EVOLUTION OF REMAINING COMPRESSIVE STRENGTH OF FIBER REINFORCED HIGH STRENGTH CONCRETE UNDER FATIGUE EFFORTS

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1 BEFIB2012 Fibre reinforced concrete Joaquim Barros et al. (Eds) UM, Guimarães, 2012 EVOLUTION OF REMAINING COMPRESSIVE STRENGTH OF FIBER REINFORCED HIGH STRENGTH CONCRETE UNDER FATIGUE EFFORTS Jesús Mínguez *, Dorys C. González *, Miguel A. Vicente * and José A. Martínez * * University of Burgos, Civil Engineering Department Polytechnic Superior C/ Villadiego s/n Burgos Spain jminguez@ubu.es, dgonzalez@ubu.es, jamartinez@ubu.es Keywords: fibers, fatigue, high strength concrete. Summary: This research tries to evaluate the remaining compressive strength of fiber and non fiber reinforced high strength concrete specimens (FRHSC and HSC) after being subjected to axial fatigue efforts. 1 INTRODUCTION Evolution in the use of concretes with increased resistance is conditioned by the demands of slender structures which are subjected to increasingly exigent tensional levels. Often these tensions are of cyclic type and therefore generate during the structure s life potential problems of fatigue in reinforced concrete. There is nowadays a very important field of application in structures subjected to high tensional levels, and where alternating load phenomena appear, such as the structural elements of concrete wind towers, high-speed train viaducts and airport runways. A well-known problem of high strength concretes is the brittleness that they may develop under certain stress conditions. In these cases, it has been necessary to incorporate fibers in order to improve the ductile behavior of concrete, among other characteristics. There are several experimental studies that analyze the fatigue life of high strength concretes with and without the addition of fibers. In those studies, samples are induced to break after having applied a certain stress level during a certain number of cycles. In the tests described in this paper we analyze the resistance behavior of concrete specimens of 75 MPa of initial strength after subjecting them to different numbers of cycles with tensional levels that do not lead the specimen s rupture. The process is repeated for samples with the same concrete mix adding polypropylene fibers. 2 EXPERIMENTAL PROGRAM The experimental study consists in the evaluation of the resistance remaining in high strength concrete specimens after subjecting them to axial fatigue cycles. We analyzed the effect produced by the incorporation of polypropylene fibers has been analyzed. An experimental program was carried out to investigate the effect of the fiber in the fatigue behavior of a high strength concrete. Different studies included in the references section analyze the fatigue behavior of high resistance concrete and fiber reinforced concrete, where the number of cycles that is capable of withstanding a test piece at a given stress level is obtained. However in this study the goal is to obtain the mechanical strength of a HSC specimen which has undergone a high stress ratio of 35%-50% of its strength resistance and that does not break before

2 2,000,000 cycles. This tensional range is greater than the fatigue strength design value written on Eurocode 2 for fatigue verification: f f = k β (t )f 1- ck cd,fat 1 cc 0 cd 250 The fatigue verification established in the Eurocode marks tensile limits that must not be exceeded. In this case, the tensile ranges applied to the specimens are well above the Eurocode s safety range shown in figure (1) c,máx/fcd,fat c,mín / fcd,fat Figure 1 Goodman s diagram showing the tensile limits established in Eurocode Specimen materials Two different mixes of 24 cylindrical specimens (100x200 mm) were produced. One of them incorporates Enduro HPP45 fibers. This is a macro-synthetic fiber designed for the reinforcement of concrete. Figure 2. Polypropylene fibers 2

3 Mix proportions used are shown in the following table: Table 1 Mix proportions Type of concrete Cement Water reducer Silica fume Fine aggregates Coarse aggregates Fiber (%) Compressive strength at 28 days (MPa) HSC ,57 FRHSC , Specimen testing Tests were performed in the University of Burgos laboratory of large structures. Four specimens were subjected to the same stress level during the same number of cycles for each trial Concrete specimens were subjected to axial compression cycles with a frequency of 6 Hz using a dynamic actuator MTS 500 KN. The stress ratio applied ranged between 35% and 50% of the static compressive strength. This stress level ensures that there is no fatigue exhaustion of the specimen before 2,000,000 cycles. Strain values were obtained during the test using a LVDT. Figure 3. Dynamic actuator Fatigue tests started 120 days after making the specimens. First, the static concrete strength of non-fatigued specimens had to be determined to obtain the stress ratio for the fatigue tests. Subsequently, the stress ratio was applied for different numbers of cycles for each type of concrete: 2,000,000, 200,000, 20,000, and 2,000. With the stress ratio applied a high stress level was guaranteed without reaching the fatigue failure of the specimen. Later on, the fatigued specimens were tested statically to estimate the static compressive strength. 3

4 Strength (MPa) BEFIB2012: Jesús Mínguez, Dorys C. González, Miguel A. Vicente and José A. Martínez 2.3 Results and discussion. Compressive strength All specimens were tested statically to obtain their static compressive strength after the fatigue tests. The values of static compressive strength for the different fatigue cycles and concrete mixes were: Table 2 Compressive strength Number of cycles Average (MPa) HSC Standard deviation (MPa) Average (MPa) FRHSC Standard deviation (MPa) 0 94,68 0,52 92,29 2, ,98 1,98 91,52 12, ,58 2,43 98,40 8, ,99 4,27 98,13 4, ,68 3,79 94,91 8,64 The statistic value is obtained with a probability of p=5% from the results shown above using the following expression. F = Average(f )-1.645σ (2) c(p0.05) c High Strength Concrete p E-1E+0 1E+1 1E+2 1E+3 1E+4 1E+5 1E+6 1E+7 N Figure 4 Comparison of compressive strengths in HSC. 4

