The effects of seawater corrosion and freeze-thaw cycles on the structural behavior of fatigue damaged reinforced concrete (FDRC) beams were experimentally studied. Results show that the residual strength of FDRC beams reduces as the fatigue load level (the ratio of maximum fatigue load to the ultimate static load) increases. The reduction in the loading capacity of FDRC beams in atmosphere environment was about 6.5% and 17.8% for given fatigue load levels of 0.2 and 0.3, respectively. However, if FDRC beams are exposed to the environment of seawater wet-dry cycles or to the environment of alternating actions of freeze-thaw and seawater immersion, as expected during the service life of RC bridge structures in coastal regions or in cold coastal regions, a more rapid reduction in the strength and stiffness of the beams is observed. The significance of an accurate simulation of working load and service condition RC bridge structures in coastal regions and cold coastal regions is highlighted.
The bearing capacity and durability of in-service reinforced concrete (RC) bridges may gradually deteriorate due to traffic loads. In particular, the expected service life of fatigue damaged RC (FDRC) bridges in cold coastal region may be greatly reduced by chloride corrosion and freeze-thaw cycles. For example, severe cracking and steel corrosion [
The fatigue behavior and mechanism of RC structures have been investigated over the last century. Chang and Kesler [
Recent studies indicate that the working environment of RC structures has a significant impact on the fatigue behavior. Erosion environment can greatly reduce the fatigue life of RC structures [
Yuan et al. [
Most of the existing studies focus on the experimental investigation of the fatigue limit of RC structures with a fatigue load level that is usually larger than 0.40. Few studies have been focused on the fatigue durability of RC structures under the practical working condition. In this study, the actual working condition of RC bridges was simulated using the following steps. First, RC beams were fatigue loaded at different load levels with a prescribed number of cycles. Next, RC beams were exposed to the seawater wet-dry environment or to the alternative environment of freeze-thaw cycles. Finally, the RC beams were statically tested and the performance degradation of the beams was studied.
The practical working condition was simulated here to investigate the fatigue durability of RC structures. The comprehensive effects of fatigue damage and environment were considered. The low fatigue load levels (0.0, 0.2, and 0.3) were selected. The maximum number of fatigue loading cycles was determined as 200,000 to represent the early stage of the fatigue life of RC beams. Next, the FDRC beams were exposed to the environment conditions of air, seawater wet-dry cycles, and alternations of freeze-thaw and seawater immersion, respectively. The effects of fatigue load level and working environments on residual strength of RC beams were studied by static loading test.
Two sets of specimens were prepared. The first set consists of a total of 9 plain concrete specimens for material property tests. The second set includes 20 RC beams for structural deterioration tests. Parameters of the concrete mix are shown in Table
Concrete mix/(kg/m3).
Water | Cement | Sand | Coarse aggregates | Fly ash | Superplasticizer | Air entraining agent |
---|---|---|---|---|---|---|
184.4 | 468.8 | 625 | 1156.3 | 37.5 | 3.1 | 0.94 |
Nine plain concrete specimens were made and were equally divided into three groups. The first group was exposed to an environment of seawater wet-dry cycles. The second group was exposed to an alternating environment of freeze-thaw and seawater immersion. The third group was exposed to the air as a reference. The dimensions for all specimens were 100 mm × 100 mm × 100 mm.
All RC beam specimens had the same dimensions of
Reinforcement detailing of beams.
A reference beam specimen was tested with a static loading at the age of 176 days, and the ultimate load was obtained as
Testing parameters of beam specimens.
Series number | Groups number | Specimen number | Fatigue load | Loading cycles | Environment condition | |
---|---|---|---|---|---|---|
Upper | Lower | |||||
A | 1 | A1, A2 | 0 | 0 | 0 | Air |
2 | A3, A4 |
|
|
2 × 106 | Air | |
3 | A5, A6 |
|
|
2 × 106 | Air | |
|
||||||
B | 4 | B1, B2 |
|
|
0 | Wet-dry |
5 | B3, B4 |
|
|
2 × 106 | Wet-dry | |
6 | B5, B6, and B7 |
|
|
2 × 106 | Wet-dry | |
|
||||||
C | 7 | C1, C2 |
|
|
0 | Freeze-thaw |
8 | C3, C4 |
|
|
2 × 106 | Freeze-thaw | |
9 | C5, C6 |
|
|
2 × 106 | Freeze-thaw |
All tests were performed in the Civil Engineering Laboratory and the Fatigue Laboratory at Beihang University in Beijing. All specimens were demolded after 24 hours of casting and were cured under standard temperature and moisture conditions. The beam specimens of 9 groups (three series) at the age of 180 days were cyclically loaded on MTS fatigue testing machine for 200,000 cycles. The detailed loading parameters are shown in Table
Setup of fatigue loading device.
