Residual compressive strength of fire‐damaged mortar after post‐fire‐air‐curing

The effects of cooling regimes and post‐fire‐air‐curing on compressive strength of mortar were investigated. Mortars were made with CEN reference sand, CEM I 42.5 R cement and natural spring water. The sand–cement and water–cement materials' ratios were chosen as 3.0 and 0.50 for all mixtures, respectively. At 28 days, the specimens were heated to maximum temperatures of 400, 600, 800 and 1000°C. Specimens were then allowed to cool in the air, furnace and water. After cooling, the specimens were air‐recured. Compressive strength test was carried out before air‐recuring and after 7 days of air‐recuring. The highest reduction in compressive strength was observed at 1000°C regardless of cooling regime. Gradual cooling regime in air and furnace without post curing showed almost no difference in terms of compressive strength reduction for four elevated temperatures. Shock cooling in water caused significant reduction in compressive strength compared with both gradual cooling regimes without post curing. After air and furnace cooling regimes, 7 days air‐recured specimens showed further reduction in compressive strength for four elevated temperatures. Specimens cooled in water and subjected to 7 days air‐recuring showed significant strength gain approximately 39, 100 and 130% for 400, 600 and 800°C elevated temperature, respectively. Copyright © 2010 John Wiley & Sons, Ltd.     

Postfire full stress–strain response of fire‐damaged concrete

This paper reports experimental data establishing the postfire full stress–strain response of fire‐affected concrete. Such data are useful in situations when redesign of fire‐damaged concrete elements is considered. Heating was carried out to various temperatures in the range 217–470°C. Cooling was carried out either by quenching or in air. The postfire strain at ultimate stress significantly increased after heating to temperatures higher than 320°C. Quenching seems to aggravate the loss in compressive strength and further increase the strain at ultimate stress. Quenching involved spraying the heated concrete with tap water for 5 min. It is evident that knowledge of maximum temperature of exposure alone is not sufficient for estimation of the postfire stress–strain relationship. Other characteristics of exposure such as method of cooling are also important in evaluating the modification in the structural behaviour of fire‐affected concrete. Copyright © 2005 John Wiley & Sons, Ltd.     

Numerical modelling of carbon fibre‐reinforced polymer and hybrid reinforced concrete beams in fire

Investigation on the fire resistance of fibre‐reinforced polymer (FRP) reinforced concrete (RC) is essential for increased application of FRP bars in the construction industry. Experimental tests for determining the fire resistance of RC elements tend to be expensive and time‐consuming. Although numerical models provide an effective alternative to these tests, their use in case of FRP RC structures is hindered because of the insufficient constitutive laws for FRP bars at elevated temperatures. This paper presents the details of a numerical modelling work that was carried out for simply supported carbon FRP (CFRP) and hybrid (steel‐FRP) bar RC beams at elevated temperatures. Constitutive laws for determining temperature‐dependent strength and stiffness properties of CFRP bars are proposed. Numerical models based on finite element modelling were employed for the rational analysis of beams using the proposed constitutive laws. The behaviour of concrete was simulated by means of a smeared crack model based on the tangent stiffness solution algorithm. The employed numerical models were validated against previous experimental results. The theoretical rebar stresses were calculated in both the FRP and steel bars, and the differences are discussed. Copyright © 2012 John Wiley & Sons, Ltd.     

The fire performance and fire‐resistance design of aluminium alloy I‐beams

In this paper, the temperature fields of unprotected and protected aluminium‐alloy I‐beams heated on three sides were calculated using the finite element method. The calculated temperature results were compared with the incremental temperature rise formulas specified in Eurocode 9. Next, finite element models were developed to simulate the flexural behaviour and the flexural–torsional buckling behaviour of aluminium‐alloy I‐beams under fire. The calculated results were validated by experimental data acquired at both ambient and high temperatures. Subsequently, simplified formulas for calculating critical temperatures for 5083‐H112 and 6060‐T66 aluminium‐alloy beams were proposed based on parametric analysis, and the results obtained using these formulas were compared against equivalent values calculated in accordance with Eurocode 9 standards. In terms of engineering applications, findings indicate that increases in the load level, the global stability coefficient, and the ratio of fireboard thickness to thermal conductivity reduce the critical temperatures of aluminium‐alloy beams. In most cases, designs using the method of checking load bearing capacity at high temperature published in the Eurocode 9 would tend to be conservative. Finally, when the global stability coefficient was greater than 0.8, the critical temperatures in some regions measured slightly higher than the calculated simplified value. Copyright © 2014 John Wiley & Sons, Ltd.     

