Prof. Dr. Sinan ALTIN
Gazi Üniversitesi Mühendislik Fakültesi İnşaat Mühendisliği Bölümü
Dept. of Civil Engineering, Faculty of Engineering, Gazi University
Abstract
Carbon fiber reinforced polymers (CFRP), and strong adhesives (i.e. epoxy) have found a major use in strengthening of reinforced concrete structures. Some of the results of experimental studies, carried out to determine performances of techniques in order to strengthen are summarized in this proceeding. Experimental studies presented here in include a series of experimental study to improve seismic behavior not only on the member level but also on the structural level. Shear deficient reinforced concrete beams and walls, non ductile frames were strengthened by various layouts of carbon fiber reinforced polymer applications and have been tested under earthquake simulated loading. Performance levels of strengthening techniques, on the improvement of behavior and load capacity of the test specimens have been introduced.
Keywords: Reinforced concrete structures; Strengthening; CFRP
1. Introduction
Almost every earthquake encountered in our country has caused collapse and damage to buildings as well as loss of lives. Design and construction defects caused by lack of inspection and insensibleness have induced inadequacies in horizontal strength, rigidity, and ductility in such buildings. Even today, a large number of existing buildings have the characteristic properties of those buildings collapsed due to earthquake. To determine and ensure safety of any building under risk of damage and collapse is a vital and crux problem in our country. Besides demolishing and rebuilding risky buildings, rehabilitation thereto by means of strengthening is also an alternative solution. Many techniques have been developed for strengthening risky buildings; some of such techniques are available in Turkish Earthquake Regulations and used in practice.
Composites have found a wide range of application in civil engineering in recent years. Fiber reinforced polymers have become an alternative material in strengthening inadequate and defected buildings due to certain advantages thereof, such as maintaining functionality in buildings, immediate usage, not causing change in geometry, and easy application in addition to having a high strength to weight ratio. In our country, there are numerous studies, completed and ongoing, on strengthening buildings and improving their earthquake performances utilizing such type of materials, particularly following the destructive Marmara earthquake.
In the paper were summarized the results of certain experimental studies conducted in Structural Mechanics Laboratory of Department of Civil Engineering, Gazi University in order to determine performances of CFRP and strong chemical in strengthening reinforced concrete buildings. Experimental studies presented here in include a series of experimental study to improve the behavior not only on the member level but also on the structural level. CFRP applications were tested with modeled specimens in various arrangements utilized for improvement of seismic behavior in reinforced concrete beams, reinforced concrete shear walls and reinforced concrete frame load-bearing systems. In line with the scope of studies, performances of CFRP used in strengthening were measured, relevant data were collected, and recommendations were made.
2. Experiments and Test Results
During experimental studies, specimens with shearing deficiency were considered. In the paper, reinforced concrete beams with deficient confinement reinforcements, reinforced concrete shear walls with shearing deficiency, reinforced concrete frames for defected and deficient for seismic loadings were tested after having strengthened with CFRP strips, and the results were discussed. CFRP and epoxy used for strengthening are identical in three successive experiments. Properties of CFRP and epoxy are summarized in Table 1.
2.1 Strengthening Shear-Deficient Reinforced Concrete Beams with CFRP Strips [1]
Shear-deficient reinforced concrete beams were strengthened with U-form CFRP strips and tested under monotonic four-point loading. Geometric dimensions and reinforcement arrangements of the beams tested are given in Figure 1. The properties of the reinforcements used are summarized in Figure 2. 10 beams of 1/2 geometric scale were tested in the scope of the study. Properties of specimens and the testing program are summarized in Table 3. There are 3 deformed reinforcements of 20 mm in diameter at the tensile region of the beam, and 2 plain reinforcements of 8 mm in diameter at the compression region. In the specimens 1-7, Ø6/300 stirrups were used. As an extreme, there are no stirrups in the specimens 8-10. In the study, three different CFRP strip intervals as 125mm, 150mm, and 200mm, were used. Strip ends affixed to the reinforced concrete beams with fan-type CFRP anchorages. Anchorage details are given in Figure 2.
