This chapter explores the complex behavior of corrosion-damaged reinforced concrete beams that have been repaired and strengthened using carbon fibre polymer (CFRP) plates. The study combines experimental and numerical methods to understand the interaction between the repaired and strengthened components, focusing on the flexural behavior of the beams. The experimental program involved casting and testing five types of beams with varying damage lengths, all repaired and strengthened using the same materials. The numerical analysis was conducted using the finite element method, modeling the nonlinear behavior of the beams under four-point bending. The results from the numerical analysis were validated against experimental findings, showing a close agreement in terms of cracking load, structural crack distribution, load-deflection relationships, and failure mechanisms. The study concludes that repair and strengthening of corrosion-damaged reinforced concrete beams significantly increase their load-carrying capacity, highlighting the potential of CFRP plates as a viable solution for strengthening such structures.
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Abstract
Carbon fibre reinforced polymers (CFRP) have emerged as an effective material for strengthening reinforced concrete structures. While many studies have been reported on CFRP strengthening of reinforced concrete elements subject to corrosion, there is a dearth of information on strengthening patch-repaired concrete elements. This paper reports on the experimental and numerical investigation of the behaviour of corrosion-damaged RC beams that have been patch repaired and strengthened with CFRP. Fifteen beams were cast; twelve of these were subjected to simulated corrosion of 5%, and the remaining three were used as control beams. The damage length was varied from 450 mm, 800 mm, 1300 mm and 1800 mm while keeping the depth of patch repair at 105 mm. All the CFRP-strengthened beams failed by intermidiate crack (IC) debonding. The control beams had a lower average crack density than the retrofitted beams. There was an increase in the average crack density for increasing damage lengths. Patch repair combined with CFRP strengthening retrofit method restored the damaged beams load carrying capacity but reduced the beams’ ductility compared to the control beams. The yield loads for patch repaired and strengthened (RS) results increased by 10%, 17% and 20% for beams with 450 mm, 800 mm and 1800 mm damage lengths, respectively. It was also observed that for increasing damage lengths; 450 mm, 800 mm and 1800 mm the peak loads increased by 13%, 20%, 20%, respectively. The experimental results correlated well with the finite element modelling (FEM) results.
1 Introduction
Corrosion of steel reinforcement is the most common cause of the deterioration of reinforced concrete elements. It leads to the reduction of the area of steel, cracking of concrete and loss of structural capacity. The typical maintenance approach for corroded reinforced concrete elements is to; 1) remove the concrete in the corrosion-affected regions, 2) clean and protect the corroded steel, 3) repair the concrete element using an appropriately designed concrete mix or cementitious grout, and 4) strengthen the element to restore the lost structural capacity. Reinforced fibre polymer plates (FRP) have emerged as a viable solution for strengthening reinforced concrete elements due to their inherent advantages such as corrosion resistance, lightweight and high strength. The behaviour of concrete elements that have been repaired and strengthened using FRP plates is complex due to their multiple-layer nature, including the old concrete layer, the new concrete layer, the epoxy layer and the CFRP layer. A good understanding of the interaction between these components is essential to understand the system's failure modes and for the development of design procedures for such systems. This study focuses on the flexural behaviour of corrosion-damaged reinforced concrete beams repaired using concrete and strengthened using carbon fibre polymer (CFRP) plates.
Malumbela et al. [1] investigated the load-carrying capacity of patch-repaired and CFRP-strengthened RC beams. They concluded that combined patch repair and strengthening is effective for retrofitting corrosion-damaged beams. They achieved up to 50% increase in load-carrying capacity. Xie and Hu [2] achieved an ultimate load-carrying capacity increase as high as 93.8% for corrosion levels between 15% and 50% compared to the control pristine beam. The above studies focused on corrosion damage localised near the mid-span of the beams.
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Numerical approaches by [3‐6] have been proposed to model the behaviour of CFRP-strengthened beams. However, these existing models do not consider the effect of the patch repair material, which introduces an additional material law to the complex multi-layer system. Such complexity for concrete is due to the nonlinear load-deformation response of concrete and difficulty in forming suitable constitutive relationships under combined stresses, progressive cracking of concrete under increasing load and the complexity in the formulation of the failure behaviour for various stress states, the consideration of steel and its interaction with concrete and time-dependent effects such as creep and shrinkage of concrete [7].
2 Experimental Study
2.1 Experimental Program
Five types of beams were cast and replicated three times. The five categories of beams cast were control beams, set 1 beams (450 mm damage), set 2 (1800 mm) beams, set 3 (1300 mm) beams and set 4 (800 mm) beams. The set numbering has to do with the order in which the beams were cast. The damaged beams were subjected to simulated corrosion of 5%. Simulated corrosion was completed by means of milling the steel perpendicular to the cross section to the longitudinal tensile reinforcement. All the damaged beams were patch repaired with the same mortar mix and strengthened with the same CFRP plate with dimensions 1700 mm × 50 mm × 1.2 mm in flexure. Identical anchorage was also provided at the CFRP plate ends with FRP wrap. After sufficient curing, all beams were tested under four-point bending. The CFRP plate used was 50 mm wide and 1.2 mm thick. The FRP wrap was 300 mm wide.
