Non-Destructive Testing (NDT) encompasses techniques that evaluate the properties of materials, components, or systems without causing damage. These methods are essential for quality control, condition monitoring, and structural integrity assessment in construction and infrastructure projects.
Training Duration: 1 week
Prerequisite Skills: Basic knowledge of construction materials, structural engineering concepts, and quality control procedures
Non-Destructive Testing provides valuable information about material properties, defects, and structural integrity without damaging the tested components. NDT is crucial for quality assurance, maintenance, and safety compliance across various construction phases.
- Evaluating concrete strength and quality in existing structures
- Detecting rebar positions and cover thickness
- Identifying voids, honeycomb, and delamination in concrete
- Locating cracks and determining their depth
- Assessing welding quality in structural steel
- Measuring moisture content in building materials
- Evaluating structural integrity after earthquakes or fires
| IS Code |
Title |
Scope |
| IS 13311 (Part 1):1992 |
NDT Methods for Concrete: Ultrasonic Pulse Velocity |
Testing concrete by ultrasonic pulse velocity method |
| IS 13311 (Part 2):1992 |
NDT Methods for Concrete: Rebound Hammer |
Testing concrete by rebound hammer method |
| IS 456:2000 |
Plain and Reinforced Concrete - Code of Practice |
Includes provisions for NDT of concrete structures |
| IS 516:1959 |
Method of Tests for Strength of Concrete |
Testing procedures for concrete strength |
| IS 1786:2008 |
High Strength Deformed Steel Bars |
Includes testing of reinforcement steel |
Important Note: NDT results should be interpreted by qualified personnel with adequate training and certification. Tests should be conducted as per the latest version of applicable standards.
The Rebound Hammer Test (also known as the Schmidt Hammer Test) is a quick, simple, and non-destructive method for assessing the compressive strength and uniformity of concrete in existing structures. It measures the rebound of a spring-loaded hammer mass after it strikes the concrete surface. Higher rebound numbers generally indicate higher concrete strength.
Principle: The test is based on the principle that the rebound of an elastic mass is dependent on the hardness and elasticity of the surface against which it strikes. For concrete, this rebound is correlated with its compressive strength.
- Original Schmidt Rebound Hammer (Type N for normal concrete)
- Abrasive carborundum stone for surface preparation
- Calibration anvil (for periodic calibration checks)
- Measuring tape and marking tools (chalk, marker)
- Data recording sheet or electronic data logger
- Surface Preparation: Select a smooth, clean, and dry area of concrete (at least 150x150 mm). Remove any loose concrete, plaster, paint, or carbonated layers using the abrasive stone.
- Calibration: Before and after each series of tests, or at least once per day, check the hammer against a steel calibration anvil. The rebound number should be within ±3 units of the manufacturer's specified value for the anvil.
- Test Grid: Mark a grid of at least 9 to 12 points (e.g., 3x3 or 4x3) within the selected test area. Points should be spaced at least 20-30 mm apart.
- Hammer Orientation: Hold the rebound hammer firmly and perpendicular to the concrete surface. For vertical surfaces, ensure the hammer is perfectly horizontal. For horizontal surfaces, ensure it's perfectly vertical.
- Applying Pressure: Apply gradual and increasing pressure until the hammer impacts the surface. The plunger will retract, and the rebound number will be indicated on the scale.
- Recording Readings: Read and record the rebound number for each impact. Avoid re-hitting the exact same spot.
- Discarding Outliers: After taking all readings (e.g., 10-12 readings per test area), discard any readings that differ from the average of the readings by more than 6 units. If more than 2 readings are outliers, the entire test area should be re-tested.
- Calculating Average: Calculate the average of the remaining valid rebound numbers for the test area.
- Strength Correlation: Use the manufacturer's or a site-specific correlation chart (developed by testing cores from the same concrete) to convert the average rebound number to an estimated compressive strength.
The rebound hammer test provides an approximate indication of concrete compressive strength. Results are affected by several factors, making it a comparative rather than an absolute measurement tool:
- Surface Conditions: Smoothness, moisture content (wet concrete gives lower rebound), presence of formwork release agents, or carbonation depth.
