Explosive Spalling Of Concrete Research Articles

The influence of the physical and chemical properties of hardened cement paste on the heat‐induced explosive spalling of concrete

AbstractThis paper summarizes the results of two research projects that were carried out between 2016 and 2022 at the VDZ (Verein Deutscher Zementwerke) in Düsseldorf. The objective of the projects was to examine the influence of the cement type as well as the physical and chemical properties of the hardened cement paste on the explosive spalling of concrete in the case of fire. A particular focus was set on studying the way in which the concrete's moisture content contributes to the spalling phenomenon. The findings obtained for the different concretes examined in the dedicated experimental program were evaluated to better explain the mechanisms causing the heat‐induced explosive spalling of concrete.

Experimental investigation of post-fire seismic behavior of reinforced high-strength concrete shear walls

Reinforced high-strength concrete shear walls (RHSCSWs) are widely used for lateral load-resisting systems in tall buildings. Due to the high possibility of explosive spalling of high-strength concrete under high temperature, the structural performance of high-strength concrete shear wall may be greatly reduced after fire, which should be paid attention to by the engineering community. In this study, the post-fire seismic performance of RHSCSWs was investigated. Twelve RHSCSW specimens were fabricated and tested under an ISO-834 standard fire. Thereafter, 10 of these specimens were subjected to an in-plane cyclic load to evaluate their post-fire seismic performance. The remaining two specimens were subjected to concentric axial loads because they were severely damaged by explosive concrete spalling in the fire. The investigated parameters of the specimens included the concrete strength, fire exposure condition (one-side or four sides), fire duration, aspect ratio, axial load ratio, surface mortar layer, and strengthening technique (i.e., whether an X-shaped steel brace was installed). Their fire and post-fire structural performances were carefully assessed. The concrete walls with a cube compressive strength of 75.8 MPa were prone to explosive spalling in fire, whereas the concrete walls with a cube compressive strength of 60.3 MPa were not. Moreover, concrete shear walls with surface mortar layers were more likely to experience explosive spalling in fire than walls without such layers. With increasing axial load ratio, the horizontal load-bearing capacities of the shear walls after the fire first increased and then decreased, with the critical value being in the range 0.2–0.25. The specimens damaged by explosive spalling retained considerable axial load-bearing capacities (i.e., 69 % of the sectional concrete compression-bearing capacity of the unfired wall).

Comparative response of tiled finishes and bonded fire resistive coatings for normal weight concrete tunnel liners under high-intensity one-sided heating

Experimental tests were performed to examine the relative fire resistance provided by several bonded coatings for normal weight concrete (NWC) panels that emulate a tunnel liner: ceramic tiling on mortar, spray-applied fire resistive material (SFRM), and intumescent paint (IP). Sixteen panel specimens (150-mm [6-in.] thick, 457-mm [18-in.] wide, and 610-mm [24-in.] tall) were subjected to one-sided heating using a gas-fired radiant panel, which can apply heat flux at intensities similar to hydrocarbon fuel fire. All specimens were cast from a single batch of NWC (approx. 40 MPa compressive strength) with a single curtain of steel reinforcing bars at 38-mm (1.5-in.) cover to the heated face. All specimens were vertically loaded to a service level of compressive uniaxial stress (approx. 14% of ambient specified compressive strength), which remained relatively constant during each test. Four bare concrete panel specimens were tested as a control group, and a ceramic tile finish was installed on six specimens in accordance with a current state department of transportation specification. The other six specimens were coated with fire resistive material (three each with SFRM and IP) at thicknesses corresponding to a 1-hr fire resistance rating per manufacturer specifications. Thermally-induced explosive concrete spalling was observed in 3 of 4 control specimens, 4 of 6 tiled specimens, and none of the 6 SFRM/IP coated specimens. The tiled finishes delaminated and eventually became fully detached from the concrete surface, thus providing negligible fire resistance compared to that provided by the SFRM or IP. Temperature time histories recorded within the outer 25-mm (1.0-in.) thickness of the heated concrete were used to evaluate the relative thermal resistance of each surface finish, as well as to validate a simplified thermal numerical model used for a brief parametric study. Post-fire removal of the surface finishes is also demonstrated.

