ost-tensioned concrete con-struction is a little differentthan standard reinforced con-

crete construction. There are severalconstruction issues with post-tensionedconcrete that I have found consistentlyamong engineers and contractors. Un-derstanding the causes of some of theseproblems should help contractors buildbetter post-tensioned structures.

Post-tensioning actively loadsthe structure

One of the main differences be-tween a post-tensioned beam or slaband a typical reinforced concrete ele-ment is that the strands drape or pro-file inside the concrete—their verticallocation changes along the length of themember. The tendon profile, which istypically in a concave shape, will try tostraighten itself out when stressed. Thiscreates an uplift load (also called a bal-ance load) on the concrete that mini-mizes and often effectively removes thedead weight of the concrete from thestress and deflection calculations.

Since post-tensioning places an ac-tive load on the structure, care musttaken during construction to make surethe locations of the tendons match theengineer’s drawings. Inaccurate tendonlocations can greatly increase the upliftforce on the slab. The most commoncause for incorrect tendon placement isdiscrepancy between the structural draw-ings and the supplier’s shop drawings.

84 © Concrete Construction © January 2006

PBy Bryan Allred

Understanding canhelp contractors avoidcommon problems

ConstructionPost-Tensioned

Issues in

Top: Post-tensioning tendons are draped within the slab cross section. Bottom:Significant over-balancing of the weight of the concrete with post-tensioning canactually lift and crack the slab.

The inspector and the contractor mustverify that the layout conforms to thestructural drawings, not just the shopdrawings.

Applying uplift loads that are largerthan the weight of the floor can causeproblems during stressing when the ten-dons begin pushing up with more forcethan the concrete weighs. This net up-ward force can result in large tensilestresses at the bottom of the slab/col-umn joint, where there is typically lit-tle or no rebar, and can actually lift theslab. Unlike with rebar, which activatesonly when loaded, too much uplift load

(whether due to the number of strandsor increased drape) can significantly im-pact the slab.

Slip connections are critical A post-tensioned system will move

20% to 30% more than convention-ally reinforced concrete. A good rule ofthumb is that a post-tensioned systemwill move about 1 inch for every 100feet of slab that is not restrained by alateral system. If the edge of the slab is50 feet away from the nearest shearwall, when post tensioned this edge willmove in about 1⁄2 inch. If this edge move-

ment is prevented, either the slab or therestraining element will most likelycrack.

Slab restraint is typically causedby concrete or masonry walls that areconnected at the perimeter of the struc-ture. In addition to having more move-ment, a post-tensioned slab will havesubstantially less rebar than a conven-tional system, which is one of the maineconomic benefits. But since a post-tensioned slab does not have excessrebar to minimize and distribute crack-ing, restraint cracks will be large andvery noticeable. The use and properconstruction of slip details is thereforecritical for the performance and aes-thetics of post-tensioned concrete.

Typical slip details use felt, build-ing paper, or plastic to eliminate thebond of the slab to the walls. Manyrestraint cracks have been created byengineers or contractors who underes-timated the strength of the bond be-tween a slab and concrete or masonrywalls. When rebar is required betweenthe slab and wall, pipe insulation orfoam rubber surrounding a portion ofthe dowel can be used to allow relativemovement without activating the dowel.

Pour stripsPour strips are specific to post-ten-

sioned structures and are typically lo-cated at the midspan or quarter pointof the bay. To provide any crack con-trol benefit, the slabs on each side ofthe pour strip have to be completelyseparate when the tendons are stressed.

Concrete Construction © January 2006 © 85

Top: To prevent restraint, plastic is placed along the top of a wall that the slab iscrossing. Bottom: The black foam rubber around the first few inches of the dowelscoming out of the adjacent wall will allow the post-tensioned slab to shorten duringtensioning without cracking the slab.

86 © Concrete Construction © January 2006

Left: Slab penetrations too close to anchors can result in blowouts. Right: This horizontal blowout resulted from tendons that werecurved in a very tight radius. On the left side of the photo, the imprint of the tendons prior to stressing can be seen approximately12 inches away from their current location. During stressing, the tendons straightened and the concrete in the curve was unableto resist the localized lateral thrust.

Any reinforcement extending from oneslab into the other will act as a tensiontie, restrain the relative movement ofthe two slabs, and most likely causecracking. All rebar and post-tensioningmust be lap spliced inside the width ofthe pour strip only after the tendonshave been stressed.

Contractors should pay special at-tention to whether the engineer requiresthe edges of the pour strip to remainfully shored after the tendons have beenstressed but before the concrete has beenplaced to tie the two slabs together. Theconfusion occurs because after a success-ful stressing, the majority of the slab (ex-cept the pour strip) is structurally stableand does not require the forms or reshoresfor stability. But without edge reshores,these midspan pour strips before theyare filled with concrete to tie them to-gether can result in large cantilevered

sections of slab oneither side that canresult in significantdeflections and crack-ing from loads thatwere never intendedor reinforced by theengineer.

