High Performance Fiber Reinforced Cement Composites (HPFRCC7), Stuttgart, Germany – June 1-3, 2015 29 INCREASED EFFICIENCY OF COLUMN STRENGTHENING WITH TRC BY ADDITION OF SHORT FIBRES IN THE FINE-GRAINED CONCRETE MATRIX R. Ortlepp Leibniz Institute of Ecological Urban and Regional Development, Dresden, Germany Abstract Textile Reinforced Concrete (TRC) can be used to improve the load carrying capacity of columns. The strengthening effect is due to the confinement effect of the textile reinforcement and the additional concrete cover. Under the multi-axial load distribution, lateral tensile stress arises from the longitudinal pressure in the column. While the fine-grained concrete in the inner range of the TRC jacket can withstand this lateral tension, the outer jacket range, where the textile is anchored in the matrix, is unable to do so. If the tensile strength of the fine- grained concrete is exceeded, the result is spalling of the concrete cover and thus to a loss of cross sectional area, as well as partial damage of the bond surface of the textile reinforcement. This may result in a zipper-like failure of the anchorage so that the textile reinforcement “decoils” from the column. The addition of short fibres to the TRC-matrix can significantly increase the resistance to bond failure of the column confinement by “needling” the fine- grained concrete layers through small openings in the textiles. A TRC-matrix with PVA short fibres was found to be the most efficient of the investigated samples. 1. INTRODUCTION Germany’s vast accumulation of buildings can be viewed as an anthropogenic material stock. The replacement value of this building stock is estimated at around 11.8 trillion Euros in the current national accounts of the Federal Statistical Office [1]. Clearly it is vital to take steps to preserve this value. Currently the average service life of buildings is 66 years. More specifically, residential buildings are used for an average of 77 years, whereas the figures for public buildings and other non-residential buildings are 57 and 53 years respectively (Schmalwasser & Weber [2]). Thus any extension in the useful economic life of buildings would greatly contribute to value retention. Furthermore, a longer utilization time is also desirable from an ecological perspective by conserving natural resources. If buildings are to be reused after the end of the original purpose then it is often necessary to make structural adjustments. This may mean altering the static system and perhaps enhancing the load-bearing capacity of structural elements. However, due to the frequent employment of monolithic building techniques, it can be either impracticable or indeed
Transcript
High Performance Fiber Reinforced Cement Composites (HPFRCC7), Stuttgart, Germany – June 1-3, 2015
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INCREASED EFFICIENCY OF COLUMN STRENGTHENING WITH TRC BY ADDITION OF SHORT FIBRES IN THE FINE-GRAINED CONCRETE MATRIX
R. Ortlepp
Leibniz Institute of Ecological Urban and Regional Development, Dresden, Germany
Abstract Textile Reinforced Concrete (TRC) can be used to improve the load carrying capacity of
columns. The strengthening effect is due to the confinement effect of the textile reinforcement and the additional concrete cover. Under the multi-axial load distribution, lateral tensile stress arises from the longitudinal pressure in the column. While the fine-grained concrete in the inner range of the TRC jacket can withstand this lateral tension, the outer jacket range, where the textile is anchored in the matrix, is unable to do so. If the tensile strength of the fine-grained concrete is exceeded, the result is spalling of the concrete cover and thus to a loss of cross sectional area, as well as partial damage of the bond surface of the textile reinforcement. This may result in a zipper-like failure of the anchorage so that the textile reinforcement “decoils” from the column. The addition of short fibres to the TRC-matrix can significantly increase the resistance to bond failure of the column confinement by “needling” the fine-grained concrete layers through small openings in the textiles. A TRC-matrix with PVA short fibres was found to be the most efficient of the investigated samples.
1. INTRODUCTION
Germany’s vast accumulation of buildings can be viewed as an anthropogenic material stock. The replacement value of this building stock is estimated at around 11.8 trillion Euros in the current national accounts of the Federal Statistical Office [1]. Clearly it is vital to take steps to preserve this value. Currently the average service life of buildings is 66 years. More specifically, residential buildings are used for an average of 77 years, whereas the figures for public buildings and other non-residential buildings are 57 and 53 years respectively (Schmalwasser & Weber [2]). Thus any extension in the useful economic life of buildings would greatly contribute to value retention. Furthermore, a longer utilization time is also desirable from an ecological perspective by conserving natural resources.
If buildings are to be reused after the end of the original purpose then it is often necessary to make structural adjustments. This may mean altering the static system and perhaps enhancing the load-bearing capacity of structural elements. However, due to the frequent employment of monolithic building techniques, it can be either impracticable or indeed
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impossible to replace individual structural elements. Thus in such cases the decision will often be made to strengthen columns. To this end, numerous strengthening measures have been developed.
