COMBINED FORMWORK AND TEXTILE REINFORCEMENT SYSTEM FOR A MINERAL CORROSION PROTECTION LAYER FOR OFFSHORE APPLICATION

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1 COMBINED FORMWORK AND TEXTILE REINFORCEMENT SYSTEM FOR A MINERAL CORROSION PROTECTION LAYER FOR OFFSHORE APPLICATION Prof. Dr.-Ing. Ludger Lohaus (1), Dipl.-Ing. Christoph Tomann (1), Dipl.-Ing. Hannes Weicken (1), (1) Institute of Building Materials Science, Leibniz Universität Hannover, Appelstr. 9A, Hannover, ( Abstract: Functional corrosion protection systems are absolutely essential for ensuring the durability of offshore wind energy converters. As an alternative to conventional corrosion protection systems, a thin layer of high performance mortar could be applied around the turbine tower to protect it from the harsh maritime conditions. A flowable mortar with a high density is required in order to prevent chloride penetration that could potentially damage the steel structure. However, such mortars have a relatively high tendency to shrink, which might lead to the formation of cracks in the mineral layer. These cracks increase the possibility for the ingress of water and chlorides penetrating through the mortar, thereby raising the risk of corrosion. To reduce these cracks respectively, the maximum crack width and to ensure an adequate layer thickness, the high-performance mortar (HPM) has to be applied by using textile reinforcement and suitable formwork. This paper presents concepts for a possible realization of a textile reinforced mineral corrosion protection system for offshore wind energy converters (OWEC). To this end, different textile reinforcements were investigated with regard to their suitability for application and crack width limiting properties by using a model steel tower. INTRODUCTION Maritime conditions provide the current corrosion protection systems for offshore wind energy converter (OWEC) for a major challenge. By the combined effects of seawater, alternating cycles of wet and dry and the mechanical abrasion, present corrosion protection systems are, especially in the area of the splash-zone (cf. Fig.1), exposed to high stresses. The currently used organic corrosion protection systems are manufactured in a multistage procedure, in a thin layer thickness with tight tolerances. In consequence of this complex manufacturing process, a constant quality is very complicated to ensure. As a result of the thin layer thickness, these protection systems are prone to transport damages and to the assembly process. In addition, maintenance measures of these systems are very complex and expensive. As an alternative to these conventional systems, a concept for a mineral corrosion protection system was developed at the institute of building materials science (IfB) at the Universität Hannover. In such a system, a high performance mortar (HPM) is applied to the foundation structure of an OWEC, in the highly stressed splashzone. Next to the initial corrosion protection, such system suits, as an offshore maintenance measure due to the hydraulicity of the cementitious binder. In addition to the various concrete technology problems concerning the durability, which were tested in literature [1] and verified for an undamaged case, cracks within the HPM layer must be analyzed due to 287

2 the part-specific requirements. Therefore, crack width limiting reinforcements are examined for their suitability by using a model steel tower is discussed in this paper. Fig. 1 schematic representation of the corrosion rate [2] MINERAL CORROSION PROTECTION SYSTEM To realize such a mineral corrosion protection system, a suitable formwork and textile reinforcement has to be applied around the steel tower and filled with a HPM [3]. The passivating properties of cementitious building materials are well known from the classical reinforced concrete design. However, since the planned layer thickness of the mineral corrosion protection system (10 mm) is significantly lower than the required minimum concrete cover in literature [4] for normal hydraulic concrete structures, the used mortar has to be very dense and durable. Due to shrinking processes, cracks can arise in the mineral corrosion protection layer because of the integrated steel tower (cf. Fig. 2). Since separation cracks can be expected in this case, in the area of cracks an increasing ingress of chlorides can occur penetrating up to the steel surface. As a result of self-healing effects of cementitious systems, where occurring cracks can be sealed, by the reaction of unhydrated cement, a precipitate CaCO3 and the plugging of tiny components, cracks don t necessarily imply a failure of the whole corrosion protection. The results of literature [5], in which the water permeability and the self-healing effects of separated cracks in concrete were investigated, show an entire sealing of dynamically stressed cracks after a duration of at least 15 weeks up to a crack width of 0.2 mm. In order to guarantee a reliable and durable corrosion protection, the maximum crack width, has to be limited by using suitable textile reinforcement. Fig. 2 cracks in the protec. layer 288

