REPAIR AND REHABILITATION OF RC STRUCTURE | NDT Testing Technique
Introduction¶
Hi everyone, in the last session you discussed about special concretes and their types. In this session, we are going to discuss about non-destructive testing techniques and provide a brief note. Along with this, we are going to discuss about various non-destructive testing techniques like:
- Rebound hammer test
- Ultrasonic pulse velocity test
- Radiography
- Electromagnetic cover measurement
Non-Destructive Testing (NDT) Overview
Core Concept: Testing concrete structures WITHOUT damaging them - like medical scans for buildings.
Key Techniques Covered:
- Rebound Hammer: Measures surface hardness (like tapping to check ripeness)
- Ultrasonic Pulse: Sends sound waves through concrete (like medical ultrasound)
- Radiography: Uses X-rays to see inside (airport scanner for concrete)
- Electromagnetic Cover: Detects steel location (metal detector for rebars)
Why Important: Assess existing structures, find hidden problems, ensure quality without destroying anything.
Types of Concrete Testing¶
Concrete testing is of two types:
- Non-destructive testing: The specimen is not destroyed and it is used for its intended purpose further.
- Destructive testing: The specimen is destroyed or failure takes place, and the specimen is not used for the intended purpose.
Testing Classification
Non-Destructive Testing (NDT)
- Structure remains intact and usable
- Like checking pulse vs. surgery
- Can test repeatedly over time
Destructive Testing (DT)
- Sample is broken/crushed to failure
- Like crash-testing a car
- Cannot reuse the specimen
Quick Memory: NDT = "Look but don't break"; DT = "Break to learn"
Why Use Non-Destructive Testing?¶
Non-destructive testing technique is used for:
- Assessment of the structure in the absence of drawings
- Quick assessment of the structure
- Quality control of construction in situ
- Determining the position of reinforcement
- Locating cracks, joints, delamination, honeycombs, etc.
- In some cases, it is also used for assessment of damage due to fire or other natural calamities
When to Use NDT
Scenario-Based Applications:
- Old buildings - No original drawings available
- Quick checks - Rapid on-site assessment needed
- Quality control - Monitor construction in real-time
- Hidden elements - Find rebar location/spacing without breaking
- Defect detection - Locate cracks, voids, honeycombing
- Damage assessment - After fire, earthquake, flooding
Key Advantage: Get information without weakening the structure - you can test the same column 100 times.
Strength Evaluation of Concrete¶
In non-destructive testing technique, the strength evaluation of the concrete is carried out using the following parameters: measurement, application, and equipment used.
[Table: Strength evaluation of concrete showing measurement, application, and equipment]
| No. | Measurement | Application | Equipment |
|---|---|---|---|
| 1 | Surface strength (rebound number) | Surface Zone Strength Assessment | Rebound Hammer |
| 2 | Homogeneity of Concrete | Quality of Concrete | Ultrasonic pulse velocity meter |
| 3 | Combined ultrasonic and rebound number determination | Uniformity/homogeneity, Location of internal defects | Ultrasonic Pulse velocity tester |
| 4 | Pull-off strength (bond strength) | Surface Zone Strength Assessment | Pull off Tester |
| 5 | Pull out force | Surface zone strength assessment | Pull out "Lock" Test (Construction Stage) Pull Out "Capo" Test (after construction |
| 6 | Break off test | The break off test at failure can be related to compressive or flexural strength | Break off tester |
| 7 | Penetration resistance | Surface Zone Strength Assessment | Windsor Probe |
Strength Assessment Methods - Quick Reference
Surface Tests (Top layer only):
- Rebound Hammer: Bounces off surface - harder = higher rebound
- Pull-off/Pull-out: Physically pulls on surface/embedded insert
- Windsor Probe: Shoots probe into concrete - depth = strength
Through-Depth Tests (Entire section):
