Mechanical properties of seawater volcanic scoria aggregate concrete-filled circular GFRP and stainless steel tubes under axial compression

2.1.1 Raw materials

Two types of coarse aggregates were obtained in this study: VSCA and gravel (Figure 1). The VSCAs were derived from Jilin Province, China, and the fine aggregates were RS and SS (Figure 2). The physical properties and chemical components of the coarse and fine aggregates were derived according to standard code [44]. It was found that the Cl⁻ content of SS is more than that of RS, and the water absorption of VSCA increases by 1,206% compared to that of gravel (Table 1). Furthermore, SW was used for mixing when preparing SVAC, and the SW was obtained from Jiaozhou Bay. Based on the ion chromatography method [28], the main chemical ions in the SW are as follows: Cl⁻ (19,631 μg·L⁻¹), Na⁺ (16,863 μg·L⁻¹), SO 4 2 − (1,926 μg·L⁻¹), Mg²⁺ (787 μg·L⁻¹), and K⁺ (878 μg·L⁻¹), respectively.

Figure 1

Coarse aggregates: (a) VSCA and (b) gravel.

Figure 2

Fine aggregates: (a) RS and (b) SS.

Two types of tubes were used in this test: stainless steel and GFRP tubes (Figure 3). GFRP was derived from a glass fibre comprising unsaturated polyester resin as the substrate, and the tube was fabricated using the filament-wound process method (the fibre volume fraction is 60%). Moreover, the fibres of the GFRP tube were angled at approximately ±75° on the axial direction of specimen. The length and inner diameter of the GFRP and stainless steel tubes were kept constant, i.e. 525 and 175 mm, respectively, whereas the tube thickness (t) was varied (3 and 4 mm).

Figure 3

Stainless steel and GFRP tubes: (a) stainless steel tube and (b) GFRP tube.

The properties of the GFRP and stainless steel tubes were tested according to standard code [45,46]. The hoop and axial strengths of the GFRP tubes were 550 and 58 MPa, respectively, and the yield and tensile strengths of the stainless steel tube were 296 and 688 MPa, respectively.

2.1.2 Concrete mix

The target concrete strength grades were C30 and C40, which are commonly used in engineering applications. Two types of concretes were fabricated: OC and SVAC. Generally, the properties of the OC mix components such as gravel and RS were different from those of the SVAC mix components (VSCA and SS). To obtain the same target concrete strength, different mix proportions of OC and SVAC were finally adopted through several preliminary experiments, as listed in Table 2. Furthermore, the cement paste volume (CPV) of SVAC [47] was also calculated to describe the changes of concrete mix (Table 2).

The SVAC and OC were fabricated and cured under similar conditions (room temperature: 20 ± 2°C), whereas fresh water and SW were adopted as the curing water for OC and SVAC, respectively. The properties of concrete such as the density and modulus were tested based on related code (Table 2) [48]. It can be observed that under similar cubic compressive strength (f _cu) conditions, the density and elastic modulus (E _c) of SVAC decreased by 15.5 and 23.2 % compared with those of OC, respectively. This is mainly because of the lightweight and porous structure of the VSCA [23].

2.2.1 Design of specimen

The mechanical behaviours of confined concrete members are significantly affected by the characteristics of concrete (concrete strength and strength) and outer tube (e.g. tube thickness). There were a total of ten groups of the SFCGT and SFCST specimens based on the four different parameters: concrete type (SVAC and OC), outer tube type (stainless steel and GFRP tubes), concrete strength (C30 and C40), and thickness (0, 3, and 4 mm). Each group had three specimens of the same size and comprised the same material. The details of the SFCGT and SFCST specimens are listed in Table 3. The length of the specimens was 525 mm, and the length-to-diameter ratio (L/D) was maintained at approximately 3.0, which could reduce the influences of loading device and L/D on the properties of confined concrete specimen. Furthermore, two groups of plain OC and SVAC specimens, SVAC-C40-T0 and OC-C40-T0, were also fabricated to determine the variations in the confined SVAC specimen properties. It should be noted that the outer tube length and strength could also affect the mechanical properties of specimens. However, the related experimental studies were not performed owing to the limitations of loading device. Further works would focus on this research field.

These specimens were denoted using the following format: “outer tube type-concrete type-strength-tube thickness.” Taking G-SVAC-C30-T3 as an example, “G” denotes the outer tube type (GFRP tube), “SVAC” represents the concrete type (SVAC), and “C30” and “T3” indicate the concrete strength and thickness of the specimen (C30 and 3 mm, respectively).