Environment simulation of FDRC beam specimens was carried out after fatigue testing. Beam specimens of A series were placed in the air for 100 days, specimens of B series were exposed to seawater wet-dry cycles, and specimens of C series were exposed to alternative actions of seawater immersed and freeze-thaw cycles. The procedure of seawater wet-dry cycles was designed to immerse the specimens in seawater for 12 h and then expose the specimens to the air for 12 h as one cycle. The procedure was repeated until 100 cycles were finished. The procedures of alternative actions of seawater immersed and freeze-thaw cycles were 300 freeze-thaw cycles and 100 times of seawater immersion, as described by Diao et al. [
All beam specimens except B7 were statically loaded until failure, while the rebar in the beam specimen B7 was tested by monotonous tension. The static loading terminated when one of the three conditions was met: the tensile reinforcement fractured, the ultimate load was decreased by the amount of 15%, and the middle deflection was larger than 1/50 of the beam span.
Table
Compression strength of concrete.
Series | Compressive strength/MPa | Average/MPa |
---|---|---|
Reference group | ||
a-1 | 42.0 | 42.1 |
a-2 | 43.5 | |
a-3 | 40.9 | |
Seawater wet-dry cycle group | ||
b-1 | 41.9 | 41.3 |
b-2 | 41.7 | |
b-3 | 40.2 | |
Seawater freeze-thaw cycle group | ||
c-1 | 38.0 | 38.7 |
c-2 | 38.9 | |
c-3 | 39.2 |
The four rebar specimens were tested by monotonous tension, named reference group in Table
Strength of rebar specimens.
Series | Number of rebar | Yield strength/MPa | Ultimate strength/MPa |
---|---|---|---|
Reference group | 1 | 560 | 705 |
2 | 545 | 735 | |
3 | 552 | 736 | |
4 | 532 | 720 | |
|
|||
Fatigue group | Fatigue 1 | 481 | 710 |
Fatigue 2 | 476 | 720 |
Tensile rebar in beam specimen B7.
Since there was no yielding point in the stress-strain curves of the reference group rebar, the yield strength was taken as the stress corresponding to 0.2% residual strain according to the Chinese code for design of concrete structures (GB 50010-2010). It can be seen from Table
The strain-stress curves shown in Figure
Stress-strain curves of rebar from reference and fatigue group.
The fatigue load level for beam specimens A3, A4, B3, B4, C3, and C4 was 0.2. All these specimens showed a crack at the bottom of middle span within the first 200 cycles of loading. Cracks initiated from the bottom were propagated to the location of the tensile reinforcement (40 mm from the bottom of the beam). As the number of cycles increased, a second crack appeared parallel to the first crack. The crack lengths of the two cracks were almost identical. The maximum fatigue load level for A5, A6, B5, B6, B7, C5, and C6 was 0.3. All these beam specimens developed a crack at the bottom of middle span in the first cycle. Cracks propagated from the bottom up to the location of compression reinforcement (110 mm above the bottom of the beam). With continuous loading parallel cracks appeared near the first crack. The lengths of the new cracks were shorter than the first crack. Figure
Crack condition of beams after fatigue loading.
Number | Fatigue | Number of cracks | Max width of crack/mm | Max length of crack on side/mm |
---|---|---|---|---|
load level | ||||
A1, A2 | 0 | — | — | — |
A3 | 0.2 | 2 | 0.07 | 43 |
A4 | 0.2 | 1 | 0.08 | 41 |
A5 | 0.3 | 3 | 0.11 | 88 |
A6 | 0.3 | 3 | 0.21 | 102 |
B1, B2 | 0 | — | — | — |
B3 | 0.2 | 2 | 0.12 | 42 |
B4 | 0.2 | 1 | 0.09 | 49 |
B5 | 0.3 | 3 | 0.10 | 83 |
B6 | 0.3 | 2 | 0.10 | 92 |
B7 | 0.3 | 3 | 0.12 | 101 |
C1, C2 | 0 | — | — | — |
C3 | 0.2 | 3 | 0.10 | 41 |
C4 | 0.2 | 2 | 0.11 | 36 |
C5 | 0.3 | 4 | 0.12 | 102 |
C6 | 0.3 | 3 | 0.11 | 124 |
Cracks distribution of beam specimen A6.
