International Concrete Abstracts Portal

This CD-ROM contains 10 papers that were presented at sessions sponsored by Joint ACI-TMS Committee 216 at the ACI Fall 2008 Convention held in St. Louis, MO, and the ACI 2010 Spring Convention in Chicago, IL. The papers present some of the latest research findings on the fire performance of concrete. They provide research results from both experimental and numerical studies on various aspects, ranging from high temperature material properties to advanced computer models for tracing the fire response of reinforced concrete structural members. Note: The individual papers are also available. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP-279

Within the last years, high- and ultrahigh-performance concrete has increasingly been used, especially for prefabricated elements. Due to its low porosity, high- and ultrahigh-performance concrete may show extensive explosive spalling in case of fire, leading to loss in cross-section and reducing the load carrying capacity significantly. The paper presents the results of an extensive testing program performed on the spalling behavior of high- and ultrahigh-performance concrete using three different concrete mixtures. The heating rate was selected as the main parameter that was systematically varied. Water evaporation was noticed to have a crucial influence on explosive spalling due to a high pore pressure. Furthermore, two different types of spalling where observed. High heating rates led to spalling close to the heated concrete surface. Low heating rates caused spalling initiated from the core. As simplification it is proposed to model concrete as “a pressure cooker”. The analysis of the test results indicates that the simplified model may become an easy to apply tool for the prediction of spalling.

A macroscopic finite element model for tracing the fire response of reinforced concrete (RC)columns is presented. The model accounts for critical factors such as fire induced spalling, various strain components, high temperature material properties, restraint effects, different fire scenarios and failure criteria, in evaluating fire resistance of RC columns. The validity of the model is established by comparing the predictions from the computer program with results from full-scale fire resistance tests. Parametric studies are performed to study the sensitivity of model predictions to different discrtization and meshing parameters and based on the results an optimum mesh size and time increment for fire resistance analysis of RC columns is recommended. Through case studies it is shown that macroscopic finite element models are capable of predicting the fire response of RC columns over a wide range of parameters as encountered in practical situations.

Recent studies indicate that high strength concrete is more susceptible to explosive spalling under high temperatures. However, more research is required to support this conclusion preferably on large scale structural elements to produce more realistic results. The first part of this paper presents the outcomes of an experimental investigation to study the fire resistance and performance of full-scale simply supported high strength concrete slabs subjected to the ISO834 fire curve. Three high strength concrete slabs 13920 psi (96 MPa) and 3 normal concrete slabs 6528 psi(45 MPa) were involved in the study. Each slab was loaded with 60% of the EC2 design load and was heated from the bottom side only. The second part of the paper presents a finite element study on the fire resistance of concrete slabs. A three-dimensional model of the reinforced concrete slabs taking into account exposure to high temperatures, steel reinforcement and cracks propagation was built. The model was validated using the tests’ results and a good correlation was obtained. The validated model was used to conduct parametric analyses to study the effect of loading level, heating rate (ISO834, Hydrocarbon and RWS fire curves) and axial restraint on the fire resistance of concrete slabs including an assessment of the thermal-mechanical stresses in the slabs.

In this study, effect of various heating and cooling regimes on the residual compressive stress-strain behavior of confined high strength concrete is investigated. To this end, a total of 57 hoop confined and 21 unconfined high strength concrete cylindrical specimens of size 5.90 x 17.71 inch (150 mm × 450 mm) were tested. The specimens were exposed to seven different temperatures ranging from room temperature to14720F (8000C). Two different heating rates (50C/minute and 150C/minute) and two cooling rates (natural air cooling and water quenching) were employed in the study. Measurements were taken for thermal gradient, spalling and residual axial load-displacement properties of confined high strength concrete. Test results indicate that the residual strength, strain corresponding to the peak stress and the post-peak strains of confined high strength concrete are affected only in the temperature range of 9320F to 14720F (500 to 8000C). Experimental results show that faster rate of heating does not have any detrimental effects on the residual behavior of confined concrete. Lesser thermal induced effects were noticed in specimens exposed to faster rate of heating than in specimens subjected to slower rate of heating irrespective of the temperature of exposure. However, the results suggest that cooling with water quenching has adverse effect on the residual stress-strain properties of confined high strength concrete. Compared with natural cooling, thermal shock induced by water quenching caused more severe damage to confined high strength concrete, in terms of greater losses in confined concrete strength and increase in strains. The results of this study may be useful for designing the confining reinforcement of reinforced concrete columns against the multiple hazards of fire and earthquake.