Introduction: The Role of Non-Destructive Testing and On-Site Challenges

In Japan, many bridges and tunnels constructed during the period of high economic growth have been in service for more than 50 years, and ensuring the safety of these aging structures has become a major social issue. In fact, there are said to be about 700,000 road bridges and more than 10,000 road tunnels nationwide, making countermeasures for deterioration an urgent task. Regular non-destructive testing (NDT: Non-Destructive Testing) is indispensable to maintaining these infrastructures in sound condition. NDT is a collective term for inspection methods that examine deterioration or damage without destroying the target, and it is used to identify cracks, corrosion, deformation, and other issues in structures so that countermeasures can be taken early. Typical methods include close-range visual inspection, tap testing, and ultrasonic testing, but these all involve significant burdens for on-site technicians.

On site, there are many inspection targets, many of which are located at height or in confined spaces, so simply carrying out regular inspections requires considerable labor and time. For example, close-range visual inspections of road bridges and tunnels are legally required roughly every five years, generating enormous inspection demand nationwide. Coupled with a shortage of technicians and an aging workforce, efficiently inspecting many structures with limited personnel is a major challenge.

Inspections also rely heavily on individual technicians’ experience and intuition (subjectivity), and recording methods remain largely analog—field notebooks written by hand or report creation by pasting photos—so not only is the workload heavy, but the accuracy and reproducibility of data tend to vary. Often, inspection results are not fully utilized and end up merely filed away on paper.

Limitations of Conventional Non-Destructive Testing Methods

Conventional infrastructure inspections required separate recording of photos taken by cameras and measurement values from instruments. For example, when investigating cracks on a concrete surface, if an anomaly is found visually, one would photograph it with a digital camera and then measure the width with a ruler or crack gauge and write the dimensions in a notebook. Because imaging devices and measurement instruments are separated, it is not intuitive later on when looking back at photos which measurement corresponded to which location, and matching them during report preparation is time-consuming. Marking damage locations on paper drawings or transcribing into spreadsheets makes recording work complicated and prone to errors.

Such manual inspection procedures leave challenges for data reproducibility and accumulated data utilization. If deterioration points noted in an initial inspection are overlooked or cannot be located during the next inspection, tracking changes over time becomes difficult. When each person records in their own format and focuses on different points, collected inspection data are not effectively utilized and often end up only summarized in reports and filed away. Furthermore, there has been no easy way to share or analyze measured values on site, causing time lags in identifying problem areas and sharing information among stakeholders. As a result, a disproportionate amount of time is often consumed by administrative tasks such as organizing hundreds of photos and preparing report drawings, sometimes more than by the on-site work itself.

“Visible, Recordable, Measurable” NDT Realized by Smartphone + LRTK

To address these challenges, digitization of inspections using ICT and robotics has been promoted in recent years. Among these, approaches that leverage everyday smartphones are expected to penetrate the field. Some modern smartphones, such as recent iPhones, are equipped with LiDAR (light detection and ranging) sensors, high-performance cameras, and GPS, enabling advanced spatial measurement in a compact device. Even smartphones without LiDAR can acquire point clouds using photogrammetry techniques that reconstruct 3D from multiple photos, but that requires many shots and processing time, so using a LiDAR-equipped device is more efficient. By combining this with a GNSS receiver device that enables centimeter-level outdoor positioning—LRTK—a smartphone can quickly become a high-precision 3D scanner and surveying instrument. LRTK is a pocket-sized RTK-GNSS module used attached to a smartphone; it enhances position information in real time by leveraging correction satellites (such as the quasi-zenith satellite Michibiki), achieving positioning accuracy of about ±2 cm horizontally and ±3 cm vertically. As a result, each point in the point cloud data acquired by the smartphone is tagged with latitude, longitude, and elevation, making 3D records with global coordinates—which were difficult before—easily achievable on-site.

With a smartphone + LRTK, the previously separate tasks of “see, measure, and record” can be performed at once. Scanning a structure’s surface with LiDAR visualizes the site as a three-dimensional model, allowing cracks and deformations to be confirmed volumetrically on the spot. Because the acquired high-density point cloud data and photos are tied to positional coordinates, important deterioration areas can be reliably recorded digitally. Of course, dimensions can be freely measured later on the point cloud model, eliminating the need for “re-measuring crack widths with a ruler while looking at drawings” later. It is fair to say that the smartphone alone can achieve inspection that is “visible, recordable, and measurable.”

