The importance of an accurate simulation of service conditions in the bond performance of reinforced concrete structures in coastal regions is highlighted. Four widths of initial crack of 0, 80, 150, and 210 microns were artificially made by inserting slice into bond specimens during concrete casting. Three bar diameters of 10 mm, 14 mm, and 18 mm were selected. At 28 days, the bond specimens were exposed to the environment of wet-dry cycles of seawater and atmosphere for another 90 days. The pull-out test was then conducted and chloride contents were tested at crack area along 40 mm depth. Results show that, for the specimen with 10 mm bar diameter, cracks width of less than 80 microns vanished rapidly during wet-dry cycles; for other specimens, cracks width of 100–150 microns decreased slightly. However the cracks of width more than 200 microns increased gradually; the chloride content decreased along the depth of concrete, and the chloride content increased as the widths of initial cracks increased or as the bar diameters increased. The ductility of bond specimens decreased as the diameter increased.
Chloride attack is one of the most severe durability problems for the marine reinforced concrete structures [
In reinforced concrete structures, concrete with fighting against compressive loads plays also the role of protection of the steel bars to the external environment. A good force transfer between the two materials can therefore only be achieved by an interaction between both materials through bond between the reinforcement bars and the concrete [
The concrete crack can be classified into the following: a crack due to load applied on structures or natural phenomena on its environment. The nonloaded cracks, especially shrinkage cracks, have more important practical significance [
Marine environment contributes to chemical reactions. When carbon dioxide diffuses into concrete in the presence of water, it reacts with calcium hydroxide to form calcium carbonate [
A pull-out load was applied to a reinforcement bar embedded in concrete; the resistance which was defined as bond strength between concrete and reinforcement was tested and it was an important factor for RC structures [
The combined effects of initial crack width, diameter of steel bar sand environments on the bond strength, and distribution of chloride ion were experimentally investigated. Three diameters of steel bars, four initial crack widths, and two types of environments were selected. The relationship between different diameters embedded in cracked concrete, chloride permeability, and bond strength between different diameter and different crack width of concrete after 90 days of wet-dry cycles of seawater were studied.
Good performances and high quality of concrete mixtures were designed here. All experiments were conducted in structural laboratory of Beihang University.
This was a concrete mixture with normal Portland cement P.O.42.5, aggregate of maximum 10 mm, medium sand 2.6, fly ash, and high performance polycarboxylate super plasticizer (standard) HY801 (water-reducer) admixture. The ratio of water/cement was 0.4, and the air content was 5.7%. The compressive strength of concrete prism with sizes of
Concrete mixture composition (kg/m3).
|
Water | Cement | Fly ash | Sand | Aggregate | Water reducer | Air entertainer |
---|---|---|---|---|---|---|---|
Quantity | 184 | 460 | 53 | 609 | 1130 | 3.94 | 100.61 |
The bond specimens between concrete and steel bars for pull-out test were designed with different steel bars of diameter 10 mm, 14 mm, or 18 mm. As shown in Figure
Detail of bond specimen for pull-out test crack design and sample design.
The chloride ingress in concrete with cracks was experimentally studied by [
Detailed information of bond specimens with 10 mm diameter bars.
COD10RE | C0D10WD | C80D10WD | C150D10WD | C210D10WD | |
---|---|---|---|---|---|
Number of specimens | 2 | 2 | 2 | 2 | 2 |
Design width of crack | 0 | 0 | 80 | 150 | 210 |
Seawater | No | Yes | Yes | Yes | Yes |
Plate sheet insertion time (hours) | 0 | 0 | 3 | 4 | 5 |
Detailed information of bond specimens with 14 mm diameter bars.
