How to evaluate and manage post-tensioned concrete

by Katie Daniel | April 26, 2018 10:31 am

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By Christopher Fulton, P.Eng.
While conventionally reinforced, concrete structures typically exhibit visible and obvious warning signs as they weaken, deterioration of post-tensioned structures often remains concealed, delaying corrective action. This deterioration can lead to individual post-tensioning strands failing (i.e. strands losing tension—similar to a stretched elastic band breaking), which instantaneously reduces the structure’s load-carrying capacity. Worse, the result of a strand failure is unpredictable. While the issue often remains concealed within the structure, strands can also be ejected from the concrete floor slab (Figure 1).

While the former scenario can result in unsafe conditions—where a structure’s load-carrying capacity is compromised without the knowledge of building owners and occupants—the latter can have serious consequences, such as property damage, personal injury, and the perception of a structurally unsafe building (which may or may not be true).

Appropriately timed condition evaluations are critical in ensuring the safety of post-tensioned structures. These assessments help engineers and building owners determine the risk of strand failure, allowing them to predict the rate of deterioration, plan, and budget for maintenance and capital repairs, and develop an overall strategy for managing the post-tensioned system over the structure’s service life. This knowledge allows for early implementation of measures that can mitigate deterioration, reduce safety risks, and limit or defer repairs—saving building owners significant cost/disruption and reducing liability. Regular review and documentation of the post-tensioned system’s condition can also enhance property value, providing prospective tenants and purchasers the confidence they are investing in a sound, well-managed structure.

What is post-tensioning?
Post-tensioning is a form of prestressed concrete in which high-strength steel strands are cast into a concrete slab or beam and tensioned to a high degree of stress following curing. The tensioned strands are anchored at either end, resulting in compression of the concrete between the anchor points. Pre-compressing the concrete reduces the material’s inherent weakness in tension, allowing for lighter structures, thinner slabs, reduced deflections, and longer spans when compared to conventionally reinforced concrete.

Post-tensioning can be bonded or unbonded. The former, most often used in bridge construction, is pressure-grouted following tensioning, which acts to bond the steel in place. Unbonded post-tensioning (commonly used in the construction of buildings and parking structures) remains separated from the surrounding concrete except where the load is transferred at the anchors. This unbonded system is the focus of this article.

An unbonded post-tensioned system includes several key components (Figure 2):

• high-strength steel strands, consisting of six wires wrapped around a central wire;
• a sheathing material surrounding the steel strand, preventing a bond between the steel and the surrounding concrete and also protecting the steel from moisture and contaminants in the concrete;
• a grease coating to protect the steel strand from corrosion and lubricate the strand, allowing it to move more easily through the sheathing during stressing;
• steel anchorages, which grip the ends of the high-strength steel strands and convert the tensile stress in the strands to a compressive stress in the surrounding concrete (the “dead end” anchorage is fully encapsulated in concrete, while the exterior side of the “live end” or “stressing” anchorage is left exposed for a jack to tension the strand following concrete curing—the exposed opening is filled with a grout plug following stressing); and
• concrete, which surrounds the post-tensioning reinforcement and forms the main structural element and surface that supports loads.

The complete assembly of the steel strand, grease, sheathing, and anchorages is referred to as a post-tensioned tendon. Each steel strand typically has its own sheathing and is referred to as a mono-strand.

Post-tensioning system evolution
First patented in the 1800s by P.H. Jackson in San Francisco, post-tensioning was refined to its contemporary form in the late 1920s by Eugene Freyssinet in France, gaining popularity in North America in the 1960s.

As with all construction technology, post-tensioning evolved as those working in the industry learned from past failures and technological advancements improved durability (Figure 3). The first system consisted of a greased strand wrapped in paper. The paper-wrap provided little protection against moisture and chloride ingress and was eventually replaced with an impermeable plastic sheathing. The initial forms of plastic-sheathed tendons were fabricated by wrapping a flat piece of plastic around the greased strands and sealing the overlapping length (referred to as “heat-sealed” or “cigarette” systems). Alternatively, the greased strands were pushed through the preformed sheathing (referred to as “stuffed” or “push-through” systems).

