Use of Fiber-Reinforced Polymer in Strengthening Reinforced Concrete Structures with Partition Walls

1. Introduction Partition walls constructed with brick elements used in the frame openings of reinforced concrete structures have been proven through numerous experimental and theoretical studies to have positive effects on the seismic behaviour of buildings when properly distributed throughout the structure. Carbon-based fibre polymers (CFRP) applied superficially to these partition walls play an extremely important role in integrating the walls into the structure and improving overall building behaviour. In this study, the effects of strengthening methods using plain and two different fibre polymer applications on the behaviour of partition wall frames were examined comparatively. For this purpose, a series of scaled partition wall frames were produced and tested under earthquake-like loads. In test specimens strengthened with diagonal application of fibre polymers on partition walls and in specimens strengthened with alternative braced diagonals, significant increases in maximum load levels achieved were observed. From the experimental study, it was concluded that partition wall frames strengthened using fibre polymers have higher energy consumption capacity compared to unstrengthened conditions. Typical partition wall applications commonly used in our country are shown in Figure 1. [gallery columns="2" size="medium" ids="eyJ1cmwiOiJodHRwczpcL1wvd3d3LnR1cmtjaGVtLm5ldFwvd3AtY29udGVudFwvdXBsb2Fkc1wvMjAxOVwvMTBcL2V2XzEuanBnIiwidGl0bGUiOiJldl8xIiwiY2FwdGlvbiI6IiIsImFsdCI6IiIsImRlc2NyaXB0aW9uIjoiIn0=,eyJ1cmwiOiJodHRwczpcL1wvd3d3LnR1cmtjaGVtLm5ldFwvd3AtY29udGVudFwvdXBsb2Fkc1wvMjAxOVwvMTBcL2V2XzIuanBnIiwidGl0bGUiOiJCXHUwMGY2bG1lIER1dmFybFx1MDEzMSBCZXRvbiIsImNhcHRpb24iOiIiLCJhbHQiOiJCXHUwMGY2bG1lIER1dmFybFx1MDEzMSBCZXRvbiIsImRlc2NyaXB0aW9uIjoiIn0="]