Moreover, measurement data obtained with a smartphone can be uploaded to the cloud for centralized management. Previously, field photos and paper reports could only be shared individually, but sharing point clouds and other data in the cloud allows the office to immediately grasp the site’s 3D situation. Stakeholders can discuss deterioration while looking at the same model in real time. Each photo also has coordinates and orientation attached on a map, so it is intuitive which structure and which location the image shows, markedly improving the accuracy of reporting materials. The intuitive operation of smartphone apps makes them accessible from veteran to novice staff, helping to eliminate dependence on individual expertise. Through such digitalization of inspection results, reproducibility and objectivity increase dramatically, enabling consistent-quality recordings of the actual condition regardless of who performs the inspection.

Benefits by Use Case: Bridges, Building Exteriors, Piping Equipment, Buried Utilities, Pavement

3D records using smartphone + LRTK are powerful across many infrastructure inspection scenarios. Here are specific benefits by use case.

• Bridges: Inspections of bridges and elevated structures face challenges in accessing high or under-girder areas. Smartphone 3D scanning can record the shape and damage of structures in detail within reachable areas without scaffolding or elevated work platforms. For example, cracks on the underside of girders can be preserved as point cloud data with accurate position and dimensions, allowing safe analysis in the office and reuse in reports. Even when close visual inspection is performed using bridge inspection vehicles, capturing 3D records alongside preserves comprehensive inspection results. Accumulating 3D data from each periodic inspection enables quantitative assessment of changes from the previous inspection, aiding repair prioritization. Reducing the number of inspection vehicle dispatches and long traffic restrictions through 3D recording can also lower costs and reduce inconvenience to users.

• Building Exteriors: For building exterior inspections, it is important not to miss tile detachments or cracks. In recent years, exterior wall surveys have been mandated for buildings above certain heights, requiring building owners to carry out regular safety inspections. While binocular visual inspection and sounding rods have been central, combining smartphone scans allows the entire façade to be recorded as a digital field notebook. LiDAR scanning while positioned near the façade with an aerial work platform enables later office-based marking of delamination or cracks on the 3D model, and report preparation can accurately indicate “what kind of deterioration exists on which floor and at what position.” Subtle deformations that are hard to convey in flat photos can be shown volumetrically, making explanations to building owners and consideration of repair scopes much easier. Given recent social issues with falling façade materials, objective records from 3D scanning increase inspection reliability and contribute to responsible maintenance.

• Piping Equipment: 3D scanning is effective for plant piping and equipment inspections. Converting complex piping and machinery into a point cloud ensures corrosion and leakage points are not overlooked. Because measurements can be taken from a distance even for high-temperature pipes or in narrow plant interiors, safety and efficiency improve. Sharing acquired data in the cloud allows remote experts to advise while viewing the site’s 3D model, facilitating remote support from inspection specialists. It is also possible to measure clearances (gaps) and pipe diameters on the point cloud and directly reflect them in repair or replacement planning. Scanning once means you no longer need to make repeated site measurements, shortening lead times from inspection to repair design. Measuring deterioration from a safe distance without stopping operations minimizes impact on production.

• Buried Utilities: Water and sewer pipes, cables, and other buried utilities become hard to locate once backfilled, but if they are 3D-recorded with a smartphone during construction, the invisible infrastructure becomes visible. Scanning the trench and surroundings at the time of laying pipes produces a 3D map faithfully reflecting burial depths and routes. Sharing this in the cloud not only aids future maintenance and excavation but can also be used for interference risk assessments with other works. The Ministry of Land, Infrastructure, Transport and Tourism is also promoting 3D recording of buried utilities, and such smartphone-measured data can contribute to building unified underground space maps in the future. In small-scale projects, companies can complete records in-house without contracting external surveyors, reducing costs and streamlining construction processes. This can also help prevent damage accidents during digging caused by inadequate records of buried utilities.

