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Reinforced concrete columns exposed to standard fire: comparison among different constitutive models for concrete at high temperature
Full text linkConcrete behaviour at high temperature was investigated in depth since the 1970s, in order to highlight the main issues linked to its mechanical performance in hot conditions, such as chemical processes, kinematic behaviour (transient and creep strains) and evolution of the physico-mechanical properties. Thanks to these studies, a few constitutive models have been proposed in the literature for concrete at high temperature, with the aim of modelling reinforced concrete structural behaviour during heating. Within this context, a Beam Finite Element code for thermo-mechanical analyses has been developed by using a Fortran solver and GID as pre- and post-processor. A number of well-documented full-scale tests on reinforced concrete columns exposed to Standard Fire (without cooling) was simulated numerically, by implementing four different constitutive models proposed in the literature for concrete at high temperature. The main goals are: to highlight the role of some critical aspects regarding reinforced concrete members in hot conditions, in particular second-order effects, transient and creep strains (a), and to make a systematic comparison between numerical and experimental results in order to assess the reliability of both 1D numerical modelling (b) and the adopted constitutive models for concrete (c). The results confirm that 1D numerical modelling is generally consistent with the experimental evidence if transient and creep strains, as well as second-order effects are carefully taken into account. Moreover, the differences among the four investigated models for concrete behaviour in compression are quite limited
FIRE PERFORMANCE OF R/C AND FRC TUNNELS EXPOSED TO FIRE: COMPARISON BETWEEN LINEAR AND NON LINEAR ANALYSES VIA A SIMPLIFIED 1D APPROACH
No full textThe mechanical response of reinforced concrete tunnels (R/C) exposed to fire can represent a critical issue in the design phase of this kind of infrastructures, due to the combination of a few negative aspects such as the development of severe fire scenarios, the development of sizable indirect actions and the severe compression state which can foster spalling phenomenon. It follows the need for a reliable evaluation of the fire performance of tunnels exposed to high temperature, considering the decay of the material properties and the evolution of the internal actions. This task is however often complex, since it can need the implementation of non-linear analyses which consider the diffusion of heat in the structural elements, the variation of the mechanical properties of the materials and the interaction between lining and surrounding soil. Such analyses are often performed by means of advanced finite element codes which can perform multi-physics simulations (as for example Abaqus or Safir). On the other hand, in the present study, a simplified approach is described for linear and non-linear analyses of deep R/C tunnels exposed to fire, based on the main assumption of axisymmetric loading and heating. This assumption is generally kept for deep tunnels, in which the ratio between vertical and horizontal pressure is usually not too far from the unit value. The assumption of axisymmetry makes it possible to describe the behavior of the lining via a sectional analysis (1D approach), in which the plane section assumption is kept. Such algorithm can be rather easily implemented in any work sheet or programming code (as for example Matlab, Fortran, or similar). This simplified approach allows to rapidly perform parametric analyses necessary for understanding the role played by different key parameters, and to understand the advantages of performing a non-linear analysis allowing the plasticization of the section
SPALLING AND TENSION STIFFENING IN HEAT-EXPOSED MEMBERS MADE OF SELF-COMPACTING CONCRETE
No full textHeat-induced damage, surface scaling and cover cracking always occur ??? with different severity - in R/C members subjected to a fire. Hence, bar-concrete bond in general and cover integrity in particular are at risk in fire, with consequences that can span from the loss of anchoring ability and tension stiffening to cover spalling, followed by the exposure of the reinforcement to the flames. In this research project, concrete spalling and tension stiffening are addressed, the attention being focused on corner spalling in reinforced and unreinforced prismatic specimens in the former case, and on reinforced tension members in the latter case. An experimental procedure to assess concrete sensitivity to spalling is proposed as well. Though very different in their nature, concrete spalling and tension stiffening are investigated within the same project, because (a) the same prismatic specimens are firstly subjected to a rather severe temperature-time ramp to provoke spalling and later to sustained high temperature to induce a generalized damage in the concrete; and (b) the bars placed in the corners are the most exposed to the risk of spalling and to the loss of bond, to the detriment of tension stiffening. Twenty one prisms made of SCC (target strength fc = 50, 80 and 90 MPa) were
