Robotic Assessment Of Post Tension Cables Help Preserve Our Nations BridgesApril 6th, 2017 Author: Doug Thaler
It is commonly accepted that our infrastructure is rapidly aging
and has not been maintained properly over the fifty years or so. With many of our bridges having outlived their intended useful life, more attention needs to be paid to conducting proper inspections and maintenance.
The FHWA (Federal Highway Administration) requires that our bridges be inspected every 2 years. Even with this mandate, the inspection services and technology used for these inspections are as old as the structures themselves. In fact, many of the procedures and services that state use to conduct bridge inspections, still include dragging a chain across a bridge to listen for issues within
the concrete deck, or hitting a hammer against a post tension cable and listen for differences in the sound of the cable. The fact that these types of services are still in use prompted IPC, Infrastructure Preservation Corporation, to develop a line of robotic inspection services.
These services utilize modern technology and robotics to specifically update these inspections to provide the asset managers with the quantitative data they need to properly budget their maintenance in order of severity. Some of IPC’s services can peer through concrete and steel to create an MRI like image of the components being inspected, and better assess various critical components of our ailing infrastructure .
One of the most important components of our infrastructure for the past 25 years are called “post tension cables”. These cables hold up long concrete structures.”Post tensioning” is a technique for reinforcing concrete.
Post-tensioning tendons (Post tension cables), which are prestressing steel cables inside plastic ducts or sleeves, are positioned in the forms before the concrete is placed.” These tendons hold up our nations bridges, dams, exit ramps, and other large concrete structures. If a post tension cable corrodes, it will compromise the structure that it is reinforcing. If enough post tension cables fail, the structure can be compromised.
Until recently, the only way an asset manager could inspect a post tension cable, would be with a helmet flashlight, camera, and pad and paper. In some cases, an inspector would hit the tendon with a hammer and listen to changes in the sound of the tendon. An electromagnetic inspection can also be made to a specific section of a post tension cable. These methods have not been found to be a commercially viable option on a large scale or even for normal maintenance until now . When nondestructive evaluations came on the scene, a more comprehensive approach started to be taken. Still to this day, IPC is the only company that we know of that has a commercially viable option to thoroughly robotically inspect the post tensioned cables on a bridge, as a standard inspection method within the current DOT’s current inspection budget.
Visual inspections complemented by bore scope inspections are a preliminary means of identifying areas of corrosion. However, to quantify the rate of corrosion, judicious use of electrochemical and nondestructive evaluation (NDE) tools must be used. These NDE tools include infrared cameras, ground-penetrating radar, ultrasonic tomography, impact echo, and corrosion-rate probes. These have all been used for spot checking and in conjunction with each other to try to verify problem areas prior to opening or drilling into a tendon and further exposing it to air causing further deterioration. Most of these techniques were employed by NDT (Nondestructive Testing) companies that were focused on NDT technology, but were not familiar with the components of the bridge that they were inspecting. They were familiar only with the technology they were employing and not the construction, nor mechanics of bridge maintenance. Options and expertise were limited, until now.
Infrastructure Preservation Corporation developed and patented two post tension inspection robots and launched an inspection service called TendonScan® . TendonScan® has two components. The first robotic device glides along a post tension cable and locates air water and bleeding grout within the post tension cable. Air, water, and bleeding grout- once they occur inside a tendon- they accelerate corrosion. They are in affect the precursors to deterioration. Locating areas that contain air, water, and bleeding grout allow for early stage problems to be identified, so they may be monitored or repaired, if required. Subsequent inspections can show deterioration progression over time to better plan for maintenance.
This breadbox size device attaches to a tendon and takes an MRI like image of the interior of the tendon that is transmitted to a control station that shows any areas of concern. The area in question is marked and a cross section view recorded showing the size and shape of any anomaly, as well as the position in the tendon.
The second robotic TendonScan unit locates loss of steel within a post tension cable. This is showing that corrosion has occurred and what percentage of deterioration or steel loss exists within the tendon. Locating steel loss is important as it shows that the tendon has been compromised and may need immediate attention. The unit locates steel loss down to .1 percent.
Both TendonScan® units are portable, lightweight wireless, and easy to operate.
By locating early stage deterioration, IPC provides the asset manager the information they need to:
1. Quantify where deterioration is and how far along the issues are.
2. Budget and plan for repairs starting with the most severe issues.
3. Make repairs earlier in an assets life to help extend the service life of that asset.
4. Schedule repairs and prevent untimely replacements of infrastructure assets, increase safety and save billions in infrastructure costs.
Post tensioning and post tension cables are a mainstay of our nations infrastructure. Many of these cables have never been properly tested and spot checking does not constitute a thorough inspection.
In 2011 a FHWA (Federal Highway Administration) paper was released titled Elevated Chloride Levels in SikaGrout® 300 PT Cementitious Grout. This grout was deemed defective. Grout with high chloride content will break down much faster and corrosion will ensue. The Marion Plant for Sika produced 20,327,300 lbs. of this product from November 2002 to March 2010 with approximately 16,000,000 lbs of the grout used in bridge project post-tensioning applications. Many of the bridges build that used sika grout started to have issues almost immediately.
It was at this time that IPC became aware of the high chloride content and immediately began work on a way to inspect post tension cables. To see if they were corroding, their TendonScan® project was born. Five years later, after patenting and field testing their process, IPC launched TendonScan®. TendonScan® was created to assist the department of transportation and asset managers worldwide to inspect critical infrastructure assets that contain post tension cables.
Early detection, repair, and replacement of Post Tension cables is critical to maintain the integrity of our nations bridges. To locate and make repairs early in an assets life cycle can help extend the service live of those assets tremendously. Post tension cables are part of the regular inspection process of the bridge, conducted every 2 years for each and every bridge. With the development of modern technology and robotics we need to update our inspection methods if we intend to repair our infrastructure. A trillion dollars in federal spending would not cover all of the bridge replacements if lack of proper maintenance continues and without being mandated, it will continue.
If you have a project that you would like to discuss with Infrastructure Preservation Corporation ,”IPC” or you would like more information on TendonScan®, contact