1

COMPARISON OF DIFFERENT STRENGTHENING TECHNIQUES OF DAMAGED AND WEAK REINFORCED‒CONCRETE FRAMES

Marin GRUBIŠIĆ1 and Vladimir SIGMUND2

ABSTRACT

As part of a broader research project, attention of this study has been focused on the existing, damaged “strong frame“ RC buildings and “weak frame“ RC buildings, strengthened with clay brick masonry infill walls (“framed‒wall”). We experimentally investigated different masonry wall application techniques within the frame that upgrade the seismic capacity of these structures. Seven one‒bay, one‒storey RC frames were built in a scale 1:2.5 and 1:2, “strong“ and “weak” frames respectively. They were tested under constant vertical and reversed cyclic lateral loading up to the drift of 1.5%. The experimental results are presented in the form of observed failure mechanism, hysteresis loops and cyclic envelope curves.

The tests showed that RC frames strengthened with masonry infill wall had the improved lateral load and deformation capacity as well as energy dissipation. The system behaved as composite “framed‒masonry” structure which damage could be controlled. Cost‒effectiveness and simplicity of the technique by the implementation of the masonry infill wall seems to be much more favourable than just the repair of the existing RC bare structure.

Keywords: RC frames, Masonry Infill, Seismic Strengthening, Framed‒masonry, Cyclic Testing

INTRODUCTION

Field and experimental evidence showed that masonry infill walls reduced damage in the reinforced‒concrete frame structures during earthquakes. Addition of the masonry infill within the reinforced‒concrete frames (“framed‒wall”), even if infill has no load‒bearing function, could contribute to the seismic performance of building. This can be an economical solution of strengthening the weak and repairing the damaged reinforced‒concrete frames. In order to test this hypothesis, a 1:2.5 and 1:2 scale one‒storey one‒bay damaged “strong” (during previous tests) and “weak” reinforced‒concrete frames were strengthened with masonry infill made of solid clay brick units and mortar. Strengthening was applied in many different ways, and tested under constant vertical and cyclic lateral loading. Test results were compared with the results of the same, undamaged, bare frame and the frame with masonry infill. The aim of this investigation was to check various strengthening techniques that would enable existing/damaged frame buildings to meet various performance criteria under expected earthquakes.

1 Ph.D. Researcher, Faculty of Civil Engineering, Department for Technical Mechanics, Josip Juraj Strossmayer University of Osijek, Drinska 16a, 31000 Osijek, Croatia, marin.grubisic@gfos.hr 2 Full Professor, Faculty of Civil Engineering, Department for Technical Mechanics, Josip Juraj Strossmayer University of Osijek, Crkvena 21, 31000 Osijek, Croatia, sigmund@gfos.hr

2

The traditional and innovative strengthening techniques were considered. Traditional methods of strengthening the infill are cheaper and easier for application, while modern ones, like the one used in these tests (produced by the MAPEI) require more specialized skill.

THE SPECIMENS

As part of a broader research project, a total of 10 previously damaged frames were tested. They were infilled with masonry that had two types of masonry units ‒ hollow and solid clay masonry units and the same general purpose mortar. Eight frames were classified as “strong” reinforced‒concrete frames (designed according to the EC8) and two as “weak” reinforced‒concrete frames (not seismically designed). Within this paper we considered only the tests conducted on the 5 specimens with solid clay (b x d x h=25 x 12 x 6.5 cm) masonry wall. All “strong” were damaged in the previous tests in which they were pushed to drifts higher than 1.5%. They were repaired by replacing the spalled concrete with fast setting concrete mortar. More details about the specimens are described in Zovkić, J. et. al (2012). Strong reinforced‒concrete frame specimens were 2.2 m wide and 1.5 m high (scale of 1:2.5) with columns and beam cross section of 20x20 cm and 12x20 cm, respectively. Span to height ratio of the opening was h/l=0.72. Materials used for concrete were C30/37 and reinforcement B500B. Longitudinal reinforcement ratio of the columns was ρcolumn=2.4% and the beams had ρbeam,mid=1.3% in the mid‒span and ρbeam,end=3.3% at its ends.