- Ultrasonic: Sound speed through concrete - faster = better quality
Remember: Surface tests = skin-deep; Ultrasonic = X-ray vision
Classification of NDT Tests¶
The NDT test is classified based on:
- Strength evaluation of concrete
- Corrosion assessment, location and diameter of the rebar
[Table: Classification showing measurement, application, and equipment]
| No | Measurement | Application | Equipment |
|---|---|---|---|
| 1 | Corrosion potential (half-cell) | Status of Corrosion activity | Half Cell Potential Meter |
| 2 | Resistivity | Rate of probable corrosion | Resistivity Meter |
| 3 | Carbonation depth | Corrosion risk and cause | Carbonation Test Kit |
| 4 | Chloride content | Corrosion risk and cause | Chloride Field Test System |
| 5 | Voids and Corrosion | Viewing interior of concrete | Endoscopy |
| 6 | Scanning Of dia. of rebar and cover | It is used for locating rebars, diameter of rebars and concrete cover | Profometer |
| 7 | Cover and re-bar measurement | Corrosion risk and cause | Micro Cover Meter |
Corrosion & Rebar Detection - Quick Guide
Corrosion Tests (Is rust happening?):
- Half-cell: Measures electrical potential - detects active corrosion
- Resistivity: Low resistance = wet concrete = faster corrosion
- Carbonation depth: pH test - concrete turns pink if safe
- Chloride content: Salt check - high chloride = future rust
Rebar Location Tests (Where's the steel?):
- Profometer/Cover meter: Electromagnetic detection - finds rebar position, size, depth
- Endoscopy: Drill tiny hole, insert camera - direct viewing
Memory Trick: "Corrosion tests answer WHY/WHEN rust, Profometer answers WHERE steel is"
Three Major Classifications of NDT¶
The non-destructive testing techniques are classified into three major categories:
-
Strength evaluation of the concrete - tabulated as measurement, application, and equipment
-
Corrosion assessment, location of the rebar, and diameter of the rebar - classified based on measurement, application, and equipment
-
Crack measurement in buildings - tabulated as measurement, application, and equipment
| No. | Measurement | Application | Equipment |
|---|---|---|---|
| 1 | Length changes | Strain measurement | digital strain gauges |
| 2 | Radiographic Images | Cracks, location of rebars | Radiographic source and detector |
| 3 | Acoustic Emission technique | To measure the location and activity of cracks | SPARTAN & MISTRAS System |
| 4 | Infra Red Images | Cracks, delamination | Infra Red Thermal Imaging Systems |
Three Main NDT Categories
1. Strength Tests - "How strong is the concrete?"
- Rebound, ultrasonic, pull-off tests
2. Corrosion & Rebar Tests - "Where's the steel and is it rusting?"
- Half-cell, profometer, carbonation tests
3. Crack Detection - "Where are cracks and are they growing?"
- Strain gauges: Measure deformation (like fitness tracker for buildings)
- Radiography: X-ray imaging (sees through concrete)
- Acoustic emission: Listens for crack sounds (stethoscope for structures)
- Infrared: Heat patterns show hidden defects (thermal camera)
Visual Memory: Think doctor's exam - Check strength (muscles), Check rust (blood test), Check cracks (X-rays)
1. Rebound Hammer Test¶
Objective¶
The objective of the rebound hammer test is to:
- Find the compressive strength of the concrete with the help of suitable correlation between the rebound index and compressive strength
- Assess the uniformity of the concrete
- Assess the quality of concrete in relation to standard requirements
What It Does
Estimates concrete strength by measuring surface hardness.
Three Uses:
- Estimate compressive strength (via correlation charts)
- Check uniformity (compare readings across structure)
- Quick quality assessment (good vs. poor concrete)
Simple Analogy: Like testing fruit ripeness by tapping - harder surface = stronger concrete
Principle¶
The principle behind rebound hammer test is that it works on the rebound of an elastic mass depends upon the hardness of the surface against which the mass impinges.