Strengthening techniques using textile reinforced concrete (TRC), which for some years has complemented traditional strengthening methods using steel reinforced shotcrete and fibre reinforced plastics (FRP), have already shown their value in practice, mainly by increasing bending capacity in situations with potentially little shear force. (i.e. Weiland et al. [3], Schladitz et al. [4], Lorenz et al. [5]). Early textile reinforcement materials were unsuited to strengthening columns. Yet with the development of efficient, coated carbon textiles, it has become possible to obtain sufficiently high levels of stiffness to achieve an effective confining effect (see Ortlepp, Lorenz and Curbach [6]). The increase of the load-carrying capacity when TRC is applied to a column results not only from the column confinement by the reinforcing layer but also from an increase in the cross sectional area.
2. SPECIAL PROBLEMS OF TRC STRENGTHENED COLUMNS
The potential increase in load carrying capacity at column head and base is largely determined by the ability to confine the column’s core. Specific design requirements have therefore to be met in the geometry of the reinforced columns to achieve a useful confinement effect. Based on single tests conducted by Bergmeister [7] and Triantafillou et al. [8], Ortlepp, Lorenz and Curbach [9] undertook comprehensive tests of TRC-confined column heads using various textile reinforcements under a wide range of geometrical parameters. In order to classify the efficiency of the TRC, the improvements in the load-bearing capacity were compared to the results of tests of CFRP-strengthened column heads. The results showed that due to the development of carbon fibre textiles (based on Carbon Fibre Heavy Tows (CFHT)) with a very high load-bearing capacity, reinforcement with TRC is a good alternative to CFRP sheets.
However, the outer jacket range of the column confinement, where the textile is anchored to the matrix, may not be able to withstand the lateral tension arising from the longitudinal pressure in the column (Figure 1). Exceeding the tensile strength of the fine-grained concrete leads not only to spalling of the concrete cover, and thus to a loss of cross sectional area, but also to partial damage to the bond surface of the textile reinforcement. This may result in a zipper-like failure of the anchorage as the textile reinforcement “decoils” from the column (Figure 2a).
This essay describes comparative tests on the enhancement of the load-bearing capacity of column heads by the addition of short fibres to the TRC-matrix of the confinement. The short fibre TRC-matrix can significantly increase the resistance to bond failure of the column confinement by “needling” the fine-grained concrete layers through small openings in the textiles. Thus the load-bearing capacity of the fine grained concrete can be enhanced. At the same time the confinement effectiveness of the textile can also be improved by avoiding unwanted failure modes, such as “decoiling” (Figure 2a), by exploiting the full tensile strength of the fibres (Figure 2b).
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Figure 1: Stresses in the TRC jacket of a strengthened column under normal forces (schematic)
a) “decoiling” of the first textile layer
b) failure by fibre rupture
Figure 2: Possible failure modes of a textile reinforced confinement
Decoiling is caused by debonding in the textile layer. Such a failure is more likely the smaller the effective area of the concrete matrix between the fibres and the higher the bond stress between the fibres and surrounding matrix. While textiles can be optimized in regard to the effective area by maintaining sufficient fibre distances, this results in a decrease of bond stresses and a decrease in reinforcement, and consequently the effectiveness of confinement. Therefore, the idea arose to enhance the load-bearing capacity of the fine grained concrete matrix in the textile layer by means of short fibres which criss-cross the textile in the openings between the textile fibres.
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3. EXPERIMENTAL INVESTIGATIONS
3.1 Materials
3.1.1 Concrete matrices of original columns and TRC-strengthening The columns were constructed of plain normal weight concrete with a maximum aggregate
size of 8 mm. After 28 days the average compressive cylinder strength was 20.7 N/mm² and the cube strength 24.9 N/mm², with a small standard deviation of 0.5 N/mm² and 0.2 N/mm² respectively.
The matrix for the TRC strengthening layer was a fine-grained concrete with a maximum aggregate size of 1 mm. Based on previous investigations by Ortlepp, Lorenz and Curbach [9], the properties of the fine grained concrete were optimised by developing a ready-mixed concrete in collaboration with the company Pagel Spezial-Beton GmbH. This mixture is easy to use and allows efficient application of the confinement layer. The improved fine grained concrete offers compressive strength of 101.9 N/mm² without short fibres. This figure decreased slightly upon the addition of short fibres to 93.2 N/mm² for carbon fibres and 90.8 N/mm² for PVA fibres. The flexural tensile strength without short fibres was 6.1 N/mm². This increased to 10.6 N/mm² with the addition of carbon short fibres and 7.6 N/mm² in the case of PVA short fibres.
