Optimized High-Performance Concrete in Grouted Connections

Transcription

1 Tailor Made Concrete Structures Walraven & Stoelhorst (eds) 2008 Taylor & Francis Group, London, ISBN Optimized High-Performance Concrete in Grouted Connections S. Anders Bilfinger Berger AG, Mannheim, Germany L. Lohaus Institute of Building Materials, University of Hannover, Hannover, Germany ABSTRACT: Grouted Connections are known in the offshore industry for a long time. In the last about ten years their technology has been projected for the foundations of Offshore Wind Energy Converters with the difference that High-Performance Concrete is applied. One of the striking advantages of Grouted Connections is the equal load-bearing capacity in tension and compression. It is shown that an increasing compressive strength of the grout, increasing shear key height and fiber reinforcement increase the load-bearing capacity of Grouted Connections. At the same time the danger of shear failure of the shear keys and yielding in the steel members raises. Equations for an appropriate design of the steel members are given, depending on the compressive strength of the grout. Thus, High-Performance Concrete for Grouted Connections can be specifically optimized for the demands of the joint. Finally, further applications e.g. for joining steel members or tower-like structures basing on Grouted Connections are presented, which suggest Grouted Connections as a joining technology for the future. 1 INTRODUCTION Grouted Connections, or Grouted Joints, are a wellknown method of fixing offshore structures to the seabed. For fixed offshore platforms usually shear connectors are used to increase the load-bearing capacity. This technology is also used for repair and strengthening of aged offshore structures. In the last decade, the technology of Grouted Connections was transferred to foundations of Offshore Wind Energy Converters (OWEC). In contrast to the offshore industry in Grouted Connections for OWECs High-Performance Concrete (HPC) is projected. Therefore extensive experiments on the static and fatigue performance of HPC in Grouted Joints were performed with focus on the effects of the grout s compressive strength, the fibre reinforcement and the height of the shear keys on the load-deformation behaviour. 2 BASIC CHARACTERISTICS OF GROUTED CONNECTIONS For the design and construction of axially loaded Grouted Connections the American Petroleum Institute (2000), the Health and Safety Executives (2002) and Det Norske Veritas (1998 and 2004) have published recommendations. Figure 1. Grouted Joints in fixed offshore platforms, characteristic properties and design with and without shear keys. According to figure 1 Grouted Joints are made of two steel tubes with different diameters that are connected using grout. The larger tube is called the sleeve, whereas the smaller one is called the pile. In general, the connection can be constructed with or without shear keys. The major parameters that characterize the load-deformation behaviour and the maximum load are the ratio of diameter to thickness (D/t) of pile, sleeve and grout, the compressive strength 369

2 of the grout and the ratio of height to spacing (h/s) of the shear keys. Some recommendations limit the h/s-ratio to 0.1. H/s-ratios exceeding 0.04 have rarely been used, neither in experiments nor offshore. The diameter to thickness ratio does not only characterize the radial stiffness of the tubes but also the confinement of the grout provided by the steel tubes. Three load bearing mechanisms can be distinguished in connections with shear keys: adhesion and friction in the interfaces between grout and steel tubes and the load-bearing capacity of compression struts between the shear keys on the pile and the sleeve. The main failure modes of connections with shear keys are shear failure along the shear connectors for too closely spaced shear keys and crushing of the grout on the stressed side of the shear keys for Grouted Joints with an appropriate shear key spacing. In this case, usually diagonal cracks occur in the grout. More detailed information on the characteristic properties and the failure mechanisms are for example described by the Department of Energy (1982), Billington (1978), Lamport (1988) or Billington et al. (1980). The fatigue performance of Grouted Connections highly depends on the loading regime. According to Hordyk (1996) the slope of the S/N-curve increases with a decreasing stress ratio (R), especially in reverse loading. In case of compression-compression loading the slope of the S/N-curve is small. In either case the number of cycles to failure exhibits large scatter. 3 EXPERIMENTAL RESULTS 3.1 Specimens and experimental programme The specimens that were used in the experiments were designed according to Det Norske Veritas (2004). Limiting conditions were the application of commercially available steel profiles and compressive loading of pile and sleeve without yielding or buckling of the steel. Figure 2 displays the resulting geometry. Compared to the limits given in the regulations the specimens are stiffer and provide a stronger confinement of the grout than covered by the recommendations. The compressive strengths of the grouts were about 60 N/mm 2 (C60), 110 N/mm 2 (C110), 150 N/mm 2 (C150) and 170 N/mm 2 (C170). All grouts were tested with and without fibre reinforcement of 2 vol.-% steel fibres, as well as shear keys with h/s ratios of and More detailed information concerning the tested materials and the testing conditions can be found in Anders (2008). Figure 2. Longitudinal section of the specimens. 3.2 Quasi-static loading In figure 3 the effects of shear keys, the compressive strength of the grout and fibre reinforcement are displayed. The pile shear strength of a specimen without shear keys is about 0.75 N/mm 2. The pile shear strength is increased to about 16 N/mm 2 at an h/s-ratio of If shear keys with h/s = are applied, the pile shear strength can be increased up to 28 N/mm 2. The stress-deformation curves of Grouted Connections show a behaviour that can be divided into two parts. The first linear-elastic part is terminated by the first slip. The second part is characterized by non-linear behaviour. This part ends with the pile shear strength. As stated by Lamport (1988), the first part is characterized by the development of diagonal cracks in the grout. The non-linear, second part of the curve is driven by gradual crushing of the grout in front of the shear keys. In this case, wedges of crushed grout develop which nevertheless transfer the load from the shear keys into the compression struts until the ultimate capacity is reached. The confinement provided by the steel tubes causes the ductility in the stress-deformation curve, even if the High- Performance Concrete is known for a brittle mode of failure. The pile shear stress is calculated by dividing the load by the surface area of the pile. 370