3 EXPERIMENTAL PROGRAMM TEST FACILITY In order to limit the crack width, different systems of different reinforcement materials with different positions in the mortar layer are applied. To investigate such systems, a model steel tower, with a diameter of about 90 cm and a circumference of about 275 cm is used (cf. Fig. 3). In this study, five different reinforcement systems with a thickness of 10 mm and a height of 20 cm are applied around the steel tower and filled with HPM. For the application a three dimensional spacer fabric (System S-I), a polymer-coated textile reinforcement (System S-II, S-III, S-IV) and a conventional reinforcement mesh, known from plaster applications, were used. The three dimensional spacer fabric of system S-I represented the reference system and simulates an un-reinforced situation. Furthermore the textile reinforcements of the other systems are applied in different position in the mortar layer. A schematic representation of the system structures the positioning of the textile reinforcement and their technical data is shown in Fig. 4. To guarantee a sufficient flow path, which is necessary for self-compacting mortars, the specimens were filled from the bottom-up, by using an inlet which is integrated into the formwork. In order to analyze the cracking behavior and to monitor the filling process, a transparent formwork surface is used. As an alternative to the closed formwork surface, a permeable stainless steel mesh was applied in system S-V. The background of this exchange was, to create an additional possibility for deaerating the mortar and to integrate the outer surface of the formwork as an additional reinforcement layer. Due to the reduced stiffness of the stainless steel mesh, the formwork has bulged during the filling process. For this reason, a defined layer thickness could not be maintained. In addition, the other systems show an adequate deaerating process of the mortar, therefore this system (S-V) is not considered hereafter. The water curing of the mortar was guaranteed by the closed formwork for 72 hours. The formed cracks were visually detected with a crack magnifier and documented directly after stripping of the formwork in regular intervals up to a supposed end of the crack initiation. Fig. 3 test facility 289

4 S-I S-II S-III S-IV S-V steel tower central zone TR1 TR1 outer zone TR1 TR2 formwork surface technical data of the textile reinforcement (TR) material TR1 alkali-resistant glass fiber TR2 carbon fiber reinforced plastic Coating epoxy resin - mesh width [mm] 10 5 mortar layer 10 [mm] Fig. 2 positioning and characteristic data of the textile reinforcement thickness [mm] 2 1 RESULTS INTEGRATION OF THE TEXTILE REINFORCEMENT The surfaces of the systems show, that an entire filling process and therefore a complete and spotless integration of the textile reinforcement is possible (S-I, S-II). Only those systems, in which the textile reinforcement is positioned in the outer surface zone (S-III, S-V), show tiny isolated defect spots in the area of the reinforcement meshes (cf. Fig. 5). System S-I System S-II System S-III System S-IV Fig. 3 surface of the specimens Due to the fact, that the surface of the formwork is directly positioned on the textile reinforcement, air voids were entrapped and for this reason the surface of the mortar layer is not homogeneously closed. Consequently the entrapped air produces voids into the layer surface, which partially reduces the cross-section of the mineral corrosion protection system and leads to a reduced durability of the whole protection system. In conclusion, the systems S-I and S-II could guarantee a complete integration of the textile reinforcement. In contrast, the systems S-III and S-IV could not archive this entire integration, but with a modification of the system structure, a significantly improved result is expected. 290

