STRUCTURAL TESTING OF STEEL FIBRE REINFORCED CONCRETE (SFRC) TUNNEL LINING SEGMENTS IN SINGAPORE 1
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John Poh , Kiang Hwee Tan , Graeme Laurence Peterson , Dazhi , Dazhi Wen
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Land Transport Transport Authority, 1, Hampshire Hampshire Road, Singapore Singapore 219428 Department of of Civil Engineering, National National University of Singapore Singapore 10 Kent Ridge Crescent, Singapore 119260
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Keywords: Land Keywords: Land Transport Authority; steel fibre reinforced reinforced concrete; tests; tunnel segments. segments.
ABSTRACT The application of steel fibre reinforced concrete (SFRC) in precast tunnel lining design is a growing trend due to its vast advantages. However, the performance performance characteristics of the material and the design procedures are still not well served by both codes and standards. The Land Transport Authority (LTA) of Singapore sees great potential and prospects in using SFRC in its Mass Rapid Transit Transit tunnels. It has taken the proactive approach approach to introduce introduce SFRC into the tunnel industry in Singapore. LTA has conducted conducted a series of laboratory tests on small scale samples (cubes, prisms and cylinders) and full size segments with the assistance of National University of Singapore (NUS) to determine the material and structural properties of SFRC in order to establish the required performance characteristics and specifications for tunnel lining applications. 3 Tests were carried out on SFRC samples and segments with steel fibre content of 30 and 40 kg/m . The 28-day characteristic characteristic compressive strength was about 60 MPa and the tensile splitting strength about 4.4 MPa. SFRC samples showed higher higher flexural tensile strength at first cracking cracking and at small crack mouth opening opening displacements. displacements. Prototype SFRC segments subjected to a transverse line load exhibited a more gradual gradual drop in load-carrying capacity after after first cracking. The residual strength was about 90% of the first crack load. SFRC segments subjected subjected to a longitudinal longitudinal point load also exhibited more gradual decrease in load carrying carrying capacity after first cracking. cracking. 1. INTRODUCTION For many years, steel fibre-reinforced concrete (SFRC) has been used as shotcrete in the mining and civil industries, industries, primarily for temporary temporary works. works. More recently, recently, it has also been used used for permanent segmental lining in prestigious tunnel projects like the Channel Tunnel Rail Link (CTRL) (King et al. 2001 and Woods et al. 2003) in UK and the Second Heinenoord Tunnel in Netherlands (Kooiman et al. 1998). Steel fibres have also been used in conjunction with conventional steel bars as reinforcement for lining segments. One such example is Barcelona Metro Line 9 in Spain, in which which the quantity of conventional conventional steel bars was reduced by the addition of steel fibres. However, the use of SFRC SFRC in the reinforcement of tunnel segments is still in a relatively early stage, with the design approaches varying with designers and projects. Lack of a generally accepted design code has in some extent hindered the full exploitation of this material.
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This paper presents the findings from an effort by the Land Transport Authority (LTA), Singapore, in collaboration with the National University of Singapore (NUS), to investigate the material and structural performance of SFRC and SFRC tunnel segments in anticipation of its use in future underground projects. 2. OBJECTIVES OF STUDY Traditionally, tunnel segments used by LTA were designed using reinforcing steel bars. In terms of durability, the LTA Design Criteria requires a service life, with appropriate maintenance, of 120 years for all permanent structures. To achieve this, the steel bars must be protected against corrosion. The risk of corrosion in regular steel bar reinforced segments could be reduced by following a stringent specification for gaskets and by using low permeability concrete. However, eliminating the presence of steel bars would be a more direct and efficient way of reducing the risk. Also, with the increasing price of steel bars and socio-economical demand for a sustainable construction industry, LTA has been constantly in earnest search for alternative construction materials. It has recognized the potential of SFRC and the associated benefits of using SFRC in tunnel lining segments (see Table 1), and is taking a proactive approach to introduce SFRC segments into the tunnel industry in Singapore. Due to the limited prototype tests done in the world, LTA has therefore conducted a series of laboratory tests on small scale specimens (cubes, prisms and cylinders) and full size segments with NUS to determine the material and structural properties of SFRC and SFRC segments so as to establish the required performance characteristics and specifications for the use of SFRC segments (Schnutgen et al. 2005). 3. MATERIAL PROPERTIES A test programme was carried out to ensure that all required properties were examined. Tests were conducted on cubes, cylinders and prisms (beams) to determine the material properties, that is, compressive strength, tensile splitting strength and flexural toughness. The tests were conducted according to BS EN 12390-3-2002, BS EN 12390-6:2002 and BS EN 14651:2005 respectively.
