By Gert Schmidt
Many post-tensioned concrete (Post-tensioned concrete | Concrete Society) repairs leave uncertainty about whether the structure’s integrity has truly been restored.
In many instances the issue has merely been concealed. There have even been cases where repairs may even increase the structure’s vulnerability to future deterioration. Even projects considered “successful” can still experience quality-related problems. Refer to Post Tensioning Repairs and Modifications.
Why do these issues occur?
Post-tensioned concrete repair projects
Post-tensioned concrete repair projects are challenging because structures depend on carefully balanced internal forces created by highly stressed steel tendons. Refer to Technote5.pdf. Any intervention risks disturbing that balance, which can affect both safety and structural performance.
One major difficulty is the risk of damaging tendons during demolition or repair work. Post-tensioning cables are often not precisely located in older drawings. They may also be embedded in unpredictable positions within the concrete. Unintentional cutting or nicking of a tendon during chipping or drilling can cause sudden force release and serious structural consequences.
Another challenge is that repairs must preserve or restore the original prestressing force path. Unlike conventional reinforced concrete, where cracks can often be patched more straightforwardly, post-tensioned systems rely on continuous tensioning. If a tendon is damaged or “detensioned”, it must be properly re-anchored and re-stressed using specialised methods. This may require strict sequencing and expertise.
Access is also a constraint. Many post-tensioned elements – such as slabs and beams –are heavily congested with steel. This leaves limited space for repair work, anchorage replacement, or stressing operations. As such, partial demolition, temporary shoring, or complex staged repairs are required.
Finally, quality control is critical but difficult to manage on site. Proper grouting, anchorage installation, stressing procedures, and curing conditions must all be tightly controlled. Small deviations can lead to long-term issues such as corrosion, loss of prestress, or cracking.
Because of these factors, post-tensioned concrete repair demands careful investigation, detailed specifications, experienced supervision, and strict adherence to procedures.
Communication on concrete repair projects
It may seem obvious but clear communication is critical on all concrete repair projects. When workers are uncertain about the instructions they receive, mistakes are more likely to occur. We have witnessed many errors made by other contractors due to poor on-site communication.
For instance, on a significant post-tensioned concrete project, workers removed an excessively large area to ensure that all necessary work had been completed. However, in concrete repair, minimising the amount of chipping and removal of the existing structure is generally considered best practice. This is especially important in post-tensioned concrete repairs.
Many of the overhead patches on the project were applied using pumped concrete. As a result, several of the larger patches began cracking shortly after the formwork was removed.
Understand concrete repair project specifications
It is also essential for contractors to carefully read and fully understand post-tensioned concrete repair project specifications. While this may also seem obvious, it remains a cause of quality issues on concrete repair projects.
For example, in post-tensioned construction, project specifications commonly require the testing of field-cured cylinders (Curing Cylinders for Acceptance Testing). This is to verify that the concrete has achieved sufficient strength before stressing spliced tendons. Contractors may also be required to submit concrete test results to the engineer within 24 hours. In practice, however, these results are often submitted two to three weeks late. There have been instances where they have not been submitted at all. This can be disastrous!
Complying with concrete repair specifications
Contractors must also ensure they are complying with concrete repair project specifications.
Supervisory personnel are responsible for enforcing these requirements. This is considering that workers carrying out the repairs often have not read the specifications. They may also not fully understand the project requirements.
As is common on post-tensioned concrete repair projects, many of the concrete pours are relatively small, typically less than 0.4 m³. Consequently, bagged concrete repair materials are used for these pours. Therefore, testing the repair material every time it is used may not be necessary. Instead, each type of bagged repair material used on a project can be tested at least once every two months. This approach facilitates variations in personnel, work areas, and weather conditions throughout the project. Refer to Testing on site | Concrete Society.
All materials must also be mixed strictly in accordance with the manufacturer’s instructions. This, while ensuring careful control over the amount of added water and aggregate.
Therefore, it is incorrect to batch concrete in a mortar mixer while a worker simply adds water using a hose. Under such conditions, it is impossible for the worker to accurately determine how much water is being added.
Well-written concrete repair specifications
Well-written concrete repair specifications are essential to the success of any repair project. Engineer’s specifications provide the contractor with clear guidance on the required repair methods. This is in addition to materials, workmanship standards, testing procedures, and quality control measures. Without clear and detailed specifications, contractors may interpret requirements differently. This results in inconsistent workmanship, delays, disputes, and repairs that fail prematurely.
Concrete repair projects are often more complex than new construction. This is because they involve working with deteriorated structures, unknown site conditions, and compatibility between existing and new materials. Thus, specifications must clearly define repair limits, surface preparation requirements, curing procedures, material properties, testing frequencies, and acceptable tolerances. They should also outline responsibilities for inspection, reporting, and compliance.
Well-prepared specifications help contractors plan the work correctly, allocate suitable resources, and ensure that site personnel understand the project requirements. This reduces the likelihood of errors, including excessive concrete removal, improper mixing, inadequate curing, or non-compliant testing procedures.
In addition, clear specifications improve communication between the engineer, contractor, and testing agencies. They provide a common standard against which the quality of the repair work can be measured. Ultimately, properly written concrete repair specifications contribute to durable repairs, improved structural performance, and reduced maintenance costs. This translates into greater confidence in the long-term integrity of the structure.
Qualified concrete repair inspectors
Qualified concrete repair inspectors who are familiar with and understand project specifications are also essential. One common method for replacing deteriorated post-tensioning anchors involves chipping a full-depth access hole approximately 0,91m from the anchor. Thereafter, the strand is “detensioned” by cutting the wires with a hand grinder. This is before continuing demolition back to the anchorage zone. A new anchorage device is installed, and a new length of strand is spliced to the existing tendon.
Where direct access to the anchorage for stressing is not available, an in-line stressing coupler (“dogbone”) must be used. In such cases, the area around the coupler is left open to provide access for stressing operations. The concrete surrounding the new anchorage is then placed and allowed to gain sufficient strength before stressing takes place. Once the strand has been stressed, the blockout around the coupler is subsequently filled with concrete.
This sequence of operations results in a construction joint between the concrete placed for the anchorage zone and the later pour around the coupler. This must be properly detailed, inspected, and executed to ensure long-term performance.
Quality in concrete repair
Guidelines, training programs, and certification offered by organisations are important first steps toward achieving quality in post-tensioned concrete repair. However, they are only the beginning. True quality also depends on the contractor’s ability to read, understand, and comply with the project specifications.
Importantly, it requires that the engineer responsible for preparing and enforcing the specifications has a thorough understanding of the project. It is unlikely to succeed when decisions are made by individuals who lack the necessary qualifications and practical experience. At a minimum, this can lead to delays, confusion, and claims for additional work. In many cases, it results in a finished project that does not fully meet the owner’s objectives.
Engineers often hesitate to bring problems to the attention of the owner’s representative. In turn, the owner’s representative may be reluctant to inform the owner. Acknowledging issues can raise concerns about whether those involved are performing their duties properly. This, while addressing them is likely to result in additional work for everyone. As a result, failures often come as a “surprise” once the repair begins to deteriorate.
Schmidt is a Director of HMP (www.hindlemason.co.za)