5 CRACK DISTRIBUTION The crack distribution of the specimen is shown in Fig. 6. Presented are the number of cracks (diagram A), the overall summed crack width (diagram B) and the maximum crack width (diagram C) for each applied system. The x-axis of all diagrams is, for reasons of clarity, scaled up using the root square of time. The test results of these investigations were documented over a period of 36 days after stripping of the formwork. System S-I, which represents the reference specimen, is the only system which showed cracks directly after stripping of the formwork (cf. Fig.6). Consequently, the integrated three dimensional spacer fabric was not able to carry the stresses in the early phase, which probably resulted from the autogenous shrinkage process of the HPM. This could be due to the low strength of the used spacer fabric. Furthermore, system S-I shows only a small increase in the number of cracks over the observed period. Systems S-II to S-IV, however, features up to 65 cracks in the first 6 days after stripping of the formwork. As a result of the high number of cracks, the integrated textile reinforcement is activated in these systems, thereby limiting the crack width. Furthermore, diagram A shows that system S-III with a positioning of the textile reinforcement in the outer zone, leads to a lower number of the cracks in comparison to system S-II where the same textile reinforcement was positioned in the central zone. The descending graph in system S-IV reveals the coalescence of different cracks. After 12 days, the number of cracks is constant in all systems which can be explained by the end of the bulk shrinkage process. Diagram B in Fig. 6 showed the overall summed maximum crack width of all cracks for each system. System S-I features, as the only system, an overall summed crack width of 1.4 mm, directly after stripping of the formwork. The other systems, however, have their first summed crack width value, one day after stripping of the formwork. Generally all systems show a potentially similar trend. Corresponding to the number of cracks, the value of the summed crack width increases within the first 12 days in all systems, up to a maximum value of 6 mm in system S-IV. After 12 days the summed crack width remains constant in all systems over the documented time period, which means, that the leading crack distribution is practically completed. In this context, system S-III, which has a polymer coated textile reinforcement, positioned in the outer zone, leads, in regard to the overall summed crack width, to the lowest value of all documented systems. System S-II and S-III shown in diagram C reveal that the used polymer coated textile reinforcement is able to limit the crack width, irrespective of its positioning in the mortar layer (central or outer zone), up to a limit of 0.1 mm over the whole time period of 36 days. Consequently, the maximum tolerable crack width required in literature [5] is maintained, which is necessary for a positive self-healing effect. System S-I with a maximum crack width of 1.3 mm and S-IV with a maximum crack width of 0.4 mm, 12 days after stripping of the formwork, however, exceeded the tolerable crack width limit. Generally, the results of crack distribution in these first tests demonstrate that the positioning of the textile reinforcement in the outer zone of the specimen is preferable. Since this application, however, may leads to voids in the mortar surface (cf. results of the integration of the textile reinforcement), a modification of the spacer bars of the textile reinforcement should be conducted to create a homogeneous closed mortar surface and to further develop the mineral corrosion protection system. In this case, a thin gap between the formwork surface and the textile reinforcement could improve the result. 291

6 SUMMARY AND OUTLOOK Since the maritime conditions, require particular measures for the corrosion protection, especially in the splash-zone, a durable protection system is required. To ensure these high durability properties with a mineral corrosion protection system, a closed and dense mortar surface is necessary. Due to the structural and geometric boundary conditions, cracks are not preventable. However, crack widths could be reduced through the use of textile reinforcement and self-healing effects of the used cementitious materials. In order to guarantee a durable corrosion protection, the textile reinforcement should be homogeneously integrated and the maximum crack width should be limited. The investigations have shown, that the textile reinforcement can be completely integrated into a thin mortar layer and as expected, the crack width can be significantly reduced. The most significant effect was found in relation to the maximum crack width. In this context, the experimental study demonstrated that a maximum crack width of 0.1 mm could be realized. This crack width is according to literature [5], in the range of a self-healing effect with a potential to a complete crack sealing. Since the study in literature [5], were carried out with tap water, the scope of the self-healing effects, needs further evaluation under offshore conditions. In conclusion the first tests shown, that the mineral corrosion protection system might work. Therefore, it offers a high potential for offshore applications either for an initial corrosion protection than for maintenance purposes. However, for the realization of such a system, the application techniques and the self-healing effects of the high performance mortar should be further examined and optimized. In this context, the Institut of Building Materials Science in Hannover is currently doing some research works with regard to the influence of chloride penetration into cracks, selfhealing mechanisms and the effects of self-healing supportive additives, especially under offshore conditions. 292

7 Fig. 4 crack distribution after stripping of the formwork note: x-axis is scaled up using the root square of time 293

8 REFERENCES [1] Weicken, H. ; Lohaus, L. (2014): Measures for autogenous shrinkage compensation and their influence on selected durability properties, International RILEM Conference on Application of Superabsorbent Polymers and Other New Admixtures in Concrete Construction, RILEM Proceedings PRO95, pp , Dresden, 2014 [2] S. Steppeler; G. Haake (2008): Forschung im ersten deutschen Offshore-Windpark, Stahlbau 77, Heft 9, S [3] Lohaus, L. ; Huhn, H. ; Weicken, H. ; Zockoll, A. ; Peters, N. C. et al. (2011): Ganzheitliches Dimensionierungs-konzept für OWEA-Tragstrukturen anhand von Messungen im Offshore-Testfeld alpha ventus, BMU GIGAWIND alpha ventus, Jahresbericht 2010, Hannover, 2011 [4] DIN (2008): Concrete, reinforced and prestressed concrete structures - Part 1: Design and construction ; German version [5] Edvardsen, Carola Katharina: Wasserdurchlässigkeit und Selbstheilung von Trennrissen in Beton. DAfStb 455, Rheinisch-Westfälische Technische Hochschule Aachen,