Expansive soil in Phoenix and the East Valley
Post tension slabs are used to create a slab that is stronger than the typical slab and foundation that is poured in stages. Post tension slabs are used in many parts of the country and Arizona where the soil has high shrink / swell potential, commonly know as expansive soil. Parts of the Phoenix area have soil that is expansive and exhibits a high shrink / swell potential. The USDA has published a shrink / swell potential map for Phoenix which identifies the cities or towns most affected by this condition. Cave Creek, Gilbert, Chandler, and north Phoenix, and parts of north Scottsdale are such areas.
Installing Post Tensioned Slabs
Concrete performs extremely well under compression (pushed together), but poorly in tension (pulled apart). The cables or tendons are stretched with hydraulic jacks up to 25,000 psi and clamped at the ends. The result is the tendons constantly keep the concrete in compression forming a stronger more reliable foundation and slab.
The plastic sheathed cables are positioned in the slab at the correct height and distance using “chairs” or little cone shaped supports. The plastic coating allows the cables to move within the concrete after it is poured and stretched. This is why you may also hear the term unbonded, since the cables can move independently of the slab. Once the concrete is poured and has reached approximately 75% of the desired strength, the cables are stretched using a hydraulic jack. Applying the pressure to the concrete after it is poured and cured is the “post tension” part of the process and name. The cables are then anchored at the ends and allowed to cure.
Identifying Post Tension Slabs In A House?
Since the cables or tendons in the slab have been stretched to high 
tensile strengths, it is extremely important that you do not drill, cut, chisel or do anything to expose the tendons. An easy way to verify if the slab is post tension is to go to the edge of the garage by the garage door and check for stamped warning in the concrete similar to the one on the right.
What About Houses Without A Post Tension Slab?
The majority of the homes in Phoenix do not have post tension slabs. Home builders may choose not to use post tension slabs if the house is not in a high risk area. As you'll notice on the map, many areas and cities are considered low risk. The best approach is to consult the shrink / swell map of the Phoenix area to identify the areas where it would be a concern. If the house uses a slab on grade type of foundation, look for severe settling cracks, especially if they are recent. Settling cracks that have not moved for a long time pose less of a concern. A qualified inspector can also be a good source of information.
A simple yet critical practive that you can control is to keep water away from the house. Many home builders suggest keeping sprinklers and landscaping at least 12 – 24 inches away from the side of the house.
Being aware of the expansive soil condition is the first step. Looking for excessive settling cracks and continued settling is important. Using a qualified inspector is key and ultimately keeping water a safe distance from the perimeter of the home will help avoid further risk.
For further questions or assistance in searching for homes, feel free to call me at 480-326-8571. For homes with in law suites, guest quarters or casitas, go to East Valley Guest Quarter Homes.
