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Contents
List of figures ............................................................................................. Error! Bookmark not defined.
1. Introduction ....................................................................................... Error! Bookmark not defined.
1.1 Objectives ................................................................................... Error! Bookmark not defined.
1.2 Background ................................................................................. Error! Bookmark not defined.
2. Equipment .......................................................................................... Error! Bookmark not defined.
3. Literature review ................................................................................ Error! Bookmark not defined.
4. Methodology ...................................................................................... Error! Bookmark not defined.
4.1 Visual Inspection ......................................................................... Error! Bookmark not defined.
4.2 Eddy Current Testing ................................................................... Error! Bookmark not defined.
5. Results and Analysis ............................................................................ Error! Bookmark not defined.
5.1 Visual inspection ............................................................................... Error! Bookmark not defined.
5.2 Covermeter....................................................................................... Error! Bookmark not defined.
5.2.1 concrete cover ........................................................................... Error! Bookmark not defined.
5.2.2 bar diameters............................................................................. Error! Bookmark not defined.
6. Discussion ........................................................................................... Error! Bookmark not defined.
7. Conclusion .......................................................................................... Error! Bookmark not defined.
8. References .......................................................................................... Error! Bookmark not defined.
9. Appendix ............................................................................................ Error! Bookmark not defined.
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List of figures
Figure Page No.
Fig.4.1 mirrors 7
Fig.4.2 flashlight 7
Fig.4.3.1 covermeter 8
Fig.4.3.2 probe 8
Fig.4.5 calibration block 9
Fig.4.6 profometer components 9
Fig.4.7 Scancar 10
Fig.5.1 tension crack 15
Fig.5.2 tension and bending cracks 15
Fig.5.3 longitudinal cracks 16
Fig.5.4 shear, tension and bending cracks 16
Fig.5.5 torsion cracks 17
Fig.5.6 shrinkage cracks 17
Fig.5.7 parallel cracks at random 18
Fig.5.8 skewed at random 18
Fig.5.9 hexagonal mesh 18
Fig.5.10 cracks due to plastic settlement 19
Fig. 5.11 accuracy ranges 22
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1. Introduction
Non destructive testing (NDT) is important in ensuring the structural integrity of both reinforced
concrete and steel structures. In the coming years Kenya will have an unprecedented growth in
infrastructure and thus the need to ensure adherence to standards in both the building and
transport sector. NDT is the evaluation of the properties of a material, component or system
without causing damage. The long list of NDT methods and techniques includes: radiographic
testing (RT), ultrasonic testing (UT), liquid penetrant testing (PT), magnetic particle testing
(MT), eddy current testing (ET), visual testing (VT) as well as leak testing (LT), acoustic
emission (AE), thermal and infrared testing, microwave testing, strain gauging, holography,
acoustic microscopy, computed tomography, non-destructive analytical methods, non-destructive
material characterization methods and many more.
The basic NDT methods which are largely used in routine services in industry are;
1. Visual inspection
2. Liquid penetrant testing
3. Magnetic particle testing
4. Electromagnetic or eddy current testing
5. Radiography
6. Ultrasonic testing
However most of these NDT methods complement and support each other and, in many cases,
must be used in a combination in order to get more accurate results. This project entailed the use
of a covermeter to determine the properties of the reinforcement steel in structural elements as
well as demonstrate how the equipment is used in reinforced concrete structures. The covermeter
was selected over other NDT methods for this function because it enables the detection of
metallic components and is completely safe for the operator compared to radiographic testing
which uses X-rays. The project included the calibration and testing of the equipment and the
principles behind its use.
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1.1 Objectives
1. To fully understand how to use the equipment; covermeter
2. To standardize the equipment
3. To determine the position and depth of rebars in an existing concrete element
4. To determine if cover is sufficient
Specific objective; to determine the position, depth and health of rebars in selected structural
elements of the civil engineering block
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1.2 Background
Although there are deterioration mechanisms that affect concrete, it is usually corrosion of the
reinforcing steel that leads to visible deterioration. Reinforcement has been used in concrete for
over a century and codes have been made to specify a minimum concrete cover depending on the
elements exposure but only recently has attention been given to chemical exposure.
Contaminants such as chlorides and carbonation break this protection causing corrosion.
Corrosion by-products expand the size of steel and create large internal bursting stresses causing
cracks in concrete. The high ph of the interior of a reinforced concrete element protects the steel
from corrosion by forming a protective layer over the steel. Sources of contaminants can be
classified as;
1. Internal contaminants
2. External contaminants
Internal Contaminants
These include the mixture of beach sand or chloride containing admixtures which shorten the
time concrete takes to set.
External contaminants
These generally come in the form of chlorides either from seawater exposure or man-made
deicing salts, atmospheric carbon dioxide or chemical processes.
Non-destructive testing is an important activity that can be used to dispel doubts about the
quality of materials used in construction as well as the methods of construction. This is because
there is no formal method of tracking the behaviour of structures in service and the need to map
the standards of buildings has increased over the years. Destructive testing on its own cannot
provide an in depth look into structural integrity as it may only allow a small number of tests to
be carried out on a large structure. This can easily lead to an inaccurate representation of the
health of the building.
As most buildings are occupied shortly after construction, destructive testing is undesirable as it
may disrupt the activities of the occupants of the building. It would thus be difficult to carry out
a comprehensive audit while preserving the use of the building.
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Non destructive testing can also aid in proving or disproving the acceptability of material
supplied that does not appear to comply with the specification and can provide information for
proposed change of use of a structure for insurance or change of ownership. It shields people
from damage of property and loss of life by ensuring that defects are located early and the
necessary precautions are taken.
The most important things to test for in a reinforced concrete building are;
1. Delamination
2. Bar location and size
3. Bar corrosion
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2. Equipment
1. Mirrors
Fig.4.1 mirrors
2. Camera
3. Flashlight
Fig.4.2 flashlight
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4. Covermeter(Profometer, proceq)
Components
a. Indicating device
This is the component of the profometer where the screen is located and output is
shown.
Fig.4.3.1 covermeter
It contains the menu that allows selection of functions for and the preselection of certain
parameters such as bar diameter that enable measurement
a. universal probe
This is the device that houses the coils required to generate a magnetic field to produce
eddy currents upon interaction with the induced magnetic field from the rebars.