How It Works
Simple Physics: Throw a ball at a wall - harder wall = higher bounce.
In Concrete: Spring-loaded mass hits concrete → bounces back → higher rebound = harder/stronger concrete
Key Point: Only tests surface (first ~30mm) - not full depth strength.
Procedure¶
- The hammer is ready for test
- The hammer is pushed against the object
- The hammer is released
- The hammer rebounds
Testing Steps
- Prepare: Press hammer against concrete surface
- Load: Push until click (spring compresses)
- Release: Automatic release of mass
- Read: Note rebound number on scale
- Repeat: Take 10-12 readings, discard extremes, average the rest
Important: Test smooth, clean, dry surfaces perpendicular to surface for accurate results.
Interpretation of Test Results¶
The average rebound number specifies the quality of the concrete.
| Average Rebound Number | Quality of Concrete |
|---|---|
| more than 40 | Very good hard layer |
| 30 to 40 | Good layer |
| 20 to 30 | Fair |
| less than 20 | Poor concrete |
If the average rebound number is very good, the concrete quality is excellent; if it is poor, the concrete quality is poor.
Quick Interpretation Guide
| Rebound Number | Quality | What It Means |
|---|---|---|
| >40 | Very Good | High-strength concrete, well-cured |
| 30-40 | Good | Normal structural concrete |
| 20-30 | Fair | Acceptable for non-critical elements |
| <20 | Poor | Weak concrete, investigate further |
Memory Tip:
- 40+ = Excellent (4 = A grade)
- 30-40 = Good (3 = B grade)
- 20-30 = Fair (2 = C grade)
- <20 = Poor (Need improvement)
Critical Note: Use correlation charts/curves to convert rebound number to actual compressive strength (MPa).
2. Ultrasonic Pulse Velocity Test¶
Objective¶
The objective of ultrasonic pulse velocity test is to:
- Determine the homogeneity of the concrete
- Find the presence of cracks, voids, and other imperfections in concrete
- Find the changes in the structure over the course of time
- Determine the quality of the concrete in relation to standard requirements
- Find the values of elastic modulus in concrete
What It Does
Sends ultrasonic waves through concrete to assess internal quality.
Five Key Uses:
- Homogeneity check - Is quality uniform throughout?
- Defect detection - Find cracks, voids, honeycombing
- Long-term monitoring - Track deterioration over years
- Quality assessment - Good vs. poor concrete
- Elastic modulus - Material stiffness calculation
Analogy: Medical ultrasound for pregnant women → Ultrasonic testing for concrete structures
Principle¶
The velocity of the ultrasonic pulse passing through any material depends upon the density of the material and modulus of elasticity of the material.
Basic Physics
Sound travels faster through dense, solid materials.
- Dense, good concrete → High velocity (>4.5 km/s)
- Cracked/porous concrete → Low velocity (<3.0 km/s)
Why? Voids and cracks slow down or deflect sound waves.
