- freely available
Fibers 2019, 7(10), 90; https://doi.org/10.3390/fib7100090
2. Research Motivation
3. Experimental Program
3.1. Material Properties
3.1.2. Steel and FRP Composites
3.2. Test Setup and Instrumentation Details
3.3. Pre-Damaging of Columns
3.4. Hybrid FRP Strengthening
4. Results and Discussion
4.1. Plain Concrete Columns
4.1.1. Load—Displacement Behavior
4.1.2. Load—CFRP Strain Behavior
4.1.3. Failure Mode of PC Columns
4.2. Reinforced Concrete Columns
4.2.1. Load—Displacement Behavior of RC Columns
4.2.2. Load—Strain Behaviour of RC Columns
4.2.3. Failure Mode of Pre-Damaged RC Columns
5. Analytical Studies for Pre-Damaged Columns
5.1. Unconfined Concrete Specimens
- The cross-section of the specimen was divided into several layers known as fibres. Fibre discretization approach helps in the use of an actual parabolic stress-strain model of concrete rather than the use of idealised rectangular stress block method.
- The nonlinear behaviour of concrete is considered through the Hognestad’s parabolic model , where the concrete cylinder compressive strength was obtained from the material characterization discussed earlier. In this context, the pre-damaged concrete specimens were assumed to have a significant reduction in strength and stiffness. Therefore, the original constitutive law was modified to represent the behaviour of the pre-damaged concrete columns.
- The stress-strain behaviour of the steel was considered linear until yielding. After that, the strain value increases until the hardening point without an increase in stress (fst). Thus, the increase in both stress and strain was observed after yielding until it reached its ultimate strength.
- Stress in FRP (fN) for a given strain was arrived based on their corresponding stress-strain behavior.
- The total load resistance of the specimen was calculated as the sum of resistance from concrete, steel reinforcement and CFRP.
- The axial displacement was computed from the value of initial strain multiplied by the gage length of LVDTs used in the experiments.
5.2. FRP Confined Concrete Specimens
- The cross-section was discretised into a large number of fibres, and the initial strain value was fixed.
- The axial stress in each concrete fibre was calculated using a modified Lam and Teng model, where fcm′ correspond to the compressive strength of pre-damaged concrete considered to be 0.75 times the compressive strength of undamaged concrete. The maximum confined compressive strength of concrete (fcc′) can be calculated in terms of maximum confinement pressure (fl), as mentioned in Equations (4)–(8).
- The contribution of NSM laminates (fN) under compression can be estimated from their respective stress-strain relationship (fN = Ef x εc).
- The total axial load contribution (PT) can be calculated from the load resistance of the concrete, steel and FRP laminates using axial force equilibrium condition.
5.3. Comparison of Analytical and Test Results
6. Three-Dimensional Finite Element Analysis
6.1. Material Properties
6.1.2. Steel Reinforcement and CFRP
6.2. Modelling Procedure and Interface Properties
6.3. Boundary Conditions and Meshing
6.4. Phased Analysis for Columns with Cyclic Damage
6.5. Comparison of FE and Test Results
6.6. Parametric Investigation from Validated FE Model
6.6.1. Effect of Pre-Damage Levels in Hybrid FRP Strengthening
6.6.2. Effect of FRP Reinforcement Ratios in Hybrid FRP Strengthening
7. Summary and Conclusions
- Both the FRP strengthening schemes (NSM and Hybrid) effectively restored the initial stiffness and load-carrying capacity of the columns after cyclic compressive damage.
- NSM strengthening of plain concrete columns increased the overall performance in terms of its load-carrying capacity and ultimate displacement. Moreover, the failure occurred in a ductile manner through the propagation of cracks in the concrete between the NSM FRP laminates.
- In the hybrid FRP strengthening of plain concrete columns, NSM laminates acted effectively as longitudinal reinforcement. The load contribution of NSM laminates was found to be significant from the measured load-strain diagram. Moreover, the effectiveness of NSM laminates significantly increased due to external confinement of CFRP fabric.
- RC columns strengthened using the NSM strengthening technique fully restored the capacity lost due to the initial cyclic damage. The increase in strength and ultimate displacement of NSM strengthened RC column was negligible and 10%, respectively, when compared to the control column element (C-RC).
- Hybrid FRP strengthening technique was able to increase the strength up to 12% and ultimate displacement was restored to 94% of its control capacity when compared to the column without any initial damage (C-RC).
- The analytical approach used in this work closely predicted the behaviour of both plain and reinforced concrete columns strengthened using NSM and hybrid FRP techniques.
