properties of steel-fibre-reinforced concrete at elevated temperatures were ... In all three batches, general purpose Portland cement for construction of concrete.
Institute for Research in Construction
lnstitut de recherche en construction ..*.~
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by T.T. Lie and V.K.R. Kodur
Internal Report No. 687
Date of issue: February 1995
CISTI/ICIST NRC/CNRC IRC Ser Received on: 02-15-95 Internal report : Institute for Research in Construction Canada InternaI report : Institute ANALYSE
--Bev Creighton 2;!.\L'fZED
This is an internal report of the Institute for Research in.Construction. Although not intended for general distribution, it may be cited as a reference in other publications.
MECHANICAL PROPERTIES OF FIBRE-REINFORCED CONCRETE AT ELEVATED TEMPERATURES ABSTRACT For the purpose of use in fire resistance calculations, the relevant mechanical properties of steel-fibre-reinforced concrete at elevated temperatures were determined. These properties, which included the strength and deformation properties of fibrereinforced concrete at elevated temperatures, were determined by canying out stressstrain tests under both steady-state and transient-state conditions. The tests were carried out on fibre-reinforced siliceous aggregate concrete as well as on plain and fibrereinforced carbonate aggregate concrete. The results indicate that the compressive strength at elevated temperatures of steel-fibre-reinforced concrete is higher than that of plain concrete. It is concluded that the mechanical properties of fibre-reinforced concrete are more beneficial to fxe resistance than those of plain concrete.
MECHANICAL PROPERTIES O F FIBRE-REINFORCED CONCRETE AT ELEVATED TEMPERATURES
In recent years, the construction industry has shown significant interest in the use of fibre-reinforced concrete due to the advantages it offers over traditional plain concrete. The use of steel-fibres as reinforcement in plain concrete not only enhances the tensile strength of the composite system but also reduces cracking under serviceability conditions. Further, steel-fibres improve resistance to material deterioration as a result of fatigue, impact, shrinkage and thermal stresses. The improvements in material properties, which improve structural performance, have extended the use of fibre-reinforced concrete to applications in the area of fire. At present, studies are in progress to determine the performance of steel-fibrereinforced concrete structural members in fire [I]. In the past, the performance of structural members at temperatures encountered in fire could only be determined by testing. Over the years, however, methods have been developed for the calculation of the fire resistance of various structural members [2,3,4], which is far less costly and time consuming than testing. To be able to perfom these calculations, knowledge of the mechanical properties at elevated temperatures of the materials of which the members are composed is essential. Previous investigations [5,6,7,8] have concentrated on the properties of fibrereinforced concrete under ambient conditions and little information [9] is available on its behaviour at high temperatures. The present study was undertaken to establish the mechanical properties of fibre-reinforced concrete under elevated temperatures for use in models [2] to predict the fire resistance of concrete-filled steel columns. The study was canied out as a part of a joint research project, on the fire resistance performance of concrete-filled hollow steel sections, between the National Fire Laboratory (NFL)of the Institute for Research in Construction, National Research Council of Canada (NRCC), the Fire Technology Laboratory of the Technical Research Centre of Finland (VTT) and the Institut fiir Baustoffe, Massivbau und Brandschutz @MI%), of the Technische Universitat Braunschweig, Germany.
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MECHANICAL PROPERTIES OF FIBRE-REINFORCED CONCRETE
The mechanical properties that determine the fue resistance of structural members are the strength and deformation properties at elevated temperatures of the materials of which the members are composed. For concrete, the important mechanical properties are the compressive strength of the concrete and the deformations caused by load, creep and thermal expansion. These properties are usually expressed in stress-strain relations, which are used as input data in mathematical models for the calculation of the fire resistance of concrete members. For the purpose of developing these stress-strain relations for fibre-reinforced concrete, tests were carried out on steel-fibre-reinforced concrete specimens made with two commonly-used aggregates, namely, siliceous and carbonate aggregate. For comparison, specimens consisting of plain carbonate aggregate concrete were also tested. The specimens were prepared at the NFL and were tested at IBMB in Germany.
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TEST SPECIMENS
Three types of concrete specimens, namely, NRCl, NRC2 and NRC3, were investigated. The NRCl specimens were made with siliceous aggregate, while the NRC2 and NRC3 specimens were made with carbonate aggregate. The NRCl and NRC3 specimens were reinforced with steel-fibres. Three batches of concrete were made for preparing the three types of samples. The concrete was designed to produce a 28-day compressive strength of 35 MPa.
