Often it is necessary to cut holes in concrete floor slabs. For slabs reinforced with rebar or wire mesh, this is considered fairly routine. However, for post-tensioned (PT) slabs, this can be mistakenly regarded as difficult, expensive, and dangerous. This misconception is perpetuated because the field procedures and hardware used to create openings and penetrations are not well understood; a concern sometimes given as a reason for not using PT construction in a particular application.
Cutting openings in PT slabs does require care and caution because of the possibility a PT tendon may get cut. Tendons carry a high amount of force, usually around 27,000 pounds for a 1/2-inch-diameter strand, and must be detensioned in a controlled manner to assure performance and safety. However, with the proper knowledge of structural behavior, repair hardware, and PT field practices, retrofitting openings and other penetrations in PT slabs is not only feasible, but achievable safely and economically.
Types of openings and penetrations
There are two types of openings that are commonly cut into existing slabs: small penetrations and large openings. Small penetrations are cut into a slab without affecting any of the existing PT tendons. Large openings require the cutting of existing tendons.
Small penetrations. As a general rule, it can be assumed the effect of small penetrations will be negligible and the slab will behave similarly to a slab without penetrations provided: none of the existing tendons are cut during the coring of the penetration; the opening is not located close to a supporting column; the opening is not located near a concentrated load; and the opening or group of openings does not significantly reduce the effective flange area for a supporting beam.
However, good judgment should be exercised. A large number of small openings in a concentrated area can have a significant impact on slab strength and stiffness, particularly if several mild reinforcing bars are interrupted.
Prior to cutting small penetrations in a PT slab, existing tendons should be located using nondestructive testing (NDT) equipment. Various NDT methods are available for locating tendons within a slab. Once the tendons have been located and marked on the slab, small penetrations can be made using core drilling equipment or chipping hammers.
The area immediately behind the anchorage should be free of sleeves, blockouts, large conduits, or other voids or congestion that could cause the concrete to fail in this high-stress zone. If a penetration must be located in this area, the use of Schedule 40 steel pipe or some other engineered solution is required to effectively transfer the stress across the opening and to avoid a possible anchor blowout. See Figure 1.
Large openings. Large openings are sometimes necessary for stairwells or large duct shafts between floors. Creating a large opening in a PT slab may necessitate cutting several tendons. An engineer should be consulted to analyze the effect that the opening will have on the slab. This analysis should show that strength is adequate and that all serviceability conditions (e.g. deflections) are met. Large openings also can significantly influence the global behavior of the structure.