What is GPR testing of concrete?
GPR testing of concrete non-invasively maps internal features of concrete structures by emitting electromagnetic pulses and recording their returns to detect differences in material properties and identify rebar, conduits, voids, and slab thickness.
GPR is used to ensure structural safety, guide repair or maintenance work, and assist in renovation projects by providing critical information without damaging the concrete.
How does GPR testing work?
The principle behind GPR testing in concrete structures involves emitting electromagnetic pulses into the concrete. These pulses reflect back upon encountering materials with different dielectric properties, such as voids, rebar, or changes in material density. By analysing these reflections, it's possible to map the internal features of concrete structures non-destructively.
What is GPR testing used for?
Deterioration process | Defects | Control of repairs |
Thickness measurement |
How do I carry out GPR testing?
To conduct a GPR survey of a concrete element it is vital to follow the procedures laid out in technical documents such as ASTM D6087-22, ASTM D6432-19 and ETSI EG 202 730, 2009. These are:
Plan the survey layout, considering the size and complexity of the concrete element, by reviewing structural drawings and schematics to understand element geometry and potential subsurface features.
Select the method of measurement. A grid system often segments the element into manageable sections, systematic scanning ensures comprehensive coverage.
Choose appropriate GPR equipment with the optimal operating frequencies, antenna configurations and system sensitivity to collect the specific data required (ETSI EG 202 730, 2009).
Calibrate the GPR equipment based on the specific composition and configuration of the concrete element under investigation and the location and type of information required (ASTM D6087-22).
Systematically scan the concrete element moving the GPR equipment uniformly across the grid layout maintaining consistent speed and direction along a predefined path to maximize coverage and minimize data gaps, ensuring the GPR waves penetrate the subsurface at consistent intervals as this facilitates the detailed mapping of internal features and anomalies within the concrete structure.
Analyse the GPR data, identifying anomalies that indicate subsurface features or differences in material properties using techniques such as tomography and 3D visualizations to depict internal features or creating graphs and 2D slices representing the scanned area.
What equipment and expertise are required for GPR testing?
For GPR testing of concrete structures, essential equipment includes GPR devices like the GSSI StructureScan Mini, MALÅ Geoscience Concrete Explorer, and Sensors & Software's Conquest 100.
GPR scanners come in various designs for specific uses, ranging from handheld units for precise investigations to wheel-mounted ones for extensive surveys. These systems are optimized for different depths and precision requirements.
Technological advancements have led to the development of drone-integrated GPR systems like Drone Arrivals Drone GPR, enabling aerial surveys that improve access to challenging terrains and large-scale areas.
Conducting GPR surveys on concrete structures demands a deep understanding of electromagnetic theory to grasp how GPR signals interact with different materials. This includes knowledge of signal propagation, reflection, and refraction principles.
Operators must be skilled in handling GPR equipment, adjusting parameters like frequency and depth resolution for optimal signal quality.
Advanced data interpretation skills are crucial, enabling the identification of subsurface anomalies, such as voids or rebar, through complex signal patterns.
This expertise requires a blend of theoretical knowledge and practical experience, often necessitating specialized training or education in geophysics or civil engineering.
What are the advantages of GPR testing?
GPR is adaptable to various structures, including historical buildings, where preserving the façade is crucial, and in complex environments like airports or bridges where safety is paramount.
Compared to core drilling or other invasive methods, GPR provides immediate results, facilitating faster decision-making and remediation planning.
GPR excels in detecting a range of internal anomalies, from moisture intrusion and delamination to voids and cracks, offering a clear picture of the structure’s health.
GPR combined with drone technology can allow scanning of extremely hard to reach or dangerous locations (Figure 5) and automated monitoring of large structures continuously over long periods of time with little user input.
What are the disadvantages GPR testing?
Accurate GPR data interpretation demands expertise, making it less accessible for general contractors without specialized training.
The presence of various materials and conditions within concrete can complicate data analysis, requiring advanced understanding to differentiate between anomalies.
The investment in GPR equipment and training can be substantial, posing a barrier for smaller projects or firms.
Factors such as extreme humidity or temperature can impact the effectiveness of GPR surveys, potentially limiting its applicability in certain climates.
How accurate is GPR testing?
The accuracy of GPR tests on concrete varies with frequency: a 1.5 GHz antenna offers a depth/thickness resolution of 5-40 cm with an accuracy of ±2 cm, suitable for detailed analysis of shallow features. A 500 MHz antenna reaches deeper, 25-150 cm with an accuracy of ±5 cm, but with decreased resolution. These figures are influenced by concrete conditions like moisture and chloride content, affecting signal penetration and clarity. High-frequency GPR provides finer detail at shallower depths, while lower frequencies offer broader, less detailed subsurface views.
What are the limits of GPR testing?
GPR testing in concrete structures encounters challenges such as sensitivity to moisture, which can scatter or absorb signals, and limitations in signal penetration, especially in dense or freshly poured concrete.
The presence of extensive metallic elements like rebar can reflect and distort GPR signals, complicating the interpretation process.
Additionally, multiple layers of reinforcement and low contrast between different materials can hinder accurate detection of subsurface features. These factors necessitate careful consideration and adaptation of GPR techniques for effective concrete assessment.
Ancillary information
Maturity of test: > 10 years
Qualification & interpretation : Specialised lab
Service disruption: No
Preliminary works: Yes
Time consumption Medium (one day)
Cost Medium
Access to element 1 face