In all three batches, general purpose Portland cement for construction of concrete structures was used. The concrete mix in Batch 1 was made with siliceous stone aggregate while the mix in Batches 2 and 3 was made with carbonate stone aggregate. The fine aggregate for all three batches consisted of silica-based sand. In order to improve workability, superplasticizer Mighty 150 was added to all three batches and retarding admixture Mulco TCDA 727 to Batch 1. Pre-mixed concrete, including cement, coarse aggregate, sand, water and retarding admixtures were supplied by Dufferin Concrete, Ottawa. RlBTEC steel-fibres of the XOREX* type [lo], supplied by Ribbon Technology Corporation, were used as reinforcement in Batches 1 and 3. XOREX is a mild carbon steel with tensile strength of approximately 960 MPa. A typical XOREX steel-fibre is shown in Figure 1. The corrugated shape of these fibres provides a strong mechanical bond to the concrete. The fibres, which were 50 mm in length with a 0.9 mm equivalent diameter, had an aspect ratio of 57. The weight percentage of steel-fibres in Batches 1 and 3 was 1.77 and 1.76, respectively. The mix proportions, together with concrete and steel data for the three batches, are given in Table 1. The steel-fibres were added to the fresh concrete and were mixed for approximately 2 minutes for uniform dispersion. Vibrators were used to consolidate the concrete. From each batch of concrete, the following specimens were made: 18 cylinders with a diameter of 80 mm and a length of 300 mm 6 cylinders with a diameter of 150 mm and a length of 300 mm 6 prisms with a cross section of 100 x 100 mm and a length of 400 mm Two thermocouples were installed in each of the 80 mm cylinders and in each prism. The thermocouplejunctions in the cylinders were located at the central axis at a distance of 100 mm from each end of the cylinder and those in the prisms at 125 mm and 275 mm from one end of the prism The specimens were de-moulded one day after coating, then soaked under water for seven days and, subsequently, conditioned in a climate room at 50% relative humidity and 20°C. At 28 days after the pouring, compression tests on the 150 mm diameter cylinders were conducted for each concrete type. The 28-day compressive strengths are given in Table 1.
Ceriain commercial products are identifiedin this paper in order to adequately specify the experimental procedure. In no case does such identification imolv recommendations or endorsement bv the National Research Council. nor does it imply that the product or m a t e 3 identified is the best available for the purpose
Afterwards, 18 of the 80 mm diameter cylinders and six prisms of each concrete type were sent to IBMB, Germany for testing, to determine the mechanical properties of the concrete. The top surfaces of these specimens werc stored in the IBMB laboratory in an atmosphere of approximately 50% relative humidity at 20°C. 4
TEST DETAILS
To determine the strength and deformation properties of the concrete at elevated temperatures, stress-strain, creep and restraint tests were canied out. The stress-strain tests were steady-state heating tests performed on the small cylindrical specimens. The creep and restraint tests were transient-heating tests performed on both the small cylinders and the prisms. In addition, the compressive strength of the concrete at ambient temperature was determined by testing three of the larger cylinders of each concrete type. The elevated temperature tests were all canied out after the specimens had reached an age of six months, to minimize variability in strength due to accelerated hydration, which is predominant in young specimens [I 11. The tests were conducted in a special testing machine, consisting of an electrical furnace in which the temperatures and rate of temperature rise could be controlled, and a loading device capable of producing controllable loads, strains and strain rates. The stress-strain tests were canied out by testing one cylindrical specimen at each of a number of selected temperatures, namely, 150,200,250,350,450,600 and 750°C. In these tests, the specimens were heated, without any load, at a rate of approximatelx 2"CImin until the desired temperature in the furnace was reached. The specimens were allowed to homogenize at the selected temperature for 2 hours to ensure the attainment of sufficient temperature uniformity within the specimen. The stress-strain tests were performed on the specimens with a constant strain rate of O.O5%/min. Because of the relatively short duration of each stress-strain test, namely, approximately 10 minutes, the creep component in the measured strain was expected to be small. . . The creep characteristics were determined in transient heating tests of longer duration. In these tests, cylindrical specimens were heated under selected loads at a rate of 2"CImin temperature rise in the fumace for at least four hours. The selected loads in these tests produced stresses in the concrete equal to 15,30,45 and 60% of the compressive strength of the concrete at 20°C. Prior to heating, the specimen was subjected to the selected load. This load was kept constant during the heating of the specimen until its failure. During the test, the axial strain of the specimen, its temperature along the central axis at the location of the thermocouples and its surface temperature were measured. To obtain information on the restraint forces that develop during the heating of the specimen, tests were also carried out in which the strain was kept constant. In these tests, the specimens were initially subjected to a stress equal to 15% of the compressive strength of the concrete at 20°C and then heated, at a rate of 2"CImin furnace temperature rise, while the strain was kept constant. W n g the test, the imposed load on the specimen, its temperature along the central axis at the location of the thermocouples and its surface temperature werc measured. The creep and restraint tests, described earlier, were carried out at a heating rate of 2"CImin temperature rise in the furnace. Under fire conditions, the heating rate is usually
higher. Therefore, rapid heating creep and restraint tests were also canied out with heating rates up to a maximum of approximately 2O0C/min temperature rise in the furnace and a duration of approximately one hour. These tests were performed on the prisms, which were subjected to an initial stress equal to approximately 45% of the compressive strength of the concrete, followed by exposure to rapid heating. In the creep tests, the load was kept constant during the heating and, in the restraint test, the strain was kept constant. During the test, the load, axial strain, central axis temperatures of the specimen and its surface temperature were measured. In the steady-state tests, the central axis temperature was regarded as the specimen temperature. In the transient tests, the specimen temperature was obtained by averaging the central axis and surface temperatures.
