trm strengthening of precast reinforced concrete wall
TRANSCRIPT
Abstract—The experimental research presented in this paper
aims to investigate the cut-out effect on the behavior of the precast reinforced concrete wall panels under in-plane seismic loading conditions. Wall openings in buildings are provided for architectural reasons or access requirements, while the cut-outs in walls are made due to change of use or simply, architectural reasons. The precast reinforced concrete wall panel presented in this paper was designed according to the 1981 Romanian code, and has an initial small window opening. In order to investigate the cut-out effect, the small window opening was enlarged to a wide window opening. The specimen was first tested in the unstrengthened condition and subsequently was repaired using high strength mortar, rehabilitated, and then tested again. The experimental tests are described and a discussion based on cut-out effect on shear walls is undertaken and future research is suggested. Numerical analysis are further needed in order to simulate the cut-out interventions made in precast reinforced concrete shear walls of different parameters.
I. INTRODUCTION ETWEEN the 1950s and 1970s, the use of large panel
structures was widely used, but then after followed the decay period until the 1990s, which marked the end in Romania. It is known that the system composed of precast reinforced concrete panels can provide a good seismic performance, but after 50 years of existence and interventions some were subjected to, detailed investigation is strongly needed. The investigated experimental specimens meet the requirements of Eurocode 8 for walls designed to medium ductility and are referred as large lightly reinforced walls.
The application of textile reinforced mortar (TRM) was investigated in this study as a rehabilitation manner, in order to restore the load bearing capacity of the specimen first tested in the unstrengthened condition. Research on textile reinforced
This work was supported in part by the Grant no. 3-002/2011, INSPIRE –
Integrated Strategies and Policy Instruments for Retrofitting buildings to reduce primary energy use and GHG emissions, Project type PN II ERA NET, financed by the Executive Agency for Higher Education, Research, Development and Innovation Funding (UEFISCDI), Romania.
C. Todut is with the Politehnica University Timisoara, 300223, Romania (phone: 0040-256-403-950; fax: 0040-256-403-958; e-mail: carla.todut@ student.upt.ro).
D. Dan is with the Politehnica University Timisoara, 300223, Romania (e- mail: [email protected]).
V. Stoian is with the Politehnica University Timisoara, 300223, Romania (e-mail: [email protected]).
mortar strenghtening were conducted by Bernat-Maso et al. [1], Triantafillou and Papanicolaou [2], Papanicolau et al. [3], San-José et al. [4], Bernat et al. [5], Elsanadedy et al. [6], Larrinaga et al. [7]. Other research presented by Mohammed et al. [8], Bing Li and Qin Chen [9], Mosoarca [10], Kitano et al. [11], Demeter et al. [12], Sas et al. [13], Doh and Fragomeni [14], Guan et al. [15], Carrillo and Alcocer [16], investigated reinforced concrete walls with openings. An experimental research on the effect of cut-out made in wall panels to the behavior of the reinforced concrete wall panel was investigated. The tests were performed under in-plane cyclic lateral loads. The specimen was tested unstrengthened, then after it was repaired, rehabilitated and tested again. A few literature on the TRM for RC wall strengthening and cut-outs made in walls is available.
The paper aims to comprehend the influence of the wall cut- out on the seismic performance of the wall without cut-out, and also the performance of the TRM system for the load bearing capacity restoration. Important aspects related to the seismic performance, lateral stiffness, horizontal displacement (drift), ductility and energy dissipation capacity are presented and discussed for the rehabilitated element in comparison with the reference one. The behavior of the tested elements also include: the failure modes, the strain analysis in reinforcement and glass fiber grid (TRM component). Some remarks are also presented for the TRM anchorage system used.
II. EXPERIMENTAL PROGRAM The experimental program consists of six 1:1.2 scale
elements, namely precast reinforced concrete wall panels PRCWP (7–12), designed and casted according to a Romanian Project Type 770-81 [17], [18]. Two specimens were selected for investigation in this paper, namely PRCWP (10-L1/L3-T), specimen having an initial small window opening and enlarged to a wide window opening, and PRCWP (11-L1-T), specimen having a small window opening. On the basis of the two experimental tests in the unstrengthened condition, important aspects related to the cut-out effect can be drawn out. Then after the specimen with cut-out opening was repaired using high strength mortar, rehabilitated and subsequently tested again, to investigate the strengthening effect and the seismic behavior efficiency. The post-damage strengthened specimen was denoted PRCWP (10-L1/L3-T/R). The experimental specimens were: 2150 mm height, 2750 mm
TRM strengthening of precast reinforced concrete wall panel with cut-out opening -
experimental investigation C. Todut, D. Dan, and V. Stoian
B
ISBN: 978-1-61804-241-5 110
width and 100 mm thickness. The small window opening was 1000 mm height and 750 mm length, while after enlargement the dimensions of the wide window opening were 1000 mm height and 1750 mm length. The wall panels were set between two reinforced steel concrete composite beams, namely a loading beam and a foundation beam. The reinforcement of the precast reinforced concrete (RC) wall panel was made of: horizontal and vertical bars, welded wire mesh in both piers, spatial reinforcement cage in the spandrel, an inclined bar at each corner of the opening, a vertical bar each side of the opening on its height and a wire mesh in the parapet. The configuration of the two specimens selected for the cut-out investigation is presented in Figure 1 (a) and (b).