The “weak” reinforced‒concrete frames were non‒seismically designed and had some typical deficiencies such as: strong‒beams weak‒columns behaviour, inadequate amount of transverse reinforcement in joints and potential hinge regions, etc. The specimens were 2.15 m (2.25 m) wide and 1.5 m high (scale of 1:2) with columns and beam cross section of 15x12.5 cm (25x12.5 cm) and

Figure 1. Reinforcement details of three different frame specimens used for experimental testing

It has been observed that addition of the masonry infill within the RC frame increased the initial stiffness and base shear strength. Improvement of the connection between the masonry and the frame elements improved the behaviour of the system. Strength deterioration has been reduced and ductility and hysteretic damping increased.

M. Grubišić, V. Sigmund 3

12.5x25 cm, respectively. Their beams were reinforced with the longitudinal reinforcement ratio ρbeam,mid =1.0%, ρbeam,end=1.9%, and the columns had the longitudinal reinforcement ratio of ρcolumn=1.1% in #1 frame and ρcolumn =1.3% in the frame #2.

Details of the specimens, cross‒sections and reinforcement details are given in Figure 1. The masonry infill wall was added within the frame by different techniques (Figure 2), as follows:

a) Unreinforced masonry infill wall was connected to the surrounding r/c frame by dowels

(Strong frame ‒ Type 1), b) Reinforced masonry infill wall was connected to the surrounding frame by dowels (Strong

frame ‒ Type 2), c) Unreinforced masonry infill wall was strengthened with additional vertical confining element

(tie‒column) and connected to the surrounding frame by dowels (Strong frame ‒ Type 3), d) Unreinforced masonry infill wall was strengthened from both sides (including columns) with

high‒performance mortar reinforced with galvanized welded mesh which is connected to the surrounding r/c frame by dowels (Weak frame #1 ‒ Type 4),

e) Unreinforced masonry infill wall was strengthened (including columns) with high‒performance mortar reinforced with galvanized welded mesh which is connected to the surrounding r/c frame by dowels (Weak frame #2 ‒ Type 5),

f) Reference specimens of undamaged bare and masonry infilled strong frames (Type 6 & 7).

All specimens had stiff foundation beam that was fixed to the reaction frame and the strong floor. The mechanical properties of the masonry walls, repairing and strengthening material are presented in Table 1.

The thicknesses of horizontal masonry joints were 1 cm and vertical joints were completely filled. Strengthening of specimens TYPE 4 and 5 was applied on both sides of the specimens, with a thickness of 1.5 cm, with galvanized mesh overlap of 15 cm, up to the beam. Mesh was attached to the masonry wall in every 30 cm in order to ensure necessary distance from the face of the wall for application of the high‒performance mortar.

Figure 2. Frames strengthened with brick masonry infill in various ways

4

Table 1. Mechanical properties of the masonry, repairing and strengthening material

Masonry solid brick infill with cement‒lime mortar

(cement:lime:sand ‒ 1:1:5)

Fast setting thixotropic mortar for concrete restoration/repairing

High‒performance mortar (17% mixing water)

Modulus of elasticity

5 533 MPa

Compressive strength (after 28 days) 35 MPa Compressive strength

(after 28 days) 15 MPa

Compressive strength

8.3 MPa Flexural strength (after 28 days) 7 MPa

Modulus of elasticity in compression (after 28 days)

10 000 MPa

Tensile strength 0.16 MPa Modulus of elasticity in compression (after 28 days)

24 000 MPa

Initial shear strength 0.15 MPa

Poissons ratio 0.15 Bond strength to substrate (brickwork)

0.7 MPa

TEST SETUP AND PROCEDURE

All specimens were tested under constant vertical and cyclic horizontal loading up to the failure (>1.5% drift). Measured were all relevant data and obtained were failure modes, hysteresis loops and hysteresis cyclic envelopes. From them the system base shear capacity, stiffness and hysteresis damping were evaluated. The tests were performed at the University of Osijek, Faculty of Civil Engineering, Osijek, Croatia. In plane static reversed cyclic lateral loading was applied at the ends of the specimen’s beam along its central axis by hydraulic actuator. Vertical loading was applied at the columns’ top and it was kept constant by means of the hydraulic valves.