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Fig.4.3.2 probe
b. Test block
This is the calibration device. It allows the user to determine if the profometer is
functioning properly and enables detection of errors before using the profometer to locate
rebars.
Fig.4.5 calibration block
c. Probe cable 1.5 m, transfer cable 1.5 m, adapter RS232/USB, carrying strap, headset,
protective sleeve for indicating device, operating instructions, carrying case.
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Fig.4.6 profometer components
d. Scan car
Used to perform specific functions and make the probe more easily mobile. It has a
correction cable to account for the air gap between the scancar and the surface to be
tested
Fig.4.7 Scancar
The profometer uses the pulse induction technique. There are coils in the probe that are
periodically charged by current pulses and thus generate an electromagnetic field. This produces
eddy currents on the surface of any electrically conductive material within the magnetic field.
The eddy currents in turn generate a magnetic field in the opposite direction to that in the probe.
This change in voltage is used for measurement. The advantage of this method is that readings
are unaffected by the non-metallic material i.e. concrete surrounding the rebars.
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The profometer can be used to determine;
- location and orientation of reinforcing bars
- concrete cover depth
- bar diameter
- bars with insufficient concrete cover
- cover depth in congested bar arrangements
3. Literature review
NDT is a test or evaluation performed to test material integrity without changing the material
characteristics or destroying it in anyway. Any kind of defects and discontinuities within the
material can affect its efficiency, maintainability and serviceability. (Hellier, 2001)
Kenya has a long standing history with NDT as its use began during the colonial era. When the
government was handed over to Kenyans, the equipment was abandoned and it was only 1982
that it was decided that personnel should be trained to use the equipment to check for defects.
Mr Abdullah Kulla (Ministry of Transport) was sent to Japan to study NDT so as to be able to
apply it in Kenya. Unfortunately, it was established that the training he received in Japan was too
advanced and mostly computer based while most of the equipment in Kenya was manual. It was
then decided that training in India might be more suitable and measures were undertaken for
training to be provided. At this time the testing was based on metallurgy and was only used to
test metals for flaws. It wasn’t until 1988 that NDT was applied to civil engineering works. It has
grown from then to be used not just in assessment of buildings but also roads using advanced
methods such as Nucleonic testing, to determine moisture and density of the road, and sand
testing. NDT has been used to assess the structural damage the 1998 bomb blast caused to the
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Co-operative building (Eng. J.K Chege, 2000) and is to be an integral part of the construction of
the nuclear power plant that is scheduled to be built in Kenya in line with vision 2030.
There is a very high demand for NDT and the NDT society of Kenya is trying to promote it as a
career. A syllabus has been developed for the training of NDT Professionals in accordance with
ISO 9712 standards. The Kenya Bureau of Standards was to do certification with the materials
department of the Ministry of Transport carrying out training but this did not come to fruition.
The University of Nairobi in conjunction with the International Atomic Energy Agency is
striving to introduce NDT to students as a viable career option and is holding seminars and
workshops to train personnel in NDT proficiency.
Non destructive testing plays a key role in minimizing the effects of the unexpected load
condition or material property. According to Stanley, due to the high cost of repairing a structure
and the safety risks involved in its failure, there is no room for uncertainty in the construction
business, but engineering is a field that is ripe with uncertainty therefore the application of
probability is important in non-destructive evaluation (Stanley,1989).
Generally NDT systems are more likely to detect large defects but according to Mordfin,
detectability of a flaw generally increases with its size but a flaw in a material or structure does
not necessarily render it unfit for use (Mordfin, 1985). It is thus important to determine the
significance of the defects. The factors to consider include the number and character of flaws,
loading environment, environmental history, residual stress levels and probable mechanism of
failure of the structure in question.
a. Number and character of flaws
b. Loading environment
c. Residual stress levels
d. Mechanism of failure/damage
There are many modes of failure in steel reinforced concrete. Failure may be due to reduction in
durability or inadequate strength. The following are the typical mechanisms that lead to the
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shortening of the design life of reinforced concrete structures, according to Morsch(1909),
Collins(1981), Nilson et al(2003);
i. Mechanical failure
Creation of cracks in concrete is almost inevitable but using the appropriate reinforcement,
proper placing of joints, proper curing and the correct mix design. Cracking allows moisture to
penetrate and corrode the reinforcement and is usually caused by inadequate reinforcement or
spacing that does not meet the minimum requirements as stated in the design codes (BS 8110
part 1). This is a serviceability failure in limit state design which can lead to overloading.
Ultimate failure can be caused by concrete crushing, yielding of the rebar or bond failure
between the concrete and the rebar
ii. Carbonation
This is a chemical reaction between carbon dioxide in air with calcium hydroxide and hydrated
calcium silicate in the concrete. This reaction is usually facilitated by inadequate concrete cover
during construction coupled with sufficient moisture and oxygen to cause electropotential
corrosion of the rebars and causes a decrease in alkalinity of the concrete.
Ca(OH)2 + H2CO3 → CaCO3 + 2H2O
3CaO•2SiO2•3H2O + 3CO2 → 3CaCO3•2SiO2•3H2O
Although dry carbon monoxide cannot react with dry calcium hydroxide, carbonation depends
on a drying atmosphere. The presence of water prevents the reactions from taking place. The
optimum moisture content for carbonation is 40-70% relative humidity. A 10o C increase in
temperature will double the rate of reaction.
iii. Chlorides
If chlorides are present in large enough concentrations, they can cause corrosion of rebars.