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Table 1 - Comparison of SFRC and Steel Bar Reinforced Concrete Segments in Singapore
Design
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Manufacturing
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Steel Bar Reinforced Concrete Segments Building code, Singapore Standard CP65, is used for design Large concrete cover (40mm) is required for durability reason Care manufacture and placing of the steel cages are required Requires large land storage area Inefficient production rate Multiple handling of steel cages
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Handling and Erection
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Care must be exercised in demoulding, transportation and erection as any cracks will compromise the durability Segments repairs required to comply with durability specifications
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Steel Fibre Reinforced Concrete Segments No national design code for steel fibre reinforced concrete yet. Only design guides are available Steel fibre is multi-directional and evenly distributed throughout the section No fabrication of steel cage is necessary. Steel fibres are added in the mix at the batch plant No obstruction in the mould Increased speed of production Less labour needed Less land required for storage Vacuum pads should be used in handling and lifting to minimise damage Minor damage will not compromise the durability Steel fibres provide better protection to edges of the segments
3.1 Mix Design The concrete mix design shown in Table 2 was adopted from an on going tunnel project which used steel reinforcing bars in the tunnel segments. Table 2 Mix Design
Characteristic Strength Cement Content Fine Aggregate Content Coarse Aggregate Content Admixture Content Silica Fume Water / Cement Ratio
60 MPa 3 380 kg/m 3 576 kg/m 3 1292 kg/m 1200 – 1400 ml per 100 kg of cement 3 20 kg/m 0.35
The steel fibres (Dramix RC-80/60-BN) were of the hooked-end type, with a length of 60 mm and diameter of 0.75 mm. In addition to the steel fibres, 1 kg of micro polypropylene fibres (PPF) (Duomix Fire M6) was added for every cubic metre of concrete (Shuttleworth, 2001). 3.2 Specimens and Tests 3 3 Three sample types with target steel fibre content of 30 kg/m (Sample A), 35 kg/m (Sample B) 3 and 40 kg/m (Sample C) were prepared. For each sample type, a. 12 cubes were tested for 28-day compressive strength, and another 12 cubes for 56-day compressive strength; b. 3 cylinders were tested at 28 days after casting to determine the stress-strain relations under axial compression; c. 15 cylinders were tested at 28 days to determine the tensile splitting tests; and
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18 prisms were tested at 28 days to determine the flexural strength at first crack (limit of proportionality) and residual flexural tensile strength.
All specimens were made using a single plant batch for each sample type in a precast factory in Johor, West Malaysia, and delivered to NUS for testing. The specimens were cured with curing compound at the factory until the day of delivery, and kept in a fog room in Workshop 1, NUS, until about 5 days before the day of testing. The tests included: (a) compression tests on seventy-two 150mm x 150mm x 150mm cubes; (b) φ 150mm x 300mm cylinders; (c) tensile splitting tests on forty-five compression tests on nine φ 150mm x 300mm cylinders; and (d) flexural toughness tests on fifty-four 150mm x 150mm x 550mm prisms. 3.3 Test Results and Discussion 3.3.1 Compressive strength The compression tests were conducted in accordance with BS EN 12390-3:2002. A loading rate of th th 450 kN/min or 0.33 MPa/s was applied. The tests were carried out on the 28 and 56 day after casting. th At 28 day (see Figure 1a), most of specimens of Sample Types A and C achieved strengths greater than the specified strength of 60 MPa. Specimens of Sample Type B however had strengths lower than 60 MPa, which was unexpected as they had steel content between those of Samples A and C. th At 56 day (see Figure 1b), the compressive strengths of Sample A and C were similar at about 70 MPa. Again, the strength of Sample B was lower. The inferiority of Sample B is consistently seen in other properties as reported herein. One of the reasons for this could be due to improper mixing of SFRC for Sample B specimens. Defining the characteristic strength as the average strength minus 1.64 times the standard deviation, that is the strength such that only 5% of test specimens would have lower strengths, the characteristic 28-day compressive strengths of Samples A, B and C are 57.4, 43.4 and 60.6 MPa respectively. The characteristic 56-day compressive strength of Samples A, B and C is 64.6, 48.7 and 68.2 MPa respectively. Sample C shows a slightly higher strength than Sample A due to a higher steel fibre content. The results for Sample B are likely to be in error due to the large standard deviation. .3.2 Tensile Splitting Strength The tensile splitting tests were carried out in accordance to BS EN 12390-6: 2002. For each sample type, 15 specimens were tested at the age of 28 days (see Figure 2). The loading rate was between 135 to 180 kN/min (0.032 to 0.042 MPa/s). The tensile splitting strength was slightly higher for Sample A than for Sample C. It was much lower for Sample B due to a corresponding lower compressive strength as reported in the earlier section. The standard deviation was about the same for all sample types. The characteristic tensile splitting strength is about 4.5 MPa, 3.8 MPa and 4.3 MPa respectively for Sample A, B and C, respectively.