- Age of Concrete: Rebound numbers generally increase with concrete age as strength develops.
- Type of Aggregate: Harder aggregates (e.g., basalt) can give higher rebound numbers than softer ones (e.g., limestone) for the same strength.
- Type of Cement: Different cement types can influence the strength gain and thus rebound.
- Hammer Orientation: The angle at which the hammer is held relative to gravity affects the rebound. Correction factors are applied for vertical, horizontal, or inclined testing.
- Presence of Reinforcement: Hitting directly over a rebar can give artificially high readings.
- Size and Shape of Member: Smaller or thinner elements may give lower readings due to insufficient mass.
Pro Tip: For more reliable results, especially when estimating strength, it is highly recommended to establish a site-specific correlation curve by testing concrete cores from the same structure and correlating their compressive strength with rebound hammer readings taken at the core locations.
| Item |
Cost Range (INR) |
Remarks |
| Equipment Cost (Standard Hammer) |
25,000 - 65,000 |
One-time investment for a good quality analog hammer |
| Digital Rebound Hammer |
70,000 - 1,50,000 |
Offers data logging and direct strength conversion |
| Test Cost (per location/grid) |
1,500 - 3,000 |
Includes 10-12 readings and basic interpretation |
| Comprehensive Report |
5,000 - 15,000 |
Depends on structure size, number of locations, and detailed analysis |
Important Consideration: The rebound hammer test is best used for assessing the uniformity of concrete and identifying areas of potentially low strength. It should not be used as the sole basis for accepting or rejecting concrete, especially for critical structural elements. It's often combined with UPV or core testing.
The Ultrasonic Pulse Velocity (UPV) test is a non-destructive method used to evaluate the quality, uniformity, and integrity of concrete, and to estimate its compressive strength. It works by measuring the time taken for an ultrasonic pulse to travel through a known distance in the concrete. Higher pulse velocity generally indicates better quality and stronger concrete.
Principle: Ultrasonic pulses are generated by an electro-acoustical transducer and transmitted through the concrete. The time of travel of these pulses is measured, and knowing the distance, the pulse velocity is calculated. The velocity is influenced by the density, elastic properties, and presence of voids or cracks in the concrete.
- UPV testing apparatus with a pulse generator and receiver
- Two transducers (typically 50 kHz to 150 kHz for concrete)
- Calibration rod (for verifying equipment accuracy)
- Coupling agent (e.g., petroleum jelly, grease, or liquid soap)
- Measuring tape, chalk, and marker for marking test points and measuring path length
- Data recording sheet or integrated data logging system
- Surface Preparation: Ensure the contact surfaces for the transducers are smooth and clean. Remove any loose material, dust, or paint.
- Calibration: Calibrate the equipment using the standard calibration rod provided by the manufacturer. The measured time of travel should match the specified value for the rod.
- Marking Test Points: Mark the exact positions for the transmitting and receiving transducers on the concrete surface. Measure the precise distance between them (path length).
- Applying Coupling Agent: Apply a thin layer of coupling agent to the faces of both transducers to ensure good acoustic contact with the concrete.
- Transducer Placement Methods:
- Direct Transmission: Transducers are placed on opposite faces of the concrete element (e.g., opposite sides of a column or slab). This is the most accurate method.
- Semi-Direct Transmission: Transducers are placed on adjacent faces (e.g., one on top, one on the side of a beam).
- Indirect (Surface) Transmission: Both transducers are placed on the same surface, at a measured distance apart. This method is less accurate but useful when only one face is accessible.
- Taking Readings: Press the transducers firmly against the marked points. The equipment will display the pulse travel time. Record this time (in microseconds).
- Calculating Velocity: Calculate the pulse velocity (V) using the formula: $V = \frac{L}{T}$, where $L$ is the path length (in km or m) and $T$ is the travel time (in seconds or microseconds).
- Multiple Readings: Take multiple readings (at least 3-5) at each test location and average them to minimize errors.
- Documentation: Record all relevant data, including test location, path length, travel time, calculated velocity, temperature, and method of transmission.
