1. HALF-CELL POTENTIAL METHOD (Electrochemical method)
The corrosion potential of the steel in reinforced concrete can be measured after the voltage differences the steel bar and a reference electrode in contact with the surface of the concrete. The concrete functions as an electrolyte and the risk of corrosion of the reinforcement in the immediate region of the test location maybe related empirically to the measured potential difference. ASTM C876-91 provides a standard test procedure and ASTM criteria for corrosion of steel in concrete.
1.1.Equipment for half-cell potential
Figure 1.0 Equipment for half-cell potential
1.2. General procedure for half-cell potential
Measurements would be made in random or grid pattern. An area with greater than 150 mV indicates that an area of high corrosion activity. A direct electrical connection is made to the reinforcing steel with a compression clamp. The rod should be brushed before connecting it to the reinforcing bar to get a low electrical resistance connection. It may be necessary to drill into the concrete to expose a reinforcing bar. Furthermore, the bar is connected to the positive terminal of the voltmeter. One end of the lead wire is connected to the half-cell and another end connected to the negative terminal of the voltmeter. The potential recorded in the half-cell measurement can be used to indicate the probability of corrosion of the steel reinforcement. The reference electrode can be moved over the concrete surface to develop a potential map that shows the possible location of active corrosion.
Figure 1.1 : System for measuring the half-cell potential
Half-cell potential is used for assessment of the corrosion of reinforced concrete members. Half –cell potential applied at location of areas of high reinforcement corrosion risk in marine structures, bridge decks and abutments. Besides, it has been use to investigating concrete contaminated by salts.
1.4. The limitation of half-cell potential
The half-cell potential measurement has been widely used in the field and this method provides a quick, simple and inexpensive method to determine the corrosion of steel in reinforcing concrete. Zones of varying degrees of corrosion risk may be identified. The limitation of this method is cannot indicate the actual corrosion rate. Besides, it has to drill a small hole to enable electrical contact with the reinforcement during examination. The measurement is very sensitive to the humidity existing in concrete. Concrete with high degree of saturation will more negative potential result. The lack of oxygen near the reinforcement result in more negative potential if compared to aerated zones. It notes that the lack of oxygen actually significantly reduces the corrosion rates and causes more negative potential. Localized corrosion can be generated by micro cracks also affect the corrosion potential measurement.
2. Covermeter (Electromagnetic method)
Covermeter is equipment utilizing eddy current effects or magnetic induction effects to locate steel reinforcing bar in concrete and estimate the thickness of the concrete cover over the reinforcement. Steel bars interact strongly with low- frequency electromagnetic waves applied at the surface of the concrete and easily to identify their location. Magnetic field generated when current passes through electrical coils to detect the reinforcing bars. Two methods had been introduced in commercial covermeters
2.1 Equipment for covermeter
Figure 2.1: covermeter
2.2 General procedure for covermeter
The first method uses an excitation coil to generate a magnetic flux. The flux travels through the concrete and its intensity is measured by a sensing coil. Concrete is not a good conductor of magnetic field flux, it has a high magnetic reluctance. When the detector move to a position near the reinforcement, the sensing coil indicates a large increase in the magnetic flux as steel is a good conductor. The intensity of the current measured at sensing coil is influence by depth of cover.
Figure 2.2: covermeter for first method
The second method use to find location of reinforcing bar by scanning the concrete surface with an electrical coil attached to an AC source and a current indicator. When the detector scan areas with no reinforcement, the current indicators remain in the same position, if the detector gets closer to the reinforcement , the indicator will show decrease in current until it reach a minimum value at the moment that the detector is on top of the reinforcement.
Figure 2.3 : covermeter for second method
2.3 Application of covermeter
Covermeter use to check whether corrects location and cover to reinforcing bars after concrete placement. Covermeter use to investigate concrete reinforcement which record is not available or need to be checked. Covermeter use to investigate of reinforcement as a preliminary step to other testing such as ultrasonic pulse velocity test. Covermeter use to identify location of buried object in concrete such as water pipes, steel joists, lighting conduits.
