Durability of seawater and sea sand concrete and seawater and sea sand concrete–filled fibre-reinforced polymer/stainless steel tubular stub columns

Abstract

This article presents an experimental investigation on the durability behaviour of seawater sea sand concrete and seawater sea sand concrete–filled fibre-reinforced polymer/stainless steel tubular stub columns. Effects of NaCl of seawater on the strength of seawater sea sand concrete and on the deterioration of fibre-reinforced polymer were studied. Accelerated degradation tests were conducted on fibre-reinforced polymer rings exposed to a combined environment of 3.5% NaCl solution and seawater sea sand concrete. Obvious hoop strength reductions were observed in glass fibre-reinforced polymer and basalt fibre-reinforced polymer rings after 6-month exposure at 60°C. Seawater sea sand concrete–filled glass fibre-reinforced polymer tubular stub columns were exposed to an indoor environment (i.e. aged in air at room temperature) for a maximum duration of 2.5 years and no degradation was found by comparing the axial compressive test results from unexposed and exposed specimens. Seawater sea sand concrete–filled stainless steel tubes did not show any deterioration in strength after a 2.5-year exposure to an indoor environment or a 1.5-year immersion in NaCl solution. This study indicated that a hydrothermal environment (e.g. full immersion in solution) is much more aggressive to fibre-reinforced polymer than a dry environment. The reliability of using accelerated degradation test data to estimate the long-term performance of fibre-reinforced polymer–related structures in a real environment may need further research.

Keywords

compressive test durability fibre-reinforced polymer hybrid sections seawater and sea sand concrete

Introduction

Utilizing seawater and un-washed sea sand is a promising way in concrete industry to achieve environmental and economic benefits by avoiding the consumption of freshwater and river sand. However, the chloride ions in seawater and sea sand concrete (SWSSC) cause noticeable corrosion on steel reinforcements (Shalon and Rapheal, 1959). Various techniques have been proposed to resolve this corrosion issue, including the use of corrosion inhibitors, coating, electrochemical techniques and replacing carbon steel by corrosion resistant materials such as stainless steel and fibre-reinforced polymer (FRP) (Goyal et al., 2018). The concept of hybrid sections consisting SWSSC, FRP and/or stainless steel was proposed for new constructions in recent years (Chen et al., 2020; Li et al., 2016; Teng et al., 2011, 2019), which is an appropriate structural form for columns as the concrete strength could be greatly enhanced by the confinement effect provided by encasing tubes. The short-term structural behaviour of SWSSC-filled FRP and stainless steel tubes under axial compression has been extensively studied and the superior structural performance of the hybrid section as compressive members has been widely demonstrated (Chen et al., 2017; Li and Zhao, 2020). Nevertheless, its long-term performance has not been well understood.

Many studies (Katano et al., 2013; Ramaswamy et al., 1982) have observed that a concrete with seawater and/or sea sand exhibited a more rapid compressive strength development during its early age than ordinary concrete. The compressive strength increase could last for 28 days (Islam et al., 2012) to 5 years (Mohammed et al., 2004). A number of studies have been conducted on the effects of seawater and sea sand on concrete durability, such as chloride penetration (Huiguang et al., 2011), carbonization (Liu et al., 2016), freeze–thaw resistance (Yamato et al., 1987) and drying shrinkage (Katano et al., 2013). Due to the complexity of concrete, it is still difficult to reach a solid consensus about the effects of seawater and sea sand on concrete properties (Xiao et al., 2017). However, it is generally agreed that the properties of plain concrete (i.e. without any reinforcement) are not significantly affected by seawater or sea sand and the major concern is chloride-related corrosion on steel reinforcements in concrete.

