The chapter delves into the critical issue of reinforced concrete (RC) infrastructure deterioration due to corrosion and higher operating loads. It presents an extensive experimental study on the fatigue behaviour of patch-repaired and CFRP-strengthened RC beams, subjected to cyclic loading. The research investigates the impact of corrosion damage and repair extent on the fatigue life, crack development, failure modes, and stiffness degradation of the beams. Notably, the study compares the performance of beams with different damage lengths and repair extents under both low and high cyclic stress ranges. The findings reveal significant improvements in fatigue life and overall structural performance with lower stress ranges and increased repair extents. The chapter also highlights the potential of Digital Image Correlation (DIC) in identifying early failure locations, contributing to the advancement of structural health monitoring techniques.
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Abstract
The service life of corrosion-damaged reinforced concrete (RC) infrastructure can be improved through patch repairs and structural strengthening. However, the effect of varying corrosion damage and patch repair extent has not yet been clearly pronounced. The fatigue performance of corrosion-damaged RC beams that have been patch repaired to varying lengths and subsequently strengthened with carbon fibre-reinforced polymer (CFRP) laminates was evaluated by conducting four-point bending tests and cyclic load tests on simply supported beams. Three criteria were identified to evaluate performance: fatigue life, crack development and stiffness degradation. Various data acquisition techniques, such as neutral axis DEMEC strain targets, strain gauges, linear variable differential transducers (LVDT) and digital image correlation (DIC) were employed to investigate these performance criteria. The experimental results indicated that an increase in corrosion damage and patch repair extent lowered the ultimate static failure load and increased fatigue life. An increase in specimen stiffness was observed for the specimens with the longer damage extent compared to the specimens with the shorter damage extent, where stiffness was gauged in terms of midspan deflection, composite material strain and neutral axis shift. Moreover, the results yielded through the DIC process showed potential to identify potential failure locations, quite early in the specimen fatigue life by comparison of tangential strain, peak vertical deflection and the eventual failure location.
1 Introduction
Reinforced concrete (RC) infrastructure deterioration is a growing global concern, whether premature failure is caused by higher operating loads that influence structural capacity or due to substandard durability design. For example, in the South African heavy-haul railway industry, there has been a drive to use longer trains which would invariably lead to higher fatigue loads [1]. In addition, reinforced concrete structures in chloride and carbon-dioxide-rich environments are susceptible to reinforcement corrosion.
A common approach to repair corrosion-damaged RC structures involves the removal of damaged concrete and corrosion reaction products, followed by applying a cementitious repair mortar. Where additional capacity is required, structural strengthening may be considered. Patch repairs restore the durability of concrete elements, and their success relies on compatibility with the concrete substrate. Fibre-reinforced polymer (FRP) strengthening has become a favourable structural strengthening method, given its high strength-to-weight ratio and ease of application [2]. Moreover, FRP has been proven effective in not only restoring the structural performance of corrosion-damaged structures in terms of immediate ultimate limit state (ULS) capacity as well as long-term serviceability limit state (SLS) fatigue performance [3].
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While the fatigue behaviour of FRP-strengthened RC structures has been extensively reported in the literature [4‐6], there is a dearth of information on the fatigue behaviour of corrosion-damaged, patch-repaired and FRP-strengthened RC elements with varying degrees of damage [5]. Considered the effect of both externally bonded reinforcement (EBR) and near-surface mounted (NSM) fibre-reinforced strengthening on quasi-full-scale RC beams under fatigue loading. The study found that although EBR specimens performed better than NSM-reinforced specimens under cyclic loading, both FRP strengthening methods improved the fatigue performance of RC beams [6]. Considered the long-term behaviour of FRP-strengthened RC beams that have been corrosion damaged and found that an increase reduced fatigue life in the degree of corrosion. However, neither of the studies above considered the effect of the patch repair component [7]. Considered this combined effect for the same damage extent [8, 9]. Considered the effect of varying damage and patch repair extent but only reviewed its performance under monotonic loading [10]. Studied the behaviour of patch-repaired beams with varying damage lengths but under impact loading. This research was focused on the fatigue behaviour corrosion damaged RC beams that have been patch repaired to varying lengths and subsequently CFRP strengthened under two different cyclic load stress ranges.