COD14RE | C0D14WD | C80D14WD | C150D14WD | C210D14CW | |
---|---|---|---|---|---|
Number of specimens | 2 | 2 | 2 | 2 | 2 |
Design width of crack | 0 | 0 | 80 | 150 | 210 |
Seawater | No | Yes | Yes | Yes | Yes |
Plate sheet insertion time (hours) | 0 | 0 | 3 | 4 | 5 |
Detailed information of bond specimens with 18 mm diameter bars.
COD18RE | C0D18WD | C80D18WD | C150D18WD | C210D18CW | |
---|---|---|---|---|---|
Number of specimens | 2 | 2 | 2 | 2 | 2 |
Design width of crack | 0 | 0 | 80 | 150 | 210 |
Seawater | No | Yes | Yes | Yes | Yes |
Plate sheet insertion time (hours) | 0 | 0 | 3 | 4 | 5 |
30 specimens of bond between concrete and steel bars were designed and cast. The 30 bond specimens were divided into three sets on the basis of the diameter of steel bars. Each set of specimens was also divided into four groups based on the width of initial cracks and exposing environments. Four groups of initial cracks width were designed as 0 microns, 80 microns, 150 microns, and 210 microns. Two environments of wet-dry cycles and atmosphere were designed. Each group contained 2 bond specimens. They were exposed in two types of environment, that is, wet-dry cycles of seawater or atmosphere. The seawater was artificially made of 3% NaCl and 0.34% MgSO4. One of wet-dry cycles includes 8 hours of seawater immersion and 16 hours of atmosphere environment. After 90 cycles of wet-dry and 10 days of atmosphere environment, totally 100 days, the pull-out test was performed.
Tables
All bond specimens were demolded after 24 h of casting and were cured under standard temperature and moisture conditions. At 28 days, except for the six reference specimens (in atmosphere), the remaining 24 bond specimens were exposed to wet-dry cycles environment. After every 30 wet-dry cycles, the crack widths of specimens were measured. Each cycle of wet-dry includes immersion in seawater for 8 h and in atmosphere for 16 h. The alternating rounds of immersion in seawater and in atmosphere environment were repeated till 90 cycles and 10 days of atmospheric environment. The standard pull-out test was conducted on all bond specimens. The pull-out forces, relative slip displacement, and bond strength of every specimen were measured.
After pull-out testing, sample of plate concrete at crack was drilled on the specimen, that is, the chloride tested zone shown in Figure
Concrete drilled slice (2.5 cm/5 cm/8 mm).
In this part, four main variables such as the crack evolution, the chloride content, ultimate tensile forces, and ultimate bond strength function with different diameter bars of 10 mm, 14 mm, and 18 mm were experimentally investigated.
After cracks were made on bond specimens, two points on each crack were selected and fixed to measure the crack width during the wet-dry cycles. The values of the two points on each crack width were measured after 0, 30, 60, and 90 wet-dry cycles. Tables
Crack width evolution of specimens with different bar diameters (80 microns).
C80D10WD2 | C80D14WD1 | C80D18WD2 | ||||
---|---|---|---|---|---|---|
Point 1 | Point 2 | Point 1 | Point 2 | Point 1 | Point 2 | |
0 | 80 | 80 | 80 | 80 | 70 | 50 |
30 | 50 | 50 | 50 | 50 | 70 | 50 |
60 | 0 | 50 | 30 | 30 | 50 | 0 |
90 | 0 | 20 | 0 | 20 | 50 | 0 |
Crack width evolution of specimens with different bar diameters (150 microns).
Wet-dry cycles | C150D10WD2 | C150D14WD2 | C150D18WD2 | |||
---|---|---|---|---|---|---|
Point 1 | Point 2 | Point 1 | Point 2 | Point 1 | Point 2 | |
0 | 100 | 110 | 140 | 110 | 150 | 150 |
30 | 100 | 110 | 140 | 110 | 130 | 140 |
60 | 100 | 110 | 130 | 100 | 130 | 130 |
90 | 100 | 100 | 130 | 100 | 130 | 130 |
Crack width evolution of specimens with different bar diameters (210 microns).
