What Is Non-Destructive Testing
Non-destructive testing (NDT), also known as non-destructive examination or in-situ non-destructive testing, is the set of inspections, checks, and measurements that make it possible to assess the condition of a building structure or one of its components without damaging the material or removing significant samples.
In civil engineering, NDT is used to identify defects, discontinuities, deterioration, voids, and the presence of reinforcement or buried utilities, to estimate the mechanical properties of concrete, steel, timber, and masonry, and to monitor the behaviour of buildings and infrastructure over time. Compared with destructive tests (core sampling, specimen extraction, failure testing), NDT offers three substantial advantages: it safeguards the integrity of the structure, it can be repeated over time to build historical data series, and it lends itself to extensive surveys over large surfaces at contained costs. It is therefore the foundation of any modern structural diagnostic campaign.
When Non-Destructive Testing Is Required
In Italy, NDT is required, either explicitly or implicitly, in numerous situations covered by technical regulations. The main ones are:
- Safety assessment of existing buildings, pursuant to Chapter 8 of the Italian Technical Standards for Construction (NTC 2018), particularly at knowledge levels LC1, LC2, and LC3.
- Seismic vulnerability assessment of public and private buildings, with particular reference to strategic buildings and schools (Ministerial Decree 65/2017, OPCM 3274/2003, and subsequent updates).
- Static load testing of new works, pursuant to Article 67 of Presidential Decree 380/2001 and the Ministerial Decree of 17/01/2018.
- Safety assessment of existing bridges and viaducts, in accordance with the “Guidelines for the classification and management of risk, the assessment of safety and the monitoring of existing bridges” (Consiglio Superiore dei Lavori Pubblici, 2020).
- Issuance of the Static Suitability Certificate (CIS) for older buildings, mandatory in some municipalities (e.g., Milan, Building Regulation art. 11).
- Verification of the load-bearing capacity of roofs and floors prior to the installation of photovoltaic systems, in accordance with CSLLPP Circular 7/2019.
- Preliminary investigations for the restoration of historic buildings that are listed or protected under the relevant Legislative Decree. 42/2004.
- Assessment of post-fire or post-earthquake damage on load-bearing structures.
- Checks on new timber construction systems (X-Lam/CLT, panel structures, light-frame timber frames) to monitor moisture and rule out deterioration phenomena.
In all these scenarios, the objective is to obtain technically reliable information on the condition of the materials and structural elements without compromising the usability of the structure. This is where the different investigation techniques come into play.
Regulatory Framework
The planning and execution of NDT in Italian construction relies on a layered system of technical standards. The main references are:
National and Mandatory Standards
- Italian Ministerial Decree 17/01/2018 — Update of the Technical Standards for Construction (NTC 2018), with particular reference to Chapter 8 “Existing buildings” and Chapter 11 “Materials and products for structural use”.
- CSLLPP Circular No. 7 of 21/01/2019 — Instructions for the application of NTC 2018.
- CSLLPP Guidelines 2020 — Classification, assessment, and monitoring of existing bridges.
- Presidential Decree 380/2001 — Italian Construction Code, Article 67 (static load testing).
European and Italian Technical Standards (UNI EN, UNI)
- UNI EN 12504-1 — Testing concrete in structures: cores — sampling, examination, and compressive testing.
- UNI EN 12504-2 — Determination of the rebound number (rebound hammer test).
- UNI EN 12504-4 — Determination of ultrasonic pulse velocity.
- UNI EN 13791 — Assessment of in-situ compressive strength in structures and precast concrete components.
- UNI 11073 — Estimation of moisture content in structural timber and visual inspection of timber elements.
- UNI EN 13187 and UNI EN 16714 — Infrared thermography of buildings.
- UNI/TR 11634:2016 — Guidelines for structural monitoring.
- UNI 9916, UNI 9614, UNI 10985, UNI 11568, UNI ISO 5348 — Vibration measurement and dynamic behaviour.
