mos for resistivity half cell potential, crack width, cover meter

CONCRETE CONSTRUCTION (ECM725-OLD)

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


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


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


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


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


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.


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


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.


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.


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


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


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

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