Experimental Investigation and Numerical Modeling of Corrosion Induced Expansive Pressure on Concrete Cover in Reinforced Concrete

Abstract

Corrosion of reinforcement is an important durability concern for the structures exposed to adverse weather conditions especially in coastal regions. The volume of corrosion products are much higher than the original volume of the corroding reinforcement, which exerts an expansive pressure on the surrounding concrete and results in cracking of the concrete cover. In this research an investigation was carried out through numerical modeling and experimentation to explore the mechanism of concrete cover cracking due to that expansive pressure. It was aimed that structural health monitoring in terms of level of corrosion might be possible through monitoring the crack width and crack propagation in the cover concrete. In addition a convenient means of corrosion protection was investigated by using zinc as sacrificial anode. A numerical model was developed using a commercial finite element modeling software ABAQUS 6.14 to evaluate the cracking pressure, crack initiation, crack propagation and radial deformation for different cover thicknesses and bar locations. The model was also used to determine the effect of bar diameter on cracking pressure and the patterns of crack for different number of bars. Numerical results were validated by experimental investigation. The cracking pressure was simulated experimentally by applying hydraulic pressure through a hole in the specimens having similar diameter as of the reinforcement. 150 mm cube specimens for three different grades of concrete with various clear cover and location of bar were used for the simulation. Impressed current technique was used to accelerate the corrosion of reinforcement. Characterization of impressed current technique involved determination of optimum chloride content and minimum immersion time of specimens for which the application of Faraday’s law could be efficient. To obtain optimum chloride content, the electrolytes in the corrosion cell were prepared similar to that of concrete pore solutions. Concrete cubes of 50 mm were used to determine the optimum immersion time for saturation. It was found that the optimum chloride content was 3.5% by mass of water and the minimum immersion time for saturation was 24 hours. Concrete prisms of 250 mm x 250 mm x 300 mm with 12 mm-Ø grade 60 plain mild steel bars were used to simulate RCC beams with various cover thicknesses of 20 mm, 37.5 mm, 50 mm and 75 mm, respectively, to observe the mechanism of crack initiation, propagation and level of corrosion with respect to the width of surface crack. Level of corrosion was measured in mg/cm2 following Faraday’s law and by gravimetric loss method, and width of crack was measured by image analysis. From numerical modeling it was found that, with the increase of cover thickness the pressure as well as the radial expansion needed to initiate crack was increased. The same pattern was found for both corner bar and side bar. A lower cracking pressure was required for corner bar with respect to side bar. On the other hand, with the increase in bar diameter, a decrease in cracking pressure was observed. It was also found that the crack was initiated from outer surface and propagated towards the steel concrete interface for a cover thickness of 20 mm as well as 37.5 mm. This result was accomplished due to heaving of cover concrete and crack initiated when bending stress exceeded the tensile strength of concrete. Whereas for a cover thickness of 50 mm as well as 75 mm, crack was initiated at the steel-concrete interface and propagated towards the cover surface. These observations were further confirmed through experimental investigation. From the experimental simulation of cracking pressure, it was found that the cracking pressure varied linearly with the increase in concrete cover-to-diameter ratio (c/d). The pressure requirement to initiate crack was also higher with higher grades of concrete. For corner bars with cover thickness 37.5 mm, the critical pressure was 6-10 MPa and it increased up to 17 MPa for cover thickness of 64 mm for different grades of concrete. On the other hand, for other bar location with cover thickness of 37.5 mm and 64 mm, the pressure required to initiate crack was about 7.6 MPa and 14.8 MPa, respectively, for C20 grade concrete. A linear relationship was found between crack width and level of corrosion. For cover thicknesses of 20 mm and 37.5 mm, the critical corrosion amount (CCA) needed to initiate crack was about 22 mg/cm2. However, a sudden increase in CCA was noticed when the cover thickness was over 44 mm. For cover thicknesses of 50 mm and 75 mm the CCA values were 129 and 211 mg/cm2, respectively. Cathodic protection of reinforcement corrosion was investigated by using zinc as sacrificial anode in the accelerated corrosion cell with two environments- (i) synthesized solutions similar to concrete pore solution and, (ii) concrete. It was found that up to 65% of the corrosion current could be diverted through the sacrificial anode. It is concluded that level of corrosion could be predicted by monitoring the surface crack width with reasonable accuracy, a minimum of 50 mm concrete cover should be provided in the corrosion prone environment and zinc as sacrificial anode could reduce as much as two-thirds of corrosion hazard to prolong the service life of reinforced concrete structures.