International Standards and Guidelines
- ASTM D2845, ASTM C597, ASTM D6432 — Sonic, ultrasonic, and ground-penetrating radar (GPR) testing.
- ACI 437.2M-13 — Load testing of concrete structures.
- BS 1881-204 — Covermeter (magnetic) testing of reinforced concrete structures.
- RILEM TC 127-MS, CEI 306-8 — Ground-penetrating radar (GPR) investigations.
- UNI EN ISO 9712 — Qualification and certification of NDT personnel.
Compliance of personnel with UNI EN ISO 9712 is increasingly required in technical specifications, especially for public tenders and acceptance testing on strategic structures.
How to Plan an Investigation Campaign
A good NDT campaign cannot be improvised: the choice of techniques, of testing points, and of the number of investigations follows a process organized into five phases.
- Documentary acquisition: collection of design drawings, calculation reports, any previous investigations, the history of the structure, and maintenance interventions.
- Preliminary visual examination: direct inspection of the structure to detect crack patterns, detachments, deterioration, moisture traces, and abnormal deformations.
- Drafting of the investigation plan: defining the techniques to be employed, their extent (number, location, depth), and the expected knowledge level in accordance with NTC 2018.
- On-site execution: site organization, performance of the tests with calibrated instruments, data recording, and any additional measurements based on intermediate results.
- Reporting and assessment: data processing, drafting of the technical report, and translation of the results into parameters usable by the designer (e.g., mean compressive strength of the concrete, elastic modulus of masonry, etc.).
The value of a campaign lies in the consistency among the different techniques used. An isolated rebound hammer test, for example, has a high margin of error; combined with ultrasonic testing and correlated with one or two targeted cores (the SonReb method), it provides an estimate of concrete strength that is reliable and accepted by the NTC.
The Main Non-Destructive Tests
Below are the main techniques used by Teknoprogetti in investigation campaigns. For each one, the scope of application, advantages, limitations, and reference standards are indicated.
Visual examination
Application: Detection, description, and cataloguing of the main issues that can be identified with the naked eye or with simple instruments. It is the starting point of every investigation campaign and guides the choice of subsequent tests.
Advantages: No cost, applicable to all structures, provides an overall picture.
Limitations: Subjective, dependent on the operator’s experience, does not quantify material properties.
Reference standards: UNI EN 1330-10 (terminology), ISTeA and CTE best practices.
Magnetometric Test (Cover Meter)
Application: Identification of the position, depth, and diameter of reinforcement bars in concrete elements; detection of ferromagnetic elements beneath plaster.
Advantages: Fully non-destructive, fast to perform, with immediate results.
Limitations: Difficulty in distinguishing bars that are very close together or arranged on multiple layers; limited investigation depth (typically 8–10 cm).
Reference standards: BS 1881-204:1988.
Rebound Hammer Test (Schmidt Hammer)
Application: Determination of the rebound number of a striking mass on hardened concrete and estimation of mechanical strength through correlation tables.
Advantages: Non-destructive, fast, inexpensive, suitable for extensive campaigns.
Limitations: Only affects the surface layer, sensitive to carbonation and moisture, and must always be correlated with other tests.
Reference standards: UNI EN 12504-2.
Ultrasonic Test
Application: Measurement of the propagation velocity of ultrasonic pulses and, through correlations, estimation of the compressive strength of hardened concrete or detection of voids and inhomogeneities.
Advantages: Non-destructive, covers the entire section, and can be applied in direct, semi-direct, or indirect transmission.
Limitations: Results influenced by reinforcement, moisture, and the presence of cracks; requires an experienced operator.
Reference standards: UNI EN 12504-4.
Sonic Test
Application: Measurement of the propagation velocity of the sonic wave in masonry to assess homogeneity, elastic modulus, and the presence of voids.
Advantages: Non-destructive, suitable for historic masonry where ultrasonic methods would be ineffective.
Limitations: Requires at least localized removal of the plaster; voids and cavities can compromise the test results.