C210D10WD1 | C210D14WD2 | C210D18WD2 | ||||
---|---|---|---|---|---|---|
Point 1 | Point 2 | Point 1 | Point 2 | Point 1 | Point 2 | |
0 | 210 | 230 | 210 | 210 | 210 | 210 |
30 | 230 | 240 | 230 | 230 | 240 | 240 |
60 | 230 | 260 | 240 | 230 | 240 | 260 |
90 | 240 | 260 | 240 | 240 | 260 | 270 |
As shown in Table
Detail of crack evolution on C80D10WD2 point 1 with maximum crack width of 80 microns.
When the initial crack widths were in the range of 100–150 microns, as shown in Table
Crack evolution of C150D10WD point 2.
Using the same procedure, the crack widths of 2 points on each crack were measured for the specimen when the initial crack widths were in the range of 210–230 microns. The specimens were chosen randomly and the testing result was shown in Table
Relation between crack width and wet-dry cycles.
Detail of crack evolution on C210D18WD2 point 2 with maximum crack width of 210 microns.
To have more clear view on the evolution of initial crack widths, the testing results of all points 1 of the specimens were shown in Figure
The experimental phenomenon of bond specimens with four classes of initial crack width of 0 microns, 80 microns, 150 microns, and 210 microns and three sizes of bar diameters of 10 mm, 14 mm, and 18 mm has shown that when initial crack widths are around 80 microns all cracks tend to decrease. Nevertheless cracks on specimens with smaller diameter bars decreased faster than those with bigger diameters. When the cracks widths were in the range of 100–150 microns, the crack widths tend to stabilize or decrease a little; that is, their initial crack width was 100–150 microns and 100–130 microns after 90 wet-dry cycles. When the initial crack widths were in the range of 210–230 microns, all initial crack widths increased as the cycle number of wet-dry increased. Evolution of crack width showed that the speed of crack growth is directly proportional to the bar diameter and it is faster for larger diameters.
The ultimate pull-out forces were gotten by pull-out test and ultimate bond strength based on the embedment length and reinforcing bar diameter [
The averages value of pull-out forces and bond strength of all specimens were listed in Table
Pull-out forces and bond strength of all specimens.
Crack width | Bar diameter 10 mm | Bar diameter 14 mm | Bar diameter 18 mm | ||||||
---|---|---|---|---|---|---|---|---|---|
Specimen name | Pull-out force kN | Bond strength MPa | Specimen name | Pull-out force kN | Bond strength MPa | Specimen name | Pull-out force kN | Bond strength MPa | |
0 | COD10RE | 19.18 | 9.95 | C0D14RE | 41.05 | 10.86 | C0D18RE | 65.46 | 10.48 |
0 | COD10WD | 23.12 | 11.99 | C0D14WD | 42.32 | 11.20 | C0D18WD | 63.87 | 10.22 |
80 | C80D10WD | 17.99 | 9.33 | C80D14WD | 33.68 | 8.91 | C80D18WD | 50.52 | 8.08 |
150 | C150D10WD | 23.08 | 11.96 | C150D14WD | 43.60 | 11.53 | C150D18WD | 64.68 | 10.35 |
210 | C210D10WD | 19.85 | 10.29 | C210D14WD | 48.22 | 12.76 | C210D18WD | 65.66 | 10.51 |
For all specimens under wet-dry cycles of seawater and with different crack width, the pull-out forces showed no clear tendency, as shown in Figure
Relationship of initial crack width and ultimate pull-out force.
Figure
Curves of pull-out force-relative slippage displacement of C0D10WD2, C0D14WD1, and C0D18WD2.
As it was previously mentioned, samples of concrete were drilled from the specimens and subdivided along the depth of sample into 5 pieces of slices and the thickness of each slice was about 8 mm. The slice was smashed by pulverization and chloride content was tested. The test and analysed results of chloride contents for different bar diameter were listed in Tables
Chloride content and incremental percentage for specimens with 10 mm bars diameter.