put inside an electric furnace at 750°C to investigate spalling (9 unreinforced and 11 reinforced with a single bar totally embedded in the concrete). Later, the reinforced prisms were cleared of the concrete at their extremities, to allow the application of opposite tensile forces and to investigate tension stiffening. On the whole, the spalling tendency was rather weak, with 10% (20%) of the specimens exhibiting severe (light) spalling, while residual tension stiffening appeared to be still effective in only 50% of the reinforced specimens, where bond stiffness exhibited a roughly linear relationship with the residual compressive strength of the concrete
Modeling R/C Columns in Fire according to Different Constitutive Models for Heated Concrete
No full textSeveral analytical formulations aimed to describe concrete behavior at high temperature have been proposed in the past two decades, all of them taking into ac-count transient and creep strains, either explicitly or implicitly. The main objective of this paper is to compare some of the different models available in the literature, with reference to the structural behavior of long R/C columns exposed to fire, since these structural members are rather sensitive to thermally-induced deformations. To this end, a number of significant examples based on real-scale fire tests, characte-rized by well-defined geometry, loads and boundary conditions, were simulated numerically, by adopting different formulations for concrete behavior in compres-sion. In terms of displacements and time to failure, the comparisons clearly show that by using accurate constitutive laws (including – implicitly or explicitly – all strain components) a correct evaluation of the time to failure is possible. At the same time, no significant differences are observed among the various models, which means that the development of further models is not fully justified
Fire spalling sensitivity of high-performance concrete in heated slabs under biaxial compressive loading
Full text linkExplosive spalling of concrete exposed to fire consists in the violent expulsion of shards from the hot surface due to the interaction between cracking and pore pressure build-up. Fire spalling relevantly increases the overall thermal damage of a structure exposed to fire, thus leading to much higher costs in the repair intervention, and in some cases it can even jeopardize the structural stability due to loss of reinforcement protection and reduction of the bearing cross-sections. High-performance concrete is particularly sensitive to spalling phenomenon due to inherent material features, such as the unstable fracture behaviour and the low permeability (favouring high values of pore pressure). In this context, an experimental campaign has been carried out on high-performance concrete (fc ≈ 60 MPa with silico-calcareous aggregate), without or with one of three different fibre types (steel fibre, monofilament or fibrillated polypropylene fibres). Tests were performed by means of a special test setup developed at Politecnico di Milano, based on slabs (800 × 800 × 100 mm) subjected to Standard Fire at the bottom and to biaxial compressive loading in the mid-plane, while monitoring pore pressure, temperature and deflection. Explosive spalling was observed in both plain concrete slabs and in one of the two slabs with steel fibre, this casting some doubts on the use of steel fibre alone against spalling. No detachment was observed when polypropylene fibre was added to the mix
Corner spalling and tension stiffening in heat-damaged R/C members: a preliminary investigation
Full text linkCorner spalling in fire and tension stiffening past a fire are addressed in this paper by investigating reinforced and unreinforced prismatic specimens in the former case, and by testing reinforced tension members in the latter case. An experimental procedure to asses concrete sensitivity to spalling due to pore pressure is proposed as well. The focus is on the material as such, in order to allow different cementitious mixes to be compared in terms of spalling sensitivity under realistic conditions regarding the thermal gradients, the pore pressure and the moisture transfer (thermal and load-induced stresses are not a primary concern). Concrete spalling and tension stiffening are investigated within the same project, because (a) a bar—whose cover has been subjected to the risk of spalling during a fire—exhibits reduced tension stiffening, and (b) the same prismatic specimens were used (firstly thermal shock at 750 °C to provoke spalling and later rest at high temperature to investigate tension stiffening past cooling). Since only 10–20 % of the twenty-one specimens suffered medium/light spalling, nearly all the reinforced specimens were later tested to investigate tension stiffening in residual conditions. The prismatic specimens were made of SCC (target strength fc = 50, 80 and 90 MPa). The results show that (a) during a thermal shock the spalling tendency is rather weak if no load-induced stresses and/or thermal self-stresses occur, (b) tension stiffening appears to be still effective in 50 % of the reinforced specimens and (c) bond stiffness is a roughly linear function of the actual compressive strength of the concrete, be it heat damaged or not
STRUCTURAL PERFORMANCE OF A LARGE-SPAN BI-DIRECTIONAL PARTIALLY PRECAST WAFFLE SLAB SYSTEM UNDER FIRE EXPOSURE
