SHEAR STRENGTHENING OF PRESTRESSED CONCRETE BEAMS WITH TEXTILE REINFORCED CONCRETE

Transcription

1 SHEAR STRENGTHENING OF PRESTRESSED CONCRETE BEAMS WITH TEXTILE REINFORCED CONCRETE Martin Herbrand, RWTH Aachen University, Institute of Structural Concrete, Mies-van-der-Rohe Str. 1, Aachen, Germany, Abstract: A large part of existing highway bridges in Germany exhibits calculative shear capacity deficits under static and cyclic loading. More structures are therefore expected to demand refurbishment and strengthening within the next years. Besides common strengthening methods, the use of textile reinforced concrete (TRC) offers an innovative alternative for strengthening measures by combining the advantages of lightweight glued CFRPstripes and additional concrete layers. Two full scale tests on I-shaped prestressed concrete beams under cyclic shear loading were therefore carried out at the Institute of Structural Concrete at RWTH Aachen University. Previous tests on identical non-strengthened beams served as a reference for the tests on beams strengthened with TRC. These preliminary tests have shown that a considerable increase of the shear fatigue and static shear strength can be obtained by strengthening the web with TRC. By increasing the shear strength with TRC, the service life of deficient concrete bridges can thus be extended efficiently. INTRODUCTION Many bridges in Germany and other European countries were built in the 1960s and 70s, making assessment, maintenance and refurbishment of the existing network increasingly important 1. Furthermore, the German design rules have changed reducing the calculative shear capacity of concrete members without shear reinforcement and increasing the required minimal shear reinforcement ratios for members with shear reinforcement. The shear design of many existing bridges was performed using the principal tensile strength criterion according to former design codes so that no shear reinforcement was necessary. Current design codes 2 require a minimum shear reinforcement that exceeds the existing shear reinforcement of many existing bridges. Hence, a large number of existing concrete bridges in Germany is deficient according to current design codes. As mentioned before, the shear design for the bridge superstructures in longitudinal direction was based on the principal tensile strength criterion according to the German code for prestressed concrete 3,4. In contrast, the shear check according to the current design code for concrete bridges 2 is based on the so-called strut-and-tie model with crack friction 5. Since the load of the load model for design of bridges has been increased several times in the past 6,7 because of rising traffic loads (Fig. 1) and due to the more conservative new shear check, more shear reinforcement is now required in the web. 465

2 9 Traffic Volume [10 tkm] Highway Railway Ship Stone; 0,6% Composite; 5,6% Steel; 7,7% Timber; 0,1% Concrete; 19,6% Prestressed Concrete; 66,4% a) b) Fig. 1: a) Heavy Goods Traffic in Germany 8; b) Materials of highway bridges 9 Therefore, more structures are expected to be strengthened within the next years. Since a replacement of these bridges is not possible or reasonable in many cases, the analysis, evaluation and development of efficient strengthening methods becomes more important. Many strengthening methods have proven to be suitable for the shear strengthening of bridges, e.g. additional external prestressing, additional concrete layers, additional steel reinforcement in slots or glued CFRP-stripes (Carbon Fiber Reinforced Polymer). However, the applicability and effectiveness of these methods are also influenced by some disadvantages. Besides these common strengthening methods, the use of textile reinforced concrete (TRC) offers an innovative alternative for strengthening measures by combining the advantages of lightweight glued CFRP-stripes and additional concrete layers, which possess better bond characteristics and lower temperature sensitivity. As the textile reinforcement does not require protection against corrosion, thin layers are possible. For the above reasons, full scale tests on I-shaped prestressed concrete (PC) beams (h = 0,7 m, l = 6,5 m) under cyclic shear loading were carried out at the Institute of Structural Concrete at RWTH Aachen University. Previous tests on non-strengthened beams served as a reference for the tests on members strengthened with textile reinforced concrete. EXPERIMENTAL INVESTIGATIONS Test setup Two full scale shear tests under cyclic loading were performed on I-shaped prestressed concrete beams strengthened with Textile Reinforced Concrete (TRC). The experimental results of the strengthened test beams were compared to similar test beams from previous research projects without TRC which served as a reference 10,11. The test setup is shown in Fig 2. The beams had a total length of 6,5 m and cross-section height of 0,7 m. The point loads were located in the third points of the beam so that the shear slenderness of the specimens amounted to a/d = 3,3. 466

