Best Practices for Post-Tensioning Elongation Records

To view the figures and tables associated with this article, please refer to the flipbook above.

Post-tensioned (PT) structural systems provide effective framing solutions for a wide range of conditions, but cost effectiveness is heavily influenced by the timing of formwork cycling. For PT structures, review and approval of elongation records is a critical part of that process. This article focuses on cast in place concrete structures reinforced with unbonded post-tensioning strands which are typically used in one-way slabs, two-way slabs and beams/girder systems.

The review process for stressing records can vary significantly across the country based on local jurisdictional requirements. In some areas, the Licensed Design Professional or Structural Engineer of Record (LDP/SEOR) does not need to be consulted if all measured elongations are within the ACI 318 allowed 7% tolerance. In these cases, the LDP/SEOR is only required to address out-of-tolerance conditions. In other areas, local jurisdictions require the LDP/SEOR to review and approve complete stressing records, even if all elongations are within tolerance, as a prerequisite step for the Contractor to cut tendon tails and continue with construction.

In the former case with out-of-tolerance conditions and in the latter case as a whole, the cycling of formwork is effectively halted until the LDP/SEOR responds. In the author’s experience, LDP/SEORs who exceed 2 days to review records do not prioritize their client and owner’s interests. In addition, reviewing elongation records for a concrete pour should take less than 60 minutes. If inattention persists, a delay claim may be forthcoming since the contractor is potentially losing significant time and money. Accordingly, the review and resolution of elongation records should be made a priority within design offices.

Timely review of stressing records also allows for faster completion of the tendon corrosion protection system. Starting with ACI 318-11, all structural building PT is required to be fully encapsulated in plastic sheathing and grease for corrosion protection. The tendon tails, however, are bare steel to accommodate stressing operations. The excess tendon tails cannot be cut until the stressing records are approved, at which time the encapsulation cap can be installed over the cut end to protect the tendon and the seated wedges. The longer the bare tendon is exposed to the elements, the potential for corrosion increases. Get the tendon ends protected as quickly as possible.

Transferring the force from the PT strands to the concrete is critical to the performance of PT structures. Contractors must demonstrate they have complied with the contract documents, and the LDP/SEOR and building official must have confidence that the structural design has been successfully implemented. ACI 318-19 section 26.10.2(e) requires two separate actions to verify prestressing force and friction losses. First is a comparison of the measured strand elongation to a theoretical, calculated value. The second is verification of the jacking force for the rams, typically calibrated using a pressure gauge. These two actions are used individually as a check on the other.

The practical benefit of elongations is they can be observed and measured after installation of the PT force. If the inspector was not present during stressing, if the gauge pressure wasn’t recorded, or something unforeseen occurred, the elongations can still be measured as a simple way to estimate the approximate strand force. Another option would be to perform a lift-off test, which effectively is re-stressing the strands until the wedges release and recording that pressure/force. While lift-offs are performed for multiple reasons, it’s more efficient and safer for the field to measure the elongation at the tendon tail and back calculate the force. Lift-off tests can also damage the tendon. This is why elongation reports are so valuable for engineers, contractors, and inspectors. However, the elongation values need to be better understood and not used to generate unnecessary work.

Measuring elongations involves spray paint or construction keel, a straight edge, and a tape measure. Prior to stressing the strand, the inspector will place the straight edge against the slab edge and mark the exposed tendon (Fig. 1). After stressing, the same straight edge is placed against the slab edge and a tape measure shows the distance the mark has moved. This distance is the measured elongation of the strand, and the inspector will record a value for each tendon. The measuring process has its own built-in tolerance, as any measurement does.

The measured elongation is then compared to the theoretical, calculated value and the difference is documented in the report. The authors highly recommend the elongation record include the allowed +/-7% deviations from the theoretical value and the actual percentage difference between the measured and calculated values (Table 1). This helps the LDP/SEOR quickly identify potential issues that may need contractor and PT Supplier input to resolve. A typical report should provide a strand identification (color code), plan location, length of each strand and the gauge pressure of the jack.

The authors also strongly encourage the LDP/SEOR to review the equipment calibration prior to the start of construction. PT suppliers should calibrate their equipment between projects and within 6 months per PTI.

The tendon force and theoretical elongation are related based upon the strength and materials equation Δ = PL / (AE), where Δ is the strand elongation, P is technically the average force in the strand, but typically approximated by engineers as the anchorage force as prescribed in ACI 318-19 section 20.3.2.5, L is the length of the strand from anchor to anchor, and A and E are the strand cross-sectional area and modulus of elasticity, respectively. The theoretical elongation for each tendon is calculated by the PT supplier using the specific material properties and strand layout and is reported on the PT shop drawing submittal.

It is important for the LDP/SEOR to understand the force in the strand is not precisely proportional to the measured elongation. They are correlated, but an under-elongated strand does not immediately indicate a reduced force. There will always be differences between the theoretical and actual elongation. For example, PT suppliers, when calculating elongations, will typically use the straight-line distance between slab edges as the tendon length. In reality, a tendon is likely not to be straight due to penetrations, embedded elements or slab openings. Similar precision arguments can be made regarding friction loss, wedge seating, actual vertical tendon profile and the actual tolerance in the inspector measurements.

