Page 193 - CW E-Magazine (27-2-2024)

Special Report

Table 3: Concrete resistivity and risk of corrosion uniform, and stress corrosion crack-
Concrete resistivity (Ohm.cm) Risk of corrosion ing.
Greater than 20,000 Negligible Only a few studies of EN for mea-
10,000 to 20,000 Low suring corrosion in concrete have been
5,000 to 10,000 High performed. In general, no distinct bene-
Less than 5,000 Very High fits of this technique compared to the
macro-cell current measurements have
dependent on concrete quality and Electrochemical noise (EN) analysis been found. A comparison between
exposure conditions, such as the rela- EN analysis is an emerging tech- some calculated parameters of mea-
tive humidity. Temperature also affects nique for monitoring corrosion of sured EN and the corrosion rate did
the degree of concrete pore saturation reinforced concrete structures(18). not, in many case, give clear results.
and so the resistivity values(12,13). It enables information on the mecha- Neither was correlation between spe-
Table-3 gives assessment of corrosion nism and rate of corrosion at areas cific EN fluctuations and the different
risk related to the value of concrete identified in concrete structures. stages in the corrosion process quite
resistivity(14). A low amplitude variation of the cor- clear. It was established that due to the
rosion potential of steel in concrete low electrical conductivity of concrete
Linear Polarisation Resistance (LPR) is measured to obtain noise data as a and lengthy electrodes (rebars) some
measurements record of potential fluctuations in the electro-magnetic disturbances could
LPR has become a well-esta- form of power spectra. The sponta- affect the measured EN signals.
blished method of determining instan- neous, random fluctuations in current
taneous corrosion rate of reinforcing flow between two identical, elec- Embeddable corrosion monitoring
steel in concrete(15,16). The tech- tronically isolated bars in concrete sensor
nique is rapid and non-intrusive, re- coupled through a zero resistance The embedded corrosion instru-
quiring only localised damage in the ammeter are recorded, together with ment (ECI) is a sensor that provides
concrete cover to enable an electrical the fluctuations in the potential of one early warning of conditions that dam-
connection to be made to the reinforc- of them, measured against a reference age steel reinforcement. By monitor-
ing steel. The data provides valuable electrode. The polarization resistance ing five key factors in corrosion, and
insight into the instantaneous corro- is given by the following expression: by communicating these through a
sion rate of steel reinforcement, giv- Rp = σ/β. digital network, the ECI provides com-
ing more detailed information than prehensive, real-time information on
a simple potential survey. LPR data The corrosion rate may be obtained structural conditions. This helps faci-
enables a more detailed assessment of from Rp. In addition, the coefficient of lity managers avert crises, save money
the structural condition and is a major variance of the current noise β is said to on maintenance, and build a detailed
tool in deciding upon the optimum indicate the type of corrosion, ranging record on each structure.
remedial strategy to be adopted. It is from 10 for general corrosion to 1.0
-3
imperative that LPR measurements for localized corrosion(19). One of
are accurate. Broad criteria for corro- the important advantages of this tech-
sion condition of rebar vs corrosion nique is lack of intrusiveness. Based
current have been developed from on the results, it is possible to charac-
field and laboratory investigations terise corrosion types such as meta-
(Table-4)(17). stable pitting, pitting & crevice,
Table 4: Corrosion current vs condition of rebar
Corrosion current Corrosion condition of rebar
(μA/cm2) Fig. 8: ECI Sensor during operation
Less than 0.1 to 0.2 Passive condition (no corrosion expected). The ECI is designed to monitor
0.2 to 1.0 Low to moderate corrosion (corrosion possible in 10-15 yrs). bridges, buildings, dams, erosion con-
1.0 to 10 Moderate to high corrosion (corrosion expected in 2-10 yrs). trol structures, flood control channels,
parking garages, road-ways, spillways,
More than 10 High corrosion rate (corrosion expected in 2 yrs or less). etc. It monitors five key factors: linear

Chemical Weekly February 27, 2024 193

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