SRG jacketing of short R/C columns: Experimental investigation

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1 SRG jacketing of short R/C columns: Experimental investigation Georgia E. Thermou Assistant Professor, AUTh, Konstantinos Katakalos Dr Civil Engineer, AUTh, George Alexiou Civil Engineer AUTh, 1. Introduction A considerable part of the existing reinforced concrete (R/C) building stock in countries around the Mediterranean basin was built either without any seismic code provisions or with low level seismic demands. Experimental evidence has demonstrated that confinement of existing R/C members with composite systems (e.g. FRP, TRM) has the potential to improve seismic resistance of substandard buildings by increasing ductility of end sections of beams and columns, increasing shear strength capacity, improving the efficiency of lap splices and delaying the occurrence of buckling of steel longitudinal bars. Towards this direction, a new composite system, the steel-reinforced concrete (SRG) jacketing, which comprises high strength steel fabrics combined with inorganic matrix that served as the connecting matrix was introduced in 2007 by Thermou and Pantazopoulou. The main advantage of the proposed system relies on the use of inorganic binders which when combined with the steel fabric provide a cost-effective, environmentally friendly and fire-resistant retrofit solution. Recent experimental studies have demonstrated the efficiency of SRG jackets (Thermou et al. 2007, 2015, 2016). The experimental study presented in this paper aims to investigate the efficiency of SRG jackets in improving the behaviour of R/C columns having different arrangements of shear reinforcement. Alternative jackets were applied depending on the density and the number of layers of the fabric. A significant number of short R/C columns was tested to failure under concentric uniaxial compression load. Test results have demonstrated that the behaviour of the retrofitted specimens was substantially modified when compared to their unconfined counterparts. Even in the case of columns susceptible to bar buckling due to the existence of sparse stirrups, the SRG jackets managed to delay bar buckling allowing thus the member to reach higher strength and deformation capacity. 2. Experimental program 2.1 Test specimens The SRG jacketing system was applied on twenty-four (24) short R/C prims which had a 200 mm square cross section representing a 1:2 scaled model of a prototype column with a 400 mm square cross section. The height of the specimens was 320 mm (Fig. 1). The longitudinal reinforcement comprised 4 12 bars and placed at the corners of the specimens as shown in (Fig. 1).

2 Parameters of the investigation were the stirrup spacing as to provide different volumetric ratios ( 6/50 mm- closed stirrups, 6/75 mm-closed stirrups, 6/150 mm-open stirrups), the density of the fabric (1.57 cords/cm and 4.72 cords/cm) and the number of fabric layers (1 and 2). Three group of specimens were defined based on the stirrups spacing (A, B, C, Table 1). In each group of specimens, three different jacketing configurations were tested defined as: (i) one layer of 1.57 cords/cm density steel fabric, (ii) two layers of 1.57 cords/cm density steel fabric, and (iii) two layers of 4.72 cords/cm density steel fabric. Two specimens of each jacket configuration were tested in each Group. The specimen details appear in Table 1. Stirrups Ø6 or Ø8 Fig. 1 Specimen reinforcement detailing. 2.2 Material properties The columns were cast in three Groups of eight using the same concrete mix. In addition, three standard cylinders (150x300mm) were cast in each group for obtaining the average compressive strength at the day of the tests. The average compressive strength was f co / =27.79 MPa. The high carbon steel fabric utilized in the experimental program was the 3X2 type which consists of wire cords made by twisting five individual wires together three straight filaments wrapped by two filaments at a high twist angle. Thanks to galvanization of the individual wires possesses high durability in a chloride, freeze-thaw and high humidity environment. The geometrical and mechanical properties of single cord as provided by the manufacturers appear in Table 2. The spacing between successive cords, which defines the density of the steel fabric, should be enough as to provide uninhibited flow of the cementitious grout through the steel fabric and enhance the developed bond between the fabric and the matrix. It is one of the crucial design parameters of the SRG

