A plumber needs to trench through the floor to reach a leak, or a remodel calls for a new floor drain in the garage. Both jobs start with cutting or drilling concrete, and on a post-tensioned slab that step carries a hazard ordinary concrete doesn’t.
This post explains what a post-tensioned slab is, how you might find out whether your house has one, what cracks in one can and can’t tell you, and what should happen before anyone puts a saw or drill to it. It’s written for La Jolla homeowners citywide. It isn’t a repair method guide, and it doesn’t replace an engineer.
What a post-tensioned slab is
A conventional slab relies on concrete and, where used, reinforcing bars that sit passively in it. A post-tensioned slab has steel tendons running through it in plastic sheathing. After the concrete is placed and gains strength, the tendons are pulled tight and anchored at the slab edges, squeezing the slab together.
Ken Bondy’s technical note for the Post-Tensioning Institute (PTI) describes these slabs this way: “Post-tensioned slabs are widely used as foundations for light residential construction in areas of expansive soils.” He adds that they are “lightly post-tensioned with unbonded tendons,” and that “A ground-supported post-tensioned slab acts as a buffer which reduces differential deformations between the soil below and the superstructure above.”
California’s building code ties the design to PTI’s standard. Section 1808.6.2 of the 2025 California Building Code says “post-tensioned slabs-on-ground, mat or raft foundations on expansive soils shall be designed in accordance with PTI DC 10.5,” and the design methods it allows have to account for “both center lift and edge lift conditions.” Those are the two ways expansive soil can move under a slab: wetter, swelling soil at the edges (edge lift), or the edges drying and dropping relative to the middle, which Bondy says is “more appropriately called edge drop.”
Our foundation types in older La Jolla homes post places post-tensioned slabs alongside the other systems you’ll find around La Jolla.
How you might find out whether you have one
From inside the house, a post-tensioned slab looks like any other slab. Some clues help, but none of them settles it alone:
- A stamped warning. A City of Palo Alto inspection handout notes that, “As a recommendation by the Post-Tensioning Institute (PTI),” plans should tell the contractor to stamp the slab “POST TENSIONED SLAB – DO NOT DRILL, CUT, OR CORE.” Garage floors are a good place to look. A stamp tells you something. A missing stamp tells you nothing, since it could have been covered by flooring or left off.
- Patched pockets at the slab edge. A construction safety talk on post-tensioned slabs says that on these slabs “there will be a hole in the foundation or slab that contains a cable end,” and advises: “Always look to see if the cable ends have been mudded over by some form of concrete patch filler.” The same talk warns that “Stucco and bricks may cover cable grout making it difficult to identify post tensioned slabs.”
- Plans and records. Structural plans and permit records, available through the City of San Diego Development Services Department’s records service, may show the foundation design. Builder documents and HOA files sometimes do too.
If there’s any doubt, treat the slab as post-tensioned until someone qualified confirms otherwise.
Why cutting or drilling is the hazard
The tendons hold a great deal of stored force. Cutting one releases it. Don Kline, writing in the International Concrete Repair Institute’s Concrete Repair Bulletin, puts it directly: “Detensioning by means of a grinder, saw, or torch results in a sudden release of energy in the slab.” The safety talk is blunter: “Do NOT cut into a post tensioned slab if there is a chance you will rupture a cable,” and “People have been dismembered and killed when cables are cut and burst out of the concrete.”
Damage to the slab is the other risk. A cut tendon no longer compresses the concrete it was designed to compress, and the slab’s behavior near the cut can change.
That’s why the guidance lines up on one point: locate first. Kline writes that before cutting small penetrations, “existing PT strands should be located using non destructive testing (NDT) equipment such as ground-penetrating radar (GPR).” For larger openings, “An engineer should be consulted to analyze the effect that the new opening will have on the slab.” The safety talk says “a competent or qualified person needs to identify the cable locations before any slab work begins.”
Kline also notes the work is doable when planned: “Modifications to existing PT slabs can be made safely and economically when the project team includes an engineer and a specialty contractor experienced in this type of application.” So the goal is planning the work, not avoiding it. A post-tensioned slab isn’t untouchable. It just isn’t a job for a rented core drill.
Our concrete leveling vs structural repair post covers one place this comes up, since injection work drills through slabs.