Enhancing the thermo-mechanical properties of calcium aluminate concrete at elevated temperatures using synergistic flame-retardant polymer fibres

This paper presents a systematic study on the feasibility of using synergistic flame-retardant polymer (SFRP) fibres for enhancing thermo-mechanical properties of calcium aluminate concrete at elevated temperatures. A series of tests were conducted to investigate the effect of SFRP fibres and cement type on thermal conductivity, compressive, splitting and flexural behaviours, and microstructural and micromechanical evolutions of concrete at different temperature levels, based on which the underlying mechanisms for enhancing the thermo-mechanical properties of calcium aluminate concrete were explored. Experimental results indicate that the combined usage of calcium aluminate blinders and SFRP fibres can significantly promote the strength-sustaining capacity (e.g., less than 45% compressive strength loss at 800 °C) and effectively prevent explosive spalling of concrete. The underlying mechanisms for enhanced thermo-mechanical properties of concrete can be explained by (i) the further reaction between hydration products and pozzolanic materials, which reduces the strength loss of concrete, and (ii) the melting, foaming and overflowing of SFRP fibres, which improves the connectivity of pore network and lowers the conductivity of concrete, and thereby improves the permeability and slows down the mechanical degradation of concrete.

Expansive spalling mechanism of concrete due to high temperature based on developed hygro-thermal-mechanical model by 3D-RBSM-TNM

Explosive spalling of concrete has been explained based on thermal stress and vapor pressure, and the mechanism of explosive spalling has been still discussed. In this study, the influence of several factors on the mechanism of explosive spalling due to high temperature was validated using the coupled analytical system: Rigid Body Spring Model-Truss Network Model (RBSM-TNM) based on hygro-thermal mechanical model. Truss Network Model can evaluate the heat transfer and vapor pressure transfer. The thermal stress and vapor pressure are applied to 3D-RBSM as an initial strain and initial stress, respectively. This proposed analysis can describe the complicated explosive spalling phenomena. Moreover, the accumulation of vapor pressure in the cracks and the change in mechanical property due to high temperature can be taken into account. In this paper, the influences of thermal stress, vapor pressure, vapor pressure transfer through cracks, and the change in mechanical property due to high temperature on the explosive spalling are precisely elucidated. As a result, it was confirmed that thermal stress causes the distributed cracks at the internal area while it does not involve the explosive spalling and a trigger of explosive spalling is the vapor pressure.

Analyzing the fire performance of concrete columns and slabs under loading and using options, preventing explosive spalling to ensure the pre-set fire resistance

Introduction. The authors focus on preventing the explosive spalling of concrete and the fireproofing of reinforced concrete structures. The relevance of this issue is explained by the insufficient number of fire tests of such structures under loading and thermal engineering calculations, needed for an objective analysis of testing results.Goal and objectives. The authors analyze the results of a series of fire tests, involving concrete columns and slabs with and without polypropylene microfiber, if no fireproofing is applied, as well as the results of the same tests involving the same items fireproofed by plates or plaster.Methods. The fire resistance of full-scale specimens of concrete was evaluated according to a standardized testing in a fire furnace under loading. It encompasses additional thermocouple measurements used to make a thermal engineering analysis. The analysis entailed both one- and two-dimensional problem formulations, methods and programmes for the numerical computation of non-stationary temperature fields in fireproof structures.Results. New data, obtained in the course of the fire experiments, show the efficiency of the polypropylene microfiber used to prevent the explosive spalling of concrete. The fire resistance limit is R 120 and R 150 under constant static loading. The fire resistance limit of similar structures, fireproofed by PROSASK Firepanel plates or IGNIS LIGHT plaster, was demonstrated. The specimens show the efficiency of methods and programmes for the one- and two-dimensional numerical analysis of non-stationary temperature fields in fireproof structures. The calculation results are presented for various fireproofing options.Conclusions. The testing results and their thermal analysis represent important items of information necessary to ensure the fire safety and the pre-set fire resistance of concrete structures under loading. They can also be used to outline the development pattern of this experimental and theoretical research project. The efficiency of thermal engineering calculations as a tool for evaluating fire protection parameters and the fire resistance of concrete structures is demonstrated, also as an option to reduce the number of expensive fire tests.

