LOAD CARRYING CAPACITY OF FASTENERS IN CONCRETE RAILWAY SLEEPERS

Transcription

1 LOAD CARRYING CAPACITY O ASTENERS IN CONCRETE RAILWAY SLEEPERS Håkan Thun*, Sofia Utsi*, Lennart Elfgren*, Paul Nilsson** and Björn Paulsson** *Division of Structural Engineering, Luleå University of Technology, Luleå, Sweden **Swedish Rail Administration, Borlänge, Sweden Abstract The horizontal load-carrying capacity of fasteners in concrete railway sleepers is investigated. Tests are performed on un-cracked as well as cracked concrete sleepers. Small cracks do not seem to influence the load-carrying capacity and it is first when cracking is very severe that the load-carrying capacity is reduced significantly. 1. Introduction Concrete sleepers are very economic as a base for rails. The fastening of the rails is usually taken care of by fasteners imbedded in the concrete. Due to bad production methods, many sleepers in Sweden, produced during the 90ies, have obtained cracking of a more or less severe kind. The cracking is believed to be cause by delayed ettringite formation, see e g Tepponen & Eriksson (1987). In order to investigate the horizontal load-carrying capacity of the fasteners, a test program is being carried out at Luleå University of Technology in Sweden, Utsi & Elfgren (2000), Thun & Elfgren (2001). The test set-up and the fasteners are illustrated in igures 1 and 2. igure 1. Concrete sleeper with fasteners. Illustration showing test set-up. 774

2 igure 2. Photos of a fastener. View in the direction of the rails (top left) and in the direction of the sleeper (top right and bottom). 2. Loads The horizontal forces that act on a sleeper are partly caused by the centripetal acceleration. It can be written as v 2 /R, for a train travelling with the speed v in a curve with the radius R. In order to reduce this force, the curve can be sloped, i.e. one of the rails is placed higher than the other one, see igure

3 igure 3. orces acting on a mass in a vehicle moving in a curve. The influence of a slope ϕ s is shown to the right. Using the level of the track as reference the horizontal force due to acceleration is: 2 v ay = cosϕ s g sinϕ s (1) R Horizontal forces a y from equation (1) are shown in igure 4 for two cases, a freight train carrying iron ore and a high-speed train. The smallest radii R are used, which exist on the railway line they traffic. rom the figure it can be seen that the maximum force from one axle is approximately 35 kn for the freight train at 70 km/h and 50 kn for the high speed train at km/h. This load is distributed by the rail to two or three neighbouring sleepers. or one fastener the maximum horizontal load will thus be of the order of 12 to 25 kn. Horizontal force [kn] Curve radius: 335 m Train type: Iron ore Axle load: 30 tons h = 0 mm h = 150 mm Train speed [km/h] 100 a) b) Curve radius: 600 m Train type: X2000 (High speed) Axle load:18.75 tons h = 0 mm h = 150 mm Train speed [km/h] igure 4. Horizontal force, a y, as defined in igure,3 as function of train speed, v, and heightening, h, of one sleeper end. (a) reight train with iron ore, R= 335 m. (b) High speed train, R= 600 m. 776

4 3. Material properties The tested sleepers are divided into three categories depending on the cracking: Class 1 Green No cracking Class 2 Yellow Some cracking Class 3 Red Severe cracking The yellow sleepers are subdivided into three categories: Group 1 No cracking on the upper side Group 2 Some cracking on the upper side Group 3 Severe cracking on the upper side The material properties of the concrete have been determined from uniaxial tensile and compression tests on drilled out cores with a diameter of 68 mm, see igure 5. The concrete was specified to have a cube strength of 60 MPa. The cement content was ordinarily 420 kg/m 3. In order to increase the production speed, the cement amount was increased to 500 kg/m 3 and heat was used during the hardening process in some of the production plants. The test results are summarised in Table 1. i = sleeper no. I:1 = tensile I:2 = compression I:3 = reserve i:3 i:2 i:1 Mid section sleeper 9:1p 9:1 12:1p a) b) igure 5. Test of material properties. (a) Location of cores. ( b) Crack planes for the test specimens 9:1p, 9:1 and 12:1p. The mean value for 16 compression tests was MPa with a standard deviation of 5.9 MPa and a coefficient of variation of The mean value for 13 tensile tests was 3.9 MPa with a standard deviation of 0.35 MPa and a coefficient of variation of Three test specimens that had cracks according to igure 5 (b) have not been included in the mean value. If these tests are also included in the mean value the results are 3.3 MPa with a standard deviation 1.29 MPa and a coefficient of variation of The sleepers are reinforced with 10 prestressed strands of 6.5 mm in diameter. 777

