Answers to Slab and Foundation Questions #1

Q. The floor in our garage is currently made up of interlocking bricks (weird, but that is what they wanted when they built it). I recently hired a contractor to replace the bricks with a poured slab. What should I be looking for when he does his work to make sure he is not doing a shoddy job? Should there be some 10-test along the walls? Should there be some sort of vapor barrier under the slab?


You will need a proper base for the slab. The interlocking bricks may be on a couple of inches of sand, which suits the flexible nature of blocks. Concrete requires a bit more robust base or suitably undisturbed soil. Your building code will specify some of these. A 6-mil vapor barrier goes under the concrete, wire mesh reinforcement. Where the bricks may not have had a slope for drainage due to the open nature of interlocking pavers, the footer for the garage may need modifying to accommodate the slope of the concrete floor. If you paved the floor well, a concrete slab of reasonable thickness will certainly be OK with only base leveling before pouring.

Your installation was not weird. Quite often heavy equipment garages are surfaced with pavers. This is because they can get into the building immediately (rather than after 24 days of concrete curing) and the pavers can be replaced because of abuse by the heavy equipment, tools and caterpillar tracks. They also do it so they can easily change underground systems. Pavers are also used for runway edging and heavy-duty taxiways at airports.
Contrary to common belief, asphalt or paving stone surface needs a better (and thicker) base than concrete. The thickness and quality of the base give the flexible pavement its strength and the asphalt or paving stones merely add to the strength and provide the desired surface. Concrete will bridge over locally weak bases. The only problem comes with differential settlement between slabs or sections.

Q. I'm looking into pre-cast foundations for modular home application. The company says 5000psi, studded and insulated. I'm wondering if anyone had thoughts or experiences with them? What is an average cost for these foundations?

A. They have been around for about 30 or 40 years, but are still not common because of lack of flexibility, price and scheduling. Forget about the 5000-psi - that is just for advertising. They have that type of concrete because it is required to strip out of the molds and handle for inventory where they cure. The height of the house is not a factor, just the height of the soil behind the wall. There's no question about the ability to keep out soil and hold up a house. Typically, the advertised insulation values are a little high (R-value wise) since they are based on one portion of the wall and not an actual test. Still, insulation for a basement is not that critical since the soil is not as cold as the air. They have been pricey except for basic boxes without too many openings, grade changes or corners. From an engineering standpoint, they do the job.

Q. I recently acid stained my cement slab using the Kemiko acid Stain. I got the results I wanted overall from the stain. Once it was completely dried, I applied the Kemiko non-yellowing clear sealer with a low nap roller according to the instructions. I let the first coat dry and then applied the second coat. It has been about two weeks since I applied the sealer and there are white areas where I seemed to have put on too much sealer. My questions are the following:
1. Will this eventually fade after some time?
2. If no is the answer to the above question, then what is my best solution for getting rid of those white areas? I tried scrubbing with a steal brush with no success. Any other ideas?

A. The white spots are not from too much sealer. More than likely, it's from moisture from below. Another possibility is dirt (bird doo for example) that wasn't completely cleaned up. It could even be that you didn't acid wash before applying the stain or didn't neutralize the acid and wash it all away before staining. It's also possible they are fibers from the roller sleeve.
1. No they will not fade, at least if the sealer is any good.
2. You can't scrub them away if the sealer is any good. The spots are under the sealer. The easiest solution is to remove the sealer over the white spots (see label directions) and reseal those areas.

Q. I have started to build an area of approx 500 sq ft using cmu and concrete. The footing and cmu wall are completed but the slab is not poured. The wall is fully grouted with drainage pipe and emulsion type waterproofing along the outside perimeter. I could use some advice on how to go about pouring the slab so it will be watertight and strong.

A. Here is a rundown on pouring a concrete slab: Usually, the slab would be poured up to the footers. Ordinarily, there is no joint inserted between the slab and the footer. 6-mil plastic is usually placed on the packed soil, under the reinforcement wire or rebar. The plastic keeps the moisture out of the slab. If there is a drainage problem, it should be corrected. Keeping the water away is cheaper and more effective than trying to keep it out of the structure, otherwise.

Q. 1. On the exterior of the house, there are some wide cracks (maybe 1/2") between some of the stones. Are these signs of other issues, or normal issues with stone foundations that can simply be repointed with mortar?

2. I have a small retaining wall running along the foundation (about 2' from the foundation, running along it for about 20'). I was hoping to plant some flowers or shrubs, but I'm worried about introducing moisture directly to that area near the foundation. I see near where the dirt line meets the foundation some large gaps in the foundation. Should I clear out the dirt, fill gaps with concrete, and then put the dirt back and plant? Should I plant at all?

A. If the cracks are in the mortar, they are most likely the result of settling and can be repointed. If your foundation wall is a curtain wall, that is, not structural, piling dirt against it may be a bad idea. In any event, piling dirt against the wall may well cause problems with moisture. Ramping some soil against the wall, say a couple of inches, to improve drainage would be beneficial because the intruding water is not good otherwise. If you plant near the wall, keep the plants from touching the wall to avoid transpiration of moisture from the plants to the wall.

Q. This is an older home which has a block foundation. The blocks have buckled inward slightly and have slight separation due to freeze and thaw. How do I go about repair?

A. There is no repair procedure for this type of problem. Best to tear the old wall down, and put in a wall that will not buckle. You can also place another wall in front of the buckling wall to stop the first wall from getting worse.

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