Standard Time-temperature Curve Research Articles

Performance of RC beams strengthened using NSM FRP and cement based adhesives after exposure to elevated temperatures

A popular method for strengthening of reinforced concrete (RC) structures is the use of carbon fiber reinforced polymers (CFRP) bonded to the structural elements using the near surface mounted (NSM) techniques. While this method possesses many advantages, poor fire resistance as a result of the low glass transition temperatures (TG) of organic resins remains a significant drawback. Replacement of organic resins with cement based adhesives (CBA) could potentially improve the fire performance of NSM FRP systems and reduce the amount of insulation required to achieve the required fire endurance period. This paper presents an experimental program consisting of fifteen RC beams strengthened in flexure using NSM FRP bonded using CBA and subjected to the standard fire time temperature curve up to a target temperature of 600 °C. Twelve of the beams were strengthened using NSM-FRP laminates and bars including both smooth and rough FRP laminates and the remaining three specimens acted as control beams. Further, the use of plasterboard was explored as fire protection to further increase the residual strength of the specimens after heating. Results showed that the reinforced concrete beams strengthened with NSM CFRP retained 82 % to 61 % of their unheated strengthened ultimate load after heat exposure. This was a result of the superior performance of the cement-based adhesive at high temperature and the fire protection provided to the CFRP through embedment in the grooves. Moreover, the use of plasterboard as fire protection decreased the CFRP temperature by 42–24 %. The insulated beams behaved similarly to strengthened beams prior to fire exposure.

Fire performance of CFRP-strengthened piloti-type columns with fire-resistant materials during standard fire exposure

This paper presents a numerical investigation into the fire endurance of carbon fiber reinforced polymer (CFRP)-strengthened columns, shielded with fire-resistant materials, in piloti-type reinforced concrete buildings. The strengthened column, equipped with a fire protection system, underwent exposure to the ASTM E119 standard time-temperature curve for a duration of 4 h. To comprehensively evaluate the thermal and structural performance of the strengthened column at elevated temperatures and substantiate the effectiveness of the fire protection system, a fully coupled thermal-stress analysis was conducted. The numerical modeling approach employed in this study was rigorously validated through previous experimental studies in conjunction with adherence to the ACI design guideline, specifically ACI 440.2R-17. Using the validated structural fire model, the thermal and structural behaviors of the RC column with an insulated CFRP strengthening system were investigated based on four key performance criteria: glass transition temperature, ignition temperature of polymer matrix, critical temperature of reinforcing bars, and the design axial load capacity at elevated temperatures. Furthermore, a comparative assessment of fire endurance was performed using diverse fire-resistant materials, including Sprayed Fire-Resistive Material (SFRM) and Sikacrete®-213 F, with insulation thicknesses ranging from 10 to 30 mm, during the 4-hour fire exposure period.

Fire Resistance of Prestressed Multispan Steel Truss Composite Slab

Prestressed steel truss composite slab is composed of a precast prestressed bottom slab, a steel truss, and a postcast concrete layer. In order to investigate the fire resistance performance of prestressed multispan steel truss composite (STPC) slabs under the coupled effect of high temperature and load, four specimens of such composite slab containing two spans with various postcast concrete layer thicknesses were fabricated. Fire experiments were conducted following the ISO 834 standard temperature–time curve. Two of the specimens underwent two-span fire experiments, while the other two underwent single-span fire experiments. During the experiment, the bottom of the multispan STPC slabs exhibited serious bursting under fire and the precast concrete layer at the bursting locations fell off. The experiment result showed that the degree of bursting was much higher than that of nonprestressed composite slabs and single-span slabs. Although bond failure was observed at the composite interface where horizontal cracks developed, the STPC slabs still performed well in terms of load bearing, which could be attributed to the contribution of the steel truss. Under the coupled action of high temperature and load, concrete cracks were prone to occurring at the cut-off position of the top reinforcement near the supports; therefore, the anchorage length of these reinforcing bars should be appropriately extended. The damage to the STPC slab specimens after fire was found to be more serious for those with a less-think postcast concrete layer. In the single-span fire experiments, the structural performance of the unfired span was not prominently affected. Due to the bursting of the precast layer, the thickness of the STPC slabs was reduced, resulting in decreases in the thermal insulation and the fire resistance limit. The fire resistance limits of the STPC slabs with 60 mm and 80 mm postcast concrete layers were 90 min and 130 min, respectively. When a high fire resistance limit of the STPC slabs is required in a design, it is recommended that the thickness of the postcast concrete layer be no less than 60 mm.

