HIGH STRENGTH SELF-COMPACTING CONCRETE AT ELEVATED TEMPERATURE

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1 HIGH STRENGTH SELF-COMPACTING CONCRETE AT ELEVATED TEMPERATURE Tao Jin, Liu Xian and Yuan Yong Tongji University, Shanghai, China Abstract The effects of high temperature on strength and stress-strain relationship of selfcompacting concrete (SCC) were investigated. Stress-strain curve tests were conducted at various temperatures under unstressed and stressed tests. The reduction in compressive strength of self-compacting concrete at elevated temperature is lower in stressed test than in unstressed test. Results from stress-strain curve tests show that plain SCC exhibits brittle properties below 600 C, and ductility above 600 C. Keywords: Self-compacting concrete (SCC), elevated temperature, strain-stress curve, stressed and unstressed test 1. INTRODUCTION High-strength concrete coffers various benefits derived from its higher mechanical properties: high compressive strength, high tensile strength, and high stiffness. It is used more widely in structural applications, especially when greater durability is desired. It can be manufactured by most concrete plants due to the availability of a variety of additives such as silica fume and high range water reducing admixtures. Self-compacting concrete was developed originally in Japan for some important properties: better flow, high resistance to segregation, and greater filling capacity. It flows through narrow openings such as spaces between steel reinforcing bars and the cover of the reinforcement. It flows under its own weight without segregation, maintaining the original concrete composition. This requires a sufficient volume of mortar to lubricate and transport the coarse aggregate particles. Self-compacting concrete can be used in production of tall walls, complex shaped members and concrete members with highly congested reinforcement. [1] As the use of self-compacting concrete becomes common, the risk of exposing it to fire increases. There are only a few investigations published on the behavior of SCC in fire. It has been shown that the risk of SCC fire spalling may be greater than HPC concrete due to the 1135

2 difference material composition. This different composition is mainly due to the higher fines content and possibly lower permeability. More research is needed in the area to determine in which cases SCC is behaving normally and in which cases there are problems for concern. It is also necessary to do more work in correlating laboratory testing with real field performance. In order to understand and eventually predict the performance of SCC structural members, the material properties that determine the behavior of the member at elevated temperatures must be known. The behavior of a structural member exposed to fire is dependent, in part, on the thermal and mechanical properties of the material of which the member is composed. [2] In the past, the fire resistance of structural members could be determined only by testing. In recent years, however, the use of numerical methods for the calculation of the fire resistance of various structural members is gaining wide acceptance. These calculation methods are far less costly and time consuming. However, for the use of these calculation methods, the material properties at elevated temperatures are required. One of the basic mechanical properties that are required for prediction of structural performance under fire conditions is the stress-strain relationship. Mechanical properties of concrete at elevated temperature are determined by testing plain concrete specimens using one of three types of steady-state temperature tests. The first method is the unstressed residual test that has been adopted in most of the published studies to measure the residual mechanical properties. In unstressed residual property tests, the specimen is heated without preload at a prescribed rate to the target temperature, which is maintained until a thermal steady state is achieved. The specimen is then allowed to cool, at a prescribed rate, to room temperature. The specimen is tested at room temperature. The second and the third test methods, namely the unstressed test and stressed test methods respectively. In unstressed tests, the specimen is heated, without preload, at a constant rate to the target temperature, which is maintained until a thermal steady state is achieved. Stress or strain is then applied at a prescribed rate until failure occurs. Briefly, in stressed tests, a preload (20 to 40% of the room temperature compressive strength) is applied to the specimen prior to heating and is sustained during heating. Heat is applied at a constant rate until a target temperature T is reached, and is maintained for a time t until a thermal steady state is achieved. Stress or strain is then increased at a prescribed rate until the specimen fails. [3, 4] Anderberg experimentally studied the relationship between thermal strains with temperatures, the stress-strain relationships, and the constant temperature creep of normal strength concrete. An experimental based constitutive model of concrete at transient thermal loads was developed. Khoury determined the stress-strain relation of HSC both in unstressed and stressed test conditions. It was found that high temperature reduced the gradient of the ascending branch of the stress-strain curves. However, such reduction was greater in the unstressed test than in stress test. Fu discussed the effects of mineral additions and test conditions on the stress-strain behaviour of HSC when exposed to the elevated temperatures. As reviewed above, there is only limited amount of test data available on the temperaturedependent stress-strain relationship of concrete under the unstressed and stressed conditions. This is probably due to the difficulties in measuring deformations at elevated temperature or applying or maintaining a load and high temperatures simultaneously [3,4]. 1136

