| Recently the world witness, a growing with conception for better assessment of existing concrete structures that have been revealed a need for improved understanding of the structural effects of deterioration. The reinforced concrete is one of the most widely used materials for tunnel lining. In long-term, the strength is one of the most important parameter of the tunnel lining that ensure a safe and reliable service life for tunnels. There are different surrounding environments such as a chloride and carbon penetration, which are common durability problems in tunnels that may have a significant effect on sustainability of reinforced concrete strength. Damage detected visually as coincident cracks along the reinforcement, which are significant result for a reduction of the re-bar, cross-section and loss of bond strength for reinforcement concrete. As a consequence, a number of models that have been developed to predict degradation for tunnel lining due to the corrosion over time are limited. The aim of this study is to deepen an understanding of the tunnel lining effects of deterioration with special attention to the bond strength and residual strength. On the basis of test result, an effort is made to develop a procedure for evaluating the strength of reinforced tunnel lining concrete with corrosion. In the other word; it is trying for developing a procedure to be beginning for publishing standards to evaluate tunnel lining design for corrosion. The experimental variables includes corrosion rate, corrosion duration, concrete strength, rebar diameter and thickness of concrete cover. The corrosion condition have been classified into three levels, light, medium and heavy corrosion, which achieved through applying three acceleration corrosion durations (4,6and8days).A total of120reinforced concrete specimens, which includes48specimens with dimension (150mm×150mm×150mm) and72specimens with dimension (1200mm×200mm×300mm). All specimens are casting by using a concrete mixture for Yelangshan Tunnel at Zhuhai. Out of120specimens,48specimens are chosen as controlled specimen, while the remaining72specimens are chosen as corroded specimens. The48specimens are tested with pullout test, while the72specimens are tested with bending-compression test. The pullout test specimens are designed to get best significant result by provided anchored length and avoid yield failure. The bearing capacity specimens are designed to carry two load forces (axial load and moment) by provided the eccentric distance. The purpose of specimen design is to reflect results for specimen test to be similar for tunnel lining element. The tester is observed the crack growing during the acceleration corrosion, also before and after tests for all specimens. The test method for all specimens is divided into many steps to compare between the corroded and controlled specimen. The main factors for strength are stress (compression and tension), bond strength, specimen displacement and crack.Tests result indicates that the product of corrosion current density (Icorr) with acceleration corrosion duration (T), which is defined as the corrosion index that is a most significant factor effect on the strength of a corroded specimen. The percentage of steel metal loss and the reduction of strength are increasing with increasing corrosion index. Moreover, the diameter of steel bar is affected on the extent of metal loss at constant corrosion index. The effect of concrete cover thickness on the loss strength is significant. There are a linear relationship between corrosion rate and residual strength. The pullout test result found that there is32%,39%and43%reduction with bond strength for concrete strength (C30) and bar diameter10for4,6and8day corrosion constantly comparison with the controlled specimen. Corrosion may reduce both the elongation and the ratio of yield to ultimate strength of the reinforcement at maximum load. This reduction leads to premature fracture of the bar before reaching to ultimate yield strength. There are no general relationships for this situation.The bending-compression test result found that the relationship between the maximum load and displacement has same curve shape for all duration. Also, there are increasing in the displacement with increases corrosion rate. The percentage of losing load for specimen is8-13%for4days corrosion,12-16%for6day corrosion and24-26%for8days corrosion.Cracks are propagating first in the direction of the smaller concrete cover for single bars. The second crack propagates to the same face or normal to the first crack depending on bar spacing and relative cover sizes. For multiple bars the cracks propagate first in the direction of the smaller cover that is depending on the spacing between adjacent bars and the relative bars size. The actual amount of corrosion is substantially greater than the amount of theoretical radial expansion. An estimate of the amount corrosion express as a percentage of the original bar area.Based on the theoretical and experimental results, the evaluation procedure includes three significant steps:bond strength, crack and specimen performance. The mechanism for tunnel lining strength is depended on the experimental tests, bending-compression test and pullout test. The results found many flow charts that give a simple method to design and evaluate the tunnel lining strength for corrosion by designing specimen with similar tunnel lining condition.With validation against further test data the procedures, which is developed in this study could be a form the basis for specification of practice for the assessment of corrosion-damaged concrete tunnel lining structures and the durability design of new concrete tunnel lining structures. |