Figure 1 the schematics and reinforcement details of the specimens
Table 1 material properties of steel
Table 2 Geometrical and mechanical properties of the grid
A. Material Considerations The specimen’s concrete quality was C16/20 class, the
reinforcement S255 for the spatial reinforcement cage, S355 for horizontal, vertical and inclined steel bars, and S490 for the steel wire mesh. The steel reinforcement properties obtained experimentally are given in Table 1. Textile reinforced mortar was used for the rehabilitation of the specimen. The system used was made of glass fiber grid and 1-component, fiber reinforced cementitious mortar with a compressive strength at 7 days of 15 N/mm2 according to the product data sheet. Table 2 summarizes the geometrical and mechanical properties of the grid used. The mentioned characteristics are based on manufacturer’s data. The mortar, used to replace the heavily damaged concrete, was Sika MonoTop 614, with a compressive strength at 28 days of 55– 60 N/mm2, according to the product data sheet.
B. Behavior and results of unstrengthened elements Similar behavior was observed for the two unstrengthened
specimens. Cracks appeared in the spandrel, piers, wings, corners of the window opening, cast in place mortar and mostly in the parapet. Failure of the tested specimens are presented in Figure 2 (a) [19] and (b) [20]. The first diagonal crack in the right pier appeared at 0.3% drift ratio, first cycle, loaded from the right for both specimens. Concrete crushing was observed in the left corners of the specimen with window enlargement, while for the specimen with small opening at the bottom left corner of the opening and parapet. Failure of the PRCWP (10-L1/L3-T) was attained at 0.65% drift ratio, while for the PRCWP (11-L1-T) it was recorded at 0.73% drift ratio.
C. Repair and strengthening of the specimen The rehabilitation strategy adopted here intended to restore
the initial load bearing capacity of the element, namely PRCWP (10-L1/L3-T/R), the solution being qualitative and based on the behavior of the reference specimen.
The PRCWP (10-L1/L3-T/R) specimen, having an initial narrow window opening and enlarged to a wide window opening, was repaired after the experimental test of the reference specimen using a repair mortar (Sika MonoTop 614) and then it was rehabilitated using TRM with GF grid and subsequently tested again. After the repair of the specimen, the rehabilitation process started with surface preparation, namely wall panel polishing, 8 mm hole drilling for the anchorage system, 20 mm rounding of the window edges and vacuum- cleaning. Then, the anchorage system composed of threaded rods having 6 cm length were fixed to the panel using resin, in order to provide a mechanical and punctual type of anchoring system (together with the nut and washer) for the transmission of stresses and deformations from the structure substrate to the TRM system. According to the rehabilitation strategy (Figure
21 50
Ø10, S255
Ø10, S255
Ø10, S255
Ø10, S255
14 395 584 206
8 424 553
16 385 613
207 8 425 507 205
OB37 S255 6 400 550 Re-bar type Grade Φ (mm) fy (N/mm2) fu (N/mm2) Es (N/mm2)
Elongation at break
Component Areal weight [g/m2]
ISBN: 978-1-61804-241-5 111
3), the GF grid was cut using scissors. A bonding primer, namely Sika Monotop 910 N was applied on the surface of the wall, followed by the first layer of mortar, GF grid and the second layer of mortar (Figure 4). Related to the glass fiber grid application (Figure 5), first were mounted the number 4 grid pieces, each side of the parapet,
Figure 2 Failure of the tested unstrengthened specimens
Figure 3 The TRM rehabilitation strategy for the wall with cut-out
Figure 4 Rehabilitation detail for the TRM use
Figure 5 Glass fiber grid application for the wall with cut-out
followed by the number 5 grid pieces, wrapped around the parapet, over the number 4 grid pieces. Then the number two grid pieces were placed each side of the parapet, followed by the number 3 grid pieces which were wrapped over the number 2 grid pieces, each side of the opening. Finally the number 1 glass fiber grid piece was wrapped around the spandrel. Strain gauges were mounted on rebar for the reference element and on the GF grid for the strengthened one.
D. Testing methodology and test set-up Detailed data related to the tests set up and testing
methodology of the precast reinforced concrete wall panel specimens are presented in Demeter [21]. A general view of the test set-up is presented in Figure 6.
The testing procedure of the specimens consisted in quasi- static reversed cyclic lateral loads - displacement controlled (using two cycles per drift), having the measure of 0.1% drift ratio, namely 2.15 mm. Vertical loading was also applied to simulate the gravity loading condition and restrain the rotation of the elements. Pressure transducers (P), displacement transducers (D) and strain gauges placed on rebars and glass fiber grid (G) were used to monitor the behavior of the precast reinforced concrete wall panel specimens (Figure 7). The same displacement transducer position was used for all the experimental specimens.
1 - 1 piece
2 pieces
12 2
3 3
glass fiber grid application
ISBN: 978-1-61804-241-5 112
Figure 7 Instrumentation layout of the specimens
Figure 8 Failure details of the TRM strengthened specimen
III. EXPERIMENTAL RESULTS AND COMPARATIVE STUDY
A. General Behavior and Failure Modes of the TRM post- damage strengthened specimen
The behavior of the strengthened and retested specimen
LA TE
RAL R
EA CT.