Chloride anions produce both localized, also known as pitting corrosion, and generalized
corrosion of rebars. This can be prevented by using only fresh or potable water for mixing
concrete and ensuring that the aggregates and admixtures used do not contain chlorides. In the
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past calcium chloride was used as an admixture but its use has decreased after the effect of
chlorides on reinforcement was discovered. In cold countries, use of de-icing salts on roads is
probably one of the causes of premature failure of reinforced or prestressed concrete bridge
decks, roadways and parking garages. Chloride ions are also found in sea water and so extra
precautions must be taken to safeguard structures that are in the sea or next to the sea.
iv. Alkali silica reaction
For this to occur there must be;
- Aggregates containing an alkali reactive constituent
- Sufficient hydroxyl ions
- Moisture above 75% humidity within the concrete
This is a reaction of amorphous silica sometimes present in aggregates with hydroxyl ions from
the cement pore solution. Poorly crystallized silica dissolves and dissociates at high Ph in
alkaline water. The soluble silicic acid reacts in the pore water with the calcium hydroxide in the
cement to form as expansive calcium silicate hydrate. This reaction causes localized swelling and
thus cracking.
v. Conversion of high alumina cement
This type of cement cures quickly and attains very high durability and strength, but can lose
strength with heat or time especially when not cured properly. The cement was banned in the UK
after the collapse of three roofs.
vi. Sulphates
Sulphates in sufficient concentration in the groundwater and soilcan react with Portland cement
in concrete causing the formation of expansive products like ettringite and thaumasite which can
lead to failure of the structure. This is typical in concrete slabs and foundation walls at grade
where the suphate ions can increase in concentration via wetting and drying. The chemical
analysis of soil borings should be done during the design of any concrete structures in contact
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with the soil of an area to check for the presence of sulphates and if large concentrations are
found, coatings can be used to offset movement of sulphate ions into the concrete.
Estimation of mode and sequence of failure may be a simple problem or a complex one in cases
where the initiating trigger is not readily apparent.
In most cases where NDT is requested the structure has not yet failed but has incurred some
damage. In reinforced concrete structures the most common types of damage include cracks,
spalling, rebar corrosion, wear and abrasion, material deterioration, impact damage, fracture,
weathering and honeycombing. Damage mechanisms include;
Structural Deficiencies
Structural deficiencies are a danger to the structural safety and thus identifying them is of great
importance. Structural deficiencies can be divided into the following four types, which can be
distinguished by their appearance:
1. Structural cracks
These are load induced and are cracks with well-defined orientation. The different types of
internal forces (bending, shear) have specific crack patterns and may be a sign of a structural
deficiency.
For reinforced concrete structures, cracks in most cases are not serious. The crack width and
spacing will indicate whether or not there is something wrong, taking the specific type of load
and type of reinforcement into consideration. Coarse cracks are an indication of over-load and/or
under-design.
Types of Structural Cracks
a. Pure tension
In a beam subjected to pure tension, cracks will form the whole cross section of the beam.
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Fig.5.1 tension crack; Non Destructive Testing and Inspection Manual
At the cracks a rise in the steel stress will occur affecting the bond between the concrete
and the bar at a distance lo (slip distance) around the cracked section, preventing the
transfer of shear stresses. The surrounding uncracked concrete reduces the steel stresses
between cracks. This is known as tension stiffening.
b. Flexural cracks
These entail two types of cracks and occur in beams subjected to bending. The first to
form are known as bending cracks. They occur on the beam face that is in tension and
extend to the neutral axis. With the increase of bending moment, other cracks form on the
face in tension and go on to just beyond the main bars. In heavily reinforced beams where
d>0.4 metre, tension cracks tend to join to form a fork-like pattern.
Fig.5.2 tension and bending cracks; Non Destructive Testing and Inspection Manual
c. Longitudinal cracks
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These may be formed by stresses in the main bars giving rise to local compression stress
in the surrounding concrete. The tension strain in a deformed main bar produces inclined
compressive stresses between the concrete and ribs of the bar, these stresses tend to split
the cross-section transversely. These cracks may occur when there are high stresses in
deformed main bars or due to anchorage failure at the end of a reinforcement bar.
Fig.5.3 longitudinal cracks; Non Destructive Testing and Inspection Manual
d. Shear cracks
These are inclined cracks that occur at the supports in beams and slabs subjected to shear
and bending. Close to the supports the angle between the cracks and the beam axis is
almost 45 degrees
Fig.5.4 shear, tension and bending cracks; Non Destructive Testing and Inspection
Manual
e. Torsion cracks
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These cracks are similar to shear cracks but are spiral and cross the entire depth of the
beam
Fig.5.5 torsion cracks; Non Destructive Testing and Inspection Manual
f. Non-structural cracks;
They are normally harmless structurally but may interfere with the durability of a
structure. The orientation of these cracks varies based on the geometric conditions
but is well defined. Drying shrinkage cracks pass through the whole cross section
Fig.5.6 shrinkage cracks; Non Destructive Testing and Inspection Manual
They look like shrinkage cracks and are caused by thermal stresses due to
temperature differences in the hardening concrete. They are developed in young
concrete and follow the surface of the coarse aggregates and stones but do not go
through them.
racks due to plastic shrinkage
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These are caused by rapid drying of the concrete surface -due to low humidity,
wind, high temperature etc. – in its plastic state probably caused by improper
curing. The cracks follow the surface of the stones like the thermal cracks. They
are wide and shallow and may form a definite pattern. In the cases of wide
surfaces, a state of hydrostatic tension will arise. Without crack guidance the
cracks form at random. The cracks are harmless from a structural point of view
but may affect durability.
Fig.5.7 parallel cracks at random; Non Destructive Testing and Inspection Manual
Fig.5.8 skewed at random; Non Destructive Testing and Inspection Manual
Fig.5.9 hexagonal mesh; Non Destructive Testing and Inspection Manual
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These are due to high concrete slump when the concrete was cast although they
are also seen in slabs with voids. They normally appear above the reinforcement
at the surface or at changes in cross section.
They are harmless structurally but affect the durability of the building as the
rebars are not sufficiently protected from environmental effects.
Fig.5.10 cracks due to plastic settlement; Non Destructive Testing and Inspection Manual
2. Excessive/unintended deflections and movements
Examples:
• Settling of foundation possibly because of poor soil condition, scour, et cetera.
• Deflection of girders, mostly caused by low stiffness, creep, poor design or improper
formwork.
• Horizontal movements of retaining walls and wing walls possibly caused by low stiffness,
creep, compaction of back fill, soil condition, under-design.
• Bearings out of position caused by wrong positioning, unforeseen movements, shrinkage,
creep, deterioration and temperature.
3. Fracture/crushing
Examples:
• Local crushing at supports/bearings caused by honeycombs, wrong positioning of bearings
and/or reinforcement, overload, inadequate initial load bearing capacity.
•Crushing of columns caused by impact during flooding.
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• Superstructure as a result of impact from vehicle.
• Local crushing at expansion joints because of inadequate joint system, wear, movements
restrained).