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TEST RESULTS
5.1
Stress-Strain Characteristics
The stress-strain curves, for the three types of concrete that were investigated, are shown for various temperatures in Figs. 2,3, and 4. In two cases, namely, for the fibrereinforced siliceous concrete at 750°C and for the fibre-reinforced carbonate concrete at 450°C, no stress-strain curves could he obtained, due to premature failure of the specimen. Generally, because of decrease of strength and ductility of the material, the slope of the stress-strain curves decreases with increasing temperature. From the stress-strain data, the compressive strength, ultimate strain and modulus of elasticity at elevated temperatures were determined for the three concrete types. Fig. 5 shows the effect of temperature on the compressive strength for the three concrete types, which is expressed as a percentage of the compressive strength at room temperature. It can be seen that the strength of the fibre-reinforced concretes, made with siliceous as well as with carbonate aggregate, is higher than that of plain carbonate aggregate concrete. The strength of both fibre-reinforccd concretes exceeds the initial strength of thc concretes at temperatures up to approximately 40OoC,whereas the strength of theplain concrete stays below the original strength. At approximately 400°C, the strengths of all concretes decrease at an accelerated rate. This loss of strength may be attributed to enhanced crack formation in the concrete, which is initiated as a result of the ex~ansiondifference between the cement paste and the aggregate [12]. For the steel fibre-reinforced concretes, a 400°C. contributing factor is the decrease of seen& - of the steel. which. at a~~roximatelv . reduces at &I accelerated rate [2,13].
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In Fig. 6, the ultimate strain for the three concretes is shown as a function of temperature. It can be seen that, at elevated temperatures, fibre-reinforced concrete attains higher ultimate strains than plain concrete. The increase in strain can be attributed to the increase in crack volume, due to the steel, which has a greater thermal expansion than the concrete [2]. In Fig. 7, the effect of temperature on the modulus of elasticity, which is expressed as a percentage of the modulus of elasticity at room temperature, is shown. It can be seen in the figure that the modulus of elasticity of all three types of concretes decreases with increasing temperature in the entire temperature range studied. The modulus of elasticity is more affected by the formation of cracks than the compressive strength. The formation of microcracks due to shrinkage of the cement paste, already leads to a decrease in the modulus of elasticity 1121. Although the fibrereinforcement contributes to increasing the compressive strength of the concrete, the
increase in strains, due to the steel fibres, results in a decrease in the modulus of elasticity. The rate of decrease in the modulus of elasticity is highest in the temperature range 0-300°C for all three concrete types. 5.2
Deformation of Loaded Concrete at Elevated Temperatures
The axial deformations at elevated temperatures of the two types of fibrereinforced concrete and plain concrete specimens, subjected to stresses of 0,30,45 and 60% of the initial compressive strength of these specimens, are shown in Figs. 8,9 and 10. The deformations corresponding to a stress level of zero represent the conventional expansion of the concrete. The results of the tests indicate that the axial deformations depend, to a high degree, on the level of the stress to which the materials are subjected. The effect of the stress is large particularly at temperatures above approximately 550°C, which can be attributed to dehydration and shrinkage of the cement paste in the concrete 121. At the higher stress levels of 45 and 60%, the deformations of carbonate aggregate concrete are four to five times higher than those of siliceous aggregate concrete. The deformations of the olain and fibre-reinforced carbonate ageregate concrete spccimcos (Figs. 8 and 9) are sirhlar in thc tcmpcrarure range undcr cggsidYeratioo. The fibre-reinforced specimens failed at highcr temperatures, however, due to thc cnhanced ductility of the concrete provided by the reinforcement, ~
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The increase in ductility indicates that the use of carbonate aggregate concrete in structures is more advantageous for obtaining a high fire resistance than the use of siliceous aggregate concrete. The addition of steel fibres further increases the ductility of the concrete. 5.3
Restraint Stresses
Fig 11 shows the development of restraint stresses in plain and fibre-reinforced carbonate concrete specimens. The specimens, which were initially subjected to a stress equal to 15% of the compressive strength of the concrete, were tested at a rate of 2OCImin temperature rise in the furnace, while the initial strain was kept constant. It can be seen in this figure that the fibre-reinforced concrete developed higher restraint stresses than plain concrete throughout the temperature range under consideration. For both types of concretes, the restraint stress increases with temperature up to a temperature of approximately 100°C and reaches a stress level of approximately 50% of the compressive strength of the concrete. The increase in restraint stresses at temperaturesup to approximately 100°C can be amibuted to the expansion of the concrete and the water in it [2,12]. After reaching a maximum level at around 10O0C,the stresses decrease rapidly, until a temperature of approximately 200°C is reached. This is due to the loss of water and, consequently, shrinkage of the concrete. Between 200 and 400°C the restrainttemperature curve remains almost flat since all the capillary water is evaporated. Beyond 400°C, the restraint stresses further decrease due to dehydration of crystal water and loss of strength of the concrete.