2 Specimen Details and Material Properties
The experimental programme comprised fifteen (15) quasi-full-scale 155×254×2000 mm RC beams with tensile reinforcement that either remained uncorroded or was subjected to accelerated corrosion and subsequently patch repaired. The dimensions of the beam were selected to conform to a series of full-scale tests previously conducted with the same size [9‐11] as shown in Fig. 1.
Fig. 1.
Beam specimen reinforcement layout
×
The concrete used in this experiment was designed yield a compressive strength of 40 MPa. The mechanical properties of the concrete are presented in Table 1.
Table 1.
Concrete 40 MPa mix material properties
28 days
Standard Deviation
Compressive Strength (MPa)
43.5
±1.1
Tensile Strength (MPa)
2.7
±0.2
Modulus of Elasticity (GPa)
39.6
±1.2
Reinforcement corrosion was induced electrochemically in a controlled laboratory environment. The electrochemical cell comprised of the tensile steel reinforcement, which served as anode and a 12 mm stainless-steel rod, submerged in the sodium chloride solution, which served as the cathode. The corrosion extent was varied to obtain damage lengths of 450 mm, 800 mm, 1300 mm and 1800 mm; however, over each damage extent, a uniform 10% degree of corrosion was maintained, which equates to 5% corrosion per tensile reinforcing bar. A corrosion pond containing 5% sodium chloride solution was assembled on the beam tensile face, as shown in Fig. 2(b). The beams were connected in series to a DC power supply to achieve uniform corrosion damage. The time required to induce 10% corrosion in the RC beams was calculated using Faraday’s law. For a constant corrosion current of 1 ampere (A), the desired degree of corrosion can be obtained in approximately 30.8 days.
Fig. 2.
Sustained load beams setup to induce a partial cracking moment
×
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All 12 beams were corroded under sustained loading through an inverse 4-point bending system where the loads applied at the beam ends, inducing a bending moment equivalent to 60% of the beam cracking moment. Pinned supports were created at the loading points to allow the beams to deflect during corrosion.
The patch repair process involved the removal of damaged cover concrete in varying lengths of either 450 mm, 800 mm, 1300 mm or 1800 mm to a depth of at least 20 mm below the corroded reinforcement and 50 mm beyond the damage extent. This research used a locally sourced cementitious grout as the patch repair mortar. The mechanical properties of the patch repair mortar are presented in Table 2.
Table 2.
Cementitious mortar material properties
28 days
Standard Deviation
Compressive Strength (MPa)
79.2
±1.2
Tensile Strength (MPa)
2.8
±0.4
Modulus of Elasticity (GPa)
35.6
±2.6
Tensile bond strength (MPa)
2.5
±0.5
The FRP strengthening for the RC beam was designed for flexure following recommendations in [2]. The RC beams were designed to resist a ULS capacity of 62.3 kNm; this capacity was reduced 10% by accelerated corrosion. The difference in performance capacity was used to calculate the FRP strengthening required. The induced capacity reduction of 6.22 kNm required a 9.71mm2 of FRP reinforcing. Two CFRP laminates, each with a cross-sectional area of 60mm2 were used. Locally sourced epoxy adhesive Sikadur 30 was used to bond CFRP strips and the patch repair mortar or concrete surface once the substrate had reached a minimum tensile strength of 1.5 MPa. The mechanical properties of the various FRP and epoxy materials used in this experiment are shown in Table 3.
Table 3.
Sika CFRP structural strengthening material properties
FRP Laminate
FRP Wrap
Epoxy resin for repair mortar
Epoxy resin for FRP laminate
Epoxy resin for FRP wrap
Compressive Strength (MPa)
-
-
60–70
70–80
-
Tensile Strength (MPa)
3100
4900
18–20
24–27
30
Modulus of Elasticity (GPa)
165
230
-
11.2
3.8
Thickness (mm)
1.2
0.127
-
2
-
Each test specimen in this study was assigned a label to identify and track it during experimental testing. These labels are listed under the identity column in Table 4.