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Figure 1 – Cube Compressive Strength
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Figure 2 – Tensile Splitting Strength
3.3.3 Flexural Toughness Tests were carried out in accordance to BS EN 14651:2005 to determine the flexural tensile strength of SFRC. For each sample type (A, B and C), 18 specimens were tested at the age of 27 or 28 days. The crack mouth opening displacement ( CMOD) was measured at a rate of 0.5 mm/min before cracking and 0.2 mm/min after cracking. The limit of proportionality ( LOP) [which corresponds to the flexural tensile strength at first crack] and the residual flexural tensile strengths f R1, f R,2, f R,3 and f R,4 [corresponding to CMOD j (j = 1, 2, 3, 4)], were determined. The values of CMOD1, CMOD2, CMO3 and CMOD4, were taken as 0.5, 1.5, 2.5 and 3.5 mm. The average values of LOP, f R1, f R,2, f R,3 and f R,4 are shown in Table 3. Sample A exhibited the highest average values for LOP, f R,1 and f R,2. The values of f R,3 and f R,4 were however highest for Sample B. Based on these values, specimens A1-12, B1-3 and C1-3 were identified as the average curves for Samples A, B and C respectively. The load- CMOD curves of these representative specimens are compared in Figure 3. The difference in steel fibre content among the three sample types was too small to result in any significance difference in the load- CMOD curves.
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Fi ure 3 – Com arison O Load-CMOD Curves Table 3 – Average Flexural Tensile Strength
4. STRUCTURAL PERFORMANCE OF SFRC SEGMENTS It was envisaged that a typical SFRC rail tunnel in Singapore would have an internal diameter of o 5.8 m and an external diameter of 6.50 m. There would be 7 nos. of 48 ordinary segments and 1 o no. of 24 bolt key in each tunnel lining. Each segment would have a uniform thickness of 350 mm and a width of 1400 mm. The clear distance between the inner straight edges was approximately 2359 mm, and the height of the segment about 600 mm. Each segment weighed about 3.3 tons. In addition to laboratory sample testing, a study was also undertaken on the ordinary SFRC segments to assess the load carrying capacity. Two tests, namely flexural and cantilever load tests were carried out. 4.1 Test Programme The flexural test was specified to investigate the load carrying capacity. The test programme comprised of three specimens each of conventional plain concrete, and steel fibre reinforced 3 3 concrete (SFRC) with steel fibre content of 30 kg/m and 40 kg/m . They were designated FP1 to FP3, FA1 to FA3, and FB1 to FB3, respectively.
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Each segment was supported on two straight edges as shown in Figure 4. One of the roller supports was restrained horizontally and the other was not restrained. Loads were applied at midspan of the segment. An incremental load of 10KN was applied each time. The deflections at midspan were measured by dial gauges. The segments were checked for cracks or distress.
Figure 4 - Flexural test set-up and instrumentation
Cantilever load test was set up to investigate the load carrying capacity at the circumferential edge. The test programme comprised of three specimens each of conventional plain concrete, and steel 3 3 fibre reinforced concrete (SFRC) with steel fibre content of 30 kg/m and 40 kg/m . They were designated CP1 to CP3, CA1 to CA3, and CB1 to CB3, respectively. Figure 5 shows the setup of the test. An incremental load of 10 kN was applied each time. Movement of the segment was monitored by gauges (at least 3 locations). The segments were checked and noted for crack or structural distress at each increment.
