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4. Method Statement For Cover Meter
If we refer to the case study, the structure needs to be monitored and tested to know
the level of damage the structure bear and why the damage occurs. One of the propose
method is by using cover meter. This method gives us the depth of the cover, whether it’s
complying with design or the structure is wrongly constructed.
This instrument is use to locate rebar’s and measure the exact concrete cover, that is
the measurement from centre of main bar to the outside surface of the concrete as shown
in Figure 4.1. The cover meter works by using the eddy current principle with pulse
induction as the measuring method. With this, drilling can be avoided as option to locate
the bars.
Figure 4.1 Concrete cover = d
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a. Equipment
The cover meter test device contains 2 major parts, as shown in Figure 4.2.
1. Reader
2. Detector
Figure 4.2. Cover Meter apparatus
This equipment has certain features that give the engineers the ability to investigate
the structural damage and the method to mitigate the problem. This equipment features:
The cover meter allows the detection of rebar’s
Measurement of the concrete cover depth and the diameter of the reinforcement
The device can give correction for the influence of neighboring bars.
The device can stored thousand of cover values and statistical analysis
b. Methodology
The location surrounding the damage structure are marked into several line, about
2-3 inches between each other and the radius of marked area are justified by the
engineers
The reader turns on, the detector are place down at marked line and data collection
can be start immediately
Data collection can be use as reference for further analysis
c. Standards
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BS 1881 part 204 DIN 1045
SN 505 262 DGZfP B2 (recommendation)
d. Conclusion
The cover meter is an easy, effective, accurate and economic way to find out the
cause of structural damage. Data collected can be use to justify the causes of the damage
occurs, whether the cover is low so that the carbonation or chlorides easily enter the
structure or the concrete covers are too thick that causes weakness and cracks to the cover
itself. From the data collected, analysis can be done to justify the condition. Figure 4.3
shows the relationship between the concrete cover thickness and time.
Figure 4.3 Reduction of the initiation time of corrosion sue to
reduction of thickness of the concrete covers
5. Method Statement For Half Cell Potential
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Other methods of determine the damage of the structure is by using half cell potential
test. This test is another non destructive test which can measure the corrosive activity in
the concrete. The test is simple and the result is dependable. Generally, the half cell
potential test gives the condition of steel embedded in the concrete with regards to the
corrosion activity. Using this method, the electrical potential difference between the real
structure and reference electrode is then measured.
a. Equipment
The equipment consists:
Reference electrode
Positive and negative connection
High impedance digital voltmeter
Figure 5.1 General diagrams of Half Cell Potential apparatus
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Figure 5.2 Half Cell Potential apparatus Figure 5.3 Half Cell Potential Test
b. Methodology
Copper sulphate solution is poured into the container
Then a copper rod is put into the container, merge with the copper sulphate
solution, creating the half cell reference electrode
The half cell is then screwed onto a voltmeter
The protective cap of the half cell container is then remove to expose the plug
at the end of the half cell, this plug is use as connector during data collection
The negative wire is connected to the voltmeter and the positive wire to the
expose steel bar in the concrete
Gridline is then marked on the surface of the structure, 1ft x 1ft or 1m x 1m.
The gridline must consistent in order to get good result then.
The data now can be collected, refer Figure 5.3
The data collected can be use to plot schematic diagrams of the structures as
contour map as shown in Figure 5.4.
Figure 5.4 Contour map plotted as per data collected
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The ASTM standards states that:
Potential level of mV % Chance of reinforcement
being corrosively active
-350 to -500
-200 to -350
<-200
95%
50%
5%
From the contour and standards given, we can estimate and justify the severity
of damage the structures have. The higher level of negative mV means
corrosive activity is active and shown in red colour in the contour map
Values between the upper and lower reading shows that the activity is uncertain
c. Standards
ASTM Standards
ASTM C876-80, "Standard Test Method for Half-Cell Potentials of Reinforcing Steel in Concrete"
G3 Practice for Conventions Applicable to Electrochemical Measurements in
Corrosion Testing
G16 Guide for Applying Statistics to Analysis of Corrosion Data
d. Conclusion
The half cell potential test is a good way to detect the activities of corrosion in the
structure, it is economic and not causing too much damage to existing structure. The data
collected can be a reference to engineers to take action accordingly to the structural
condition.