Bevels of 15mm in diameters were formed at the beam corners where CFRP strip is to be folded. For anchorages, holes of 14mm in diameter and 50mm in depth were opened, and the insides thereof were cleaned off dust. Gaps on concrete surface were filled with epoxy binder, and the places prepared earlier for binding the CFRP strips were covered up completely with binder. Afterward, CFRP strips were placed on binding layer; any air bubbles under fixed pressure were discharged making use of a cylinder; and the binder was let be absorbed by fabric. For anchorages, CFRP fabric in 120mm long and 80mm width was wrapped around a straight steel bar of 10mm in diameter at its one end, and placed into the hole filled with epoxy binder. The end of fabric in 70mm long remaining outside the hole was sliced lengthwise into finer strips and combined with the strip by laying it down, like blades of a fan, on the CFRP strip already glued. Upon completion of strengthening, beams were prepared for testing. The testing technique and a layout sketch of measuring instruments are given in Figure 3.
Crack forms and collapse mechanisms for specimens are provided in Figure 4. Shearing force – beam midpoint displacements measured during tests are grouped and given in Figure 5. Strengthening reinforced concrete beams against shearing using CFRP strips is a simple and efficient technique. Having CFRP strips not completely wrapping T-section beams requires strip ends to be held by means of anchorages. Despite increasing shearing capacity of beams, non-anchored CFRP strips were failed to prevent collapse of beams before shearing. Strengthening with anchored CFRP strips have ensured both increasing the shearing strength and improvement of shearing behavior as well as improvement of ductile deflection behavior.
Anchorages prevented stripping of CFRP strips from concrete surface and controlled enlargement of shear cracks. Non-anchored specimens collapsed due to stripping of ends of CFRP strips. Anchoring applied at CFRP strip end was effective for improving shear capacity and behavior of specimen. Strain capacity of specimens strengthened with anchored strips increased. As an extreme example, beams without shear reinforcements also reached to deflection capacity, and CFRP strips successfully controlled widening of shear cracks on beams.
Strain values of specimens increased after shear loading of 40 kN. Maximum strain measurements obtained from strain gauge are given in groups based on CFRP strip widths in Figure 6. Average strain values taken from CFRP strips pulled out from beam surface of the specimens 2, 3, and 4 are measured as 0.0041mm/mm, 0.0036 mm/ mm, and 0.0039mm/mm, respectively. Due to wide shear cracks occurred on the left shear gap of the specimen 7, a greater strain measure was taken from that specimen. Maximum mean strain took place as 0.0060mm/mm and 0.0071mm/mm for the specimens 7 and 10, respectively. When compared to the specimens strengthened with CFRP strips of 20 mm/mm in width, mean strains measured on
the CFRP strips of the specimens 9 and 10 were 54% and 82% greater, respectively, than that on the strips of the specimen 4.
2.2 Strengthening Reinforced Concrete Shear Walls with CFRP Strips [2]
In practice, shear deficient reinforced concrete shear walls are frequently encountered. Shear wall geometry in existing old buildings is observed as a little larger than column and generally, with a wall height to width ratio of 1.5. Although not collapsed, reinforced concrete buildings with such type of shear walls were severely damaged during earthquakes. To increase horizontal strength and rigidity in reinforced concrete shear walls of such buildings may mitigate damage on buildings. In the experimental program developed for this purpose, five reinforced concrete wall specimens with shearing deficiency were produced, and after strengthening them with CFRP strips in four different configurations, tested under reversible loadings simulating seismic loadings.
Having taken into account the distribution of moment and shear force in a building with reinforced concrete wall in the design of specimens, the lowest part of the building within the critical wall height where shearing effects are at maximum was taken into consideration. Geometric dimensions and reinforcements of the test specimens are given in Figure 7. The part of the reinforced concrete wall, remaining between foundations and the first floor pavement and under the impact of maximum moment and shearing force, was designed as the specimen at a scale of 1/2. Properties of reinforcements used in the specimens were summarized in Table 4, and the specimens and the testing program in Table 5.
The shear wall to be strengthened in the specimen is between a rigid base beam and loading beam. Reinforced concrete shear wall was designed in such a way that shear strength is to be insufficient and that a brittle shear fracture is to occur and deflection strength to be greater than shear strength. Loading beam was reinforced as a rigid beam with longitudinal deformed reinforcements of 8Ø12 in diameter and 300x300mm in size, and shear reinforcements at 100 mm intervals. Likewise, the base beam was produced with longitudinal deformed reinforcements of 16Ø16 in diameter and 400x500mm in size and shear reinforcements of Ø10 at 100mm intervals so that it would not be damaged under the impacts of a feasible shear force and deflection moment. Deformed reinforcements of Ø16mm in diameter were used as deflection reinforcement at the end part of reinforced
concrete wall with a cross section of 1000×100 mm, and plain reinforcements of Ø6mm in diameter in the main part. The vertical reinforcement ratio of RC panel is pv=0.0183, and the horizontal reinforcement ratio is ph=0.0015. Along 1500mm height of the wall, web reinforcements of Ø6/400 were used. in the panel of the specimen, a wrapped end component was not formed, but the ends of web reinforcements were bended at 90o. This way it was aimed to simulate an existing shear wall with insufficient seismic properties.