Table 4.
Test specimen notation and details
Identity
Corrosion level (%)
Patch Repair Length
FRP strengthening
Test Regime
S_CNTRL 1
0
No patch repair
Strengthened
Monotonic testing
S_CNTRL 2
0
No patch repair
Strengthened
Fatigue 40% stress range
S_CNTRL 3
0
No patch repair
Strengthened
Fatigue 60% stress range
S_450 mm 1
10
450mm
Strengthened
Monotonic testing
S_450 mm 2
10
450mm
Strengthened
Fatigue 40% stress range
S_450 mm 3
10
450mm
Strengthened
Fatigue 60% stress range
S_800 mm 1
10
800mm
Strengthened
Monotonic testing
S_800 mm 2
10
800mm
Strengthened
Fatigue 40% stress range
S_800 mm 3
10
800mm
Strengthened
Fatigue 60% stress range
S_1300 mm 1
10
1300mm
Strengthened
Monotonic testing
S_1300 mm 2
10% corrosion
1300mm
Strengthened
Fatigue 40% stress range
S_1300 mm 3
10% corrosion
1300mm
Strengthened
Fatigue 60% stress range
S_1800 mm 1
10% corrosion
1800mm
Strengthened
Monotonic testing
S_1800 mm 2
10% corrosion
1800mm
Strengthened
Fatigue 40% stress range
S_1800 mm 3
10% corrosion
1800mm
Strengthened
Fatigue 60% stress range
3 Instrumentation
Each test specimen was instrumented with four strain gauges. One strain gauge was placed on each of the following surfaces: the compression surface, tension steel surface, tension concrete (or patch repair) surface and the CFRP laminate surface. DEMEC strain targets were placed on one side of each specimen to track the neutral axis migration. An extensometer was used to measure the relative movement of the DEMEC targets.
Crack behaviour was monitored using two different techniques. The first method entailed visual monitoring, where crack patterns were tracked using a permanent marker at pre-determined load intervals. The second method to used monitor crack development was DIC. This method involved spraying and painting a stochastic matt black pattern on the test surface. A high-resolution 5-megapixel (MP) monochrome Basler digital camera captured the test surface from 2m away from the calibrated test specimen. The images were then post-processed using Dantec software specimen.
4 Test Setup and Procedure
Monotonic and fatigue tests were conducted under a four-point bending simply supported configuration. One specimen from each patch repair extent was tested under monotonic loading to establish and verify ULS failure loads. The remaining two specimens of each damage extent were subjected to cyclic loading under different stress ranges, where a minimum load of 6kN was chosen to avoid impact loads and the maximum loads of either 40% or 60% of the beam ULS capacity.
Two-point loads 450mm apart were applied on the compression surface using an Instron actuator through a spreader beam. The sinusoidal loads were applied at a frequency of 4Hz which is relatively high, but an acceptable test frequency for fatigue testing of RC concrete [2, 5, 7, 12–14].
5 Experimental Results
5.1 Accelerated Corrosion
Subsequent to fatigue testing tension steel was retrieved to examine the extent of corrosion damage in terms of mass loss, type of corrosion as well as the locality of the corrosion damage. Table 5 presents findings from the post-fatigue assessment of corrosion damaged tension steel.
Table 5.
Accelerated corrosion results
Identity
Corrosion Type
Measured Mass Loss (g/m)
Equivalent Uniform Depth (mm/bar)
Percentage Mass Loss (%)
Standard Deviation (%)
S_450 mm 1
Pitting
189.96
0.31
9.87
±0.30
S_800 mm 1
Pitting
136.85
0.22
7.11
±1.46
S_1300 mm 1
Pitting
136.97
0.22
7.11
±0.95
S_1800 mm 1
Pitting
109.33
0.18
5,68
±0.26
5.2 Monotonic Behaviour
The static test results indicated that the specimens with the shortest damage length outperformed those with longer damage lengths. The 450 mm specimen had an 18.6% higher failure load than the 0 mm (control) specimen, whereas 1800 mm specimen only had a 5.8% higher failure load than the control specimen. The results further show that the ULS capacity of the 450mm damage length specimen was 8.3%, 10.9% and 12.1% higher than the 800 mm, 1300 mm and 1800 mm specimens, respectively.