Formula concept: Velocity ∝ √(Elastic Modulus / Density)
Remember: Dense material = Fast sound = Good concrete
How It Works¶
- Ultrasonic waves are similar to light waves that get reflected, refracted, and focused
- Reflection and refraction occur when the sound waves interact with interfaces of different acoustic properties
- Ultrasonic reflections from the presence of discontinuities or geometric features enable detection of defects and their locations
- A pulse of longitudinal vibration is produced by electro-acoustical transducer which is held on the surface of the concrete
- After traveling a known length, the pulse of vibration is converted into electrical signal by another acoustical transducer
- The electronic timing circuit enables them to capture the transit time of the pulse, and this is how the pulse velocity is measured
Testing Process
Step-by-Step:
- Transmitter → Sends ultrasonic pulse into concrete
- Pulse travels → Through concrete (affected by defects)
- Receiver → Catches pulse on other side
- Timer measures → Transit time
- Calculate → Velocity = Distance / Time
Three Arrangements:
- Direct: Transmitter and receiver opposite sides (most accurate)
- Semi-direct: Adjacent surfaces at 90° (moderate accuracy)
- Indirect: Same surface, different positions (least accurate)
What Affects Speed: Cracks, voids, moisture, aggregate type, curing quality
Think: Sonar in submarines detecting obstacles underwater
Interpretation of Test Results¶
The pulse velocity determines the concrete quality:
| Pulse Velocity | Concrete Quality |
|---|---|
| Above 4.5 km/s | Excellent |
| 3.5 to 4.5 km/s | Medium |
| Below 3.0 km/s | Doubtful |
Quick Quality Assessment
| Velocity (km/s) | Quality | Interpretation |
|---|---|---|
| >4.5 | Excellent | Dense, well-compacted, no defects |
| 3.5-4.5 | Good/Medium | Normal concrete, minor imperfections |
| 3.0-3.5 | Questionable | Possible voids, poor curing |
| <3.0 | Poor/Doubtful | Serious defects, investigation needed |
Memory Aid:
- 4.5+ = "Excellent Express" (fast train)
- 3.5-4.5 = "Medium Metro" (normal speed)
- <3.0 = "Doubtful Delay" (slow, problematic)
Note: Values can vary with aggregate type, moisture - compare with reference samples from same concrete mix.
3. Electromagnetic Cover Measurement¶
Basic Principle¶
The presence of steel affects the field of an electromagnet.
Simple Concept
Steel rebars disturb electromagnetic fields - like how metal detectors work at airports.
Physics:
- Device creates magnetic field
- Steel rebar changes the field
- Sensor detects the disturbance
- Calculates depth and size
Applications¶
- Used to measure the concrete cover and the bar diameter of the existing RC structure
- Can also be used to identify the location of the rebar and their spacing
Three Main Uses
- Cover measurement - Distance from surface to rebar (critical for corrosion protection)
- Bar diameter estimation - Size of reinforcement (16mm, 20mm, etc.)
- Rebar location & spacing - Mapping reinforcement layout
Why Important?
- Inadequate cover → Early corrosion risk
- Unknown layout → Helps plan drilling/cutting safely
- Verify construction → Check if built as designed
Common Tool: Profometer or Cover Meter
Limitation: Accuracy decreases with depth and congested reinforcement
4. Half-Cell Electrical Potential¶
Purpose¶
Used to detect the corrosion in the reinforcement of the structure.
Primary Use
Detects active corrosion in steel reinforcement without breaking concrete.
What It Identifies: Areas where rust is currently happening vs. areas still safe.
Working¶
- The in-situ corrosion potential is observed
- The electrical charges that pass through the reinforcement are also observed
- The corrosion activity is thus determined
- No corrosion in rebars - no potential difference occurs
- Corrosion present - potential difference occurs
How It Works
Electrochemical Principle: Corroding steel creates electrical potential difference.
Testing Process:
- Place reference electrode (half-cell) on concrete surface
- Connect to rebar via drilled hole or existing exposure
- Measure voltage between half-cell and rebar
- Map readings across entire surface
- Identify zones of high corrosion risk
The Science:
- No corrosion → Small/neutral voltage
- Active corrosion → Large negative voltage
Think: Like a voltmeter checking battery health - corroding steel is like a weak battery generating unusual voltage.
Interpretation of Test Results¶
The test result depends upon the potential that flows in the concrete and indicates high corrosion or low corrosion activity.
Reading the Results (using Copper-Copper Sulfate reference electrode)
| Potential (mV) | Corrosion Risk | Action |
|---|---|---|
| >-200 | Low (<10% probability) | No immediate concern |
| -200 to -350 | Uncertain | Monitor, further investigation |
| <-350 | High (>90% probability) | Active corrosion likely, repair needed |
Visual Mapping:
- Create contour maps showing potential distribution
- Hot spots (very negative) = corrosion zones
- Cool zones (less negative) = protected areas
Important Notes:
- More negative = worse corrosion
- Wet concrete gives more negative readings (not always corrosion)
- Use with other tests (chloride, carbonation) for confirmation
Memory: Think temperature map - "hot" spots need urgent cooling (repair)
5. Radiography¶
Principle¶
When radioactive rays are directed towards the object, some of the photons interact with the particles of the matter and that energy is either absorbed or scattered. This absorption and scattering is called attenuation.