FRP generally has the features of high corrosion resistance, high strength, low maintenance, anisotropic mechanical properties, transparent to magnetic fields, low elastic modulus, linear-elastic stress–strain response, low ductility and low creep-rupture threshold (Bank, 2006). The durability of FRP has been extensively studied in recent decades, and the external factors affecting the long-term behaviour of FRP include moisture, alkali, acid, salt, temperature, stress, fatigue and ultraviolet radiation (GangaRao et al., 2006; Liu et al., 2020; Raman et al., 2020). The deterioration of FRP results from the degradation of matrix, fibre and the fibre–matrix interphase (Nkurunziza et al., 2005). Since excessively long-term tests can be prohibitive, accelerated degradation tests are widely adopted to assess the durability performance of FRP, in which the material deteriorates at a higher rate using high ageing temperatures. Prediction models, mostly derived from Arrhenius’s law (Litherland et al., 1981) or Fick’s law of diffusion (Katsuki and Uomoto, 1995), have been proposed to correlate the short-term accelerated degradation test data to the long-term performance of FRP. As an other alternative to traditional reinforcement, stainless steels that exhibit a superior corrosion resistance (Flint and Cox, 1988) have been successfully used to reinforce concrete structures (e.g. Broadmeadow Bridge) subjected to harsh environments (BA 84/02, 2002; Cochrane, 2003).

A number of studies have been conducted on the durability of concrete-filled FRP wraps subjected to alkaline/acidic/saltine solutions (Eldridge and Fam, 2014; Kshirsagar et al., 2000; Liu et al., 2002; Micelli and Myers, 2008), ultraviolet radiation, salt fog cycles (Silva, 2007), wet/dry cycles, freeze/thaw cycles (Toutanji and Balaguru, 1998; Toutanji and Deng, 2002) and sustained load (Wang and ElGawady, 2019). Eldridge and Fam (2014) found that the reduction of strengthening ratio (i.e. confined-to-unconfined concrete strength ratio f_cc’/f_c’) of bioregion-glass fibre-reinforced polymer (GFRP) wrapped concrete cylinders was about 25% after 300-day immersion in saltine solution and the effect of solution temperature on the degradation was insignificant. Durability of SWSSC-filled FRP tubes in 40°C saltine solution was investigated by Li et al. (2018b). The averaged reduction of f_cc’/f_c’ after 6-month exposure was 45%, 28% and 44% for SWSSC-filled glass/carbon/basalt FRP tubes, respectively.

Currently, the long-term durability data of FRP or concrete-FRP composites are rather scarce. An 11-year outdoor exposure test on carbon fibre-reinforced polymer (CFRP) plates indicated no clear change in the tensile strength along fibre direction and only small reductions occurred in tensile strength perpendicular to fibre direction and in-plane shear strength (Nishizaki et al., 2005). The relaxation levels of FRP cables after 17-year outdoor exposure were investigated in Sasaki and Nishizaki (2012): 10%∼30% load relaxation was observed for CFRP under preload of 0.8P_u, 10% reduction was observed for GFRP under 0.25P_u preload and GFRP cables ruptured under 0.4P_u preload, where P_u is the ultimate tensile capacity of FRP cables. By examining the microstructure of GFRP bars embedded in concrete for 8 years, Mufti et al. (2007) concluded no degradation. Based on the study of Xie et al. (2018), the strength reduction of CFRP wrapped concrete cylinders subjected to 30-month natural environment exposure was about 10% and most of the reduction occurred within the first 18 months. By comparing the accelerated degradation test data and long-term data, it seems that the accelerated laboratory environment was much harsher than real environments due to unlimited supply of hydroxyl ions and full saturation for conditioned specimens (Mufti et al., 2007).