2.4 The limitation of covermeter
Application of covermeter is very slow and labor intensive. The result from covermeter test are affected by the presence of more than one reinforcement bar in the tests area, by laps, metal tie wires and bar supports. This test is unsuitable in case of closed packed bar assemblies.
3.0 Ultrasonic pulse velocity (stress wave propagation method)
The ultrasonic pulse velocity consists of measuring the travel time of a pulse of longitudinal ultrasonic waves passing through the concrete. Longitudinal waves with frequency in range of 20 to 150 khz are normally used to testing. When the pulse generated is transmitted into concrete from the transducer using a liquid coupling material such as grease or cellulose paste. The first waves to reach the receiving transducer are the longitudinal waves, which are converted into an electrical signal by a second transducer. The traveling times between the initial onset and reception of the pulse are measured electronically.
3.1 Equipment for Ultrasonic pulse velocity
Figure 3.1: equipment for Ultrasonic pulse velocity
3.2 General procedure for Ultrasonic pulse velocity
A minimum path length of 100 mm is recommended for concrete of maximum aggregate size 30mm and 150 mm when the maximum aggregates size is 45mm. Generally the transducers used should be in range of 20 to 150 KHz. The receiving transducer detects the arrival of that component of the pulse, which arrives earliest. This is generally the leading edge of the longitudinal vibration. The direction in which the maximum energy is propagated is at right angles to the face of the transmitting transducer. It is possible to make measurement of pulse velocity by placing the two transducers on either:
a) Opposite faces (direct transmission)
b) Adjacent faces ( semi-direct transmission)
c) The same face ( indirect or surface transmission)
figure 3.2 a (direct transmission) figure 3.2 b (semi direct transmission)
figure 3.2 c (indirect or surface transmission)
3.3 Application for Ultrasonic pulse velocity
Measurement of the velocity ultrasonic pulses of longitudinal vibrations passing through concrete may be used for the following applications:
- Determination of the uniformity of concrete in and between members.
- Measurement of change occurring with time in properties of concrete.
- Correlation of pulse velocity and strength as a measure of concrete quality.
- Determination of the modulus of elasticity and dynamic poison’s ratio of the concrete.
- Application in prestressed concrete
The velocity of an ultrasonic pulse is influenced by those properties of concrete which determine its elastic stiffness and mechanical strength. The variations obtained in a set of pulse velocity measurements made along different paths in a structure reflect a corresponding variation in the state of the concrete. When a region of low compaction, voids or damaged material is present in the concrete under test, a corresponding reduction in the calculated pulse velocity occurs and this enables the approximate extent of the imperfections to be determined. As concrete matures or deteriorates, the changes, which occur with time in its structure, are reflected in either an increase or decreases respectively in the pulse velocity. This enable changes to be monitored by making tests at appropriate intervals of time.
Pulse velocity measurement made on concrete structures may be used for quality control purposes. In comparison with mechanical tests on control sample such as cubes or cylinders. Pulse velocity measurement has the advantage that they relate directly to the concrete in the structure rather than to samples, which may not be always truly representive of the concrete in situ. Ideally, pulse velocity should be related to the results of tests on structural components and, if a correlation can be established with the strength or other required properties of these components, it is desirable to make use of it.
The ultrasonic pulse velocity has a very important application in prestressed concrete. It can be used to assess the strength of concrete in a structure. The strength of concrete will determine whether it is safe to release tension in the prestressing wires.
3.4 Limitation of ultrasonic pulse velocity
Ultrasonic pulse velocity has some limitation. Bars in concrete may lead to more pulse velocity, especially when bars are parallel to pulse path. When this test is applied to a reinforced concrete an engineer should take care of the bars. It is a important to identify the bars before performing this test. Besides, moisture condition will affect the result and should be noticed.