Residual compressive behavior of alkali‐activated concrete exposed to elevated temperatures

This paper reports the effect of elevated temperature exposures, up to 1200^°C , on the residual compressive strengths of alkali‐activated slag concrete (AASC) activated by sodium silicate and hydrated lime; such temperatures can occur in a fire. The strength performance of AASC in the temperature range of 400–800^°C was similar to ordinary Portland cement concrete and blended slag cement concrete, despite the finding that the AASC did not contain Ca(OH)₂ , which contributes to the strength deterioration at elevated temperatures for Ordinary Portland Cement and blended slag cement concretes. Dilatometry studies showed that the alkali‐activated slag (AAS) paste had significantly higher thermal shrinkage than the other pastes while the basalt aggregate gradually expanded. This led to a higher thermal incompatibility between the AAS paste and aggregate compared with the other concretes. This is likely to be the governing factor behind the strength loss of AASC at elevated temperatures. Copyright © 2008 John Wiley & Sons, Ltd.     

Residual mechanical resistance of concrete blocks and laying mortars after high temperatures

This paper reports an experimental campaign to evaluate the residual mechanical resistance after high temperatures of two structural masonry components: block and mortar. Residual compressive strength and deformation modulus of four different hollow concrete blocks and two different mortar mixes after heating at high temperatures are investigated. The test method used was the one recommended by RILEM TC 200 for mortars and an adaptation of the same method proposed by Medeiros et al. suitable for the geometry of hollow blocks. Despite the sharp drop in the deformation modulus after heating blocks and mortar, no different behaviours are observed in the deformability of the materials caused by the variables studied. The same cannot be said in relation to the variation of the residual compressive strength of the blocks, which is affected by the variables: initial nominal compressive strength and width of the concrete block. Regarding laying mortars, the results confirmed the small influence of compressive strength on the evolution of residual mechanical strength. The data and analyses reported here on the residual mechanical properties of hollow concrete blocks produced from a concrete mixture of very dry consistency, vibro-pressed and with normal weight aggregates are relevant, since the data found in the literature generally refer to the wet cast concrete material and in cylindrical bodies.     

The effect of various fire‐exposed surface dimensions on the spalling of concrete specimens

Concrete spalling due to fire exposure is often defined as the sudden detachment of fragments from a concrete surface. It can be quantified by various parameters of which weight loss and spalling depth are the most common ones. The risk of spalling is influenced by many factors such as concrete composition, heating rate and applied testing methods. A reduced scale testing method should be developed to analyse the spalling behaviour and to understand its effectiveness in more detail. As a subsection of this development, this study aimed to analyse the effect of different‐sized, circular heated areas in semi full‐scale fire tests. Therefore, vermiculite slabs with varying cut‐outs in their centre were placed between a specimen made of a spalling‐sensitive concrete and the combustion chamber. The combustion chamber was heated following a standard fire curve. Our experimental results show that the thermal expansion inside of equal‐sized specimens is strongly dependent on the size of the heated area. In addition, this area also affects thermal stresses. They decrease as a result of lower temperature gradients for tests with smaller unheated boundary areas. Apart from this, the analysis of fragments shows no correlation between their relative volume distribution and the heated area. Copyright © 2014 John Wiley & Sons, Ltd.     

Toughening study of fire‐retardant high‐impact polystyrene

Fire‐retardant high‐impact polystyrene (HIPS) was modified by melt blending with varying amounts of three types of tougheners. The effects of the tougheners on the properties of the fire‐retardant HIPS were studied by mechanical, combustion tests, and thermogravimetric analysis. The morphologies of fracture surfaces and char layers were characterized through scanning electron microscopy. The results show that the impact properties of styrene–butadiene–styrene (SBS)‐containing composites were better than those of ethylene–propylene–diene monomer (EPDM)‐containing or ethylene–vinyl acetate copolymer (EVA)‐containing composites. The tensile strength and flexural modulus of the fire‐retardant HIPS decreased evidently with the addition of tougheners. It is found that the compatibility between SBS copolymer and HIPS matrix was best among the three types of tougheners. The addition of SBS had little influence on the thermal property, residue, flammability, and morphology of char layer of the fire‐retardant HIPS, but the addition of EPDM rubber or EVA brought adverse influence on the residue, flammability, and morphology of char layer of the fire‐retardant HIPS, especially for EPDM. Copyright © 2009 John Wiley & Sons, Ltd.     