F RAM
1910 1080 280 1000 1250 1000 280 1080 1910 9790
D2
G
D1
D3
D1
ISBN: 978-1-61804-241-5 113
PRCWP (10-L1/L3-T/R) under reversed cyclic lateral loads, revealed an expected behavior in accordance with the design strategy. During the experimental test, the PRCWP (10-L1/L3- T/R) specimen developed cracks in the spandrel, piers, corners of the opening and parapet. The TRM system exhibited mortar detachment from 0.3% drift ratio, followed by mortar exfoliation and system excessive detachment (about 3 cm between the TRM system and the wall was observed) between 0.4-0.6% drift ratio. Subsequent detachments and mortar crushing appeared between 0.7-0.8% drift ratio. At the end of the experimental test, the TRM system was removed from each side of the opening and thick inclined cracks were observed together with concrete crushing (Figure 8). The load bearing capacity of the initial system was restored even if the anchorage system used turned to be inefficient. It can be concluded that the punctual mechanical type of anchorage is a cheap alternative to the surface type of anchorage [21] but smaller distances between the threaded rods are necessary.
B. Load displacement response diagrams The obtained lateral loads versus the drift ratio envelopes
are presented in Fig. 9a for the cut-out effect investigation and in Fig. 9b for the TRM performance. It can be seen that significant strength reduction (≈ 50 %) was induced by the cut-out made in the experimental specimen. The TRM rehabilitation strategy restored the initial load bearing capacity of the specimen with cut-out opening, despite the inefficiency of the anchorage system used.
C. Energy dissipation The cumulative energy dissipation was obtained by the
continuous integration of the load-drift hysteretic response using an iterative equation, as presented in [21].
A comparison between the cumulative dissipated energy (CED) per half-cycle versus drift ratio within each test performed is presented in Fig. 10. It can be concluded that the PRCWP (10-L1/L3-T) specimen with cut-out opening developed a significant lower energy dissipation (≈ 57 %) compared to the reference specimen, namely PRCWP (11-L1- T). In the case of the post-damage strengthened specimen using TRM, namely PRCWP (10-L1/L3-T/R), the energy dissipation was higher (≈ 33 %) compared to the unstrengthened specimen, PRCWP (10-L1/L3-T).
Figure 9 Load-drift ratio response diagrams
Figure 10 The cumulative energy dissipation of the specimens
(b)
(a)
ISBN: 978-1-61804-241-5 114
D. Strain analysis During the experimental tests, strain was measured on the
vertical, horizontal and inclined reinforcing bars, and horizontal on the glass fiber grid. In Figure 11 are presented the strain ε (‰) versus drift ratio for the current tested specimens. The position of strain gauges is shown in Fig. 7. It can be seen that, yielding of the reinforcement was attained for the unstrengthened wall panels, in the top left corners of the opening and parapet. In the case of the TRM strengthened specimen, the grid debonded beyond 2 ‰ strain due to the inefficiency of the anchorage system used.
E. Stiffness degradation According to the stiffness versus drift ratio diagram (Figure
12), the cut-out made in wall produced a significant reduction in the initial stiffness (50%) compared to the reference specimen, PRCWP (11-L1-T). In the case of the wall with cut- out opening the initial stiffness was similar for the unstregthened and post-damage strengthened condition.
F. Ductility considerations The ductility of the wall specimens was evaluated using the
μ0.85 method, which defines the ductility (μ = Δu/Δy) as the ratio between the ultimate displacement (Δu - the displacement when the horizontal load falls to 80% of the maximum horizontal force) to the displacement corresponding to 0.85 of the maximum load on the ascending branch of the monotonic envelope (Δy - the displacement at yielding). The ductility coefficient μ0.85 for the tested specimens is presented in Figure 13. It can be concluded that the specimen with cut-out opening, namely PRCWP (10-L1/L3-T) exhibited a lower ductility than the reference one, PRCWP (11-L1-T). The TRM strengthened specimen, PRCWP (10-L1/L3-T/R), exhibited a considerable higher ductility than the reference specimen, PRCWP (10-L1/L3-T).
IV. CONCLUSIONS The work presented in this paper refers to the experimental
results on cut-out effect investigation and post-damage strengthening of a precast reinforced concrete wall panel using textile reinforced mortar.
