Half Cell/Surface Potential Test:
Measuring the half-cell /surface potential at the selected location on RCC members of the structures covered under the study will help in understanding the extent of reinforcement corrosion as per ASTM C 876.
REINFORCEMENT LOCATION, SIZE AND CORROSION
Steel shares about 40 to 70% of the load in RCC. During last few decades it has been observed that, corrosion of reinforcement in severe in structures near seashore and in the vicinity of chemical industries. A lot of attention is needed for detecting this deterioration and protecting it with proper treatment. Thus due importance shall be given for measuring the size of bar and the amount of corrosion.
PROFOMETER / REBAR LOCATOR AND BAR SIZER
Principle and Procedure: The reinforcement bar is detected by magnetising it and inducing a circulating “eddy current” in it. After the end of the pulse, the eddy current dies away, creating a weaker magnetic field as an echo of the initial pulse. The strength of the induced filed is measured by a search head as it dies away and this signal is processed to give the depth measurement. The eddy current echo is determined by the depth of the bar, the size of bar and the orientation of the bar. This detection of location of reinforcement is required as a pre process for core cutting.
Profometer is a portable battery operated magnetic device that can measure the depth of reinforcement cover in concrete and detect the position of reinforcement bars, Fig-5. The basic principle in this method is that the presence of steel affects the field of electromagnet. Fig-5 shows a typical circuitry diagram to locate rebars and cover includes the probe unit and display unit.
In the typical Proformeter, the probe unit consists of a high permeable U-shaped magnetic core on which two coils are mounted. An alternating current is passed through one of these coils and the current induced in the other coil is measured. The induced current depends upon the mutual inductance of the coils and upon the nearness of the steel reinforcement.
Profometer is available in three models namely Model ‘S’, Model ‘S+’, and Model ‘SCANLOG’. Model ‘S’ is standard equipment and is used for locating rebars, measuring concrete cover, storing and evaluation of data. It displays location of rebar and concrete cover on a LCD monitor with x/y meter scale and values obtained can be printed and down load to PC also.
Model ‘S+’ is similar but this software can print cyber scan data without PC. Model ‘SCANLOG’ is similar to S+ but it also includes integrated software for grey-scale display of concrete cover and can give direct print out without PC. Using any of above model rebars can be scanned over a defined area by connecting the mobile probe first and following procedure is as follows :
- Select defined area from ‘Basic Steps’ with scan area option
- Set bar diameter of first layer
- Select option ‘Scanning Bar’ from menu.
- Press ‘start’ to locate the rebars over selected area.
- The starting position of a mobile probe can be defined with the cursor and the cursor is moved with arrow keys to locate the rebars. The cursor position is then transferred to the measuring area. In similar way, other rebars in first layer is marked Fig-5. The rebars in second layer is also marked by moving probe in other direction as shown in Fig.5, the Cover is also simultaneously measured.
- Store the diagram showing the position of rebars in first and second layer and concrete cover. Cyber scan print out can be obtained on a printer.
In the similar manner diameter of bar can also be determined. A typical arrangement for measurement of bar diameter by using diameter prob. There are various factors, which affect the Profometer results. These factors are: arrangement of reinforcement, variation in the iron content of cement and use of aggregate with magnetic properties, metal ties also affects the magnetic field. These factors should be considered in interpretation of observations obtained from this instrument.
Applications of Profometer
Profometer is used for:
- Determination of bar arrangement
- Determination of cover of reinforcement
- Determination of bar diameter
Fig.5 :
Diameter of Rebar in I & II Layer
Reliability and Limitations: With this instrument a cover to reinforcement can be measured up to 100 mm with an accuracy of + 15% and a bar diameter with an accuracy of less than 2 to 3 mm. Proper calibration of these instruments is very essential. The factors which affect the accuracy are – very closely spaced bars or bundled bars, binding wire, aggregate containing iron or magnetic propertiesReinforcement less than 10mm diameter, high tensile steel or deformed bars. In these cases the indicated cover is likely to be higher than true value.
- Cover measured lower than the true value when special cement, including high alumna or added pigments is used.
- Rebars in excess 32mm distance may require a recalibration.
CORROSION MAPPING
Reinforcement in concrete will not corrode if the protective iron oxide film formed by the high alkaline condition of the concrete pore fluid with a pH around 13 is maintained. This film gets destroyed by chlorides or by carbonation, if moisture and oxygen are present, resulting in corrosion. In the corrosion process anodic and cathodic areas are formed on the reinforcement, causing dissolution of the steel and the formation of expansive corrosion products at the anode.
