As seen in NDT.org, November 8, 2017 Edition
Proper Post Tension Tendon Inspections
Help Prevent Bridge Disasters
Proper Post Tension Tendon Inspections Help Prevent Bridge Disasters
Since its initial development, the use of post-tensioned reinforcement has been used to construct concrete structures so that concrete slabs can result in thinner sections and/or longer spans between supports. Modern bridges would not be able to be what they are without post tension tendons being utilized. The prestressed tendons are critical in the load bearing ability of the concrete and the reason the concrete doesn’t crack under its own weight. Post tensioning technique allows you to build almost any shape or structure in less time, material and money as compared to the older methods of construction.
Some of the benefits of post tensioning are:
Flexibility of design
Reduced material and maintenance cost
Reduced construction time
Durable structure with enhanced life span
Reduced environmental impact
How post tensioning works?
As we all know, concrete is the basic material used in construction of our bridges for its compressive strength, that is the amount of load the structure can take prior to cracking or failing under its weight. This means that it can bear its own weight within a structure but can tend to sag or deflect as soon as you introduce the concrete structure to live loads, like vehicular load on bridge. Deflection in engineering terms; “deflection is the degree to which a structural element is displaced under a load. It may refer to an angle or a distance.” The sagging and deflection is caused due to the daily usage and changes in the load. This can result in cracks which can weaken the structure.
Another major problem with a concrete structure is that it has low tensile strength and low flexibility. To overcome this problem, steel reinforcement bar or “rebar” is used to limit the cracks by embedding them in the concrete structure. But this rebar doesn’t share any live load of the concrete structure until it is cracked.
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 post tensioned tendon which is an assembly of high strength pre-stressing steel bar or strand, sheath or protective duct often made up of plastic, grout or corrosion preventing coating and anchorage.
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 on the structure and fixed into its place by anchoring components and thus prevents any occurrence of cracking and prolongs the life of the structure. In addition, the utilization of post tensioning has allowed us to build buildings and bridges as well as other structures using thinner concrete slabs due to concretes increased strength.
This is very important in building complex structure like bridges because it involves demanding geometry requirements like variable super elevation, grade changes and complex curves as well as load changes.
What causes post tensioned tendon failure?
Post tensioned tendon 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 passivization and HDPE duct prevent the tendon from external moisture and other aggressive chemicals hoping to prevent deterioration and corrosion. However, cases of PT tendon corrosion have been found in Florida and elsewhere.
In some cases corrosion occurs due to improper grouting procedure and accumulation of bleed water. New grout materials have been formulated especially for post tensioning to minimize bleed water formation but occurrence of corrosion also happened with the use of these new grouts. There have been numerous cases where the wrong grout mixture has caused premature deterioration.
Some of the PT bridges in Florida with the new grout have been found to have material deficiencies in the grout that was responsible for the PT tendon failure in just eight years of service. It was found that the grout in these tendons had a high moisture content, high pore water pH, low chloride concentration and enhanced sulphate concentration. This corrosion is found in regions where grout segregation was severe. This corrosion is enhanced with formation of macrocells between the regions of dissimilar aerated condition caused by interstitial crevice environment and varying moisture content. A study on corrosion caused by bad grout conducted by the Florida department of transportation can be found here.. https://www.fdot.gov/research/Completed_Proj/Summary_SMO/FDOT-BDV29-977-04-rpt.pdf
How tendon failures can be avoided?
The last step in any construction is the most crucial as it typically includes cutting of strand tails, installation of protective end caps, cleaning of stressing pocket as well as patching it with high quality mortar. These processes assure the durability of the PT tendons but if these processes are done improperly or if the grout is not mixed properly, then it can compromise the durability of the tendons causing tendon failure. Therefore, it must be mandatory for the inspector to check the tendon finishing.
Besides inspecting the tendons at finishing, its regular inspection is also critical for its proper functioning. The most common current method of inspecting post tension tendons in manual in nature, subjective and prone to inconsistencies.
Infrastructure Preservation Corporation (IPC) has developed the first post tensioned tendon scanning system named “TendonScan”which can detect even the slightest fault in the PT tendons by using non-destructive testing methods.
How TendonScan works?
TendonScan utilizes modern technology and robotics incorporating non destructive testing methods to locate section loss and corrosion within external tendons. The information recorded from this inspection provides the asset owners/department of transportation with a quantitative post tension tendon inspection of the bridge tendons. The tendon scanning system couple itself with the tendon and travel along the whole length of it while an inspector is present on the mobile control unit consisting of sensors. The data collected from these sensors can be collected for real time assessment and the information is also stored for later use.
An IPC company representative stated ” Asset owners, typically the department of transportation needs better information in order to make more intelligent budgetary decisions.” TendonScan® can provide the quantitative information required for the DOT to make precise budgetary decisions. and precise information about corrosion, voids, bleeding grout and any other problem long before the condition of the tendon becomes threatening. TendonScan helps the decision makers make the right decision on time about repairing or replacing a PT tendon in the structure and it does this within the DOT’s current maintenance budgets.”