The use of drones for detecting changes in bridge condition brings a new dimension to structural condition monitoring
Hundreds of thousands of aging road and rail bridges spread across vast territories of the USA have been causing a headache for highways authorities in assessing their condition and planning maintenance work to prevent potentially catastrophic failure.
The problem is intensified by the “critical mass” of bridges falling into an age range during which they’re beginning to show signs of failure after the bridge building boom 20-30 years ago.
Historically, bridges have always been inspected on the ground using visual methods, measuring and core sampling the bridge decks at points where the sub-surface is suspected to be deteriorating.
Now, drones are increasingly being used with sensing apparatus to detect defects from afar, but is it enough on its own?
US Drone Trials
Electronics giant, Intel has been collaborating with two departments of transport in the USA to improve bridge inspections, supplementing manual inspections of the Daniel Carter Beard Bridge connecting Ohio and Kentucky and the Stone Arch Bridge in Minnesota. Throughout the inspections, Intel’s advanced automated commercial drone hardware and software increased efficiency and produced more reliable data in a fraction of the time and cost of traditional methods.
According to Intel’s vice president in charge of the drone team, Anil Nanduri, bridges throughout the world have been experiencing undetected structural issues due to inefficient inspection and monitoring processes and unreliable data for rehabilitation.
“With aging infrastructure, it’s critical to address the real-world concern of transport safety and with Intel’s drones, not only is speed and accuracy improved through increasing automation of existing workflows, but also safety risks are reduced through providing engineers and transport agencies with more reliable, actionable insights for maintenance planning,” he says.
To achieve this on the high traffic volume Daniel Carter Beard Bridge, the Intel drone captured about 2,500 high-resolution aerial images, generating 22GB of data that was uploaded into the Intel cloud platform. Using the images, a 3D model – or a digital twin of the structure – was generated to aid with analyses and visualisation that can also be applied to monitoring the paint deterioration and cable stability of the bridge over time.
In Minnesota, Intel worked alongside structural engineering company, Collins Engineering to inspect several bridges of different types and compare the results to manual inspection methods.
In these cases, different sensors were deployed so that still images, video and Infrared images could be used to compare the value received from them.
Evaluation is continuing throughout 2019 with benefits already having been achieved in the visualisation of changes in structural integrity, wear and surface deterioration as well as finding potential sub-surface defects in the bridge decks using infrared imagery.
Overall, the visual inspection element alone saves considerable time over manual inspections due to the ability to cover more bridges over a wider area in less time. However, the sub-surface defect detection capabilities of IR sensors could yield bigger benefits in reducing ground-level inspections with associated closures and traffic disruption.
Deck level inspection
US company, Infrasense has long been engaged in the technology of bridge inspection and has reduced lead times for performing bridge deck assessments through the use of sensing devices to replace the need for labour-intensive coring operations, which require extended road closures.
There are two main issues that are investigated at the time of inspection – delamination and reinforcing bar (rebar) corrosion.
Ground Penetrating Radar (GPR)
Radar antennas fixed to a vehicle are driven along the bridge driving lanes to transmit electromagnetic energy pulses into the bridge deck. Several passes are often required with different energy frequencies to gain the optimum combination of penetration depth and resolution.
The return signal is analysed to provide insight into the bridge deck layer thicknesses and properties and is of particular benefit in detecting rebar problems, which often occur deep inside the deck.
IR Thermography is also a vehicle based technique and can be performed in a single pass of each driving lane at normal speeds. The thermal properties are evaluated and mapped onto an image of the bridge deck to visualise the temperature profile and identify areas of delamination or debonding of the road surface.
The benefit of the vehicle based approached is that heavier cooled cameras can be used, which provide greater resolution and clarity at high speed. The alternative microbolometer IR cameras are lighter and cheaper but don’t provide as much detail.
Infrared Thermography Pilot Study
Recognising the more widespread use of drones for aerial visual inspections, Infrasense has moved into the use of UAV technology for making IR inspections of bridge decks. Due to weight considerations, microbolometer IR cameras are used.
The company recently completed internal research to assess the feasibility and accuracy of aerial imaging for mapping bridge deck conditions. The results of this study provided support for implementation on future projects. The primary advantage of the aerial method is its efficiency. Data can be collected for hundreds of bridge decks in a single day, and, using Infrasense’s processing and analysis methods, defects can be quantified and mapped very quickly.
The two methods used for aerial surveys of bridge decks are infrared thermography (IR) and high-resolution visual (HRV). The infrared technology was able to detect delaminations just under the surface of the bridge deck, which were then plotted onto an infrared map, where delaminated boundaries could be delineated. HRV was coupled with this method to rule out any thermal anomalies on the deck such as debris, shadows, wet spots or staining.
Keen to adopt the technology, Infrasense believes that aerial imaging will open the door to comprehensive routine bridge deck evaluations at a relatively low cost.
This fills a real need in the inspection market as accurate, network-level data is required to facilitate asset management decisions, according to an Infrasense spokesperson.
A Combination of Approaches
Creating a map of potential problems is only the first part of making bridge maintenance plans and more data is always needed from the ground. The main advantage of the use of drones is the speed with which initial assessments can be made.
Once this has been done, highways agencies can prioritise the next phase of work to make detailed assessments based on coring and physical inspections, from which a maintenance plan can be made. By only focusing on assets identified as needing attention, time and money is saved.
To obtain the greatest benefit from the use of drones, there needs to be a combination of technologies and skills. With Intel’s expertise in drones and data management, the structural engineering expertise of companies such as Collins Engineering and the sensing and imaging expertise of Infrasense, a detailed and structured approach can be taken to the way different highways agencies can manage the hundreds of thousands of assets that are under their care.