Bond Strength Degradation Due to Impressed Current Cathodic Protection in Reinforced Concrete

210th ECS Meeting , Abstract #792, copyright ECS

Bond strength degradation due to impressed current cathodic protection in reinforced concrete

D.Koleva1_{, O.Copuroglu}1_{, K.van Breugel}1_{, J.H.W.de Wit}2

1_{Delft University of Technology, Dept. Civil Engineering}

&Geosciences, Stevinweg 1, 2628 CN Delft, Netherlands

2_{Delft University of Technology, Dept Materials Science}

&Engineering, Mekelweg 2, 2628 CD Delft, Netherlands Corrosion of reinforcement in concrete structures is known to cause structural deterioration as result of concrete cover cracking and spalling. The damage occurs as consequence of the volume expansion of corrosion products, penetration of the latter into the restricted empty space (pores, voids or micro-cracks) in the steel/paste interface and further penetration into the bulk material. Splitting stresses are thus induced, thereby influencing the bond strength of the reinforcement and the bulk concrete matrix. Cathodic protection (CP) is one of the protection techniques, applied to reinforced concrete structures. It has side effects however, one of them being the influence on bond strength. This study presents some research outcomes on the application of an alternative of CP, denoted as pulse CP, aiming at revealing the effectiveness of a pulse regime in protecting the steel reinforcement, as effectively as the CP on one hand, and minimizing the side effects on the other.

The materials used were reinforced concrete cylinders, cast from OPC CEM I 32.5, w/c ratio 0.6, containing two embedded construction steel bars. The specimens were maintained in certain conditions for corrosion initiation first (salt spray cabinet, 5% NaCl for 420 days). CP (both conventional and pulse) were applied at 120 days of cement hydration, when corrosion was already on accelerated stage. Microstructural analysis was performed (using environmental SEM, equipped with EDAX) for investigating the bulk matrix properties and structural alterations in the steel/cement paste interface.

Fig.1 Steel/cement paste interface in corroding (left) and non-corroding specimen (right)

Fig.1 presents the steel/paste interface (and up to ~700 m into the bulk matrix) in corroding (C) and non-corroding (R) specimens. Fig.2 depicts the interface regions in specimens under pulse CP (A) and CP conditions (B). The structural alterations in the latter two specimens would depend on the density of current flow through the cells. For this particular

case, CP was in the range of 2 to 10 mA/m2 _{steel surface,}

the pulse was achieved using 12.5 to 50 % duty cycle, frequency 1kHz. Obviously, the constant current flow in CP conditions (being always at least 50 % higher as amount of current passing through the cell compared to pulse CP) is leading to structural deterioration. The visual observations are supported my pore structure analysis, performed on cross sections at the interfaces and in the bulk, in BSE mode. The porosity data are averaged over 75 sub-areas, (physical size of each image is 226 m in length and 154 m

in width) located at the peripherical interface around the steel reinforcement (radius of the steel bar =0.6cm), following a systematical sampling strategy.

Fig.2 Cross/sections of CP protected specimen(right) and pulse CP protected specimen (left)

As seen from the micrographs on Figs.1-2, the most damaged interface is as expected in the corroding specimen, where due to volume expansion of corrosion products, the interfacial gap is increasing and at 420 days of age is in the range of 200 – 500 m away from the steel surface. For the specimens under CP, an adhered to the steel surface layer of 100 to 200 m cement paste is observed, after which in radial direction (almost all around the steel cross section) an irregular gap of 200 – 300 m appears.

Evolution of concrete resistance, derived by AC 2 pin method

70 80 90 100 110 120 130 140 150 15 45 75 95 ₁₂0 15 0 17 5 19 5 21 0

time (days of cement hydration)

re si st an ce [O hm ] B A C R 0 20 40 60 80 100 120 re si st an ce (o hm )

B(CP) A(pCP) C(corr.) R(ref.) Concrete resistance derived from Impedance spectroscopy

Fig.3 Concrete electrical resistivity, derived from AC 2 pin method (left) and impedance spectroscopy (right).

Na2O(w .%) K2O(w .%) MgO(w .%) Cl(% c.w .)

Chloride & alkali concentrations derived wet chemically

B(CP) A(pCP) C R

Na2O(w .%) K2O(w .%) MgO(w .%) Cl(% c.w .)

Chloride%alkali concentrations derived wet chemically

6(CP) 7(pCP) 3(corr.) R

Fig.4 Alkali and chloride concentrations in the vicinity of

the steel bar for all specimens. (CP current 2mA/m2_-left,

10mA/m2 –right)

The latter considerations are supported by derived pore structure parameters as gradient of porosity from the steel surface into the interface and bulk material, revealing higher structural heterogeneity for CP, compared to pulse CP conditions, which is evidenced by electrical properties as well (Fig.3). In the 50 m region adherent to the steel surface, the cement paste in the corroding specimens is characterized by 8 % porosity, the specimens under CP – by 4.8 % porosity and the specimens under pulse CP – by 6.2 % porosity. In the 150 to 200 m region, the porosity for the corroding specimen is about 23 %, for the specimens under CP – about 12 % and for the pulse CP – about 8 %.