This paper investigated bond-slip characteristics of chloride-induced corroded reinforced concrete incorporating different levels of recycled concrete aggregates (RCA). Pullout tests were adopted to evaluate the bonding and debonding behaviors of the embedded rebar experiencing different corrosion levels. Both high- and low-strength concrete were considered. Bond-slip curves were recorded to determine the influences of rebar corrosion levels and RCA replacements on the bond strength and debonding energy of the specimens. Test results indicate that increasing rebar corrosion level gradually weakens the antisliding ability of reinforced recycled aggregate concrete (RAC) except for a small level corrosion and the degradation rate of ultimate bond strength increases with a decrease of compressive strength at 0.5% rebar corrosion. The results also demonstrate that the ultimate bond strength of reinforced RAC slightly decreases with an increase of RCA replacement. However, the relative bond strength between uncorroded rebar and RAC is little affected by RCA content, while it decreases with an increase of RCA replacement in high-strength specimens after rebar corrosion. The debonding energy between deformed rebar and RAC is found decreasing with the increment of the rebar corrosion level and increasing with an increase of RAC content.
Modern constructions require the conservation of natural resources and the preservation of environment due to the gradual depletion of natural resources and the disposal crisis of growing wastes, for example, demolition and construction wastes. One of most common ways to mitigate these problems is to incorporate demolished wastes like recycled aggregates in new concrete for construction uses. So far, recycled aggregates have been primarily limited to low-value applications such as road filler materials or backfill for retaining wall in China [
Incorporation of recycled aggregates in structural members has to guarantee a tight bond between the rebar and the surrounding concrete so that external loadings are carried effectively. Bond behavior between RAC and the rebars has raised wide attention in the structural engineering world [
This paper aims to test the bond behavior of reinforced RCA-containing concrete under different levels of chloride-incurred corrosion. Both low- and high-strength concrete samples were prepared with different w/c ratios. Bond-slip curves were recorded to determine the influences of RCA and corrosion levels on bond behavior.
Demolished old concrete pavement slabs in the urban area of Nanning, China, were jaw broken and then sieved for 5–31 mm coarse recycled aggregates, while natural aggregates (crushed limestone) were bought from a local commercial aggregate plant. Both aggregates were compliant with the Chinese standard GB50152-92. Basic physical properties of these aggregates were tabulated in Table
Physical properties of coarse aggregates.
Aggregate type | Gradation | Observed density (kg/m3) | Bulk density (kg/m3) | Absorptivity (%) | Crushed index (%) |
---|---|---|---|---|---|
Recycled | 5–31 mm | 2430 | 1260 | 5.96 | 19.5 |
Natural | 5–31 mm | 2760 | 1429 | 1.35 | 13 |
Two series of specimens were prepared. One was targeted as high-strength concrete with a designed water-to-cement (w/c) ratio of 0.28 (HRC), in which some amounts of the fly ash and ground granulated blast-furnace slag were added as cementitious binder. For comparison, the other was objected as low-strength concrete specimens with a designed w/c ratio of 0.50, in order to evaluate the specimen strength on the bond behavior of corroded reinforced concrete containing RCA. For both, 0%, 50%, and 100% of natural aggregate were replaced by the RCA, referred to hereafter as HRC1, HRC2, and HRC3 for high-strength concrete specimens, respectively, and as RC1, RC2, and RC3 for low-strength concrete samples, respectively. It should be noted that the mixture water contained 5% weight of NaCl in order to disseminate uniform chloride seeds in the concrete specimens. All ingredients were mixed in a concrete mixer for about 5 min and were then poured into 150 mm3 cubes wooden moulds (Figure
Geometry of the fabricated specimens; all units in mm.
The embedded rebar typed HRB335 was the quenched steel bar with a yield stress of 420 MPa and a diameter of 20 mm. Before being placed into the moulds, the reinforcing bars were cleaned with a 12% hydrochloric acid solution, cleaned by pure water, neutralized by calcium hydroxide solution, and finally cleaned by pure water again. Rebars were then dried for 4 hrs in a dryer cabin and were weighed with a resolution of 0.1 g. The embedment length of the rebar was adopted as 5
In total, six groups of concrete were casted with detailed mixtures tabulated in Table
Mixtures of specimen.
Number | w/c | Ingredients (kg/m3) | Slump (cm) | ||||||
---|---|---|---|---|---|---|---|---|---|
RA | NA | C | FA | KF | S | W | |||
HRC1 | 0.28 | 0 | 1050 | 420 | 90 | 90 | 670 | 168 | 21.8 |
HRC2 | 0.28 | 525 | 525 | 420 | 90 | 90 | 670 | 168 | 16.5 |
HRC3 | 0.28 | 1050 | 0 | 420 | 90 | 90 | 670 | 168 | 9.3 |
RC1 | 0.5 | 0 | 1150 | 420 | 0 | 0 | 750 | 210 | 12.0 |
RC2 | 0.5 | 575 | 575 | 420 | 0 | 0 | 750 | 210 | 4.1 |
RC3 | 0.5 | 1150 | 0 | 420 | 0 | 0 | 750 | 210 | 2.8 |
Specimens prepared for corrosion tests were further immerged in 5% NaCl solution for 3 days and then were corroded to the targeted corrosion level using electrochemical method. In the artificial corroded setup, parallel circuits were adopted during the corrosion process (Figure
Parallel circuits used to accelerate the corrosion of the rebars.