2.2 Test Beams
All test beams had identical cross-section dimensions. The reinforcement layout was also identical except for the tensile longitudinal reinforcement milled in the maximum bending regions to simulate the various damage lengths. The beam section and reinforcements for a typical beam are shown in Fig. 1.
Fig. 1.
Reinforcement layout, patch repair and CFRP plate for typical 800 mm patch
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2.3 Material Characterisation
The materials used for this study included concrete, steel, epoxy, CFRP plate and wrap and their mechanical properties are summarised in Table 1.
Table 1.
Mechanical properties of materials
Material
Compressive strength
(MPa)
Tensile strength (MPa)
Modulus of Elasticity (GPa)
CFRP Plate
-
3100
165
CFRP Wrap
-
4900
230
Steel bar (tension and compression)
-
630
200
Steel bar (Stirrups)
-
300
200
Epoxy/FRP plate
70–80
24–27
11.2
Repair mortar
70
5.5
Concrete
50
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3 Finite Element Modelling
The finite element analysis performed in this study consisted of modelling the nonlinear behaviour of the same reinforced concrete beams of the experimental studies, which are patch repaired and strengthened with CFRP bonded to their tension face to investigate the behaviour of such RC beams under four points bending. The commercial finite element package ABAQUS software was used.
3.1 Material Properties and Constitutive Models
Concrete
Concrete was modelled using the concrete damaged plasticity model in Abaqus to capture both inelastic deformation and stiffness degradation that concrete undergoes at low confining pressure. The material properties specified in Table 1 were used for modelling. The elastic parameters necessary for establishing the first part of the model were the secant modulus of elasticity \(E_{cm}\) and mean axial tensile strength, \(f_{ctm}\) and were calculated according to [8]. The post-peak behaviour in tension was represented with tension stiffening to simulate the effects of concrete/steel effects such as bond slip and dowel action. The nonlinear uniaxial compression stress-strain curve was constructed based on the expression proposed by [8]:
where \(\eta =\frac{{\varepsilon }_{c}}{{\varepsilon }_{c1}}\), \(k=1.05\frac{\left|{\varepsilon }_{c1}\right|}{{f}_{cm}}\) and \({\varepsilon }_{c1}=0.7{f}_{cm}^{0.31}\le 2.8\)‰, \({f}_{cm}\) is the mean value of concrete cylinder compressive strength derived from concrete cube strength and \(\sigma_{c}\) is the compressive stress in concrete.
Steel Reinforcement
Tension, compression and shear reinforcements were assumed to behave in an elastic perfectly plastic manner in both compression and tension.
Fibre Reinforced Polymers Material and Adhesive-Concrete/CFRP Interface
CFRP material behaves in a linear elastic manner up to failure. For flexural strengthening, the elastic modulus in fiber direction is of most importance. The properties presented in Table 1 were assigned to CFRP during modelling.The layer of adhesive between concrete and CFRP of 1 mm thick was modeled using cohesive zone model and the values suggested by [9] were assigned.
3.2 Numerical Analysis
In this study, different parts of the complete model were assembled in Abaqus/Standard in 3D modelling space. A typical model for 800 mm patch is shown in Fig. 2.
Fig. 2.
Finite element model for patch-repaired and CFRP strengthened RC beam (800 mm-Patch)
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4 Results and Discussions
As stated earlier, the main objective of this study was to investigate the behavior of reinforced concrete beams patch repaired and strengthened with FRP composites under static loading. Such behavior was studied in terms of crack initiation and propagation, load-deflection relationships, and failure mechanisms. A comparison with experimental results obtained from the same beams is done for validation.
4.1 Cracking Initiation and Evolution
Cracking was initiated whenever the maximum principal stress was greater than the tensile strength of concrete which was 3.5 MPa for concrete and 4.3 MPa for repair material (Figs. 3 and 4).
Fig. 3.
Cracking loads
Fig. 4.
Structural crack pattern: (a) Control beam, (b) 1800 mm patched beam
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4.2 Load Deflection Relationships
Load deflection curves obtained from control beam and four patch repaired and strengthened beams are shown in Fig. 5 in comparison to experimental results. As it can be seen, there is a close agreement between numerical results and experimental findings and it is seen that patch repair and FRP strengthening increases load carrying capacity of damaged reinforced concrete beams.
Fig. 5.
Load-deflection relationships
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4.3 Failure Mechanism
From both numerical and experimental studies, the failure mode was intermediate crack induced debonding followed by concrete crushing as shown in Fig. 6.
Fig. 6.
Intermediate-crack induced debonding.
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5 Conclusions
In this study, a finite element model was developed for analysis of reinforced concrete beams patch repaired and strengthened with FRP composites by varying the length of the patch. Results from finite elements analysis agreed with experimental findings regarding cracking load, structural crack distribution, load deflection relationships and failure mechanism. Repair and strengthening of corrosion-damaged reinforced concrete beams was found to increase the load-carrying capacity. Despite good results obtained from this study, future researches are necessary putting much effort in the energy approaches to study the behavior of patch-repaired and strengthened reinforced concrete beams, particularly the debonding failure.
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