How to evaluate and manage post-tensioned concrete

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.

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