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Re-deterioration of ASR-affected Concrete Bridge Pier and Proposal for Rehabilitation Method
Abstract
The brittle fracture of steel bars in ASR-affected concrete bridge piers has recently been reported in various places in Japan. At present, the development of maintenance and rehabilitation techniques for such severely damaged concrete piers becomes a great concern for civil engineers. The authors have carried out the series of investigation on both the deterioration of concrete and the fracture of steel bars in concrete piers which were severely deteriorated again after the surface coating had been applied. This report describes the survey on the deterioration of ASH-damaged concrete bridge piers, and the reconstruction method of piers proposed based on the survey.
Using concrete cores taken from the abutments of K and T bridges in Toyama Prefecture, the judgement of potential expansion capacity by accelerated expansion test in 1N-NaOH sol. was verified from the ASR reactivity of the used aggregate and the results of detailed texture observation of the concrete. Fine aggregates used in K bridge has a high content of andesite showing a high ASR reactivity and ASR-gel filled cracks were observed in andesite. By contrast, fine aggregates used in T bridge has a high content of rhyolitic welded tuff showing a low ASR reactivity and no cracks due to ASR were observed. As a result of the alkali solution immersion test, the concrete of K bridge was assessed as “harmful” and that of T bridge was as “innocuous”. From microscopic observations, it was considered that accelerated expansion in T bridge was not promoted due to original absence of cracks in concrete and the progress of carbonation in cement paste.
In the Hokuriku district, in which very reactive river sands and gravels have been used as concrete aggregates for a long time, large numbers of alkali-silica reaction （ASR）- affected structures such as the bridge, the tunneling, and so on are found out. When the river sands and gravels are both reactive, their ASR reactivity of concrete are well characterized by each river from one place to another in which a wide range of rock types is composed and proportionally mixed. In the former research work, especially in this district, the main reactive stones contained in them are both the volcanic rocks such as andesite, rhyolite and welded tuff, and the sedimentary rocks as shale and slate. It is well known that the evaluation of cracking and deformation by visual inspection is essential in in-situ survey for ASR-deteriorated structures. However, for the structures such as foundations or caissons in bridge piers located underground or in water, such a visual inspection is not usually applied. For this reason, there have been very few studies concerning the survey for ASR-affected concrete foundation structures underground or in water. So, this study investigated the ASR degree of concrete foundation structures in the Hokuriku district by the survey of surface cracking of both structures and cores from them. The change in ASR degrees in cores taken from different conditions of water supply underground or in water was investigated by means of fluorescence and polarization microscopes using the polished fragment or thin section of cores with a fluorescence epoxy. Furthermore, the fractal dimension analysis was also applied for the cracking of both structures and cores. The relationships between the ASR degree determined by petrographic examination and their results by fractal dimension analysis were discussed.'
Sino-atrial node (SAN) function was evaluated in 46 patients, three of whom had the sick sinus syndrome. Patients were paced from the right atrium for 15 to 180 sec at rates of 90, 110, 130, and 150 beats/min. The rapid cessation of pacing was associated with suppression of the SAN at all paced rates and at all durations of pacing. The observed pause was terminated by a sinus beat in all instances. The duration of pacing had little influence on the duration of the observed pause. The pause increased as the pacing rate was increased until, at a rate of 150 beats/min, a marked decrease in the pause was noted. Atropine (1.5-3.0 mg iv) diminished but did not eliminate the SAN suppression. Subthreshold pacing did not suppress SAN function. Three patients with sick sinus syndrome had a greater degree of SAN suppression than normal patients (4732 ± 415 msec [SSS] M ± sem; 1041 ± 56 msec for normal patients).
The determination of the duration of the pause following cessation of atrial pacing provides a technique for recognition of abnormalities of SAN function.