3.1.2 Reinforcements of TRC confinement
Textile A carbon fibre heavy tow (CFHT) textile was used as reinforcing textile (Figure 3). During
textile production it was particularly important that the long carbon fibres display as little undulation as possible in order to reduce the unfavourable initial strain and to improve the efficacy of the confinement. The textile’s mechanical properties in the principal direction of confinement are given in Table 1.
Figure 3: Used CFHT reinforcement textile
The textile was applied by wrapping it around the test columns, with the first and last layer anchored within the fine grained concrete matrix. The required anchorage lengths generally depend on the particular textile. After conducting yarn pull-out tests, the anchorage length was set at 450 mm.
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Table 1: Mechanical properties and parameters of textile fabric
name property
unit
CFHT
yarn fineness [tex] 3,500
interstice between yarns (see Figure 3) [mm] 10.8
cross sectional area [mm²/m] 180
Young’s modulus [N/mm²] 204,000
weight without coating [g/m²] 324.1
density [kg/dm³] 1.8
stiffness per layer [kN/m] 36,728
tensile strength [N/mm²] unknown
effective area factor [-] 0.39
Short fibres Concrete mixes containing either polyvinylalcohol (PVA) or disperse carbon fibres at
0.6 % of total volume were tested and compared to a plain concrete mix (Table 2). The desired consistency was achieved by addition of a superplasticizer, whereby the PVA fibre mixture was easier to handle than the carbon fibre mixture.
Table 2: Mechanical properties and parameters of short fibres
name property
unit
carbon PVA
filament length mm 6 12
filament diameter μm 7 40
density kg/dm³ 1.7 1.3
tensile strength N/mm² 3,950 1,620
Young’s modulus N/mm² 238,000 40,000
3.2 Test series, specimen and test setup In order to test the properties of the various concrete mixtures, four test series SF (short
fibres) were conducted with 3 specimens each: SF 1 = unstrengthened, SF 2 = TRC Pagel, SF 3 = TRC Pagel + short carbon fibres and SF 4 = TRC Pagel + short PVA fibres. Series SF 1 served as a reference for comparison to the strengthened columns. Series SF 2
was a second reference mixture to identify the effects of the two short fibre mixtures. In the
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strengthened series SF 2–4 the CFHT textile was wrapped around the columns five times. In view of the previously mentioned anchorage length, this produced three effective textile layers for load-bearing purposes.
The tests were conducted on short columns to exclude the danger of buckling. In order to minimise the lateral confinement effect arising from the clamping of the column ends, the geometry of the specimen was designed according to Eurocode 2 (EN 1992-1-1 [10], [11]), which prescribes a width/diameter to height ratio of 1:2 when testing the uniaxial compressive strength of a concrete cylinder. Here the specimen height was 300 mm and the diameter 150 mm.
The tests were executed using a compression test machine with a maximum load capacity of 1 MN. The test speed was selected according to the Young’s modulus test from EN 12390-1 [12], [13]. The tests were carried out deformation controlled in order to be able to examine the course of failure in the core and the reinforcing layers of the test specimen. At column head the load was only applied through the core cross-section; at column base the full cross section was loaded to reproduce the stress distribution of a continuous column.
3.3 Test Results The maximum load before failure of the series SF 2 specimens (TRC matrix without short
fibres) was on average 144 % of that achieved by the unstrengthened reference series SF 1 (Figure 4). The computational model of Ortlepp, Schladitz and Curbach [14] allows us to differentiate the loading capacities provided by the textile confinement and the fine-grained concrete matrix. Thus 67 % of the total load increase could be attributed to the effective cross sectional area of the fine-grained concrete of the TRC-jacket. Consequently, 77 % of the improved strength can be attributed to the confinement of the textile reinforcement.
Figure 4: Share of load increase from concrete matrices vs. textile confinement
The results for series SF 3 and 4 show that significant load increases can be achieved by the addition of short fibres. The most efficient matrix was concrete with PVA short fibres, for which the maximum load before failure was 41 % above that of pure Pagel matrix.
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Regarding the matrix with carbon short fibres, the load increase compared to the Pagel matrix can be attributed to a moderate increase in the strength of the textile confinement, whereas the strength of the fine-grained concrete layer declined slightly. In contrast, the fine grained concrete matrix with PVA short fibres showed a clear increase in the load capacity of confinement of 34 % compared with the pure Pagel matrix.