Residual cube strength of coarse RCA concrete after exposure to elevated temperatures

An experimental investigation was carried on the residual cube strength of concrete made with coarse recycled concrete aggregate (RCA) after exposure to temperatures of 20°C to 800°C. A total of 360 cube specimens were made with 2 water/cement ratios (w/c = 0.31 and 0.45) and 5 replacement percentages (r = 0%, 25%, 50%, 75%, and 100%) of coarse RCA. Effects of different cooling regimes (natural cooling, water cooling) on the residual compressive strength of coarse RCA concrete after exposure were also investigated. Experimental results show that the cube compressive strength and splitting tensile strength of coarse RCA concrete diminish with increasing temperature, of which the splitting tensile strength declines quicker than the compressive strength. The effects of coarse RCA replacement percentage and w/c ratio on losses in relative strength after being exposed to high temperatures are found to be insignificant. The results also reveal that the relative compressive strength of coarse RCA concrete cooled in water after heating process is lower than that of specimens cooled naturally.     

Thermal and mechanical properties of light‐weight concrete exposed to high temperature

The extensive studies devoted so far to normal‐strength light‐weight aggregate concrete (LWAC or LWC) have exhaustively clarified its behaviour in ordinary conditions. However, the introduction of high‐performance light‐weight aggregate concrete (HPLWAC or HPLWC), containing such pozzolanic components as microsilica and fly ash, raises some concerns, for instance about the behaviour at high temperature and after cooling. To investigate the temperature‐induced mechanical damage, both in compression and tension, of silica‐fume HPLWCs, a systematic research program was carried out at the Politecnico di Milano on materials residual behaviour (testing after cooling down to room temperature). Three concrete mixes (Normal‐Strength Concrete = NSC, f_c²⁰ = 30 MPa; Light‐Weight Concrete = LWC, f_c²⁰ ≈ 40 MPa; and High‐Performance Light‐Weight Concrete = HPLWC, f_c²⁰ ≈ 60 MPa), five temperature levels (20, 105, 250, 500 and 750 °C, no loads applied during heating), one thermal state (after cooling), three nominally‐equal tests for each case (for repeatability) were planned, bringing the total number of specimens to 120 (45 tested in compression, 45 in direct tension and 30 in indirect tension by splitting). At the same time, the thermal diffusivity of the materials was evaluated up to 750 °C (4 specimens). The results show that HPLWC is somewhat more temperature‐sensitive than both NSC and LWC, but this extra sensitivity is counterbalanced by HPLWC's lower diffusivity. Its better insulation properties are advantageous for the concrete in axially‐loaded members and for the tension bars in the beams, as demonstrated by the thermo‐mechanical analyses of three typical R/C sections (rectangular, T and slab sections) carried out in the second part of the paper. Copyright © 2012 John Wiley & Sons, Ltd.     

Effect of temperature and cooling regime on mechanical properties of high‐strength low‐alloy steel

This paper presents results from experimental studies on the effect of temperature on mechanical properties of high‐strength low‐alloy ASTM A572 steel commonly used in structural members in bridges. A set of high‐temperature tensile strength tests and post‐temperature exposed residual strength tests is carried out on ASTM A572 steel coupons in 20–1000 °C temperature range. The residual strength tests on high‐temperature exposed steel coupons are carried out after subjecting the coupons to two methods of cooling, namely, air cooling and water quenching. Results from these tests indicate that temperature‐dependent strength and stiffness degradation in A572 steel follow the same trend as that of carbon steel but with some variations. A572 steel recovers almost 100% of its room temperature yield strength when heated to temperature up to 600 °C, regardless of the method of cooling, while the extent of strength degradation in coupons subjected to heating beyond 600 °C is dependent on heated temperature and method of cooling. Data generated in these tests are utilized to generate high‐temperature stress–strain and residual stress–strain response of A572 steel. These results are also utilized to propose temperature‐dependent strength, elastic modules, and residual strength reduction factors of A572 steel, which can be used in evaluating residual response of fire‐exposed steel structures. Copyright © 2015 John Wiley & Sons, Ltd.     

