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Thu, April 18

Column: Prescott has two kinds of concrete: 'cracked and not cracked yet'

I have three hard questions this week; they're all about concrete.

Q: I have a post tension slab and I was told I cannot drill into the cement. I want to add an interior wall and need to nail it to the floor. Is this OK, and what is a post tension slab?

A: A post tension slab has tendons (cables) routed through it. The tendons are installed all the way through the slab before the concrete is poured. After the concrete is poured the tendons are tightened- hence the name post tension slab. The tendons will strengthen the slab and reduce the chance of significant cracking in the concrete.

If a home has a post tension slab there should be a stamp in the concrete in the garage floor stating something like "post tension slab - do not drill or core." If you drill into the slab and break one of the tendons, it could cause damage to the slab.

You said you wanted to make an interior wall and need to secure it to the floor. The main reason to secure an interior wall to the concrete floor would be to keep it from moving back and forth. So, if you use two-inch long nails they will only go into the concrete 1/2 inch (assuming the sole plate is 1-1/2 inch), and should not hit a tendon.

By the way, you have a concrete floor, not a cement floor. Cement refers to the mix - you buy bags of cement mix at the hardware store. Cement is just one component, when mixed with the other components (i.e. sand and water) it forms concrete.

Q: I have a year old home with a lot of cracks in my garage floor and patio. The builder says these are not a concern, but I'm not so sure. How many cracks can you have before it becomes a structural concern?

A: All concrete cracks. In fact, my inspection reports state "we only have two types of concrete in Prescott, cracked and not cracked yet." We call these common cracks, for obvious reasons. Concrete will shrink as it cures, and will crack. The grooves in a concrete slab are called control joints, and what they control is where the concrete cracks. The grooves will be the weakest point in the concrete, so the cracks should occur in these grooves.

How many cracks you get in a concrete slab depends on many things - the mix (did the truck driver or concrete finishers add more water to the concrete to make it easier to work with?), the weather when the concrete was poured, etc. The number of cracks does not determine if they are significant, as long they are all small (i.e. no wider than a dime, although in this market many home inspectors only have a penny to use). It is more important to check for displacement (unevenness) at the surface of the crack. Displacement would indicate the concrete on one side of the crack has moved up or down. This is not due to shrinkage and is no longer a common crack.

Technically, in your situation these would not be a 'structural concern' anyway. A garage floor and a patio are not structural components - they are just a floor and are not holding anything up. Your garage floor or patio could be dirt, wood, crushed beer cans or anything else you desire.

Q: The third question was actually part two of question two. It asked why the concrete porch and sidewalk were delaminating. There are two different conditions that can occur on the surface of concrete slabs. One is scaling and the other is spalling. Scaling is more like delaminating because it is basically the surface of the concrete failing.

A: Scaling is usually blamed on the freeze/thaw cycles. But that's not the underlying reason, because most concrete in our area does not scale.

Excessive scaling can be caused by any number of things when the concrete is poured, such as the wrong concrete mix (no air entrainment or adding too much water again), improper or over finishing, and insufficient curing. I won't try to describe all these in detail since there's nothing you can do about them at this point.

Once the concrete is poured, there are some things you can do to reduce the chance or spread of scaling. Never put salt or de-icers on a slab for the first year, only use sand if needed for traction. Always use salt or de-icers sparingly, and only ones approved for use on concrete. Some contain chemicals that can damage concrete. There is some debate about whether salt and chemical de-icers can damage concrete,

but most agree you should not use it in the first

year.

Never use fertilizers as a de-icer/ There's not much debate about this. Fertilizers, especially those with ammonium nitrate or ammonium sulfate, will very likely damage your concrete.

Sweep water or snow off the concrete surfaces as soon as possible in freezing weather. You might want to caulk cracks to keep water from getting in them and freezing, and you might want to seal the concrete with an appropriate sealer.

If the scaling has already progressed to the point that it's a major cosmetic concern or possibly even a trip hazard, there are ways to repair scaled concrete. These are best left to an appropriate professional. For one thing, the surface needs to be properly prepared by high pressure washing, sandblasting, etching or cleaning with acid. The concrete can be surfaced with special Portland cement, latex modified concrete resurfacing, or a Polymer-modified mortar. The professional will know what repair material will work best for your concrete and situation.

As you now know, concrete porches and sidewalks are not structural components. This is not to say you don't have a right to ask that this be corrected on a new slab if defective concrete or workmanship is the cause.

Randy West owns Professional Building Consultants in Prescott. West is president of the Arizona Chapter of the American Society of Home Inspectors and currently serves on the Home Inspector Rules and Standards Committee for the Arizona Board of Technical Registration. Contact him at editorial@prescottaz.com.

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