Another factor to consider in NDT is the sample size to be tested, that is the character
information that may emerge from a small data set as opposed to a large one and the difference
within the data sets between the probability of existence of a defect and distribution of defects
within the sample set. According to Mordfin(1985), if a small data set is used, it must be
considered how well the data represents the overall population. Larger data sets provide better
insight into the actual condition as well as providing a well defined mean and standard deviation
in analysis. It is also important to consider the accessibility of the members as proper testing of
an element may be hindered if elements do not have exposed surfaces.
It must however be noted that increasing the size of a data set will also increase the amount of
time to be taken in non destructive testing as well as the cost. The cost of implementing an NDT
program is high. The equipment is expensive and storage costs must be considered as well as the
cost of training the staff to operate the equipment. The cost varies depending on sensitivity and
frequency of testing. The cost of inspection dominates programs that depend on low sensitivity
and frequency while in programs utilizing high sensitivity equipment and performing frequent
checks the cost is dominated by costs associated with failure. Therefore, when choosing a sample
size, the person carrying out testing must be conscious on the urgency of the data required and
the cost of carrying out the tests. In cases such as nuclear power plants and testing facilities, the
sample size should be larger because of the repercussions of any flaws created in the
construction or during the design life of the facility. In other cases, cost may be the main factor
to consider as the risk may not be as high.
Factors affecting the optimization of inspection include;
1. minimum detectable flaw size by equipment
This depends on the sensitivity of the equipment. Some models are superior to others
2. operator reliability
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Training of operators should be done according to ISO9712 and caution should be exercised
during inspection so as to limit human error in the collection of data
3. frequency of inspection
This depends on the nature of the structure and the general practice of the country. In Kenya
there are no regulations concerning NDT and thus testing is done upon request.
4. risk of defects missed
This risk is reduced with an adequate sample size, proper equipment and a well trained
operator.
In Kenya there are two scenarios to consider when carrying out Non Destructive testing on a
structure;
Scenario 1; when there are no drawings
In this case, 10 to 20% of the members are selected for testing. The sample group should include
the most heavily loaded members of the structure and members at corners as they are subjected
to torsion.
Scenario 2; when drawings are available
Less than 10% of the members can be selected for testing and the sample group should have the
same criteria as in scenario 1.
One must also consider the maximum depth to which the equipment such as the eddy current
probe can scan. The profometer has a range of up to 180mm but if the range exceeds 60mm for a
small range and 120mm for a large range, the accuracy is less than 5% which is below the
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accuracy required in BS1881 part
204
Fig. 5.11 accuracy ranges
4. Methodology
4.1 Visual Inspection
A detailed visual inspection was carried out using the following steps;
1. rating the condition of the building
2. describing observed damage
3. stating if there is a need for special inspection
4. taking photos
4.2 Eddy Current Testing
1. Calibration
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The device was calibrated using the test block by verifying that the measured distances were
equal to the known distances of the cover and diameter of the bar within the test block
2. Measure with statistics
Determining the Bar Diameter
A. Determining diameters without correction
For precise determination of the bar diameter, a place on the structure where there is sufficient
spacing between the rebars was selected. This is because, if the spacing is too small, the resulting
value will be too high. To measure the bar diameter in the first and second layer, the minimum
spacings a and b as shown in Tab. 6.1 are required.
• The «Measure w. Statistics» function was selected.
• The RESET procedure was carried out
• The probe was placed parallel over the bar and the ↑ key was pressed.
• The result of the bar diameter determination were displayed in mm or inch.
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Table 6.1 minimum spacing for bar diameter determination without correction
B. Determining diameters with correction
• The parallel rebars were located carefully and marked on the surface of the concrete.
• The rebar spacings were measured and the data was entered in the «Corrections» → «Neighb.
Bar Corr.»
• The «Measure w. Statistics» function was selected.
• The probe was held in the air and the «RESET process» was carried out
• The probe was placed parallel over a bar and the ↑ key was pressed.
As bar diameter (d=...), the display shows the value corrected by the influence of the neighboring
bars.
• In addition to the bar spacing, the measured bar diameter was entered in the menu.
RESET process
The probe was held in the air and the START/RESET key was pressed. A bar appeared in the
display that informed on the progress of the procedure. The probe was not moved before the bar
had disappeared and «0» was displayed. This check procedure was repeated from time to time.
Cover s1
(mm)
Rebar of 1st
layer Cover s2
(mm)
Rebar of 2nd
layer
a (mm) b(mm) a(mm) b(mm)
15 90 200 15 90 180
30 110 200 30 110 220
45 130 210 45 130 240
60 150 250 60 150 260
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Locating rebars and measuring concrete covers
This function can be used to locate rebars, measure concrete covers and determine bar diameters.
The cover values can be stored under object numbers.
• The bar diameter was entered.
The bar diameter was not known so the default of 16 mm was entered.
• The object number was entered.
Limit value: To avoid confusions, the limit value was set to «0» during the measuring process.
The value required for the building was entered after completion of the series of measurement.
The percentage of the covers that are too small were be displayed in the statistical evaluation.
• The desired audible locating aid was selected («MENU» →
«Basic setups» → «Audible locating aid»): (Short) beep tone or variotone.
• The requested values were entered under the menu option «Corrections» → «Neighbouring Bar
Correction» or «2- Layer-Correction» if the correction is required.• Select the «Measure w.
Statistics» function.
• The START/RESET key was pressed.
• The probe was moved from a starting position in one direction with the locating aids being
observed: current concrete cover, flow bar, (short) beep, variotone, signal value. As long as the
flow bar moves to the right, the probe is approaching a rebar. If the flow bar stops moving, the
probe is directly over the rebar axis. If the centerline of the probe has overshot the rebar axis
somewhat, acoustic and visual indication is given in the «beep» setting by a short beep and by
«—» in the «Current cover» display field. At the same time, the flow bar moves to the left and
the cover is temporarily stored in the field Memo».
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• When having activated the audible locating aid «Vario- tone» («MENU» → «Basic Setups» →
«Audible locating aid»), the audio frequency increases as the probe approaches a rebar. In this
operation mode the cover of the scanned rebar is also temporarily stored in the field «Memo».
• In the case of loud noises from the surroundings, use the headset to hear the acoustic signals.
• The bar direction was detected by moving the probe in the direction of its longitudinal axis
along the rebar. It was made sure that the signal value and the current cover remain as constant
as possible.
Storing the measured values
To store the measured values, an object number in the menu is selected.