5 Table 1. Summary of the results from the compression and tensile tests. Index p means that the core comes from that half of the sleeper that has been exposed to the horizontal pull test. Tensile Sleeper no. Sleeper Compression Sleeper strength a) strength Class No σ c No. σ t [MPa] mean [MPa] mean 8 8: : green 8:2p :1p : :1 (0.90) green 9:2p :1p (0.99) : : yellow/group1 10:2p :1p : : yellow/group1 11:2p :1p : : yellow/group2 12:2p :1p (0.38) 13 13: : yellow/group2 13:2p :1p : : yellow/group3 14:2p :1p : : yellow/group3 15:2p :1p a) Test evaluation according to the Swedish Code BBK 94 (1996). 4. Test results The test arrangement is shown in igure 1. The sleeper was placed on a steel girder and tightened to prevent movement. A hydraulic jack and a load cell were mounted on a bar. To measure the displacement a LVDT was placed horizontally against the fastening. Typical failures are shown in igures 6 and 7 and typical test results are given in igures 8 to 9. In Table 2 a summary of all horizontal shear tests is presented. The horizontal load carrying capacity, kn, for the fasteners in the green and yellow sleepers are much beyond the load imposed by the trains, cf. section 2. Even the red sleeper with a maximum capacity of 18 kn for a deformation of 5 mm may function if it is surrounded by green and yellow sleepers. 778

6 igure 6. ailure of sleeper no. 10. igure 7. ailure of sleeper no

7 Horizontal force [kn] Displacement fasteners [mm] Tested sleepers Nr 7 - red Nr 5 - green Nr 8- green Nr 9 - green Nr 10 - yellow, group1 Nr 11 - yellow, group1 Nr 12 - yellow, group2 Nr 13 - yellow, group2 Nr 14 - yellow, group3 Nr 15 - yellow, group3 igure 8. Result from horizontal shear test of fasteners. Horizontal force [kn] Displacement fasteners [mm] Tested sleepers Nr 5 - green Nr 8- green Nr 9 - green Nr 10 - yellow, group1 Nr 11 - yellow, group1 Nr 12 - yellow, group2 Nr 13 - yellow, group2 Nr 14 - yellow, group3 Nr 15 - yellow, group3 igure 9. Result from horizontal shear test of fasteners. Enlarged detail of figure ailure mechanism The load-carrying capacity according to the Ψ-method, Eligehausen et.al. (1994), can be written as: 0.2 ' href 1.5 Vu =Ψc d fcc c1 d c2 75 á 100 where ψ c = = = á 0.208, see igure10, 1.5c d = diameter, varies between 12 and 60 mm, f cc = concrete cube strength, 100 MPa, and h ref = effective depth, 110 mm. 780

8 Table 2. Summary of all horizontal shear tests. Sleeper no. Class a) Group b) Max. horizontal force max, [kn] 7 red green green green yellow yellow yellow yellow yellow yellow a) Classification performed by the Swedish Rail Administration. b) Classification performed by Luleå University of Technology (LTU). V c 2 c 2 c 1 c 1 = 320 [mm] c 2 = 75 to 100 igure 10.ailure mode of a single anchor loaded in shear when located in a narrow member, Eligehausen et.al. (1994). or c 2 = 75 to 100 mm and d = 16 to 60 mm, the ultimate load V u varies between 52 and 104 kn which can be compared to the test results of kn for the sleepers in classes 1 and 2 (green and yellow). When the failure mechanism is compared for the three classes, the red sleeper shows a completely different failure process then the green and yellow ones. The failure process for the red sleeper is calm and steady, i.e. the fastening was slowly pulled out with no large concrete parts falling of. On the other hand, the failure process for the yellow and green sleepers was explosive. The failure started with a crack growing from the fastening and down towards the base where it was divided into two horizontal cracks, one going 781

