T3-4: Testing and NDT Methods Flashcards
Name different types of non-destructive tests (x*) for concrete?
- Covermeter
- Indirect strength tests (rebound hammer, ultrasonic pulse velocity, near surface tests)
- Half-cell potential
- Concrete resistivity
- Infrared thermography (advanced)
- Impulse radar/GPR (advanced)
- Impact/Pulse echo (advanced)
What are three types of stage two (physical samples) methods?
- Dust sample
- Core
- Lump sample
How does a covermeter survey work?
- Electromagnetic technique
- The presence of steel disturbs the magnetic field, with the degree of disturbance dependent on steel mass (diameter) and proximity to surface
What is a covermeter survey used for?
- Finding the depth of concrete cover to rebar
- Interpreting carbonation depths
- Analysing chloride ion problems
- Checking if rust stains have underlying bar
What are three things to be aware of during covermeter testing?
Lapped or crossing bars
- after finding the laps/crossings, measure the cover of single bars
The cover is rarely uniform
- leads to differential corrosion cells
Need to calibrate the covermeter depth
- drill into the concrete and measure the depth of cover at the breakout
[NAQ] covermeter survey results
- Cover depths are plotted on a drawing of the element
- Cover depths marked depend on specification (e.g. depth < 20mm)
Name three types of indirect “strength” tests
- Rebound (Schmidt) hammer
- Ultrasonic pulse velocity (UPV)
- Near to surface tests
What does a rebound hammer test do?
- It estimates the compressive strength of concrete
- Measures the rebound of a spring-loaded mass after impacting the concrete surface
How does UPV testing work?
- It measures the time it takes for a high-frequency sound wave (ultrasonic pulse) to travel through the concrete
- Two transducers (one transmitting and one receiving) are placed on opposite sides of the concrete element, and the time it takes for the pulse to travel between them is measured
What is UPV testing used to determine (x5)?
- Homogeneity of concrete
- Presence of voids, cracks or other imperfections
- Changes that occur with time (e.g. due to the cement hydration)
- Action of fire, frost or chemical attack
- Quality of concrete (generally strength)
What do higher velocities in UPV testing indicate?
Good quality, density, and homogeneity of the concrete
What does UPV testing require?
A couplant (e.g. silicone grease)
What is the equation for UPV testing?
UPV = L/T
Name five near surface testing methods
- Internal fracture test
- Penetration resistance
- Pull-out test
- Break-off test
- Pull-off test
What NDT method measures the likelihood of reinforcement corrosion?
Half-cell potential
How do half-cell potential survey tests work?
- The corrosion in steel develops macrocells: passive and corroding areas
- Results in electrochemical behaviour and flow of current
- Current flow is accompanied by an electric field, which can be measured at the concrete surface
- Compare the electrode potential of the steel beneath the concrete surface with the output of the half-cell electrode potential (which is a constant value)
What is the preferred thing to have half-cells based on?
Silver-Silver Chloride
How are half-cell potential survey results processed?
- Results of the test (500mm grid) shown on a drawing of the structural element
- Contours of half cell potentials are then drawn at -100mV intervals
In half-cell potential testing, describe the range of results?
- Dry, non-corroding concrete can give low positive readings (+100mV)
- Saturated, salt-contaminated concrete can give high negative readings (-500mV)
What testing is used in conjunction with half cell potential testing, for assessing corrosion potential?
Concrete resistivity
What is concrete resistivity testing used to indicate?
- The moisture state of concrete
- The likely rate of corrosion of reinforcement
Describe the process of concrete resistivity testing?
- Resistivity measurements are taken at node points where the half cell potential is > -350mV (ie. high corrosion likely)
- Resistivity is measured using a Wenner Four Electrode Resistivity Meter
- Probes may be inserted 6mm into holes into the concrete for accurate readings
What is concrete resistivity affected by?