As part of the experimental study, 4 specimens were strengthened with CFRP strips affixed on exterior of the reinforced concrete wall. Strip arrangements are given in Figure 8. The variable in the experiment program is the CFRP strip arrangement used for strengthening. Reinforced concrete panel in the specimen 2 was horizontally wrapped with CFRP strips of 100mm in width. Along the wall height, 8 CFRP strips of 100mm in width were placed horizontally on reinforced concrete panel at 200mm intervals equal from center to center thereof. The reinforced concrete panel of the specimen 3 was strengthened with 4 CFRP strips of 200 mm in width placed on both surfaces of the panel symmetrically in diagonal direction. Strengthening detail applied in the specimen 4 is similar to the specimen 3. In addition to cross CFRP strips of 200mm in width placed on the specimen 3, the reinforced concrete panel was wrapped with 2 CFRP strips of 200mm in width at its bottom and top edges. In the details of strengthening applied for the specimen 5, 20 CFRP strips of 100mm in width were placed symmetrically at 270mm intervals from center to center thereof on both wall surfaces in parallel direction to the wall diagonal. Afterwards, the bottom and top ends of the wall were wrapped with CFRP strips on 100mm in width. CFRP strips were anchored to the concrete surface utilizing fan-type anchorages. Anchorage details are given in Figure 9. For horizontal and diagonal strips, anchoring distances are 300mm and 270mm, respectively. The fan-type anchorages were produced by rolling a CFRP strip of 80mm in width and 200mm in length along short edge thereof. The extensions of 50mm at both ends of the fabric which passed through a hole in 10mm diameter drilled on the wall for anchoring were laid down like fan blades and affixed on CFRP strip previously affixed.
All specimens were produced on horizontal position and in two stages in laboratory environment. At the first stage, the concrete of reinforced concrete shear wall was poured into a horizontal formwork set on the laboratory floor, and subjected to a 28-day curing. At the second stage, the specimens were strengthened by utilizing CFRP strips. For this purpose, the positions of CFRP strips and anchorage points were marked on the specimen. Afterwards, holes of 10mm in diameter were drilled for anchoring, and the surfaces of concrete, whereon CFRP strips are to be affixed, were roughened for better binding. During this procedure, bevels of 15mm in diameter were formed at the corners where CFRP strip is to be folded. Then, concrete surfaces and inside of holes drilled for anchoring were cleaned using compressed air and a brush, paying attention to that anything loose may not adversely affect affixing. After cleaning surfaces, epoxy binder prepared according to the manufacturer’s instructions was spread over concrete surface in 1.5mm thickness, and anchoring holes were filled with the binder. Afterward, CFRP strips were placed on the binder layer; any air bubbles under fixed pressure were discharged making use of a cylinder; and the binder agent was allowed to be absorbed by fabric. Anchorages were placed into the holes drilled and the extensions left outside were affixed to existing CFRP strip after laying them down like fan blades in 100mm diameter. Before the testing, all the specimens were subjected curing for at least 15 days in the laboratory for letting the binder layer to dry.
These strengthening procedures were applied to all the specimens. The specimens were tested under reversible loads simulating seismic loads. Experimental arrangement and the layout of measuring instruments are given in Figure 10.
After collapse, appearance of the specimens is given in Figure 11, and envelope curves obtained from testing of the specimens comparatively in Figure 12. The technique for strengthening reinforced concrete shear walls with insufficient shear strength utilizing CFRP strips has significantly improved horizontal strength, horizontal rigidity, ductility, and drift ratios. For the wall collapsed without shearing, ductile deflection dominated the behavior after strengthening. It was observed that CFRP arrangements used for strengthening were highly influential on reversible repetitive behavior displayed by the specimens and the mode of collapse. All the CFRP layouts applied to reinforced concrete shear walls with insufficient shear strength were successful, and with the strength increase attained, the specimens that must collapse by displaying brittle shear fractures had reached to a level to display a ductile deflection behavior. Moreover, they displayed certain rates of ductility depending on the CFRP strip arrangements applied. CFRP strips were not effective on improvement of starting rigidity of the specimens. However, CFRP strips prevented widening of shear cracks on the entire wall and controlled loss of horizontal rigidity of the specimen.