Reference standards: ASTM D2845-08, ASTM C597-02.
Load tests
Application: Verification of the actual behaviour of structures under controlled static or dynamic loads. They include tests on floors, bridges, balconies, and cantilevers.
Advantages: Provides direct evidence of structural behaviour, essential for the static load testing of new works and for the assessment of existing structures.
Limitations: Demanding in terms of organization; they require specific test design and safety devices.
Reference standards: ACI 437.2M-13, ReLUIS Guidelines 2/2015, NTC 2018 §9.
Vibration Measurement
Application: Verification of the dynamic behaviour of the structure, identification of natural frequencies, validation of numerical models, and monitoring of vibrations induced by construction sites or traffic.
Advantages: Quick to perform, the characteristic frequencies are independent of the forcing input, and results can be compared with regulatory thresholds.
Limitations: Requires specific instrumentation and qualified signal analysis.
Reference standards: UNI 11568, UNI ISO 5348, UNI 9916, UNI 9614, UNI 10985.
Structural Monitoring
Application: Time-based measurement of displacements, crack openings, inclinations, and vibrations. It is divided into static and dynamic monitoring, either continuous or periodic.
Advantages: Enables remote control, the setting of alarm thresholds, and tracking of how characteristic parameters change over time.
Limitations: Requires the design of the data acquisition and management system.
Reference standards: UNI/TR 11634:2016, CSLLPP Guidelines 2020 (for bridges).
Ground-Penetrating Radar (GPR) Survey
Application: Exploits the reflection of electromagnetic waves to identify buried utilities, voids, reinforcement, and delaminations between layers. Also used on pavements and bridge decks.
Advantages: Non-destructive, with investigation depth that varies according to the antenna used (0.1–3 m); provides both 2D and 3D outputs.
Limitations: Output is not intuitive, the signal is shielded by ferromagnetic materials and water, and requires a certified operator.
Reference standards: RILEM TC 127-MS, ASTM D6432, CEI 306-8.
Infrared Thermography
Application: Acquisition of images in the infrared spectrum to detect thermal bridges, water infiltration, plaster detachments, masonry texture beneath plaster, and temperature anomalies.
Advantages: Non-destructive, contactless, rapidly covers large surfaces, and useful for both structural and energy diagnostics.
Limitations: Requires favourable thermo-hygrometric conditions and a certified operator (Level II per UNI EN ISO 9712).
Reference standards: UNI EN 16714, UNI EN 13187.
Typical Application Cases
The choice of techniques depends on the objective of the campaign. Some recurring examples:
School Buildings and Seismic Vulnerability
For school buildings, which are the subject of diagnostic campaigns promoted by MIUR (the Italian Ministry of Education) and the Regions, the typical combination includes visual inspection, rebound hammer + ultrasonic testing (the SonReb method) on columns, cover meter on the reinforcement, and occasional targeted core sampling. The objective is to reach the LC2 knowledge level required by NTC 2018 for seismic vulnerability assessment.
Existing Bridges and Viaducts
The CSLLPP Guidelines 2020 prescribe a multi-level approach that integrates visual inspections, non-destructive testing on concrete and reinforcement, dynamic monitoring of natural frequencies, and, where necessary, load tests. Diagnostics support the risk classification of the structure.
Roofs for Photovoltaic Systems
The installation of photovoltaic systems on existing roofs requires verification of the structure’s load-bearing capacity. Investigations include cover-meter scanning of the reinforcement, ultrasonic testing on prestressed concrete (PRC) joists, thermography to detect water infiltration, and, in specific cases, load testing on floors.
Historic and Masonry Buildings
For listed buildings, the priority is to preserve the structure. Sonic and GPR testing are preferred for investigating masonry texture, thermography for detecting hidden elements, and monitoring of crack patterns over time. Core sampling is limited to the strict minimum.