• The PRINT/STORE key is pressed to store the measured value shown in the field «Memo».
• The ↓ key is pressed to delete the measured value, or in the case of several values, the last of
the remaining measured values.
Note that the deletion of a value cannot be undone
Detecting insufficient concrete cover
Settings
• The bar diameter was entered.
• The limit value of the cover was measured.
• The bar spacing under the menu option «Neighb. Bar Corr.» was entered. The set limit value is
not displayed in this case
The setting of the audible locating aid is not of importance.
Measuring the concrete cover
With a preselected limit value, the probe can be moved at a maximum search speed of 0.25 m/s
without having to watch the display. If the current cover displayed is less than the limit value, an
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acoustic alarm sounds. When the probe is over the bar, «—» is indicated in the «Current concrete
cover» display field.
5. Results and Analysis
5.1 Visual inspection
The floor plan is included in the appendix to aid in identifying the locations of the columns. The
following defects were noticed during the visual inspection of the columns;
1. Chipping
Fig. 7.1 chipped column (B1)
This was noted to be at the corners of the columns which are within the classrooms. It is most
likely caused by abrasion due to movement of desks by students as they were at desk level. Such
damage is crucial to note as it reduces the cover to the bars at the corners of the columns. If the
damage continues it may lead to the exposure of rebars to the atmosphere. No rebar was exposed
but other methods of NDT should be used to determine the extent to which the cover has been
damaged
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2. Cracks
The cracks run diagonally from the joint between the wall and the columns, terminating before
the edge of the column face. Their diagonal appearance may indicate that they are shear or
torsion cracks but for this to be verified other methods of NDT should be employed.
Fig.7.2 cracks
Fig.7.3 cracks
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5.2 Covermeter
5.2.1 concrete cover
Column 1 (A2)
face1
Object no: 100000
Bar diameter:16
Neighbouring bar correction: a=70mm
n=4
Mean=33mm
Minimum cover= 23mm
Maximum cover=44mm
Sa(standard deviation)=+/-9.4mm
23mm
44mm
36mm
27mm
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Face 2
Object no: 100001
Bar diameter:16
Neighbouring bar correction: a=70mm
n=4
Mean=44mm
Minimum cover= 38mm
Maximum cover=56mm
Sa=+/-7.4mm
38mm
38mm
56mm
45mm
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Face 3
Object no: 100002
Bar diameter:16
Neighbouring bar correction: a=70mm
n=5
Mean=28mm
Minimum cover= 19mm
Maximum cover=38mm
Sa=+/-8.1mm
31mm
38mm
39mm
19mm
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Face 4
Object no: 100003
Bar diameter:16
Neighbouring bar correction: a=70mm
n=5
Mean=48mm
Minimum cover= 38mm
Maximum cover=56mm
Sa=+/-6.7mm
48mm
47mm
56mm
52mm
38mm
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Column 2(A3)
Face1
Object no: 100004
Bar diameter:16
Neighbouring bar correction: a=70mm
n=4
Mean=33mm
Minimum cover= 23mm
Maximum cover=44mm
Sa=+/-9.4mm
23mm
44mm
36mm
27mm
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Face 2
Object no: 100005
Bar diameter:16
Neighbouring bar correction: a=70mm
n=10
Mean=45mm
Minimum cover= 29mm
Maximum cover=59mm
Sa=+/-9.1mm
43mm
48mm
47mm
30mm
29mm
49mm
47mm
59mm
49mm
48mm
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Face3
Object no: 100006
Bar diameter:16
Neighbouring bar correction: a=70mm
n=4
Mean=45mm
Minimum cover= 42mm
Maximum cover=52mm
Sa=+/-4.7mm
43mm
42mm
52mm
43mm
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Face 4
Object no: 100007
Bar diameter:16
Neighbouring bar correction: a=70mm
n=4
Mean=53mm
Minimum cover= 46mm
Maximum cover=61mm
Sa=+/-6.4mm
50mm
46mm
36mm
61mm
53mm
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Column3(A5)
Face 1
Object no: 100008
Bar diameter:16
Neighbouring bar correction: a=70mm
n=5
Mean=47mm
Minimum cover= 39mm
Maximum cover=53mm
Sa=+/-5.1mm
49mm
48mm
48mm
53mm
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Face 2
Object no: 100009
Bar diameter:16
Neighbouring bar correction: a=70mm
n=6
Mean=27mm
Minimum cover= 20mm
Maximum cover=31mm
Sa=+/-5.3mm
31mm
31mm
28mm
30mm
20mm
20mm
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Face 3
Object no: 100010
Bar diameter:16
Neighbouring bar correction: a=70mm
n=4
Mean=52mm
Minimum cover= 46mm
Maximum cover=59mm
Sa=+/-6.8mm
46mm
46mm
59mm
56mm
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Face 4
Object no: 100011
Bar diameter:16
Neighbouring bar correction: a=70mm
n=5
Mean=42mm
Minimum cover= 24mm
Maximum cover=52mm
Sa=+/-10.6mm
44mm
44mm
52mm
46mm
24mm
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Column 4(A6)
Face 1
Object no: 100012
Bar diameter:16
Neighbouring bar correction: a=70mm
n=4
Mean=38mm
Minimum cover= 35mm
Maximum cover=41mm
Sa=+/-2.5mm
41mm
38mm
39mm
35mm
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Face2
Object no: 100013
Bar diameter:16
Neighbouring bar correction: a=70mm
n=4
Mean=47mm
Minimum cover= 37mm
Maximum cover=58mm
Sa=+/-10.6mm
37mm
58mm
53mm
38mm
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Face3
Object no: 100014
Bar diameter:16
Neighbouring bar correction: a=70mm
n=4
Mean=57mm
Minimum cover= 49mm
Maximum cover=68mm
Sa=+/-8.0mm
56mm
49mm
68mm
55mm
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Face4
Object no: 100015
Bar diameter:16
Neighbouring bar correction: a=70mm
n=4
Mean=52mm
Minimum cover= 47mm