The time‒history of the applied cyclic lateral and constant vertical loads (160 kN for “weak” frame #1, 280 kN for “weak” frame #2, and 365 kN for “strong” frames in each column), are shown in Figure 3. The tests were at the initial stage force controlled and after the first cracks occurred displacements controlled. Cyclic quasi‒static lateral load was applied in increments of 10 kN and each loading cycle was repeated twice. In the displacements phase, the displacements were increased in the increments of 1.0 mm.

Figure 3. Time‒history of the applied lateral cyclic load (kN) and vertical load oscillation history (%)

TEST RESULTS

The results of these tests were compared with the reference specimens (“strong” frame with infill and bare frame‒ Type 6 and 7). Hysteresis curves and cyclic envelopes are presented in the Figure 4. The results showed that addition of the masonry infill with proper detailing, increased the base shear capacity and ductility. Strengthened weak reinforced concrete frames behaved well during

M. Grubišić, V. Sigmund 5

the tests, and successfully avoided the columns and beam failure. The observed cracks on the specimens TYPE 4 and TYPE 5 are shown in Figure 5. Measured cumulative hysteretic energy and stiffness degradation are shown in Figure 6 and 7, while discrete cumulative hysteretic energy in terms of inter‒storey drift (IDR) levels are shown in Figure 8.

6

Figure 4. Overview of the hysteresis curves of all tested specimens

Figure 5. Cracks on the specimens with surface strengthening, TYPE 4 (left) and TYPE 5 (right)

Figure 6. The hysteresis envelopes and stiffness degradation curves of all specimens

M. Grubišić, V. Sigmund 7

Figure 7. The cumulative hysteretic energy for cyclic loading

Figure 8. Discrete cumulative hysteretic energy at specific IDR levels

CONCLUSIONS AND REMARKS

Presented are the results of six RC frames infilled with masonry infill for strengthening reasons. Four of the specimens were “strong” frames damaged in previous tests and two specimens were “weak” frames. The infill was made of solid clay brick units and general purpose mortar. It was connected to the frame elements by use of various traditional and modern methods. In some cases the planar strengthening technique was used. The specimens were tested under cyclic lateral and constant vertical loading. The test results were compared with the results of the reference frames tested under the same conditions.

Addition of the masonry infill ‒ with some contribution of seismic strengthening ‒ directly affected the strength, stiffness and displacements of the structure under cyclic excitation. Increase in the base shear capacity depended on the strengthening type. Strength of the repaired “strong” frames did not deteriorate even up to drifts of 1.5%. There was no significant contribution of the applied strengthening technique to the stiffness. The biggest contribution of the strengthening was that the failure of the “framed‒masonry” system was delayed and that the columns did not fail.

The strengthened weak (“nonductile”) frames had fat hysteresis loops and pronounced strength deterioration after the drifts of 0.9%. In all specimens, the major cracks were formed in the area of

8

masonry infill, while the cracks on the frame members remained mostly within the limits of concrete cover.

ACKNOWLEDGEMENT

The research presented in this paper is a part of the research project “Seismic design of infilled frames”, No. 149‒1492966‒1536, supported by the Ministry of Science, Education and Sports of the Republic of Croatia and its support is gratefully acknowledged. MAPEI Croatia provided the material for the strengthening of the specimens Type 4 and 5 and we thank them for that.