Although FRP has potential to reinforce SWSSC, confident experimental or field evidence is still missing to demonstrate the superior durability of SWSSC-FRP composites. In the authors’ previous study (Li et al., 2018b), noticeable strength reduction was observed for SWSSC-filled FRP tubes immersed in 40°C saltine solution for 6 months. However, the natural environment is probably not as severe as the accelerated degradation environment as indicated by the limited field data of FRP (Mufti et al., 2007; Nishizaki et al., 2005; Sasaki and Nishizaki, 2012). The long-term performance prediction by accelerated degradation test data is yet to be amply supported by field test demonstration. It is worthwhile to investigate the long-term behaviour of SWSSC-filled FRP/stainless steel tubes subjected to ‘real’ natural environments to promote a reliable application of SWSSC in infrastructure. This article presents an experimental study on the long-term behaviour of SWSSC-filled FRP/stainless steel tubular stub columns subjected to natural indoor environment up to 2.5 years. In order to understand the mechanism of the capacity change of stub columns, the compressive and tensile strength development of plain SWSSC and the degradation of FRP alone were also investigated.

SWSSC

Concrete mixtures

SWSSC investigated in this study was made of alkali-activated slag (binder), seawater, sea sand (collected in Brighton beach, Melbourne), coarse aggregate (∼14 mm) and hydrated lime slurry to improve workability (Table 1). The activator is sodium metasilicate in powder form, which contained 46.6% of SiO₂ and 35.8% of Na₂O. Chemical compositions of slag, sea sand, seawater and sodium metasilicate were determined and reported in Li et al. (2018c). Freshwater and river sand concrete (FWRSC) was also prepared and tested in this study and its composition is shown in Table 1. Before mixing, the solid sodium metasilicate was preblended with slag in dry state. The mixing procedures for SWSSC and FWRSC were the same.

Table 1.

Concrete mixture (kg/m³).

Constituents	SWSSC	FWRSC
Slag	360	360
Seawater	190	N/A
Freshwater	N/A	190
Sea sand	830	N/A
River sand	N/A	830
Coarse aggregate	1130	1130
Sodium metasilicate	38.4	38.4
Hydrated lime slurry	14.4	14.4

SWSSC: seawater and sea sand concrete; FWRSC: freshwater and river sand concrete.

It is known that the grading of aggregate could somewhat affect the workability and strength of concrete (Mehta and Monteiro, 2006). To guarantee the quality of concrete, standards (ASTM C33/C33M, 2018; BS 882, 1992) specify the limits of particle size distribution of aggregates. Figure 1 shows the particle size distribution curves of sea sand collected in Melbourne and Sydney, normal sand available in laboratories as well as the limits in BS 882 (1992). Sand samples obtained from different locations have different particle size distributions but all of them are within the limits. As shown in Figure 1, a considerably less amount of sea sand could pass through the sieve with 0.3 mm aperture indicating sea sand has fewer fine particles. By calculating the fineness modulus of sand, it is clear that sea sand in this study is coarser than river sand available in the laboratories.

Figure 1.

Particle size distribution curves of sea sand.

Mechanical properties

Concrete cylinders (Ф100 mm × 200 mm) and prisms (100 mm × 100 mm × 350 mm) were cast and tested to measure the compressive strength f_c’ (AS 1012.9:2014, 2014), elastic modulus E_c (AS 1012.17-1997 (R2014), 2014) by axial compressive test and flexural tensile strength f_ct,f (AS 1012.11-2000 (R2014), 2014) by four-point bending test, respectively. One day after concrete casting, the specimens were demoulded and sealed with plastic films to avoid moisture evaporation. All specimens were stored at room temperature for 28 days. Thereafter, parts of the specimens were tested to get the reference compressive and tensile strength and other specimens were fully immersed in different solutions at room temperature for durations of 1 month to 1 year. Three identical specimens were tested for each case and the average results were reported in this article. Four combinations of concrete types and solution types were adopted in this study: SWSSC in freshwater, SWSSC in 3.5% saltwater, FWRSC in 3.5% saltwater and SWSSC in seawater.

