The composite type anti-seismic structure developed in Japan for multistory construction generally entails structural members with an exceedingly high relative quantity of steel cast in concrete. This dense placement of steel sections and rainforcement bars in concrete results in a high resistance to the flow or plastic deformation of fresh concrete when poured into forms, and eventually leads to appearances of honeycomb or voide defects especially at soffit portions directly under lower corde flange plates of girders. In this report the term "Fill-up-ability" is used for expressing a property pertaining to fresh concrete which is basically the accumulative result of the following three conditions: (1) Material properties of concrete mixture itself comming from its flowability or plastic deformability. (2) Suppressive conditions within forms which hinder the flow or plastic deformation of concrete mixture when it is placed. (3) External forces which are applied to facilitate the flow or plastic deformation of concrete mixture within forms, such as impulses caused by dropping or falling at the time of placement, and vibration applied by mecanical vibrators. Test girder sections were made imitating actual sections of concrete structures. They were given varied suppressive conditions against the flow of concret mix, i.e., with different arrangement of steel sections and reinforcement bars within the test forms. Keeping external force conditions the same, concrete mix with differing material properties was placed in those test forms. The fill-up-ability of concret mix with various properties when poured into the forms of gider type sections of varied suppressive conditions was observed both quantitativly and qualitatively. An index termed "Degree of Fill-up-ability" was created by the author to measure quantitatively the fill-up-ability. Summary Conclusion 1. It was observed that, under same external force conditions and same suppresive conditions, though concrete mix may have idential slumps, its degree of fill-up-ability will differ considerably due to other material properties, and that the existance of a broad range of difference is evident. 2. The fill-up-ability becomes more favorable as the slump is increased, but there are different allowable limit value of slump corresponding to the different strength of suppresive condition and the different material propertie of fresh concrete. 3. The fill-up-ability becomes more favorable as the sand ratio is increased (experiments were performed from 27% up to 41.5%) and the unit cement content is increased, air-entrained concrete shows better fill-up-ability than plain concrete, 4. Concrete with light sand and light gravel show instable qualities of separating easily, and its fill-up-ability was unfavorable in contrast with concrete with river sand and light gravel which shows stable qualities and favorable fill-up-ability. 5. Among the sample girder section of the three structural type, namely, reinforced concret structur, concret convered steel frame structure and composite structure, fill-up of concrete was most difficult for composite structure. Fill-up-ability for girder section types generally used in practice for composite structures was proved poor. The exissance of reinforcement bars in addition to steel sections makes it very difficult for concrete mix to reach portions under the flanges of steel sections. 6. The degree of difference of fill-up-ability of concrete mix, as stated herinabove under item 1,2 and 3, decrease considerably when the strength of suppressive conditions are increased like as the case of the composit structures. 7. It was recognised that the detail design of sections or connections of structural members is very important as well as the serection of material properties of concrete mix is very important to expect favorable fill-up ability, and slump shall be serect relatively

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