On measuring techniques of pore pressure in concrete at elevated temperature

Entrapment of water vapor pressure is a major reason leading to explosive spalling of concrete subjected to elevated temperature. However, the measurement techniques for pore pressure inside concrete specimens were not consistent from different studies. This paper presents an extensive review of pore pressure measurement techniques first. Three main differences (viz., medium to transfer pore pressure, type of pressure gage head, and placement of thermocouples) were found in previous studies. An experimental program comprising twelve pore pressure gages was designed to evaluate effectiveness of all the combinations of factors by conducting one-dimensional heating test of concrete slabs. The test results showed that heat transfer properties and temperature measurements were not influenced by the different measurement techniques. Filling steel tubes with rods or leaving the tubes empty would impair build-up of vapor pressure. On the contrary, filling it with silicon oil will create extra hydraulic pressure due to thermal expansion of the oil itself. The use of sintered metal as a gage head could compensate additional pressure by introducing free volume and collect pore pressure evenly. Placing thermocouples outside the steel tube would not compromise accuracy of temperature measurement and the complexity of the measurement system could be greatly reduced. At last, two requirements are proposed for correctly measuring vapor pressure: (1) expansion of oil must be compensated by free volume; (2) free volume should not interfere with moisture accumulation and vapor pressure build-up.