- Concrete quality
- Moisture content
- Chloride ion content (ie. contaminants)
- Aggregate types and underlying steel
Does higher resistivity lead to higher or lower likelihood or corrosion rate?
Higher resistivity = lower likelihood
What is an advanced NDT method, which is used to detect subsurface defects?
Infrared thermography
What does infrared thermography testing show (x2)?
How is this reflected in the results?
Delamination (due to corrosion)
- laminated areas heat up and cool down faster
Areas of dampness/poor insulation:
- evaporation cools the surface
How does infrared thermography testing work?
- All objects emit infrared radiation, with the amount increasing/decreasing with changes in temperature
- Thermographic imaging measures this radiation and displays it as a visual heat image of the surface
NB. thermography measures surface temperature, not actual temperature
What are four other types of advanced NDT methods?
What does each one do?
Impulse radar/GPR (image):
- uses electromagnetic pulses to non-destructively map internal features, including rebar, conduits, voids, and slab thickness
Impact/pulse echo:
- assesses the condition of concrete structures by analysing the surface motion resulting from a short-duration impact
Radiography:
- uses X-rays or gamma rays to create images of the concrete’s internal structure, revealing defects like voids, cracks, or improper placement of reinforcing steel
Indirect UPV:
- measuring the time it takes for ultrasonic waves to travel along the surface of a material, rather than through it
What three principal types of alteration are responsible for fire damage?
- Cracking (and microcracking) in the surface zone
- Alteration of the phases in aggregate and paste
- Dehydration of the cement hydrates
In fire damage, what does alterations lead to (x4)?
- Reduced strength
- Increased porosity
- Desiccation of pore water
- Reduced durability
For each type of sample (lump sample, core, dust sample), classify the respective equipment types and possible tests
What is drilled dust sampling used for?
Analysing chloride content and sulfate content
Describe how drilled dust samples are taken
- Rotary percussion drill used
- Samples taken in depth increments (e.g. 15-25mm); also will determine if it is cast-in or penetrating chloride
How are results from chloride content tests used?
- Chloride ion content is expressed by weight of cement (usually estimated)
- Results are presented as a chloride/depth profile
- Results above 0.4% bw cement indicate a high corrosion risk
Describe the process of testing for carbonation depth (x3 steps)
- Carbonation depth is measured on a freshly broken surface
- the ‘throat’ between two close-spaced drill holes is broken off to make a fresh fracture face - Broken surface is sprayed with indicator (phenolphthalein)
- Carbonation depth is measured between the surface and colour change interface
For carbonation depth measuring, at what pH does the concrete sample turn from colourless to pink
Above pH 9.2
[NAQ] carbonation depth vs age of structure graph
What are important uses of core samples (x6)?
- Calibration (used in combination with NDT tests for calibration)
- Density and compressive strength (BS EN 13791)
- Visual assessment of concrete quality
- Petrographic and chemical analysis
- Crack depths and widths
- Cement content
What is the range of a typical core diameter?
50mm - 150mm
What steps should be taken to ensure core locations do not damage a structure?
Cover surveys and structural analysis
What are four common laboratory tests on concrete?
- Strength tests on cores
- Chemical testing:
- cement content
- chloride content
- sulfate content - Permeability, diffusion, absorption
- Petrographic examination
Describe the process of petrographic analysis
- Samples are taken from cores or lump samples
- They are impregnated in resin and reduced to 30 microns thickness (or polished)
- Samples are examined using a petrological microscope in transmitted or reflected light
What does petrographic analysis determine (x10)?
- Aggregate type, grading, shape and condition
- Nature of cement paste (PC, PFA, GGBS etc)
- Volumetric proportions of constituents
- Nature and degree of paste hydration
- Aggregate bond
- Porosity of cement paste
- Air entrainment
- Depth of carbonation
- Cracks, voids and inclusions
- Evidence of sulfate attack, frost damage, ASR
For corrosion testing, what would you use
- Half-cell
- Covermeter
- The two principle tests on site (chloride content, carbonation depth)
