Title: Degradation of Bond Between Reinforcing Steel and Concrete Due to Cathodic Protection Current

Author(s): Rasheeduzzafar, Mohammad Golam Ali, And Ghazi J. Alsulaimani

Publication: Materials Journal

Volume: 90

Issue: 1

Appears on pages(s): 8-15

Keywords: bonding; cathodic protection; chlorides; reinforced concrete; Materials Research

DOI: 10.14359/4028

Date: 1/1/1993

Abstract:
Results from a study on the effect of cathodic protection (CP) current on deterioration of steel-concrete bond are presented. Constant current densities of 03, 10, 20, and 50 ma/ft 2 (32.3, 108, 215, and 538 ma/m 2), based on the reinforcement surface area, were maintained for a period of 14 months. Chloride contents of 2 and 8 lb/yd 3 (1.19 and 4.76 kg/m 3) were inducted into concrete through the mix water to investigate the effect of chloride concentrations on the steel-concrete bond when CP current is applied to protect reinforcing steel from chloride-based corrosion. A cantilever method of bond testing was used to insure a bond stress distribution along the steel-concrete interface similar to that in reinforced concrete flexural members. Chemical analysis was performed for the determination of sodium, potassium, and chloride ions present in concrete near the reinforcing steel surface. Bond strength and chemical analysis results obtained from current-treated specimens were compared with the corresponding values measured in the control specimens. It was found that sustained impressed current on reinforcing steel will cause a deterioration in the bond between steel and concrete. The magnitude of bond deterioration was found to be a function of the current density and chloride content of concrete from NaCl source. Bond loss data show that, for a practical CP current density of the order of 3 ma/ft 2 (32.3 ma/m 2) applied for a 14-month period, the bond losses are insignificant 1.5 and 3 percent for 2 and 8 lb/yd 3 (1.19 and 4.76 kg/m 3) chloride cements, respectively. However, when the current density is increased to 50 ma/ft 2 (538 ma/m 2), the bond losses were found to be 19 and 33 percent, respectively, for the 2 and 8 lb/yd 3 (1.19 and 4.76 kg/m 3) chloride contents. A 50-ma/ft 2 (538-ma/m 2) current applied for 14 months required 504 amp-hr/ft 2 (5426 amp-hr/m 2) to cause 19 to 33 percent bond loss. At a cathodic protection current density of 2 ma/ft 2 (21.3 ma/m 2), 504 amp- hr/ft 2 is equivalent to come 30 years. Chemical analysis data show a significant electromigration of cations and anions present in the pore solution of concrete. This results in an accumulation of Na + and K + in the vicinity of the reinforcing steel and a movement of chlorides away from the steel- concrete interface. The buildup of Na + and K + at the steel-concrete interface causes softening of concrete, which results in the degradation of bond between steel and concrete just mentioned. Bond reduction is shown to be almost proportional to the accumulation of sodium and potassium ions in the vicinity of reinforcing steel. However, the migration of chlorides away from the steel significantly brings down the chloride ion concentration at the steel-concrete interface, thereby reducing the corrosion risk. Increase in the NaCl-supplied chloride content of concrete decreases bond due to the additional accumulation of Na + near the steel surface.