3 Fig 2: Test setup and position of the point loads The cross-section of the test beams had a total width of 0,6 m and a web width of 0,1 m. The cross-section and the reinforcement layout of the test specimens are shown in Fig 3. The first test was performed on a member without shear reinforcement strengthened by several layers of TRC (I-O-5 TRC). The second test was performed on a member with a low amount of shear reinforcement (ρ w = 0,22%) which was also strengthened by TRC (M-22-7 TRC). The test beams were subjected to 1,2 to 3,1 million load cycles using different highest and lowest loads Ø Ø10 2Ø8 StØ8/12 5 2Ø8 BüØ8/ TRC Layer (25 mm) TRC Layer (25 mm) 100 StØ6/25 ( =0,22%) ρ w 15 2Ø8 2Ø StØ8/25 6Ø StØ8/25 6Ø10 a) 400 b) 400 [mm] Fig. 3: a) cross-section of the test specimen without shear reinforcement (I-O-5 TRC) b) cross-section of the test specimen with shear reinforcement (M-22-7 TRC) Material properties The steel reinforcement in each beam consisted of normal strength steel bars (f yk = 500 MPa). The beams were prestressed using two tendons, each consisting of three 0,6 (15,2 mm) strands of prestressing steel St1570/1770 with a cross-sectional area of mm². An additional layer of 25 mm of TRC was applied on each side of the web after prestressing. Normal strength concrete with a compressive strength of f cm = 43 MPa similar to many existing bridges was used. The concrete was planned to be similar to the test beams without TRC (I-O-5 and M-22-7). However, the concrete strength of the members with TRC turned out to be higher than of the previous beams. The compressive cylinder strength f cm,cyl and the modulus of elasticity E cm were measured on concrete cylinders 467

4 (300 mm height, 150 mm diameter) and the centric tensile strength f ctm on core samples (91 mm height, 45 mm diameter) of a small sample beam on the day testing started (Tab. 1). Additionally, the yield strength f 0,2, the tensile strength f t and the modulus of elasticity E s of the Ø6 stirrups are also given in Tab. 1. Tab. 1: Material properties Specimen f cm,cyl Concrete f ctm E cm f 0,2 The shear reinforcement ratios of the test beams ρ w, the prestressing force P mt on the day testing began and the initial shear crack force V crack are summarized in Tab. 1 for each specimen. For the members without shear reinforcement V crack is a calculated value, since the fatigue of the concrete was investigated. For the members with shear reinforcement V crack is a measured value, since shear cracks occurred during the initial loading. Textile reinforcement material In order to ensure an adequate combination of matrix and textile reinforcement for the strengthening of the beams, tensile tests were conducted on specimens with dimensions of 100 mm width, 880 mm length and a thickness of approximately 25 to 30 mm (Fig 4, b)). Combinations of two different matrices (shotcrete with a maximum aggregate size of 4 mm and polymer-modified dry-spray mortar (SPCC) with a maximum aggregate size of 2 mm) and one to four layers of textile grid (alkali-resistant glass /carbon impregnated with epoxy resin / styrene-butadiene / unimpregnated carbon grid) were investigated. In total, 32 tensile tests were performed. The stress-strain curve of the two specimens V12-1 and V12-2 with SPCC and four layers of unimpregnated carbon grid (Fig 4, a)) is shown in Fig 4, c). These test specimens exhibited the highest initial cracking stresses of all specimens (σ c = 2,6 MPa). Moreover, the unimpregnated carbon grid was easy to apply during the pretests. Therefore, unimpregnated carbon grid with an area of a t = 55 mm²/m, which was also previously used in an innovative structural project 12, was used as textile reinforcement material. Theoretically, single carbon fibres have a tensile strength of up to 3950 MPa with a modulus of elasticity of MPa. However, the actual tensile strength of unimpregnated carbon grid in combination with concrete can be much low er. The mean tensile strength of the carbon grid in these tests was σ t = 1136 MPa. Steel f t E s ρ w [%] P mt σ cp,mt I-O-5 29,4 2, , I-O-5 TRC 42,5 2, , M ,0 2, , , M-22-7 TRC 43,0 3, , , V crack 468

