Performances of concrete structures should be decreased with time due to some mechanisms of deterioration such as chloride attack, carbonation of concrete and alkali aggregate reaction. Considering the establishment of an appropriate maintenance system for such structures, there are many problems to be solved. In this paper, deterioration cases of concrete structures are outlined, followed by the explanation of some investigations to build the maintenance system and remained issues, regarding mainly the problem of steel corrosion in concrete.
The brittle fracture of steel reinforcement caused by an excessive expansion of concrete due to the alkali-silica reaction (ASR) has recently been detected especially in large numbers of about 40 bridge piers in Japan. At present, concerning the rehabilitation of such severely deteriorated bridge piers with the fracture of steel reinforcement, their diagnosis and maintenance procedure has been a matter of concern for civil engineers. The author has engaged in the strengthening work of bridge piers on the Noto expressway in Ishikawa prefecture. This report introduces the recent survey on actual conditions of ASR-deteriorated bridge piers with the fracture of steel reinforcement and the countermeasure applied to them.
There has been a growing demand of cathodic protection (CP) for the rehabilitation of salt damaged concrete structures. A brief introduction is given for the major CP systems used for concrete structure and the criteria of protection. In the present paper discussion is also given on the mechanism of protection, materials flow and electrical conduction within concrete, a solid system with low mobility and high electrical resistivity placed in atmosphere. Cathodic polarization protects steel reinforcement not only by shifting the metal potential in cathodic direction but also by modifying the environment factors favorable for steel protection such as an increase in pH and a decrease in chloride ion in the vicinity of metal surface resulting in protection conditions that satisfy the criteria. The environmental modification is thought to be the principal effect of CP. It is not seldom in concrete CP that a system that has been in operation below the criteria in the early stage would be changed to be satisfactory after long run owing to the modified environments. It is thus required to establish design and maintenance criteria in consideration of chronic changes.
Hydrogen embrittlement of mild steel was examined by a slow strain rate test (SSRT) in a solution simulated the concrete with alkaline silica reaction (ASR), and the effects of strain rate and hydrogen evolution reaction current on the fracture strain were investigated. A reduction of the fracture strain was observed when the strain rate was small enough with some extent of hydrogen evolution current. Quasi-cleavage fracture surfaces were observed under hydrogen charging conditions, suggesting hydrogen embrittlement. The entry of hydrogen was effective only when the specimen deformed with necking. On the other hand, the amounts of hydrogen entering in the elastic and the plastic deformation regions before the maximum stress had very little effect against the reduction of fracture strain.
Fracture of reinforcing rebar due to alkali silicate reaction (ASR) of concrete has been investigated by slow strain rate test (SSRT) using a V-notched specimen simulating preexistent crack. Cathodic hydrogen charging reduced the fracture strain and facture surface showed three regions of different fracture mode, quasi-cleavage, dimple and cleavage surface from outer side to center. Specimens without cathodic charging showed only dimple and cleavage fracture surface. It suggests that hydrogen entered into rebar caused quasi-cleavage surface. Cleavage surface observed at the center part in the both cases was a result of rapid deformation and fracture of the specimen occurred just before rupture. For the hydrogen charged specimen having a sharp slit as observed on rebar fractured in ASR concrete, distribution of the three fracture mode coincided with that observed on fractured rebar in ASR concrete and hydrogen embrittlement of rebar was suggested.
In order to clarify the fracture mechanism of steel bars through an alkali-silica reaction (ASR), the mechanical properties and fracture observation of steel bars ruptured by ASR and the delayed fracture resistance was investigated in detail, and a stress analysis during the bending process of steel bars was also performed. The macroscopic fracture morphology of steel bars was as follows : the first crack was initiated near the ridge of the steel bar in the inner side of the bending area. Then, the second and third cracks were generated and propagated in the direction of the outer side of the bend. The first crack was a kind of ductile fracture, and the second and third cracks were cleavage fractures ; quasi-cleavage fractures were not observed anywhere on the entire fractured surface. In addition, surface seams, large non-metallic inclusions and voids were observed with some frequency in the fractured steel bar. As a result, low Charpy absorbed energy in the unbent portions was obtained, and surface hardness in the bent portions remarkably increased through strain aging. Residual tensile stress existed in the inner surface after bending; in particular, the existence of high residual tensile stress near the ridge was confirmed by FEM analysis. Furthermore, delayed fractures did not occur in the constant loaded test using the notched specimen with diffusible hydrogen of 0.97 ppm. Thus, it was concluded that the fracture of steel bars through ASR produced brittle fractures. Such fractures were caused by the deterioration of fracture toughness, which resulted in strain aging due to bending; the existence of large non-metallic inclusions and voids; the existence of residual tensile stress after bending; and the increase in applied stress to the steel bar through ASR. Initiation and the propagation behavior of the second and third cracks in the steel bar were also consistently explained by the brittle fracture mechanism.