No full textThis paper describes the evaluation of the structural performance under fire exposure of a partially precast waffle slab system employed in the 1970s for the construction of residences in high-mountain skying stations in the Italian Alpine arch. Full information is available about this structural arrangement, including geometry and reinforcement shop drawings, and material properties, adjuvated by on-site non-destructive detection tests. The bi-directional waffle slab system at study was not designed according to any specific rule or calculation concerning fire resistance. The performance of the slab system under fire exposure is investigated through numerical analysis employing non-linear equivalent beam elements under the standard ISO 834 temperature-time exposure, where sectional non-linear temperature distribution and moment-curvature diagrams are evaluated separately and attributed to the numerical model. The modelling is aimed at evaluating the structural performance of the waffle slab under fire exposure with an advanced method taking into account of indirect actions caused by the temperature rise in this highly-statically-undetermined structural arrangement, cheking the actual safety as well as stress and deformation profiles of the slab members under different time exposures. The simulation also aims at assessing the benefits brought in by structural redundancy and stress/force redistribution capacity, this being of primary importance for structures not designed to withstand the recent provisions in case of fire
Preliminary results on tension stiffening in heat-exposed R/C tension members
No full textTension stiffening and - more generally – bond has attracted scanty attention in the recent past with reference to fire and high temperature, probably because of scholars’ and designers’ opinion that sectional collapse due to the strength loss in the rebars be more critical than the overall collapse due to bond loss. (As a matter of fact, bond losses involve the length of the reinforcement and not a single section, as bending). However, since many R/C structures survive a fire and require their residual safety level to be assessed, knowing bond properties and having information on tension stiffening in residual conditions is a necessary step to reliably analyze the structural behavior past a fire.
In this research project, tension stiffening is investigated by testing a number of reinforced tension members initially exposed to a rather severe fire (750°C) to investigate concrete spalling. Later, the specimens were kept at 750°C for a time length sufficient to have a uniform thermal field in excess of 600°C, in and around the bar. An electric furnace pre-heated to 750°C was used.
After the removal from the furnace, each specimen was tested by applying opposite forces at the end sections, to check the effectiveness of tension stiffening in residual conditions. The total number of specimens in this preliminary experimental campaign was 11. Three self-compacting mixes were used (fc = 50, 80 and 90 MPa). Only 5 specimens showed a non-negligible tension stiffening, and the justification of such weak effect was found in the marked bond sensitivity to high temperature, something that was also checked by means of a rather simple slip-based model
A new test method to study the influence of pore pressure on fracture behaviour of concrete during heating
No full textFracture behaviour of concrete at high temperature is one of the factors governing explosive spalling, namely the expulsion of chunks due to both pressure build-up in the pores and stress induced by thermal gradients and external loads. In this context, a special experimental setup has been developed aimed at performing simple indirect-tension tests under different levels of sustained pore pressure. A cubic specimen is heated on two opposite faces, whereas the lateral sides are sealed and thermally insulated, so as to instate a mono-dimensional thermo-hygral transient field. In the splitting test, fracture develops along the symmetry plane, where both temperature and pressure are monitored by means of a customized probe. The results show that pore pressure has a significant influence on the mechanical response of heated concrete, though the concurrent contribution of external load and thermal strain is required for triggering explosive spalling
Heated slabs under biaxial compressive loading: a test set-up for the assessment of concrete sensitivity to spalling
No full textExplosive spalling of concrete members in fire is the violent expulsion of shards from the exposed face caused by the combined effect of pressure build-up in the pores due to water vaporization and of in-plane stress induced by both external loads and thermal gradients. Spalling progression leads to the reduction of bearing cross-section and often to the direct exposure of rebars to the flames. Since established predictive models are not available yet, experimental studies appear to be the most effective means of investigation on this phenomenon. To this purpose an experimental setup has been developed for the assessment of concrete sensitivity to spalling. The specimen is a concrete slab (800Â ÃÂ 800Â mm) with a thickness comprised between 100 and 200Â mm. The bottom face is heated via a horizontal furnace, in which a propane burner is actively controlled in order to follow the prescribed fire curve. During heating, a biaxial compressive load can be applied thanks to hydraulic jacks restrained by a steel frame. Load and slab thickness can be adjusted in order to represent the mechanical conditions achieved in the hottest region of thicker concrete members such as tunnel lining segments. The setup proved to be very effective in comparing spalling sensitivity among different concrete mixes, as is often required in initial material testing for strategic infrastructures such as tunnels
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