A post tension slab is not an ordinary concrete floor. Inside it run steel tendons, high-strength cables each holding roughly 26 kip of force after losses, and that force is what keeps the slab in compression, controls its cracking and limits its deflection. Cut one and the energy releases at once. The strand can burst out of the slab edge, and the slab loses part of the compression it was designed around.
A plumber routing a drain, an electrician cutting a conduit trench: routine in an ordinary slab, a serious hazard in a post tension slab.
The five mistakes below are the failure points we see most often on post tension slab projects across Texas. If you are a homeowner, the short version is this: do not cut, core or drill a PT slab yourself, and do not let anyone do it without a scan and an engineer's approval.
First: Which Kind of Post Tension Slab Is It?
Most Texas houses sit on a post tension slab on ground. Its tendons usually run in both directions at a fairly even spacing, about 3 to 4 ft apart, through a slab of about 5 in with deeper stiffening ribs, according to STRUCTURE magazine's guide to PT slabs on ground (2010). Elevated slabs in condos and garages are different: tendons are often bunched in bands along the column lines in one direction and spread out in the other. Where the tendons are depends on which slab you have.
Mistake 1: Skipping the Post Tension Slab Scan
This is the most common error we encounter, and it has the most immediate consequences. Contractors estimate tendon positions from drawings or typical layouts, but tendons shift during placement, especially around blockouts and plumbing rough-ins. No drawing records final positions in a post tension slab precisely enough for a safe cut.
Ground-penetrating radar (GPR), run by a qualified scanning technician, maps tendons, rebar and conduit before anyone cuts. STRUCTURE magazine's guidance on existing PT foundations calls for scanning and marking strand positions before drilling, in its article on existing foundations (2010).
- What it does well: locates tendons and rebar without damaging the slab, with results marked directly on the surface.
- What to watch: telling a tendon from rebar in congested zones takes an experienced operator, and it is not a do-it-yourself tool.
On a residential slab in a Dallas-area home built in the late 1990s, GPR shifted proposed cut lines by as much as 8 in from the positions estimated from the original permit drawings. That gap is the difference between a clean penetration and a severed tendon. Our full field procedure is in the checklist to locate and avoid post-tension cables before drilling.
Mistake 2: Cutting Near a Post Tension Slab Anchorage
Each tendon ends in an anchor at the slab edge, usually visible as a small patched pocket. The concrete around it carries the full tendon force in bearing and is the most highly stressed concrete in the slab. Cutting there can crack the anchor zone, dislodge the plate or cut the strand just behind it, and the tendon loses its force.
The STRUCTURE article on existing foundations recommends keeping penetrations about 24 to 36 in from the anchor face. Locate every pocket before setting a cut line. Why anchors sit where they do is explained in how anchorage placement affects load distribution in a post tension slab.
Mistake 3: Cutting Through a Tendon Band
In an elevated post tension slab, banded tendons run grouped together along the column lines. A trench or large core through a band can cut several tendons at once, and losing even one changes how the slab carries load along that line. Route new penetrations through the zones between bands, where tendons are widest apart. Any opening in or near a band needs a structural engineer's review first.
Mistake 4: Cutting With No Repair Plan
Even with a clean scan, the engineer of record or a qualified engineer should approve the penetration before work starts, and a repair plan should exist for the case nobody wants: a nicked or severed tendon.
We have seen post tension slab projects where a saw blade grazed the plastic sheathing of an unbonded tendon without cutting the strand. The strand runs in grease inside that sheathing; a nick lets moisture in and starts corrosion that is invisible from the surface and can weaken the tendon over years.
Repairs of unbonded tendons typically involve exposing the damaged length, splicing in new strand with a coupler, re-stressing and restoring the corrosion protection. That is specialist work. The Post-Tensioning Institute certifies personnel in unbonded PT repair, and its guides PTI DC80.3 and DC80.4 cover evaluation and repair of existing unbonded structures. Ask any repair contractor for PTI repair certification.
Mistake 5: Trusting the Original Drawings Alone
This one catches experienced contractors. Original PT drawings show design intent at permit time. They do not show tendons rerouted around plumbing, profiles adjusted on the day of the pour, or bands shifted after a late column change. In Texas, where residential PT slabs are poured quickly, those field deviations are common and often undocumented.
Old drawings also reference grid lines and walls that may no longer exist after a remodel. A dimension from a column on a 1988 drawing is hard to find once that column is buried in a wall. The defensible workflow: get the drawings, use them to plan the scan, scan, reconcile the two, and document every discrepancy.
Why the Tendons Matter So Much
In a post tension slab, the tendons are the primary reinforcement. ACI 318-19 Section 8.6.2.1 requires at least 125 psi of average precompression in two-way PT slabs. A slab designed close to that minimum has little reserve to lose a tendon, which is why a cut is an engineering problem, not just a repair.
Post Tension Slab Pre-Cut Checklist
- Gather the original PT drawings, stressing records and any as-built markups.
- Commission a GPR scan from a qualified technician and ask for a marked-up plan of everything detected.
- Reconcile the scan with the drawings and investigate discrepancies of more than 3 in before proceeding.
- Mark every anchor pocket on the edge and keep cuts well clear of anchor zones.
- Route penetrations between bands; get written engineering approval for anything near one.
- Line up a PTI-certified repair contractor and materials before cutting starts.
- Document findings and approvals, and keep them with the property records for the next person who cuts the post tension slab.
Before Anyone Picks Up a Saw
The tendons you are protecting were stressed under strict procedures, set out in our contractor's checklist for stressing and grouting and post-tension slab stressing safety protocols. Slab geometry is covered in the maximum recommended span for a residential post-tension slab.
This article is general guidance; it does not replace an on-site scan or a licensed engineer's approval of your specific slab. Our sourcing rules are in our editorial policy. If you already have a scan report or a repair quote and want a second opinion, a technical consultation on cutting or repairing a post-tensioned slab reviews it with you before work starts.