2.2.2 Specimen fabrication

The detailed process for the SFCGT and SFCST fabrication was as follows. Firstly, the tube was placed upright on the ground, and a plastic film was used to cover the bottom of the specimen to prevent mortar leakage. Secondly, the outer tube was filled with fresh concrete, and vibration was performed using a poker vibrator. Additionally, polyethylene sheets were used to cover the top surface of the specimen to prevent humidity loss in the fresh concrete during the initial age (24 h). Thirdly, the specimens were cured over 28 days under standard conditions (20 ± 2°C). Finally, the top and bottom of concrete were polished using an angle grinder to make the specimen surface smooth.

The fabrication of SVAC-C40-T0 and OC-C40-T0 (plain concrete specimens) was similar to that of SFCGT and SFCST. A polyvinyl chloride (PVC) tube (height: 525 mm, inner diameter: 175 mm) was used as a mould and filled with concrete; after 28 days, the PVC tube and plain concrete specimens were removed (Figure 4).

Figure 4

Illustration of SFCGT and SFCST specimens: (a) SFCGT and plain concrete specimens and (b) SFCST.

3.1.1 Failure phenomenon and pattern

3.1.1.2 Failure pattern

The failure patterns varied with the type of concrete and outer tube (Figure 6). It was obtained that the GFRP tube of the SFCGT completely fractured and the core concrete was crushed. Additionally, the ends bulged and a mid-span shear slip line was observed in the SFCST specimens, which is attributed to the properties of steel and the confinement between core concrete and outer tube. However, the failure pattern of the plain SVAC specimen (SVAC-C40-T0) differed from those of the SFCGT and SFCST specimens, the macrocracks crossed the specimen, and no obvious deformation was observed owing to the inferior deformability of the SVAC (Figure 6).

Figure 6

Typical failure patterns in the specimens: (a) SFCGT, (b) SFCST, and (c) SVAC-C40-T0 (left) and OC-C40-T0 (right). Note: The mid-span shear slip lines of SFCST specimen was marked by red line.

The SVAC also influenced the failure of the specimen. Compared with gravel, the strength and modulus of VSCA were inferior to those of cement mortar, causing that VSCA could not effectively restrict the propagation of cracks. The main cracks would cross cement mortar, interfacial zones, and VSCA. The coarse aggregates of the SVAC specimens were almost fractured owing to the lightweight and porous structure of the VSCA; however, the gravel in the OC was seldom destroyed. The failure surfaces of the SFCGT and SFCST were smooth compared to that of the specimen using OC (Figure 7).

Figure 7

Destruction of the core concrete: (a) SVAC and (b) OC.

3.1.2 Axial stress–strain curve

Axial stress–strain (σ–ε) curves are usually used to characterize the variations in the macromechanical performances of specimens, where the strain ε is the ratio of the axial deformation to the mid-span of specimen. The axial stress (σ) is equal to the ratio of the axial load (N) to the cross-sectional area of the specimen (σ _c = N/(A _c + A _t)). The averaged σ–ε curves of SFCST and SFCGT are displayed in Figure 8.

Figure 8

σ–ε curves of the specimens: (a) SFCGT and (b) SFCST.

Generally, the σ–ε curves of SFCGT and SFCST were significantly different from those of the plain SVAC specimen (SVAC-C40-T0). Before the peak point, the modulus and curvatures of curves of SFCGT and SFCST specimens increased, and the peak strains and stresses (strengths) of the confined SVAC specimens were 47.3 and 46.7% higher than those of SVAC-C40-T0, respectively.

After the peak point, the decline in the σ–ε curves of the SFCST became smooth, and the deformability and strength of the SFCGT and SFCST were improved owing to the restriction of crack propagation in the core concrete by the passive confinement.

3.2.1 Typical effects of parameters on σ–ε curves

3.2.1.1 SFCGT

The GFRP tube and core SVAC have significant influences on the shape of the σ–ε curve. Cui [49] and Dabbagha et al. [25] reported that the σ–ε curve of concrete-filled circular FRP tubes can be categorized into two types considering the level of confinement: lightly confined and heavily confined (Figure 9). In this study, the curve of the specimen adopting OC was classified into the heavily confined category, whereas that of the SFCGT was considered in the lightly confined category under the same condition. The σ–ε curve of G-OC-C40-T3 was characterized by a bilinear response in the form of a low stiffness ascending section after the initial branch (Figure 8); however, for the SFCGT specimens, a descending stage was observed after the initial branch of the curve followed by another hardening part (Figure 8). A fluctuation was also observed in the σ–ε curve of the SFCGT and not in the specimen adopting the OC. This is mainly attributed to the inferior deformability and brittle properties of SVAC, which causes the confining pressure to decrease and delays its activation. Furthermore, GFRP is an anisotropic material with low axial strength.

