To examine the relaxation phenomena of PC steel wire used for pretension-concrete method over long periods, the authors measured the relaxation of 2.9mm dia. PC steel wire taking the following factors, which are relative to the relaxation factor of prestressed-concrete, into consideration.(1) Loading time; i.e. the time from O load to the given measured load value.(2) Initial load to relax the PC steel wire(3) Bluing (one of the heat treatments of PC steel wire)(4) Prestretching of PC steel wire(5) Measuring temperatureFrom this experiment, the authors arrived at the following results:(1) The relaxation value must be corrected in accordance with changes in the loading time.(2) The relaxation value in percent is the same for loads up to 80% of the yielding point load.(3) Bluing treatment greatly reduces relaxation.(4) Prestretching significantly reduces the relaxation, Prestretching is particularly effective in the case of 10% overload.(5) The measuring temperature is an important factor for relaxation tests. Therefore the tests should be held at a constant temperature.The authors also were able to locate the turn over point in the semi-log graph of the time-relaxation characteristic curve. The log-log plot indicates the straight line characteristics and the turn over point corresponds to the change in slope point.Based on their investigation, the authors suggest an estimation method for the relaxation phenomena over long periods. Experimental results indicate that the relaxation values estimated with this method are consistent with the actually measured values over long periods.

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