REFERENCES Al‒Chaar, G. (1998), Non‒Ductile Behaviour of Reinforced Concrete Frames with Masonry Infill Panels

Subjected to In‒Plane Loading, University of Illinois at Chicago, Chicago. Brokken, S.; Bertero, V. V. [1981], Studies on Effects of Infills in Seismic Resistant RC Construction. Report

UCB/EERC, 81‒12, University of California, Berkeley, CA. Calvi, G. M., Bolognini, D. (2001), Seismic Response of Reinforced Concrete Frames Infilled with Masonry

Panels Weakly Reinforced, Journal of Earthquake Engineering; 5:153‒85 Fiorato, A. E., Sozen, M. A., Gamble,W. L. (1970), An Investigation of the Interaction of Reinforced Concrete

Frames with Masonry Filler Walls. Civil Engineering Studies, University of Illinois, Urbana, IL, 525p. Gazić, G., Sigmund, V. (2013), Experimental Investigation of Weak R/C Frames with Masonry Infill,

SE‒50EEE, International Conference on Earthquake Engineering, Skopje, Republic of Macedonia. Griffith, M. (2008), Seismic Retrofit of RC Frame Buildings with Masonry Infill Walls: Literature Review and

Preliminary Case Study, JRC European Commission Report, JRC 44166. Grubišić, M., Kalman Šipoš, T., Sigmund, V. (2013), Seismic Fragility Assessment of Masonry Infilled

Reinforced Concrete Frames, SE‒50EEE, International Conference on Earthquake Engineering, Skopje, Republic of Macedonia.

Grubišić, M., Sigmund, V. (2012), Seismic Strengthening of Masonry Infilled Reinforced Concrete Frames, Bauhaus Summer School, Course Model Validation and Simulation, Bauhaus‒Universität Weimar, Weimar, Germany.

Grubišić, M., Sigmund, V. (2014), Experimental Studies of Single Bay RC Frames with Strengthened Masonry Infill, YOUNG SCIENTIST 2014, 94‒107, Technical University of Košice, Košice, Slovakia.

Hashemi, A., Mosalam, K. M. [2006], Shake‒Table Experiment on Reinforced Concrete Structure Containing Masonry Infill Wall. Earthquake Eng. Struct. Dyn. 35: 14, 1827‒1852.

Kalman Šipoš, T., Sigmund, V., Hadzima Nyarko, M. (2013), Earthquake Performance of Infilled Frames Using Neural Networks and Experimental Database, Engineering Structures. 51, 113‒127.

Kalman Šipoš, T., Sigmund, V., Žarnić, R. (2010), Evaluation of Masonry Panel Models for Nonlinear Analysis of Infilled RC Frames, Proceedings of 14th European Conference On Earthquake Engineering, Ohrid, Macedonia.

Lepage, A. (1997), A Method for Drift‒Control in Earthquake‒Resistant Design of RC Building Structures. PhD Thesis, University of Illinois, Urbana, 251p.

Markulak, D., Radić, I., Sigmund, V. (2013), Cyclic Testing of Single Bay Steel Frames with Various Types of Masonry Infill. Engineering Structures. 51, 267‒277.

Negro, P., Verzeletti, G. (1996), Effect of Infills on the Global Behaviour of Frames: Energy Considerations from Pseudodynamic Tests. Earthquake Engineering and Structural Dynamics, 25: 8, 753‒773

Penava, D., Sigmund, V. (2013), Influence of Openings with and without Confinement on Cyclic Response of Infilled RC Frames. Journal of Earthquake Engineering, 18, 1; 113‒146.

Shimazaki, K, Sozen M. A., (1988), Strong Ground Motion Drift and Base Shear Coefficient for RC Structures. Proc. 9th World Conference on Earthquake Eng., Tokyo and Kyoto, Japan, 5, 165‒170.

Wood, S. (1991), Performance of Reinforced Concrete Buildings During the 1985 Chile Earthquake: Implications for the Design of Structural Walls. Earthquake Spectra, 7:4, 607‒639.

Žarnić, R, Gostič, S, Crewe, A. J., Taylor C. A. (2001), Shaking Table Tests of 1:4 Reduced‒Scale Models of Masonry Infilled Reinforced Concrete Frame Buildings. Earthquake Engineering and Structural Dynamics 30:6, 819‒834.

Zovkić, J., Sigmund, V., Guljaš, I. (2012), Cyclic Testing of a Single Bay Reinforced Concrete Frames with Various Types of Masonry Infill, Earthquake Engineering & Structural Dynamics, 41, 14, 41‒60.