How to evaluate and manage post-tensioned concrete

Unbonded post-tensioned evaluation techniques
It is often impractical and cost-prohibitive to review every strand in a post-tensioned structure. A systematic approach provides adequate assurance of safety in a cost-effective manner. This typically begins with a small sample size and low cost-evaluation techniques, then expanding the sample size and implementing more cost-intensive techniques as required (based on initial findings).

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Often, these evaluations are carried out in operational structures (e.g. office spaces, parking garages, etc.). Careful planning and co-ordination is required to minimize disruption to the facility operation.

Non-destructive field evaluation and document review
The first step in any post-tensioned evaluation is to understand construction details and existing conditions. This includes a review of available documentation (i.e. specifications, as-built drawings, previous evaluation reports, and maintenance reports). This is followed by a visual review of the site to identify any evidence of post-tensioned system deterioration or conditions that could contribute to deterioration, including:

• evidence of structural distress (e.g. noticeable deflections, cracking, spalled/delaminated concrete, and ruptured tendons that have broken through the slab);
• rust or grease staining on the slab underside;
• reports of unexplained noise similar to a gunshot;
• exposed sheathing;
• evidence of moisture penetration in the building (e.g. leaking cracks or efflorescence, water staining, or leakage through the building envelope);
• missing or deteriorated grout plugs or evidence of moisture infiltration at these areas;
• deteriorated waterproofing systems, including traffic topping system and expansion joints;
• inadequate drainage (e.g. ponding water or plugged drains); and
• post-construction penetrations (e.g. retrofitted drains or slab openings).

Corrosion of post-tensioned tendons due to moisture is the most common cause of failure. High-risk areas for moisture ingress should be identified, including:

• expansion and construction joints;
• below-grade slabs where anchors can be exposed to soil contaminants and groundwater at foundation walls;
• exposed balcony slabs; and
• areas below landscaping (e.g. roofs, pools).

The identification of these conditions provides insight into the condition of the post-tensioning system and the level of deterioration risk. However, this information alone is not enough to determine the post-tensioning condition or the deterioration rate. Similarly, absence of these conditions does not necessarily indicate the post-tensioning condition is acceptable.

Structural analysis to understand breakage tolerance
Under conditions where tendon failures are suspected or known, a structural analysis is performed to understand the structure&#;s load-carrying capacity, as well as the level of tolerance available for post-tensioning failure before the structure no longer provides an acceptable level of safety. Post-tensioned systems are sometimes designed with excess capacity, allowing for some failure of individual strands. This information is valuable to owners and engineers, as it aids in repair planning. While slab areas with low tolerance for tendon failure may require immediate repair, repairs in higher tolerance areas can often be deferred (based on engineering analysis) until budgets allow or until a larger repair scope can be carried out, achieving economies of scale and avoiding multiple operational disruptions.

Exploratory openings and penetration testing
Small openings are made in the concrete slab to expose the post-tensioning strands (Figure 4). The openings are typically made at the slab/beam underside to limit the risk of moisture ingress following the evaluation. Where possible, the most likely location for strand deterioration is selected for review. For heat-sealed and stuffed systems, this is typically at mid-span of the bay, closest to the exterior slab edge. This area is a low point in the tendon profile where moisture that has entered the sheathing (typically through the live end anchor assembly) is most likely to accumulate and cause corrosion. For paper-wrapped or encapsulated systems, moisture does not travel along the sheathing, so locations of leaking cracks or control joints are often selected. Per PTI guidelines, openings should never be made near the anchorage zone, as there are highly-concentrated compressive stresses from the anchorages in these areas and disrupting concrete can result in the anchors rupturing through the slab. (Refer to PTI DC80.3-12/ICRI 320.6, &#;Guide for Evaluation and Repair of Unbonded Post-Tensioned Concrete Structures.&#;)

Prior to concrete removal, strands are located using ground-penetrating radar (GPR) or similar method. Once removals are complete, the sheathing is cut to expose about 200 mm (7.9 in.) of the post-tensioning strand. The strands are visually reviewed to document the condition of the steel, quantity and condition of protective grease, and the presence of moisture. However, one must be careful when interpreting these observations, as they are not necessarily representative of the post-tensioning condition beyond the extent of the opening.