Experimental proof of moisture clog through neutron tomography in a porous medium under truly one‐directional drying

AbstractStructural failure of concrete buildings on fire and complete destruction of the monolithic refractory lining during their drying stage are dangerous examples of the effect of explosive spalling on partially saturated porous media. Several observations in both cases indicated the presence of moisture accumulation ahead of the drying front, which are in tune with the most common theories on the explosive spalling of concrete. Previous studies have shown evidence of the existence of this phenomenon, however, they were biased by artifacts and experimental limitations (such as the beam hardening effect and changes in the microstructure of the material due to the presence of pressure and temperature sensors). In the current work, rapid neutron tomography was used to investigate the in‐operando drying behavior of a high‐alumina refractory castable, proposing a novel experimental layout aimed at a truly one‐dimensional drying front. This setup provided more realistic boundary conditions, such as the behavior of a larger wall heated from one of its sides, while also preventing some nonphysical artifacts (notably beam hardening). By eliminating these aspects, a direct proof that moisture accumulates ahead of the drying front was obtained. This work also lays the basis for further studies focusing on the response sensitivity analysis to boundary conditions and other parameters (e.g., heating rates and properties of the sample related to the moisture clog formation), as well as useful data for the validation and characterization stages of numerical models of partially saturated porous media.

Experimental investigation on fire resistance of high-strength concrete encased steel composite columns

This paper presents the experimental findings on fire resistance of concrete encased steel (CES) composite columns made of C120 concrete and S500/S690 high-strength steel section. Eight full-scale CES columns were tested under concentrated load and heated under the ISO834 fire until failure. The fire performance of each CES column specimen was evaluated by analysing the fire resistance time, axial displacement-time curve, temperature-time curve, rotational angle of end support, post-fire conditions of the concrete surface and failure mode. The experimental study showed that the addition of polypropylene fibre was effective in minimizing explosive concrete spalling in high-strength CES columns to achieve comparable fire resistance time as those of normal-strength CES columns. A database containing 40 CES specimens was established to assess the accuracy of the current design methods in predicting the fire resistance of CES columns. The limitations of each design method were discussed based on whether they can provide safe and accurate predictions of the fire resistance of tested CES column specimens. Finally, recommendations were provided to modify the current methods for fire resistant design of CES columns made of high-strength concrete.

Experimental and numerical study of temperature developments of composite joints between concrete-encased concrete-filled steel tube columns and reinforced concrete beams

Previous studies of concrete-encased concrete-filled steel tube (CFST) columns have demonstrated their enhanced fire performance due to the passive protection of the outer encasement. To facilitate the design and application of concrete-encased CFST structures, the fire performance of full-scale reinforced concrete beam to concrete-encased CFST column joints subjected to full-range fire including heating and cooling phases was studied experimentally. This paper presents the experimental programme and reports the experimental results related to temperature. Finite element analysis (FEA) models of varying complexities were also incorporated to complement the fire tests and to better interpret the temperature developments. Results show that beam failure was observed for all tested specimens. Concrete explosive spalling was insignificant due to the low strength of the outer concrete. The temperature of the joint zone was tremendously reduced compared to the connected members due to the passive protection of outer concrete and the heat sink effect. For the region close to the steel tube, the temperature-time curves showed a significant moisture plateau. The FEA models produced satisfactory temperature predictions when the thermal properties in Eurocode 4 were used.

Neural network models to predict explosive spalling of PP fiber reinforced concrete under heating

Predicting explosive spalling of concrete containing polypropylene (PP) fibers at elevated temperature is a challenging problem. It is difficult for traditional FEM or DEM methods to tackle this problem at the moment due to unclear mechanism of PP fibers in mitigating explosive spalling and difficulty to measure instantaneous hot permeability of concrete. This paper describes development of two Artificial Neural Network (ANN) models for assessment of explosive spalling risk of concrete. One model (ANN1) was concrete mix-based, the other model (ANN2) was concrete strength-based. A total of 306 and 300 test records collected from literature were used to train ANN1 and ANN2, respectively. Twenty groups of heated tests were conducted on high performance concrete and ultra-high performance concrete containing PP fibers to validate ANN1 and ANN2. The two ANN models were successfully trained and validated, with a prediction accuracy of 100% and 90% for ANN1 and ANN2, respectively. Excellent prediction performance demonstrated the feasibility of ANN models for predicting explosive spalling tendency of PP fiber reinforced concrete.

