticed, any testing of concrete, and any other thing you might think pertinent. Pretty much everything you know about the concrete would get the ball rolling.

   Don't mean to sound patronizing but have to be careful. An inaccurate answer is worse than no answer.

1. 
   IanDGilham_ | Mar 02, 2002 10:59am | #2

   *Some of the floor areas are dusting because of a weak mix.The usual reasons for the surface of finished concrete 'dusting' are that it has been rained on after laying and before setting or that it has had water added to the surface during trowelling -- both create a weak, sandy surface. The cure for this condition is to grind the surface of the concrete back to remove the weak portion and re-surface with a self-levelling compound.If you are correct that the problem is caused by a weak mix then that is a major structural defect and requires an engineer to inspect and develop a solution.If you are unsure which of these conditions apply it is best to have a qualified person inspect the slab.

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   teresa_greer | Mar 03, 2002 08:23am | #3

   *IanD,&Rich: We batched the sand and portland on site as a topping to cover our radiant tubing over an existing slab in good condition.What I meant by weak was the mix between the sand and portland was not consistent.We should have used a mortor mixer instead of a concrete mixer.The mix was rich--six bag with very little surface work until setup was complete. Don L.

1. 
   IanDGilham_ | Mar 03, 2002 09:58pm | #4

   *In that case I wouldn't lay ceramic on top of the faulty areas -- especially with radiant floor heating and the associated expansion/contraction.As far as I know, there is nothing that will bond together a weak slab, I'm afraid that you'll end up having to replace the weak spots.

1. 
   Rich_ | Mar 04, 2002 04:34am | #5

   *Okay, the way I understand this is that you have placed a cement topping over an existing concrete slab. This work was done to cover your radiant heating system. This sand/cement topcoat was field mixed and placed by labor you provided.Now, more questions: was this work done in preparation for tiling or was the tile an afterthought? How thick is the pour you placed over the concrete slab? How did you prepare the existing concrete for this cement topcoat? Besides sand and cement are there any other materials used in the topcoat? How much water did you add to the sand/cement mix? Did you test for compaction? How was the topcoat worked/tooled into place? What was ambient temperature during the work period? While your answers to those questions are important I'm going to go out on a limb and hazard a guess. You probably did not add enough water to the sand/cement mix. Also, you did not allow enough time for what water there was to fully hydrate the cement. Further, either the temperature that day was at least 70F or the air was quite dry. Both would have the effect of reducing an already amount of water by evaporation. Even further, by not having enough water in the mix the mixer was producing 'flour balls' in the mix. You attempted to flatten these into the topcoat but they were never fully hydrated, hence the dusting.I could be way off base but it sounds like not enough water in the mix. At this point I would hazard another guess. Either the dusting is superficial and easily remedied or the dusting is a sign of very weak spots perhaps as deep as the thickness of the topcoat.I'll wait for your answers before I further expose myself to foolishness. BTW: I disagree w/Ian about tiling over radiant heat. I have successfully done this for years and such work is within the designed use of such a floor heating system.

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   teresa_greer | Mar 04, 2002 06:16am | #6

   *Rich;The surface of these portions of the slab can be raked off using a sharp object down to about 1/8th inch.Below that is solid.Have heard about sealers,some that contain sodium silacate(liguid glass) that will cure our problem.Was hoping our original post might expose someone who had solved this problem in other ways Thanks,Don

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   IanDGilham_ | Mar 04, 2002 06:39am | #7

   *Rich,Please read my post properly instead of correcting what I DIDN'T say.

4. 
   Rich_ | Mar 04, 2002 08:54am | #8

   *Ian, my bad.

5. 
   Dr._'Crete | Mar 05, 2002 07:05pm | #9

   *RE: Dusty topcoat. If the problem is spots of dust (efflourecense in concrete talk) the first culprit is a funky mix or finishing technique, as stated above. The next suspect is water percolating through the old slab, the topcoat carrying lime with it. The clue here is a seasonal problem - does the dust appear some time after spring run-off? Check for water by setiing a piece of poly over the spot for a few days. On well-cured concrete there should be little or no water condensed on the poly. If there is, the cost of a fix just went up - hold off on the tile.If no water appears, try a slurry of Portland cement, carpenter's glue (Elmer's)and water. Prep the surface by scouring with a wire brush, sucking up the dust and painting on glue diluited with a little water. Follow with a coat of the slurry. This should give you something your tile substrate will stick to.Oh, there is an old saying in the world of concrete: an empty drum makes the most noise...The Ole Concrete Doc.