Figure 9

Lightly confined and heavily confined σ–ε curves.

To verify the causes of the decrease in the stress of the SFCGT before the peak point, the lateral deformation factor (β) and axial strain (ε) relations were analysed, as shown in Figure 10. The β (β = ε _h/ε) is the hoop strain (ε _h)/ε-ratio, which can reflect the degree of changes in the hoop strain and confining pressure. During the early stages of loading (15–20% of the peak stress [approximately 0.3–0.4 f _c]), the β of the SFCGT nonlinearly increased compared to that of the specimen adopting OC (Figure 10(b)–(d)), which is attributed to the high shrinkage of SVAC. Beyond this point, β remained stable. When σ approached the end of the initial branch of the SFCGT curve, β drastically increased, and this increase became negligible after the descending stage of curve (Figure 10). This phenomenon indicates that, compared to the specimen adopting OC, the activation of the confining pressure of the SFCGT is delayed, and the changes in the confining pressure and hoop strain are drastic owing to the brittle properties of SVAC. The more delayed the activation of the confining pressure, the more drastic the development of β, and the higher the degree of the changes in the confining pressure and hoop strain. Therefore, after the initial ascending stage in the SFCGT curve, a sharp drop occurred in the stress. However, the next stage of the σ–ε curve had an ascending tendency for the strain-hardening behaviour owing to the rapid increase in the core concrete lateral deformation and passive pressure provided by GFRP tubes.

Figure 10

Typical β–ε relations of the SFCGT: (a) G-OC-C40-T3, (b) G-SVAC-C40-T3, (c) G-SVAC-C40-T4, and (d) G-SVAC-C30-T3.

Furthermore, the slope of the curve in the hardening and initial stages increased with the GFRP tube thickness (Figure 8). It can be obtained that the larger the thickness, the higher the confining pressure provided by the tube, and the greater the curve slope. The enhancement in the concrete strength accelerated the decline in the SFCGT curve and reduced the curve slope of the hardening stage because high-strength concrete reduces the confinement between the outer tube and core concrete (Figure 10(b) and (d)) [42].

3.2.1.2 SFCST

Compared to SFCGT, the σ–ε curve of the SFCST can be divided into four stages (Figure 8):

1. An elastic stage where the curve is approximately a straight line;

2. An elasto-plastic ascending stage in which σ nonlinearly increases with an increase in ε;

3. A declining stage where σ decreases with increasing ε;

4. A steady stage in which the variation of σ is negligible and the growth of ε is rapid.

The concrete type has also been found to have an influence on the shape of the σ–ε curve. During the elastic and elasto-plastic ascending stages, the slope and curvature of the specimen adopting OC (S-OC-C40-T3) increased compared to those of SFCST, even under the same strength grade condition, owing to the inferior modulus and deformability of the SVAC. After the peak point, the curve of S-OC-C40-T3 almost became stable and the σ of the SFCST rapidly declined before stabilizing. This is because the σ of SVAC suddenly decreases, and the core concrete lateral deformation is small compared to that of OC when the σ reaches the peak point [7], causing a decrease in the confining pressure provided by the steel tube.

To identify the causes for the decrease in the σ of the SFCST after the peak stress, the development of β is displayed in Figure 11. The β of S-OC-C40-T3 was higher than that of S-SVAC-C40-T3 at the different ε levels, indicating a lower hoop strain and confining pressure of the specimen adopting SVAC. It was also seen that the β-axial strain relations of the SFCST are different from those of the SFCGT (Figure 10) owing to the isotropic properties of steel.

Figure 11

Typical β–ε relations of the SFCST.

Furthermore, the thickness can decrease the influence of the SVAC on the curves of the specimen. The slope of S-SVAC-C40-T4 curve increased and the decline in the curve became gentler compared with that of S-SVAC-C40-T3 because of the high confinement provided by a larger thickness (Figure 11). However, the decline in the curve became steeper as the concrete strength increased, and the higher the strength, the more brittle the concrete and the inferior the deformability of the specimen (Figure 11).

3.2.2 Strength and strength enhancement factor

3.2.2.2 Strength enhancement factor

The test results indicate that the bearing capacities of the SFCST and SFCGT are greater than the sum of the bearing capacities of the independently loaded concrete and outer tube owing to the composite action of the confined specimens. Therefore, the strength enhancement factor (λ) was adopted to describe the influences of the outer tube thickness, concrete type, and strength on the composite action of the specimen (Table 4), where λ is expressed using the following equation:

(1) λ = N max N c + f A t ,

where N _max and N _c are the peak loads of the confined concrete and plain concrete specimens, respectively, and f and A _t denote the strength and cross-sectional area of the outer tube, respectively.

