Seawater sea sand concrete (SWSSC) is a promising alternative to ordinary concrete in terms of saving valuable natural resources of freshwater and river sand. Basalt fiber reinforced polymer (BFRP) rebars can be a good solution to corrosion of steel rebars in SWSSC. This paper presents an experimental study on the bond behavior between SWSSC and BFRP rebars through pullout testing. Concrete mixed with freshwater and river sand was also prepared for comparison with SWSSC. BFRP rebars with two different surface configurations were selected, that is, ribbed surface and sand-coated surface. Fly ash as a replacement of cement was also investigated in terms of its effect on bond behavior. Failure modes, bond-slip relationships, and bond strengths were reported and discussed in terms of the previously mentioned parameters. It was found that ribbed surface of BFRP rebar could achieve better mechanical interlocking with surrounding concrete. SWSSC could have comparative bond strength with BFRP rebar compared with ordinary concrete. However, using fly ash to replace cement is not recommended because it would significantly reduce concrete strength leading to much lower bond at the interface between SWSSC and BFRP rebar.
Global concrete construction consumes great amount of freshwater and river sand, which represent valuable natural resources and are expected to become short by 2050 [
It is reported that SWSSC can achieve similar mechanical properties to ordinary concrete mixed with freshwater and river sand [
The corrosion issue of steel reinforcements in SWSSC can be solved by using fiber reinforced polymer (FRP) rebars. FRP rebars have high specific strength and desirable corrosion resistance [
Successful combination of SWSSC with BFRP rebars relies on the reliable bond behavior between the two [
In order to address this knowledge gap, this paper presents an experimental study on the bond behavior between BFRP and SWSSC through pullout testing. A total of 24 pullout tests were conducted considering effects of various parameters on the bond behavior. Two types of BFRP rebars were selected with different surface configurations, that is, ribbed surface or sand coated surface. SWSSC was compared with ordinary concrete mixed with freshwater and river sand. Cement was also partially replaced with fly ash in some specimens to see its effect on bond behavior. Failure modes, bond-slip curves, and bond strengths were reported. It was found that SWSSC could achieve comparative bond strength with BFRP rebar compared to ordinary concrete. However, replacing cement with fly ash is not desirable due to significant reduction in bond strength between SWSSC and BFRP rebars.
Two types of BFRP rebars were selected with different surface preparation methods. Concrete mixed with seawater and sea sand was compared with that mixed with freshwater and river sand. Fly ash was also considered as an alternative material partially replacing cement to improve the greenness of concrete material.
Two types of concrete materials are considered in this study. One is mixed with river sand and freshwater. The other is mixed with sea sand and seawater. Sea sand was transported from coast of Tianjin, the Bohai Sea, China. Seawater was artificially made by mixing NaCl powder in freshwater with a concentration of 3.5 wt. %. Similar NaCl solution has been frequently used as artificial seawater in the literature [
The particle size distributions of sea sand, river sand, and coarse aggregate were obtained through a sieving process according to ASTM C136 [
Particle size distributions of (a) coarse aggregate and (b) sea sand and river sand.
In addition, relative densities of coarse and fine aggregates were measured according to ASTM C127-15 [
Physical properties of aggregates used in this study.
Aggregates | Relative density (g/cm³) | Bulk density (g/cm³) | Fineness modulus | Moisture content (%) |
---|---|---|---|---|
Coarse aggregate | 2.84 | 1.60 | — | 1.22 |
Sea sand | 2.33 | 1.56 | 2.78 | 0.83 |
River sand | 2.38 | 1.67 | 2.66 | 0.73 |
The mix proportions of the four scenarios for concrete are shown in Table
Details of pullout testing groups and concrete mix design.
Group ID | Concrete mix | BFRP rebars | |||||||
---|---|---|---|---|---|---|---|---|---|
Component (kg/m³) | Slump (mm) | Compressive strength (MPa) | |||||||
Cement | Fly ash | Coarse aggregate | Fine aggregate | Water | NaCl | ||||
OR-Rib | 939 | — | 2842 | 1338 | 469 | — | 45 | 50.6 | Ribbed |
OR-Sc | Sand-coated | ||||||||
OS-Rib | 939 | — | 453 | 16 | 42 | 47.4 | Ribbed | ||
OS-Sc | Sand-coated | ||||||||
FR-Rib | 657 | 282 | 469 | — | 50 | 42.1 | Ribbed | ||
FR-Sc | Sand-coated | ||||||||
FS-Rib | 657 | 282 | 453 | 16 | 48 | 40.9 | Ribbed | ||
FS-Sc | Sand-coated |
An HJW-60 single-horizontal concrete mixer was used for mixing the concrete materials. Slump of concrete was measured according to ASTM C143 [
It can be seen in Table
Two types of BFRP rebars were used in this study with different surface preparation methods as shown in Figure
(a) Two types of BFRP rebars and (b) microscopic image of sand coating.