The specimens strengthened with horizontal and X pattern were strained without any significant decrease in their deflection strengths until 2% drift ratio. By means of the strengthening technique applied, the strain capacities of the specimens were improved without any significant decrease in their load bearing capacities. Anchorages delayed stripping off CFRP strips from reinforced concrete panel surface whereon they were affixed. After CFRP strips or anchorages ruptured, there was a decrease in load bearing capacities of many specimens and the specimens collapsed. Anchorages played a significant role in active load bearing of CFRP strips until rupture strain values.
Typical examples taken from the strips during the tests for unit strain measurements are given in Figure 13. Average of maximum values measured from horizontal strips is 0.0090mm/mm for the specimen 2, and 0.00748mm/mm maximum unit deformations measured from cross strips. Unit deformations of the specimens 3 and 4 were measured as 0.00840mm/mm. Deformations and cracks developed in the wall cause stripping off CFRP strips from concrete surface and strip ruptures before limit values under combined strains.
2.3. Strengthening Reinforced Concrete Frames with CFRP Strips [3]
Testing frame is a non-ductile reinforced concrete frame of one story, with single opening built at 1/3 geometrical scale. It was designed and produced as to represent general insufficiencies in buildings of Turkey. Geometrical features and reinforcement details are given in Figure 14. Inside the frame was walled using hollow bricks. The infill wall was aligned to the frame exterior to represent exterior wall of a building. On both surface of the infill wall a plaster layer was added in 7.5mm thickness matching to the frame’s geometric scale. Properties of reinforcements used in the specimens were summarized in Table 6, and the specimens and the testing program in Table 7.
Experimental parameters of the study are widths and applications of CFRP strips. Strips were applied symmetrically on the specimens 2-4, on the interior surface of the infill wall on the specimens 5-8, and on the exterior surface of the infill wall on the specimens 8-10. Strips of three different widths were affixed on the infill wall as 200mm on the specimens 2, 5, and 8, 300mm on the specimens 3, 6, and 9, and 400mm on the specimens 4, 7, and 9. Strips arrangements are given in Figure 15 and anchorage details in Figure 16. Affixed strips were anchored to both reinforced concrete frame and the infill wall. For the anchorage applied to the reinforced concrete frame from interior surface of the infill wall, firstly, holes of 14mm in diameter and 100mm in depth were drilled to the frame columns and beams. Drilled holes were cleaned utilizing compressed air and a brush. A piece of CFRP fabric in 300mm long and 100mm width was wrapped around a straight steel bar of 12mm in diameter at its one end, and placed into the anchoring hole filled with epoxy binder. Part of the fabric remained outside the hole was joined, using epoxy binder, to the CFRP strip affixed on the filling epoxy. For each 100mm strip width joined on the infill wall, 1 anchorage was applied to the reinforced concrete frame. For the anchorages applied on exterior surface of the infill wall to the reinforced frame, holes of 14mm in diameter and 100mm in depth were drilled and cleaned on reinforced frame. For the anchorages on the exterior surface, CFRP fabric pieces were used in 240mm long and 30mm width.
One end of the fabric is sliced into finer strips lengthwise in 100mm depth. The other end is wrapped around a steel rod in shorter direction, and inserted into a previously drilled anchorage hole filled with epoxy binder. In order to minimize the stress buildup, a piece of CFRP fabric in 300mm long and in the width of the strip used as strengthening was affixed to the anchoring location with epoxy binder.
The specimens were tested under reversible loads simulating seismic loads. The testing technique and a layout of measuring instruments are given in Figure 17. Testing was terminated after the specimens lost their horizontal load bearing capacity.
