Resistivity imaging for hidden voids beneath concrete
Hidden voids beneath rigid concrete pavements can remain invisible until cracking, spalling, settlement, or ponding exposes the problem. Erosion beneath a slab, a failed joint, leakage around drainage penetrations, and poorly compacted fill can all create a loss of support without producing a clear fracture pattern. Surface symptoms identify distress, but they rarely reveal its depth or extent with enough certainty for targeted repair. The risk is greatest where the slab carries heavy traffic or spans drainage and service corridors, because a small loss of support can trigger rapid deterioration.
Resistivity imaging, commonly delivered as electrical resistivity tomography (ERT), offers a non-destructive way to screen for these concealed changes. It maps variations in subsurface electrical resistivity rather than photographing a void directly, so the result is evidence of an anomaly that must be interpreted and, where consequences are significant, verified. Used with drilling, cores, ground-penetrating radar, and drainage inspection, it helps focus investigation and reduce unnecessary excavation.
How electrical contrast reveals subsurface changes
ERT normally uses electrodes placed along a survey line. A controlled current is injected through a pair of electrodes, voltage differences are measured by other electrodes, and the apparent-resistivity values are inverted to construct a two-dimensional model. Electrode spacing and array geometry influence the trade-off between shallow detail and depth of investigation; resolution generally becomes coarser with depth.
Sound concrete and dry granular material commonly show relatively high resistivity. An air-filled void may therefore appear as a high-resistivity zone, while a water-filled void can produce a low-resistivity feature, especially against dry fill. Saturation, salinity, temperature, compaction, and material type all affect the signal. ERT resolves underlying support conditions more readily than the slab itself, and nearby reinforcement, utilities, and metal fittings can distort the response. Resistivity values should be considered in ohm-metres and relative to the local baseline; no single value reliably defines a void in every material.
For agencies familiar with Jakarta’s public works testing market, UP. PPP’s published service fees can help frame a specialist-services budget, although Australian buyers should obtain current local quotations and confirm inclusions, accreditation, traffic control, and follow-up investigation in writing.
Designing a field survey for the target
A reliable survey starts with the construction and drainage record. Review as-built drawings, slab joints, reinforcement layouts, utility locations, flood levels, previous test results, and reports of leakage or settlement. Transects should cross the suspected defect and, where possible, the direction of groundwater movement. Line spacing and electrode spacing must be fine enough for the expected target yet remain practical around live roads and constrained urban sites. One transect may resolve a linear feature, while parallel transects are needed for wider or irregular areas; a full grid may be justified at complex sites.
On site, the array needs uninterrupted electrode contact, low contact resistance, and a stable background. Where concrete cover prevents good soil contact, specialists should use suitable access points or complementary methods rather than forcing a poor-contact array. Traffic control, lane closures, overhead clearances, and worker safety arrangements can determine whether a dense line is practical. A sound section of nearby pavement should also be measured as a local baseline.
Moisture control is essential because a shower, irrigation event, or dry season can change the electrical field before, during, or after acquisition. Calibrate the instrument, record electrode positions and contact resistance, repeat selected readings, and log temperature, weather, drainage conditions, and nearby sources of interference. Data rejected for poor contact should be reacquired, not concealed through smooth inversion settings.
Distinguishing voids from other anomalies
A low-resistivity feature is not automatically a water-filled void. It may represent leakage, saturated clay, a buried service, a damp interface, or an inversion artefact. High resistivity can also arise from dry aggregate, sound concrete, compacted fill, or the masking effect of a wet background. A credible void interpretation therefore depends on anomaly shape, depth continuity, comparison with sound pavement, and agreement with the distress mechanism.
ERT should be treated as anomaly detection. Ground-penetrating radar can add information about reinforcement, joints, and shallow layering, although conductive clay may strongly attenuate its signal. Impact echo and ultrasonic methods can check slab thickness or internal discontinuities. Cores, boreholes, and test pits provide direct confirmation, while drain cameras or flow testing may identify the process that created the loss of support.
