Polymers (Basel).
2022 Sep; 14(18): 3877.doi:
10.3390/polym14183877PMCID:
PMC9504286
PMID: 36146021
1College of Civil and Transportation Engineering, Shenzhen University, Shenzhen 518060, China
2Guangdong Provincial Key Laboratory of Durability for Marine Civil Engineering, Shenzhen University, Shenzhen 518060, China
3Key Laboratory for Resilient Infrastructures of Coastal Cities, Ministry of Education, Shenzhen University, Shenzhen 518060, China
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1College of Civil and Transportation Engineering, Shenzhen University, Shenzhen 518060, China
2Guangdong Provincial Key Laboratory of Durability for Marine Civil Engineering, Shenzhen University, Shenzhen 518060, China
3Key Laboratory for Resilient Infrastructures of Coastal Cities, Ministry of Education, Shenzhen University, Shenzhen 518060, China
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1College of Civil and Transportation Engineering, Shenzhen University, Shenzhen 518060, China
2Guangdong Provincial Key Laboratory of Durability for Marine Civil Engineering, Shenzhen University, Shenzhen 518060, China
3Key Laboratory for Resilient Infrastructures of Coastal Cities, Ministry of Education, Shenzhen University, Shenzhen 518060, China
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1College of Civil and Transportation Engineering, Shenzhen University, Shenzhen 518060, China
2Guangdong Provincial Key Laboratory of Durability for Marine Civil Engineering, Shenzhen University, Shenzhen 518060, China
3Key Laboratory for Resilient Infrastructures of Coastal Cities, Ministry of Education, Shenzhen University, Shenzhen 518060, China
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1College of Civil and Transportation Engineering, Shenzhen University, Shenzhen 518060, China
2Guangdong Provincial Key Laboratory of Durability for Marine Civil Engineering, Shenzhen University, Shenzhen 518060, China
3Key Laboratory for Resilient Infrastructures of Coastal Cities, Ministry of Education, Shenzhen University, Shenzhen 518060, China
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Author information Article notes Copyright and License information PMC Disclaimer1College of Civil and Transportation Engineering, Shenzhen University, Shenzhen 518060, China
2Guangdong Provincial Key Laboratory of Durability for Marine Civil Engineering, Shenzhen University, Shenzhen 518060, China
3Key Laboratory for Resilient Infrastructures of Coastal Cities, Ministry of Education, Shenzhen University, Shenzhen 518060, China
*Correspondence:Correspondence: nc.ude.uzs@nauyf
Copyright © 2022 by the authors.Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( https://creativecommons.org/licenses/by/4.0/ ).
All data, models, or codes generated or used during the study are available upon request.
Using locally available raw materials for preparing concrete, such as coral reefs, seawater, and sea sand, is conducive to compensating for the shortage of construction materials used on remote islands. Jacketing fiber-reinforced polymer (FRP), as passive confinement, is a practical approach to enhance the strength, ductility, and durability of such coral aggregate concrete (CAC). Rational and economical CAC structural design requires understanding the interactions between the CAC fracture process and FRP confinement. The coral aggregate size is the critical parameter of their interaction since it affects the crack propagation of CAC and FRP confinement efficiency. This study conducted axial compression tests on FRP-confined CAC cylinders with varying coral aggregate sizes and FRP confinement levels. The test results indicate that the coral aggregate sizes affected the unconfined CAC strength. In addition, the dilation behavior of FRP-confined CAC varied with aggregate sizes, showing that CAC with smaller coral aggregate featured a more uniform hoop strain distribution and larger FRP rupture strain. These coupling effects are epitomized by the variation in the transition stress on the stress–strain curve, which makes the existing stress–strain models not applicable for FRP-confined CAC. A modified stress–strain model is subsequently proposed. Finally, the practical and environmental implications of the present study are discussed.
Keywords:
coral aggregate concrete, aggregate size, fiber-reinforced polymer, axial compression, confinement
In the South China Sea, there are many coral islands and reefs in the ocean, which are coral polyp skeletons composed of thousands of tons of calcium carbonate. This coral debris washed ashore by seawater provides a new type of building material for engineering construction. Meanwhile, there is an extreme lack of sand, gravel, and freshwater resources on the islands hundreds or even thousands of kilometers away from the mainland. The transportation of these raw materials for concrete will greatly increase the cost of projects and extend construction periods due to the disturbance of natural weather conditions. On the premise of not destroying the natural ecosystem of coral reefs, it is of great significance to prepare the required concrete by using local rich coral aggregate resources [1,2].