CONCRETE CONSTRUCTION (ECM725-OLD)
6. Method Statement For Resistivity
The corrosion in steel can be specified as an electro-chemical process that causes
current flow. These current flows causing metal to dissolve and lower its strength. To
evaluate the probability of the reinforcement corrosion, we can run the resistivity test which
by using this test, we can evaluate the electrical resistance of the concrete so that we can
know how good the concrete is. This method can be applied in this case study so that the
engineers can detect the causes of the damage occurs offers an indication of the concrete
permeability. Generally, the probability of the steel corrosion increases, the concrete
resistivity deceases and it’s not a good things.
The electrical resistivity of the concrete structure usually decreases because of:
Increasing temperature
Increasing chloride content
Decreasing carbonation depth
Increasing concrete water content
Increasing concrete porosity
a. Equipment
The equipment consists:
Resistant meter reader
Current sensor
Current source
Resistivity measuring electrodes
Figure 6.1 Resistivity test apparatus
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b. Methodology
The surface of the concrete are marked into several lines, creating an equal
gridline
The sensor are put onto the structure, it’s better to put wet sponge under the
sensor to create better reading
Place the apparatus onto the structure along with the reference block
The meter reader is then turn on and the data can now be collected
Data collected can be use for further analysis and reference to match
rehabilitation method
Generally, the result can be justified as:
Meter reading Corrosion conditon
When ρ ≥ 120 Ω-m corrosion is unlikely
When ρ = 80 to 120 Ω-m corrosion is possible
When ρ ≤ 80 Ω-m corrosion is fairly certain
Thus, the corrosion increases when the electrical resistivity of the concrete is
low, the opposite condition happen when the electrical resistivity of the
concrete is high, the corrosion is decrease.
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c. Standards
ASTM Standard C1202-94: Standard Test Method for Electrical Indication of
Chloride’s Ability to Resist Chloride Ion Penetration
d. Conclusion
The resistivity test gives the engineer the idea and condition of the concrete condition,
whether the resistant to electricity is high that shows the concrete and its covers is in good
condition and when the electrical resistivity is low which means the corrosion is
increasing. The data shall provide engineers to understand the situation and comes out
with proper solution.
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7. Method Statement For Crack Width Measurement
The structure in this case study might have visible crack line that occurs because of
structural problem. The crack line needs to be monitored and the proposed method is by
using crack width measurement. The crack width measurement is use to monitor the crack
occurs on structure. The equipment comes with microscope that can be adjusted to view
the crack and torch to make the crack line visible. The apparatus are powered by batteries
and easy to operate.
a. Equipment (refer to Figure 7.1)
The equipment consist a very simple apparatus:
Adjustable lens
Small torch attached to the lens
Measurement line at the lens
Figure 7.1 crack width measurement apparatus
b. Methodology
The surface of structure been clean so that the crack line can be exposed
The crack width measurement apparatus being set on the crack line
Turn on the torch
The crack width data can be taken by using the measurement line
c. Standards
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The crack width data gathered have to be refer to BS Standards, ASTM or equivalent
standards to check the limitation of crack for particular structure measured.
d. Conclusion
This test gives the engineers the width of crack line the structure have. The data
collected can be compared to the standard data so that the engineers can justify the
severity of the damage.
Reference
CONCRETE CONSTRUCTION (ECM725-OLD)
1. Corrosion Of Steel In Concrete-Prevention, Diagnosis, Repair- Luca Bertolini,
Bernhard Elsener, Rob Polder, Wiley-Vch, Germany 2004
2. Significance of test and properties of concrete & concrete-making materials-
Joseph F.Lamond, James H. Pielert, ASTM International, West Conshohocken,
PA, 2006