The relationship between the intensity of the photon beam received and transmitted is given by the formula:
I = I₀ × e^(-μx)
Where: - I = transmitted intensity - I₀ = initial intensity - μ = attenuation coefficient - x = thickness
Basic Physics
X-rays or gamma rays pass through concrete - denser material absorbs more radiation.
Key Concept - Attenuation:
- Dense concrete → More absorption → Less reaches film → Lighter image
- Voids/cracks → Less absorption → More reaches film → Darker image
Formula Simplified:
Transmitted intensity decreases exponentially with material thickness and density.
Real-World Parallel: Medical X-ray - bones (dense) appear white, air/soft tissue (less dense) appear dark.
How It Works¶
- The effect of attenuation is used to determine the defects in the concrete
- As the radiation passes through the member, the intensity is reduced according to the thickness, density, and characteristics of the material
- The quantity of radiation passed through the member is recorded on a film
Testing Setup
Configuration:
- Radiation source (X-ray or gamma ray) on one side
- Concrete member in middle
- Detector/film on opposite side
Process:
- Radiation passes through concrete
- Different materials absorb differently:
- Steel rebar → High absorption (appears light/white)
- Dense concrete → Medium absorption (gray)
- Voids/cracks → Low absorption (appears dark)
- Film/detector captures the pattern
- Developed image shows internal structure
What You Can See: Rebar position, voids, honeycombing, cracks, variations in density
Detection of Cracks and Voids¶
Cracks and voids absorb less radiation and show up as a dark zone on the film. Cracks parallel to the radiation direction are more readily absorbed than cracks perpendicular to the radiation direction.
Based on the absorption of radiation, the cracks are determined on the following basis: - Cracks and voids absorb less radiation and show up as dark zones on the film - Cracks parallel to the radiation direction are more readily absorbed than cracks perpendicular to the radiation direction
Interpreting Images
On Film/Detector:
- Dark areas = Voids, cracks, less dense concrete
- Light areas = Steel, dense concrete
- Gray areas = Normal concrete
Crack Detection Rules:
- Perpendicular cracks (to radiation beam) → Most visible (dark lines)
- Parallel cracks (to radiation beam) → Less visible (harder to detect)
- Optimum: Orient radiation perpendicular to suspected crack direction
Practical Tip: May need multiple angles to catch all crack orientations.
Limitations:
- Requires access to both sides
- Safety concerns (radiation)
- Expensive equipment
- Trained operators needed
- Good for thin sections (<300mm typically)
Conclusion¶
We have come to the end of the session. I hope you now understand about the non-destructive testing techniques and their types like:
- Rebound hammer test
- Ultrasonic pulse velocity test
- Electromagnetic cover measurement
- Half-cell potential test
- Radiography
In our upcoming lecture series, we are going to see about epoxy injection.
NDT Quick Recap
Five Key Methods Learned:
- Rebound Hammer → Surface hardness test (bounce test)
-
Remember: Higher bounce = stronger concrete
-
Ultrasonic Pulse → Sound wave through concrete
-
Remember: Faster sound = better quality
-
Electromagnetic Cover → Metal detector for rebars
-
Remember: Finds steel location and cover depth
-
Half-Cell Potential → Electrical test for corrosion
-
Remember: More negative voltage = active rust
-
Radiography → X-ray vision for concrete
- Remember: Dark spots on film = voids/cracks
The Big Picture: These tools let engineers diagnose structural health without damaging buildings - like medical tests for structures.
Next Topic: Epoxy injection (repair technique for cracks)