Residual properties of three-span continuous reinforced concrete slabs subjected to different compartment fires

This paper presents an experimental study on the post-fire residual behavior of continuous reinforced concrete (RC) slabs. The mechanical performance of five post-fire continuous RC slabs is investigated, including load-deflection curves, concrete and reinforcement strains, cracking patterns, and failure modes. The results indicate that the residual material properties of heated compartments and concrete spalling significantly affect the ultimate load and failure mode of the fire-damaged continuous RC slabs. The deflection failure criterion is suitable for determining the ultimate state of each span in fire-damaged continuous RC slabs. Apart from the flexural failure mode, punching shear failure also occurred in the fire-damaged continuous slab, particularly in the span with considerable explosive concrete spalling. Compared with the edge spans, the middle span in the continuous slab tends to exhibit better ductility performance owing to continuity at both supports of the span. In addition, several theoretical methods are used to estimate the residual performance of the tested slabs. The reinforcement strain difference and ACI methods are proposed to help predict the residual limit loads of the post fire continuous RC slabs.

Experimental studies on continuous reinforced concrete slabs under single and multi-compartment fires with cooling phase

This paper experimentally investigates the effect of number and position of heated compartments on the fire behaviour of continuous reinforced concrete slabs. A total of five continuous three-span slabs are tested under single compartment fire, two-compartment fires and three-compartment fires. The measurements involve furnace temperatures, slabs’ temperature, displacements, restraint forces, cracks and failure modes. The results show that the cracking pattern, displacement and reaction force of continuous slabs greatly depend on the furnace temperature, the position and number of heated compartments. Transverse cracks and short cracks appear on the top and bottom surfaces of the heated compartments, respectively, and the number of cracks increases with increasing heated compartments. The heated middle compartment or the unheated edge compartments may experience upward or downward deflections. Explosive concrete spalling is observed in the tests, which affects the failure mode of slabs and water evaporation from the unexposed surface. The selection of the worst fire scenario depends on the furnace temperature, the layout of heated compartments and concrete spalling. The conventional failure criteria (temperature or deflection failure criteria) can result in conservative or unconservative predictions of fire resistance of continuous slabs, which should be improved to include the favourable effect of structural continuity.

Empirical model for determining fire resistance of Reinforced Concrete columns

Fire resistance of Reinforced Concrete (RC) columns is affected by explosive spalling of concrete. Existing models for determining fire resistance of reinforced concrete columns do not generally account for the effects of spalling. The data from tests conducted in this study and other studies available from the literature have been used to develop an empirical model. This study attempts to develop a model for determining fire resistance of reinforced concrete columns by incorporating the influence of various key parameters including pre-mature spalling of concrete during fire. The model also incorporates the effect of local buckling of longitudinal reinforcement triggered by explosive spalling. The model further incorporates the role of reinforcement configuration in determining fire resistance. A standard equation is developed for a full ISO-834 fire scenario in RC columns. The model is seen to predict greater than 75% safe results. These are better predicted for range of influencing parameters.

A Simplified Model to Predict Thermo-Hygral Behaviour and Explosive Spalling of Concrete

A Simplified Model to Predict Thermo-Hygral Behaviour and Explosive Spalling of Concrete

Effect of PVA/SiO2 NPs Additive on the Structural, Durability, and Fire Resistance Properties of Geopolymers

This exertion introduces polyvinyl alcohol fiber/silica nanoparticles (poly vinyl alcohol (PVA)/SiO2 NPs) in the fly ash-based geopolymer at ambient curing temperature. The present study aims at investigating the structural properties (compressive, bond strength, fracture parameters (fracture toughness (KIc), crack mouth opening displacement (CMOD)), cyclic compression), durability (freeze-thaw), and fire resistivity of the newly developed PVA/SiO2 NPs mediated geopolymer. The outcomes suggest that geopolymers incorporated with 5% PVA fibers showed improved structural properties and durability as compared to other specimens. Investigation on the fire resistivity of the geopolymers exposed to different heating temperatures (400 °C, 600 °C, 800 °C), showed that geopolymers with PVA/SiO2 NPs significantly prevented the explosive concrete spalling. Microstructural studies confirmed that PVA fibers in the geopolymeric matrixes were well distributed and developed a fiber-bridging texture with improved performance. Addition of the nano-silica particles accelerated the heat evolution during the hydration process and the geopolymeric reaction (formation of sodium aluminosilicate N-A-S-H gel) at ambient curing environment.