Depth | Free (cl−) by cement (%) | Percentage of intercracks | |||||
---|---|---|---|---|---|---|---|
Reference | C80D10WD | C150D10WD | C210D10WD | 50–80 | 100–110 | 210–230 | |
8 | 0.013 | 0.091 | 0.137 | 0.183 | 86% | 91% | 92% |
16 | 0.007 | 0.021 | 0.025 | 0.058 | 67% | 72% | 88% |
24 | 0.007 | 0.013 | 0.016 | 0.029 | 46% | 56% | 76% |
32 | 0.007 | 0.008 | 0.013 | 0.020 | 8% | 46% | 65% |
40 | 0.007 | 0.006 | 0.011 | 0.017 | −9% | 36% | 56% |
Chloride content and incremental percentage for specimens with 14 mm bars diameter.
Depth | Free (cl−) by cement (%) | Percentage of intercracks | |||||
---|---|---|---|---|---|---|---|
Reference | C80D14WD | C150D14WD | C210D14WD | 50–80 | 110–150 | 210–230 | |
8 | 0.013 | 0.175 | 0.212 | 0.283 | 93% | 94% | 95% |
16 | 0.007 | 0.026 | 0.032 | 0.078 | 73% | 78% | 91% |
24 | 0.007 | 0.020 | 0.024 | 0.040 | 65% | 71% | 83% |
32 | 0.007 | 0.017 | 0.021 | 0.033 | 59% | 67% | 78% |
40 | 0.007 | 0.015 | 0.017 | 0.026 | 53% | 59% | 73% |
Chloride content and incremental percentage for specimens with 18 mm bars diameter.
Depth | Free (cl−) by cement (%) | Percentage of intercracks | |||||
---|---|---|---|---|---|---|---|
Reference | C80D18WD2 | C150D18WD2 | C210D18WD2 | 50–80 | 100–110 | 210–230 | |
8 | 0.013 | 0.171 | 0.239 | 0.332 | 92% | 95% | 96% |
16 | 0.007 | 0.109 | 0.135 | 0.212 | 94% | 95% | 97% |
24 | 0.007 | 0.069 | 0.069 | 0.111 | 90% | 90% | 94% |
32 | 0.007 | 0.056 | 0.064 | 0.074 | 87% | 89% | 91% |
40 | 0.007 | 0.033 | 0.042 | 0.069 | 79% | 83% | 90% |
The chloride content of specimens with different bar diameters and different width of initial cracks was shown in Figure
Chloride content of specimens with different bar diameters and different width of initial cracks.
As shown in Figure
This paper presents an experimental study on the impact of the combined actions of wet-dry cycles, initial cracks, and different bar diameters on the cracks evolution, chloride penetration, and the bond strength of reinforced concrete specimens. The following conclusions can be drawn from the results of the current study: When the bond specimen is with small diameter of steel bars and with crack width less than 80 microns, after 90 days of wet-dry cycles of seawater, the cracks vanished rapidly compared to those with bigger diameter bars. When cracks width were in the range of 100–150 microns, after 90 days of wet-dry cycles of seawater, the width of cracks decreased slightly; however the cracks width increased if the crack widths were larger than 200 microns. After 90 days of wet-dry cycles of seawater, the chloride content decreased along the depth of concrete, and the chloride content increased as the widths of initial cracks or bars diameter increased. The combined effects of crack widths and bar diameters on the bond strength have shown no clear tendency in the current study; however the ductility of bond specimens decreased as the diameter increased.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This work is part of the projects financially supported by the Chinese National Natural Science Foundation (NSFC) Grant no. 51578031 and by the open topics of State Key Laboratory of Subtropical Architecture Science (SKLSAS) in South China University of Technology (2016ZA03). The authors gratefully acknowledge the financial support received from the NSFC and SKLSAS.