The pulse velocity is used to assess the quality of concrete based on empirical correlations. IS 13311 (Part 1) provides a general guide for concrete quality grading:
| Pulse Velocity (km/s) |
Concrete Quality Grading |
General Condition |
| Above 4.5 |
Excellent |
High quality, well compacted, dense concrete |
| 3.5 - 4.5 |
Good |
Standard quality concrete, generally sound |
| 3.0 - 3.5 |
Medium |
May contain some voids or minor cracks, average quality |
| 2.0 - 3.0 |
Poor |
Contains significant defects, honeycombing, or extensive cracking |
| Below 2.0 |
Very Poor |
Severely deteriorated concrete, large voids, or significant damage |
Factors Influencing Results:
- Moisture Content: Wet concrete generally gives higher velocities.
- Presence of Reinforcement: Steel has a much higher pulse velocity than concrete. If the pulse travels through rebar, it will give an artificially high reading. Correction factors or alternative path lengths may be needed.
- Aggregate Type: Dense, hard aggregates result in higher velocities.
- Temperature: Higher temperatures can slightly increase velocity.
- Presence of Voids/Cracks: These will cause the pulse to detour, increasing travel time and thus decreasing velocity, indicating poor quality.
| Item |
Cost Range (INR) |
Remarks |
| Equipment Cost (Digital UPV) |
1,00,000 - 2,50,000 |
High-quality digital UPV with various transducers |
| Test Cost (per location/grid) |
2,500 - 5,000 |
Includes multiple readings and basic interpretation |
| Comprehensive Report |
10,000 - 25,000 |
With defect mapping, strength correlation, and detailed analysis |
Important: UPV is excellent for detecting internal defects, assessing uniformity, and identifying areas of poor concrete. While it can be correlated with compressive strength, this correlation is empirical and highly dependent on the concrete mix. For definitive strength assessment, especially in critical cases, core testing is often required in conjunction with UPV.
Core testing involves extracting cylindrical samples from hardened concrete for laboratory testing. While slightly destructive, it provides the most reliable assessment of in-situ concrete strength and quality.
- Core drilling machine with diamond bits
- Water supply for cooling during drilling
- Measuring equipment (calipers, scale)
- Rebar detector to avoid cutting reinforcement
- Non-shrink repair grout
- Compression testing machine
- Core capping materials
- Locate suitable extraction points avoiding reinforcement
- Secure the drilling machine firmly to the structure
- Drill at right angles to the surface
- Extract cores with L/D ratio preferably 2.0 (minimum 1.0)
- Mark cores with location, orientation, and date
- Repair core holes with non-shrink grout
- Measure dimensions and examine cores visually
- Prepare cores for testing by trimming and capping
- Test cores for compressive strength
- Apply correction factors for L/D ratio, reinforcement, etc.
Core strength needs to be adjusted using correction factors:
- For L/D ratio less than 2.0: Apply correction factor as per IS 516
- For presence of reinforcement: Adjust as per code requirements
- For moisture condition: Apply 1.05 factor for air-dried cores
- For core diameter: Apply correction for cores smaller than 100mm
Equivalent cube strength = Core strength × (Correction factors)
| Item |
Cost Range (INR) |
Remarks |
| Equipment Cost |
50,000 - 1,50,000 |
Core cutting machine with various bit sizes |
| Core Extraction (per core) |
3,500 - 6,000 |
Including extraction and hole repair |
| Core Testing (per core) |
1,500 - 3,000 |
Compressive strength testing |
| Complete Analysis Report |
15,000 - 30,000 |
For 3-6 cores with detailed assessment |
Caution: Core extraction weakens the structure locally. Obtain structural engineer's approval before drilling, especially in critical load-bearing elements.
Rebar locators and cover meters help detect reinforcement bars, measure concrete cover, and estimate bar diameter without damaging the structure. These tools are essential for structural assessment, renovation planning, and quality control.