3 Figure 3. Effects of shear keys, the h/s-ratio, the compressive strength of the grout and fibre reinforcement on the static behaviour of Grouted Joints. If steel fibres are used, the stress at first slip as well as the pile shear strength can be increased significantly. The transferable stresses can be increased using a higher compressive strength of the grout, higher shear keys or fibre reinforcement. The effect of steel fibre reinforcement on the stress at first slip and the pile shear strength is an increase of about 25% in the load bearing capacity compared to the grout without fibres, independent of the compressive strength of the grout and the h/s-ratio. In addition, it seems as if the three possibilities to enhance the loadbearing capacity namely the compressive strength of the grout, the fibre reinforcement and the h/s-ratio are independent and can be used additively. 3.3 Fatigue loading In addition to the static tests numerous fatigue tests were performed. The upper and lower stress levels were chosen as percentages of the stress at first slip. Thus, the behaviour of the different concrete mixes subjected to fatigue loading can be compared. The lower stress level is kept constant at 5% of the static stress at first slip throughout the tests. It was found that the ultimate numbers of cycles show a significant scatter as already reported by Hordyk 1996 for tests in compressioncompression. Except for the C110 with h/s = and fibres, all specimens show comparable ultimate numbers of cycles to failure. At least no significant differences between the S/N-curves were found. Consequently the increased static pile shear strengths of Grouted Connections with HPC can be transferred to fatigue loading. More detailed information concerning the fatigue performance of the tested Grouted Connections can be found in Anders (2008). 4 INTERACTIONS BETWEEN CONCRETE AND STEEL MEMBERS The most important interactions between the grout and the steel tubes can be summarised as follows: The forces transferred by the shear keys are the reasons for the compression struts. On the other hand the compression struts can cause shear failure of the shear keys as can be seen in Figure 4. The inclined compression struts induce circular stresses in the pile and the sleeve, which, in turn provide a confining pressure for the grout (Figure 6). The ring-forces in the pile and the sleeve might cause yielding of the steel, because the longitudinal stresses have to be transferred as well. Thus, the equivalent stress in the steel tubes has to be considered. With an increasing compressive strength of the grout, the stresses in the steel members increase, too. Anders (2008) describes a model in which the limiting criteria: shear failure of the shear keys and the grout as well as yielding of pile and sleeve are expressed depending on the compressive strength of the grout. This model is developed on the basis of the PhD-Thesis of Lamport (1988) and the regulations by theamerican Petroleum Institute (2000). According to the American Petroleum Institute the shear keys can be designed by 371

4 Figure 5. Minimum shear key width depending on the compressive strength of the grout. Figure 4. Shear failure of the uppermost shear key, compressive strength of the grout 190 N/mm 2. applying a stress of 2.5 times the compressive strength of the grout on the stressed side of the shear keys. Based on this approach, the minimum shear key width min (w) can be calculated as follows: In this equation A SR,P equals the stressed area of the shear keys, D P the outer diameter of the pile, f y,k the yield stress of the steel, h the shear key height and f cu the unconfined compressive strength of the grout. As an example, the minimum shear key width is given in figure 5, depending on the compressive strength of the grout. In the tests described in chapter 3 a shear key width of 2.5 mm was used. Therefore, failure could be expected at a compressive strength of about 175 N/mm 2. For comparative reasons two specimens with compressive strengths of 150 N/mm 2 and about 190 N/mm 2 are included. The specimen that was grouted using the C150 did not show failure of the shear keys. In the specimen with a compressive strength of the grout of 190 N/mm 2 the uppermost shear key failed. Figure 6 displays the cracks that developed in the grout during a fatigue test. Marked compression struts have developed between the shear keys on the pile and the sleeve. The forces transferred by friction (F R )at the steel-grout interface and the normal stresses acting horizontally on the pile (F N ) and the sleeve can Figure 6. Forces acting on a compression strut in a Grouted Connection according to Lamport (1988). be calculated by the model introduced by Lamport (1988). The normal forces can be calculated using the geometry of the grouted connection. The force transferred by friction depends on the stresses that are induced by the shear keys F SR. To estimate F SR, Lamport applied two plasticity models. A simpler approach has already been given by the American Petroleum Institute (2000) using 2.5 times the compressive strength of the grout. Apart from the 372