Analiza utjecaja protupožarne obloge na bazi gipsa na pougljenje drvenih nosivih elemenata

Fire resistance of load-bearing wooden structures is evaluated by the charring of surface layers of wooden bearing elements. The degree of resistance is determined by the onset and course of charring; these basic parameters are affected by the cover structure as well as by cavity fillers between the individual bearing elements. This article focuses on analysing the structure of fire protection covers based on gypsum boards, namely gypsum plasterboards and gypsum fibreboards. The foundation for the current analysis, based on the ignition temperature of the load-bearing timber element under the fire cover, includes the results of experimental fire tests carried out on test specimens in a fire furnace with a standard time-temperature curve set to simulate a fire. In addition, a numerical analysis of the temperature behaviour in the structure was carried out in order to compare the experimental results with the values obtained by mathematical analysis. Finally, the influence of the cover layers on the resulting fire resistance of the analysed structures is evaluated.

Analysing the impact of insulating material properties on the thermal performance of a novel composite precast concrete walling system exposed to elevated temperatures

The use of prefabricated elements is widely adopted in the construction sector to reduce the necessary on-site construction works and time, to maximise efficiency in material use and to mitigate on-site fabrication and assembly errors. The established use of such methodologies necessitates ongoing investigation and improvement of their performance both under normal loading conditions and when exposed to high temperatures. The present research proposes and assesses an innovative precast concrete wall composite section, incorporating insulation layers of varying thickness and position in relation to the fire source. Considering the combustibility of the incorporated insulation layer, sheds new light on the impact of the insulating material properties on the thermal performance of such innovative wall assemblies when exposed to fire. Quantifiable indicators are established, and qualitative conclusions are drawn, to aid with appropriate specification of materials and assemblies. Wall samples are subjected to a standard temperature–time curve as per EN1991-1-2 and their thermal and structural performance is reviewed and commented upon. It is concluded that the positioning of rockwool insulation against the fire front can reduce the temperature of the protected/non-exposed to fire side of the wall by up to 60 °C. The thickness and position of the insulation also lead up to a staggering 520 °C difference in the steel frame temperature when protected with 10 cm of rockwool compared to the same thickness of insulation installed on the non-exposed side of the assembly. A comparison between the performance of the surface-mounted expanded polystyrene to rockwool shows a temperature difference of up to 155 °C on the non-exposed boundary, for the same insulation thickness and positioning. The analysis is carried out through an experimentally validated 3D finite element model developed on COMSOL Multiphysics®.

The load carrying capacity of reinforced concrete columns subjected to elevated temperature using simplified method

The present work proposes a method to determine the load-carrying capacity of the reinforced concrete column at elevated temperatures. The method uses the zone method mentioned in Eurocode to determine the temperature-dependent material properties of steel and concrete. Later the design equations, as per Indian standard guidelines, are used to determine the load-carrying capacity of the column. The loads considered here include both axial loads as well as uniaxial moments. The results obtained are validated with experimental data from the literature. Once Pu and Mu are determined using temperature-dependent materials properties, interaction curves are plotted at various temperatures mentioned in the standard time–temperature curve. The results obtained are validated against the experimental results from the literature. The Paper also investigates the behaviour of reinforced columns by varying design parameters like the size of the column and the grade of concrete. The work also discusses the relevancy of the 500° Isotherm method and the zone method mentioned in Eurocode when designing columns at elevated temperatures per Indian Standard guidelines.

