Bond in RC structures at high temperature and in fire: lessons from the past and hot issues still open to investigation

High temperatures and fire are definitely among the various exceptional load situations RC structures are required to resist, as demonstrated by the extensive research activity performed so far, from the behavior of cementitious materials to that of single members and entire structures. Reinforcement-concrete bond, however, has become a hot issue at a relatively late stage, with an acceleration in the new millennium. The objective of this paper is to recall some of the major issues treated in the literature since the last conference “Bond in Concrete” (Brescia, Italy, 2012) and still open to investigation, such as: (1) bond micromechanics in fire; (2) bond stress-slip law as a function of the temperature; (3) the role of polypropylene, steel and hybrid fibers; (4) tension stiffening in fire; and (5) bonded fasteners in fire. This reexamination takes advantage of the tests performed in Milan in the last decade on bond-stress distribution along anchored bars in fire, pull-out vs. splitting failures, in-fire capacity of postinstalled fasteners and tension stiffening at high temperature. Only by improving the knowledge – and the modelling - of the basic resisting mechanisms, bond included, today’s refined FE codes will provide rational structural responses based on clearly-recognizable contributions

Some key issues concerning RC fire design

none1P. GambarovaGambarova, PIETRO GIOVANN

FRC at high temperature: recent advancements in the knowledge of thermal and mechanical properties

none1P. GambarovaGambarova, PIETRO GIOVANN

Properties of Concrete Subjected to Extreme Thermal Conditions

Durability, high-temperature resistance, impact and blast resilience, radiation-shielding properties, irradiation endurance and – of course – good mechanical properties are required of the cementitious composites to be used in a variety of high-performance structures. Among these, tall buildings, road and railway tunnels, off-shore platforms, gasification plants, wind and solar mills for the production of “clean” energy should be mentioned, as well as nuclear power plants, and radioactive- and hazardous-waste repositories. Hence, understanding, measuring and modelling concrete behavior under extreme environmental conditions is instrumental in making concrete structures safer and more efficient. To this end, the hot and residual properties associated with the exposure to high temperature, fire and thermal shock are treated in this paper. Reference is made to ordinary vibrated concrete (Normal-Strength Concrete - NSC), as well as to a number of innovative cementitious composites, such as Fiber-Reinforced Concrete - FRC, High-Performance/High-Strength Concrete - HPC/HSC, Ultra High-Performance/Very High-Strength Concrete - UHPC /VHSC, Self-Compacting/Consolidating Concrete - SCC, Light-Weight Concrete - LWC, shotcrete and high-strength mortars. It is shown that these materials can be “tailored” according to a variety of requirements and functions, even if several aspects of their behavior (like spalling in fire and long-term mechanical properties under sustained high temperature) are still open to investigation

High-Bond Bars in NSC and HPC: a Study on Size Effect and on the Local Bond Stress-Slip Law

Size effect is studied here with reference to the bonding of short, deformed bars, embedded in normal-strength concrete (NSC) and high-performance concrete (HPC). Tests on 48 cylindrical specimens reinforced with a single bar and subjected to a pull-out or push-in force show that bond exhibits a strong size effect, which is well described by Bazant's general-type size-effect law. Four diameters are considered (db =5, 12, 18, and 26 mm), with bonded length-to-bar diameter ratios equal to 3.5 (HPC) and 5 (NSC). All specimens are highly confined by means of a steel jacket to prevent or control cover splitting and to investigate bond behavior in highly confined conditions. Test results on short, anchored bars were instrumental in working out the local bond stress-slip law, taking into account size effect, which appears in the formulation of the maximum bond stress through the bar diameter. Short embedments also prevented bar yielding. The proposed local bond stress-slip law (1) is formulated as an extension of the law suggested in European Model Code MC90, also including some later proposals; (2) fits quite well with the available test data on short, well-confined anchored bars; (3) introduces the favorable effects that the confining reinforcement (generally consisting of stirrups and longitudinal bars) has on bond strength; and (4) may be easily introduced into a design code