Figure 1. Existing typical low-rise partition wall frames

Although the use of fibre polymers in structural strengthening is frequently studied in academic circles as a scientific topic, CFRP application methods particularly aimed at integrating partition walls into structures are limited in real-world applications. Existing studies in the literature indicate that strengthening methods using CFRP provide significant advantages to partition wall frames in terms of strength, stiffness and energy damping characteristics. Reinhorn and Madan [1] investigated the effect of CFRP applied asymmetrically to both sides of masonry wall elements on behaviour. They demonstrated that CFRP increases the horizontal load-carrying capacity and energy dissipation capacity of masonry wall elements. Elgawady and others [2] conducted tests under reversible cyclic static loads to determine the behaviour of 1/2 scale masonry wall elements. The study results concluded that energy consumption increased due to friction movement between cracks formed in the wall and deformations in the fibre polymers. Santa-Maria and others [3] conducted experiments on wall elements with different CFRP applied in alternative configurations, and significant increases in shear strength and stiffness were observed in specimens where CFRP was applied diagonally. Yuksel and others [4] experimentally examined the behaviour of partition wall reinforced concrete frames strengthened with CFRP under reversible cyclic loads. They concluded that partition wall reinforced concrete frames have higher strength and stiffness than plain frames, and that strengthening with FRP prevented corner crushing and reduced crack widths. Erdem and others [5] investigated the effect of diagonally applied CFRP on the behaviour of partition wall reinforced concrete frames. As a result of this study, they found that strength, stiffness and energy consumption capacities increased significantly in CFRP-strengthened specimens, and that damping characteristics showed important improvements. Hashemi and Mosalam [6] tested 3/4 scale partition wall reinforced concrete frames on a shaking table under real earthquake loads. Partition wall reinforced concrete frames were observed to have 4 times greater stiffness than plain frames, a 50% decrease in natural vibration periods, and damping ratios increased from 4% to 13% when partition walls were added to plain frames. Altin and others [7] tested FRP-strengthened partition wall reinforced concrete frames under reversible cyclic loads. In this experimental study, they examined the effect of FRP strip widths and anchors used in these strips. Almusallam and Al-Salloum [8] experimentally investigated the strengthening of partition wall reinforced concrete frames with GFRP. They applied GFRP strips horizontally on both surfaces of specimens. The experimental results found that when GFRP strips were applied horizontally, specimens exhibited more ductile behaviour compared to others. Wei and others [9] experimentally investigated the effects of different types of FRP applications on masonry wall elements on overall behaviour. They applied FRP to wall elements vertically and diagonally. The experimental results show that diagonal application caused more ductile behaviour compared to other applications. The studies summarized above have shown that fibre polymer applications in partition wall reinforced concrete frames improve existing building behaviour. CFRP strips limit crack widths, prevent brittle failure of walls, keep scattered wall pieces together, and prevent out-of-plane movement of walls. Preventing partition walls used in reinforced concrete frames from becoming inactive during earthquakes provides significant contributions to the overall building behaviour in terms of stiffness, strength and energy consumption. The main objective of this study is to transform partition wall reinforced concrete frames that are insufficient in terms of strength into a composite system capable of carrying high lateral loads with high stiffness, strength, particularly high damping and energy consumption capability by strengthening them with fibre polymers. With the application proposed in the study, lateral loads acting on the building system can be transferred to CFRP strips without creating additional load concentration in column-beam connection areas and while maintaining continuity with the reinforced concrete elements surrounding the partition walls. Compared to other alternative strengthening methods, this approach practically strengthens partition walls within their own planes and significantly reduces labour and material costs.

2. Experimental Study

Within the scope of the study, two identical scaled reinforced concrete frame specimens were produced and constructed with scaled perforated bricks to represent partition walls and to be suitable for laboratory conditions. One of the specimens was an unstrengthened plain partition wall frame, while the other was strengthened with CFRP application. The test specimens were produced and tested in the Istanbul Technical University Building and Earthquake Engineering Laboratory. The test specimens were designed and produced to represent old structures by using low-strength concrete without complying with current earthquake code rules and without reinforcement densification in beam-column connection areas. The plain partition wall reinforced concrete frame and the strengthened frame used in the tests are shown in Figure 2.

Figure 2. Unstrengthened partition wall frame and strengthened test specimens

In the test specimens, the longitudinal reinforcement ratio in columns and beams was 1% while the transverse reinforcement ratio was 0.4%. No special measures such as densification were taken in and around the column-beam area. In this experimental study, to ensure compliance with the 1/3 scale, the dimensions of brick elements used in the construction of partition walls were specially produced as 88x84x57 mm. Material tests were conducted to determine the stress-strain relationships of concrete and steel materials. The 28-day concrete compressive strength was found to be 19 MPa, while the yield and maximum strengths of the steel material were determined as 420 MPa and 500 MPa, respectively. Uniaxial CFRP material was used in the strengthened specimens. According to the manufacturer's technical data, the unit weight of CFRP is 300 g/m², and the fibre density is 1.79 g/cm³. The CFRP elastic modulus is 230 GPa, maximum tensile strength is 3,900 MPa and maximum unit strain is 1.5%. The manufacturer recommends a usage amount of 1.0 kg/m² for two-component epoxy resin. CFRP strips were surface-prepared before application and then bonded to the surface using epoxy resin with a width of 150 mm. Anchor rods made of CFRP material 300 mm in length were used to connect CFRP strips to each other on both surfaces of the wall. As a result of the material tests conducted, the 28-day concrete compressive strength was 19 MPa, steel yield and maximum strength were 420 MPa and 500 MPa, respectively. Photographs taken during application are shown in Figure 3.

