Cracking is the most common issue experienced with slab-on-grade foundations. Even under the best conditions, concrete naturally forms cracks as it shrinks during the curing stage. These shrinkage cracks appear as a displacement or separation in the surface of the concrete and are hard to prevent. While shrinkage cracks aren’t detrimental to the performance of a slab, they can be unsightly and cause cosmetic damage to interior floor finishes like tile, wood, vinyl, and carpeting.
Expansive soils
Certain regions of the U.S. make constructing a traditional slab-on-grade foundation difficult. These regions have challenging soil conditions that can lead to costly problems with the foundation. One type of soil that’s particularly challenging is expansive soil. Expansive soil shrinks and swells as the water content changes, creating serious problems for the home, especially in the form of cracks. If the cracks run deep, they can reduce the slab’s strength and durability. To prevent these problems, it’s common for builders to reinforce and strengthen a slab-on-grade foundation through a process called post-tensioning.
Post-tensioning
To post-tension a slab, steel strands are run through the concrete in a grid pattern. Each strand is housed inside plastic sheathing that protects the steel from corrosion and allows it to move during the stressing operation. Each strand is gripped by an anchorage on each end, one of which is called the fixed-end anchorage and the other the stressing-end anchorage. The combination of the strand, sheathing, and anchorages is called a tendon. In some cases, tendons are stressed from both ends, but the more common technique is to stress from just one end. As the tendon is stressed, the steel stretches and is held in place by the anchorage, keeping the tendon tensioned for the life of the foundation.
The post-tensioning process compresses the concrete, enabling it to resist forces like expansive soil that would normally cause cracks to open up. However, for the tensioning process to work, the foundation has to be designed and constructed properly. Otherwise, issues like tendon failures, concrete blowouts, and cracking are likely to develop, leading to costly repairs and unhappy homeowners down the road. If the foundation is designed and constructed properly, it’s then essential to make sure no errors are made during the post-tensioning process. Certain errors in particular can lead to cracking, including improperly supporting the tendons, stressing the tendons too late, and incorrectly installing the slip sheet.
Installation error: Improperly supporting the tendons
Tendons should be properly supported on chairs. If a tendon isn't properly supported, it won't be at the correct position within the slab once concrete is poured. As a result, the stressing operation will cause the tendon to lift the slab in localized areas, creating stresses that appear as cracks in the top of the concrete.
To prevent this issue, inspect the position of each tendon before the concrete is poured. Make sure they're properly supported on chairs. The supports should be placed at each tendon intersection and halfway between any intersections where the space between tendons exceeds 4'6". If cracks appear in the slab after it's been poured, install a crack isolation membrane on top of the slab before installing the floor finish.
Installation error: Stressing the tendons too late
Stressing the tendons too late means the slab has already advanced significantly through the curing stage and plastic cracks have opened up. At this point, the slab is too strong for the stressing operation to close the cracks.
To prevent this problem, start the stressing operation as soon as the concrete attains the proper strength. Typically, this strength is reached during the first 4 to 10 days of the curing stage. If plastic cracks have already occurred, install a crack isolation membrane on top of the slab before installing the floor finish.
Installation error: Incorrect installation of the slip sheet
The slip sheet (10 mil polyethylene) reduces the friction between the slab and bearing surface, allowing the tensioning of the tendons to move the slab smoothly across the ground. When the slip sheet is installed incorrectly, such as beneath the gravel layer or on top of an uneven surface, the slab will resist movement, preventing post-tensioning from closing any cracks.
To prevent this issue from occurring, make sure the bearing surface beneath the slip sheet is level and that the slip sheet fully covers the foundation footprint. Overlap all seams in the slip sheet at least 12", and seal the seams with 3" construction tape. Make sure the slip sheet is directly under the slab.
In the end, always remember to carefully inspect a post-tension slab as it’s installed. If you find errors or have concerns about its construction, discuss them with the design engineer. With foundations, it’s very important to take care of any issues before it’s too late.