The relationship of the number of loading cycles and deflection at midspan of beam specimens was shown in Figure
Max deflection of beams at load level 0.2.
The relationship of the number of loading cycles and the maximum tensile strain and residual strain of tensile rebar are shown in Figure
Max and residual strain of rebar at load level 0.2.
The midspan deflection of beams and the strain of tensile rebar are shown in Figures
Max midspan deflection at load level 0.3.
Max and residual strain of rebar at load level 0.3.
Figure
At the fatigue load level of 0.2, the COV of the maximum midspan deflection, the maximum tensile strain, and the residual tensile strain of rebar were 0.043, 0.055, and 0.148, respectively. At the fatigue load level of 0.3, the COV of the maximum midspan deflection, the maximum tensile strain, and the residual tensile strain of rebar were 0.110, 0.117, and 0.181, respectively. This indicates that as the fatigue load level increased, the plastic deformation of the specimen increased and the variation of the test results became larger.
Table
Ratio of residual strain to max strain.
Number | Ratio of residual strain to maximum strain | |||
---|---|---|---|---|
3 cycles | 1000 cycles | 20,000 cycles | 200,000 cycles | |
A3 | 0.128 | 0.210 | 0.270 | 0.406 |
A4 | 0.136 | 0.172 | 0.204 | 0.271 |
A5 | 0.281 | 0.516 | 0.509 | 0.525 |
A6 | 0.348 | 0.675 | 0.612 | 0.682 |
B3 | 0.053 | 0.359 | 0.330 | 0.364 |
B4 | 0.015 | 0.219 | 0.227 | 0.333 |
B5 | 0.333 | 0.563 | 0.553 | 0.651 |
B6 | 0.045 | 0.705 | 0.591 | 0.574 |
B7 | 0.344 | 0.538 | 0.536 | 0.604 |
C3 | 0.043 | 0.328 | 0.347 | 0.345 |
C4 | 0.085 | 0.314 | 0.315 | 0.341 |
C5 | 0.345 | 0.435 | 0.383 | 0.407 |
C6 | 0.327 | 0.420 | 0.417 | 0.509 |
The surface color of beam specimens of B series turned from light gray to pale yellow when increasing the number of wet-dry cycles. Specimens B3, B4, B5, B6, and B7 showed a significant phenomenon of self-healing in seawater wet-dry cycles. It can be seen in Figure
Surface self-healing of B5 in seawater wet-dry process (16 times’ magnification).
B5 before seawater dry-wet cycles (0 cycles)
B5 after 100 dry-wet cycles in seawater
The number of observed fatigue cracks during the procedures of seawater wet-dry cycles is shown in Table
Variation of fatigue cracks with different wet-dry cycles.
Number | Fatigue | Numbers of cracks | Max crack width/mm | ||||||
---|---|---|---|---|---|---|---|---|---|
load level | 0 cycles | 33 cycles | 66 cycles | 100 cycles | 0 cycles | 33 cycles | 66 cycles | 100 cycles | |
B3 | 0.2 | 2 | 0 | 0 | 0 | 0.12 | 0 | 0 | 0 |
B4 | 0.2 | 1 | 1 | 1 | 0 | 0.09 | 0.05 | 0.04 | 0 |
B5 | 0.3 | 3 | 2 | 2 | 2 | 0.10 | 0.08 | 0.05 | 0.05 |
B6 | 0.3 | 2 | 1 | 1 | 0 | 0.10 | 0.07 | 0.06 | 0 |
B7 | 0.3 | 3 | 2 | 2 | 0 | 0.12 | 0.08 | 0.08 | 0 |
The surface color of beam specimens of C series turned from light gray to taupe as the number of freeze-thaw cycles increased. In this series, cracks of beam specimens did not show an obvious self-healing. Some cracks even became wider and clearer after applying the freeze-thaw cycles. It can be seen in Figure
Surface self-healing of C5 in freeze-thaw process (16 times’ magnification).
C5 before freeze-thaw cycles
C5 after 300 freeze-thaw cycles
The variation of fatigue cracks during the alternating freeze-thaw and seawater immersion test is shown in Table
Variation of fatigue cracks with different freeze-thaw cycles.