Mechanical and physical properties of BFRP bars.
Properties | Diameter (mm) | Tensile strength (MPa) | Elastic modulus (GPa) | Fiber content (%) | Fiber tensile strength (MPa) | Fiber elastic modulus (GPa) |
---|---|---|---|---|---|---|
Ribbed BFRP rebars | 14.29 | 1351 | 43.08 | 91.09 | 4840 | 110 |
Sand-coated BFRP rebars | 15.58 | 37.78 |
Specimens were prepared for pullout testing to assess the bond behavior between BFRP rebar and concrete. Four specimens of concrete combined with two types of BFRP rebars yield eight testing groups, which are also shown in Table
Details of the specimen for pullout testing are shown in Figure
Specimen for pullout testing.
When preparing pullout specimen, BFRP rebar was firstly cut into a specific length. Then, one end of rebar was inserted in a plastic tube of 80 mm long. The gaps between rebar and plastic tube were filled with silicone gel. After that, the rebar was installed vertically in the center of a cubic wooden box, which served as concrete mold. Concrete was then poured in the mold. When pouring concrete, a bubble level was used to insure rebar straightness. Concrete was demolded after one day. The plastic tube at the end of rebar was then removed creating debonding at the end of rebar (Figure
The pullout test setup is shown in Figure
Setup and instrumentation of pullout testing.
Each testing group was repeated for three times, and the experimental results were listed in Table
Experimental results of all pullout specimens.
Specimen ID | Bond strength (MPa) | Average bond strength (MPa) | Failure mode |
---|---|---|---|
OR-Rib-1 | 7.39 | 7.60 | Concrete splitting |
OR-Rib-2 | 7.80 | Concrete splitting | |
OR-Sc-1 | 8.67 | 8.53 | Pullout |
OR-Sc-2 | 8.19 | Concrete splitting | |
OR-Sc-3 | 8.72 | Pullout | |
OS-Rib-1 | 11.75 | 10.71 | Concrete splitting |
OS-Rib-2 | 9.66 | Concrete splitting | |
OS-Sc-1 | 9.82 | 9.20 | Concrete splitting |
OS-Sc-2 | 9.15 | Pullout | |
OS-Sc-3 | 8.64 | Pullout | |
FR-Rib-1 | 7.17 | 6.43 | Concrete splitting |
FR-Rib-2 | 5.68 | Pullout | |
FR-Sc-1 | 5.89 | 6.19 | Pullout |
FR-Sc-2 | 6.10 | Pullout | |
FR-Sc-3 | 6.58 | Pullout | |
FS-Rib-1 | 10.92 | 9.91 | Concrete splitting |
FS-Rib-2 | 8.47 | Concrete splitting | |
FS-Rib-3 | 10.33 | Concrete splitting | |
FS-Sc-1 | 6.31 | 7.29 | Pullout |
FS-Sc-2 | 8.53 | Pullout | |
FS-Sc-3 | 7.02 | Pullout |
Two typical failure modes were observed as shown in Figure
Typical failure modes of pullout specimens with (a) sand coated rebar and (c) rebar with ribs. The detailed examinations of surfaces of these two rebars are shown in (b) and (d), respectively.
For specimens with BFRP rebars of ribbed surface, the failure was much brittle with concrete block split into halves as shown in Figure
Typical bond-slip curves are chosen and plotted in Figures
Typical bond-slip curves for specimens with (a) ribbed rebar and (b) sand coated rebar.
The relative slip between rebar and concrete can be obtained from readings of the two LVDTs. It should be noted that the rebar extension should be excluded from LVDT readings due to the instrumentation setup as shown in Figure
It can be seen in Figure
In addition, specimens with ribbed rebar showed more scattering nature in the bond-slip curves than specimens with sand coated specimens. This is also related to their different failure modes. Specimens with ribbed rebar failed because of concrete splitting (Figure
Peak stress of the bond-slip curve is defined as the bond strength of corresponding specimen. The bond strengths of all specimens are listed in Table
Comparison of bond strengths of all specimens.