- The results from the staged FE modelling exhibited a good correlation with the test results showing the reliability of the FE modelling approach to include the effect of pre-damage. The detailed parametric investigation from this study highlights an important observation, namely that the different levels of pre-damage have no significant impact on strength improvement due to hybrid FRP strengthening.
Conflicts of Interest
|Ac = total area of the concrete section (mm2).|
|Ae = total area of the effectively confined concrete section (mm2).|
|Ag = total gross area of the concrete section (mm2).|
|b = breadth of the member (mm)|
|d = sectional depth excluding the concrete cover (mm).|
|d′ = thickness of concrete cover (mm).|
|di = distance of steel reinforcement with respect to the position (mm).|
|dN,i = distance of NSM reinforcement with respect to the position (mm).|
|E2 = slope of linear portion in the stress-strain curve for the concrete confined with FRP (MPa).|
|Ec = modulus of elasticity of concrete (MPa).|
|Ef = modulus of elasticity of FRP in tension (MPa).|
|εc = compressive strain in the top face of the section.|
|εc′ = maximum compressive strain of concrete without confinement.|
|εcu = failure strain level in concrete without confinement.|
|εcm = ultimate strain of unconfined concrete with pre-damage.|
|εccu = failure strain level in concrete with FRP confinement.|
|εfu = strain in FRP corresponding to rupture.|
|εfe = effective strain level in FRP reinforcement at failure.|
|εs,i = strain in the steel at different levels of the section.|
|εt′ = transition strain in the stress-strain curve of CFRP confined concrete.|
|εy = yield strain of steel reinforcement in tension.|
|fc = average cylinder compressive strength of concrete without confinement (MPa).|
|fc′ = compressive stress in concrete (MPa).|
|fcc′ = compressive strength of FRP confined concrete (MPa).|
|fcm′ = modified compressive strength of unconfined concrete with pre-damage (MPa).|
|fl = maximum confining pressure owing to CFRP fabric (MPa).|
|fst = overall stress contribution from steel reinforcement (MPa).|
|fN = overall stress contribution from NSM laminates (MPa).|
|κε = strain efficiency factor of FRP.|
|n = number of plies of FRP reinforcement.|
|PT = total sectional capacity in terms of load (kN).|
|rc = corner radius of non-circular sections confined using FRP (mm).|
|ψf = Reduction factor for FRP.|
|tf = nominal thickness of FRP for one ply (mm).|
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|Type of Specimen||Series Name||Specimen Details (mm)||No of Specimens||Rebar Details||Strengthening Details|
|Plain Concrete Columns||C-PC||230||230||1||Control specimen with no Pre-damage|
|P-PC||1||Pre-damaged and no strengthening|
|P-NSM||2||Pre-damaged and strengthened with NSM|
|P-HYB||2||Pre-damaged and strengthened by a hybrid technique|
|Reinforced Concrete Columns||C-RC||230||230||1||8 # 12 mm dia.||10 mm dia. @ 100 mm c/c||Control specimen with no Pre-damage|
|R-RC||1||Pre-damaged and no strengthening|
|R-NSM||2||Pre-damaged and strengthened with NSM|
|R-HYB||2||Pre-damaged and strengthened by a hybrid technique|
|Material||Coupon Size||Tensile Strength (MPa)||Rupture Strain (%)||Elastic Modulus (GPa)|
|Hand layup CFRP||15 × 1.5||1300||1.15||113|
|Pultruded CFRP Laminates||12.5 × 1.4||2300||1.40||165|
|Type of Specimen||Specimen ID||Axial Stiffness (kN/mm)||Load (kN)||Axial Displacement (mm)||Average Axial Displ. (mm)||Axial Displ. Ductility||Improvement Ratio|
|Peak||Average Peak||Peak||Failure||Peak||Failure||Axial Stiffness||Peak Load||Failure Displ.|
|Plain Concrete Column||C-PC||8037||1825||1825||0.25||0.59||0.25||0.59|
|Reinforced Concrete Column||C-RC||8780||2281||2281||0.42||1.50||0.42||1.50||3.66|
|Specimen ID||Column Type||Exp. Peak Load (kN)||Anal. Peak Load (kN)||FE Peak Load (kN)||PEXP/PANAL||PEXP/PFEM|
|P-PC||Pre-damaged PC column||1724||1587||1789||1.08||0.96|
|R-RC||Pre-damaged RC column||2006||2004||2049||1.00||0.98|
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