Number | Fatigue | Number of cracks | Max crack width/mm | ||||||
---|---|---|---|---|---|---|---|---|---|
load level | 0 cycles | 100 cycles | 200 cycles | 300 cycles | 0 cycles | 100 cycles | 200 cycles | 300 cycles | |
C3 |
|
3 | 3 | 3 | 3 | 0.10 | 0.10 | 0.10 | 0.11 |
C4 |
|
2 | 2 | 2 | 2 | 0.11 | 0.10 | 0.10 | 0.11 |
C5 |
|
4 | 4 | 4 | 4 | 0.12 | 0.12 | 0.12 | 0.12 |
C6 |
|
3 | 3 | 3 | 3 | 0.11 | 0.15 | 0.16 | 0.17 |
Comparisons between the beam specimens of series B and series C revealed the following facts. The width of crack of specimens in series B (i.e., seawater wet-dry cycles) decreased gradually, and the width of cracks of specimens in series C (i.e., alternating freeze-thaw and seawater immersion) did not decrease; some even became larger.
Beam specimens without fatigue loading were A1, A2, B1, B2, C1, and C2. In the static loading process, the first vertical cracks appeared in the midspan, and cracking load was 5.42 kN, 5.65 kN, 5.64 kN, 5.79 kN, 4.71 kN, and 4.03 kN, respectively. With the increasing load cracks gradually developed, and the parallel cracks appeared. When cracks propagated to the location of the compression reinforcement, the tensile reinforcement yielded and the concrete compression zone crushed.
Another observation is that in the static loading process the original fatigue cracks of the test specimens further propagated, and new cracks appeared (Figure
Static loading process of beam specimen C5.
C5 installed for testing
C5 yielded
C5 broken
Figures
Testing results of beam specimens of A series.
Testing results of beam specimens of B series.
Testing results of beam specimens of C series.
Tables
Residual capacity results of beams of A series.
Number | Fatigue | Yield load |
|
Ultimate load |
|
Ductility factor | Yield modulus |
---|---|---|---|---|---|---|---|
load level | kN | kN | kN/mm | ||||
A1 | 0 | 51.80 | 2.59 | 64.29 | 5.68 | 3.58 | 20.00 |
A2 | 0 | 54.21 | 2.57 | 65.87 | 5.28 | 2.73 | 21.13 |
A3 | 0.2 | 50.28 | 2.28 | 62.98 | 4.27 | 5.61 | 22.07 |
A4 | 0.2 | 46.40 | 2.24 | 58.69 | 5.28 | 6.24 | 20.69 |
A5 | 0.3 | 46.18 | 2.69 | 54.72 | 5.27 | 5.21 | 17.18 |
A6 | 0.3 | 45.54 | 2.53 | 52.28 | 4.17 | 5.53 | 17.98 |
Residual capacity results of beams of B series.
Number | Fatigue | Yield load |
|
Ultimate load |
|
Ductility factor | Yield modulus |
---|---|---|---|---|---|---|---|
load level | kN | kN | kN/mm | ||||
B1 | 0 | 52.12 | 2.67 | 62.05 | 5.27 | 6.34 | 19.52 |
B2 | 0 | 49.81 | 2.58 | 58.5 | 4.89 | 4.24 | 19.31 |
B3 | 0.2 | 43.39 | 2.42 | 53.44 | 5.43 | 4.87 | 17.95 |
B4 | 0.2 | 45.25 | 2.20 | 58.1 | 5.38 | 5.69 | 20.62 |
B5 | 0.3 | 39.66 | 2.50 | 51.74 | 6.88 | 5.61 | 15.89 |
B6 | 0.3 | 40.75 | 2.49 | 48.71 | 4.54 | 5.61 | 16.34 |
Residual capacity test results of C series.
Number | Fatigue | Yield load |
|
Ultimate load |
|
Ductility factor | Yield modulus |
---|---|---|---|---|---|---|---|
load level | kN | kN | kN/mm | ||||
C1 | 0 | 49.10 | 2.73 | 56.2 | 3.90 | 4.38 | 17.98 |
C2 | 0 | 46.37 | 2.53 | 55.27 | 4.94 | 3.24 | 18.32 |
C3 | 0.2 | 43.82 | 2.78 | 51.47 | 5.57 | 3.11 | 15.78 |
C4 | 0.2 | 45.41 | 2.86 | 53.41 | 4.56 | 5.46 | 15.88 |
C5 | 0.3 | 41.17 | 2.65 | 49.28 | 6.19 | 5.28 | 15.53 |
C6 | 0.3 | 40.10 | 2.68 | 45.97 | 4.80 | 5.22 | 14.96 |
Figures
Load-deflection curves of A series.
Load-deflection curves of B series.
Load-deflection curves of C series.
It can be seen from Table
It can be seen from Table
It can be seen from Table
Based on the results shown in Tables
Figures
Load-deflection curves in different environments.
Load-deflection curves at load level 0.2.
Load-deflection curves at load level 0.3.
