Houston slabs often use ‘Post-Tension’ cables (steel tendons under high tension) instead of rebar. If water enters the cable sheath (via a ‘pop-out’ or crack), the cable corrodes and can snap with explosive force, compromising the home’s structural integrity. Signs include rusty cable ends on the foundation perimeter or localized heaving. Repair involves re-tensioning and epoxy encapsulation.
‘That rusty wire isn’t trash; it’s holding your house together. If a post-tension cable snaps, your foundation loses its spine.’
— Structural Forensic Engineer
In the Houston area, monolithic slabs are the standard. Unlike traditional rebar-reinforced concrete, post-tensioned slabs rely on a grid of high-strength steel tendons. These cables are pulled to high tension after the concrete has cured, providing the compressive strength needed to resist the movement of volatile clay soils.
The first sign of trouble is often a small patch of concrete breaking away from the edge of your slab, revealing a rusty wire. This is known as a “pop-out.” When the protective grease inside the cable’s plastic sheath fails or the sheath itself is breached, oxidation begins. As the steel rusts, it expands, putting pressure on the concrete until it blows out the end of the cable pocket.
| Symptom | Cause | Urgency |
|---|---|---|
| Rusty Cable End | Sheath Failure | High |
| Loud ‘Bang’ | Cable Snap | Critical |
| Slab Cracking | Loss of Tension | High |
Repairing a compromised tendon is a highly technical process that involves more than just patching the concrete. Contractors must excavate around the cable, cut back the corroded section, and install a new “splice” using a specialized coupler. Once the new section is integrated, the tendon is re-tensioned to its original design specifications, typically around 33,000 lbs of force, and then sealed with epoxy to prevent future moisture intrusion.
The foundation loses structural rigidity, leading to severe cracking and instability. It can also cause physical injury if it snaps explosively.
Understanding the soil beneath your slab: Montmorillonite Clay Mechanics and Slab Heave.