The designed corrosion amount in terms of the mass loss of corroded rebar was estimated by [
The actual corrosion of a rebar may deviate somewhat from the designed one. Upon completion of pullout tests, the corroded rebar was dismantled from the specimen. Corrosion products were scrapped from the rebar and weighted to evaluate the actual corrosion level, which was expressed as
Bond-slip behavior of the rebar was tested by use of pullout tests. Loading frames were imposed by a RMT-201 testing machine as shown in Figure
Pullout test system.
The bond stress was assumed to be uniform along the anchorage length, and the average bond stress at each loading level was calculated as follows:
Although corrosion of some specimens was occasionally less than the designed one due to the heterogeneousness of concrete materials, measured corrosion was in most cases greater than designed corrosion levels (Table
Corrosion of different specimens.
Specimen number | Current (A) | Time (h) | Crack (mm) | Measured corrosion (%) | Designed corrosion (%) |
---|---|---|---|---|---|
HRC1-0 | — | — | 0 | 0 | 0 |
HRC1-1 | 0.03 | 35.0 | 0 | 0.57 | 0.50 |
HRC1-2 | 0.02 | 123.0 | 0.2 | 1.96 | 1.50 |
HRC1-3 | 0.02 | 256.6 | 0.65 | 2.90 | 2.50 |
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HRC2-0 | — | — | 0 | 0 | 0 |
HRC2-1 | 0.04 | 26.0 | 0.1 | 0.75 | 0.50 |
HRC2-2 | 0.06 | 52.4 | 0.2 | 1.71 | 1.50 |
HRC2-3 | 0.02 | 256.6 | 0.6 | 2.54 | 2.50 |
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HRC3-0 | — | — | 0 | 0 | 0 |
HRC3-1 | 0.04 | 26.0 | 0 | 0.67 | 0.50 |
HRC3-2 | 0.05 | 62.9 | 0.2 | 1.37 | 1.50 |
HRC3-3 | 0.05 | 102.6 | 0.7 | 2.7 | 2.50 |
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RC1-0 | — | — | 0 | 0 | 0 |
RC1-1 | 0.09 | 12.0 | 0.25 | 0.44 | 0.50 |
RC1-2 | 0.10 | 24.4 | 0.60 | 1.42 | 1.50 |
RC1-3 | 0.13 | 38.0 | 1.35 | 2.63 | 2.50 |
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RC2-0 | — | — | 0 | 0 | 0 |
RC2-2 | 0.12 | 20.3 | 0.50 | 1.70 | 1.50 |
RC2-3 | 0.12 | 41.0 | 1.30 | 2.54 | 2.50 |
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RC3-0 | — | — | 0 | 0 | 0 |
RC3-1 | 0.10 | 11.0 | 0.15 | 0.73 | 0.50 |
RC3-2 | 0.12 | 20.3 | 0.60 | 2.10 | 1.50 |
RC3-3 | 0.17 | 29.0 | 1.40 | 2.90 | 2.50 |
Note: the specimen labeled RC2-1 was broken.
The measured bond-slip curves of different corrosion levels and different RCA containments are shown in Figure
Bond-slip parameters of specimens under the pullout tests.
Number |
|
|
|
|
|
Crack style |
---|---|---|---|---|---|---|
HRC1-0 | 61.8 | 16.50 | 10.50 | 0.0979 | 0.64 | Splitting |
HRC1-1 | 19.63 | — | — | — | Yield | |
HRC1-2 | 12.96 | 7.94 | 0.0944 | 0.61 | Splitting | |
HRC1-3 | 5.31 | 3.16 | 0.0696 | 0.59 | Splitting | |
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HRC2-0 | 64.9 | 16.4 | 11.41 | 0.0575 | 0.70 | Splitting |
HRC2-1 | 13.64 | 9.83 | 0.0913 | 0.72 | ||
HRC2-2 | 11.04 | 6.31 | 0.1448 | 0.57 | ||
HRC2-3 | 6.57 | 2.37 | 0.1345 | 0.36 | ||
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HRC3-0 | 52.6 | 14.30 | 9.49 | 0.0429 | 0.66 | Splitting |
HRC3-1 | 12.33 | 10.46 | 0.1525 | 0.85 | ||
HRC3-2 | 9.87 | 6.01 | 0.1575 | 0.61 | ||
HRC3-3 | 7.63 | 3.53 | 0.1600 | 0.46 | ||
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RC1-0 | 39.7 | 13.01 | 9.75 | 0.3451 | 0.75 | Splitting |
RC1-1 | 7.98 | 5.59 | 0.1540 | 0.7 | ||
RC1-2 | 6.19 | 4.39 | 0.2397 | 0.71 | ||
RC1-3 | 4.80 | 2.83 | 0.0535 | 0.59 | ||
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RC2-0 | 40.6 | 12.31 | 7.93 | 0.0743 | 0.64 | Splitting |
RC2-2 | 5.31 | 2.13 | 0.0406 | 0.4 | ||
RC2-3 | 4.79 | 2.35 | 0.0144 | 0.49 | ||
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RC3-0 | 32.0 | 11.61 | 7.98 | 0.0677 | 0.69 | Splitting |
RC3-1 | 5.75 | 3.90 | 0.2528 | 0.68 | ||
RC3-2 | 5.02 | 2.95 | 0.2272 | 0.59 | ||
RC3-3 | 4.49 | 2.14 | 0.1107 | 0.47 |
Bond-slip curves of different specimens.