- Electromagnetic cover meter or GPR-based locator
- Calibration block/rod
- Marking tools (chalk, marker)
- Measuring tape
- Data recording sheet or electronic device
- Calibrate the equipment as per manufacturer's instructions
- Define the test area and grid pattern
- Hold the device perpendicular to the concrete surface
- Move the probe slowly in a grid pattern to locate rebars
- Mark detected rebar positions on the surface
- Measure and record concrete cover at multiple points
- Use specific functions to estimate bar diameter
- Document findings with sketches or photos
- Accuracy decreases with increasing cover depth
- Closely spaced rebars may not be individually detected
- Surface-mounted metal objects can interfere with readings
- Some aggregates with magnetic properties affect accuracy
- High moisture content may influence measurements
| Item |
Cost Range (INR) |
Remarks |
| Basic Cover Meter |
75,000 - 1,50,000 |
Electromagnetic cover meter with standard features |
| Advanced Rebar Scanner |
2,00,000 - 5,00,000 |
GPR-based with imaging capabilities |
| Survey Cost (per sq.m) |
1,000 - 2,500 |
Depends on access and detail required |
| Rebar Mapping Report |
8,000 - 20,000 |
With detailed drawings and analysis |
Pro Tip: For critical structural investigations, combine cover meter survey with selective chipping to confirm actual rebar details at key locations.
Carbonation reduces concrete alkalinity, compromising the passive protection of steel reinforcement. The carbonation test measures the depth of this chemical process to assess potential corrosion risk and remaining service life.
- Phenolphthalein indicator solution (1% in alcohol)
- Spray bottle or dropper
- Drilling or chiseling tools
- Clean water for surface preparation
- Measuring scale (mm)
- Repair materials
- Camera for documentation
- Identify test locations, preferably avoiding reinforcement
- Create a fresh break or drill a hole in the concrete
- Clean the exposed surface with brush or compressed air
- Spray or apply phenolphthalein solution to the fresh surface
- Observe color change immediately (within 30 seconds)
- Measure the depth from surface to the pink/purple color boundary
- Record measurements at multiple points around the perimeter
- Document with photographs showing scale
- Repair test locations appropriately
Uncarbonated concrete (pH > 9) turns pink/purple, while carbonated concrete (pH < 9) remains colorless.
- Compare carbonation depth with concrete cover thickness
- If carbonation reaches reinforcement, corrosion risk is high
- Calculate carbonation rate using √t law: d = k√t
- Where d = depth (mm), k = carbonation coefficient, t = time (years)
- Estimate remaining time before carbonation reaches reinforcement
| Item |
Cost Range (INR) |
Remarks |
| Indicator Solution |
500 - 1,500 |
Per 100ml bottle |
| Test Cost (per location) |
1,500 - 3,000 |
Including sample preparation and repair |
| Comprehensive Survey |
8,000 - 20,000 |
Multiple locations with analysis report |
Safety Note: Phenolphthalein solution contains alcohol and is flammable. Avoid contact with skin and eyes. Use in well-ventilated areas.
The half-cell potential test measures the electrochemical potential difference between reinforcing steel and a reference electrode to assess the probability of corrosion activity in reinforced concrete structures.
- Half-cell reference electrode (usually copper/copper sulfate)
- High impedance voltmeter (>10 MΩ input resistance)
- Electrical connection to reinforcement
- Spray bottle with water
- Sponge or contact medium
- Marking grid for measurement locations
- Data recording equipment
- Pre-wet the concrete surface in the test area
- Establish electrical connection to reinforcement
- Mark a grid pattern (typically 0.5-1.0m spacing)
- Connect voltmeter between reference electrode and rebar
- Place reference electrode at each grid point
- Record potential readings in millivolts (mV)
- Check reference electrode regularly with calibration cell
- Create potential contour map of the tested area
| Potential Reading (mV CSE) |
Corrosion Probability |
Interpretation |
| More positive than -200 |
< 10% |
Low corrosion risk |
| -200 to -350 |
Uncertain |
Intermediate corrosion risk |
| More negative than -350 |
> 90% |
High corrosion risk |
Note: CSE = Copper/Copper Sulfate Electrode
- Concrete moisture content
- Oxygen availability
- Concrete resistivity
- Presence of stray currents
- Presence of surface coatings or treatments
- Carbonation or chloride contamination
| Item |
Cost Range (INR) |
Remarks |
| Equipment Cost |
50,000 - 1,50,000 |
Complete half-cell test kit |
| Test Cost (per sq.m) |
800 - 1,500 |
Depends on grid density |
| Comprehensive Survey |
15,000 - 40,000 |
With potential mapping and analysis report |
Best Practice: Combine half-cell potential measurements with concrete resistivity tests for more reliable assessment of corrosion risk.