3 2. RESEARCH SIGNIFICANCE One of the key properties needed for the assessment of the overall response of structures under fire conditions is the stress-strain relationships. Whereas the stress-strain curves at elevated temperatures have been established for various types of NSC and HSC, this is not the case with SCC, specifically SCC with polypropylene fibers. Temperature is one of the main factors that influence the strength, as well as the stressstrain relationships of concrete. The present study was undertaken to investigate the effect of high temperature on the strength and stress-strain relationship for SCC under stressed and unstressed conditions. Stress-strain curve tests were conducted at various temperatures(200, 400, 600 and 800 C) for polypropylene fiber-reinforced SCC. The data, on mechanical properties, presented in this paper can be used to develop stressstrain relationships, as a function of temperature, for SCC. These relationships can be used input to numerical models, which in turn can be used to determine the behavior of SCC structural members at high temperatures. 3. EXPERIMENTAL PROGRAM 3.1 Materials and specimens The program to measure mechanical properties comprises of two series of tests on cylinders, corresponding to two steady-state temperature conditions, or two test methods (namely stressed, unstressed property test methods). The stressed and unstressed test methods were designed to provide measurements of property data at elevated temperatures and required simultaneous application of loading and heating. In the stressed test, specimens were restrained by a preload equal to 20 percent of their room temperature compressive strength prior to and throughout the heating process. In the unstressed property tests, the specimens were heated without restraint. Both stressed and unstressed specimens were loaded to failure at elevated temperature under uniaxial compression, when the steady-state temperature is reached within the specimen. There are two types of concrete specimens used in tests, including self-compacting concrete, self-compacting concrete with the addition of polypropylene fiber with the same water-binder ratio. The size of cylinder is 150mm diameter, 300mm height. In all batches, the specimens are cast with normal Portland cement, natural river sand, crushed limestone with 15 mm maximum size. In order to improve workability, superplasticizer (Glenium) and limestone power are used in three batches of concrete mix. The mix proportions and mean compressive cube strengths for 28 days are included in table 1. The parameters of polypropylene fiber are listed in the table 2. The specimens are de-molded one day after casting, and then cured in a moist environment at 20 C, 95% relative humidity for a period of 28 days. To reduce the difference of the water content between specimens arising from a long test period, all specimens are tested after 90 days. 1137

4 Table 1 Mix Proportion and Cube Compressive Strength of Concrete Type Cement (kg/m 3 ) Water (kg/m 3 ) Sand (kg/m 3 ) Curse aggregate 4-8mm (kg/m 3 ) 8-16mm (kg/m 3 ) SCC SCCPPF Type Cube Compressive Limestone power Superplasticizer PPF (kg/m 3 ) (liter) (kg/m 3 strength at room ) temperature (MPa) SCC SCCPPF Polypropylene fibers were used in SCCPPF batch of concrete mix. The fiber was 12 mm in length with an 18 diameter. The parameters of PPF are shown in table 2. Table 2 PPF parameters Diameter Tensile strength Elastic modulus length Specific area Melting point 18 m μ 365MPa 3300MPa 12mm 225m 2 /kg 160 C 3.2 Testing equipment and procedure The test set up comprises a cylindrical furnace, a hydraulic testing machine with a support arrangement for load application and a specially designed device for deformation measurements. The temperature in the test specimens are measured in three points by chromel-nisiloy thermocouples and recorded continuously. One is situated at the surface of specimen, one at the mid section of specimen, whereas a third is placed about 38mm from surface in the mid section. To obtain deformation at very high temperature, the measurements at the high-temperature zone are transformed to the room temperature zone through special attachments, which are fixed at the top and bottom compression attachments. Two blankets are welded to the bottom of the upper attachment and the other two are welded on the top of the bottom attachment, respectively, for setting up LVDT to measure the relative deformation of the specimen [2]. For each type of concrete and target test temperature, two specimens were tested and the average of these results was used as the final result. The target test temperatures were determined to be 20 (room temperature), 200, 400, 600 and 800 C, respectively. All tests were conducted under hot conditions and no residual strength tests were carried out. Before undertaking stress-strain curve tests, the temperature of the furnace was calibrated against the specimen temperatures. To facilitate this, the concrete cylinders, with thermocouples, were heated in the furnace and the furnace temperatures were measured when the center of the specimens reached the specified target temperature. These temperatures were used as the representative highest furnace temperature for the following stress-strain tests. 1138