3 jacketing technique as has been highlighted by Thermou et al. (2007, 2015, 2016). The density of the fabrics examined herein correspond to 1.57 cords/cm and 4.72 cords/cm. The equivalent thickness per unit width, t s, were defined equal to 0.084mm and 0.254mm, respectively. A commercial one component mortar was used as the substrate material applied to the concrete surface of the specimens, the bonding material between the applied layers of the steel fabric and as a final cover. It is a geo-mortar with a crystalline reaction geobinder base, with very low petrochemical polymer content and free from organic fibers. The mechanical properties of the mortar appear in Table 3. Table 1: Specimen details No Specimen Notation Fabric density (cords/cm) Layers 1 A A RL1A RL1A RL2A RL2A RM2A RM2A B B RL1B RL1B RL2B RL2B RM2 B RM2B C C RL1C RL1C RL2C RL2C RM2C RM2C Group A Group B Group C Table 2: Geometrical and mechanical properties of single cord as provided by the manufacturer. Fabric type Cord diameter (mm) Cord area (mm 2 ) Break load (N) Tensile strength ffu,s, (MPa) Strain to failure εfu,s, (mm/mm) 3X Table 3: Mechanical properties of the mortar at 28 days. Mortar Modulus of elasticity, Em (MPa) Flexural strength, fmf (MPa) Compressive strength, fmc (MPa) Adhesive bond, fmb (MPa)

4 2.3 Construction of the R/C columns Application of the jackets Wooden moulds were utilized for the construction of the square specimens with rounded edges with a corner radius r=25 mm. The steel reinforced fabrics were pre-bent before application in order to facilitate the application procedure taking into account the corner radius. The width of the tape was 300 mm whereas the height of the specimens 320 mm. A gap of 10 mm was left at the top and bottom of the specimens. After removing the moulds the substrate of the columns was roughened. It was cleaned and saturated with water before proceeding to the application of the mortar (Fig. 2). The cementitious grout was applied manually with the help of a trowel directly onto the lateral surface of the specimens (Fig. 2). The metallic fabric was placed immediately after the application of the cementitious grout (Fig. 2). The grout was squeezed out between the steel fibers by applying pressure manually (Fig. 2). After the application of the fabric to one full-cycle the remaining length was lapped over the lateral surface. The overlap length was defined equal to one and a half side. A final coat of the cementitious grout was applied to the exposed surface. The effect on the geometric dimensions of the jacketed specimens was small. The grout layer including the steel reinforced jackets was 5-10 mm thick. The thickness of the grout layer was such as to guarantee that the steel fabric was fully embedded in the cementitious matrix. Fig. 2 SRG jacketing application: roughening and saturation of the interface; application of a thin layer of mortar; application of the steel-reinforced fabric. 2.4 Test setup The specimens were subjected to monotonically increasing concentric uniaxial compression load up to failure and both axial and lateral strains were measured (Fig. 3). The loading was applied at a rate 0.15 MPa/sec in load control, using a 6000 kn compression testing machine. Axial and lateral strain were calculated using the measurements of eight linear variable differential transducers (LVDTs) mounted on the four sides of the specimens with the help of a custom made metallic frame structure attached to the specimen (Fig. 3). The upper and lower frames were fixed to the specimen by tightening the screws, ensuring that the frames were almost symmetric about the center of the specimen (Fig. 3). The distance between the center of the upper and lower frames was then measured as the gauge length of the LVDTs. The four horizontal LVDTs, were mounted on the middle of the specimen to measure the lateral

5 expansion. Axial load was measured from a load cell placed at the top of the specimen (Fig. 3). Herein, only the results related to axial strains are presented. Fig. 3 Test setup. Table 4: Summary of test results. No / / Specimen f cc f ccu,80% Failure ε Notation (MPa) (MPa) cc ε ccu,80% mode 1 A A RL1A D 4 RL1A D 5 RL2A R 6 RL2A R 7 RM2A R 8 RM2A R 9 B B RL1B R 12 RL1B D 13 RL2B R 14 RL2B R 15 RM2 B R 16 RM2B R 17 C C RL1C R 20 RL1C R 21 RL2C D 22 RL2C R 23 RM2C R 24 RM2C R Group A Group B Group C