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Figure 5 - Cantilever load test set-up and instrumentation
4.2 Test Results and Discussion 4.2.1 Flexural tests For the plain concrete (Type FP) segments, an elastic behaviour was initially observed. The displacements increased linearly with the applied loads. Near the peak loads, the stiffness of the segment started to decrease. A vertical crack quickly formed at the midspan of the segment as the peak load was reached, and this led almost immediately to t he segment being broken into two at the midspan. Figure 6 shows a close up view of the cracked section. For the SFRC (Types FA and FB) segments, the displacements increased linearly with the applied load until a crack formed at the midspan section. For Type FA segments with a steel fibre content 3 of 30 kg/m , the applied load dropped immediately upon the occurrence of the crack until it stabilized at about 80% of the peak load. Thereafter, the load decreased more gradually with increasing displacements.
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Figure 6 - Close-up views of cracked sections 3
For Type FB segments with a steel fibre content of 40 kg/m , the behaviour of FB-3 was similar to Type FA segments while for FB-1 and FB-2, a slight drop in applied load was observed upon the occurrence of the crack at midspan before the load started to pick up again. A second peak in the load-deflection curves was observed before the load started to decrease gradually again. The residual strength of each SFRC segment was determined as the average of the applied load at first cracking and the load at a deflection equal to 10.5 times the first-crack deflection. Type FA 3 segments with a steel fibre content of 30 kg/m had a residual strength of about 140.0 kN, or 85% 3 of the first-crack load. Type FB segments with a higher steel fibre content of 40 kg/m had a
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correspondingly higher residual strength of about 164.5 kN, or 92% of the first-crack load. Figure 7 shows the plots and Table 4 shows the values.
Figure 7 - Load-midspan deflection curves
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Table 4 - Flexural Test Results
The toughness indices and residual strength factors were determined from the adjusted loadmidspan deflection curves following the procedure in ASTM C1018-97. The toughness indices are I 5, I 10, and I 20, which are defined as the areas under the load-deflection curve up to a deflection of 3.0, 5.5, and 10.5 times, respectively, the first-crack deflection, divided by the area up to first crack . Table 5 shows the results from the tests.
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Table 5 - Toughness Index and Residual Strength Factor Results
4.2.2 Cantilever load tests For CP (plain concrete) segments, the segments suddenly broke into two with a loud bang when the displacement reached about 13 to 14 mm (see Figures 8 and 9). For Type CA segments with a steel 3 fibre content of 30 kg/m , the maximum load occurred when the vertical displacement at point C reached about 9 to 11 mm, upon which a crack suddenly appeared next to the support edge, extending from the top to almost the bottom of the segment. The load dropped dramatically to about 120 and 160 kN in CA-1 and CA-2 respectively and to about 40 kN in CA-3 where it stabilized.
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Figure 8 – Appearance of segments after tests
Figure 9 - Close-up views of cracked sections 3
The behaviour of Type CB segments with a steel fibre content of 40 kg/m was similar to CP and CA segments up to the maximum load-carrying capacity (see Figure 10), which occurred at a vertical displacement at point C of between 9 and 11 mm. A crack was seen to appear from the top of the segment near the support edge. However, in contrast to CP and CA segments, the applied load did not drop drastically. Instead it decreased at a decreasing rate with increasing displacements.
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The crack widened as loading was continued. No other cracks were observed. The vertical displacement at point C reached about 25 mm while the crack widened to more than 25 mm when the load has dropped to 100 kN. The steel fibres could be seen to bridge the crack. Figure 8 and 9 show the segment after the tests and a close up of the cracked section indicating the bridging action of the fibres respectively.
Figure 10 - Comparison of Load –Deflection Curves
The ultimate strength (load-carrying capacity) and the corresponding displacement at point C of all segments are tabulated in Table 6. The load-carrying capacities of the three types of segments are comparable to each other considering the variations in concrete strength and experimental errors. However, as mentioned earlier, both Types CP and CA segments failed suddenly with a drastic drop in the load-carrying capacity whereas the load carried by Type CB segments decreased gradually after the occurrence of a single crack at the support edge. It is deduced that the steel fibre content in Type CA segments was insufficient as to prevent the sudden drop in load-carrying capacity.
