Long T. Phan, NIST 1 High-Strength Concrete at High Temperature − An Overview Long T. Phan ABSTRACT This paper presents results of NIST’s experimental program on the effects of elevated temperature exposure on the mechanical properties and potential for explosive spalling of high strength concrete (HSC). Mechanical properties of HSC were measured by heating 100 x 200 mm cylinders at 5 °C/min to temperatures of up to 600 °C, and heat-induced pore pressure buildup was measured by heating 100 x 200 x 200 mm blocks to the same temperature level at the same heating rate. The results of NIST mechanical property measurement are compared with results obtained in other studies as well as with existing code provisions to evaluate their applicability to HSC. The paper also presents results of measurement that indicate the efficacy of polypropylene fibers for mitigation of explosive spalling. 1 Introduction Degradation of mechanical properties of concrete due to short-term exposure to elevated temperature has been studied as early as the 1950s. Among the early studies were those of Abrams [1], Malhotra [12], and Schneider [16,17]. Results of these studies constituted the technical basis for the provisions and recommendations for determining concrete strength at elevated temperature in many existing codes. While these studies provided valuable information on concrete strength as a function of temperatures, almost all used specimens made with normal strength concrete (NSC, f 23 C ≤ 40 MPa). Thus, in light of the results of recent studies, which have shown that high-strength concrete (HSC) behavior at elevated temperature may be significantly different from that of NSC [14,15,18-20], question may be raised as to whether existing design rules and recommendations are applicable to HSC. The behavioral differences between HSC and NSC are found in two main areas: (1) the relative strength loss in the intermediate temperature range (100 °C to 400 °C) and (2) the occurrence of explosive spalling in HSC specimens at similar intermediate temperatures. In terms of strength loss, studies [14,15,18,19] have shown that, for intermediate temperatures between 100 °C and 400 °C, the compressive strength of HSC could be reduced by close to 40 % of the room-temperature strength – a reduction of approximately 20 to 30 percentage points more than in NSC exposed to the same temperatures. In terms of explosive spalling, which refers to a sudden and violent breaking away of a surface layer of heated concrete, it has been observed in laboratory tests that HSC has a significantly higher potential for explosive spalling than NSC, even at heating rates less than 5 °C/min which is lower than that would occur during real fires [4,7-10,15-18]. The phenomenon,