Fire resistance of a fully concealed, moment-resisting new timber connection utilizing mechanically-fastened steel rods

One major hindrance to more utilization of timber in the construction of tall buildings is the limited fire resistance of the connections between its structural elements. In an experimental study that aimed to investigate the structural behaviour of a new connection configuration that utilizes fully concealed, mechanically fastened steel rods, eight (8) full-size glued-laminated timber beam-end connections were tested under the CAN/ULC-S101 standard time-temperature curve. Test variables investigated in this study included rod anchorage length (200 and 250 mm) and washer size (38.1 × 38.1 and 50.8 × 50.8 mm). Results show that using the smaller washer provided more wood side cover to protect the steel components from the effects of elevated temperatures. Also, utilizing longer rod anchorage length provided larger internal surface area of the wood prism to better resist shear stresses as well as longer length for the rods to bear against the wood at high rotation values of the connection. By combining the said two aspects (longer anchorage length and smaller washer) a fully concealed glulam beam-end connection that utilizes only two mechanically-fastened steel rods can be successfully designed to achieve a 1-h fire resistance without the application of additional fire protection.

Analytical Equations Applied to the Study of Steel Profiles under Fire According to Different Nominal Temperature-Time Curves

Some analytical methods are available for temperature evaluation in solid bodies. These methods can be used due to their simplicity and good results. The main goal of this work is to present the temperature calculation in different cross-sections of structural hot-rolled steel profiles (IPE, HEM, L, and UAP) using the lumped capacitance method and the simplified equation from Eurocode 3. The basis of the lumped capacitance method is that the temperature of the solid body is uniform at any given time instant during a heat transient process. The profiles were studied, subjected to the fire action according to the nominal temperature–time curves (standard temperature-time curve ISO 834, external fire curve, and hydrocarbon fire curve). The obtained results allow verifying the agreement between the two methodologies and the influence in the temperature field due to the use of different nominal fire curves. This finding enables us to conclude that the lumped capacitance method is accurate and could be easily applied.

Fire protection assessment for the steel beams of a road bridge

The impact of a fire in an open car park situated under a road bridge was evaluated. In a highly ventilated space such as an open car park, fire may develop by spreading from one car to another. High temperatures may appear locally above burning cars and, in the case of flames impacting the bridge deck, the heat flux from each burning car may be added to different sections of the steel beams of the bridge. The purpose of the study was to determine the most severe fire scenario leading to the highest temperatures in unprotected steel beams of a bridge situated above an open car park. As a function of these temperatures and using standard temperature-time curve equivalence, the necessary fire protection of the steel beams was assessed. Temperatures were determined using the localised fire models of BS EN 1991-1-2:2002, together with the results of some fire tests on isolated cars and car parks existing in the literature. Considering the positions of cars in the car park, the maximum temperatures in the unprotected steel beams were obtained considering no more than five cars in the fire, as corresponding to statistics regarding fires in open car parks.

Experimental Study on the Temperature Distribution and Residual Strength of Fire-damaged RC Columns Depending on Heated Areas

Most of the columns in actual fire conditions are heated on partial faces rather than all four sides due to the floor plan, which results in asymmetric behaviors of columns. The asymmetric behaviors of fire-damaged columns may cause more vulnerability to the structural performance. In this study, temperature distribution and residual strength of reinforced concrete columns exposed to fire were investigated according to various heated areas. To achieve the objective, columns were heated for 2 h according to ISO-834 standard time-temperature curve and subsequently tested under the axial loading after a week. The test results show that the residual strength of the fire-damaged columns decreased as the heated area increased, and the residual strength reduced additionally due to asymmetric heating.

Beyond the standard time-temperature curve: Assessment of intumescent coatings under standard and deviant temperature curves

Nowadays there are intumescent coatings available for diverse applications. There is no established assessment of their protection performance besides the standard time-temperature curve, but natural fire scenarios often play an important role. A reliable straightforward performance-based assessment is presented. The effective thermal conductivity per thickness is calculated based on intermediate-scale fire tests. The optimum thermal insulation, the time to reach it, and the time until contingent failure of the coating are used for an assessment independent of the heating curve. The procedure was conducted on four different commercially intumescent coatings for steel construction, one solvent-based, one waterborne, one epoxy-based, and a bandage impregnated with a waterborne coating. The performance was studied under four different but similar shaped heating curves with different maximum temperatures (standard time-temperature curve, hydrocarbon curve and two self-designed curves with reduced temperature). The thermal protection performance is crucially affected by the residue morphology. Therefore, a comprehensive morphology analysis, including micro-computed tomography and scanning electron microscopy, was conducted on small-scale residues (7.5 × 7.5 cm2). Two different types of inner structures and the residue surface after different heat exposures were discussed in terms of their influence on thermal protection performance.