Maximum cover=61mm
Sa=+/-6.5mm
50mm
48mm
61mm
47mm
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Column5(D4)
Object no: 100016
Bar diameter:16
Neighbouring bar correction: a=70mm
n=8
Mean=58mm
Minimum cover= 51mm
Maximum cover=65mm
Sa=+/-5.5mm
62mm
60mm
56mm
65mm
52mm
51mm
64mm
54mm
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Column 6(D6)
Object no: 100017
Bar diameter:16
Neighbouring bar correction: a=70mm
n=11
Mean=47mm
Minimum cover= 39mm
Maximum cover=59mm
Sa=+/-7.3mm
44mm
59mm
57mm
39mm
47mm
56mm
50mm
39mm
43mm
45mm
41mm
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column Column face Average cover(mm) Less screed and
plaster(5mm)
1 1 33
2 44
3 28
4 48
Column average 38.25 33.25
2 1 33
2 45
3 45
4 53
Column average 41 36
3 1 47
2 27
3 52
4 42
Column average 42 37
4 1 38
2 47
3 57
4 52
Column average 48.5 43.5
5 58 53
6 47 42
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5.2.2 bar diameters
column 1
face1 face2
bar1 bar2 bar3 bar4 bar1 bar2 bar3 bar4 bar5
reading1 12.4 16.7 25.6 16 15.8 16.3 16.1 19.8 11.9
reading2 12.2 17 23.9 16.1 16.1 15.9 16.2 20.3 12.3
reading3 11.9 15.8 24.9 16.4 15.9 16 15.5 21 12.2
reading4 12.5 15.9 24.5 15.9 15.9 16.1 15.8 20.1 12
average 12.25 16.35 24.725 16.1 15.925 16.075 15.9 20.3 12.1
bar size Y12 Y16 Y25 Y16 Y16 Y16 Y16 Y20 Y12
face3 face4
bar1 bar2 bar3 bar4 cover too thick
reading1 15.8 20.4 20.9 12.1
reading2 15.8 19.8 20.5 12
reading3 15.9 19 20.1 12.5
reading4 15.7 19.7 20.1 11.7
average 15.8 19.725 20.4 12.075
bar size Y16 Y20 Y20 Y12
column 2
face1 face2
bar1 bar2 bar3 bar4 bar1 bar2 bar3 bar4 bar5
reading1 11.9 12.3 12.2 11.9 12 19.8 20 15.8 9.8
reading2 12.1 12.1 12.3 12.1 12.5 20.1 20.4 16.2 10.3
reading3 12.9 12.1 11.7 12.5 12.3 21 20.4 16.3 10.8
reading4 12.5 12.3 12 12.1 12.1 20.3 20.2 16.2 9.7
average 12.35 12.2 12.05 12.15 12.225 20.3 20.25 16.125 10.15
Y12 Y12 Y12 Y12 Y12 Y20 Y20 Y16 Y10
face3 face4
bar1 bar2 bar3 bar4 cover too thick
reading1 12.5 15.9 16.4 12.2
reading2 12.3 16.1 16.5 12.3
reading3 12.1 16.2 15.9 11.8
reading4 12 16.1 15.8 12.3
average 12.225 16.075 16.15 12.15
Bar size Y12 Y16 Y16 Y12
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column 3
face1 face2
bar1 bar2 bar3 bar4 bar1 bar2 bar3 bar4 bar5
reading1 11.8 16.5 19.8 15.9 15.7 16.1 16 20.5 12.1
reading2 12.1 16.3 20.1 16.1 16.1 16 16.2 20.9 12.1
reading3 12.4 16 20.9 16.1 16 16.3 16.1 19.9 12
reading4 12.2 16.1 20.5 15.9 15.9 16.3 16.2 20.1 12.5
average 12.125 16.225 20.325 16 15.925 16.175 16.125 20.35 12.175
bar size Y12 Y16 Y20 Y16 Y16 Y16 Y16 Y20 Y12
face3 face4
bar1 bar2 bar3 bar4 cover too thick
reading1 16.3 21 19.9 13.5
reading2 16.1 20.5 20.4 12.9
reading3 15.8 20.3 21 13.2
reading4 16.1 20 20.2 12.6
average 16.075 20.45 20.375 13.05
bar size Y16 Y20 Y20 Y12
column 4
face1 face2
bar1 bar2 bar3 bar4 bar1 bar2 bar3 bar4 bar5
reading1 12 11.8 12.2 12.1 12.5 21 19.9 16.1 11.5
reading2 12.5 12.3 12.3 12.5 12.1 21.1 20.1 16.2 12
reading3 12.3 11.8 12.2 13 12.4 22 20.1 15.9 10.8
reading4 12.3 12.1 12.1 12.3 12 19.8 20.2 16.2 12.8
average 12.275 12 12.2 12.475 12.25 20.975 20.075 16.1 11.775
Y12 Y12 Y12 Y12 Y12 Y20 Y20 Y16 Y12
face3 face4
bar1 bar2 bar3 bar4 cover too thick
reading1 12.2 16.1 16.3 11.9
reading2 12.3 16.1 16.5 12.3
reading3 12.1 15.7 16.1 12.1
reading4 12 16.1 16.3 12.1
average 12.15 16 16.3 12.1
Bar size Y12 Y16 Y16 Y12
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6. Discussion
The objectives of the project were to fully understand how to use the equipment, to standardize
the equipment, to determine the position and depth of rebars in an existing concrete element and
to determine if cover is sufficient.
The equipment was calibrated prior to testing. According to BS1881 part 204 and the operator’s
manual, calibration of equipment should be carried out every 6 months to ensure that it is still up
to standard.
The visual inspection was carried out to check for obvious defects such as
Structural Cracks: Caused by excess weight placed upon columns or shortfalls in the
original design
d by over exposure to sun, rain, snow, and other extreme weather
conditions
The inspection yielded results to the effect that there are a few observable defects in the building
such as chipping of the columns and cracks along the walls at the points where they meet the
column. Even though the chipping may not seem structural, it is important that other methods of
non destructive testing, such as ultrasonic testing and x-ray imaging, are used to verify this. The
cracks must also be assessed to determine exactly what type of cracks they are and if they pose a
structural problem to the building.
The sample size of the columns on the first floor of the civil engineering block was selected to be
10% which is 6 columns. The columns chosen were A2, A3, A5, A6, D4 and D6 located in the
4
th
and 5th
year classrooms. They were selected because of;
1. Accessibility. Four of the columns have all 4 faces exposed, and the other 2 have 3 faces
exposed
2. Capability of the device. During a test run of the equipment it was discovered that some
of the columns could not be tested as the cover provided for the reinforcement was
probably too thick for the equipment to obtain readings, as anything greater than 55mm
gave result ‘Cover Too Thick’. The columns joined to the walls had a very thick cover
providing sufficient protection to the rebars even though they are the ones with the
chipped corners and cracks. To determine the cover to the bars and the size of the rebars
other methods such as x-ray imaging can be used. It is however safe to say that the cover
to these columns is greater than 55mm.
