The paper presents the results of the theoretical analysis of explosive spalling mechanism of precast- and prestressed concrete members in fire.

The explosive spalling is not caused by the steam pressure of the heated water within concrete voids but caused by thermal stress of concrete near the surface.

As the distribution of temperature in concrete near the surface is not linear, thermal stress occurred. The explosive spalling does not occur until thermal stress becomes greater than compressive strength of concrete.

According to above opinion, it is possible to explain clearly every phenomenon of explosive spalling of prestressed concrete in fire.

Explosive spalling of prestressed concrete member can be avoided by adding fire proof covering to the member surface or applying some kinds of aggregates, which have a small coefficient of thermal expansion at high temperature, to its concrete.

* Manuscript received on May 8, 1965.
** Research Member of B.R.I , Ministry of Construction