Fire Spalling Research Articles

Effects of Recycled Steel and Polymer Fibres on Explosive Fire Spalling of Concrete

Modern high-performance concrete, increasingly used in tunnels and other important infrastructure, is susceptible to explosive fire-induced spalling. To prevent fire spalling, modern codes recommend the use of small quantities (e.g. 2 kg/m3 as recommended by the Eurocodes) of polypropylene fibres in the concrete mix. This paper presents an experimental study investigating, the effect of cleaned recycled fibres extracted from end-of-life tyres on the explosive fire-induced spalling of concrete. This paper presents 24 spalling tests, indicating that recycled tyre polymer fibres, at dosages equal to or larger than 2 kg/m3, might help prevent fire spalling. Recycled tyre steel fibres also show the potential of preventing fire spalling damage by keeping spalled concrete attached to the heated surface, thus protecting the main steel reinforcement. The use of these fibres might lead to safe and sustainable fire spalling mitigation solutions.

Fire spalling of ultra-high performance concrete: From a global analysis to microstructure investigations

This article presents the behaviour under fire of a specific ultra-high performance concrete (UHPC) called BCV. Several compositions with synthetic additions such, as polypropylene (PP) fibres and powder or acrylic (PAN) fibres, are mixed to produce these specimens. As a first step, a blowtorch test on prismatic specimens shows that small PP fibres are more efficient in resisting fire. Next, scanning electron microscopic observations after heating reveal significant behavioural differences between PP fibres and PAN fibres during their vaporisation process. Lastly, a mercury intrusion porosimetry investigation following a heating cycle at 350°C indicates that concrete porosity is not a sufficient parameter for determining whether or not a given material composition is resistant to fire spalling. A critical factor dependent on pore size distribution, is proposed herein; its threshold value, i.e. under which the material exhibits a very high probability of being prone to fire spalling, is also estimated.

Preventive Effect on Fire Spalling of High-Strength Concrete With Jute Fibre in Ring-Restraint Specimen

High-strength concrete (HSC) is widely used to construct high-rise buildings, bridges and other heavy structures in the world. However, the best way to reduce spalling risk of HSC structures when exposed to high temperature, such as fire is still ongoing. According to the researches, thermal stress and vapour pressure are the major factors that cause concrete spalling at high temperatures. In addition, Japan Concrete Institute Technical Committee previously examined the potential performance of concrete in high-temperature conditions (ref. JCI-TC154A), and proposed the ring restrain heating test. As a preventive way to reduce explosive spalling of HSC, commonly Polypropylene (PP) fibres are used. On the other hand, many research shows that, natural Jute fibre are also effective however, only few researches and experimental studies regarding the study of preventive effect of spalling. Therefore, this study aims to confirm the difference of spalling property on various fibre-reinforced high-strength concrete (HSC) using ring-restraint specimen. Jute and PP fibre of 0.1 vol% and the heating condition is a RABT 30 rapid heating curve. The restrained stress and vapour pressure were measured. In result, spalling was observed for HSC and HSC with PP fibre specimen, no spalling was occured HSC with Jute fibre specimen.

Explosive spalling of concrete in fire: novel testing to mitigate design risk

Heat-induced explosive spalling in fire poses a credible risk to concrete structures, and has received considerable research attention in recent decades. However, no validated guidance to enable the design of concrete mixes to prevent spalling, nor any established, widely verified, repeatable test methods are yet available to confidently quantify or demonstrate spalling resistance for a particular mix in a given application. As a result, no models yet exist that can predict spalling with sufficient confidence to be used in design. This paper summarises contemporary research on heat-induced concrete spalling, with particular emphasis on design for fire of concrete-lined tunnels. The topic is also relevant for modern concrete buildings. A novel, repeatable and economical testing method to reduce project risk by quantifying the propensity of concrete mixes for spalling under a range of different thermal and mechanical conditions is described. The intent of this paper is to present the limitations of knowledge to enable design for heat-induced spalling, and to highlight research currently under way to overcome some of the issues faced in practice.