6 3 Test results and discussion The axial stress -strain responses of the tested specimens are presented in Figs 4 and 5. A summary of the peak stress, f / cc, and the corresponding strain, ε cc, as well as the ultimate compressive strength, f / ccu,80%, which corresponds to 20% drop in compressive strength f / cc, and the corresponding strain ε ccu,80% appear in Table 4. The failure mode is also presented where D stands for debonding and R for rupture of the fabric. Control columns: The effect of the three different stirrups spacing selected ( 6/50 mm, 6/75 mm, 6/150 mm) is evident through the comparison of the stress-strain curves presented in Fig. 4. Specimens of Group A failed due to bar buckling since the low level of confinement provided by the 6 mm bar diameter stirrups with open legs spaced at 150 mm could not provide support to the longitudinal bars. The response of Group B specimens was slightly improved owed to the closer stirrup spacing. The response of Group C specimens is significantly affected by the close stirrup spacing of 50 mm selected. In this case, the behaviour is considered ductile with no bar buckling being observed. The response of specimens of Group A is representative to the response of members designed according to old construction practice whereas the response of specimens of Group C resembles the response of specimens designed following modern codes design The control specimens at failure state are shown in Fig. 6. Stress (MPa) A1 A2 15 B1 10 B2 C1 5 C Strain (mm/mm) Fig. 4 Control columns at failure state. Group A columns: The application of the SRG jackets has substantially modified the response of the existing specimens as shown in Fig. 5. The application of one layer of the 1.57cords/cm fabric in specimens R1LA managed to delay buckling and the average increase in strength and strain capacity was 10.78% and 472.2%, respectively. The two layers of the 1.57cords/cm fabric in specimens R2LA led to 35.4% and 599.4% increase in strength and strain capacity, respectively. The two layers of the denser steel fabric (4.72 cords/cm, columns RM2A) had the same influence in strength increase (35.2%) but almost doubled the strain capacity when compare with the effects of the two layers of the 1.57 cords/cm fabric (columns RL2A).

7 Stress (MPa) Stress (MPa) Stress (MPa) A1 A2 15 RL1A1 RL1A2 10 RL2A1 RL2A2 5 RM2A1 RM2A Strain (mm/mm) B1 B2 15 RL1B1 RL1B2 10 RL2B1 RL2B2 5 RM2B1 RM2B Strain (mm/mm) C1 C2 15 RL1C1 RL1C2 10 RL2C1 RL2C2 5 RM2C1 RM2C Strain (mm/mm) Fig. 5 Control columns at failure state.

8 A B C Fig. 6 Control columns at failure state. Group B columns: The effect of the SRG jacketing in case of the columns of Group B seems to be more pronounced in terms of strength when compared to columns of Group A. Strength capacity when compared with that of the unconfined specimens (columns B) increased by 17.2% in columns R1LB, 32.8% in columns R2LB and 42.2% in columns R2MB (Fig. 6). Strain capacity increased up to 584.9% for the two-layered SRG jackets with fabric density equal to 4.72 cords/cm. Group C columns: Wrapping with the steel fabric improved the axial stress and strain capacity of Group C columns. The strength when compared to the unconfined counterparts increased by 25.8%, 38.9% and 50.5% for the R1LC, R2LC and R2MC columns. In case of the well confined columns of Group C ( 6/50 mm, closed stirrups) the application of the SRG jackets contributed in a further increase of the axial strain capacity of columns according to 19.2% for R1LC, 152.4% for R2LC and 223.7% R2MC. SRG jacketed specimens at the end of the tests are depicted in Fig. 7. Fig. 7 SRG confined columns at failure state. 4 Conclusions The effectiveness of the SRG jackets in modifying the response of short columns reinforced with different shear reinforcement configurations was studied experimentally and the first results are presented in current paper. Three of groups of specimens were examined corresponding to three levels

9 of confinement area ratio. The types of jackets applied were differentiated as per the density of the fabric and the number of layers. In all cases, wrapping with the steel fabric improved substantially both strength and strain capacity. In case of the columns with the sparse confinement reinforcement (Group A), SRG jacketing managed to delay bar buckling occurrence and increase substantially strength (up to 35%) and strain capacity (up to 1031%). Tt was observed that as the shear reinforcement area of the columns increased (Groups B and C), the effect in strength increase was more pronounced. The opposite was the case regarding strain increase. The results of this experimental sequence support the use of SRG jackets as an efficient and promising retrofit solution for seismic retrofitting applications. Acknowledgements The program was conducted in the Laboratory of Strength of Materials and Structures, Civil Engineering Department, Aristotle University of Thessaloniki. The authors wish to thank Mr. T. Koukouftopoulos for his assistance in the experimental program. The materials were donated by Kerakoll and Interbeton. References Thermou, G.E., Katakalos, K. and Manos, G. (2016) Influence of the cross section shape on the behaviour of SRG-confined prismatic concrete specimens. Materials and Structures, 49(3), Thermou, G.E., Katakalos, K. and Manos, G. (2015). Concrete Confinement with Steel-Reinforced Grout Jackets. Materials and Structures, 48(5), Thermou, G.E., and Pantazopoulou, S.J. (2007) Metallic fabric jackets: an innovative method for seismic retrofitting of substandard R/C prismatic members. Structural Concrete, Journal of the fib, 8(1),