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Concrete answers on post tensions slabs

Question: I have a label in my garage floor saying I have a post tension slab – “do not cut or drill.” I want to build some interior walls in my home. I was planning on securing the walls to the floor with a ram set. Is there any concern using a ram set on a post tension slab? (And by the way, what exactly is a post tension slab)?

Answer: I’ve been asked about post tension slabs before. Before I explain post tension slabs, I need to explain two forces. These forces are tension and compression. These are very important in the construction industry, and a home inspector must be very aware of these forces. Compression and tension affect any building material in different ways.

An easy way to explain this would be compression is trying to squeeze something together, and tension is trying to pull something apart. Some materials have better compression strength than tension strength, and vice versa. Some materials in a building are under both forces at the same time. For example a beam that is sagging in the center has compression along the top and tension along the bottom.

Concrete has very good compression strength. It’s pretty darn hard to damage concrete by compressing it, (although not impossible). Concrete does not have good tension strength. Concrete can be damaged by pulling it apart easier than by pushing it together. So concrete makes a great column, since gravity will ensure the column is under compression. And concrete makes a good driveway to support heavy vehicles. But a driveway can be damaged by a heavy vehicle on the edges – because it does not have good tension strength.

Now think of a steel cable, which has excellent tension strength. Imagine two people having a tug of war with a steel cable. The cable will not “give,” the stronger person will pull/move the weaker person. But if those two people walk toward each other, the cable will simply sag because it has virtually no compression strength.

So if we put a cable with excellent tension strength in a concrete slab that has good compression strength, we should end up with stronger concrete. We have always done this to some extent, usually with rebar (metal bars) in the concrete. In fact, rebar is short for reinforcing bar.

Using cables, which are actually called tendons, should provide more strength than rebar. The tendons are in plastic sleeves, and protrude from each end of the slab. The tendons are tightened after the concrete is partially cured. Because the tendons are literally tightened and under tension, they provide more tension strength than rebar. The “post” in post tension means the tension was put on the tendons after (post) the concrete was poured.

There is usually a stamp in the garage floor stating there is a post tension slab and saying not to drill into the concrete. You can usually spot a post tension slab without seeing the stamp. There will be small round patches on the outside edges of the slab. This is where they tightened and then cut off the tendons.

I would never drill or saw into a post tension slab. The tendons are under a lot of tension. If you cut through a tendon there is a possibility of injury, and it will weaken the concrete. The question was regarding a ram set. This is a device that shoots nails into concrete using powder shell similar to a gun. I personally would not use a ram set either. If you are adding interior walls and they are not bearing walls (e.g. they are not supporting the weight above), I would clean the concrete under the new wall thoroughly, and use adhesive to secure the new wall to the concrete. Newer adhesives are amazingly effective.

Assuming the adhesive is effective, the only reason you would need nails would be to resist extreme lateral movement. So unless the wall is a backstop for football practice, or is in a location where the kid with poor brakes parks his car, the adhesive should be adequate. If I was really concerned about lateral movement of the new wall, I would use an adhesive and perhaps install a few short concrete nails. I would make sure the nails don’t go more than a 1/4 inch into the concrete. But I strongly recommend not using any nails or mechanical fasteners on a post tension slab.

I heard somebody comment recently that gutters were not needed on his home because the home has a post tension slab that would not settle. This is inaccurate.

Post tension slabs are used more often where there is expansive soil. We have expansive soil in Arizona, and, as a matter of fact, we have expansive soil in Prescott and Prescott Valley. So we have some homes with post tension slabs in our area.

Post tension slabs are less susceptible to movement but not immune. And site drainage is just as important on homes with post tension slabs. In fact, it could be argued, site drainage is more important because the post tension slab suggests there may be expansive soil.

No masonry product is waterproof. Water will seep through concrete, blocks, bricks, stucco, etc. So if there is water ponding against your home or foundation, the water can seep through into the soil under the slab. If there is expansive soil, water can cause the soil to expand and damage any slab, including a post tension slab. It is very important to maintain good drainage around any home, no matter what type of foundation or slab you have.