Concrete propensity to fire spalling: testing and observations

This article presents results of fire spalling tests on small concrete slabs and studies of material parameters that may increase its occurrence. Experimental techniques enabling to study and determination of material features are presented and discussed. Experimental studies on spalling behaviour of elements were carried out on seven different concrete mixes with constant content of cement paste and mortar. Research aimed at determining influence of the following parameters: w/c ratio (0.30; 0.45; 0.60), cement type (CEM I, CEM III) and type of aggregates (riverbed gravel, granite, basalt) on fire concrete spalling. Paper discusses also the influence of cold rim that forms while testing slab-like element is subjected to one-side heating.

Increase of fire resistance of reinforced concrete structures with polypropylene microfiber

The increase in the construction of high-rise, technically complex buildings and structures is a prerequisite for the widespread use of structures of heavy concrete. In this work, a special type of destruction of this type of concrete is considered in the fire action explosive spalling. One method of protection is polypropylene microfiber, the objective of which is to increase the fire resistance of concrete and reinforced concrete structures. The fire resistance tests of the reinforced concrete structure with the use of microfiber and without it have been carried out. It is shown that polypropylene microfiber can completely prevent explosive spelling of concrete. In addition, the introduction of additives in the form of fibrous materials into the concrete mix is the most optimal from the point of view of labor intensity and material costs.

Fire Protection of Concrete Tunnel Linings with Waste Tyre Fibres

The damages caused by fire-induced explosive spalling of concrete in tunnels can be tremendous; it could result in enormous economic cost and potential loss of human life. For this reason, the structural fire protection of concrete tunnel linings plays an important role in the tunnel design. Polypropylene fibres have been used in concrete to prevent explosive spalling of concrete exposed to fire. On the other hands, thousands of tonnes of polymer fibres are generated worldwide as a by-product of the recycling of end-of-life tyres. Storage of these fibres is a problem, since it is flammable, of low density (and so very large in volume) and can be carried away by wind and pollutes the surrounding environment. They are also too agglomerated or contaminated with rubber to find any alternative use, and are generally disposed of by incineration. The polymer fibre recycled from tyres has equal high quality and durability as manufactured fibres. Finding ways of introducing these fibres in concrete can potentially reduce the use of virgin fibres and delivery a more environmental-friendly spalling-mitigation solution. This paper shows the preliminary outcomes of this research, which indicates the potential of using these recycled fibres to prevent fire spalling instead of manufactured polypropylene.

Fracture behaviour of steel fibre reinforced recycled aggregate concrete after exposure to elevated temperatures

In the past decade, research on applications of recycled aggregate concrete (RAC), which uses recycled concrete aggregate (RCA), has attracted world-wide interest due to their social, environmental and potential economic benefits. One of the main concerns for using such concrete in buildings is its spalling in fire and post-fire residual mechanical properties. This concern may be alleviated by adding steel fibres to form steel fibre reinforced recycled aggregate concrete (SFRAC). In this study, 60 notched beams were tested to evaluate the fracture behaviour of SFRAC after exposure to elevated temperatures, including the modulus of elasticity (E), fracture energy (GF), initiation fracture toughness KIcini and unstable fracture toughness KIcun. The effects of steel fibre volume content (0%, 0.5%, 1% and 1.5%) and exposure temperature (room temperature, 200°C, 400°C and 600°C), were investigated. The test results show that the addition of steel fibres postpones the initiation of cracks and limits their opening in RAC, and thus significantly enhances the fracture energy and fracture toughness of RAC after exposure to high temperatures.

Concrete Permeability and Explosive Spalling in Fire

Permeability of concrete is a good indicator of the risk of explosive spalling, concrete with low permeability is more prone to explosive spalling. To study explosive spalling of concrete, experimental tests on the concrete permeability have been carried out at ETH. The influences from temperature and moisture content have been investigated. The permeability of concrete is found to increase with the temperature and to decrease with moisture content. Based on the test results, a permeability model has been proposed. The explosive spalling has been predicted and an engineering boundary permeability for the liability to spalling is recommended to be 2 × 10-17 m2 for a concrete slab heated according to ISO fire curve. The boundary permeability is influenced by moisture content, tensile strength and heating rate.

