Impact of Deterioration of Post Tensioned Tendons in Bridges
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An October 2016 report by the FDOT stated “Recent corrosion failures of post-tensioned (PT) tendons in the Ringling Causeway Bridge. Severe corrosion was accommodated by segregated grout that was characterized as having high moisture content…” Did the bridge built in 2003 take 13 years to go bad?
Why weren’t current inspection practices sufficient enough to find issues before corrosion became so severe?
An online Transportation Research Board (TRB) report details the collapse of a segmental post-tensioned concrete bridge due to corrosion. https://onlinepubs.trb.org/Onlinepubs/trr/1989/1211/1211-005.pdf.
The report points out how difficult it is for a manual subjective inspection to reveal corrosion in post-tension tendons.
What is Post Tensioning?
Post-tensioning is a technique for reinforcing concrete. Post-tensioning tendons, which are prestressed steel cables inside plastic ducts or sleeves, are positioned in the forms before the concrete is placed. Afterward, once the concrete has gained strength but before the service loads are applied, the cables are pulled tight, or tensioned, and anchored against the outer edges of the concrete. https://www.concretenetwork.com/post-tension/
Post-tensioning is used in all large concrete structures. In bridge construction it allows you to build very large concrete structures of almost any shape. Conventionally without Post Tensioning concrete would have to be poured very thick to handle loads, in addition, the size and length of concrete structures were kept short to manage the concrete weight and load-carrying ability. With today’s post-tensioning techniques and materials, amazing structures are appearing worldwide. Post-tensioning allows construction to be conducted in less time and at a lower price.
Some additional benefits of building with post-tensioning are:
- Flexibility of design
- Reduced material and maintenance cost
- Reduced construction time
- Durable structure with an enhanced life span
- Reduced environmental impact
How does post-tensioning work?
As we all know, concrete is the basic material used in construction for its compressive strength. This means that it can bear its own weight. Once formed within a large structure it tends to sag or deflect as soon as it is introduced to live loads like a vehicular load on a bridge. This sagging and deflection is caused due to the daily use of the concrete structure and can result in cracks that can weaken the structure.
Another major problem with the concrete structure is that it has low tensile strength and low flexibility. To overcome this problem, a steel reinforcement bar or “rebar” is used to limit the cracks by embedding them in the concrete structure. But this rebar isn’t enough to manage the live load of the concrete structure.
Post-tensioning is an active reinforcing technique used to place the concrete structure under compressive stress where it can face tensile stress. Post-tensioning is applied with the help of a post-tensioned tendon which is an assembly of high-strength prestressing steel bar or strand, sheath, or protective duct often made up of plastic, grout, or corrosion preventing coating and anchorage.
The tensile strength of the prestressed steel used in post-tensioning is four times higher than the common rebar used in regular concrete structures. It stretches as the active load is applied to the structure and fixed into its place by anchoring components and thus preventing the occurrence of cracking and can help prolong the life of the structure and keep it safe under loads.
This is very important in building complex structures like bridges because it involves demanding geometry requirements like variable super elevation, grade changes, and complex curves.
What causes post-tensioned tendon failure?
Post-tensioned tendons are generally embedded in grout material with highly alkaline pore water filled within a High-Density Polyethylene (HDPE) duct to prevent corrosion. The high alkaline environment steel passivation and HDPE (Polyethelene) duct prevent the tendon from external moisture and other aggressive chemicals. However, cases of PT tendon corrosion have been found worldwide.
This corrosion occurs due to improper grouting procedures and the accumulation of bleed water. New grout material has been formulated especially for post-tensioning to minimize bleed water formation but the occurrence of corrosion also happened with the use of new grouts.
Some of the PT bridges in Ohio, Florida, South Carolina, and Tennessee have been found with high chloride grout and have been responsible for the PT tendon failure after just eight years of service. It was found that the grout had high moisture content, and high pore water pH and was most likely mixed improperly by the manufacturer. Current manual inspection procedures would not detect this type of corrosion until it is too late.
How tendon failures can be avoided?
The finishing step in any construction is a very crucial step. It includes cutting strand tails, installation of protective end caps, cleaning of stressing pocket, and patching it with high-quality mortar.
These processes assure the durability of the PT tendons but if these processes are done improperly then it can compromise the durability of the tendons causing tendon failure. Therefore, it must be mandatory for the inspector to check the tendon and be extremely diligent when finishing their work
Besides inspecting the tendons at finishing, its regular inspection is also critical for its proper functioning. In the US bridges are inspected every two years and bridges with any deficiencies are typically inspected every year. Infraspect, Clearwater Florida has developed and patented the first post-tension tendon scanner and inspection service aptly named “TendonScan®”.
TendonScan can detect even the slightest fault in the PT tendons by using non-destructive testing methods.
How TendonScan work?
TendonScan is Infraspect’s trademarked name for their post-tension tendon inspection service. TendonScan is composed of two patented devices. One that utilizes magnetic flux and one that utilizes Electrical Capacitance Tomography (ECT) which performs an MRI-like assessment of the material in the PT tendon.
The devices separately can locate the precursors to corrosion, air, and water which is great for the normal inspection cycle. The second unit locates section loss and corrosion. Typically when it is too late and the tendon needs immediate attention.
The tendon scanning systems couple themselves with the tendon and travel along the whole length of it while an inspector monitors progress from a remote control unit. The data collected from these sensors can be collected for real-time assessment and also stored for later comparison that can show progression in deterioration.
How does it help?
TendonScan for the first time gives the bridge or asset owner the information they need to properly allocate their assets to make repairs in a timely manner. Asset owners need better information in order to make more intelligent budgetary decisions.”
TendonScan can provide accurate and precise information about corrosion, voids, bleeding grout, and any other problem long before the condition of the tendon becomes threatened. It helps the decision-makers to make the right decisions about repairing or replacing a PT tendon in the structure. It also ensures the safety of the structure and of the public.