In recent years, numerous studies have been devoted to investigating the properties of coral aggregate concrete (CAC) [3,4,5,6,7,8,9]. Compared to natural aggregates, coral aggregates exhibit lower density, rough surface texture, higher flakiness index values, and higher water absorption due to their low intensity, fragility, porous nature, and nonhomogeneity [3,4,5]. Owing to these unique characteristics of coral aggregate, CAC exhibits lower strength, more pronounced brittleness, and poorer workability than natural aggregate concrete (NAC) [6,7,8,9]. Da et al. [6] and Wang et al. [7] tested the uniaxial compressive behavior of CAC and found that it showed a more brittle failure mode than NAC. The post-peak stress decreased suddenly to 0.30–0.50 of the peak stress, and the enclosed area of the uniaxial compressive stress–strain curve of CAC was only half the enclosed area of the uniaxial compressive stress–strain curve of NAC. Zhou et al. [8] tested the uniaxial and triaxial compression behavior of CAC and proposed the corresponding constitutive relationships. Ma et al. [9] investigated the effect of the strain rate on the mechanical characteristics of CAC and found that the rate dependence on the compressive strength was more remarkable for CAC than for other cement-based composites. The obvious brittleness of CAC severely limits its application in engineering practices. The other major problem in the application of coral aggregate, seawater, and sea sand is that of durability. The rich chloride ions in these raw materials make them unsuitable for traditional steel-reinforced structures [10,11].
Fiber-reinforced polymers (FRPs) are attracting widespread attention in engineering practices because of their excellent strength and immunity to chloride-induced corrosion [12,13]. External jacketing using FRP sheets will put concrete in a state of triaxial compression, thereby greatly enhancing the mechanical properties of the concrete [14]. Recently, several studies have investigated the axial compressive properties of FRP-confined CAC [7,15,16]. Ying et al. [15] investigated the axial compressive behavior of glass FRP (GFRP)-confined CAC and compared it with that of GFRP-confined NAC. They found that the glass FRP (GFRP)- and GFRP-confined NAC exhibited similar failure modes, and the strength and deformation capacity of CAC could be significantly increased through FRP confinement. Wang et al. [7] and Zhang et al. [16] used prefabricated GFRP tubes to confine CAC and found an evident increase in ductility. The transition segment of FRP-confined CAC was lower than the transition segment of FRP-confined NAC. Actually, the difference in the mechanical properties between CAC made with coral aggregate, seawater, and sea sand and NAC prepared from natural gravel, freshwater, and river sand lies in the differences in the aggregates. Previous studies have indicated that the strength and working performance of seawater sea sand concrete are similar to the strength and working performance of freshwater river sand concrete, where the former exhibited slightly higher early compressive strength [17,18,19]. The mechanical performance of CAC is sensitive to the properties of coral aggregates, where the size of the aggregate should be the priority. Previous studies on the aggregate size effect of NAC indicated that the strength of unconfined CAC [20,21,22] as well as the stress–strain response and confinement efficiency of FRP-confined NAC were affected by the aggregate size [23,24]. However, there are few reports regarding the effect of coral aggregate size on the properties of CAC with or without FRP confinement.
This paper is an attempt to investigate the influence of aggregate size on the characteristics of FRP-confined CAC subjected to uniaxial compression. First, a group of FRP-confined CAC specimens with the test parameters of aggregate size and FRP thickness were prepared and tested. The experimental results, including the failure modes, axial stress versus axial/lateral strain response, lateral strain distribution, and ultimate conditions, were analyzed in detail. A modified model suitable for FRP-confined CAC was subsequently proposed based on the regression analysis of the acquired test results.
A total of 27 cylinder specimens with a height (H) of 300 mm and diameter (D) of 150 mm were manufactured. The test variables included the coral aggregate size (da) and the FRP thickness (tf). The test specimens consisted of three groups according to the continuous gradation of the coral aggregate involving 5–10 mm, 10–16 mm, and 16–26 mm. Each group of specimens was confined by 1-ply or 2-ply carbon FRP (CFRP) sheets. The unconfined cylinders were also prepared as control specimens for each group. Each test configuration included three identical nominal specimens. gives detailed information on the designed specimens. The naming rules of the specimens are as follows: (1) the first two letters, CA, followed by a number, represent the coral aggregate size; (2) the letter P, followed by a single-digit number after the first hyphen, indicates the number of plies of FRP sheets; (3) the number after the second hyphen (1/2/3) differentiates the three nominal identical specimens. For example, CA10-P2-3 represents the third of the three nominally identical FRP-confined CAC cylinders with aggregate sizes of 10–16 mm and two-ply CFRP wraps.
The specimens of the same concrete mix were cast from the same batch of concrete. After 28 days of curing, the carbon fiber sheets were impregnated with the epoxy resin adhesive and wrapped gradually onto the specimen through a wet lay-up process, with an overlap region of 150 mm. The CFRP fibers were only oriented in a hoop direction to guarantee the maximum confinement efficiency. After 7 days of room temperature maintenance, the gypsum was placed on the cylinder to flush the concrete surface with the loading plate.