HRC specimens with 0% RCA
HRC specimens with 50% RCA
HRC specimens with 100% RCA
RC specimens with 0% RCA
RC specimens with 50% RCA
RC specimens with 100% RCA
The relationships of ultimate bond strength versus corrosion level are drawn in Figure
Except for HRC1-1 specimen, similar to reinforced normal aggregate concrete, it can be observed that the ultimate bond strength of RAC decreases with an increase of corrosion level (Figure
The ultimate bond strength decreased with an increase of RCA containment before and after rebar corrosion (Figure
Bond strength versus RCA replacement.
Ultimate bond strength
Relative bond strength
Because the compressive strength of concrete with different RCA ratio is not uniform for a specific w/c ratio, it seems reasonable to compare the relative bond strength of RAC to analyze the influence of RCA replacement on ultimate bond strength. The relative bond strength is defined as follows [
The relationship between
Although the bond behavior between uncorroded rebar and RAC is wildly researched in previous studies, the debonding energy is seldom considered as a surrogate of the bond performance of reinforced RAC [
Due to the increase of the corrosion level that discounted the ultimate bond strength, the
Though incorporating RCA in reinforced concrete sacrificed the ultimate bond strength somewhat, energies required to debond rebars fully from RCA-containing concrete increased with RCA replacement, as indicated on the
Specimens | Rebar type | 0% RCA | 50% RCA | 100% RCA |
---|---|---|---|---|
HRC in this study | Deformed | 8.7 | 9.8 | 11.0 |
RC in this study | Deformed | 12.0 | 4.8 | 7.5 |
Xiao and Falkner, 2007 [ |
Deformed | 36.9 | 71.7 | 76.0 |
Plain | 33.9 | 30.3 | 29.9 |
Note: integral of the debonding energy from Xiao and Falkner work was evaluated from the reported load-slip spectrums; some minor errors may happen during the data reproducing process.
The interpretation mentioned above seems plausible because Xiao and Falkner work demonstrated that the debonding energy decreased with the increase of RCA contents when the plain rebar was embedded (Row 5 in Table
The experimental conditions herein may be somewhat different from those conditions in other studies. The mix proportion and raw materials may be different and will affect the results. As our study represents typical experimental parameters for the correction of reinforcing concrete with different level of RCA, the authors believe any variations of the concrete mixture, pullout tests procedure, and others will challenge the reproduction of the experiment and will not cause substantially different or reverse conclusions. High-strength RAC tends to crack in a smaller scale due to its relatively higher tensile strength. Corrosion products at the low-strength RAC may ooze preferentially through those big-scale pores and cause larger surface cracking width at the concrete surface. A small level corrosion in the reinforced RAC first increases the ultimate bond strength of the rebar, and then, with the increment of rebar corrosion, the ultimate bond strength decreases, while the degradation rate increases with a decrease of compressive strength at a 0.5% rebar corrosion but decreases until corrosion level increases towards 1.5% and 2.5%. Additionally, increasing corrosion rate gradually weakens the antisliding ability of reinforced RAC. The ultimate bond strength of reinforced RAC slightly decreases with an increase of RCA replacement. However, the relative bond strength between uncorroded rebar and RAC is little affected by RCA containment, while it decreases with an increase of RCA replacement in high-strength specimens after rebar corrosion. The debonding energy is found decreasing with an increase of the rebar corrosion level and increasing with an increase of RCA content; however, this type of increase vanishes when a RCA-containing concrete embedded with plain rebars.
Designed and measured corrosion level of rebar, respectively
Molar mass of rebar, 56 g/mol
Average current
Avogadro constant, 6.02 × 1023 mol−1
Electron charge, 1.6 × 10−19 C
Weight of rebar before and after corrosion, respectively
Bond stress and applied load
Debonding energy
Diameter and embedded length of uncorroded rebar
Peak bond stress and corresponding slip
Cubic compressive strength of concrete
Bond stress corresponding to first slip occurring
Relative bond strength (
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to appreciate the funding from Natural Science Foundation of China (no. 51308135) and the Program of Guangxi Natural Science Foundation (no. 2014GXNSFBA118242).