Influence of fibers on bond strength of concrete exposed to elevated temperature

Concrete is a building material having good fire resistance and the resistance depend on many factors including the properties of its constituent materials. Fiber Reinforced Concrete (FRC) apart from improving mechanical properties has better fire resistance than conventional concrete. Bond strength of concrete is one of the important properties to be considered by structural engineers while designing reinforced concrete cements. In this research, an experimental investigation has been carried out to determine the effect of fibers on the bond strength of different grades (M20, M30, M40 and M50) of concrete subjected to elevated temperature. Different types of fibers such as Aramid, Basalt, Carbon, Glass and Polypropylene were used in the concrete with a volume proportion of 0.25% to determine the bond strength by pull-out test. Prior to the pull-out test, the specimens were kept in a furnace and subjected to elevated temperatures following standard fire curve as per ISO 834. Based on the test results of the investigations, type of fiber, grade of concrete and duration of heating were found to be the key parameters that affect the bond strength of concrete. The contribution of carbon fiber in enhancing the bond strength was found to be more significant compared to other fibers. An empirical relationship has been developed to predict the bond strength of FRC at a slip of 0.25?mm. This empirical relationship is validated with experimental results.     

Physical and mechanical properties of heat‐damaged structural concrete containing expanded polystyrene syntherized particles

Materials recycling for sustainable concrete constructions are favoring the replacement of ordinary aggregate with waste materials, in the form of either solid particles or small void‐formers. This is the case of expanded polystyrene syntherized (EPS) concrete containing EPS particles—or beads—whose high‐temperature residual behavior is investigated in this project. Two EPS mixes and one reference mix (f_c = 25–30 MPa) are tested in compression and tension after a thermal cycle at the reference temperature of 20°C, 150°C, 300°C, 500°C, and 700°C. The thermal diffusivity and the mass loss up to 700°C are investigated as well. The normalized mechanical decay of the two EPS concretes turns out to be slightly higher than that of ordinary concrete, but on the whole, the behaviors are rather close, while the thermal diffusivity of EPS concrete is definitely lower, to the advantage of its insulating capability. Damage indexes are also worked out on the basis of the elastic modulus and of the ultrasonic velocity, in order to have information on the possible void‐induced nonlinear effects. The still open problem, however, is whether the game (polystyrene recycling and mass reduction) is worth the candle (more cement and additives). Copyright © 2014 John Wiley & Sons, Ltd.     

Effect of glass fiber‐reinforced polymer and epoxy injection on compressive strength of elevated temperature damaged concrete

Glass fiber‐reinforced polymer (GFRP) materials have received a great deal of interest among civil engineers during the past decade. This paper presents an overview of experimental studies carried out on GFRP‐wrapped and epoxy‐injected concrete samples exposed to elevated temperatures. For this purpose, 0.30, 0.35 and 0.40 water to binder (w/b) ratios were used. For each w/b ratio, normal aggregates were replaced by lightweight aggregates with a size fraction of 0–2 mm at three different volume fractions such as 10%, 20% and 30% of total aggregate volume. At the same time, a group of air‐entrained samples was also cast for each w/b ratios. Prepared samples were exposed to 600 °C for 3 h. The damaged samples were separately repaired by GFRP and epoxy injection. Before and after elevated temperature exposure, water absorption and compressive strength were tested. After repairing with GFRP and epoxy injection, only the compressive strength test was carried out. GFRP improved the compressive strength between 1–22% and 348–1403% for samples before and after being exposed to elevated temperatures, respectively. Epoxy injection increased the compressive strength of the samples, exposed to elevated temperature, between 1% and 123%. However, the epoxy injection process failed to recover the compressive strength of the samples before elevated temperature exposure. Copyright © 2012 John Wiley & Sons, Ltd.     

Characterization and thermal degradation of protective layers in high‐rating fire‐resistant glass

Fire‐resistant glass products are considered to have better performance against fire. They are developed to replace conventional glass products. However, the smoke emitted from these products can be potentially harmful during fires and cause injuries or even deaths. Six samples of insulating glass available in the local market were selected. A variety of techniques, including X‐ray photoelectron spectroscopy, Fourier transform infrared spectroscopy (FTIR), thermogravimetric FTIR, pyrolysis–gas chromatography–mass spectrometry and tubular furnace coupled FTIR spectroscopy, were employed. According to the test results, there are two types of protective layers of the fire‐resistant glass. One consists of water, water‐soluble salt and polyamide with possible presence of alcohols, and the other consists of water metal silicates with possible presence of carboxylate and alcohols. Gases emitted from the protective layers heated in air, in argon and in vacuum are similar. Water vapor, carbon dioxide and hydrogen chloride are the main components of the gases emitted. Copyright © 2014 John Wiley & Sons, Ltd.     