The materials used in the casting of the CAC consisted of ordinary Portland cement (P.0.42.5), coral aggregate, seawater, sea sand, and polycarboxylate superplasticizer. shows the chemical composition of the seawater. The physical properties and chemical composition of the sea sand are shown in . The coral aggregate was purchased from Lingshou, Hebei Province, China, and originally produced in the Philippines. In order to filter the aggregates by size, standard sieves were used in this study, which were 26 mm (sieve hole), 16 mm, 10 mm, and 5 mm. Three kinds of coral aggregates with sizes between two adjacent sieves were obtained, which were 5–10 mm, 10–16 mm, and 16–26 mm ( ). The properties of the coral aggregates were determined based on the Chinese standard GB/T 14685 [25], as shown in . shows the mixture composition of the CACs with different aggregate sizes. Notably, due to the larger specific surface area and higher water absorption of coral aggregates with smaller sizes, the required amount of superplasticizer was relatively larger to ensure the same workability. Three cubes with the dimensions of 100 × 100 × 100 mm were cast for each type of CAC. The average compressive strengths (fcu) for the CA5, CA10, and CA16 concrete-mix cubes were 33.5 MPa, 32.9 MPa, and 27.2 MPa, respectively. The epoxy resin adhesive hardener and epoxy resin adhesive base resin were E2500S from Kony Bond, and the mixing weight ratio of the base resin and hardener was 2:1. The CFRP sheet possessed a thickness of 0.167 mm, produced by Toray Industries, Inc. Nihonbashi Muromachi, Chuo-ku, Tokyo. The fiber weight was 300 g/m2 and the real density was 1.80 g/cm3. The mechanical properties of the CFRP were determined by the average value of the five tensile coupon tests according to ASTM D3039M [26]. The results are summarized in .
Open in a separate windowThe load was applied through the load-based control option with a load rate from 2 kN/s to 250 kN followed by the displacement-based control option with a displacement rate of 0.3 mm/min until the failure of the specimens. Four linear variable displacement transformers (LVDTs), 90° apart along the circumference of the cylinder, were arranged to obtain the axial deformation/strain of the specimens. The measured length of the LVDTs was 185 mm, aligned along the axial direction. Two strain gauges (SGs) were symmetrically and vertically attached outside the overlapping region to measure the axial strains of the cylinders (SG1, SG2). The digital image correlation (DIC) systemproduced by Correlated Solutions, Inc. Irmo, USA was employed to measure the strain field on the specimen, which has proven to be highly accurate and effective for continuously capturing the strain fields during loading [27,28]. The test setup and instrumentations are shown in . The measured data were recorded by a DEWESOFT dynamic acquisition system produced by Dewesoft, Trbovlje, Slovenija.
Open in a separate windowFrom the previous analysis, we can infer that a modification should be carried out based on the existing stress–strain models to reflect the mechanical properties of FRP-confined CAC well. Because the properties of CAC are closer to the properties of lightweight aggregate concrete, the stress–strain expression employed by Wang et al. [7] and Zhou et al. [31] was used as the benchmark, which is given by:
σc=E1εn−f0e−εcεn+f0+E2εc1−e−εcεn
(3)
where σc = the axial stress of the concrete, εc = the axial strain of the concrete, E1 = the slope of the first branch of the stress–strain curve, εn = nεo, where εo = f0/E1, and n is the curve shape parameter that controls the curvature of the transition zone, which satisfies 0 < n < 1. According to the previous analysis, the model of fcc proposed by Wu and Wei [30] and the model of εcc proposed by Wang et al. [7] were used to predict the ultimate state of the FRP-confined CAC, which is given in . In addition to the fcc and εcc, there are four parameters (E1, f0, E2, n) to be determined in Eq. (3) to develop the stress–strain model of the FRP-confined CAC. E1, E2, and n can be determined accordingly as in Zhou et al. [31] (E1 = Ec, n = 0.5, E2 = (fcc–f0)/εcc). Through the comparison between the measured stress–strain curves and the existing models in , none of the four models performed well in predicting the f0. Therefore, it is necessary to propose a new model for the f0 of FRP-confined CAC. Through careful examination of the test results, the value of f0/fco was found to be proportional to fl/fco and the confinement stiffness (2Eftf/D), as shown in . Through regression of the test results, the following model is thus proposed:
f0fco=1+2.235flfco0.87−0.00072EftfD
(4)
Open in a separate windowAs seen in , the modified model was much more accurate than these available models in representing the stress–strain response of the FRP-confined CAC with various aggregate sizes because the suggested modified model properly considers the effect of the unconfined compressive strength of CAC resulting from different coral aggregate sizes. Unfortunately, because of the delay in expansion and the utilization of the bilinear stress–strain relationship of CAC, the slight stress drop following the transition zone cannot be captured by the modified model. However, this slight stress drop does not affect the accurate representation of the ultimate condition by using the modified model.