Figure 3. CFRP fabric applications to test specimens

Reversible cyclic horizontal displacement cycles were applied to the test specimens. The loading apparatus is shown in Figure 4. Horizontal load was applied to the specimen using a 280 kN capacity hydraulic actuator powered by a servo-controlled hydraulic system and mounted on a reinforced concrete reaction wall. Each displacement level was applied once in push and pull, and displacement amplitudes were increased incrementally.

Figure 4. Test apparatus for testing strengthened specimens

3. Evaluation of Test Results

The plain partition wall frame was used as a reference for comparing the results obtained in strengthened specimens. The force-displacement relationships enabling performance evaluation of the strengthened condition and the plain specimen are given in Figure 5. The strengthened frame produced larger and more stable cycles compared to other specimens during the test. This indicates better energy consumption capacity of the strengthened frame. The strengthened frame exhibited better behaviour in terms of horizontal load-carrying capacity and ductility compared to the other specimen.

Figure 5. Force-displacement curves obtained from test specimens

Comparison of the strength and displacement capacities of the specimens under identical displacement cycles was performed by considering the envelope curves of the base shear force-peak displacement cycles obtained from the experimental study. The maximum strengths reached in the specimens under push and pull conditions are shown in Figure 6. In the plain partition wall frame, a horizontal strength level of 119.9 kN was reached in push, and this capacity then decreased rapidly. The test was stopped when the relative storey drift ratio reached 2.8%. In the strengthened frame, a horizontal strength of 203.0 kN was generated at a relative storey drift ratio level of 1.1%.

Figure 6. Envelope curves of load-displacement relationships obtained from test specimens

The damage conditions observed in the specimens at the end of the tests are shown in Figure 7. In the partition wall specimen, the first flexural crack in the columns was observed at a relative storey drift ratio of 0.22% when the horizontal strength reached 76.2 kN. The first diagonal crack on the partition wall was observed at a relative storey drift ratio of 0.55% when the horizontal load was 105.5 kN. In the strengthened frame, the first crack in the column-beam connection area was generated at a relative storey drift ratio of 0.3% when the horizontal strength measured 172.4 kN. In the strengthened condition, the first diagonal crack on the partition wall occurred at a relative storey drift ratio of 1.60% when the horizontal load was 194.5 kN.

Figure 7. Damage observed in plain partition wall frame and strengthened frame

The variation of cumulative cyclic energy calculated for all specimens according to relative storey drift ratio is shown in Figure 8. Energy consumption capacity is an important characteristic in evaluating the performance of structures under earthquake effects. Cumulative cyclic energy is obtained by summing the areas enclosed by the base shear force-peak horizontal displacement cycles. At the relative storey drift ratio of 1.1% where the maximum horizontal strength was reached, the strengthened frames consumed 2.9 times more energy compared to the reference specimen.

Figure 8. Comparison of energy consumption between partition wall and strengthened conditions

4. Conclusions

The applied strengthening method significantly increases the horizontal strength of reinforced concrete frames. In the strengthened specimens, less damage occurred for the same relative storey drift levels compared to other specimens. When the energy consumption capacities achieved were compared, significant advantages were provided in terms of strengthening. As a result of testing a limited number of specimens under earthquake-like loads in their own planes, it was understood that the application carried out using CFRP in partition wall reinforced concrete frames could be an effective strengthening method. The proposed strengthening method with CFRP applied outside the column-beam connection points not only prevented sudden and brittle failure of the wall but also prevented additional force transfer to the still weak but non-ductile node points.

Acknowledgements

This study was conducted within the framework of the 106M050 numbered TUBITAK and 31966 ITU BAP research projects completed under the supervision of Prof. Dr. Ercan Yüksel, Head of the Istanbul Technical University Faculty of Civil Engineering Building and Earthquake Engineering Laboratory. I acknowledge my esteemed professor and the relevant institutions. Assoc. Prof. Hasan Özkaynak Faculty Member Beykent University Faculty of Engineering and Architecture Department of Civil Engineering    

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