The following conclusions can be drawn within the limitation of the current research:
Figure 11 Steel strain (ε) versus drift ratio of the specimens
Figure 12 The stiffness versus drift ratio diagram of the specimens
Figure 13 The normalized ductility coefficient for specimens
-3.5 -3
-2.5 -2
-1.5 -1
-0.5 0
0.5 1
1.5 2
2.5 3
3.5
-1.1 -0.9 -0.7 -0.5 -0.3 -0.1 0.1 0.3 0.5 0.7 0.9 1.1
strain ‰
3.5
-1.1 -0.9 -0.7 -0.5 -0.3 -0.1 0.1 0.3 0.5 0.7 0.9 1.1
strain ‰
3.5
-1.1 -0.9 -0.7 -0.5 -0.3 -0.1 0.1 0.3 0.5 0.7 0.9 1.1
strain ‰
ISBN: 978-1-61804-241-5 115
- the corners of the opening and the parapet exhibited concrete crushing in the case of the reference specimens; - the dissipated energy was significantly lower for the wall with cut-out compared to the wall without cut-out, while the post-damage strengthened specimen dissipated more energy compared to the unstrengthened one; - according to the μ0.85 method, the ductility of the wall with cut-out was inferior to the ductility of the wall without cut-out, while the post-damage strengthened specimen proved to be more ductile compared to the reference specimen; - vertical wire mesh yielding was recorded in the parapet for the unstrengthened specimen; while in the case of the TRM strengthened specimen, the grid debonded beyond 2 ‰ strain; - the cut-out made in wall produced a significant reduction in the initial stiffness (50%) compared to the reference specimen, while the TRM strengthened specimen exhibited a comparable initial stiffness to the reference one; - TRM system can be an effective solution for strengthening elements; the punctual anchorage system used turned to be inefficient, allowing for system local debondings. Further studies related to the numerical modelling of the tested elements are in progress. The studies aims to establish the seismic performance of PRCWP having different parameters and the most convenient solutions of strengthening.
ACKNOWLEDGMENT The author acknowledges the following research grant for
the support of this study: 1. Grant no. 3-002/2011, INSPIRE – Integrated Strategies and Policy Instruments for Retrofitting buildings to reduce primary energy use and GHG emissions, Project type PN II ERA NET, financed by the Executive Agency for Higher Education, Research, Development and Innovation Funding (UEFISCDI), Romania.
2. This work was partially supported by the strategic grant POSDRU/159/1.5/S/137070 (2014) of the Ministry of National Education, Romania, co-financed by the European Social Fund – Investing in People, within the Sectoral Operational Programme Human Resources Development 2007-2013.
REFERENCES [1] E. Bernat-Maso, C. Escrig, C. A. Aranha, L. Gil, “Experimental
assessment of Textile Reinforced Sprayed Mortar strengthening system for brickwork wallettes”, Construction and Building Materials, 50, (2014), pp. 226–236.
[2] T. C. Triantafillou, C. G. Papanicolaou, “Shear strengthening of reinforced concrete members with textile reinforced mortar (TRM) jackets”, Materials and Structures, 39, (2006), pp. 93–103.
[3] C. G. Papanicolaou, T. C. Triantafillou, M. Papathanasiou, K. Karlos, “Textile reinforced mortar (TRM) versus FRP as strengthening material of URM walls: out-of-plane cyclic loading”, Materials and Structures, 41, (2008), pp. 143–157.
[4] J. T. San-José, D. García, T. El Hadid, R. San-Mateos, A. Al Far, I. Marcos, “Novelty FRP and TRM Strengthening Systems Applied to Stone Masonry Walls: Experimental Programme Presentation (I)”, Asia- Pacific Conference on FRP in Structures (APFIS 2007), pp. 271-276.
[5] E. Bernat, L. Gil, P. Roca, C. Escrig, “Experimental and analytical study of TRM strengthened brickwork walls under eccentric compressive loading”, Construction and Building Materials, 44, (2013), pp.35-47.
[6] H. M. Elsanadedy, T. H. Almusallam, S.H. Alsayed, Y. A. Al-Salloum, “Flexural strengthening of RC beams using textile reinforced mortar – Experimental and numerical study”, Composite Structures, 97, (2013), pp. 40-55.
[7] P. Larrinaga, C. Chastre, J. T. San-José, L. Garmendia, “Non-linear analytical model of composites based on basalt textile reinforced mortar under uniaxial tension”, Composites: Part B, 55, (2013), pp.518-527.
[8] B. S. Mohammed, L.W. Ean, M.A. Malek, “One way RC wall panels with openings strengthened with CFRP”, Construction and Building Materials, 40, (2013), pp. 575-583.
[9] B. Li, Q. Chen, “Initial stiffness of reinforced concrete structural walls with irregular openings”, Earthquake Engineering and Structural Dynamics, 39, (2010), pp. 397–417.
[10] M. Mosoarca, “Seismic Behavior of Reinforced Concrete Shear Walls with Regular and Staggered Openings After the Strong Earthquakes Between 2009 and 2011”, Engineering Failure Analysis, 34, (2013), pp. 537-565.
[11] A. Kitano, O. Joh, Y. Goto, “Experimental Study on the Strengthening and Repair of R/C Wall-Frame Structures with an Opening by CF- Sheets or CF-Grids”, FRP Composites in Civil Engineering - CICE, (2004).
[12] I. Demeter, T. Nagy-György, V. Stoian, D. Dan, “Seismic Retrofit of Cut-out Weakened Precast RC Walls by Externally Bonded CFRP Composites”, 15 WCEE, Lisboa, (2012).
[13] G. Sas, I. Demeter, A. Carolin, T. Nagy-György, V. Stoian, B. Täljsten, “FRP Strengthened RC Panels With Cut-out Openings”, Challenges for Civil Construction, Porto, (2008).
[14] J.H. Doh, S. Fragomeni, “Ultimate Load Formula for Reinforced Concrete Wall Panels with Openings”, Advances in Structural Engineering, 9 (1), (2006), pp. 103-115
[15] H. Guan, C. Cooper, D. Lee, “Ultimate strength analysis of normal and high strength concrete wall panels with varying opening configurations”, Engineering Structures, 32, (2010), pp. 1341-1355.