5 Strength (MPa) BEFIB2012: Jesús Mínguez, Dorys C. González, Miguel A. Vicente and José A. Martínez Fiber Reinforced High Strength Concrete p E-1E+0 1E+1 1E+2 1E+3 1E+4 1E+5 1E+6 1E+7 N Figure 5 Comparison of compressive strengths in FRHSC The linear fit of the stadistic value of the results obtained is shown in the following expressions: For HSC For FRHSC f c = 93,60-0,96log 10 N f = 84,8-0,38log N (4) c 10 The obtained results show that the compressive strengths obtained are lower in fiber reinforced concrete specimens. In addition, those values show a greater dispersion of results also. Strain behavior (3) 6. Specimen strain is evaluated using first and last fatigue cycle (2,000,000) data as shown in figure. De = ( e s (50%) ) N=1 -(e s (50%) ) N= (5) Figure 6. Strain increase obtained from fatigue hysteresis curve 5

6 The following table shows the strain increment values obtained in the specimens subjected to 2,000,000 cycles of fatigue, for the two types of concrete. Table 3 Fatigue strain Type Strain (%) Average strain (%) HSC 0,036 0,036 0,055 0,049 0,044 FRHSC 0,050 0,043 0,036 0,020 0,037 A typical curve showing longitudinal strain development under the maximum load is presented in fig. 7. Three stages are observed: Stage I is the rapid increase of strain up to 10% of the fatigue life. Stage II is the stabilization between 10% and 80% of the fatigue life Stage III is the rapid increase due to failure. Fig. 7 Typical strain development curve The following figure shows the strain evolution for each concrete in their fatigue life at the maximum load (50% of compressive strength). 6

7 Figure 8 Cycles-strain curves If it corresponds to the curve of figure 7 it can be assumed that half the fatigue life has been exceeded without even reaching stage III (rapid increase due to failure). Fatigue strain shows a fast increase during the first 200,000 fatigue cycles, and then it develops a steady growth. 3 CONCLUSIONS A fatigue study of two types of high strength concrete was performed, one without fibers and another with polypropylene fibers in its matrix. Both types of concrete have been subjected to the same stress ratio and frequency of fatigue. The conclusions obtained are as follows: - Limit values for fatigue testing included in Eurocode 2 are very conservative because for higher stress values than those set in the Eurocode, the remanent concrete s resistance varies very little after being subjected to fatigue from that of a concrete specimen that has not been fatigued. - For the same mix proportions, reinforced concrete with polypropylene fibers presents a lower resistance before conducting the fatigue test, maybe because in high strength concrete the soft fiber content may represent a weakness in the concrete matrix. - Dispersion in compressive strength values is greater in fatigued specimens, so it may be an indicator for the specimen s fatigue damage. This dispersion is greater for FRHSC than HSC. - There is a reduction in concrete s compressive strength when fatigued, compared to non-fatigued. This decrease is more pronounced in concrete without fibers. - The strain increase under fatigue efforts is lower in fiber concrete, which may imply that its fatigue life is higher. - After 2,000,000 cycles the beginning of strain development curve s stage III has not been reached. REFERENCES [1] Abid A. Shah, Y. Ribakov. Recent trends in steel fiber high-strength concrete. Materials and Design 32 (2011) [2] A.E. Naaman & H. Hammoud. Fatigue Characteristics of High Performance Fiber-reinforced Concrete. Cement and Concrete Composites 20 (1998) [3] Handong Yan, Wei Sun, Huisu Chen. The effect of silica fume and steel fiber on the dynamic mechanical performance of high-strength concrete. Cement and Concrete Research 29 (1999) 7

8 [4] Jin-Keun Kim and Yun-Yong Kim. Experimental study of the fatigue behaviour of high strength concrete. Cement and Concrete Research 26, 10 (1996) [5] Paulo B. Cachim, Joaquim A. Figueiras, Paulo A.A. Fatigue behavior of fiber-reinforced concrete in compression. Cement and Concrete Composites 24 (2002) [6] He Xia, Nan Zhang and Guido de Roeck, "Dynamic analysis of high speed railway bridge under articulated trains," Computers and Structures, vol. 81, p , [7] Thomas T. C. Hsu, "Fatigue and microcracking of concrete," Matériaux et Constructions, vol. 17, no. 97, [8] A.E Naaman and H.W. Reinhardt, "High performance fiber reinforced cement composites HPFRCC-4," vol. Cement & Concrete Composites, 2004 [9] C. Zanuy, L. Albajar and P. de la Fuente, "El proceso de fatiga del hormigón y su influencia estructural," Materiales de Construcción, vol. 61, no. 303, pp , 2011 [10] ACI Committee 544, "State-of-the-Art Report on Fiber Reinforced Concrete," American Concrete Institute,