In both these configurations, the sheathing diameter was larger than the strand, allowing for potential air voids and creating a direct path for moisture and oxygen to access the steel strand and drive corrosion. Additionally, the sheathing was often cut short at the ends to facilitate anchorage installation, which provided an access point for moisture and left the strand ends vulnerable to corrosion. These weaknesses were addressed in 1985 when the Post-tensioning Institute (PTI) introduced Specifications for Un-bonded Single Strand Tendons, which required a tightly fitted extruded plastic sheathing. In the years since, post-tensioned systems have been further improved with the introduction of encapsulated anchorages wrapped in a protective coating, resulting in a system completely isolated from the surrounding concrete.

It is important to understand the age and type of post-tensioning system installed in a given structure, as this plays a significant role in the deterioration risk and scope of evaluation.

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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).

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.

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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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After review is complete, the exposed strand must be protected from fire and moisture. This can be achieved by patching the opening with concrete or, more commonly, repairing the cut sheathing and filling the opening with non-combustible insulation before sealing it with an engineered metal plate. The latter option provides fire and moisture protection, and also permits easy and low-cost access for future evaluations. The metal plate is engineered to withstand the force of an erupting strand should a break occur.

Where accessible, a select number of grout plugs can be removed to expose live-end anchorages to review for corrosion. Removed plugs should be replaced with non-shrink grout, and one should consider installing a waterproofing membrane over the slab edge.

The goal of penetrative testing is to review enough tendons to have a representative sample of the building condition. The number of exploratory openings is initially selected based on engineering experience and other contributing factors (i.e. the extent of conditions which could contribute to deterioration, risk tolerance, budget, and operational constraints). On review of the initial results, a statistical analysis is carried out to determine if the findings are considered representative (typically assuming 95 per cent confidence).

This analysis includes variables for the total number of tendons in a structure, the number of tendons tested, and the number of failed tendons identified. If it is determined further openings are required, additional testing is performed, variables are updated, and the statistical analysis is re-run. The process repeats until the required sample size is achieved. As tendons in different areas of the structure have different likelihoods of breakage (determined from conditions observed and knowledge gained from the non-destructive field evaluation and document review), tendons are separated into sub-populations (e.g. floor by floor, below-grade versus above-grade) for statistical analysis purposes.

Quantitative tension testing techniques
Techniques that provide a quantitative measurement of strand tension are available. The results from the tests are compared to the design stresses found on structural design drawings or shop drawings to determine if the system is behaving as expected.

Where the tendon anchorage can be accessed, a lift-off test can be performed. This involves using a calibrated hydraulic jack to pull on the end of the strand. With enough force, the anchor wedges release and the tension in the tendon can be determined based on the hydraulic pressure of the jack. If the anchor ends are not accessible or the strand end has been cut too short for the jack to grip, an in-situ tension test can be carried out (Figure 5). In this test, a force is applied perpendicular to the direction of the strand and deflection is measured. The tension in the strand is calculated based on the correlation between the force and deflection measurements. Quantitative testing is more costly and time-consuming than qualitative, and is typically used to supplement penetration testing where results are unclear.

Strand extraction and property testing
Strands can be extracted to visually review their full length, providing additional information about conditions unobservable using exploratory openings alone (Figure 6). Extraction of failed strands is typically carried out to help understand the location and cause of failure. To review the condition of strands near anchorage points where it is unsafe to make exploratory openings, it is sometimes necessary to extract strands that are fully tensioned. In this case, structural analysis is required prior to extracting tensioned strands to determine if temporary support or shoring is required. When tensioned strands are cut for extraction purposes, they will shorten as tension is released. This shortening can be measured and used to calculate the tension within the strand prior to cutting.

The extracted strands can be sent to a laboratory for testing to learn more about their properties, such as:

• tensile strength;
• chemical composition;
• fracture characteristics; and
• susceptibility to failure as a result of hydrogen embrittlement.

Corrosion potential evaluation
The corrosion potential evaluation technique (CPE) identifies the probable degree of tendon corrosion based on measured humidity levels within the sheathing. Inlet and outlet ports are created at either end of the plastic sheathing and dry air is forced through. This air is then collected and sampled for humidity levels. A representative sample of strands are extracted and visually graded to develop a correlation between condition and humidity readings. This is used to determine a probable degree of corrosion for each of the remaining tested tendons. This evaluation technique cannot be used on paper-wrapped or extruded systems, as there is not enough space between the sheathing and strand to permit airflow.