[16] J. Carrillo, S. Alcocer, “Degradation Properties of Reinforced Concrete Walls with Openings”, Dyna, ISSN 0012-7353 (2011), vol. 78, pp. 106- 115.
[17] IPCT: Cladiri de locuit P+4 din panouri mari. Proiect 770-81, Vol. C: Elemente prefabricate, Bucuresti, Romania, (1982). IPCT: Precast reinforced concrete large panel buildings P+4. Project type 770-81, Vol. C: Precast elements, Bucharest, Romania, (1982).
[18] IPCT: Cladiri de locuit P+4 din panouri mari. Proiect 770-81, Vol. D: Elemente prefabricate - Armari, Bucuresti, Romania, (1982). IPCT: Precast reinforced concrete large panel buildings P+4. Project type 770- 81, Vol. D: Precast elements – Reinforcing, Bucharest, Romania, (1982).
[19] Seismic Strengthening of a Precast Reinforced Concrete Wall Panel using Textile Reinforced Mortar – Todut C., Stoian V., Demeter I., Fofiu M., in Proceedings of the International Conference on Earthquake Engineering, Skopje, Republic of Macedonia, (2013), paper 323.
[20] Glass Fiber versus Carbon Fiber Grid used in Textile Reinforced Mortar Strengthening of Precast RC Walls – Todut C., Stoian V., and Demeter I., IACSIT International Journal of Engineering and Technology, vol.5, no.5, (2013), pp. 622-626.
[21] Demeter, I. (2011), Seismic retrofit of precast RC walls by externally bonded CFRP composites. PhD Thesis, Politehnica University of Timisoara.
C. Todut was born in Satu Mare, Romania on the 13th of April 1986. She earned the Bachelor’s degree in Civil Engineering at the Politehnica University Timisoara, Romania in 2009, the Master of Science degree in Structures at the Politehnica University Timisoara, Romania in 2011. Currently she is a PhD Student at the Civil Engineering Department of the Construction Faculty, Politehnica University Timisoara, Romania. During Faculty some of the author’s achievements are obtaining a scholarship at the University of Edinburgh, Scotland, 3rd place at Carpatcement contest, 1st place in the County Olympics for the Strength of Materials contest, Timisoara and 2nd place in the National Olympics for the Strength of Materials contest, Iasi. Previous publications of her appear in the Proceedings of fib 2013, FRP RCS 2013 and Structural Faults and Repair 2012. Her current research is based on precast reinforced concrete wall panels, seismic performance, weakening induced by cut-outs and strengthening possibilities. PhD Student Todut was a student member of the American Concrete Institute and the American Society of Civil Engineers.
Advances in Engineering Mechanics and Materials
ISBN: 978-1-61804-241-5 116
aims to investigate the cut-out effect on the behavior of the precast reinforced concrete wall panels under in-plane seismic loading conditions. Wall openings in buildings are provided for architectural reasons or access requirements, while the cut-outs in walls are made due to change of use or simply, architectural reasons. The precast reinforced concrete wall panel presented in this paper was designed according to the 1981 Romanian code, and has an initial small window opening. In order to investigate the cut-out effect, the small window opening was enlarged to a wide window opening. The specimen was first tested in the unstrengthened condition and subsequently was repaired using high strength mortar, rehabilitated, and then tested again. The experimental tests are described and a discussion based on cut-out effect on shear walls is undertaken and future research is suggested. Numerical analysis are further needed in order to simulate the cut-out interventions made in precast reinforced concrete shear walls of different parameters.
I. INTRODUCTION ETWEEN the 1950s and 1970s, the use of large panel
structures was widely used, but then after followed the decay period until the 1990s, which marked the end in Romania. It is known that the system composed of precast reinforced concrete panels can provide a good seismic performance, but after 50 years of existence and interventions some were subjected to, detailed investigation is strongly needed. The investigated experimental specimens meet the requirements of Eurocode 8 for walls designed to medium ductility and are referred as large lightly reinforced walls.
The application of textile reinforced mortar (TRM) was investigated in this study as a rehabilitation manner, in order to restore the load bearing capacity of the specimen first tested in the unstrengthened condition. Research on textile reinforced
This work was supported in part by the Grant no. 3-002/2011, INSPIRE –
Integrated Strategies and Policy Instruments for Retrofitting buildings to reduce primary energy use and GHG emissions, Project type PN II ERA NET, financed by the Executive Agency for Higher Education, Research, Development and Innovation Funding (UEFISCDI), Romania.
C. Todut is with the Politehnica University Timisoara, 300223, Romania (phone: 0040-256-403-950; fax: 0040-256-403-958; e-mail: carla.todut@ student.upt.ro).
D. Dan is with the Politehnica University Timisoara, 300223, Romania (e- mail: [email protected]).
V. Stoian is with the Politehnica University Timisoara, 300223, Romania (e-mail: [email protected]).
mortar strenghtening were conducted by Bernat-Maso et al. [1], Triantafillou and Papanicolaou [2], Papanicolau et al. [3], San-José et al. [4], Bernat et al. [5], Elsanadedy et al. [6], Larrinaga et al. [7]. Other research presented by Mohammed et al. [8], Bing Li and Qin Chen [9], Mosoarca [10], Kitano et al. [11], Demeter et al. [12], Sas et al. [13], Doh and Fragomeni [14], Guan et al. [15], Carrillo and Alcocer [16], investigated reinforced concrete walls with openings. An experimental research on the effect of cut-out made in wall panels to the behavior of the reinforced concrete wall panel was investigated. The tests were performed under in-plane cyclic lateral loads. The specimen was tested unstrengthened, then after it was repaired, rehabilitated and tested again. A few literature on the TRM for RC wall strengthening and cut-outs made in walls is available.