The post-tensioning strands are qualitatively checked for tension using the penetration testing method. This involves attempting to insert a flathead screwdriver or similar tool between each of the six outer wires of the strand. If the wires can be penetrated, one or both of the wires is broken. Further, if all the wires can be penetrated, the strand is either under-stressed (i.e. carrying less tension than for which it was designed, due to either not being properly stressed at the time of construction or the strand having slipped at the anchorage) or fully de-stressed (i.e. broken or released from the anchorage). Determination of an under-stressed or fully de-stressed condition is based on engineering judgement and may require further quantitative testing to confirm.

Limitations of the penetration test method include:

• results are subjective to interpretation, based on the skill of the tester and the force they apply in an attempt to penetrate between wires;
• false results&#;for example, testing may not detect a tension deficiency if failure and test locations are far apart, as this distance could cause friction accumulation (either due to undulations in the tendon profile or possible corrosion product build up) to simulate tension in the strand, or corrosion product between wires can impede penetration; and
• cause or rate of failure cannot be determined.

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Tendon Repairs and Modifications

Post-tensioning is the most significant development in the concrete construction industry since steel reinforcement was first employed in the mid-s. Post-tensioning (PT) delivers roughly four times the tensile strength compared to conventional reinforcement and significantly reduces (or eliminates) concrete cracking, thus enabling thinner slab construction &#; reducing the environmental impacts, saving material and labor costs, and shortening construction schedules. Post-tensioning also brings a host of seismic advantages to a structure and enables architects to employ concrete in artful shapes and sizes once thought impossible.

Consequently, post-tensioning in new construction has blossomed in the United States since the s. And, as with any technological advent, the decades following its introduction saw significant improvements in PT techniques and materials. The original button-headed wire with heavy wax paper wrappings has been replaced by higher grade steel strand, excellent anchorages, injection ports, grout caps, polypropylene ducts, improved grouts, and an array of customizable installation techniques to best suit the slab or beam&#;s design and use.

However, construction concerns arise as structures built in the industry&#;s formative years, and newer structures for that matter, are remodeled, repurposed, or repaired. The PT remodel industry steadily grows because generations of structures erected using this technology are coming to the ends of their useful lives. These are mostly parking garages and mid-rise buildings erected in the s and s. Construction from the s tends to be particularly &#;light,&#; with design engineers saving every foot of rebar and pound of concrete possible, making for underbuilt structures by today&#;s standards. By the s, PT had been primarily refined into the processes and materials employed today, making remodeling and repairs easier. Also, in previous generations, life-cycle calculations were not given the importance or precision that they are today, so it is little wonder that older buildings require work to extend their usefulness.

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Even recently erected structures may require significant work. Perhaps a landlord wishes to add or change utilities in a building or redesign the floor layout. Perhaps a cracked slab has exposed a section of strand, or an exposed anchorage has corroded, or a contractor has cut into a slab and unintentionally severed a PT strand. This article provides an overview of a few common situations and solutions for PT tendon replacement, repair, and remodeling in older and modern commercial structures alike.

Contemporary Concerns

Each project brings unique challenges, so begin by consulting an experienced and competent post-tensioning engineer. Simply because one has successfully repaired or remodeled a building from the same region or date as a previous project does not mean that the prior experience can inform the final details of a new situation. In fact, it is possible for an older building to show dramatic PT variation within a single building, such as button-headed PT on one level and steel-rod PT on another. Also, without the PT shop-drawings, even in a newer building, the actual placement of the tendons and interactions of the tendons are not known without some investigation informed by experience. So, before any slab or beam is touched, seek professional guidance. What follows are general thoughts based on years of industry experience.

First, slab and beam cracking is a common concern and an indication that attention and repairs are needed. Are the cracks deep? If so, the engineer must ascertain the cause. Often the culprit is not the concrete or post-tensioning work. Most likely, it is the original design. Design issues likely mean expansive, expensive repairs rather than a localized and inexpensive solution. To the surprise of many engineers, it is often possible to replace the tendons in existing slabs and beams to increase their strength and close the cracking. One caveat worth mentioning involves epoxy. If the cracks are old, an owner may have filled them with epoxy to improve aesthetics or to protect the slab or beam from water intrusion. However, if the filler epoxy reached the strand, PT replacement is significantly more challenging. A second, clear sign of trouble in an older building is deflection in any region: slab, beams, columns, walls, or ceiling. Again, the engineer must discover the cause of the deflection and draw up the remodel action plan accordingly. Deflection generally indicates a more serious design issue than cracking.