The ACI Code tolerance for elongations was implemented decades ago to account for numerous factors in stressing and measuring a PT strand. Concrete buildings are not constructed in a vacuum, and nothing is perfect. The 7% tolerance is not stating the code is “ok” with 93% of the force shown on the drawings. It acknowledges there will be slight differences between the theoretical and measured elongations, and provided they are small enough, that is expected and acceptable.

When elongation measurements deviate from the allowable range, the design and construction team must collaboratively identify the cause. Generally, the PT supplier is to first reconfirm the elongation calculations by checking material properties and other calculation inputs. For example, on a recent project, a last-minute change to a construction joint location caused tendon lengths to differ from that used in the elongation calculations. The problem was quickly identified, and the elongation calculations were revised. If there are broken tendons or major elongation deviations, the PT supplier can conduct a more complex variable force calculation to provide additional information to the design team.

The role of the concrete contractor is to identify anomalies such as malfunctioning jacking equipment, observed anchorage slip, localized concrete defects, or other issues that could affect elongations. The role of the LDP/SEOR is to determine the impact of potentially deficient tendon forces on their structural design. All parties should collaborate on appropriate repair techniques should it come to that, to fix the problem and minimize further damage.

Another commonly misunderstood aspect of elongation reports is an out-of-tolerance result for short length tendons. Tiny differences between small elongation values can result in large percentage deviations. A 20-foot-long strand should elongate approximately 1-5/8 inches. However, an extra 3/16-inch wedge seating loss will produce a 12% out-of-tolerance deviation. This condition may incorrectly indicate a low force and possibly result in unnecessary remedial work. The Post-Tensioning Manual 7th Edition recommends ¼ inch be added to the theoretical elongation for strands shorter than 40 feet to account for these effects. In addition, the authors recommend relying more on verifying the applied jacking force and to view short tendon elongations with an informed eye.

The PT supplier prepares friction-loss calculations, which do not constitute a “design” of the PT system but are a verification of the theoretical tendon force for a given configuration. The tendon force is a function of many variables, but the material modulus of elasticity, angular coefficient, and wobble coefficient are specific to a given PT Supplier’s material.

The LDP/SEOR must assume a final effective PT force to produce a rationale design based on a finite number of tendons in a concrete member. Most seasoned PT designers assume a force of about 27 kips per tendon. This effective force assumption must be documented on the structural contract documents and should be verified by the friction loss calculations. The supplier is demonstrating their product can provide the force assumed as the basis of design by the LDP/SEOR. The PT supplier is not “designing” the PT system. An analogy would be rebar mill certs for deformed reinforcing steel. In many cases, LDP/SEOR insist the PT supplier is the “PT designer” because of the friction loss calculations, which they then use as an improper basis to demand additional submittals and evaluations that are not properly the purview of the PT supplier.

In some areas of the U.S., PT suppliers are required by LDP/SEORs to determine and “certify” the final installed forces based upon the measured elongations—even for tendons within the allowable 7% deviation range. Table 2 is an example of this process. This does not have a useful purpose other than to spread liability and slow the elongation review process. It is questionably unethical to require the PT supplier to “certify” construction put in place by others. The implied liability is enormous and may violate some State professional licensing laws. The authors strongly encourage all who engage in this practice to cease immediately.

Structural contract documents also might mandate: “Post-Tensioning Supplier shall have an Engineer supervise the stressing operations and issue a letter certifying that the prestressing forces have been transferred to the structure. The letter should also address and resolve any discrepancies.”

This specification is wildly inappropriate in the author’s opinion. The LDP/SEOR should understand the proper role of the PT supplier prior to requiring these types of letters. The PT Supplier controls the manufacturing process. The PT material is typically required to be fabricated at a PTI-certified plant, which is an ANSI certified program. Additionally, the PT Supplier has shared control over stressing equipment. After the initial calibration, onsite equipment maintenance is beyond the control of the PT supplier. The PT Supplier does not have a consistent field presence to supervise or monitor the construction phase. In the past, the PT Supplier might conduct a pre-pour jobsite visit, but this practice has become mostly obsolete due to the commonplace adoption of recognized PTI certification programs for installers, stressors and inspectors. Furthermore, the PT Supplier is not involved with the formwork, PT installation, rebar installation, concrete placement, stressing operations or elongation measurement and reporting.

Requiring the PT supplier to certify forces is not required by any code that we are aware of, adds time to the elongation review process, and increases the PT supplier’s liability for out-of-scope construction (which increases their cost to cover that risk). If the PT supplier is contracted to fulfill the requirements shown on the structural documents, then they have already committed to providing the listed force.

The collective industry goal should be to advance practices and procedures that contribute to the sound design and construction of cost-effective PT structures, in a manner that is reliable, fair, and reasonable for all. ■