International Concrete Abstracts Portal

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.

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.

One of the new techniques to reduce explosive spalling in concrete subjected to fire is to add a cocktail of polypropylene fibers and steel fibers into the concrete mixture. This method is still in the early stages of development and requires more research to investigate the efficiency of introducing such a combination of fibers in reducing explosive spalling in fire. The purpose of this paper is to present the results of an experimental study conducted to investigate the performance of reinforced concrete columns containing steel and polypropylene fibers under different loadings and subjected to severe fire conditions. Two loading levels were investigated representing 0.6 and 0.76 of the ultimate strength limits of ACI 318. Columns containing polypropylene (1 kg/m3) and steel fibers (80kg/m3) showed a higher fire resistance by an average factor of 1.76 compared to columns containing PP fibers (1 kg/m3) only. The paper also assesses the effect of adding steel and polypropylene fibers on the severity of concrete explosion under fire. Measurements of axial displacements and concrete temperatures are presented in this paper. The paper compares the obtained experimental values of the axial displacements with theoretical values calculated using a previously developed simple approach.

Adequate information may not be readily available to justify the development of alternative methods for evaluating and predicting the performance of concrete exposed to fire and elevated temperatures. The recent trends to use alternatives to ASTM E119 such as small-scale testing and computer models is driven primarily by the costs associated with ASTM E119 tests and a desire by some to be able to model the performance of concrete in actual fires. Increased measurement and reporting may be needed to validate these alternative test methods, and analytical computer models and simulations methods to correlate the results of alternative testing techniques and models are needed. These considerations are becoming increasingly more important due to recent efforts to refine existing and develop new methods for the design of buildings to resist fire, including being able to undergo total burnout without collapse.

Unbonded post-tensioned (PT) concrete slabs have been widely used in Canada and the United States since the 1960s, as they allow increased span-to-depth ratios and excellent control of deflections compared to non-prestressed reinforced concrete flexural members. The satisfactory fire performance of unbonded, PT concrete slabs in North America was established by a series of standard fire tests performed in the United States during the 1960s. However, there is a paucity of data on the effect of elevated temperatures on cold-drawn prestressing steel, both in terms of post-fire residual mechanical properties and high-temperature stress relaxation, which can lead to significant prestress loss both during and after a fire. To aid in the post-fire evaluation of PT concrete floors, a series of high-temperature residual tension tests on prestressing steel is presented, along with a second series of tests that illustrate the irrecoverable and significant loss of prestress force that may result from steel relaxation (creep) during a fire. A preliminary model is presented that can be used to predict the change in prestress force and allow for the computation of flexural capacity of a PT slab after a fire.