5 The cylindrical specimens were heated, without any load, at a constant heating rate of 5 C/min in the furnace chamber until the temperature at the center of the specimen reaches the target temperature. During the heating period, moisture in the test specimens was allowed to escape freely. For generating data in the descending part of the stress-strain curve of concrete, a strain control technique was adopted. After the specimen reached the steady state, it was loaded under uniaxial compression at a rate of 0.2 mm/min until the specimen could no longer sustain the axial load. In unstressed tests the specimens were heated up to the designed temperature and maintained for 3h to attain a thermal steady condition. The specimens were then loaded to fail. In the stressed tests the specimens were loaded at room temperature before heating. The stress, 20% of the ultimate compressive strength at room temperature, was maintained during the heating-up period. 4. RESULTS AND DISCUSSION 4.1 Spalling During the heating at 5 C/min, only self-compacting concrete specimens presented an instable behavior. All cylindrical specimens spalled explosively during the heating under stressed tests. All the spalling took place when the specimen s surface temperature was between 300 and 450 C. Some specimens experienced surface spalling in unstressed tests. No spalling occurred when heating concrete containing polypropylene fibers SCCPPF. The test confirms that polypropylene fiber enhances the thermal stability of high-strength concrete and self-compacting high strength concrete. The tests showed that severe fire spalling could occur with self-compacting high-strength concrete even at a low heating rate. Calcareous filler was used for the composition of self-compacting concrete. It is suspected that calcareous filler used in the composition of self-compacting concrete decreases the permeability of the concrete matrix and thus increased the risk of spalling. The other reason for spalling is the applied loads increase the susceptibility of concrete members to spalling. The 1982 revision of the FIP/CEB recommendations suggest that compressive stresses should be limited [5,6,7]. An increase in compressive stress, either by reduction in section size or an increase in loading, encourages explosive spalling. The initial compressive stress in the exposed layer of concrete may not by itself promote spalling. However, high compressive stresses caused by restraint to thermal expansion develop when the rate of heating is such that the stresses cannot be relieved by creep quickly enough. Combinations of compressive stresses (above 2 N/mm2) and moisture contents (above 3.3% by weight) make the occurrence of spalling likely in a fire.[8] 1139

6 Fig.1: Explosive spalling and surface spalling of SCC 4.2 Compressive strength Almost all self-compacting concrete showed explosive spalling in stressed test. So in this paper only SCCPPF concrete is discussed. The results of unstressed and stressed tests are shown in Fig.2 for self-compacting with addition of PP fiber. Fig.2: Strength loss of SCCPPF at elevated temperature under unstressed and stressed test The relative strength ratio is defined as the ratio of the maximum compressive strength, at a specified temperature, to the maximum compressive strength at room temperature (original strength). The compressive strengths under unstressed and stressed conditions change similarly. As shown in the figure, the compressive strength-temperature relationship can be characterized by the following stages: (1)initial strength-loss stage, which begins from room temperature to 200 C (2)stabilizing and regaining stage, which begins from 200 to 400 C. (3) permanent strength-loss stage, which begins 400 to 800 C. A distinct pattern of strength variation can be observed in each of the stages. Initially, as the temperature increased to 200 C, the strength decreased very slowly. With further increase in temperature, the strength at 400 C, is about 94% of the original strength (at 20 C). In the 1140

7 temperature range of 400 C to 800 C, the strength drops sharply, reaching to a low level of 67 and 26 percent of initial strength at 600 C and 800 C, respectively. The strength-temperature relationships observed in the unstressed test data are similar in trend with those of the stressed test data, except that the strength losses in the unstressed tests are slightly larger at each target temperature. This conclusion is agree with many conclusions existed for NSC and HSC for the two test methods. [3,4] Fig.3 and Fig.4 show the published results of compressive strength at elevated temperature under unstressed test and stress test respectively. The strength-temperature relationships appear to follow similar trends as for SCC and HSC, expect that the loss of SCC is smaller than that of HSC. The difference is narrowed in the permanent strength-loss stage. Overall, SCC at elevated temperatures loses a significant amount of its compressive strength above 400 C and attains a strength loss of about 75% at 800 C. The loss of strength in the temperature range of C is marginal and C is minimal, and lower than that of high strength concrete. Fig.3: Published unstressed test results for HSC and NSC Fig.4: Published stressed test results for HSC and NSC 1141