Effects of geometry and fraction of polypropylene fibers on permeability of ultra-high performance concrete after heat exposure

This paper investigated the effect of geometry and fraction of polypropylene (PP) fiber and the induced microcracks on permeability of ultra-high performance concrete (UHPC) subjected to elevated temperature. Residual permeability and microcrack networks of fifteen UHPC mixes were characterized and an analytical model correlating residual permeability of UHPC with fiber fraction and geometry was proposed. Results showed that increasing fiber length and dosage had much stronger effect than increasing fiber diameter on enhancing permeability due to greater enhancement on percolation of fiber tunnels. It was found that permeability of UHPC is positively correlated with both the aspect ratio and dosage of PP fibers. However, at low fiber aspect ratio, increased fiber dosage does not increase the permeability of UHPC. Similarly, at low fiber dosage, solely increasing fiber aspect ratio does not contribute much to increasing permeability of UHPC. The proposed model thus provides insight for PP fiber selection and optimization to prevent explosive spalling of concrete.

Polypropylene Microfibers Against Explosive Spalling of Concrete. Strength Properties of Concrete with Fiber

Polypropylene Microfibers Against Explosive Spalling of Concrete. Strength Properties of Concrete with Fiber

Tests on the Steel–Concrete Bond Strength in Steel Reinforced Concrete (SRC) Columns After Fire Exposure

Bond behaviour between the steel and concrete is important in ensuring the interaction of different components in steel reinforced concrete (SRC) columns under various working conditions, and sufficient bond strength should be kept in these columns even after fire exposure. This paper presents the experimental results of bond strength between the embedded steel and concrete in SRC columns after fire exposure. A total of eight push-out specimens were prepared, including two unheated and six fire exposed specimens. The fire exposed specimens were heated and left cooling down in a fire furnace, and then were tested for residual bond strength. Concrete explosive spalling was found when the test specimens were heated. The elevated temperatures at the steel–concrete interface and at the location of stirrups were recorded by thermocouples. Effects of different parameters, such as fire scenario, lateral confinement, and encased steel type, on the bond behaviour were analysed. Time delay was found in the temperature increase across the concrete section, and the inner maximum temperature reached at a measurement location was 430°C during the cooling phase, which was much higher than the temperature of 180°C measured at the end of heating phase for the same location. The reduction in bond strength was as high as 54.2%, whereas the corresponding ultimate slip increased significantly from around 0.3 mm to over 4 mm due to fire exposure. An existing model was revised to predict bond stress-slip curves for SRC columns after fire exposure, and a reasonably good agreement was obtained between the measured and predicted results.

Analysis of fire resistance of concrete with polypropylene or steel fibers

The decrease in concrete resistance and the expansion generated in reinforced concrete structures by direct exposure to fire at 400°C maximum temperature serves as the basis for the present research. The aim is to improve these problems by the addition of steel fibers or of polypropylene fibers in concrete. From the results analysis of compression fracture tests on cylindrical concrete specimens, it can be concluded that concrete with addition of polypropylene fibers or steel fibers are a good alternative to traditional concrete, because both its strength, and its behavior in case of fire are improved, delaying the appearance of fissures and explosive concrete spalling.

Performance of Steel-Reinforced Concrete Column after Exposure to Fire: FEA Model and Experiments

This paper studies the postfire performance of steel-reinforced concrete (SRC) columns subjected to an entire loading and fire phase, including ambient temperature loading, heating and cooling with constant external loads, and postfire loading. A sequentially coupled thermal-stress analysis module was employed in a finite-element analysis (FEA) program to establish temperature field analysis and structural-analysis models of SRC columns subjected to the entire loading and fire phase. To improve the precision of the FEA model, the influence of fire-induced concrete explosive spalling (CES) was considered by using element change technique. Besides the existing fire-resistance test data of SRC columns, to further validate the accuracy of the FEA model in the postfire phase, a set of fire tests on SRC columns, including a fire-resistance test and postfire test subjected to the entire loading and fire phase, were conducted. Compressive failure and bending failure were observed in the test. Comparison between predicted and test results indicates that the accuracy of the FEA model was acceptable, and then the FEA model was extended to simulate a full-scale SRC column in a real project subjected to the entire loading and fire phase. The temperature distribution, load versus axial deformation relations, and load redistribution of the full-scale SRC column during the entire loading and fire phase were investigated. It was found that the postfire peak load of the calculated column can be 26.7% lower than that at ambient temperature. The load carried by shaped steel rises from 20% to the peak value of 93% during the heating and cooling phase, indicating the significant contribution of shaped steel to resist the external loads during fire exposure.