5 Fig. 4: a) unimpregnated carbon grid; b) test setup for tensile tests; c) stress-strain curve of TRC with two layers of unimpregnated carbon grid Fig. 5: a) Web of specimen after sandblasting ; b) determination of depth of roughness; c) application of shotcrete; d) application of textile reinforcement Strengthening of the test beams The strengthening of both beams was performed 2-3 weeks after prestressing. In the first step both sides of the web were roughened by sandblasting (Fig 5a) after which the depth of roughness was measured by applying gypsum to an area of cm on the web (Fig 5b). The depth of roughness was calculated from the amount of plaster that was necessary to create a planar surface on the web. The values had a range of 1,1 to 2,4 mm. In the se- 469

6 cond step, shotcrete and the textile reinforcement were applied alternately. Two layers of textile reinforcement and three layers of shotcrete were applied on each side of the web of both specimens (Fig 5c and d). The used shotcrete was a polymer-modified dry-spray mortar (SPCC) with a maximum grain size of 2 mm. The thickness of each layer of shotcrete averaged about 8 mm so that the total thickness of the reinforcement amounted to about 25 mm. The mechanical properties of the shotcrete were determined by prism testing with a compressive strength f cm,prism = 49,9 MPa and a flexural tensile strength of f ctm = 5,6 MPa. After the strengthening procedure the surface of the TRC layer was moisturised for at least three days in order to ensure proper hydration. Test program In the first step, both specimens were subjected to an initial static loading until the highest load V max was reached. The specimens were then subjected to cyclic loading with a frequency of 1,0 to 2,5 Hz depending on the maximum deflection. The initial highest load V max of the member without shear reinforcement (I-O-5 TRC) was defined as 75 % of the theoretical shear crack load V crack according to the principal tensile stress criterion in order to induce fatigue failure in the concrete. After 10 6 load cycles the maximum load was increased to 90 % of V crack (Tab. 1). The test was then aborted after an additional 0, load cycles. Tab. 2: Cyclic loading on the specimens Specimen I-O-5 Load cycles 10 3 V max N i V min ΔV ΔN i I-O-5 TRC M M-22-7 TRC The specimen with shear reinforcement (M-22-7 TRC) was subjected to a maximum load of 110 % of the actually measured shear crack load V crack (Tab. 1) in order to induce fatigue failure in the stirrups. The number of load cycles N i and the corresponding maximum and minimum loads of each specimen are summarized in Tab. 2. After the cyclic testing the remaining shear capacity of both specimens was determined. The applied load was determined by measuring the hydraulic pressure in the jack. Two displacement transducers were placed in the third points to measure the deflections of the beams. Photogrammetry was used for measuring crack widths in the web in a rectangle of 300x225 mm (Fig 6a). Four sets of three displacements trancducers were used on the web to measure crack widths and the angle of the principal stresses. In addition, the concrete strains on the surface of the TRC were measured by strain gauges (Fig 6b). 470