GPR uses electromagnetic waves to detect objects and anomalies within concrete structures. It provides non-invasive imaging of reinforcement, conduits, voids, and other embedded objects.
- Detecting rebar layout and depth
- Locating post-tensioning cables and ducts
- Finding embedded utilities and conduits
- Identifying voids, honeycomb, and delamination
- Determining concrete thickness
- Mapping structural elements prior to drilling or cutting
- GPR unit with appropriate antenna (900-2600 MHz for concrete)
- Data acquisition system or recorder
- Grid marking tools
- Distance measuring wheel
- Data processing software
- Mark grid or scan lines on the survey area
- Set up GPR parameters (dielectric constant, range, gain)
- Perform calibration scan over known object or void
- Scan systematically along grid lines
- Ensure complete coverage of the area of interest
- Record data with position references
- Process data to generate 2D/3D images
- Interpret results and mark findings on structure
- Depth penetration limited (typically 300-500mm in concrete)
- Resolution decreases with depth
- Closely spaced objects may not be individually resolved
- High moisture or salt content affects signal penetration
- Requires skilled operator for proper data collection and interpretation
| Item |
Cost Range (INR) |
Remarks |
| Equipment Cost |
8,00,000 - 25,00,000 |
Complete GPR system with software |
| Survey Cost (per sq.m) |
2,000 - 5,000 |
For standard resolution scanning |
| High-Resolution Survey |
5,000 - 8,000 |
Per sq.m, with detailed imaging |
| 3D Modeling & Report |
15,000 - 40,000 |
Advanced processing and visualization |
Pro Tip: GPR results are most reliable when verified with selected physical investigations at representative locations.
Infrared thermography detects variations in surface temperature that may indicate subsurface defects, moisture ingress, or thermal bridges in building components.
- Detecting delaminations and debonding
- Locating moisture intrusion and leakage
- Identifying thermal bridges and insulation defects
- Detecting voids in grouted tendons or injection areas
- Evaluating heating/cooling systems
- Infrared thermal camera (resolution ≥ 320×240 pixels)
- Surface temperature probes for reference measurements
- Heating sources (for active thermography)
- Temperature and humidity meter
- Analysis software
- Reference targets and markers
- Passive Approach:
- Plan inspection during optimal environmental conditions
- Record ambient conditions (temperature, humidity, etc.)
- Set camera parameters (emissivity, temperature range)
- Take systematic thermal images of the structure
- Active Approach:
- Heat/cool the surface uniformly
- Record thermal images during heating/cooling cycles
- Analyze temperature differences and patterns
- Document findings with thermal and visual images
Interpretation requires understanding of heat transfer principles and material properties:
- Areas with subsurface defects typically show different cooling/heating rates
- Moisture appears cooler in hot conditions (due to evaporation)
- Thermal bridges appear warmer in winter and cooler in summer
- Measure temperature differentials to assess defect severity
- Compare thermal patterns with structural features
| Item |
Cost Range (INR) |
Remarks |
| Basic Thermal Camera |
80,000 - 2,00,000 |
Entry-level resolution (160×120) |
| Professional Camera |
3,00,000 - 8,00,000 |
High resolution (640×480) with advanced features |
| Inspection Cost (per day) |
15,000 - 30,000 |
Standard passive thermography |
| Active Thermography |
30,000 - 60,000 |
Per day, with heating/cooling equipment |
| Analysis & Report |
10,000 - 25,000 |
Detailed thermal analysis and documentation |
Important: Environmental conditions significantly affect results. Avoid inspections during rain, strong winds, or when the surface is directly exposed to sunlight.
NDT personnel must be properly qualified and certified to ensure reliable testing and interpretation of results.
