チャート質岩のアルカリ骨材反応性

抄録

The damage to concrete due to alkali-aggregate reaction of chert has recently been discovered in several regions in Japan. Cherts are included in river and pit gravels and they are used in large quantities as concrete aggregate. In this paper, alkali reactivity of chert gravels was investigated with polarizing microscope, scanning electron microscope, X-ray diffractometer, and chemical (ASTM C289) and mortar bar (ASTM C227) test methods.
Results obtained are summerized as folows:
(1) The chert particles used were composed largely of cryptocrystalline to microcrystalline quartz, with some chalcedony. Especially, the amount of the chalcedony showed a considerable variation with the types of chert particles. Quartzite particles containing strained quartz with structural defect were also found in small quantities.(2) The texture of chalcedony under polarizing microscope was radiating fibers. They were embedded in the matrix of apparently non-fibrous silica (opal). Under scanning electron microscope, chalcedony showed still a fibrous texture at a low magnification, but appeared at a high magnification as an assemblage of rod-like particles with uneven surfaces and of different sizes. The rod-like particles were seen clearly after etching with hydrofluoric acid, which dissolved preferentially, but partly, the filling of opal.(3) The X-ray diffraction patterns of chalcedony were different in features from those of quartz: the 2.46 Å(110) peak of chalcedony was less intense and the 1.382 Å(212), 1.375 Å(203) and 1.372 Å(301) peaks were much less resolved than quartz. Such features of X-ray diffraction patterns could be utilized to establish the presence of chalcedony in some cherts.(4) Expansion of mortar bars was negligible when the alkali content (Na₂0 equivalent) was kept at 0.8%. Increasing the alkali content to 1.5 %, however, expansion occurred and its magnitude was in the order of white-> green-> dark gray-> reddish brown-colored cherts.(5) ASTM C289 chemical test revealed that cherts were deleterious or potentially deleterious aggregates, though mortar bars did not expand when normal Portland cement of about 0.8 % Na₂0 equivalent was used. The expansion of mortar bar made with chert in pit qravels changed remarkably upon the change of alkali content and curing temperature.(6) The crystals showing appearance of staurolite twin were observed around the reaction products in chert aggregate in damaged concrete bridge of eighteen years old.