Parking garage projects account for a great deal of PT work because the technology suits the structures so well. Transportation needs also change over time, requiring garage modifications. Required live load increases can create challenging situations. It is more likely that a garage will experience live loads near or over capacity than a mid-rise residential building. However, the PT remodel process is usually much easier than in a tenanted building as there is so much working room, and the garage can usually be closed while work is done. Both slabs and beams can be remodeled. Repairs are common too.

Mid-rise buildings with PT slabs rarely experience the live load issues of parking garages. However, slab repairs required to address broken or damaged tendons, or modifications required for remodeling, are often more challenging to engineer solutions for, primarily because the impact on tenants must be considered. In general, there are two basic techniques for accomplishing a PT project in a mid-rise building &#; clearing a large portion of the building to do all of the work in one phase or performing the work in multiple phases over a more extended period.

The first approach involves shoring three to four stories below the repair floor. This will mean the evacuation of a significant part of the building, interrupting work and rents for the tenants and owner. Also, the shoring itself is costly to erect. The main advantage of this technique is for the PT repair crew since they have free access to the floors and their work can be accomplished quickly. Additionally, if a building has serious design flaws, this technique will probably be the only remodel means available to an owner.

The second approach can be accomplished without traditional shoring. Most structures, even the &#;lighter&#; s structures, can usually be remodeled by working on every third strand, one at a time, and proceeding down the slab. This means the crew will usually make at least three passes over the slab, spending significantly more time on the floor than with the shoring method. However, the combined benefit to tenants (they can remain on all other floors) and the crew (they do not need to erect costly shoring) usually makes the added time and labor for the PT crew on the individual floors well worth it, from a cost analysis and tenant relationship standpoint. This method requires evacuation of the remodel floor to clear that floor&#;s live load. Live load elimination means that a single strand being replaced in any given location will not affect the slab negatively. Usually, the slab has enough tension reinforcement to absorb the relatively minor, temporary changes in force due to remodel activity. This technique has been employed for applications from simple single-floor, single-strand replacement to multi-floor cut-outs for elevator shafts and upgraded escape routes. The amount of actual disruption such remodeling may cause tenants largely depends on the state of the slab. For instance, if the strand is snagging in its ductwork for some reason, small windows to the strand will be jackhammered in the slab to identify and remove the issue, thereby increasing potential tenant disruption.

This process of removing a live load and treating the work one strand at a time probably suggested itself by early repair of failed strands, whether due to crumbling concrete at an anchor head, a slipping head from a now-obsolete coil anchor, or an unintentional cut through a strand from a trade worker. In these cases, the tension from one strand was lost, but the slab did not fail because building specifications called for at least that much extra carrying capacity, even in older structures. It is readily inferred, then, that one may approach a repair with a similar mindset, one strand at a time. However, if a strand fails or is unintentionally cut, seek professional guidance. It cannot be left cut or un-tensioned simply because one sees no cracking, buckling, or other negative signs in the slab. The force changes will have an effect over time, and it is essential that the slab tension capacity is put back into balance to ensure safe and consistent operation for present and future use of the structure. If one discovers deflections of any kind (sagging, buckling, leaning, etc.) in a building, consult an engineer. If the deflection occurs while a crew is working or people are in the building, the building must be evacuated immediately until an engineer investigates the situation and clears the building for reoccupation. This, of course, is true of buildings constructed with or without post-tensioning.

Example Repair: Simple Repair

Often the best approaches to forming a sounder general understanding of a subject are to consider specific cases. First, consider a common issue: the repair of a single broken tendon, and second: the creation of a new opening as part of a remodel project.