The paper aims to comprehend the influence of the wall cut- out on the seismic performance of the wall without cut-out, and also the performance of the TRM system for the load bearing capacity restoration. Important aspects related to the seismic performance, lateral stiffness, horizontal displacement (drift), ductility and energy dissipation capacity are presented and discussed for the rehabilitated element in comparison with the reference one. The behavior of the tested elements also include: the failure modes, the strain analysis in reinforcement and glass fiber grid (TRM component). Some remarks are also presented for the TRM anchorage system used.
II. EXPERIMENTAL PROGRAM The experimental program consists of six 1:1.2 scale
elements, namely precast reinforced concrete wall panels PRCWP (7–12), designed and casted according to a Romanian Project Type 770-81 [17], [18]. Two specimens were selected for investigation in this paper, namely PRCWP (10-L1/L3-T), specimen having an initial small window opening and enlarged to a wide window opening, and PRCWP (11-L1-T), specimen having a small window opening. On the basis of the two experimental tests in the unstrengthened condition, important aspects related to the cut-out effect can be drawn out. Then after the specimen with cut-out opening was repaired using high strength mortar, rehabilitated and subsequently tested again, to investigate the strengthening effect and the seismic behavior efficiency. The post-damage strengthened specimen was denoted PRCWP (10-L1/L3-T/R). The experimental specimens were: 2150 mm height, 2750 mm
TRM strengthening of precast reinforced concrete wall panel with cut-out opening -
experimental investigation C. Todut, D. Dan, and V. Stoian
B
ISBN: 978-1-61804-241-5 110
width and 100 mm thickness. The small window opening was 1000 mm height and 750 mm length, while after enlargement the dimensions of the wide window opening were 1000 mm height and 1750 mm length. The wall panels were set between two reinforced steel concrete composite beams, namely a loading beam and a foundation beam. The reinforcement of the precast reinforced concrete (RC) wall panel was made of: horizontal and vertical bars, welded wire mesh in both piers, spatial reinforcement cage in the spandrel, an inclined bar at each corner of the opening, a vertical bar each side of the opening on its height and a wire mesh in the parapet. The configuration of the two specimens selected for the cut-out investigation is presented in Figure 1 (a) and (b).
Figure 1 the schematics and reinforcement details of the specimens
Table 1 material properties of steel
Table 2 Geometrical and mechanical properties of the grid
A. Material Considerations The specimen’s concrete quality was C16/20 class, the
reinforcement S255 for the spatial reinforcement cage, S355 for horizontal, vertical and inclined steel bars, and S490 for the steel wire mesh. The steel reinforcement properties obtained experimentally are given in Table 1. Textile reinforced mortar was used for the rehabilitation of the specimen. The system used was made of glass fiber grid and 1-component, fiber reinforced cementitious mortar with a compressive strength at 7 days of 15 N/mm2 according to the product data sheet. Table 2 summarizes the geometrical and mechanical properties of the grid used. The mentioned characteristics are based on manufacturer’s data. The mortar, used to replace the heavily damaged concrete, was Sika MonoTop 614, with a compressive strength at 28 days of 55– 60 N/mm2, according to the product data sheet.
B. Behavior and results of unstrengthened elements Similar behavior was observed for the two unstrengthened
specimens. Cracks appeared in the spandrel, piers, wings, corners of the window opening, cast in place mortar and mostly in the parapet. Failure of the tested specimens are presented in Figure 2 (a) [19] and (b) [20]. The first diagonal crack in the right pier appeared at 0.3% drift ratio, first cycle, loaded from the right for both specimens. Concrete crushing was observed in the left corners of the specimen with window enlargement, while for the specimen with small opening at the bottom left corner of the opening and parapet. Failure of the PRCWP (10-L1/L3-T) was attained at 0.65% drift ratio, while for the PRCWP (11-L1-T) it was recorded at 0.73% drift ratio.
C. Repair and strengthening of the specimen The rehabilitation strategy adopted here intended to restore
the initial load bearing capacity of the element, namely PRCWP (10-L1/L3-T/R), the solution being qualitative and based on the behavior of the reference specimen.
