Fire Technology, 39, 47–61, 2003 © 2003 Kluwer Academic Publishers. Manufactured in The United States. Flexural and Split Cylinder Strengths of HSC at Elevated Temperatures R.V. Balendran, Abid Nadeem, Tayyab Maqsood, and H.Y. Leung Department of Building and Construction, City University of Hong Kong, Hong Kong Abstract. This paper investigates the effect of elevated temperature on the flexural strength (FS) and split cylinder strength (SCS) of high strength concrete (HSC). Four concrete mixes of 50, 90, 110, and 130 MPa grade were prepared and subjected to elevated temperature exposure of 200  C and 400  C, and cooled under slow and quick cooling conditions. In addition, 130 MPa grade concrete specimens were also subjected to 100  C and 600  C exposure temperatures to compare FS and SCS under elevated temperatures. It was observed that with the increase in the elevated temperature, the FS and SCS experienced significant losses. The loss was found to be higher for richer concretes. FS was observed to experience a sharp loss at low temperatures that became gradual later at high temperatures. SCS, however, experienced a gradual loss, though sharper than FS, with the increase in temperature. The results indicated that cooling had a significant effect on the residual values and quick cooling caused greater loss in FS and SCS, than slow cooling at elevated temperatures. The quick cooling was noted to produce maximum loss over slow cooling at temperatures around 400  C. Key words: high strength concrete, elevated temperatures, flexural strength, split cylinder strength, quick cooling, slow cooling 1. Introduction Fire remains one of the serious potential risks to most building and structures. The exten- sive use of concrete as a structural material has led to the demand to fully understand the effect of fire on concrete. Concrete is generally considered as a non-combustible and fire resistant material, but excessive fire and heat may substantially alter its mechani- cal properties. A sizeable amount of literature exists for the fire performance of normal strength concrete. With the advent of High Strength Concrete (HSC) in 1970s, its application in the indus- try is becoming wide spread. HSC offers significant economic and architectural advan- tages over the normal strength concrete (NSC), and is also suited for special construction requiring high durability but it has been observed that HSC has certain shortcomings in the form of increased brittleness and decreased fire resistance [5, 22]. The reason being the increase in strength and silica fume content [26]. With low water/cementitious mate- rial ratios and addition of silica fume, HSC becomes denser, and it is more difficult for the vapors to escape in the case of fire. Therefore, the risk of spalling increases for HSC. Moreover, the strength of HSC degenerates more sharply than the conventional concrete