- IS 13805: Qualification and certification of NDT personnel
- ISO 9712: International standard for NDT personnel certification
- ASNT SNT-TC-1A: American Society for NDT recommended practice
- PCN: Personnel Certification in NDT (UK certification)
| Level |
Capabilities |
Requirements |
| Level I |
Perform specific NDT according to written instructions and under supervision |
Basic training + experience (typically 3-6 months) |
| Level II |
Set up equipment, perform tests, interpret results, prepare reports |
Level I + advanced training + experience (9-12 months) |
| Level III |
Develop procedures, interpret codes, supervise personnel, approve reports |
Level II + extensive experience (2-4 years) + comprehensive examination |
- ISNT: Indian Society for Non-Destructive Testing
- BARC: Bhabha Atomic Research Centre
- NTB: National Test House
- RITES: Rail India Technical and Economic Service
- NPCIL: Nuclear Power Corporation of India Limited
| Certification Type |
Cost Range (INR) |
Duration |
| Level I (Single Method) |
15,000 - 25,000 |
1-2 weeks |
| Level II (Single Method) |
25,000 - 40,000 |
2-3 weeks |
| Level III (Single Method) |
40,000 - 75,000 |
3-4 weeks |
| Refresher Course |
10,000 - 20,000 |
3-5 days |
| Recertification |
8,000 - 15,000 |
Every 3-5 years |
Industry Requirement: Construction projects often specify minimum certification levels for NDT personnel in quality control plans and contract documents.
On-Job Training is crucial for NDT professionals to bridge the gap between theoretical knowledge and practical application. It allows trainees to gain hands-on experience with equipment, understand site-specific challenges, and develop critical problem-solving skills under the guidance of experienced mentors. High-quality OJT ensures that NDT personnel are competent, confident, and capable of performing accurate tests and interpretations in real-world scenarios.
- Structured Curriculum: A clear, well-defined OJT plan outlining learning objectives, tasks, and expected outcomes for each NDT method.
- Qualified Mentors: Experienced Level II or Level III certified NDT personnel who can provide effective guidance, demonstrate techniques, and offer constructive feedback.
- Hands-on Practice: Ample opportunities for trainees to operate NDT equipment, perform tests on various concrete elements, and collect data.
- Real-world Scenarios: Exposure to diverse site conditions, types of defects, and structural configurations to develop adaptability and problem-solving skills.
- Data Analysis & Reporting: Training in interpreting raw data, performing necessary calculations, and preparing comprehensive NDT reports as per industry standards.
- Safety Protocols: Thorough instruction and adherence to safety procedures related to NDT equipment operation and site work.
- Performance Evaluation: Regular assessments and feedback sessions to track progress, identify areas for improvement, and ensure competency before independent work.
- Documentation of Training: Maintaining detailed records of OJT hours, tasks performed, and mentor evaluations, which are often required for formal certification.
- Alignment with Standards: Ensure OJT content and duration meet or exceed the requirements of relevant certification bodies (e.g., ISNT, ISO 9712).
- Dedicated Resources: Allocate sufficient time, equipment, and qualified personnel to facilitate effective OJT.
- Feedback Mechanism: Implement a system for trainees to provide feedback on the OJT program, allowing for continuous improvement.
- Continuous Learning: Encourage mentors and trainees to stay updated with the latest NDT technologies, standards, and best practices.
- Simulated Environments: Where real-world exposure is limited, use simulated environments or mock-ups to practice complex NDT procedures.
- Cross-Functional Exposure: Provide opportunities for trainees to understand how NDT integrates with other construction processes and quality control measures.
Benefit of Quality OJT: Well-implemented on-job training not only enhances the technical skills of NDT personnel but also fosters a culture of quality, safety, and continuous improvement within the construction project team.
Non-Destructive Testing provides crucial information about the condition, quality, and performance of construction materials and structures without causing damage. By implementing appropriate NDT methods at various stages of a project lifecycle, engineers can ensure structural integrity, compliance with codes, and long-term durability.
The selection of NDT methods should be based on the specific requirements of the project, the information needed, and the accessibility of the structure. Often, a combination of complementary techniques provides the most comprehensive assessment. Always ensure that personnel conducting NDT are properly trained and certified according to relevant standards.
Regular NDT as part of a maintenance program can identify potential issues before they become critical, extending the service life of structures and reducing lifecycle costs.
Key Takeaway: NDT is an indispensable tool for modern construction, ensuring safety, quality, and longevity of infrastructure. Continuous learning and adherence to standards are paramount.