What cracks in a post-tensioned slab can mean
Search “post tensioned slab foundation cracks” and it’s easy to come away thinking any crack is a failure. Bondy’s technical note pushes back on that. Of these slabs he writes that “some cracks can be found in virtually all of them,” and that this “suggests that cracking is not detrimental to their structural behavior.” He also cautions that “the ramifications of cracking must be carefully considered.”
He sorts cracks by cause:
- Restraint-to-shortening (RTS) cracks. Bondy calls these “The most common type of cracking found in post-tensioned slabs on ground.” He writes that “They occur shortly after the concrete is placed, often before the tendons are stressed,” and “They are substantially vertical and penetrate the entire depth of the concrete slab.” His conclusion is that “RTS cracks are of no structural consequence in post-tensioned slabs on ground.”
- Flexural cracks from soil movement or loads. These are “located at points of maximum slab moments,” and “They do not penetrate the full depth of the slab.”
Location and direction carry the weight in his analysis. He writes that “everything structurally significant that happens in post-tensioned ground-supported slabs happens between the slab edge and a point just inside” a design line set back from the perimeter. Within that perimeter zone, the cracks with potential structural consequence are the ones running parallel to the slab edge.
Two cautions. Bondy’s note was written for design professionals, and the size of that perimeter zone comes from the slab’s design, not a rule of thumb. A homeowner can’t sort RTS from flexural cracks by looking at tile. What you can do is record where cracks are relative to the outside walls, which way they run, and whether they’re changing. That record is useful to the engineer who makes the call. If you’re tracking cracks, our hub on foundation crack repair explains how cracks are evaluated, and slab foundation repair covers slab systems.
Keeping water in check
Post-tensioned slabs on expansive soil are designed around soil moisture behaving as expected. A PTI convention presentation on construction and maintenance puts site maintenance on the property owner and lists a few points: “Do not alter the drainage pattern of the site,” and “Roof drains should not discharge water at the perimeter of the foundation.” It says long-term slab performance depends on good drainage and a moisture maintenance program.
Practical habits include keeping downspouts discharging away from the house, watching for new low spots against the foundation after landscaping, and fixing irrigation leaks near the slab edge. Our exterior drainage maintenance calendar lays out a year of checks.
Questions to ask before slab work
Before a plumber, remodeler, or anyone else cuts or drills:
- Has anyone confirmed whether this slab is post-tensioned, and how?
- If it is, how will tendon locations be identified before work starts?
- Is an engineer reviewing the opening, and who is it?
- Is the crew experienced with post-tensioned slabs?
- Does the work need a permit?
A contractor who can’t answer the first two shouldn’t be cutting. Our structural inspection and structural engineer pages explain when an engineer belongs on the job, and when to involve a structural engineer goes into more detail.
Cost factors, not prices
We don’t publish prices. What shapes the cost of work on or around a post-tensioned slab: whether the slab type has to be confirmed first, tendon locating (such as radar scanning), the size and number of openings, engineering review, the specialty crew required, flooring and finish removal and restoration, permits, and any drainage corrections that come first.
Where we fit
We’re a foundation and structural repair company serving La Jolla citywide. On a visit we can document slab cracks, drainage, and the clues that point toward a post-tensioned slab, and explain what we see during a foundation inspection. We aren’t engineers, and we don’t locate tendons or design openings ourselves. We work with verified contractors and coordinate with licensed engineers as needed. This post is general education, not an assessment of any property.
Next step
If you have slab work coming up or cracks you want looked at, request a Free Quote for an on-site visit anywhere in La Jolla, or call (650) 739-6537. Hold off on cutting or drilling until the slab type is confirmed.
Sources (accessed October 26, 2026)
- Post-Tensioning Institute, Technical Notes Issue 6, Ken Bondy, “Cracking in Ground-Supported Post-Tensioned Slabs on Expansive Soils” (1995)
- 2025 California Building Code, Section 1808.6.2 Slab-on-Ground Foundations (via UpCodes)
- City of Palo Alto Development Services, “Inspection Requirements: Post Tension Slab”
- Don Kline, “Creating Openings in Unbonded Post-Tensioned Slabs,” International Concrete Repair Institute, Concrete Repair Bulletin (May/June 2013)
- Safety Toolbox Talks, “Hazards of Post Tensioned Concrete Slabs”
- Post-Tensioning Institute convention presentation, “Construction & Maintenance of Post-Tensioned Slab-on-Ground Foundations” (2015)
- City of San Diego Development Services, Records