Effect of heated areas on thermal response and structural behavior of reinforced concrete walls exposed to fire

The purpose of this study was to investigate variations in the thermal and structural behavior of reinforced concrete (RC) walls according to the fire exposure time, compressive strength of the concrete, and the heated area. To this end, fire and axial loading tests were performed to obtain the temperature distributions and residual strengths. Heat was applied to the walls according to the ISO 834 standard time–temperature curve for 1 or 2 h. The fire test results demonstrated significant dependence of the heat propagation through the wall along the thickness on the moisture clog formed during heating. In order to show the effect of moisture clog on the heat propagation, heat transfer analyses were performed, and the results of temperature distributions obtained from the finite element (FE) model including the moisture clog zone were in good agreement with experimental results. The axial loading test results also indicated that the structural behaviors of the fire-damaged walls were affected by the fire exposure time, concrete strength, and heated area. In particular, the wall heated only on the front surface showed eccentricity, which reduced the residual strength considerably. Numerical studies for the structural behaviors of the fire damaged concrete walls were also conducted considering the eccentric loading effect due to fire. As results from the validation, the proposed simplified modeling approach was able to predict the asymmetric behaviors of fire damaged concrete walls owing to fire damage.

Comparative study of LightWeight and Normal Concrete composite slabs behaviour under fire conditions

This paper presents the experimental study of lightweight concrete (LWC) and normal concrete (NC) composite slabs with steel decking profile exposed to standard time-temperature curve. LWC composite slabs are very efficient in terms of energy savings and sustainability due to their low values of thermal transmittance. In this study NC is used to compare the fire behaviour of LWC composite slabs. Dimensions of the slab are 160 mm thick, 1120 mm wide and 2030 mm long. In order to perform fire tests in the intermediate-scale samples, a non-standard furnace was designed and assembled. The furnace is composed of a heating chamber, a gas evacuation system and a portal frame. The heating chamber is made of a steel framework and refractory layers. The evacuation system, which has an auxiliary vein with a fan to introduce air, extracts hot gases. The portal frame is composed of steel profiles with a rotating beam equipped with a linear actuator to apply the load on the samples. This beam is opened to allow the slabs to be placed and closed during the test. Finally, experimental results show that the time of fire resistance is close to 30 min for both types of slabs. Free water within the slabs affects the temperature increase of concrete. Thus, the temperature distribution inside the slab follows a non-constant trend, due to the movement and evaporation of the free water. Furthermore, although there is de-bonding between the steel and the slabs, no spalling is detected.

Investigating the behaviour of hybrid fibre-reinforced reactive powder concrete beams after exposure to elevated temperatures

This study investigated the structural behaviour of reinforced reactive powder concrete (RPC) beams under service load and fire exposure. The beams were composed of hybrid fibres (50% polypropylene fibres and 50% steel fibres) at different volume fractions relative to nonfibrous-reinforced RPC beams. The bottom and both sides of the beams which were simply supported and loaded with two-point loads were exposed to a controlled fire for 120 min in accordance with ASTM E 119 standard time–temperature curve. The midspan deflection was recorded every 5 min. The experiment also included loading tests on fire-damaged beams after cooling. The nonfibrous-reinforced RPC beams failed during the fire test after 38 min because of the spalling of the reinforcement cover which directly exposed the reinforcing steel to elevated temperatures. By contrast, the beams with hybrid fibres could resist failure during the entire test period. The rate of increase in deflection during fire exposure declined with an increase in hybrid fibre content. Increases in fibre volume fraction from 0.25% to 0.75% and 1.25% decreased the midspan deflection of the reinforced RPC beams by 33% and 36%, respectively. Adding hybrid fibres could considerably improve the residual stiffness of fire-damaged beams.

