Reported herein are the results of study on the method of making compression test specimens of prepacked concrete. In many cases the tests on intrusion-mortar or prepacked concrete have been performed in accord with the General Specification for Prepakt-Concrete, some times introducing modifications to it when necessary. As recognized generally, however, the specification is considered in adequate or inappropriate in some cases to be applied to prepacked concrete, if not necessarily to Prepakt Concrete, and the method of making test specimens is regarded as one of the cases At present the specimens are manufactured by pumping the mortar into the molds prefilled with coarse aggregate soaked in water. However it has not yet been guaranteed if it is the best method to prepare the reliable and representative samples of prepacked concrete to be tested for comprissive strenegth.
　Since the pumping method requires certain types of heavy molds and equipments, the studies of prepacked concrete have been conducted in a limited number of well-equipped experimental stations or laboratories. To provide more representative and reliable samples of prepacked concrete and also to promote the studies on it at the same time, the simpler method of making the specimens is desirable if it will yield the equivalent test results to those by the present practice of pumping.
　From this point of view, the authors discussed the requirements for making test specimens of prepacked concrete, investigeted the effects of some factors accompanied in the pumping method upon the compressive strength of the concerte, and tried alternative methods of making cylinders using conventional type of steel or cast iron molds employed for casting ordinary concrete cylinders in laboratory as well as in field. The authors conclude that the following method can be employed as the alternative, which does not require any special mold or equipment except those required for casting ordinary concrete cylinders with the equivalent results to those by other methods.
　Suggested Method of Molding Compresseion Test Specimens of Prepacked Concrete
1. The molds for casting prepacked concrete cylinders shall be steel or cast iron molds which meet with the reqirements in the Japan Industrial Standard A1108 1950.
2. The coarse aggregate shall be of saturated-surface-dry condition. Before coarse aggregate is prep laced,the split cylinder shall be closed tight by bolts or clamps.
3. The test specimens shall be formed by placing the coarse aggregate and intrusion-mortar in the mold in three layers of approximartely equal volume. In placing each layer of the aggregate and mortar, the following procedures shall be taken.
　i) The coarse aggreggate shall be placed in the mold and rodded with 25 strokes of the tamping rod. The strokes shall be distributed uniformly over the cross-section of the mold and shall penetrare into the underlying layer. The bottom layer shall be rodded throughout its depth.
　ii) After each layer of the coarse aggregate has been rodded, the intrusion-mortar shall be poured over the aggregate preplaced from the top of the mold. Pouring of the mortar shall be continued until the layer is filled and the surface of the poured mortar appears atop the preplaced aggregate. The pouring of the mortar shall then be stopped and the sides of the mold shall be tapped with a mallet 20 times in order to insure the voids of the prepacked aggregate be compeltely filled with the mortar.
　iii) The next layer of the aggregate and mortar shall then be placed iin the same manner as specified above.
4. After the top layer has been placed, the surface of concrete shall be leveled off and covered with a glass plate or metal plate to prevent evaporation. On the top of the plate a proper weight shall be placed to restrain expansion of the prepacked concrete during the hardening period.
Note α): A wight of approximately 12 kg or an oldconcrete specimen of φ15×30-cm is considered sufficient for the specimen of φ15×30-cm to restrain the expansion.