Timber and X-Lam Structures
For new timber constructions (light-frame timber frames, X-Lam/CLT panels), investigations focus on moisture content (contact moisture meters, in accordance with UNI 11073), the integrity of metal connections, and the ruling out of biological deterioration. Thermography makes it possible to identify zones of moisture accumulation that are not visible to the naked eye, as illustrated in our case study on the school building in Frassinoro (X-Lam, March 2025).
Frequently Asked Questions about Non-Destructive Testing
When is non-destructive testing mandatory on a building?
NTC 2018 (Chapter 8) requires diagnostic campaigns for every intervention on existing structures that substantially modifies the static or seismic behaviour. They are also mandatory for the issuance of the Static Suitability Certificate in those municipalities that require it (e.g., Milan), for seismic vulnerability assessments on strategic and school buildings, and for static load testing.
What is the difference between destructive and non-destructive testing?
Destructive tests (core sampling, steel specimen extraction, failure tests) require the removal of material and cause localized damage to the structure, but they provide direct strength values. Non-destructive tests do not damage the structure but provide indirect values that must be correlated. A good campaign always combines the two: a few destructive tests for calibration plus extensive non-destructive testing.
How long does a non-destructive testing campaign take?
It depends on the size of the structure and the techniques chosen. A campaign on a three-storey school building may require one to two weeks of on-site work, plus two to three weeks for data processing and the technical report. For bridges and infrastructure, timelines extend depending on access conditions.
How much does a non-destructive testing campaign cost?
The cost depends on the number of tests, the complexity of access, and any need for calibration cores and for scaffolding or aerial platforms. It is always advisable to request a free preliminary site visit so that the investigation plan can be calibrated to the actual objective, avoiding both over-specification and insufficient investigation.
Does non-destructive testing damage the structure?
By definition, no. Some techniques (rebound hammer, cover meter, GPR, thermography) leave no trace at all. Others, such as direct-transmission ultrasonic testing, may require a small localized removal of plaster of a few square centimetres, which is easily restored.
Who is qualified to perform non-destructive testing in a formally valid manner?
UNI EN ISO 9712 defines three qualification levels for NDT personnel. For investigations on in-service structures, Level II is typically required for the operator carrying out the tests, and Level III for the person planning the campaign. Technical supervision must be entrusted to a structural engineer registered with the professional Order.
Do non-destructive tests replace structural analysis?
No. NDT provides the input parameters (strengths, actual geometry, condition of the reinforcement) that the designer uses to model the structure and verify its safety. They are complementary to structural analysis, not a substitute for it.
Can non-destructive testing be performed on occupied or in-service buildings?
Yes, this is one of the main advantages. Techniques such as the rebound hammer, cover meter, thermography, and GPR do not require any interruption of the building’s use. Load tests, on the other hand, do entail the temporary evacuation of occupants from the area concerned.
How do you choose among the different non-destructive testing techniques?
The choice depends on the objective of the campaign and the type of structure. For reinforced concrete structures, the SonReb combination (rebound hammer + ultrasonic testing) is the standard for estimating strength; for masonry, sonic and GPR testing are preferred; for timber structures, the focus is on moisture content in accordance with UNI 11073; for energy diagnostics and the detection of voids, thermography is unmatched.
What happens after non-destructive testing?
The results are consolidated into a technical report signed by a structural engineer, who translates the data into design parameters and provides recommendations: continued use, strengthening interventions, long-term monitoring, or further in-depth investigations. The report serves as a reference document for the designer and for the supervising authorities.
Choosing Teknoprogetti for Structural Diagnostics
Teknoprogetti Engineering has been operating since 2003 in the field of structural diagnostics and non-destructive testing on public and private buildings, bridges, viaducts, school buildings, and structures of historical value in Lombardy and neighbouring regions. Diagnostic campaigns are designed on a case-by-case basis, carried out with calibrated instruments and qualified personnel, and delivered in technical reports signed by structural engineers.
To request a preliminary site visit and a proposed investigation plan, you can fill in the contact form or call +39 039 2142477. For more operational information about the service, please visit the dedicated Non-Destructive Testing page in our Services section.
