4. Conclusions
The effects of the type, dosage and length of fibers on the spalling resistance of ultra-high-strength concrete with highest compressive strength of 198 MPa under the condition of rapid heating and rapid cooling were demonstrated. The mechanism of the spalling resistance of ultra-high-strength concrete mixed with PP fiber was discussed. The main conclusions drawn from this study are as follows. (1) Steel fiber can delay the spalling time but does not obviously ease the spalling phenomenon. PP fiber can improve the spalling resistance of concrete. (2) The PP fiber dosage affects the spalling resistance of concrete. The degree of explosive spalling is related to the PP fiber dosage. The spalling resistance is poor when the dosage is lower than 0.20%. Meanwhile, the spalling resistance is best for a PP fiber dosage of 0.20% or more, in which case the compressive strength residual rate exceeds 70% and the mass loss rate is within 8%. (3) The PP fiber length affects the spalling resistance of the concrete specimen. Specimens with PP fiber lengths of 6 and 9 mm undergo appreciable explosive spalling, while specimens with PP fiber length of 12 and 19 mm have appreciable spalling resistance at high temperature. The compressive strength residual rates are more than 70% and mass loss rates are within 8%. (4) PP fiber improves the spalling resistance of ultra-high-strength concrete mainly through the formation of tubular channels from the external to internal via the melting of PP fiber. Additionally, there is an increase in capillary porosity due to the dehydration of hydration products. The combination of these behaviors relieves internal vapor pressure and prevents specimen spalling.