5.3 Fatigue Life
Table 6 presents the relative fatigue life cycles of specimens tested under 40% and 60% stress ranges.
Table 6.
Monotonic and fatigue loading test results
Identity
Load Range (kN)
Total No. Cycles
Ultimate Static Load (kN)
Fatigue Failure Mode
Static Failure Mode
Static Load after Fatigue Testing (kN)
S_CNTRL 1
Static
-
274
-
CC & FD
-
S_CNTRL 2
6–109.6
1000000
-
No Failure
276
S_CNTRL 3
6–164.4
256000
-
SR, FD & CC
-
-
S_450 mm 1
Static
-
325
-
CC & FD
-
S_450 mm 2
6–130
788303
-
SR (2 bars), FD & CC
-
S_450 mm 3
6–195
119716
-
SR (1 bar), FD & CC
-
S_800 mm 1
Static
-
300
-
CC & FD
-
S_800 mm 2
6–120
1150000
-
No Failure
300
S_800 mm 3
6–180
102750
-
SR, FD & CC
-
S_1300 mm 1
Static
-
293
-
CC & FD
-
S_1300 mm 2
6–117.2
1083935
-
SR, FD & CC
-
S_1300 mm 3
6–175.8
201450
-
SR, FD & CC
-
S_1800 mm 1
Static
-
290
-
CC
-
S_1800 mm 2
6–116
2000000
-
No Fatigue, SR
216
S_1800 mm 3
6–174
247000
-
SR, FD & CC
-
*IC = inconclusive results, CC = concrete crushing, FD = carbon fibre debonding, SR = steel rupture
Figure 7 shows the predicted fatigue life cycles based on the Helgason and Hanson model.
Overall, the experimental results suggest that as the stress range is reduced by 20% from medium cycle fatigue stress (60%) to low cycles fatigue stress (40%), the fatigue life can be increased by 5 to 8 times. Under both stress range conditions, as the damage extent was increased from 450mm to 1800mm, the fatigue life was extended by as much as 76.7%.
5.4 Crack Development and Failure Mode
All specimens tested under fatigue loading exhibited a similar crack propagation and failure mode, as summarized in Table 7. A few unique stages can characterize this process:
1.
A crack pattern with predominantly flexural cracks and shear and flexural-shear cracks was clearly defined during the first load cycle. This crack pattern remained relatively unchanged throughout the test.
2.
Crack propagation remained low until the rupture of the tension steel. The rupture of tension steel caused a rapid increase in crack propagation in terms of crack heights and densities.
3.
After the rupture of steel, FRP laminates started to delaminate at the position of the main cracks. Shortly after FRP debonding, compression concrete would crush, leading to the ultimate failure of the section.
Table 7.