Increased coverfor fire protection

Post-tensioningstrands have differ-ent cover require-ments depending on

whether they are located in restrainedor unrestrained spans. Whether a slabis restrained or unrestrained is a func-tion of the expansion potential of theslab due to fire. An unrestrained spanis the first or last span in the directionof the tendon.

For a three-hour fire rating, accord-ing to Table 720.1 of the InternationalBuilding Code, the minimum bottomcover for post-tensioning tendons is 2inches, while it is only 1 inch for rebar.This significant difference in cover isfrequently overlooked and is a directcode violation. One retrofit option isto have plaster or other fire-rated ma-terial applied to the slab or beam aroundthe low point of the tendon to increasethe fire rating. In post-tensioned slabs,the cover to the strands should typi-cally be greater than the cover to therebar in the end (unrestrained) bays.

Waterproofing A common myth is that post-ten-

sioning will create waterproof concrete.Unfortunately, this is not true. Concreteby nature is a porous material andadding some amount of compressiondoes not suddenly make it non-porous.When designed and built correctly, post-tensioning will enhance the natural watertightness that concrete possesses, but itwill never be waterproof.

In fact, one could argue that morepost-tensioning actually makes the sys-tem less waterproof. For higher valuesof post-tensioning, the slab will expe-rience larger movements, which can in-duce more restraint cracks. No matterhow much reinforcement is put into theslab, a crack is never waterproof.

Concrete blowouts and tendon profiles

Whether in a small residential slabon grade or a 30-story hotel, each ten-don will be loaded to roughly 33,000pounds during stressing. This force istransferred to the concrete through thebearing of the ductile iron casting an-chor against the concrete. If the con-crete is not well vibrated near the an-chors, if penetrations are located infront of the anchors, or if congestedrebar prevents a uniform bearing sur-face, a concrete blowout is likely. Theconcrete will literally explode duringtensioning as the anchors crush into theslab or beam, which then can cause thehydraulic jack to move suddenly and

Reinforcement must be discontinuous across a pour strip.

violently. For this reason, only trainedprofessionals should operate or be any-where near the jack during stressing. Ifpenetrations can’t be relocated awayfrom the anchors, steel pipes should beplaced around the openings to resist theanchorage force.

Blowouts can also occur away fromthe anchors in the middle of the slab.These blowouts typically are caused byextreme horizontal or vertical tendoncurvatures that create localized forcesin the slab. Horizontal curving of thetendons is often done to avoid penetra-tions, slab openings, and embeddeditems that prevent typical strand place-ment. During stressing, the tendons tendto straighten and the concrete in thecurve is unable to resist the localizedlateral thrust.

For horizontal curves, a good prac-tice is to start curving the tendons inthe middle part of the slab so sufficientconcrete and rebar are present to resistany lateral thrust. Curving tendons nearthe top of the concrete has led to prob-lems since the concrete has minimumcover and rebar can’t be added. If ex-treme curves are unavoidable, hairpinsare often used to “pull back” the thrustinto the main slab.

Vertical draping of the strands isone of the main benefits of post-tension-ing, but this needs to be done smoothlyand gradually. Tendon discontinuitieswill produce localized point loads on theslab. Tendons draped in a reverse curveover other tendons will create a down-ward force. Instead of picking up theconcrete, this reverse curvature will push

down into the lightly reinforced slabbelow, most likely causing cracks or asmall blowout. Reverse curvatures aretypically caused by the tendons beingkicked off their chairs, wrong chairsbeing used, or the strand being tied offto a piece of rebar that has been movedafter the tendon was secured.

Drilling into a post-tensioned slab

Another popular myth with post-tensioned slabs is that it is very difficultto drill into an existing slab because ofthe unknown location of the tendonsand anchors. But as long the tendons

and the concrete in front of the anchorsare not damaged, drilling into a post-tensioned slab is a fairly routine issue.

Existing tendons can be locatedusing a pacometer (handheld metal de-tector) or an X-ray device. With the X-ray in hand, a technician can mark thetendon locations directly on the con-crete surface. I have used a $40 store-bought metal detector and located ten-dons in a 12-inch-wide beam. For build-ings where tenants change frequently,we recommend marking one side of theslab so future tenants will know ex-actly where the strands and anchorswere placed.

The above list of “issues” is nowherenear complete but represents commonitems that I frequently come across. Forother questions, the Post-Tensioning In-stitute has recently completed the sixthedition of its Post-Tensioning Manual,which covers a large range of topics forpost-tensioned buildings and is usefulfor contractors as well as engineers. n

— Bryan Allred, S.E. is vice pres-ident of Seneca Structural Engineering,Inc. in Laguna Hills, Calif. Allred spe-cializes in reinforced concrete buildingsusing post-tensioned floor systems, theretrofit of existing buildings using ex-ternal post-tensioning, and post-ten-sioned slabs on grade. Over the pasttwo years, he has participated in a post-tensioned concrete seminar series forthe Post-Tensioning Institute.

88 © Concrete Construction © January 2006

This small blowout is a result of a localized low point in the tendons between twohigher points. Note the gap beneath the tendons and the slab, indicating how muchthe tendons lifted.

These tendons will create a downward force that is nowhere near a band or a verticalsupport and that can cause cracking or a small blowout.