Structural fire performance of wood-steel-wood bolted connections with and without perpendicular-to-wood grain reinforcement

PurposeIn fire condition, the time to failure of a timber connection is mainly reliant on the wood charring rate, the strength of the residual wood section, and the limiting temperature of the steel connectors involved in the connection. The purpose of this study is to experimentally investigate the effects of loaded bolt end distance, number of bolt rows, and the existence of perpendicular-to-wood grain reinforcement on the structural fire behavior of semi-rigid glued-laminated timber (glulam) beam-to-column connections that used steel bolts and concealed steel plate connectors.Design/methodology/approachIn total, 16 beam-to-column connections, which were fabricated in wood-steel-wood bolted connection configurations, in eight large-scale sub-frame test assemblies were exposed to elevated temperatures that followed CAN/ULC-S101 standard time-temperature curve, while being subjected to monotonic loading. The beam-to-column connections of four of the eight test assemblies were reinforced perpendicular to the wood grain using self-tapping screws (STS). Fire tests were terminated upon achieving the failure criterion, which predominantly was dependent on the connection’s maximum allowed rotation.FindingsExperimental results revealed that increasing the number of bolt rows from two to three, each of two bolts, increased the connection’s time to failure by a greater time increment than that achieved by increasing the bolt end distance from four- to five-times the bolt diameter. Also, the use of STS reinforcement increased the connection’s time to failure by greater time increments than those achieved by increasing the number of bolt rows or the bolt end distance.Originality/valueThe invaluable experimental data obtained from this study can be effectively used to provide insight and better understanding on how mass-timber glulam bolted connections can behave in fire condition. This can also help in further improving the existing design guidelines for mass-timber structures. Currently, beam-to-column wood connections are designed mainly as axially loaded connections with no guidelines available for determining the fire resistance of timber connections exerting any degree of moment-resisting capability.

FIRE RESISTANCE PROVIDING OF HOLLOW‐CORE CONCRETE SLABS USING SHEET BUILDING MATERIALS

Introduction. This paper deals with the analysis of world experience in fire resistance providing of hollow‐core concrete slabs. To protect concrete structures are used many structural applications: thermal coatings and materials. The research first analyzes main methods and hypothesis using to make temperature analysis of solution fire resistance of concrete structures. Problem of making temperature analysis of hollow‐core concrete slabs are nonlinear thermal material properties and radiation heat transfer in the hollow-cores. The aim of this paper is to establish the temperature distribution in hollow‐core concrete slab considering radiation heat transfer in the hollow-cores in case of fire in compartment that is spreading by standard temperature-time curve. In addition, the aim is to substantiate the possibility of using gypsum panels to provide fire resistance of hollowcore concrete slabs. Material statement. The paper reports the results of modeling the process of heat transfer in hollow‐core concrete slab, between compartment space and slab surface and in hollow‐cores. To calculate temperature fields in hollow‐core concrete slab considering nonlinear thermal material properties and radiation heat transfer in the hollow-cores was used finite element model. At addition, the results of finite elements simulations show temperature fields in hollow‐core concrete slab and gypsum panels that installed under concrete slab. Scientific novelty. The paper reports results of theoretic substantiated of possibility of using gypsum panels to protect of hollow‐core concrete slabs considering nonlinear thermal material properties, radiation heat transfer between surfaces and radiation heat transfer in the hollow-cores. The results indicate that using gypsum panels to protect of hollow‐core concrete slabs reduces speed heating of concrete elements to critical temperatures that increase fire resistance of hollow‐core concrete slabs to 20.4 %.

Evaluation of zero-strength layer depths for timber members of floor assemblies with heat resistant cavity insulations

Fire resistance of timber members can be evaluated by an analytical design model known as Effective Cross-Section Method (ECSM). In this method, the residual cross-section in fire conditions is evaluated by decreasing the original cross-section by a notional charring depth and a so-called zero-strength layer. The ECSM for timber frame assemblies (TFA) exposed to standard fire conditions has been already presented. This design model considers the fire protection provided by claddings and cavity insulations, for which design values for zero-strength layers are still missing. In this paper, zero-strength layer depths for different set-ups of TFA are determined by means of a series of thermo-mechanical simulations. Model-scale furnace experiments were performed in order to validate the simulations. The study presents an analysis of the parameters that influence zero-strength layer depths for load-bearing members of timber frame assemblies exposed to the standard time-temperature fire curve.