Crack behaviour and failure mode results
Identity
Average Crack Spacing (mm)
Average Crack Height (mm)
Predominant Crack Type
Fatigue Failure Mode
Failure Position
S_CNTRL 2
58.06
63.81
flexural
No Fatigue Failure
Centre
S_CNTRL 3
85.71
74.20
flexural
SR, FD & CC
Right Pin Load
S_450 mm 2
69.23
59.14
flexural
SR,FD & CC
Centre
S_450 mm 3
75.00
65.70
flexural
SR,FD & CC
Left Pin Load
S_800 mm 2
48.65
54.22
flexural
No Fatigue Failure
Centre
S_800 mm 3
75.00
59.81
flexural
SR (1 bar), FD & CC
Right Pin Load
S_1300 mm 2
46.15
63.66
flexural
SR (2 bars), FD & CC
Centre
S_1300 mm 3
69.23
52.61
flexural
SR (2 bars), FD & CC
Centre
S_1800 mm 2
45.00
75.08
flexural
No Fatigue, SR (1 bar)
Centre
S_1800 mm 3
66.67
47.72
flexural
SR (2 bars), FD & CC
Right Pin Load
*IC = inconclusive results, CC = concrete crushing, FD = carbon fibre debonding, SR = steel rupture
Figure 3 shows the crack pattern sketched from an actual specimen at ultimate failure as well as a DIC crack pattern of the specimen at maximum load during the first load cycle. The comparison of these two different images was done intentionally for two reasons. The first reason being that it was found overall that the crack patterns did not change significantly after the first load cycle. The second reason was to evaluate the possibility of using DIC to identify possible failure locations early in the structural service life. The DIC crack patterns were obtained by plotting tangential strain in the x-direction, where positive strain concentrations indicate areas where cracks were likely to form, and conversely negative strain concentrations indicate areas where concrete crushing was likely.
Fig. 3.
450 mm specimen: Final actual crack pattern versus DIC crack pattern at 1 load cycle (60% ULS load)
×
The DIC crack pattern correlates with the actual specimen's crack pattern, as shown in Fig. 8 above. It does not show the shear cracks quite as accurately as the flexural cracks. This may be due to the fact that the correlation algorithm considered only relative movement in the x-direction when it computed tangential x-strain.
The average crack spacing of the specimens tested under the 40% and 60% stress ranges followed similar patterns. For the 40% stress range specimens, as the damage extent increased from 450 mm to 1800 mm there was a 53.9% reduction in the average crack spacing. In comparison to the 60% stress range test specimens the 40% stress range test specimens had a 47,6% lower average crack spacing as well as a lower average crack spacing reduction as the damage extent was increased from 0 mm to 1800 mm.
If one considers the overall performance of the 40% stress range specimen, the reduction of average crack spacing indicates an increase in the total number of cracks as the damage extent was increased. The increase in average crack height with the damage extent indicates that there may be a stiffness reduction as the damage extent is increased.
5.5 LVDT vs DIC Deflections
A summary of the maximum deflection results obtained from LVDT measurements and theoretical design calculations is presented in Table 8. The results are presented for midspan span deflection measurements after 100 000th load cycle. There is a good correlation between theoretical and measured deflections.
Table 8.
Summary of midspan deflection results from LVDT measurements and theoretical calculations
Identity
Maximum Test Load Pmax (kN)
Calculated Deflection (mm)
LVDT Measurement
100 000th load cycle (mm)
S_CNTRL 2
109.6
2.57
2.77
S_CNTRL 3
164.4
3.86
2.46
S_450 mm 2
130
6.56
2.74
S_450 mm 3
195
9.84
2.26
S_800 mm 2
120
6.06
2.58
S_800 mm 3
180
9.09
7.15
S_1300 mm 2
117.2
5.92
3.11
S_1300 mm 3
175.8
8.87
2.42
S_1800 mm 2
116
5.85
3.44
S_1800 mm 3
174
8.78
8.09
6 Concluding Remarks
This paper presented an experimental study of the fatigue behaviour of corrosion-damaged RC beams that were patch-repaired and CFRP-strengthened. Fatigue performance was assessed in terms of fatigue life, crack development, failure mode and stiffness degradation. The experimental results indicated that under low cyclic stress, the fatigue life of rehabilitated beams was up to eight times higher than the high-stress range. Under both stress range conditions, as the damage extent was increased from 450 mm to 1800 mm, the fatigue life increased by as much as 76.7%. As the damage extent was increased from 450 mm to 1800 mm, average crack spacing and average crack height were reduced, culminating in an overall stiffer section. Moreover, crack densities were found to increase under the lower stress range as those specimens experienced a longer fatigue life. Crack densities tended to increase at the location of steel rupture. The location of steel rupture often coincided with the points of maximum.
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