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.

Novel concrete elements are emerging utilizing high performance self-consolidating concrete (HPSCC) reinforced with high-strength, lightweight, and non-corroding carbon fiber reinforced polymer (CFRP) prestressed reinforcement. The fire performance of these elements must be understood before they can be used with confidence. In particular, the bond performance of the novel CFRP reinforcement at elevated temperatures requires investigation. This paper examines the bond performance of a specific type of CFRP tendon as compared with steel prestressing wire. The results of transient elevated temperature bond pullout and tensile strength tests on CFRP tendons and steel prestressing wire are presented and discussed, and show that bond failure at elevated temperature is a complex phenomenon which is influenced by a number of interrelated factors, including the type of prestressing, degradation of the concrete, CFRP, and steel, differential thermal expansion, thermal gradients and stresses, release of moisture from the concrete, and loading. It is shown that CFRP tendons are more sensitive to bond strength reductions than to reductions in tensile strength at elevated temperature.

An earlier paper demonstrated that the cover requirements given in ACI 216.1 will not consistently lead to adequate fire resistance of simply supported, unrestrained, slabs, with current load factors and reinforcement materials. This study leads to a suggested replacement for Table 2.3 of ACI 216.1. Greater cover thicknesses would be required for all fire exposure times beyond one hour, with significant increases for the uncommon design case of 4 hours required fire resistance.

Fire safety is a critical criterion for designing reinforced concrete (RC) structures. As new design codes are moving towards performance-based design, analytical tools are needed to help engineers satisfy code criteria. These tools are also needed to assess the fire performance of critical structures. As full scale experiments and finite element simulations are usually expensive and time consuming options for designers to achieve specific fire performance, a simplified sectional analysis methodology that tracks the axial and flexural behavior of RC square sections subjected to elevated temperatures from their four sides was previously developed and validated by the authors. In the first part of this paper, the proposed methodology is extended to cover rectangular beams subjected to standard ASTM-E119 fire from three sides. An extensive parametric study is then conducted to study the distribution of the concrete compressive stresses at different ASTM-E119 fire durations. Based on the parametric study, simple equations expressing the equivalent stress-block parameters at elevated temperatures are presented. These equations can be utilized by designers to accurately estimate the flexure capacity of simply supported and continuous beams exposed to fire temperatures.

This paper presents an experimental study on the fire behavior of seven restrained RC beams strengthened with carbon fiber sheet (CFS) and provided with fire insulation. The influence of some parameters (i.e., axial and rotational restraint stiffness, thickness of the fire insulation, and load ratio) on the deformations and internal forces of the beams is analyzed. The test results indicate that: (a) for a restrained beam in fire, the maximum axial elongation decreases with increasing stiffness of the axial restraint, and increasing thickness of the fire insulation, while the maximum additional axial force increases with increasing stiffness of the axial restraint, and decreasing thickness of fire insulation. The maximum additional bending moment at the beam ends increases with decreasing thickness of the fire insulation, but on the whole the influence of the thickness of the fire insulation within the investigated range (10~20 mm [0.394~0.787 in]) is rather limited; (b) the additional axial force in a restrained beam recovers slightly during the cooling phase, while the additional bending moment at beam end of a restrained beam recovers significantly in the cooling phase; (c) the peak value of the bending moment of a strengthened and insulated beam occurs much later than in the case of an ordinary RC beam (i.e., without strengthening and fire insulation), while the maximum additional bending moment at beam end is lower; and (d) the influence of the rotational restraint stiffness on the maximum additional bending moments at beam ends is rather limited.