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Table 6 - Cantilever Load Test Results
5. CONCLUSIONS 3 From the specimen tests, the results obtained were very satisfactory albeit Sample B (35 kg/m of fibres) showed very dubious results. Generally, the samples were able to attain the compressive strength of 60 MPa required at 28 days. As a result, it was possible to achieve an average tensile splitting strength of 4.5 MPa. In terms of flexural toughness, based on average values, Samples A and C showed higher and comparable flexural tensile strength at first cracking and at small crack mouth opening displacements. Sample B showed higher residual flexural tensile strengths at high crack mouth opening displacement. The difference in load- CMOD curves is not significant among the three sample types due to the small difference in steel fibre content. Based on the flexural tests on segments, plain concrete segment do not have the residual strength characteristics and failed suddenly at peak. It can be concluded that for SFRC segments, the steel fibres were effective in bridging the crack, thereby resulting in a more gradual drop in load-carrying capacity and hence more ductile behaviour after the occurrence of first cracking. The first-crack load was higher for segments with a higher steel fibre content due to the bridging effect of steel fibres across microcracks in the concrete that finally led in the formation of the critical major crack at midspan. The residual strength, calculated as the average load-carrying capacity at first crack and at a deflection equal to 10.5 times the first-crack deflection, was 85% and 92% of the first3 3 crack load, respectively for Type FA (30 kg/m ) and Type FB (40 kg/m ) segments. It is also seen that the toughness indices and residual strength factors increased with steel fibre content. The cantilever load tests showed that the displacement increased with the applied load at a gradually increasing rate for all segments, up to the maximum load-carrying capacity. For plain concrete segments, the segments suddenly broke into two with a loud bang at maximum loadcarrying capacity. For SFRC segments, a crack suddenly appeared next to the support edge at maximum load, extending from the top to near the bottom of the segment. The load-carrying capacity dropped dramatically thereafter in CA segments which had a steel fibre content of 30 3 3 kg/m . For Type CB segments with a steel fibre content of 40 kg/m , however, the applied load did
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not drop drastically but instead it decreased at a decreasing rate with increasing displacements. The load-carrying capacities of all three types of segments are comparable to each other considering the 3 variations in concrete strength and experimental errors. A steel fibre content of 40 kg/m is required as in Type CB segments to prevent the sudden drop in the load-carrying capacity after cracking. These results have given the LTA sufficient confidence that the there is enough structural capacity in SFRC for use in tunnel linings. These results will form the basis in specifying the minimum requirements in using SFRC tunnel linings for future rail lines.
ACKNOWLEDEGMENT The authors would like to express their sincere gratitude to the Land Transport Authority for the permission to publish this paper and the assistance rendered by personnel from SPC Industries Sdn. Bhd and Bekaert Singapore Pte Ltd, without which, the tests would not have been such a success.
REFERENCES ASTM C1018-97 (1997), “Standard test method for flexural toughness and first-crack strength of fiber-reinforced concrete (using beam with third-point loading)”, American Society for Testing of Materials. BS EN 12390-3 (2002), “Testing hardened concrete – Part 3: Compressive strength of test specimens”, European Committee for Standardization. BS EN 12390-6 (2002), “Testing hardened concrete – Part 6: Tensile splitting strength of test specimens”, European Committee for Standardization. BS EN 12390 (2000), “Testing Hardened Concrete – Part 5: Flexural Strength Of Test Specimens”, European Committee for Standardization. BS EN 14651 (2005), “Test Method For Metallic Fibered Concrete – Measuring The Flexural Tensile Strength (Limit of Proportionality (LOP), Residual)”, European Committee for Standardization. Kooiman, A.G., Van der Veen, C. & Djorai, M.H. (1998), “Steel fibre reinforced concrete (SFRC) tunnel segments suitable for application in the Second Heinenoord Tunnel”, Proceedings of the XIII Congress on challenges for concrete in the next millennium , Amsterdam, the Netherlands, pp. 719-722. King M. R., Alder A. J. (2001), “The Practical Specification of Steel Fibre Reinforced Concrete (SFRC) For T unnel Linings”, Proceedings of Underground Construction 2001 Conference, London , published by Brintex Ltd. Schnutgen, B., Vandewalle, L. (2003), “Test and design methods for steel fibre reinforced concrete – Background and Experiences”, Proceedings of RILEM TC 162-TDF Workshop, Bochum Germany, 20-21 March 2003. Shuttleworth, P. (2001), “Fire P rotection of Concrete Tunnel Linings”, Tunnel Fires and Escape from Tunnels Conference, pp. 157-165 Woods, E., May, R., Hurt, J., Watson, P. (2003) , “Design of Bored Tunnels on Channel Tunnel Rail Link”, UK, Proceedings Rapid Excavation and Tunnelling Conference , pp. 230-244
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