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3. Labour intensive nature of measurement. The process is repetitive and requires precision
and thus using a sample size of more than 10-20% is uneconomical
4. Use of the rooms. One of the advantages of NDT is that it does not disrupt the goings on
of the building’s occupants. Thus the columns were selected keeping in mind the
building’s functionality. The rooms containing the columns were chosen because they
have fixed timelines for use and are always open
5. Loading pattern. The chosen columns carry the load over a significant span.
According to BS 8110 part 1 nominal cover is the design depth of concrete cover to all steel
reinforcement, including links. It is the dimension used in design and indicated on the drawings.
The actual cover to all reinforcement should never be less than the nominal cover minus 5 mm.
The nominal cover should:
a) Be in accordance with the recommendations for bar size and aggregate size for concrete cast
against uneven surfaces (see 3.3.1.2, 3.3.1.3 and 3.3.1.4);
3.3.1.2 Bar size
The nominal cover to all steel should be such that the resulting cover to a main bar should not be
less than the size of the main bar or, where bars are in pairs or bundles, the size of a single bar of
cross-sectional area equal to the sum of their cross-sectional areas. At the same time the nominal
cover to any links should be preserved.
3.3.1.3 Nominal maximum size of aggregate
Nominal covers should be not less than the nominal maximum size of the aggregate. The
nominal maximum size of coarse aggregate should not normally be greater than one-quarter of
the minimum thickness of the concrete section or element. For most work, 20 mm aggregate is
suitable. Larger sizes should be permitted where there are no restrictions to the flow of concrete
into sections. In thin sections or elements with closely spaced reinforcement, consideration
should be given to the use of 14 mm or 10 mm nominal maximum size.
3.3.1.4 Concrete cast against uneven surfaces
In such cases the specified nominal cover should generally be increased beyond the values given
in Table 3.3 to ensure that an adequate minimum cover will be obtained. For this reason, the
nominal cover specified where concrete is cast directly against the earth should generally be not
less than 75 mm. Where concrete is cast against an adequate blinding, a nominal cover of less
than 40 mm (excluding blinding) should not generally be specified.
b) Protect the steel against corrosion (see 3.3.3);
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The cover required to protect the reinforcement against corrosion depends on the exposure
conditions and the quality of the concrete as placed and cured immediately surrounding the
reinforcement. Table 3.4 gives limiting values for the nominal cover of concrete made with
normal-weight aggregates as a function of these factors. There may be cases where extra
precautions are needed beyond those given in 3.3.4 in order to ensure protection of the
reinforcement. Further information is given in 3.1.5.
c) Protect the steel against fire (see 3.3.6);
Cover for protection against corrosion may not suffice as fire protection. The values given in
Table 3.4 and Figure 3.2 of BS 8110 part 1 will ensure that fire resistance requirements are
satisfied.
d) Allow for surface treatments such as bush hammering.
According to table 3.2 the exposure condition is mild as the columns protected against weather
or aggressive conditions. Thus from table 3.3 the minimum cover is 25mm.
Using figure 3.2 and table 3.4, columns 1-4 have a b of 35mm and are fully exposed thus the
value of h is 2.5 and thus the minimum required cover is 25mm and for columns 5 and 6 h=2
thus minimum cover is 25mm.
4.3.4 Spalling of nominal cover
If the nominal cover, i.e. the cover to the outermost steel exceeds 40 mm for dense or 50 mm for
lightweight aggregate concrete, there is a danger of concrete spalling.
After measuring the selected columns it was determined that the average cover for all columns is
more than 25mm.According to BS 1881 it is thus sufficient to protect reinforcement from
corrosion and fire.
The bar diameters were determined for the four columns within the classrooms for 3 faces out of
the four that were exposed. All columns had one side that had a cover too thick to allow the
reading of measurements. Although the covermeter is supposed to have a range of up to 180 mm,
whenever the cover exceeded 55mm the device would display that the cover was too thick
regardless of the range settings. On the columns where the ‘Cover Too Thick’ result was
obtained, over 10 readings were taken over all the exposed surfaces of the columns. It was not
able to be determined whether this is a general problem or if the specific covermeter used was
faulty as there was only one device available.
Locating the bars was easy but the actual measurement of the bars posed a problem as variations
between results were at times very large and thus many readings had to be taken. In the last two
columns (D4, D6) the interference from the metal plates within the column made reading the bar
diameters impossible and may have interfered with the measurement of the cover.
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Possible sources of error of the experiment include;
1. Inexperienced personnel; there are training standards set by ISO9712 and other than a
couple of workshops on NDT attended, the personnel handling the equipment had limited
training and no experience
2. Use of metallic aggregates or additives in concrete; they may interfere with the electrical
field and influence readings
3. Lack of drawings; this required the diameter to be set to a default value of 16mm. if the
actual bar diameter is known, the readings will be more accurate.
Advantages of eddy current testing
Disadvantages
on can go undetected.
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7. Conclusion
The covermeter was calibrated according to the standards set in BS1881 part 204 and the tests
were carried out to check if the concrete cover was within the limit of nominal cover set by BS
8110 part 1. It was determined that the cover was sufficient in all columns. The bar diameters
were problematic to obtain and for one side of the columns were unreadable because the cover
was too thick. They were however determined and the accuracy of the device was +/- 2mm. It
must be noted that there are many sources of error for the experiment including the
inexperienced personnel carrying it out, possible presence of metallic aggregates and lack of the
building drawings.
It is recommended that any personnel undertaking this task should be trained in line with the
standards of ISO9712 so as to improve the accuracy of results. It is also important that other
methods of NDT are incorporated in determining the structural health of the building such as
ultrasonic testing and x-ray imaging to provide a detailed account of how the structure is and if
there are any problems that need to be addressed, especially in the case of the cracks. In the case
of the chipped columns, it is recommended that students be brought up to speed on the effect the
abrasion they’re causing may have on the building elements. This may help prevent further
damage. It is also important to consider the use of other models of covermeters to compare
sensitivity and determine the accuracy of the Profometer 5+ in relation to other models
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8. References
Hellier. C. (2003). Handbook of Non-destructive Evaluation . McGraw Hill Series, p.1.1, New
York.