Typically, commercial buildings are built with 0.5-inch diameter, 7-wire monostrand tendons. Modern materials and techniques make most unbonded tendon repair or replacement a routine event. Older button-headed tendons usually feature grease and paper conduit. This conduit is known for inconsistencies that make tendon replacement difficult and, therefore, most button-headed repairs will be splices. If button-headed tendon replacement is required, it may only be possible to run a smaller gauge tendon in its place, and engineers will need to be consulted to ensure that the result remains safe and balanced. In some cases, the new strand may be welded onto the end of the old tendon and pulled through, enabling a 0.5-inch diameter monostrand tendon to replace a 0.5-inch diameter button-headed tendon, despite some conduit issues.

Finding the break is usually not a problem, even when hidden in the slab. A scan with a noninvasive ground penetrating radar device provides a wealth of information concerning the location, size, and material of all slab-encased elements. Also, if the broken tendon is to be pulled out for removal, the lengths of each segment can be determined to verify the break location.

The first step in construction is to locally shore the work area and to chip away the concrete to expose the broken tendons (Figure 1). This must be done with care to avoid damaging adjacent tendons. In most situations, a single broken tendon will be repaired with a splice and re-tensioned rather than entirely replaced. However, if the tendon shows rust or other material damage, it will need replacement.

Example Repair: Complicated Case

If a single-strand repair or replacement represents the basic end of the repair spectrum, a large, multi-floor slab cutout for a new stairway or elevator may serve as the more complicated and challenging end of the spectrum. A few warnings should be heeded. First, plan on tenant evacuation and building shoring. Retrofits of this nature require open access to ensure safe and timely project completion. Second, review the original plans and scan the slabs to avoid locations of bundled tendons.

Third, even if extensive shoring is temporarily taking load from any columns, avoid destructive construction techniques near them. Besides the obvious need to protect essential building structures, tendon work near columns poses two risks: 1) tendons are often bundled near columns, and bundled tendons are more challenging to work with than single tendons, and 2) tendons are often nearest the top of the slab surface at column locations. Because splice and anchor retrofits need to be fully buried in the slab once the work is complete, much more of the slab must be broken out when repairing any tendon that approaches the surface of the slab. Such tendons must be re-profiled by breaking out and excavating under a tendon, resulting in a redistribution of stress and incurring extensive slab demolition.

As mentioned above, tendons are more challenging to repair when bundled. Therefore, when possible, engineers should avoid bundled locations when selecting a work location, especially when working with a multi-floor cutout. Sometimes surrounding tendons will pinch a tendon in the bundle and hinder the work. The tendon may bind when being de-tensioned. The conduit may collapse when the tendon is removed (again, welding on a new tendon to the old for pull-through replacement may prevent this). Any work done at a bundle location runs a higher risk of damage to other tendons and of increasing labor, material costs, and project-completion time.

Once planning, blocking, de-tensioning, cutting, and demolition are complete, the tendons are replaced or spliced with appropriate anchors, in a manner similar to that for the single tendon project, and the slab is re-constructed. Figure 2 shows the de-tensioning of the tendons at the end of a beam. The process of slab tendon de-tensioning is similar. A safety item to note is that slab ends must be guarded with perimeter blocking (typically with a heavy wood or steel beam) when the tendons are first cut. The force released by a cut tendon may cause the tendon to break through its grout cap and pose a threat to nearby people or equipment. The contractor must ensure that, before any cutting takes place, safety blocking protects any strand ends that might release force. Once the de-tensioning and demolition are complete, the profile of the tendons can also be adjusted within the work area to address the new structural spans and geometry. Once the tendon work is finished, including the addition of new end anchorages, the concrete floor slab is recast with rebar reinforcement (Figure 3). Bonding agents help rapid set repair mortars tie into the existing slab. The slab opening may call for a turned down beam to rim the cutout for reinforcement, aesthetic look, or other mounting needs (such as serving as an anchor for railing attachments). Once the concrete cures, tendons are re-tensioned as in new construction (Figure 4), and the cutout is ready for new service (Figure 5).

Conclusion

Hopefully, these brief thoughts and examples will serve to clarify basic PT repair and remodeling processes. Like the development of many industries since the s, construction methods and materials have refined dramatically, even within one of the most ancient building materials on earth: concrete. The post-tensioning skill and experience of engineers, crew leaders, and crews means that new and renovated post-tensioned structures can grow in scale, number, and elegance.&#;

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