• Pavement: Road and pavement inspections involve many tedious tasks such as counting cracks and measuring rut depths. Smartphone 3D scanning of pavement can digitize damaged areas on the spot, allowing necessary indicators to be calculated freely back at the office. For example, surface steps and unevenness can be analyzed for cross-sectional shape on the point cloud, and settlement amounts or pothole volumes can be calculated automatically. If inspectors carry smartphones during patrol inspections, roadway conditions can be saved in 3D on the spot and defect data centrally managed on a GIS map. This makes it easy to judge repair necessity immediately at the scene and to calculate repair material quantities with high accuracy, speeding up the PDCA cycle of maintenance management. Accumulated pavement inspection data can be analyzed and used for repair planning.

Furthermore, acquired 3D data can be easily integrated with existing drawings and other survey data. Overlaying point clouds with CAD drawings or BIM models makes it smooth to visualize deviations from the design by color-coding, facilitating repair planning. Advanced analyses, such as comparing past and new point clouds to quantify deterioration progression, are also achievable. Data stored in the cloud can be linked with GIS maps and asset management systems, enabling consistent information sharing across sites, offices, owners, and contractors. As data become interconnected, the efficiency and accuracy of inspection work will rise further. Integration with AI for automatic crack detection and displacement analysis is also expected. Smartphone inspection data will play an increasingly important role in the advancement of infrastructure maintenance management.

In fact, at the Noto Peninsula earthquake in 2024, technicians who rushed to the affected sites scanned collapsed and deformed locations with LRTK-equipped smartphones and immediately shared the high-precision point cloud models and geotagged photos via the cloud. They measured on site the settlement of utility poles tilted by liquefaction and the depth of road cracks, enabling related agencies to quickly grasp the situation. This is a good example of how digital inspection methods contributed to speedy decision-making in disaster response, where compiling survey results used to take a long time. High-precision measurements enabled by one smartphone-per-person surveying devices are also proving powerful in emergency infrastructure inspections and disaster surveys.

Conclusion: The Future of Infrastructure Inspection Expanded by Simple Surveying with LRTK

The NDT solution using a smartphone + LRTK not only solves on-site inspection challenges but also opens new possibilities for infrastructure maintenance. Because it has lower introduction costs and is easier to operate than specialized equipment, it can significantly lower the barrier to on-site DX. With a single smartphone capable of high-precision surveying, measurement, and data sharing, the various tasks surrounding inspections have become more broadly applicable.

The main benefits of introducing smartphone + LRTK can be summarized as follows:

• Efficiency: Substantial reductions in the labor and time required for inspections. Paper records and manual dimension measurements are reduced, allowing a small team to inspect many facilities efficiently.

• Higher Accuracy: RTK positioning records location information with centimeter-level precision, improving the reliability of inspection data. Accurate dimensional data enable quantitative evaluation of deterioration. Human reading errors and transcription mistakes are prevented, facilitating quality control.

• Improved Safety: Measurements can be taken from a distance, reducing the risks of working at height or in hazardous locations. Compared with conventional methods that rely on scaffolding or aerial work platforms, this contributes to worker safety. Reducing the need for scaffolding or lane closures also lowers impacts on users and costs while ensuring worker safety.

• Information Sharing: Inspection data can be shared immediately via the cloud, smoothing collaboration between site and office and between owners and contractors. Experienced technicians can remotely review data to support newcomers. Past data are accumulated and can be used for long-term asset management.

• Versatility: Acquired 3D data can be used for not only inspections but also repair planning, as-built management, and other purposes. With a single smartphone + LRTK, surveying and design support can be handled, expanding work scope while limiting additional equipment investment. Applications in disaster surveys and as-built drawing preparation are also anticipated.

Moreover, 3D data obtained from inspections can be used for simple pre-repair “as-built surveying” (taking dimensions and calculating quantities). Tasks that previously required arranging a separate surveying team can now be completed quickly with a single smartphone, smoothing the transition from investigation to construction. Using LRTK, it is also possible to stake out and transfer coordinates from design drawings on site and to handle pile-driving layout and marking out (position setting) alone. AR functionality can overlay 3D models of the completed image onto the real scene to give instructions like “install a bolt at this position” or “repair this area,” enabling a seamless workflow from identifying defects in inspection to planning repairs.

In this way, the combination of smartphone and LRTK powerfully accelerates the DX of infrastructure inspection. Equipping each field worker with a high-precision “digital eye” is expected to prevent overlooked deterioration, enhance record quality, and speed up decision-making. Why not adopt this new technology that realizes non-destructive testing with a smartphone and open a new era of safety, efficiency, and accuracy at your site?