Mordfin,L. (1985). Non-Destructive Evaluation, Materials and Processes, McGraw Hill
Series,New York.
Stanley, B. (1989) Non-destructive Evaluation; A tool for design, manufacturing and services.
McGraw Hill Series in Mechanical Engineering,NewYork
Crouzen, P., Verstijnen, W., Hulsey, R. C. & Munns, I. J. (2006). Application of Pulsed Eddy
Current Corrosion Monitoring in Refineries and Oil Production Facilities. CORROSION
2006. San Diego Ca: NACE International.
IAEA (2002).Guidebook on non-destructive testing of concrete structures, (Training Course
Series No. 17).
Davis, A. G. (1998). Non-destructive Test Methods for Evaluation of Concrete in Structures
Reported by ACI Committee 228.
George J Riley(2003). Rebar locators how do they measure up
http://www.structuremag.org/OldArchives/2003/july_august/RebarLocators.pdf StructureMag
Magazine, July/August 2003
Reuben Barnes, Tony Zheng (2008). Research on Factors Affecting Concrete Cover
Measurement http://www.ndt.net/article/v13n12/zheng.pdf NDT.net, December 2008
Alldred, John C. (1995). Quantifying the losses in cover-meter accuracy due to congestion of
reinforcement, Extending the Life of Bridges, Civil & Building Structures, London.
V.M. Malhotra, Carino J Nicholas (2003). Handbook on Non Destructive Testing of Concrete.
CRC Press, Florida, U.S.A.
Sivaprakasam.Palani, Karthikeyen.S, and Hariharen.P (2012). A Study on Non Destructive
Evaluation of Materials Defects by Eddy Current Methods, International Conference on
Mechanical, Automotive and Materials Engineering (ICMAME'2012) Dubai, UAE.
Ghambir, M.L. (2004). Concrete Technology: Theory and Practice 5th Edition, McGraw Hill
Series, New York.
Shetty M.S. (2005). Concrete Technology, S. Chand and Co., New Delhi, India.
Verma Sanjeev K., Bhadauria Sudhir S., Akhtar Saleem (2013). Review of Nondestructive
Testing Methods for Condition, Journal of Construction Engineering, March.
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Central Railway(2006). Non Destructive Testing and Inspection Manual, Denmark
Einav Isaac, AWARENESS of NDT TECHNIQUES for CIVIL ENGINEERING, STAR.IK Ltd,
Canada
ISO9712, Non-destructive testing- Qualification and certification of NDT personnel
BS8110 part 1, Code of Practice for Design and Construction
BS1881 part 204, Recommendations on the use of electromagnetic covermeters
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9. Appendix
Other NDT methods
Penetrant Testing
Penetrant solution is applied to the surface of a precleaned component. The liquid is pulled into
surface-breaking defects by capillary action. Excess penetrant material is carefully cleaned from
the surface. A developer is applied to pull the trapped penetrant back to the surface where it is
spread out and forms an indication. The indication is much easier to see than the actual defect.
Used to locate cracks, porosity, and other defects that break the surface of a material and have
enough volume to trap and hold the penetrant material. Liquid penetrant testing is used to inspect
large areas very efficiently and will work on most nonporous materials
Magnetic Particle testing
A magnetic field is established in a component made from ferromagnetic material. The magnetic
lines of force travel through the material and exit and reenter the material at the poles. Defects
such as crack or voids cannot support as much flux, and force some of the flux outside of the
part. Magnetic particles distributed over the component will be attracted to areas of flux leakage
and produce a visible indication. It is used to inspect ferromagnetic materials (those that can be
magnetized) for defects that result in a transition in the magnetic permeability of a material.
Magnetic particle inspection can detect surface and near surface defects.
Ultrasonic testing
High frequency sound waves are sent into a material by use of a transducer. The sound waves
travel through the material and are received by the same transducer or a second transducer. The
amount of energy transmitted or received and the time the energy is received are analyzed to
determine the presence of flaws. Changes in material thickness and changes in material
properties can also be measured. It is used to locate surface and subsurface defects in many
materials including metals, plastics, and wood. Ultrasonic inspection is also used to measure the
thickness of materials and otherwise characterize properties of material based on sound velocity
and attenuation measurements.
Radiographic testing
X-rays are used to produce images of objects using film or other detector that is sensitive to
radiation. The test object is placed between the radiation source and detector. The thickness and
the density of the material that X-rays must penetrate affects the amount of radiation reaching the
detector. This variation in radiation produces an image on the detector that often shows internal
features of the test object. It is used to inspect almost any material for surface and subsurface
defects. X-rays can also be used to locates and measures internal features, confirm the location of
hidden parts in an assembly, and to measure thickness of materials
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Nucleonic Gauge Testing
A nuclear density gauge is a tool used in civil construction and the petroleum industry, as well as
for mining and archaeology purposes. It consists of a radiation source that emits a directed beam
of particles and a sensor that counts the received particles that are either reflected by the test
material or pass through it. By calculating the percentage of particles that return to the sensor, the
gauge can be calibrated to measure the density and inner structure of the test material.
Schmidt hammer
This method can be used for concrete and masonry structures. It is most commonly used for
concrete structures. The Schmidt hammer is used for testing the strength of hardened concrete.
The device consists of a spring loaded steel mass that is automatically released against a plunger
when the hammer is pressed against a concrete surface. Part of the energy is absorbed by the
concrete through plastic deformation and part of the energy causes a rebound of the hammer. The
rebound of the hammer depends on the hardness and thereby the strength of the concrete.
Ground penetration radar
This method can be used for concrete and masonry structures. Ground penetration radar makes
use of high frequency electromagnetic pulses which are directed by a transceiver towards the
surface. Waves are reflected back to a receiver. The waves received indicate the composition of
the considered component. As the wave propagates through a component and encounters an
interface between two materials with different dielectric constants, a portion of the energy is
reflected back. The remaining energy continues through the component.
Spraying indicators (pH)
This method can be used for concrete structures. This test is performed by applying an indicator
solution to concrete surfaces just fractured. The colour of the solution will change with
corresponding changes in pH of the concrete. The carbonation depth is then measured by means
of a scale.
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