5 longitudinal stresses σ L the circumferential stresses σ R have to be taken into account as follows: σ R equals the circumferential stresses, σ L the longitudinal stresses, D P the outer pile diameter, t P the wall-thickness of the pile, t g the wall-thickness of the grout, s the spacing of the shear keys, h the shear key height, µ the coefficient of friction, F SR the force applied by the shear keys, A P the area of the pile in the cross-section, n the number of shear keys on the pile, α the angle of the compression strut against the horizontal and finally F max the ultimate load-bearing capacity of the connection. The compressive strength of the grout material is implicitly considered in the forces F SR and F max. The experiments and the estimation of the loadbearing capacity of the shear keys, pile and sleeve have shown that a separate design of the named parts of the Grouted Connections is necessary in order to ensure the load-bearing capacity of the whole connection. This is especially true for High-Performance Grouts. 5 FURTHER APPLICATIONS AND RESEARCH NEEDS The technology of Grouted Connections is already applied for the rehabilitation and strengthening of aged offshore structures. In this case usually Grouted Connections without shear keys are chosen. Not only thinking about offshore structures Grouted Connections could also be used in large steel structures onshore for rehabilitation, strengthening or joining. Figure 7 shows an example where Grouted Connections are used to connect the substructure and the superstructure of the research platform FINO 1. Similar connections are possible onshore, if hollow steel members have to be connected. One of the major advantages of these connections is that their load-bearing capacity in tension and compression is the same. Another aspect are double-walled concrete filled hybrid members, which could also be connected using grouting technology. Not only small struts in frameworks are imaginable, even tower structures for Wind Energy Converters onshore could be designed using double-walled members. For instance Schaumann et al. (2006) use polymer foams to fill the gaps between the steel walls. Due to their low compressive Figure 7. Grouted Connections at the FINO 1 platform used to connect the sub- and superstructure F+Z Baugesellschaft. strength, High-Performance Concrete should be used when the segments are connected. One has to bear in mind, that in this application very high (D/t)-ratios, thus very low confinement of the grout, are present which are not covered by the existing design rules. In either case, further tests are needed concerning the effects of the radial stiffness on the load-bearing capacity if smaller or larger steel members with a bigger or smaller radial stiffness than in the offshore industry are used. The outlined approaches for the estimation and design should be complemented with further instrumented tests to fix the local stresses and deformations. In addition, the coefficient of friction between steel and grout with further compressive stresses should be investigated. Another aspect is a suitable estimation of the angle of the compression struts. 6 SUMMARY The performed tests have clearly shown that it is possible to increase the load-bearing capacity of Grouted Connections significantly by using High-Performance Concrete. Moreover, the strength and composition of the grout can be adjusted and optimised with respect to the needed load-bearing capacity and ductility of the connection. In addition the interactions between steel and grout were shown. It is described that a sufficiently high compressive strength of the grout can cause shear failure of the shear keys or yielding of the pile and sleeve. The necessary equations to estimate the named failure criteria are also given. Finally, further areas of application namely joining of hollow steel members e.g. in frameworks 373

6 and double-walled construction member for frameworks or tower-like structures are discussed. To sum up, Grouted Connections have proven their potential of becoming a connection technology for future applications. ACKNOWLEDGEMENTS The financial and non-material support of the Stiftung Industrieforschung, Cologne is kindly acknowledged. REFERENCES American Petroleum Institute Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms Working Stress Design. 21st edition, Washington. Health & Security Executives Pile/Sleeve Connections. Offshore Technology Report 2001/016, Norwich. Det Norske Veritas Rules for fixed Offshore Installations. Det Norske Veritas. Det NorskeVeritas DNV-OS-J101 Design of Offshore Wind Turbine Structures. Det Norske Veritas. Department of Energy Report of the Working Party on the Strength of Grouted Pile/Sleeve Connections for Offshore Structures. Offshore Technology Paper, OTP 11. Billington, C.J The Integrity of Jacket to Pile Connections. In, Oceanology International: Billington, C.J. & Tebbett, I.E The Basis for new Design Formulae for Grouted Jacket to Pile Connections. In 12th Offshore Technology Conference: OTC Houston. Lamport, W.B Ultimate Strength of Grouted Pile-to- Sleeve Connections. PhD-Thesis. University of Texas at Houston, Houston. Hordyk, M The Static and Fatigue Strength of Grouted Pile-Sleeve Connections. In Fatigue in Offshore Structures, Volume 2. Oxford & IBH Publishers. Lohaus, L. & Anders, S Static and Fatigue behaviour of High-Performance Concrete in Grouted Joints for hybrid structures. In Proceedings of the 2nd Int. fib Congress, Naples. Anders, S. Betontechnologische Einflüsse auf das Tragverhalten von Grouted Joints (in german). PhD-Thesis, Hannover, Schaumann, P., Keindorf, C., Matuschek, J. & Stihl, T Schalenbeulen von Sandwichzylindern mit einem neuen Elastomer alsverbundwerkstoff. In Stahlbau,Vol. 75, Heft 9, Ernst & Sohn Verlag. 374