Generally, the λ of the SFCGT is, on average, 2.02 (Table 4) and is approximately 35.69% higher than that of the SFCST (1.30). This is because the high passive confinement is provided by the GFRP tubes. Passive confinement can be reflected by the distribution of hoop strains, which were obtained using the hoop strains recorded at different loading levels on the perimeter of the specimens (Figure 12). It can be found that the hoop strain around the circumference of the GFRP tube is higher than that of the stainless steel tube at the same loading level, verifying that a higher confining pressure distribution can be created around the circumference of the core concrete of GFRP tube compared to the steel tube. Moreover, a higher confining pressure results in a better composite action of the specimen and larger λ.

Figure 12

Distribution of the hoop strains over the perimeter of the specimens: (a) G-SVAC-C40-T3, (b) S-SVAC-C40-T3, (c) G-OC-C40-T3, and (d) S-OC-C40-T3.

It was also observed that the λ of the SFCGT and SFCST increases with increasing thickness owing to the influence of passive confinement. The λ of the G-SVAC-C40-T4 and S-SVAC-C40-T4 is 9.04 and 0.13% higher than that of the G-SVAC-C40-T3 and S-SVAC-C40-T3, respectively, and the hoop strain around the circumference of specimen using OC is larger than that of the specimen using SVAC (Figure 12), resulting in a higher λ in G-OC-C40-T3 and S-OC-C40-T3 by 32.03 and 13.06% compared with that of G-SVAC-C40-T3 and S-SVAC-C40-T3, respectively. The reduction in the composite action of the confined concrete members is mainly attributed to the brittle properties of SVAC and inferior deformation.

Based on the aforementioned test analyses, the ascending order of the influence of the parameters on the σ _cc and λ of the confined SVAC members is as follows: concrete strength, thickness, concrete type, and type of outer tube.

3.2.3 Deformability

The deformation properties of the SFCGT and SFCST varied significantly from those of plain concrete. In this study, the peak strain (ε _cc) and deformation enhancement factor (μ) were used to investigate the variation in the deformability of the SFCGT and SFCST, which could provide a basis for the practical applications of confined SVAC members.

3.2.4 Toughness

The toughness (K) denotes the capacity of the material to absorb energy during the loading period. Generally, K is represented by the area under the σ–ε curve of a specimen (Figure 13) [51]. As indicated in Table 4, the K of the plain SVAC specimen (SVAC-C40-T0) is 520.8, which is 2.0% lower than that of the plain OC specimen (OC-C40-T0). However, the K can be significantly enhanced using tube confinement. Therefore, the K values of the SFCGT and SFCST are 38.1 and 9.6 times higher on average compared with those of SVAC-C40-T0.

Figure 13

Area under the σ–ε curve.

Furthermore, the K increases with increasing thickness owing to the improved confinement conditions. The K values of S-SVAC-C40-T4 and G-SVAC-C40-T4 are 27.6 and 22.1% higher than those of S-SVAC-C40-T3 and G-SVAC-C40-T3, respectively. However, the high-strength concrete has a negative effect on the K. Based on the test results, the K of G-SVAC-C40-T3 is 18056.7 and that of G-SVAC-C30-T3 is 18387.1. This is mainly attributed to the brittle properties and inferior confinement of high-strength concrete. Compared with the specimen adopting OC, the K of the SFCST is approximately 89.1% lower and that of the SFCST is 3.7% higher. Moreover, the fluctuation in the σ–ε curve of the SFCGT before the peak point enhances the K of the specimen.

4.2.1 Theoretical model of SFCGT

The σ–ε curve in the theoretical model of the SFCGT is shown in Figure 14 with a description of the critical parameters, where the relation of the curve is characterized by an analytical formula (Eq. (4)) [42,54]:

where σ (ε) is the stress (strain) of SFCGT; σ _cc and ε _cc denote the strength and corresponding strain of specimen, respectively; E ₁ = E _c, in which E _c is the elastic modulus of plain concrete; and f _o and E ₂ are the parameters calculated using Eqs. (5)–(8):

where t, D, and f are the thickness, diameter, and hoop strength of the GFRP tube, respectively; ε n = n ε o , ε o = f o / E c , and n = 1.

Figure 14

σ–ε curve model and related parameters of SFCGT.

The σ–ε curves provided by the theoretical model are shown in Figure 15, along with the real curves for comparison. The differences are small, and the predictions are highly accurate. However, the decrease in the stress following the initial stage in the SFCGT curve is not accurately characterized by the model because the suggested σ–ε relation was obtained using a design-oriented model. This problem can be solved in future using an analysis-oriented model for the SFCGT. However, the theoretical model used in this study can be adopted to obtain the general properties of SFCGT.