Acoustic monitoring
Acoustic monitoring is a non-destructive test method that records the rate and location of breaks over time. The structure is fitted with a grid of sensors (called accelerometers), which record the acoustic energy released by a rupturing strand or individual wires (Figure 7). The sensors can differentiate wire/strand breaks from other energy sources (i.e. vehicles and pedestrians). The break location is triangulated based on the time the energy source takes to arrive at each sensor (similar to how the epicentre of an earthquake is calculated). Sensor data is constantly processed by a central monitoring station. When the system detects a break, a notice is immediately sent to assigned parties—generally the building management and its post-tensioning engineer. Following a break notification, penetration testing is performed to confirm the breakage occurrence and location.

Acoustic monitoring is used where the deterioration rate is high and/or the tolerance for breakage is low. Timely breakage notifications allow unsafe conditions to be addressed immediately—or prevented from developing altogether if repairs can be made before breakage tolerance is exceeded. This system is also used to assess deterioration rate and any changes in this rate over time, helping owners forecast expenditures for post-tensioning repairs and measure the effectiveness of a repair or preventative maintenance program.

This evaluation technique does not identify the condition of existing tendons or the number and locations of previous breaks—only breakage occurring subsequent to the monitoring system installation.

Management strategies for post-tensioned structures
Once the condition of a post-tensioned structure is understood, the next step is implementing a strategy to manage the post-tensioning system over the structure’s service life.

The most effective way to manage deterioration is to mitigate it. In a study conducted by this author, 95 per cent of post-tensioned failures in parking decks and 50 per cent in building structures were determined to be the result of in-service conditions that could have been avoided with proper maintenance and operational protocols. (This information was included in C. Fulton and S. Trepanier’s study “Dispelling the Myth About Unbonded Post-Tensioned Buildings,” published in the August 2015 issue of PTI Journal.) These practices include implementing preventative maintenance programs to limit moisture ingress, such as maintaining waterproofing components (e.g. membranes, expansion joints, and building envelope seals), replacing deteriorated grout plugs and ensuring adequate drainage, and implementing protocols to prevent mechanical damage to tendons (i.e. requiring service contractors to scan slabs prior to coring, fastening through, or removing concrete).

If deterioration has already begun, techniques such as tendon drying, re-greasing, injecting with an epoxy resin, and cathodic protection of anchorages can be introduced to limit further progression.

In cases when tendon failure has already resulted in reduction in load-carrying capacity, review of the structural analysis in conjunction with engineering judgement is used to assess the need for repair. Repairs should be performed as required to maintain the structure in a safe and serviceable condition. A licensed design professional, along with a qualified post-tensioning contractor, must be engaged for this complex work, as it can be dangerous if improperly executed. Repairs typically involve removing and replacing sections or full-lengths of strands.

Ongoing evaluation and monitoring is essential to tracking the rate of post-tensioning deterioration and the effectiveness of any implemented measures, as this allows management strategies to be adjusted as needed. The frequency and extent of ongoing evaluation is based on engineering judgement, consideration of the rate and extent of deterioration previously observed, and the tolerable risk level.

Conclusion
Post-tensioned structures have specific deterioration characteristics and require specialized evaluation techniques in order to gain a true understanding of the structural condition. With a number of available evaluation techniques, it is the engineer’s responsibility to develop an appropriate evaluation methodology based on the specific conditions for each structure. With the implementation of proper evaluation and management practices, engineers and owners can maximize a post-tensioned structure’s potential, enhance property value, and ensure a safe and durable structure throughout its service life.

Christopher Fulton, P.Eng., is a project manager and post-tensioning practice leader at WSP Group in Hamilton. He is an active member of the Post-tensioning Institute (PTI) DC-80 committee for the repair, rehabilitation, and strengthening of unbonded post-tensioning. Fulton has six years of experience in the evaluation and repair of post-tensioned slabs and has performed this work in various cities across North America, including Toronto, Washington D.C., and Chicago. He can be reached via e-mail at christopher.fulton@wsp.com[3].

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