9 towards the end and the other towards the mid section. When enough energy was built up large sections of the concrete fell of. Possible failure mechanisms are illustrated in igures l b = 320 e = 130 mm b = 200 igure 11. Possible failure mechanism at shear test of fasteners. If a simplified stress distribution according to figure 12 is assumed, where the tensile stress decreases linearly along the length, l b, an equilibrium equation around A gives: [mm] e = 130 σ l b /3 2l b /3 l b = 320 A H σ l 2l t b b A: e b = e σ = = 1,98 MPa t b l b : τ l b= 0 b e = 130 σ t τ A τ = = 1,62 MPa b l b [mm] l b /3 2l b /3 l b = 320 igure 12. Simplified stress distribution. The stress is distributed along the length l b. The lengths l b and e are measured on the tested sleepers. =103.8 kn (sleeper no. 11). [mm] e = 130 σ l b /9 l b /3 2l b /3 l b = 320 A σ ( ) l 8l 27 e t b b A: e b = 0 σ = t 2 H b l 27 e σ = = 4,45 MPa t b l igure 13 Simplified stress distribution. The stress is distributed along the length l b /3. The lengths l b and e are measured on the tested sleepers. =103.8 kn (sleeper no. 11). b b 782

10 A more realistic assumption is that the tensile stress, σ t, is distributed on a distance of l b /3, see figure 13. The propagation of the crack may be studied if the softening properties of the concrete are taken into consideration, cf. igure 14 and Elfgren (1989, 1998). [mm] e = 130 l b /3 2l b /3 σ t igure 14. ailure process if the softening properties of the concrete is taken into consideration. A l b /3 11l b /18 l b /6 l b /2 l b = 320 H 6. Discussion and Conclusions Small cracks, corresponding to class 2 (yellow sleepers), do not seem to influence the horizontal load carrying capacity of the tested fasteners significantly. It is first when the cracking is very severe (red sleepers) that the load-carrying capacity is reduced so much that it is approaching the level of the applied load. The sleepers produced with inferior methods are now inspected annually in order to see at what rate the cracking is progressing. 7. References BBK 94 (1996): Swedish Handbook on Concrete Design, Vol. 1 and 2. (Boverkets Handbok om Betongkonstruktioner, In Swedish), Boverket, Karlskrona 1994, 1996, 185 pp and 116 pp, ISBN , Elfgren, Lennart, Editor (1989): racture mechanics of concrete structures. rom theory to applications. Chapman & Hall, London, London 1989, 407 pp. ISBN Elfgren, Lennart, Editor (1998): Round Robin Analysis and Tests of Anchor Bolts in Concrete Structures. RILEM Technical Committee 90-MA racture Mechanics of 783

11 Concrete - Applications. Research Report 1998:14, Division of Structural Engineering, Luleå University of Technology, pp. Eligehausen, Rolf, Editor (1994): astenings to concrete and masonry structures. State of the art report. Comité Euro-International du Béton, CEB Bulletin 216. Thomas Telford, London 1994, 249 pp. ISBN Tepponen, Pirjo and Eriksson, Bo-Erik (1987): Damages in concrete railway sleepers in inland, Nordic Concrete Research, Oslo, V , pp Thun, Håkan and Elfgren, Lennart (2001): Testing of concrete sleepers (In Swedish). Project report :1, Preliminary version, ebruary Division of Structural Engineering, Luleå University of Technology, 26 pp. Utsi, Sofia and Elfgren, Lennart (2000): Testing of concrete sleepers (In Swedish). Test Report. May 2000, Division of Structural Engineering, Luleå University of Technology, 20 pp. 784