Envelope curves obtained from load-horizontal displacement values of the specimens measured during tests are given in Figure 18. Diagonal CFRP strips substantially increased horizontal rigidity and strength of the specimens. The reference specimen reached to maximum horizontal load with 0.40% horizontal drift. After the infill wall inside the reinforced concrete frame was squeezed at its top corners under compressive forces, the specimen lost its horizontal load bearing capacity. The specimens 2, 3, and 4, which were strengthened on both surfaces of the infill wall, displayed similar behaviors and exhibited the same mode of collapse. Diagonal CFRP strips on the interior surface of the infill wall for these specimens were torn, and the anchorages of the strips on the exterior surface of the infill wall were ruptured (Figure 19, Figure 20). Horizontal load level tearing diagonal CFRP strips is the maximum horizontal load level of these specimens. After this stage, the specimens suddenly lost their horizontal load bearing capacities and horizontal rigidities. In the specimens strengthened only on one surface of the infill wall utilizing CFRP strips, the CFRP strip exposed to diagonal compressive forces between 0.50-0.61% of horizontal drift ratio both in forward and reverse cycles was detached from the infill wall together with plaster at the top corner of frame. Horizontal load level detaching the strips from the filling is the maximum horizontal load that may be reached during testing of the specimens. At this level of load, joint mortars between bricks were crushed at top corners of the infill wall. The level of load ruptured CFRP anchorages is lower the maximum load in all specimens.
CFRP strips used for strengthening were effective for improvement of horizontal strength and rigidity of reinforced concrete frames walled with bricks. Success of this technique depends basically on successful application of anchorages to the infill wall and reinforced concrete frame utilizing CFRP strips. Horizontal strength of the specimens strengthened with symmetrical CFRP strips increases 2.18 to 2.61 times in comparison to the specimens strengthened on one surface only. Horizontal rigidity increase in the same specimens was 4 to 6 times. Horizontal strengths of the specimens strengthened on single surface increased 1.57 to 1.85 times and the rigidity thereof 3.81 to 5.70 times. In terms of horizontal strength and rigidity, gains of the specimens strengthened on single surface with similar CFRP strips occurred at the same level. Increase in strength and rigidity may be limited with wider strip width. When strip width is increased from 0.13dw to 0.20dw, the rigidity of the specimens strengthened symmetrically increased by 50%. In comparison to the specimens strengthened on single side, rigidity increase was in the range of 31% to 36%. Floor drift ratios of all specimens were obtained greater than the limit value of 0.35% recommended by new Turkish seismic regulations. Until such limit value, the specimens maintained their horizontal load bearing capacities considerably.
The maximum deformations measured from CFRP strips are given in Figure 21. CFRP strips affixed to the interior of the infill wall of the specimens 2, 3, and 4were torn at a unit strain level varying between 0.0042 and 0.0054. Due to horizontal loading having impact on the specimen, cracks were formed on the infill wall and accordingly, directions of CFRP strips changed. Therefore, CFRP strips were strained not only under axial force, but also under shear loading, and lost their strengths at a lower unit strain level. CFRP strips on the panels strengthened on single surface were strained at a maximum unit strain levels in the range of 0.0028 to 0.0032. These values correspond to rupture of anchorages on CFRP strips. Considering average tensile strains, unit strain values measured from CFRP strips on the panel strengthened on both surfaces are 1.6 times higher than those of the specimens strengthened on single surface.
3. Conclusions
Carbon fiber polymers are an alternative material for strengthening inadequate and defected buildings due to certain advantages thereof, such as maintaining functionality in buildings, immediate usage, not causing change in geometry, and easy application in addition to having a high strength to weight ratio. Effective strengthening techniques were developed utilizing this material, and such techniques are presently used in practice. It is known that a great majority of existing building inventory in Turkey has inadequate earthquake safety due to numerous reasons. Such buildings are of similar characteristics with the buildings collapsed during past earthquakes and have high risk of collapsing in case of an earthquake. Ensuring earthquake safety for such buildings is essential for avoiding economic losses as well as loss of lives.
In this paper, some of the general issues encountered with the buildings in our country were addressed and adapted to specimens modeled in the laboratory environment. Strengthening techniques that were developed for elimination of the problems were applied to the specimens, and the performances of these techniques were tested under the loadings simulating earthquake loadings. The test results revealed that CFRP strips with the held ends met the expectations, and shear-deficient reinforced concrete beams and shear walls increased strength and rigidity on reinforced concrete frames as well as improving behavior.
References
[1] Improving shear capacity and ductility of shear-deficient rc beams using cfrp strips, Journal of Reinforced Plastics and Composites, 29(19): 2975-2991.
[2] Hysteretic behavior of rc shear walls strengthened using cfrp strips, Composites Part B: Engineering Journal, 44(1): 321-329.
[3] An experimental study on strengthening of masonry infilled rc frames using diagonal cfrp strips, Composites Part B: Engineering Journal, 39(4):680-693.