Firstly, comparing specimens with ordinary cement and specimens with fly ash replacement, it is obvious that ordinary cement specimens always achieved higher bond strength than those with fly ash replacement. For example, OR-Rib strength is 7.60 MPa, which is higher than FR-Rib (6.43 MPa). Similar comparisons can be made for OR-Sc versus FR-Sc (8.53 MPa vs. 6.19 MPa), OS-Sc versus FS-Sc (9.20 MPa vs. 7.29 MPa), and OS-Rib versus FS-Rib (10.71 MPa vs. 9.91 MPa). This is reasonable because fly ash replacement may reduce the concrete strength as indicated in Table
Secondly, comparing specimens with ribbed rebar and specimens with sand coated rebar, it is observed that most ribbed rebar specimens yielded greater bond strength than sand coated specimens. For example, comparisons can be made for OS-Rib versus OS-Sc (10.71 MPa vs. 9.20 MPa), FR-Rib versus FR-Sc (6.43 MPa vs. 6.19 MPa), and FS-Rib versus FS-Sc (9.91 MPa vs. 7.29 MPa). This can be associated with their different failure modes in Figure
Thirdly, comparing specimens mixed with sea sand and seawater with specimens mixed with river sand and fresh water, it is interesting to notice that seawater specimens always exhibited greater strength than freshwater specimens. For example, this observation can be made when comparing OR-Rib versus OS-Rib (7.60 MPa vs. 10.71 MPa), OR-Sc versus OS-Sc (8.52 MPa vs. 9.20 MPa), FR-Rib versus FS-Rib (6.43 MPa vs. 9.91 MPa), and FR-Sc versus FS-Sc (6.19 MPa vs. 7.29 MPa). However, when looking at the compressive strength, concrete mixed with seawater is marginally weaker than concrete mixed with fresh water. Then, the question becomes why weaker concrete achieved stronger bond with BFRP rebar. This may be associated with the chemical reactions of BFRP rebar in the chloride environment. In chemistry, the following reactions will happen when basalt fiber is in contact with chloride solution [
The previously mentioned reactions may improve the chemical bonding at the interface between BFRP rebar and surrounding concrete. In addition, formation of Fe2O3.nH2O may lead to geometry expansion of BFRP rebar whose diameter would increase. This geometry expansion of BFRP rebar is constrained by surrounding concrete and thus mechanical interaction at the interface may be enhanced.
In order to verify the abovementioned mechanism, basalt fibers were immersed in NaCl solution for 28 days as shown in Figure
Diameter of basalt fiber (a) before and (b) after immersion in sea water.
This paper presents an experimental study on the bond behavior between BFRP rebar and concrete mixed with seawater and sea sand. Concrete mixed with freshwater and river sand was also prepared for comparison purpose. Fly ash was used to partially replace cement in some specimens to understand its effect on bond behavior. Pullout tests were conducted to quantify the bond behavior in terms of bond strength. Failure modes, bond-slip curves, and bond strength were reported, and the effects of various parameters on the bond strength were discussed. Based on the experimental results in this paper, the following conclusions can be drawn. Replacing cement with 30 wt.% fly ash may significantly reduce compressive strength of concrete (max by 20%). On the other hand, mixing concrete with seawater and sea sand may also reduce compressive strength of concrete, but the strength reduction was only marginal. Two failure modes were observed for pullout specimens. For specimens with ribbed rebar, they failed by concrete splitting. For specimens with sand coated rebar, the failure was interface debonding and no cracking was observed in concrete. Concrete splitting was more brittle than interface debonding. Specimens with ribbed rebar showed more scattering nature in the bond-slip curves due to significant uncertain factors such as concrete strength and cracking path introduced by concrete splitting. This is also related to their different failure modes. Replacing 30 wt.% cement with fly ash would result in lower bond strength between concrete and BFRP rebar, which was associated with the reduced strength of concrete. Ribbed surface seams more effective than sand-coated surface in terms of enhancement in bond strength. Ribs may provide a better mechanical interlocking with surrounding concrete than sand coating. Specimens with seawater sea sand concrete always showed higher bond strength than those with freshwater river sand concrete. The improvement in bond behavior was attributed to chemical reactions of basalt in chloride environment, which led to improved chemical interaction as well as enhanced mechanical interactions at the rebar-concrete interface due to geometry expansion of rebar. This is only based on the short-term measurement in the current study. How the chemical reaction and rebar expansion under seawater environment will affect the long-term bond behavior requires a more comprehensive future study. One interesting finding of the current study is that sand-coated rebar specimens have more ductile failure than the ribbed rebar specimens. Ribbed rebar specimens are failing near the split tension levels of concrete which means the concrete cannot take any more bond strength than was offered by rebar. This indicates that if the design bond strength can be compromised a little bit using sand-coated rebar, ductile failure is possible to be achieved.
This study proposes an environmentally friendly solution for construction and alleviates resource shortages and pollution to a certain extent. By proper design, it is recommended that seawater sea sand concrete (SWSSC) reinforced with BFRP rebars be a promising alternative to steel reinforced ordinary concrete, especially when the access to freshwater and river sand is limited.
The experimental data used to support the findings of this study are included within the article.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The first author acknowledges the financial support provided by the National Natural Science Foundation of China (51911530208 and 51978025) and Thousand Talents Plan (Young Professionals). The last author acknowledges the support from the National Natural Science Foundation of China (51808020) and the China Postdoctoral Science Foundation (2017M620015 and 2018T110029).