5.4
Rapid Heating Deformations and Stresses
The axial deformations and stresses measured during the rapid heating tests are shown in Figs. 12 and 13 for the three types of concretes examined. In Fig. 12, the deformations are shown as a function of temperature for the tests in which the specimens were subjected to a constant stress of approximately 45% of the compressive strength of the concrete. This corresponds to a constant stress of 18.5 MPa for the fibre-reinforced siliceous concrete, 16 MPa for the plain carbonate concrete and 19.5 MPa for the fibrereinforced carbonate concrete. The deformations in the fibre-reinforced siliceous aggregate concrete increases with temperature up to 650°C and rapidly dropped after reaching a maximum, until failure of the specimen. In the cases of the plain and the fibrereinforced carbonate concrete specimens, the deformations remain flat until approximately 250°C, and then gradually reduce with increasing temperature up to approximately 675OC. Similar to the behaviour in the transient creep tests described earlier, which were conducted at a much lower heating rate, the ductility of the carbonate concretes, indicated by the higher deformations, was considerably greater than that of the siliceous aggregate concrete. In Fig. 13, the restraint stresses in the concretes, heated under a constant strain, are shown as a function of temperature. The strains, produced by subjecting the concrete to a stress of approximately 45% of the concrete compressive strength, are 0.51% of the initial length of the specimen for the fibre-reinforced siliceous concrete, 0.54% for the plain carbonate concrete and 0.44% for the reinforced carbonate concrete. It can be seen that the restraint stresses increase rapidly initially up to a temperature of approximately 100°C. Above this temperature, up to approximately 200°C, the stresses remain more or less constant, which is likely caused by shrinkage due to loss of water. At temperatures above approximately 200°C, another increase in stresses takes place. In the case of the siliceous concrete, the stresses continue to increase up to approximately 600°C. The stresses in the more ductile carbonate concrete, however, started to decline after reaching approximately 350°C. At temperatures above approximately 600°C the stresses declined in all concretes due to cooling of the concrete.
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CONCLUSIONS
Data on the mechanical behaviour of steel-fibre-reinforced concrete at elevated temperatures is presented in this report. Results from experimental studies on two types of steel-fibre-reinforced concrete, siliceous and carbonate aggregate concretes, are compared to each other and to plain carbonate concrete. Based on the information obtained from the experiments, the following conclusions can be drawn:
1. The compressive strength at elevated temperatures of fibre-reinforced concrete is higher that that of plain concrete. The compressive strength of fibre-reinforced concrete increases considerably with temperature, up to a temperature of approximately 400°C; whereas the compressive strength of plain concrete continuously declines with increasing temperature. 2. The presence of steel fibres increases the ultimate strain and improves the ductility of a fibre-reinforced concrete member. 3. The axial deformations of plain and fibre-reinforced carbonate aggregate concrete specimens under a load are substantially higher than those of fibre-reinforced siliceous aggregate concrete for temperatures above 400°C. The thermal expansion of fibrereinforced siliceous aggregate concrete is higher than that of plain and fibre-reinforced carbonate aggregate concrete. 4. Fibre-reinforced concrete develops higher restraint stresses than plain concrete.
5. Overall, fibre-reinforced concrete exhibits, at elevated temperatures, mechanical properties that are more beneficial to fire resistance than those of plain concrete. 6. Additional work, such as interpolation or extrapolation and curve fitting, is needed to derive, from the data presented in this report, stress-strain relations for fibre-reinforced siliceous and carbonate aggregate concrete at elevated temperatures that are suitable for use as input data in mathematical models for the calculation of the fire resistance of concrete structural members. REFERENCES
1. Kodur, V.K.R. and Lie, T.T., "Fire Resistance of Hollow Steel Columns Filled with Steel-Fibre-Reinforced Concrete", Second University-Industry Workshop on Fibre Reinforced Concrete and other composites, Proceedings, Toronto, Ontario, Canada, 1995 A