8 4.3 Stress-strain relationship Unstressed Test The stress-strain curves under different temperatures are shown in Fig. 5. It is very clear that for the ascending phase of the curves, with an increase in temperature, the slope gradually becomes smoother. Hence, the ascending phase of the curves at the higher temperature is always below that at lower temperature. From room temperature to 200 C, an increase in temperature slightly changed the gradients of the curves and the peak strength. Self-compacting concrete were brittle and the curve had short descending branches. Above 400 C, the strains, corresponding to peak strength, increased considerably. The strains attained, corresponding to peak strength at 600 C and 800 C are twice and five times the strain at room temperature. A progressive softening after peak strength and a ductile failure was observed at 600 and 800 C. The descending phase of the curves behaves differently from the ascending phase. This is because the rate of decrease of stress at the higher temperature is slower compared to that at the lower temperature. Thus, the gradients of the descending phase of curves at the higher temperature will be flatter than those at the lower temperature with an increase of strain. Therefore, the entire stress-strain curves become smoother and flatter with an increase of temperature. Stressed Test The stress-strain curves under different temperatures are shown in Fig. 6. The shapes of curves in stressed tests are different from that of unstressed tests. The curves from 200 to 600 C are close to the room temperature curve. All the strains are less than those measured in instressed tests. The ascending branches of curves are steeper than those of unstressed tests. It shows that a smaller reduction in E values from 200 to 600 C. This is consistent with the results of HSC obtained by Khoury.[9] From 200 to 800 C, all SCCPPF behaves as a brittle materials and had only a short descending branch. Fig. 5: Stress-strain curves in unstressed test condition Fig. 6: Stress-strain curves in stressed test condition 1142

9 4.4 Failure mode SCC specimens with PP fiber showed a very brittle type of failure at temperatures less than 400 C. It was impossible to define the complete stress-strain curves, especially in the descending portion. The specimens failed soon after reaching their peak strength. The failure surfaces are "neat surfaces" and pass through the "broken aggregate". The end portion of the failed specimens resembled "double cone pattern" (at the top and bottom), as can be seen in Figure 7. When exposed to a temperature of 600 C and 800 C, concrete specimens exhibited a gradual failure and complete stress-strain curves could be defined. These specimens failed in an irregular pattern (Figure 8). Since HSC is a brittle material, it fails soon after crack propagation is initiated. If the number of voids is increased, the propagation of cracks occurs more gradually, resulting in less brittle failure. When temperature increases, the pore water is removed and more voids appear. The ductility of specimens increased with increased temperatures. Fig.7: Failure Mode of SCC specimen (with PP fiber) at 400 C Fig.8: Failure Mode of SCC specimen (with PP fiber) at 600 C and 800 C 1143

10 5. CONCLUSIONS Based on the results obtained in this study, and within the limitations of the test parameters, the following conclusions can be drawn: 1. Addition of limestone power and applied load may increase the chance of spalling. Addition of PP fiber can reduce the chance of spalling. 2. Self-compacting concrete with addition PP fiber loses a significant amount of its compressive strength above 600 C and attains a strength loss of about 75% at 800 C. The change of strength in the temperature range of C is marginal. 3. The reduction in compressive strength of self-compacting concrete at elevated temperature is lower in stressed test than in unstressed test. 4. Ductile deformation increase with increased temperature, especially above 400 C in the unstressed test. REFERENCES [1] A. Noumowé, et al. High-Strength Self-Compacting Concrete Exposed to Fire Test, Journal of materials in civil engineering, 2006, 18,No. 6, [2] Fu-Ping Cheng,et al. Stress-strain curve for high strength concrete at elevated temperature, NRCC-46973, Journal of Materials in Civil Engineering, v. 16, no. 1, 2004, pp [3] Long T. Phan et al, High-Strength Concrete at High Temperature An Overview, NIST. [4] Y.F. Fu, et al. Stress-strain curve of high strength concrete at elevated temperature, Magazine of concrete research,2005,57, No. 9, [5] Meyer-Ottens,C. The question of spalling of concrete structural elements of standard concrete under fire loading. PhD Thesis, Technical University of Braunschweig, Germany, [6] Institution of Structural Engineers and Concrete Society Design and detailing of concrete structures for fire resistance. Interim Guidance by a Joint Committee of the Institution of Structural Engineers and The Concrete Society, Institution of Structural Engineers, London, [7] Fédération Internationale de la Précontrainte, Recommendations for the design of reinforced and prestressed concrete structural members for fire resistance. FIP/CEB Guides to Good Practice, Publication , [8] Gabriel Alexander Khoury, Yngve Anderberg. Concrete Spalling Review, Fire Safety Design,2000 [9] Khoury G A, et al. Mechanical Behaviour of HPC and UHPC Concretes At High Temperature In Compression And Tension. ACI International Conference on State-of-Art In High Performance Concrete, Chicago,