Confidence should rise only when independent evidence supports the interpretation. The final report should separate observations from inferences, provide coordinates and depth ranges, state the inversion settings and fit error, identify areas of poor data quality, and explain what remains uncertain. A useful confidence system can label anomalies as probable, possible, or unresolved, with each label tied to supporting observations rather than visual intensity alone. A small core can confirm material properties without proving that an entire mapped anomaly is a void, so validation should be designed around the question being answered.
Adapting surveys to Australian conditions
Field conditions differ materially from Jakarta. Salt and storm-driven wetting can affect some Sydney corridors; reactive clay and fluctuating moisture influence many Melbourne sites; clay-rich Brisbane catchments may keep voids saturated and reduce electrical contrast; and Perth’s dry sandy profiles can create very high-resistivity baselines outside short wet periods. These contrasts should be characterised locally rather than assumed from a generic resistivity threshold.
Project delivery also follows Australian authority requirements, council specifications, Austroads guidance, relevant editions of AS 1742 for traffic management, and the AS 1012 series where concrete testing is required. Asset owners include state transport departments, Main Roads Queensland, Main Roads Western Australia, and local councils. Utility locating, work-zone safety, electrical equipment checks, and local drilling or coring availability belong in the method statement. Laboratories and contractors should be selected against the project’s required accreditation scope, with NATA accreditation commonly sought for testing conducted in Australia.
UP. PPP is a technical testing and measurement unit under Jakarta’s Dinas Bina Marga, applying SNI ISO/IEC 17025 and ISO 45001 to its laboratory and field work. Its accredited laboratory and field investigations cover asphalt, soil, concrete, elevation, and infrastructure assessment for road, bridge, drainage, and pedestrian projects. For Australian or international teams, these standards provide useful evidence of structured quality and safety systems, while local procurement still needs to address currency, insurance, work health and safety, data requirements, and Australian market pricing.
Clear records also prevent ambiguity when several parties must approve a repair. The Pennsylvania claim-status portal illustrates the value of a transparent status view in another public-service setting. A pavement survey register can apply the same principle by recording each anomaly, check performed, reviewer, and disposition.
Turning survey evidence into targeted decisions
The goal is not to produce a coloured anomaly map in isolation. It is to identify where support may have been lost, rank the likelihood and consequence, and select sensible verification or repair actions. ERT is particularly useful when many suspects are hidden beneath a continuous slab, when defects appear to follow drainage paths, or when a staged investigation would reduce disruption. High-consequence findings still need direct verification.
| Method | Main strength | Useful output | Important limitation |
|---|---|---|---|
| Electrical resistivity tomography | Continuous screening beneath the pavement | Depth-based anomaly map and prioritised targets | Indirect, non-unique interpretation |
| Ground-penetrating radar | High-resolution dielectric and reinforcement contrasts | Joints, rebar, layering, and some shallow defects | Strong attenuation in wet or clay-rich soil |
| Impact echo or ultrasound | Slab response and internal discontinuity checks | Thickness and indications of delamination | Does not directly prove a supporting void |
| Cores and boreholes | Direct material and support observations | Samples, stratigraphy, and local ground truth | Destructive and spatially limited |
| Test pits | Direct visual access | Exposed fill, voids, interfaces, and drainage | Disruptive and costly across a network |
Practical field actions that reduce uncertainty include:
- Define the target void size, plausible depth, pavement type, and decision required before choosing the electrode spacing.
- Reconcile drawings, utility records, drainage plans, distress mapping, and previous interventions into one pre-survey hypothesis.
- Establish background readings on sound pavement and repeat them across the survey to identify drift and contact problems.
- Schedule around rainfall, irrigation, traffic peaks, and heat so the electrical conditions are as consistent as practicable.
- Use GPR, cores, boreholes, or drainage inspection to verify credible anomalies rather than treating every colour contrast as a void.
- Deliver raw data, electrode coordinates, inversion controls, quality notes, anomaly confidence, limitations, and a clear verification plan.
Contact UP. PPP with the pavement location, slab thickness, distress photographs, construction history, suspected drainage issues, site-access constraints, and required completion date. Request a written ERT proposal covering array design, traffic and safety controls, calibration, complementary verification, and reporting that separates measured anomalies from the assessed likelihood of a void. This gives project teams a defensible basis for targeted investigation rather than broad, speculative excavation.