Figure 15

Comparison of the experimental and predicted curves: (a) SFCGT and (b) SFCST.

4.2.2 Theoretical model of SFCST

The σ–ε model of the SFCST was established based on Han et al. [55], Xiao et al. [56], and the test results. The specific expressions are outlined in (Eq. (9a–c)):

where E sc = f scp / ε scp ; a, b, c, and d are the parameters for the model obtained from the study by Han et al. [55]; and ω , ε scp , f scp , and ε scy are the variables obtained from (Eqs. (10)–(13)) based on the related experimental data:

where f _cu and f _c denote the cubic and prismatic compressive strengths of plain concrete, respectively, ξ represents the constraining factor [38,39], f _y is the steel tube yield strength, and γ is the VSCA replacement percentage ( 0 ≤ γ ≤ 100 % ). A comparison between the calculated and experimental relations is given in Figure 15, where the calculations agree reasonably well with the actual results. Therefore, this theoretical model can be adopted for practical design and analysis.

5.2.1 Concrete

The concrete damage plasticity model was adopted in this study, and the theoretical relationship between the SVAC stress–strain curve was proposed based on the study by Huang et al. [6] as follows:

where y = σ / f c and x = ε / ε c ; f _c denotes the SVAC prismatic strength; ε _c is the strain corresponding to f _c (Eq. (14)); and δ and ψ are the critical parameters considering the coupled effects of VSA and SW [6]. Furthermore, the damage variables corresponding to the tension and compression were obtained based on the stress–strain relation of concrete.

5.2.2 Outer tube

For the GFRP tube, the Hashin failure criterion and elastic constitutive model were adopted because of its brittleness. The circumferential elastic modulus and ultimate tensile strain of GFRP tube were 30.3 GPa and 1.83%, respectively.

Furthermore, a nonlinear elasto-plastic model was suggested for stainless steel tubes, and the stress–strain relation of the steel was obtained as follows:

where E s is the elastic modulus; σ 0.2 and E 0.2 denote the nominal yield strength and tangent modulus when the residual strain reaches 0.2% [45]; m and s are the key parameters [58]; and e = σ 0.2 / E 0 ; ε 0.2 = σ 0.2 E s + 0.002 .

5.4.1 Strength and thickness of outer tube

To investigate the variations in the mechanical properties of the specimens with different tube thicknesses and strengths, FE analyses were undertaken and the following research findings were obtained:

The typical influences of the tube strength on the performances of the SFCGT and SFCST are shown in Figure 18. It was observed that the strength and peak strain of specimen increase with increasing tube strength. The strength of the SFCGT adopting GFRP tube with 500 MPa is 31.1% higher than that of specimen using GFRP tube with 400 MPa. This phenomenon is attributed to the confinement between the outer tube and concrete, where the higher the tube strength, the more the passive confining pressure and the greater the strength and peak strain. Generally, the slope of the curve (in the initial stage) of the SFCGT increases, while the curvature of the SFCST curve decreases in the elasto-plastic ascending stage with an increase in the tube strength owing to the improved constraining action of the specimen.

Figure 18

σ–ε curves under different tube strengths: (a) SFCGT and (b) SFCST.

The effects of the tube thickness on the properties of the SFCGT and SFCST were similar to those of the tube strength (Figure 19). Generally, the strength (peak strain) of specimen adopting a 6 mm tube thickness approximately increases by 28.1% (16.2%) compared with that of specimen adopting a 2 mm tube thickness. Therefore, the thicker the tube, the greater the confinement, and the better the strength and deformability. Furthermore, the declining section of the SFCST curve would be significantly improved by increasing the tube thickness, which is attributed to a higher passive confining pressure.

Figure 19

σ–ε curves under different tube thicknesses: (a) SFCGT and (b) SFCST.

5.4.2 Concrete strength

Through the analyses of the SFCGT and SFCST with concrete strengths of C40, C50, and C60 (Figure 20), it was found that a decrease in concrete strength can reduce the strength of the specimen. The strength of specimen using C60 is averagely 20.1% more than that of specimen with C40. However, the effects of the concrete strength on the ε _cc and ductility of the SFCGT and SFCST are negative. It was further observed that the ε _cc of the C60 specimen decreases by 7.8 and 16.1% compared with that of the C50 and C40 specimens, respectively. This is attributed to the inferior deformability of the high-strength concrete.

Figure 20

Typical influence of the concrete strength on the σ–ε curves: (a) SFCGT and (b) SFCST.