Figure 5 shows photographs of the second specimen of each series after column failure. While the specimen SF 2-2 with the plain Pagel mixture failed by decoiling, this kind of failure was not observed in case of series SF 3 with added carbon short fibres. The most significant effect was observed in the TRC jacket with PVA short fibres, where a finer distribution of longitudinal cracks appeared, indicating a more uniform stress distribution and thus a more efficient load-bearing capacity through the addition of short fibres.
SF 2-2 SF 3-2 SF 4-2
Figure 5: Photographs of the specimens after testing
4. CONCLUSIONS
TRC can be used as an effective confinement system for concrete columns to improve strength and ductility. The increase in the load-bearing capacity through a TRC layer is largely due to the confinement of the concrete core. Compared to the confinement effect, the share of the load transferred through the normal forces within the fine grained concrete jacket has previously been comparably low. This, however, can be increased by optimization of the fine-grained concrete matrix to produce higher compressive strength.
Concerning the load bearing of the textile confinement, suitable measures to avoid early failure by decoiling may have a positive effect on the load-bearing capacity of columns by exploiting the maximum tensile strength of the textiles. Tests were undertaken on the potential increase in the efficiency of TRC column reinforcement by the addition of short fibres to the concrete matrix. It was determined that such treatment provides an additional load increase compared with a standard TRC matrix. However, the extent of the load increase is dependent on the particular kind of short fibre. A TRC-matrix with PVA short fibres was found to be the most efficient of the investigated samples.
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ACKNOWLEDGEMENTS
The author gratefully acknowledges the funding provided by the Deutsche Forschungsgemeinschaft (DFG) within the framework of the collaborative research centre SFB 528. She also thanks Prof. Victor Mechtcherine and, in particular, his laboratory manager Dr. Marko Butler and former research assistant Dr. Rabea Barhum from the Institute of Building Materials of the Technische Universität Dresden for their support of this work in the context of a joint supervision of a master thesis.
[2] Schmalwasser, O., Weber, N., 'Revision der Anlagevermögensrechnung für den Zeitraum 1991 bis 2011', in Wirtschaft und Statistik (2012).
[3] Weiland, S., Ortlepp, R., Hauptenbuchner, B., Hankers, Ch. and Curbach, M.: 'Textile Reinforced Concrete for Flexural Strengthening of RC-Structures – Part 2: Application on a Hypar Concrete Shell', in 'Design & Applications of Textile-Reinforced Concrete', ACI SP-251-3, 2008, 41–58.
[4] Schladitz, F., Hoffmann, A., Graf, W., Lorenz, E. and Jesse, F., 'Strengthening of a Barrel Shell with Textile Reinforced Concrete – Part I: Dimensioning and Design', in Proceedings of the ACI 2010 Spring Convention, Chicago, March, 2010.
[5] Lorenz, E., Schladitz, F., Jesse, F. and Curbach, M., 'Strengthening of a Barrel Shell with Textile Reinforced Concrete – Part II: Practical Experience', in Proceedings of the ACI 2010 Spring Convention, Chicago, March, 2010.
[6] Ortlepp, R., Lorenz, A. and Curbach, M., 'Column Strengthening with TRC: Influences of the Column Geometry onto the Confinement Effect', Adv. Mater. Sc. Eng. (2009), Article ID 493097.
[7] Bergmeister, K., 'Verstärkung mit Kohlenstofffasern', Beton- und Stahlbetonbau Spezial (2005) 69-73.
[9] Ortlepp, R., Lorenz, A. and Curbach, M., 'Geometry Effects onto the Load Bearing Capacity of Column Heads Strengthened with TRC', in 'Concrete Engineering for Excellence and Efficiency', Proceedings of the 2011 fib Symposium, Prague, June, 2011 (CEB-FIP) Vol. 2, 1193-1196.
[10] BS EN 1992-1-1:2004: Design of concrete structures. General rules and rules for buildings. [11] DIN EN 1992-1-1: Eurocode 2: Bemessung und Konstruktion von Stahlbeton- und
Spannbetontragwerken – Teil 1-1: Allgemeine Bemessungsregeln und Regeln für den Hochbau; Deutsche Fassung EN 1992-1-1:2004 + AC:2010 (Berlin, Beuth, 2011).
[12] BS EN 12390-1:2000: Testing hardened concrete. Shape, dimensions and other requirements for specimens and moulds.
[13] DIN EN 12390-1: Prüfung von Festbeton – Teil 1: Form, Maße und andere Anforderungen für Probekörper und Formen; Deutsche Fassung EN 12390-1:2000 (Berlin, Beuth, 2001).
[14] Ortlepp, R., Schladitz, F. and Curbach, M., 'Textilbetonverstärkte Stahlbetonstützen', Beton- Stahlbetonbau 106 (2011) 9, 640-648.
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