As a former PT manager, and PT engineer executive ...

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I've seen alot of concern about post-tensioned residential slabs. Most people don't know about how severe the concrete will crack during curing. The normal process of curing envolves a shrinking of the over-all dimension of the foundation. In flatwork this isn't a big problem because it will just slide together as it shrinks, however ground supported foundations won't slide. The trenched grade beams act like cleets in the soil, restraining the foundation from shrinking towards the middle ... thus the foundation will crack in the middle and slide towards the outside. The technical term for this problem is a "Restraint To Shortening" or "RTS" crack.

RTS cracks happen at the center or quadrapoints of the long or short deminsion of the post-tensioned house. They are typically perpendicular to the circumfrance of the house. RTS cracks are not superficial, and typically section the (otherwise monolithic) foundation into multiple parts. RTS cracks are frequent in foundations that have one or more overall deminsion in the footprint of the house that exceeds 35 feet (even if the RTS crack is not noticed.) RTS cracks tend to arrive at weak/thin points of the slab (for instance the crack will be coincidental with the location of a plumbing penetration or cable placement, or just a thinner part of the slab.) RTS cracks are cosmetic problems.

RTS cracks are typically overcome by the stressing opperation if the tendons are sufficiently stressed. Grit may keep the crack from "closing-up" if the crack was exposed to saw-dust or sand before the cables were stressed. Filth from the construction of the superstructure may conceal the existence of the crack until later stages of construction, but the cracks will not occur after the stressing operation is complete. RTS cracks do not have virtical displacement across the crack (one side is not at a higher elevation then the other.) RTS cracks are not structurally significant, and the do not foretell of structural failure.

The RTS problem is magnified by three main factors. First, house foundations are not normally etched or scored to control the cracking in nice straight lines (like the lines in sidewalks or driveways.) Secondly, most of the shrinking of the slab is done before the cables are tensioned, and thus the cracks can show up during the first week, while the slab is un-reinforced. Thirdly, residential homes do not typically envolve "pre-stressing" which applies a fraction of the stressing to the tendons a day or two after concrete placement so that there is compression during the first week of hardening.

My advice is to shoot a benchmark LEVELNESS/FLATNESS of the foundation elevations throughout the house (20 or so readings.) So that you can measure how much your slab is flexing if it is flexing at all. That way you have registered the initial deformity of the slab to compare with later readings. Your problem with the brick might be due to foundation flexure, or it might not. There are limitless reasons for a crack in the brick, and most of them have nothing to do with floor movement. Maybe you simply have a sap-laden green limb that is warping and buckling behind your brittle exterior. Then you would not want to blame the foundation for something that is caused by a member of the stick frame.

For more information about cracking in foundation that are reinforced by post-tension cables, I suggest the following resources:

Cracking in Ground-Supported Post-Tensioned Slabs on Expansive Soils

Confessions of a warranty
inspector during foundation inspection