Figure
Load-bending stiffness curves of the beams.
Air
Wet-dry cycles
Freeze-thaw cycles
Comparing curves with the same fatigue load level in Figure
Bending stiffness analysis.
Fatigue | Specimen | Initial stiffness | amplitude reduction | Yield stiffness | amplitude reduction | Ultimate stiffness | amplitude reduction |
---|---|---|---|---|---|---|---|
load level | kN⋅m2 | kN⋅m2 | kN⋅m2 | ||||
0 | A1 | 414 | 0 | 190 | 0 | 89 | 0 |
B1 | 343 | 17.1% | 130 | 31.6% | 68 | 23.6% | |
C2 | 318 | 23.2% | 104 | 45.3% | 57 | 36.0% | |
|
|||||||
0.2 | A4 | 341 | 0 | 190 | 0 | 82 | 0 |
B3 | 324 | 5.0% | 131 | 31.1% | 55 | 32.9% | |
C3 | 260 | 23.7% | 95 | 50.0% | 53 | 35.4% | |
|
|||||||
0.3 | A6 | 298 | 0 | 152 | 0 | 79 | 0 |
B6 | 320 | −7.4% | 110 | 27.6% | 53 | 32.9% | |
C5 | 245 | 17.8% | 91 | 40.1% | 48 | 39.2% |
Table
Residual capacity results analysis.
Number | Fatigue | Yield strength |
Reduction |
Yield modulus |
Reduction |
Ultimate strength |
Reduction |
---|---|---|---|---|---|---|---|
load level | kN | kN/mm | kN | ||||
A1, A2 | 0 | 53.01 | 0 | 20.57 | 0 | 65.08 | 0 |
A3, A4 | 0.2 | 48.34 | 8.8% | 21.38 | −3.8% | 60.84 | 6.5% |
A5, A6 | 0.3 | 45.86 | 13.5% | 17.58 | 17.0% | 53.5 | 17.8% |
B1, B2 | 0 | 50.97 | 3.8% | 19.42 | 5.6% | 60.28 | 7.4% |
B3, B4 | 0.2 | 44.32 | 16.4% | 19.29 | 6.6% | 55.77 | 14.3% |
B5, B6 | 0.3 | 40.21 | 24.1% | 16.12 | 27.6% | 50.23 | 22.8% |
C1, C2 | 0 | 47.74 | 9.95% | 18.15 | 11.8% | 55.74 | 14.4% |
C3, C4 | 0.2 | 44.62 | 15.8% | 15.83 | 23.0% | 52.44 | 19.4% |
C5, C6 | 0.3 | 40.64 | 23.3% | 15.25 | 34.9% | 47.63 | 26.8% |
It can be seen from the 3th and 4th columns in Table
It can be seen from the 5th and 6th columns in Table
The data in the 7th and 8th columns of Table
Figure
Yield load and ultimate load with fatigue load level.
Figure
Yield modulus with fatigue load level.
(1) The residual bending capacity of FDRC beams in the air reduces as the fatigue load level increases. The reduction of bearing capacity is mainly due to the reduction of yield strength of the reinforcement bars, and the decrease of bending stiffness is caused by fatigue damage accumulation.
(2) The residual yield load and ultimate load of the beams without fatigue damage can be decreased by the seawater wet-dry cycles and the alternating freeze-thaw and seawater immersion cycles. This is mainly due to the decrease of the concrete compressive strength.
(3) The residual yielding strength and ultimate strength of FDRC beams under the environment of seawater wet-dry cycles reduce as the fatigue load level increases. The residual yielding strength and ultimate strength of FDRC beams under the environment of alternating freeze-thaw and seawater immersion cycles reduce as the fatigue load level increases. Contributions from the three factors to the reduction, from smaller to larger, are air, seawater wet-dry, and alternating freeze-thaw and seawater immersion, respectively.
(4) The fatigue cracks of beams exhibit self-healing in the environment of seawater wet-dry cycles; however, the self-healing is not obvious in the environment of alternating freeze-thaw and seawater immersion cycles.
200,000 cycles of fatigue loading at small fatigue load levels, for example, 0.2 and 0.3 in this study, can introduce serious deteriorations of mechanical behavior of RC beams due to the synthetic actions of fatigue damage and erosion environment and can jeopardize the structural integrity. The quantitative modeling of combined effect to FDRC structures will be investigated in the future.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was financially supported by the National Science Foundation of China (Grants nos. 51108015 and 51178020) and the Open Project of State Key Laboratory of Subtropical Building Science, South China University of Technology (no. 2012KA03).