The PRCWP (10-L1/L3-T/R) specimen, having an initial narrow window opening and enlarged to a wide window opening, was repaired after the experimental test of the reference specimen using a repair mortar (Sika MonoTop 614) and then it was rehabilitated using TRM with GF grid and subsequently tested again. After the repair of the specimen, the rehabilitation process started with surface preparation, namely wall panel polishing, 8 mm hole drilling for the anchorage system, 20 mm rounding of the window edges and vacuum- cleaning. Then, the anchorage system composed of threaded rods having 6 cm length were fixed to the panel using resin, in order to provide a mechanical and punctual type of anchoring system (together with the nut and washer) for the transmission of stresses and deformations from the structure substrate to the TRM system. According to the rehabilitation strategy (Figure
21 50
Ø10, S255
Ø10, S255
Ø10, S255
Ø10, S255
14 395 584 206
8 424 553
16 385 613
207 8 425 507 205
OB37 S255 6 400 550 Re-bar type Grade Φ (mm) fy (N/mm2) fu (N/mm2) Es (N/mm2)
Elongation at break
Component Areal weight [g/m2]
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3), the GF grid was cut using scissors. A bonding primer, namely Sika Monotop 910 N was applied on the surface of the wall, followed by the first layer of mortar, GF grid and the second layer of mortar (Figure 4). Related to the glass fiber grid application (Figure 5), first were mounted the number 4 grid pieces, each side of the parapet,
Figure 2 Failure of the tested unstrengthened specimens
Figure 3 The TRM rehabilitation strategy for the wall with cut-out
Figure 4 Rehabilitation detail for the TRM use
Figure 5 Glass fiber grid application for the wall with cut-out
followed by the number 5 grid pieces, wrapped around the parapet, over the number 4 grid pieces. Then the number two grid pieces were placed each side of the parapet, followed by the number 3 grid pieces which were wrapped over the number 2 grid pieces, each side of the opening. Finally the number 1 glass fiber grid piece was wrapped around the spandrel. Strain gauges were mounted on rebar for the reference element and on the GF grid for the strengthened one.
D. Testing methodology and test set-up Detailed data related to the tests set up and testing
methodology of the precast reinforced concrete wall panel specimens are presented in Demeter [21]. A general view of the test set-up is presented in Figure 6.
The testing procedure of the specimens consisted in quasi- static reversed cyclic lateral loads - displacement controlled (using two cycles per drift), having the measure of 0.1% drift ratio, namely 2.15 mm. Vertical loading was also applied to simulate the gravity loading condition and restrain the rotation of the elements. Pressure transducers (P), displacement transducers (D) and strain gauges placed on rebars and glass fiber grid (G) were used to monitor the behavior of the precast reinforced concrete wall panel specimens (Figure 7). The same displacement transducer position was used for all the experimental specimens.
1 - 1 piece
2 pieces
12 2
3 3
glass fiber grid application
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Figure 7 Instrumentation layout of the specimens
Figure 8 Failure details of the TRM strengthened specimen
III. EXPERIMENTAL RESULTS AND COMPARATIVE STUDY
A. General Behavior and Failure Modes of the TRM post- damage strengthened specimen
The behavior of the strengthened and retested specimen
LA TE
RAL R
EA CT.
F RAM
1910 1080 280 1000 1250 1000 280 1080 1910 9790
D2
G
D1
D3
D1
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PRCWP (10-L1/L3-T/R) under reversed cyclic lateral loads, revealed an expected behavior in accordance with the design strategy. During the experimental test, the PRCWP (10-L1/L3- T/R) specimen developed cracks in the spandrel, piers, corners of the opening and parapet. The TRM system exhibited mortar detachment from 0.3% drift ratio, followed by mortar exfoliation and system excessive detachment (about 3 cm between the TRM system and the wall was observed) between 0.4-0.6% drift ratio. Subsequent detachments and mortar crushing appeared between 0.7-0.8% drift ratio. At the end of the experimental test, the TRM system was removed from each side of the opening and thick inclined cracks were observed together with concrete crushing (Figure 8). The load bearing capacity of the initial system was restored even if the anchorage system used turned to be inefficient. It can be concluded that the punctual mechanical type of anchorage is a cheap alternative to the surface type of anchorage [21] but smaller distances between the threaded rods are necessary.
B. Load displacement response diagrams The obtained lateral loads versus the drift ratio envelopes
are presented in Fig. 9a for the cut-out effect investigation and in Fig. 9b for the TRM performance. It can be seen that significant strength reduction (≈ 50 %) was induced by the cut-out made in the experimental specimen. The TRM rehabilitation strategy restored the initial load bearing capacity of the specimen with cut-out opening, despite the inefficiency of the anchorage system used.
C. Energy dissipation The cumulative energy dissipation was obtained by the
continuous integration of the load-drift hysteretic response using an iterative equation, as presented in [21].
A comparison between the cumulative dissipated energy (CED) per half-cycle versus drift ratio within each test performed is presented in Fig. 10. It can be concluded that the PRCWP (10-L1/L3-T) specimen with cut-out opening developed a significant lower energy dissipation (≈ 57 %) compared to the reference specimen, namely PRCWP (11-L1- T). In the case of the post-damage strengthened specimen using TRM, namely PRCWP (10-L1/L3-T/R), the energy dissipation was higher (≈ 33 %) compared to the unstrengthened specimen, PRCWP (10-L1/L3-T).
Figure 9 Load-drift ratio response diagrams
Figure 10 The cumulative energy dissipation of the specimens
(b)
(a)
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D. Strain analysis During the experimental tests, strain was measured on the
vertical, horizontal and inclined reinforcing bars, and horizontal on the glass fiber grid. In Figure 11 are presented the strain ε (‰) versus drift ratio for the current tested specimens. The position of strain gauges is shown in Fig. 7. It can be seen that, yielding of the reinforcement was attained for the unstrengthened wall panels, in the top left corners of the opening and parapet. In the case of the TRM strengthened specimen, the grid debonded beyond 2 ‰ strain due to the inefficiency of the anchorage system used.