Fire Spalling Behaviour of Concrete: Role of Mechanical Loading (Uniaxial and Biaxial) and Cement Type

Fire poses one of the most severe environmental conditions that can act on concrete structures as an external load and can induce severe damages (cracks, spalling) or even lead to collapse. Fire spalling of concrete is a complex phenomenon, which might occur due to pressure build-up in the pores, thermal and load-induced stresses. In this context, ordinary concrete specimens (B40-II and B40-III: fc28days ≈ 40 MPa) were exposed to standard fire curve (ISO 834-1), while a constant uniaxial or biaxial compressive load was applied. Six different levels of uniaxial compressive stress on cubes and four different levels of biaxial compressive stress on slabs have been investigated. The test results showed that loaded specimens are more susceptible to spalling than unloaded specimens, with increasing amount of spalling for higher values of applied load. It has been found that biaxially loaded specimens are more prone to spalling than uniaxially loaded specimens. B40-II concrete (3% of slag) exhibited higher spalling than the B40-III concrete (43% of slag).

Effect of Moisture Content and Fiber on the High-Temperature Properties of Tunnel Second Lining Concrete

The critical moisture content of concrete with different fibers and the quality loss of different moisture contents are analyzed by simulating the real heating curve of the building material in a fire, and a fire test on the concrete slab is conducted. Results show that (1) moisture content greatly influences the surface burst degree, number of cracks, time of water seepage, and quality loss of the specimen after a fire; (2) the critical moisture content of fireproof fiber concrete burst is higher than that of three other kinds of concrete (normal concrete, polyethylene fiber concrete, and polypropylene fiber concrete), and has the highest crack resistance; and (3) the observation of fire spalling and the burn crisp degree of the concrete slab with the same moisture content verified that the fireproof fiber has a better anti-burst effect. Testing the post-fire side face of concrete slabs and the regional rebound value show that rebound value losses are not the same because of different temperature distributions on the side faces of the concrete slabs. The loss of mechanical property of the concrete is related to the temperature field.

A Comprehensive Review of Spalling and Fire Performance of Concrete Members

Spalling of concrete in fire is due to dislodgement of small pieces of concrete. This phenomenon is one of the most detrimental effects causing damage or even failure of the concrete members during or after a fire by decreasing the residual mechanical properties and durability of the structure. In spite of many experimental and numerical studies, the real mechanism of spalling is still not well understood, which is mainly due to the erratic nature of this phenomenon and inhomogeneity of the concrete structures. To establish a more clear view of the achievements in this area and also the areas in need of more investigations, this paper critically reviews the latest findings on concrete performance during heating in general and specifically the fire spalling behaviour of concrete members and different views on governing factors that affect this phenomenon.

Real-time measurements of temperature, pressure and moisture profiles in High-Performance Concrete exposed to high temperatures during neutron radiography imaging

High-Performance Concrete (HPC) is particularly prone to explosive spalling when exposed to high temperature. Although the exact causes that lead to spalling are still being debated, moisture transport during heating plays an important role in all proposed mechanisms. In this study, slabs made of high-performance, low water-to-binder ratio mortars with addition of superabsorbent polymers (SAP) and polypropylene fibers (PP) were heated from one side on a temperature-controlled plate up to 550°C. A combination of measurements was performed simultaneously on the same sample: moisture profiles via neutron radiography, temperature profiles with embedded thermocouples and pore pressure evolution with embedded pressure sensors. Spalling occurred in the sample with SAP, where sharp profiles of moisture and temperature were observed. No spalling occurred when PP-fibers were introduced in addition to SAP. The experimental procedure described here is essential for developing and verifying numerical models and studying measures against fire spalling risk in HPC.

Corner spalling and tension stiffening in heat-damaged R/C members: a preliminary investigation

Corner 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.

Reliability Analysis of High-Strength Concrete Columns during a Fire

A reliability analysis was conducted on high-strength concrete (HSC) columns during a fire. The influences of fire’s randomness and explosive spalling of concrete were investigated. The fire resistance for axial loading capacity of HSC columns was in terms of steel yield strength and concrete compressive strength with considering the effect of elevated temperatures. The load random variables included dead load and sustained live load. The JC method was applied to calculate the reliability index of the fire resistance of axially loaded HSC columns. It was found that the randomness of fire and explosive spalling of concrete had a significant influence on reliability of HSC columns.

