Post-tensioned concrete construction is a little different than standard reinforced concrete construction. There are several construction issues with post-tensioned concrete that I have found consistently among engineers and contractors. Understanding the causes of some of these problems should help contractors build better post-tensioned structures.
Top: Post-tensioning tendons are draped within the slab cross section. Bottom: Significant over-balancing of the weight of the concrete with post-tensioning can actually lift and crack the slab.
Post-tensioning Actively Loads The Structure
One of the main differences between a post-tensioned beam or slab and a typical reinforced concrete element is that the strands drape or profile inside the concrete—their vertical location changes along the length of the member. The tendon profile, which is typically in a concave shape, will try to straighten itself out when stressed. This creates an uplift load (also called a balance load) on the concrete that minimizes and often effectively removes the dead weight of the concrete from the stress and deflection calculations.
Since post-tensioning places an active load on the structure, care must taken during construction to make sure the locations of the tendons match the engineer's drawings. Inaccurate tendon locations can greatly increase the uplift force on the slab. The most common cause for incorrect tendon placement is discrepancy between the structural drawings and the supplier's shop drawings. The inspector and the contractor must verify that the layout conforms to the structural drawings, not just the shop drawings.
Applying uplift loads that are larger than the weight of the floor can cause problems during stressing when the tendons begin pushing up with more force than the concrete weighs. This net upward force can result in large tensile stresses at the bottom of the slab/column joint, where there is typically little or no rebar, and can actually lift the slab. Unlike with rebar, which activates only when loaded, too much uplift load (whether due to the number of strands or increased drape) can significantly impact the slab.
Top: To prevent restraint, plastic is placed along the top of a wall that the slab is crossing. Bottom: The black foam rubber around the first few inches of the dowels coming out of the adjacent wall will allow the post-tensioned slab to shorten during tensioning without cracking the slab.
Slip connections are critical
A post-tensioned system will move 20% to 30% more than conventionally reinforced concrete. A good rule of thumb is that a post-tensioned system will move about 1 inch for every 100 feet of slab that is not restrained by a lateral system. If the edge of the slab is 50 feet away from the nearest shear wall, when post tensioned this edge will move in about ½ inch. If this edge movement is prevented, either the slab or the restraining element will most likely crack.
Slab restraint is typically caused by concrete or masonry walls that are connected at the perimeter of the structure. In addition to having more movement, a post-tensioned slab will have substantially less rebar than a conventional system, which is one of the main economic benefits. But since a post-tensioned slab does not have excess rebar to minimize and distribute cracking, restraint cracks will be large and very noticeable. The use and proper construction of slip details is therefore critical for the performance and aesthetics of post-tensioned concrete.
Typical slip details use felt, building paper, or plastic to eliminate the bond of the slab to the walls. Many restraint cracks have been created by engineers or contractors who underestimated the strength of the bond between a slab and concrete or masonry walls. When rebar is required between the slab and wall, pipe insulation or foam rubber surrounding a portion of the dowel can be used to allow relative movement without activating the dowel.
Reinforcement must be discontinuous across a pour strip.
Pour strips are specific to post-tensioned structures and are typically located at the midspan or quarter point of the bay. To provide any crack control benefit, the slabs on each side of the pour strip have to be completely separate when the tendons are stressed. Any reinforcement extending from one slab into the other will act as a tension tie, restrain the relative movement of the two slabs, and most likely cause cracking. All rebar and post-tensioning must be lap spliced inside the width of the pour strip.
Contractors should pay special attention to whether the engineer requires the edges of the pour strip to remain fully shored after the tendons have been stressed but before the concrete has been placed to tie the two slabs together. The confusion occurs because after a successful stressing, the majority of the slab (except the pour strip) is structurally stable and does not require the forms or reshores for stability. But without edge reshores, these midspan pour strips before they are filled with concrete to tie them together can result in large cantilevered sections of slab on either side that can result in significant deflections and cracking from loads that were never intended or reinforced by the engineer.