Open in a separate windowWith the increasing demand for the construction of global island projects, coral concrete is one of the hot research topics in the field of civil engineering worldwide. China has regarded the strategy of marine power as the basic national policy. There are more than 200 islands and countless coral reefs and amounts of sea sand in the South China Sea. Using these locally available raw materials for preparing concrete is beneficial for reducing energy consumption and significantly avoiding harm to the environment during transportation. Although any non-renewable resource, including coral reefs and sea sand, unavoidably will face the problem of resource depletion, the planned and rational application of these raw materials will have a negligible negative impact on the local ecological environment. Currently, scarce work is available with respect to the optimized treatment of the coral aggregate in improving the mechanical behavior of CAC. The main attempt of the present work is to explore the influence of coral aggregate size on the axial compression characteristics of unconfined and FRP-confined CAC. A new stress–strain model is also proposed for the sustainable design of eco-friendly CAC members. The results from the present test indicate that a smaller coral aggregate size is more desirable for the enhancement of the properties of both unconfined and confined CAC. Since the tailoring of the size of coral aggregate is easily available, the research findings of the present study are conducive to further promoting the low-carbon sustainable application of coral aggregate in remote island construction.
This study investigated the effect of coral aggregate size on the axial compressive properties of the FRP-confined CAC. A group of FRP-confined CAC cylinders with test parameters of aggregate size and FRP thickness was prepared and tested under uniaxial compression. The experimental results, including the failure modes, axial stress versus strain response, hoop strain distributions, and ultimate conditions are discussed in a detailed manner. Several conclusions are summarized as follows:
(1)
Owing to the low strength of the coral aggregate, the failure mode was rather brittle for the unconfined CAC. The aggregate size had a pronounced influence on the mechanical properties of the CAC. The larger the size of the coral aggregate was, the lower the strength and the smaller the ultimate axial strain of the CAC. Under the same mixture proportions, when the coral aggregate size increased from 5–10 mm to 16–26 mm, a 43.9% and 51.9% reduction of the stress and the ultimate strain were observed, respectively.
(2)
The strength and ductility of CAC could be enhanced significantly by using external FRP confinement. The coral aggregate size had a pronounced effect on the dilation property of the CAC. The CAC with a smaller aggregate size dilated more uniformly than the CAC with the same FRP thickness but a larger aggregate size, suggesting a higher FRP confinement efficiency. When the coral aggregate size decreased from 16–26 mm to 5–10 mm, the average values of the εh,rup/εfu improved by 19.0% and 18.9%, respectively, for the columns confined by one and two-ply FRP jackets.
(3)
The ultimate axial stress of the FRP-confined CAC was inversely proportional to the coral aggregate size. When the coral aggregate size increased from 5–10 mm to 16–26 mm, the average peak stress decreased by 42.9%, 14.2%, and 19.1%, respectively, for the columns without confinement, confined by one-, and two-ply FRP jackets. However, the coral aggregate size had little influence on the strength and ultimate axial strain enhancements as the confinement ratio increased.
(4)
None of the existing models can capture the stress–strain behavior of FRP-confined CAC well. A modified stress–strain model is subsequently suggested for FRP-confined CAC with the careful consideration of the models of the ultimate condition as well as the model of the stress at the transition segment (the value of f0). The modified model provided a reasonable prediction of the stress–strain curve of the FRP-confined CAC.
(5)
A smaller coral aggregate size is more desirable for the property enhancement of both unconfined and confined CAC. Since the tailoring of the size of coral aggregate is easily available, the research findings of the present study are conducive to further promoting the low-carbon sustainable application of coral aggregate in remote island construction.
This research was funded by the National Natural Science Foundation of China (No. 52078299), the Guangdong Basic and Applied Basic Research Fund Project (Grant No. 2020A1515011552), the Shenzhen Science and Technology Program (Grant No. KQTD20200820113004005, 20200807104705001) and the Open project of the Key Laboratory of Concrete and Prestressed Concrete Structures of Ministry of Education of Southeast University (Grant No. CPCSME2022-04).
Conceptualization, P.L.; methodology, P.L. and F.Y.; software, D.H.; validation, R.L. (Ruiyi Li); investigation, D.H., R.L. (Ruiyi Li) and R.L. (Rongkang Li); data curation, R.L. (Rongkang Li); writing—original draft preparation, D.H.; writing—review and editing, P.L. and F.Y.; supervision, F.Y.; funding acquisition, P.L. All authors have read and agreed to the published version of the manuscript.
All data, models, or codes generated or used during the study are available upon request.
The authors declare no conflict of interest.
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