EFFECT OF COOLING MODE ON SERVICEABILITY OF PARTIALLY PRESTRESSED CONCRETE BEAMS EXPOSED TO FIRE FLAME

To investigate the effect of fire flame on the behaviour of partially prestressed concrete beams and the residual ultimate strength, seven pretensioned concrete beams were prepared, cast and tested. Each pair were exposed to the same burning temperatures of 300, 500 and 700ᴼC, respectively but cooled in different manners (gradually or suddenly), while the other beam was left without burning as a reference beam. Test results show that fire flame affects the camber of the pretensioned concrete beams due to concrete deterioration. As the burning temperature increased, the camber increased as well as the residual camber in both modes of cooling. At burning temperature of 300, 500 and 700ᴼC, the residual cambers, compared to that of the reference beam, were 260, 304 and 510%, respectively for gradually cooled beams. While they were 244, 270 and 600%, respectively for suddenly cooled beams. In addition, increasing burning temperature leads to decreasing beam stiffness. Likewise, as the exposing temperature increased the residual ultimate load carrying capacity decreased. Compared with that of the unburned beam, the residual ultimate load was 73, 63, and 46% at burning temperature 300, 500 and 700ᴼC, respectively for gradually cooled beams and it was 67, 56, and 45%, respectively for suddenly cooled beams. Sudden cooling had a worse effect on the residual carrying capacity than the gradual cooling. This difference between the two modes of cooling increases as the burning temperature increased, it was 8 and 11% at burning temperature 300 and 500ᴼC, respectively, while it was 2% at 700ᴼC. Slip of prestressing rebar did not happen in the burned pretensioned concrete beams at a temperature of 300ᴼC while at 500 and 700ᴼC it was observed. At burning temperature of 700ᴼC, the slip initiated at loading stage of 10 and 10.4 kN when the burned beams were cooled gradually and suddenly, respectively. Accordingly, the maximum slip value was 0.35 and 0.41 mm, respectively