E. Stiffness degradation According to the stiffness versus drift ratio diagram (Figure
12), the cut-out made in wall produced a significant reduction in the initial stiffness (50%) compared to the reference specimen, PRCWP (11-L1-T). In the case of the wall with cut- out opening the initial stiffness was similar for the unstregthened and post-damage strengthened condition.
F. Ductility considerations The ductility of the wall specimens was evaluated using the
μ0.85 method, which defines the ductility (μ = Δu/Δy) as the ratio between the ultimate displacement (Δu - the displacement when the horizontal load falls to 80% of the maximum horizontal force) to the displacement corresponding to 0.85 of the maximum load on the ascending branch of the monotonic envelope (Δy - the displacement at yielding). The ductility coefficient μ0.85 for the tested specimens is presented in Figure 13. It can be concluded that the specimen with cut-out opening, namely PRCWP (10-L1/L3-T) exhibited a lower ductility than the reference one, PRCWP (11-L1-T). The TRM strengthened specimen, PRCWP (10-L1/L3-T/R), exhibited a considerable higher ductility than the reference specimen, PRCWP (10-L1/L3-T).
IV. CONCLUSIONS The work presented in this paper refers to the experimental
results on cut-out effect investigation and post-damage strengthening of a precast reinforced concrete wall panel using textile reinforced mortar.
The following conclusions can be drawn within the limitation of the current research:
Figure 11 Steel strain (ε) versus drift ratio of the specimens
Figure 12 The stiffness versus drift ratio diagram of the specimens
Figure 13 The normalized ductility coefficient for specimens
-3.5 -3
-2.5 -2
-1.5 -1
-0.5 0
0.5 1
1.5 2
2.5 3
3.5
-1.1 -0.9 -0.7 -0.5 -0.3 -0.1 0.1 0.3 0.5 0.7 0.9 1.1
strain ‰
3.5
-1.1 -0.9 -0.7 -0.5 -0.3 -0.1 0.1 0.3 0.5 0.7 0.9 1.1
strain ‰
3.5
-1.1 -0.9 -0.7 -0.5 -0.3 -0.1 0.1 0.3 0.5 0.7 0.9 1.1
strain ‰
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- the corners of the opening and the parapet exhibited concrete crushing in the case of the reference specimens; - the dissipated energy was significantly lower for the wall with cut-out compared to the wall without cut-out, while the post-damage strengthened specimen dissipated more energy compared to the unstrengthened one; - according to the μ0.85 method, the ductility of the wall with cut-out was inferior to the ductility of the wall without cut-out, while the post-damage strengthened specimen proved to be more ductile compared to the reference specimen; - vertical wire mesh yielding was recorded in the parapet for the unstrengthened specimen; while in the case of the TRM strengthened specimen, the grid debonded beyond 2 ‰ strain; - the cut-out made in wall produced a significant reduction in the initial stiffness (50%) compared to the reference specimen, while the TRM strengthened specimen exhibited a comparable initial stiffness to the reference one; - TRM system can be an effective solution for strengthening elements; the punctual anchorage system used turned to be inefficient, allowing for system local debondings. Further studies related to the numerical modelling of the tested elements are in progress. The studies aims to establish the seismic performance of PRCWP having different parameters and the most convenient solutions of strengthening.
ACKNOWLEDGMENT The author acknowledges the following research grant for
the support of this study: 1. Grant no. 3-002/2011, INSPIRE – Integrated Strategies and Policy Instruments for Retrofitting buildings to reduce primary energy use and GHG emissions, Project type PN II ERA NET, financed by the Executive Agency for Higher Education, Research, Development and Innovation Funding (UEFISCDI), Romania.
2. This work was partially supported by the strategic grant POSDRU/159/1.5/S/137070 (2014) of the Ministry of National Education, Romania, co-financed by the European Social Fund – Investing in People, within the Sectoral Operational Programme Human Resources Development 2007-2013.
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[21] Demeter, I. (2011), Seismic retrofit of precast RC walls by externally bonded CFRP composites. PhD Thesis, Politehnica University of Timisoara.
C. Todut was born in Satu Mare, Romania on the 13th of April 1986. She earned the Bachelor’s degree in Civil Engineering at the Politehnica University Timisoara, Romania in 2009, the Master of Science degree in Structures at the Politehnica University Timisoara, Romania in 2011. Currently she is a PhD Student at the Civil Engineering Department of the Construction Faculty, Politehnica University Timisoara, Romania. During Faculty some of the author’s achievements are obtaining a scholarship at the University of Edinburgh, Scotland, 3rd place at Carpatcement contest, 1st place in the County Olympics for the Strength of Materials contest, Timisoara and 2nd place in the National Olympics for the Strength of Materials contest, Iasi. Previous publications of her appear in the Proceedings of fib 2013, FRP RCS 2013 and Structural Faults and Repair 2012. Her current research is based on precast reinforced concrete wall panels, seismic performance, weakening induced by cut-outs and strengthening possibilities. PhD Student Todut was a student member of the American Concrete Institute and the American Society of Civil Engineers.
Advances in Engineering Mechanics and Materials
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