Compressive strength of fly‐ash‐based geopolymer concrete at elevated temperatures

This paper presents the compressive strength of fly‐ash‐based geopolymer concretes at elevated temperatures of 200, 400, 600 and 800 °C. The source material used in the geopolymer concrete in this study is low‐calcium fly ash according to ASTM C618 class F classification and is activated by sodium silicate (Na₂SiO₃) and sodium hydroxide (NaOH) solutions. The effects of molarities of NaOH, coarse aggregate sizes, duration of steam curing and extra added water on the compressive strength of geopolymer concrete at elevated temperatures are also presented. The results show that the fly‐ash‐based geopolymer concretes exhibited steady loss of its original compressive strength at all elevated temperatures up to 400 °C regardless of molarities and coarse aggregate sizes. At 600 °C, all geopolymer concretes exhibited increase of compressive strength relative to 400 °C. However, it is lower than that measured at ambient temperature. Similar behaviour is also observed at 800 °C, where the compressive strength of all geopolymer concretes are lower than that at ambient temperature, with only exception of geopolymer concrete containing 10 m NaOH. The compressive strength in the latter increased at 600 and 800 °C. The geopolymer concretes containing higher molarity of NaOH solution (e.g. 13 and 16 m ) exhibit greater loss of compressive strength at 800 °C than that of 10 m NaOH. The geopolymer concrete containing smaller size coarse aggregate retains most of the original compressive strength of geopolymer concrete at elevated temperatures. The addition of extra water adversely affects the compressive strength of geopolymer concretes at all elevated temperatures. However, the extended steam curing improves the compressive strength at elevated temperatures. The Eurocode EN1994:2005 to predict the compressive strength of fly‐ash‐based geopolymer concretes at elevated temperatures agrees well with the measured values up to 400 °C. Copyright © 2014 John Wiley & Sons, Ltd.     

The influence of aggregate type on the physical and mechanical properties of high‐performance concrete subjected to high temperature

This paper presents the results of an experimental study on the behaviour of high‐performance concretes after high temperature exposure. The high temperature exposure is related to the potential risk of fire, and mechanical properties analysis is needed afterwards to assess the residual strength of the material. The results presented in the paper show the properties evolution of four concretes made with four different aggregate types: basalt, granite, dolomite and riverbed gravel. The mix compositions allow comparisons, because the cement paste and mortar compositions and their volumes remain the same for all the four concretes. Moreover, the aggregate particle size distribution was chosen to be quasi identical so that this factor does not affect the concrete behaviour. The decrease of tensile strength value with the increase of temperature is more pronounced than compressive strength reduction thus, the exponential and power function equations were proposed to describe f_tT–f_cT relationship. The change of modulus of elasticity in relative values is similar, although the initial values of modulus are different and correspond to the aggregate type. Copyright © 2015 John Wiley & Sons, Ltd.     

Effects of heat treatment on tensile properties of high‐strength poly(vinyl alcohol) fibers

The tensile properties of high‐strength poly(vinyl alcohol) (PVA) fibers after heat treatment in air, water, and engine oil were studied. The results show that heat treatment in air, water, and engine oil have a different influence on the tensile properties of high‐strength PVA fibers. After heat treatment in air, the fibers possess excellent heat stability of the tensile properties. But in water, especially in hot water, the tenacity, Young's modulus, and specific work of rupture of the fibers decrease, while the elongation at break of the fibers increases. Similarly, engine oil has a significant influence on the tensile properties of the fibers. When the temperature of engine oil is above 120°C, the tensile properties of the fibers decrease drastically. We also discuss the influence of heat, water, and engine oil on the tensile properties of high‐strength PVA fibers in relation to the structure of the fibers. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 237–242, 2000     

Effect of elevated temperatures and cooling regimes on normal strength concrete

The compressive strength of normal strength concrete at elevated temperatures up to 700^°C and the effect of cooling regimes were investigated and compared in this study. Thus, two different mixture groups with initial strengths of 20 and 35 MPa were produced by using river sand, normal aggregate and portland cement. Thirteen different temperature values were chosen from 50 to 700^°C. The specimens were heated for 3 h at each temperature. After heating, concretes were cooled to room temperature either in water rapidly or in laboratory conditions gradually. The residual strengths were determined by an axial compressive strength test. Strength and unit weight losses were compared with the initial values. Throughout this study, ASTM and Turkish Standards were used. It was observed that concrete properties deteriorated with the heat; however, a small increase in strength was observed from 50 to 100^°C. Strength loss was more significant on the specimens rapidly cooled in water. Both concrete mixtures lost a significant part of their initial strength when the temperature reached 700^°C. Copyright © 2008 John Wiley & Sons, Ltd.