Probabilistic thermo-mechanical finite element analysis for the fire resistance of reinforced concrete structures

This paper presents a probabilistic methodology based on the thermo-mechanical finite elements analysis to investigate the impact of the variability of the thermal properties of the concrete in the fire safety of structures. This is meant to evaluate if characteristic values or safety factors for the conductivity and specific heat are required during the semi-probabilistic structural fire safety assessment. To illustrate the use of the proposed methodology, this work includes a case-study with a tunnel lining which considers the uncertainties related to the thermal and mechanical properties of the concrete, the soil load, and the temperatures described by the standard temperature-time curve. Two failure criteria are considered: one was the maximum temperature of 300 °C at the reinforcement and the other based on the temperature-dependent strength as provided in the Eurocode EN 1992-1-2. Several finite element analyses are performed. The design of experiments is executed by a Correlation Latin Hypercube Sampling. The calculated probability of failure has different values depending on the adopted failure criteria. A sensitivity analysis using the Spearman's rank correlation coefficient was carried out and demonstrates that the uncertainty related to the specific heat has the greatest impact on the results.

Experimental study on the onset of swelling for thin intumescent coatings

Intumescent coatings (also known as reactive coatings) are widely used to protect load-bearing steel structural elements during fire. Upon heating, these initially thin coatings swell to form a low density, highly insulating foamed char that insulates steel substrate. Within the context of modern fire safety engineering design of steel structural systems, thermal insulation during fire may be key for assuring that steel does not reach ‘critical’ temperatures that may yield local structural instability and/or progressive failure (i.e. global instability).Traditionally, the fire performance of intumescent coatings is typically based on compliance (and to some extent their performance) to the standard fire resistance test in furnace, where single coated elements are exposed to the standard temperature-time curve. Numerous research studies have emphasised the influence of heating conditions on the onset of swelling and the overall effectiveness of intumescence. Moreover, some studies have shown that slow growing fires or low heating regimes may cause an incomplete swelling of the intumescent coating, or even melting or delamination. Consequently, the onset of swelling of is key for assuring its effectiveness in providing thermal insulation to the steel substrate during fire. Past researchers have shown that the onset of swelling occurs for temperatures in the range of 200-300°C. There are limited research studies that have look at the effects of varied, non-standard heating regimes on the onset or effectiveness of swelling of thin intumescent coatings.The study presented herein investigates the onset of swelling for a commercially available thin intumescent coating exposed to a wide range of heating regimes. Experiments are performed by using radiant panels and controlling incident radiant heat flux at the exposed surface of coated steel samples. This allows for the direct and precise control of the thermal boundary conditions at the exposed surface of tested samples. The onset of swelling criteria were defined in two ways: (1) visual observation of swelling during heating or (2) time-history of the protected steel temperature.The outcomes of this work aim at yielding a sound understanding of the true effectiveness of thin intumescent coatings. Experimental results mainly show that onset of swelling is directly influenced by the heating regime and it occurs for test samples heated at constant incident radiant heat flux above 20 kW/m2. The onset of swelling for the tested thin intumescent coating occurs for steel temperatures between 150 and 300°C and ‘mean coating temperature’ between 350 and 450°C. However, the steel temperature at onset of swelling is inversely proportional to the amount of constant incident heat flux at the exposed surface of tested samples, while the ‘mean coating temperature’ is directly proportional. As a conclusion, the time-history of net heat flux at the exposed surface of tested samples is not the only parameter that governs the onset of swelling for thin intumescent coatings.

An experimental study for the thermal behavior of composite concrete beam with slab compared with isolated beam under fire

As the number of fire accidents is gradually increasing, many experimental studies on fire performance of concrete have been reported by focusing on thermal behaviors of isolated structural members under fire. However, experimental approaches still have issues due to experimental conditions, such as heating and loading limits. One of them is about using isolated specimens for experiments, because when structural member is solely tested under fire, the behaviors may be different from those of structural members connected to each other. It is worth verifying whether isolated beam shows different behaviors under fire compared to the case with beam-slab connections, considering most structural members are connected to each other in real buildings. With these considerations, this paper aims at evaluating thermal behaviors between isolated specimen and composite specimen from experiments. Toward that goal, fire tests are performed on concrete beams connected with slab and isolated beams by applying high temperatures according to ISO-834 standard time-temperature curve. This study uses two different parameters; the first is fire exposure time and the second is whether it is joint with other structural members. From the experiments, time-temperature curves are obtained at several locations and differences of thermal behaviors between beam with slab and isolated beam are compared to each other.