International Concrete Abstracts Portal

The aim of this SP is to present some of the latest research in the area of fire performance of concrete. The ten papers in this SP present state-of-the-art review and results from both experimental and numerical studies on the various aspects ranging from material properties at elevated temperatures to optimum solutions for overcoming spalling in HSC concrete members exposed to fire. Fire represents one of the most severe conditions encountered during the lifetime of a structure and, therefore,the provision of appropriate fire safety measures for structural members is a major safety requirement in building design. The basis for this requirement can be attributed to the fact that, when other measures for containing the fire fail, structural integrity is the last line of defense. Generally, concrete structural members exhibit good performance under fire situations. In most cases, structural members used to be made of conventional concretes, often referred to as normal-strength concrete (NSC). However, in the last two decades, there have been significant advances in concrete material technology. These advances have lead to new concrete types, often referred to as high-strength or high-performance concrete. The construction industry has shown great interest in the use of high-strength concrete (HSC) due to improvements in structural performance, such as high strength and durability, that it can provide, compared to conventional NSC. HSC is typically characterized by high strength, good workability, and durability. Studies show, however, that the performance of HSC is different from that of NSC, and may not exhibit the same level of performance in fire. Furthermore, the spalling of concrete under fire conditions is one of the major concerns in HSC. Fire-induced spalling in concrete has been observed under laboratory and real fire conditions in HSC specimens. Spalling is theorized to be caused by the buildup of pore pressure during heating. HSC is believed to be more susceptible to this pressure buildup because of its low permeability compared to NSC. Data from various studies show that predicting the fire performance of HSC, in general, and spalling, in particular, is very complex because it is affected by a number of factors. In the aftermath of the September 11 terrorist attacks on the World Trade Center and the Pentagon, several issues relating to building performance under extreme conditions (structural, material, fire) have come to the forefront. Since intense fires played a major role in the collapse of the Twin Towers of the World Trade Center and other buildings, the issue of material performance under extreme fire conditions has attracted significant attention from the research and engineering community. Consequently, a number of new research programs in structural fire safety area are leading to new design provisions and solutions for enhancing the fire resistance performance of steel structures.

Effects of elevated temperature exposure and various factors, including water-to-cementitious material ratios (w/cm), curing conditions, heating rates, test methods, and polypropylene (PP) fibers, on (1) pore pressure buildup and potential for explosive spalling and on (2) degradation of mechanical properties in normal-strength (NSC) and high-strength concrete (HSC) are presented. Degradations of mechanical properties were measured using 100 x 200 mm cylinders, heated to temperatures of up to 600 °C at 5 °C/min, and compared with results of other studies and existing codes. Pore pressures were measured using 100 x 200 x 200 mm blocks, heated to 600 °C at 5 °C/min and 25 °C/min. Experimental evidences of the complex, temperature-dependant moisture transport process that significantly influenced pore pressure and temperature developments are described.

Whereas traditionally the verification of fire safety is based on prescriptive measures and criteria, an evolution toward performance-based design can be noticed, which is reflected in the design approaches given in the fire parts of Eurocodes 1 and 2. In Part 1-2 of Eurocode 11, general design aspects of structures exposed to fire are given as well as specific load combinations, design values of thermal and mechanical material properties, fire models, and heat transfer models. Most of these design principles are applicable to all types of construction materials. In Part 1-2 of Eurocode 22, specific approaches related to concrete structures are given, i.e., models giving the influence of high temperatures on material characteristics, a method based on tabulated values, simplified verification methods, and the basic principles of advanced calculation methods. In this paper, a review is presented of the most relevant clauses of the mentioned documents. For practical applications, the complete documents should be consulted.

Fire is one of the common events that might occur during the lifetime of any concrete structure. At elevated temperatures, mechanical properties of concrete and reinforcing bars experience significant deterioration. Following a fire event, these properties improve with time toward their original values. The paper focuses on the flexural behavior of unreinforced or lightly reinforced siliceous concrete slabs after exposure to elevated temperatures. Such behavior is controlled by the concrete tensile behavior. Models to predict related concrete and steel mechanical properties during and after exposure to elevated temperatures are presented. When needed, new models are developed based on available experiments data. A case study involving flexural testing of 11 concrete slabs after 85 days from exposure to fire is presented. The slabs were protected by a thin sprayed liner (TSL). The case study allowed evaluating the presented models and assessing the effect of the TSL layer on the slabs’ behavior.

A hydrocarbon fire test was conducted on nine concrete slabs incorporating three different types of binders: 100% ordinary portland cement (OPC), 50% OPC, and 50% ground-granulated blastfurnace slag (GGBFS), and alkali-activated slag (AAS). The specimens (780 mm [30.71 in.] x 360 mm [14.17 in.]) were made with three different thicknesses (100 mm [3.94 in.], 200 mm [7.87 in.] and 400 mm [15.75 in.]). Specimens were tested at an age of six months when the strengths were about 75 Mpa (10,877 psi). The specimens were exposed to the hydrocarbon fire on one side. Explosive spalling only occurred in the 400 mm (15.75 in.) AAS concrete specimen that had a lower moisture content and higher permeability than the OPC and OPC/slag concretes. This suggests that the well-renowned moisture clog theory is unlikely to be a predominant mechanism of spalling in AAS concrete. It is speculated that high thermal gradients caused explosive spalling in the AAS concrete specimen.