Compressive behavior of steel fiber reinforced recycled aggregate concrete after exposure to elevated temperatures

For sustainability considerations, the use of recycled aggregate in concrete has attracted many interests in the research community. One of the main concerns for using such concrete in buildings is its spalling in fire. This may be alleviated by adding steel fibers to form steel fiber reinforced recycled aggregate concrete (SFRAC). This paper presents an experimental investigation into the compressive properties of SFRAC cylinders after exposure to elevated temperatures, including the compressive strength, Young’s modulus (stiffness), stress–strain curve and energy absorption capacity (toughness). The effects of two parameters, namely steel fiber volume content (0%, 0.5%, 1%, 1.5%) and temperature (room temperature, 200°C, 400°C and 600°C) on the compressive mechanical properties of concrete were investigated. The test results show that both compressive strength and stiffness of the concrete are significantly reduced after exposure to high temperatures. The addition of steel fibers is helpful in preventing spalling, and significantly improves the ductility and the cracking behavior of recycled aggregate concrete (RAC) after exposure to high temperatures, which is favorable for the application of RAC in building construction.

Experimental discussion on the mechanisms behind the fire spalling of concrete

SummaryThe behaviour of six concretes at high temperature (600 °C) and in particular the risk of fire spalling is studied. Tests are performed with two sizes of samples: small samples (300 × 300 × 120 mm3) and small slabs (700 × 600 × 150 mm3). Different storage conditions (pre‐drying at 80 °C, air and water storing) are used to highlight the effect of the initial water content. Thanks to different scenarios of heating, the influence of the heating curve is studied.Results enabled to identify parameters that highly influence the risk of fire spalling: initial water content and concrete permeability during heating. The permeability of concrete can increase during heating due to the melting of the polypropylene fibres or by thermal damage. This thermal damage is important when heating is violent (ISO 834 or increased hydrocarbon fire), or when concrete is made with silico‐calcareous aggregates (flint).Fire spalling cannot be explained by either the only thermo‐mechanical behaviour of concrete, or only by the appearance of high pore gas pressure. Based on the recent hypothesis of the critical zone, the formation of a saturated layer of liquid water is consistent with the results obtained. Copyright © 2014 John Wiley & Sons, Ltd.

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.

Reduction of fire spalling in high-performance concrete by means of superabsorbent polymers and polypropylene fibers: Small scale fire tests of carbon fiber reinforced plastic-prestressed self-compacting concrete

High-performance concrete (HPC) is prone to explosive spalling when exposed to fire, which may lead to failure of the concrete elements. Polypropylene fibers (PP) are often added to HPC, as upon their melting they create channels through which water vapor is evacuated, preventing the build-up of high vapor pressures. In self-compacting HPC (HPSCC), the amount of PP fibers needs to be limited in order to keep the self-compacting properties, which may reduce the fire resistance.In this paper, a novel strategy to reduce fire spalling in HPSCC is illustrated, based on adding small particles of superabsorbent polymers (SAP) during mixing. The SAP end up as empty macropores, similar to air voids, in the HPSCC matrix. The PP fibers-SAP voids system percolates at a lower fiber loading than the fibers alone, allowing maintenance of the self-compacting properties while reducing substantially the fire spalling. In particular, in this paper it is shown how addition of SAP is able to reduce fire spalling in thin-walled HPSCC slabs prestressed with carbon fibre reinforced plastic reinforcement.

Concrete Structures Restoration from the Fire Safety Point of View

The need of concrete structures restoration in order to ensure their fire resistance can, in engineering practice, occur for reasons of damage to the structure by fire (spalling of concrete cover layer) or by the failure to cover the load-bearing structure during construction. The paper is devoted to the evaluation of the structure cover of the evaluated structure, the determination of temperature distribution in the structure with consideration to the choice of probable fire scenario, subsequent determination of critical temperature of the structure and the optimization of possible methods of the restoration of damaged or wrongly made constructions.