Influence of two-directional ingress of corrosion agents on the initiation and propagation of steel corrosion in concrete

• Main Author: Zakka, Ze Gyang
• Format: Others
• Language: en
• Published: 2021
• Online Access: https://hdl.handle.net/10539/31202

A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, in fulfillment of the requirements for the degree of Doctor of Philosophy in Engineering, 2020 === Most research in steel reinforced concrete corrosion over the past 40 years has predominantly focused on corrosion due to uni-directional (1D) chloride ingress. This thesis presents results of measurements that are related to steel corrosion in cracked and uncracked concretes that are exposed to 1D chloride ingress and bi-directional (2D) chloride ingress. The concrete exposed to 1D chloride ingress typifies reinforcement within elements such as walls and slabs while the 2D chloride ingress is assessed in reinforcement bars that are located at orthogonal edges of concrete elements such as square edged beams and columns. 72 beam specimens of dimension 150 × 150 × 625 mm were used in this study. Two equal number of beams were cast using a binder blend of plain Portland cement (PC) with fly ash (FA) (70%PC:30%FA) and ground granulated blast furnace slag (SL) (50%PC:50%SL) with water-to-binder ratio = 0.40. The beams were reinforced with 10 mm diameter high yield reinforcement bars embedded 20 mm from the exposed face(s) of the beam. Three reinforcement arrangements were used: a single isolated reinforcement bar placed at an orthogonal edge, in the middle of the specimen, or at an orthogonal edge and electrically connected to a stainless-steel rod near the other face of the concrete. After casting, the specimens were cured by immersion in potable water for 28 days after which the chloride ingress direction was modified (1D or 2D) using an epoxy paint. A single mechanical crack of width 0.16 mm to 0.40 mm was induced in one-half of each specimen set using the 3-point bending technique. A sustained flexural load was imposed on the cracked specimens by clamping them in pairs(back-to-back)in a 3-point bending technique. All the specimens were exposed to a 2-week wetting-drying in 5% NaCl solution then air-drying in an ambient laboratory conditions (temperature of 20 ±5oC and relative humidity of 40 ± 10%) for the total experimental duration of 110 weeks. The corrosion potential, corrosion rate, and concrete resistivity were measured 24 hours after the wetting cycle and 24 hours before the end of the drying cycle. In general, corrosion initiation occurred earlier in the cracked and uncracked PC/SL specimens that were exposed to 2D chloride ingress while there was no significant difference in the time-to-corrosion initiation in the PC/FA specimens. After corrosion initiation, the corrosion rate of specimens with reinforcement bars at the orthogonal edges proceeded at a rate that was higher than that of specimens with reinforcement bars in the middle of the concrete beam. This trend was more significant in the cracked specimens as they attained a mean corrosion rate that was more than 2 times that of the uncracked specimens. The cracked PC/FA and PC/SL concrete specimens exposed to 2D chloride ingress attained a corrosion rate of 11.0 μA/cm2 and 4.3 μA/cm2 while the specimens exposed to 1D chloride ingress attained 4.8 μA/cm2 and 3.9 μA/cm2 respectively. On the other hand, the uncracked PC/FA and PC/SL concrete specimens exposed to 2D chloride ingress attained corrosion rates of 5.0 μA/cm2 and 1.5 μA/cm2 while the specimens exposed to 1D chloride ingress attained corrosion rates of 2.2 μA/cm2 and 1.6 μA/cm2 respectively. The cracked and uncracked specimens with stainless-steel rods had lower corrosion rates. The lower corrosion rate can be explained by the limiting of the corrosion rate by the cathodic reaction. The concrete resistivity of concrete exposed to 2D chloride exposure signified a high risk of corrosion (<10 kΩ.cm) while that of specimens exposed to 1D chloride ingress signified lower corrosion risk (>10 kΩ.cm). In line with the corrosion rate of the specimens, the corrosion-induced damage in the concrete, corrosion pit factors, and steel mass loss in the concrete with steel bars at orthogonal edges and exposed to 2D chloride ingress were significantly higher than that of concrete with reinforcement bars that are away from the concrete edge. The results of this study show that reinforcement bars which are placed at orthogonal edges of cracked and uncracked concrete elements and exposed to 2D ingress of corrosion agents are prone to corrode faster than those which are farther away from the concrete edges. The higher corrosion rate also results in longer and wider corrosion-induced cracks. In order to improve the corrosion free life of steel bars at orthogonal edges, the minimum recommended cover should be adopted. The corrosion of the edge reinforcement bars can also be limited by using cathodic protection, surface treatments to limit ingress of aggressive agents of corrosion, galvanized or stainless-steel bars should be provided at the edges of concrete elements that are severely exposed === CK2021