7 Fig. 6: a) Photogrammetry for determining shear crack widths; b) displacement transducers and strain gauges on the web Experimental results The deflection of the beams during the cyclic loading is illustrated in Fig 7. It shows that the beam I-O-5 TRC exhibits larger deflections than the beam I-O-5 due to the highest load, which was increased by 40 %. However, the beam strengthened with TRC did not show any signs of fatigue failure. In the following step the loading was increased further so that the maximum load was almost equal to the initial shear crack load of I-O-5. At this level the original beam without TRC would have failed immediately, whereas the beam with TRC sustained another load cycles. Although additional load cycles would have been possible, the testing was aborted at this point due to large deflections. The remaining capacity of the specimen I-O-5 TRC was determined to V u = 233 kn, whereas the original specimen without TRC had a remaining capacity of V u = 158 kn. Deflection [mm] 20,0 15,0 10,0 5,0 I-O-5 TRC I-O-5 V max/min = 141/79 kn V max/min = 103/47 kn Increase of loading Modification of test setup Deflection [mm] 0, a) Load cycles b) 25,0 20,0 15,0 10,0 5,0 0,0 Modification of test setup Increase of loading V max/min = 204/147 kn V max/min = 160/103 kn V max/min = 204/118 kn M-22-7 TRC M Load cycles Fig. 7: Deflection at maximum load subject to the number of load cycles for a) beams without shear reinforcement; b) beams with shear reinforcement The highest load of the specimen M-22-7 TRC was increased by about 30 % compared to the previous specimen M In the previous experiment various stirrups failed during the first 10 6 load cycles which can be seen from the progression of the load deflection curve (Fig 7b). In contrast, the beam strengthened with TRC did not exhibit any damage on the stirrups after load cycles, after which the amplitude was increased further. After the increase of the amplitude some stirrups failed and the deflection grew moderately. Even then, the beam was able to sustain another 1, load cycles after which the test 471

8 was aborted. The remaining capacity of the beam M-22-7 TRC amounted to V u = 350 kn. The remaining of the original beam M-22-7 was not determined due to its considerable damage in the stirrups. However, another previous test beam M-22-3 with the same prestressing but subjected to smaller highest loads had a remaining capacity of V u = 264 kn. It can therefore be concluded that the TRC strengthening had a considerable effect on the remaining shear capacity for the beams with shear reinforcement as well. CONCLUSIONS AND FUTURE WORK The use of textile reinforced concrete (TRC) offers an innovative alternative for strengthening measures by combining the advantages of light glued CFRP-stripes and the better bond characteristics of an additional concrete layer. A possible field of application has been investigated and described in the paper: Firstly, a considerable increase of the shear fatigue strength can be obtained by strengthening the web with TRC. Secondly, the static shear capacity also increases considerably due to the TRC strengthening. In addition to this preliminary comparison of the tests, a more detailed investigation is required regarding the actual fatigue design check of beams strengthened with TRC. Here, further experimental investigations can show to what extent the models for the fatigue design of prestressed concrete beams with and without shear reinforcement can be applied if textile reinforced concrete is used. REFERENCES 1. Naumann, J. (2010), Brücken und Schwerverkehr - Eine Bestandsaufnahme ( Bridges and Heavy Goods Traffic an Inventory ), Bauingenieur Vol. 85, No.1, pp DIN EN (2013), Eurocode 2: Design of concrete structures - Part 2: Concrete bridges - Design and detailing rules; German version, April DIN 4227 (1953), German Design Code Prestressed concrete - Guidelines for design and construction. 4. BMV (1969), German Design Guideline: Additional provisions to DIN 4227 for prestressed concrete bridges, Federal Ministry of Transport, Building and Urban Development. 5. Reineck, K.-H. (2001), Hintergründe zur Querkraftbemessung in DIN für Bauteile aus Konstruktionsbeton mit Querkraftbewehrung, Bauingenieur 76, Iss. 4, pp DIN Fachbericht 101 (2009), German Design Code Actions on Bridges. 7. DIN EN (2012), Eurocode 1: Actions on structures - Part 2: Traffic loads on bridges; German version, August Ickert, L. (2007), Abschätzung der langfristigen Entwicklung des Güterverkehrs in Deutschland bis 2050, Final Report for the Federal Ministry of Transport, Building and Urban Development of Germany, Basel. 9. Kaschner, R.: Auswirkungen des Schwerlastverkehrs auf die Brücken der Bundesfernstraßen. Heft B68, Bundesanstalt für Straßenwesen (BASt), Teworte, F.; Hegger, J (2013a), Fatigue of prestressed beams without web reinforcement under cyclic shear.beton- und Stahlbetonbau 108, Iss. 1, pp Teworte, F.; Hegger, J (2013b), Fatigue of prestressed beams with web reinforcement under cyclic shear.beton- und Stahlbetonbau 108, Iss. 7, pp Scholzen, A. et al.: Thin-walled shell structure made of textile reinforced concrete: design, dimensioning and realization, Beton- und Stahlbetonbau 107, Iss. 11, pp