Some Problems on High Strength Concrete for Prestressing

This paper describes some problems on high strength concrete studied in our laboratory in recent years. Summary of this report is as follows:(1) In order to make high strength concrete for prestressing, it is necessary to lower the water-cement ratio. But, in the case of water-cement ratio lower than 40%, increase of strength of concrete is little for lowering water-cement ratio. Investigation results shows that one of this cause is due to increase of entrapped air by lowering water-cement rotio and lowering consistency of concrete. Therefore, it is necessary to place compactly by vibrator of good performance for concrete with low water-cement ratio.(2) One of the causes of that the increase of strength of concrete is little for lowering water-cement ratio in low water-cemente ratio, is due to the decrease of the rate of hydration of cement. That is, the structures of floccules of cement particles in paste in so densified by lowering water-cement ratio that the spase for hydration products around cement particles is decreased, and the rate of hydration of cement is decreased. Therefore, in order make high strength concrete rationally, it is necessary to mix thoroughly so that cement particles may be dispersed.(3) In the case of structures used rich mixture, the influences of temperature rise of concrete due to heat of hydration of cement can not be ignored.To remove the deffects of this temperature rise, considerations should be given to the selection of cement, proportion of concrete, making and curing temperature and procedures of concrete.(4) It is effective to use alumina cement for concrete or mortar which is used in joint of block at post-tensioning as the case may be.