Elastic modulus (E_c) is an important parameter in theoretical models for predicting the stress–strain response of FRP-confined concrete. The relationship between E_c and f_c’ is presented in Figure 2 for SWSSC tested in this study and from the existing literature as summarized in Table 2 (Chen et al., 2017; Yang et al., 2019; Zeng et al., 2020), in which SW represents seawater and SS refers sea sand. The E_c–f_c’ relationship specified in ACI 318-11 (2011) (i.e. $E_{c} = 4730 \sqrt{f'_{c}}$ in MPa), which applies for normal concrete, is also plotted in Figure 2. As shown in Figure 2, almost all points fall within the 20% error of the ACI curve regardless of concrete types or curing regimes. It could be concluded that the E_c–f_c’ relationship of SWSSC is similar as that of normal concrete and existing standard (e.g. ACI 318) can be applied for SWSSC to estimate E_c.

Figure 2.

Relationship between E_c and f_c’ of concrete.

Table 2.

Details of concrete specimens from the existing literature.

Data source	Binder	Mixing water	Sand	Curing	Quantity^a	Age
Zeng et al. (2020)	OPC	Seawater or simulated seawater	Sea sand	Laboratory environment	2	More than 35-day
Chen et al. (2017)	OPC	Seawater or freshwater	Sea sand or river sand	Spray with water	3	28-day
Yang et al. (2019)	Alkali-activated slag	Seawater or freshwater	Sea sand or river sand	N/A^b	3	28-day
Younis et al. (2018)	Slag/OPC = 1.85	Seawater	Normal sand	Seawater	1	28-, 56-day
Katano et al. (2013)	Slag/OPC = 1	Seawater	Sea sand	N/A	3	28-, 91-day
Wegian (2010)	OPC	Seawater	Normal sand	Seawater	6	28-, 90-day
Xiao et al. (2017)	OPC	Seawater	Sea sand	Seawater	4	28-, 90-, 180-day
Kaushik and Islam (1995)	OPC	Seawater	Normal sand	Seawater	2	1-, 3-, 6-, 12-, 18-month
Ghorab et al. (1989)	OPC or sulphate-resisting cement	Seawater	Normal sand	Seawater	2	12-, 18-month
Mohammed et al. (2004)	OPC or slag/OPC = 0.05~0.7 or fly ash/OPC = 0.1~0.2 or slag cement	Seawater	N/A	Seawater	14	1-, 5-, 10-, 15-, 20-year

OPC: ordinary Portland cement.

Quantity of specimen groups.

Not given in the literature.

Compressive strength (f_c’) and flexural tensile strength (f_ct,f) of concrete at 28-day and 1-year exposure are presented in Figure 3(a), in which the error bar stands for the standard deviation. In generally, the compressive or tensile strength increased by 33% to 56% after 1-year immersion in solutions. SWSSC in saltwater and seawater shows larger compressive strength increase ratio than SWSSC in freshwater and FWRSC in saltwater. It seems that NaCl in concrete and curing water is beneficial to the compressive strength development probably because NaCl improves the pore structure of concrete (Islam et al., 2012). However, the effect of curing solution type on the flexural tensile strength is not obvious. However, the NaCl in mixing water and aggregate could improve the flexural tensile strength as evidenced by a higher f_ct,f of SWSSC than that of FWRSC.

Figure 3.

Strength development of plain concrete in seawater: (a) data of this study, (b) data of literature and (c) data of Mohammed et al. (2004).

Changes in the compressive strength of seawater and/or sea sand concrete from this study and existing literature (Ghorab et al., 1990; Katano et al., 2013; Kaushik and Islam, 1995; Mohammed et al., 2004; Wegian, 2010; Xiao et al., 2017; Younis et al., 2018) are plotted in Figure 3(b) and (c), in which the normalized strength is the ratio of compressive strength after exposure to the 28-day compressive strength (i.e. unexposed strength). Details of the concrete specimens are summarized in Table 2 where specimens were exposed to corrosive environment at ambient temperature. As shown in Figure 3(b) and (c), the strength increase could last for 3 months to 5 years depending on the concrete mixtures (e.g. binder type, water-to-binder ratio). Study of Mohammed et al. (2004) indicated that slag is helpful to improve the durability but the effect of fly ash is insignificant. Among all the types, concrete with alkali-activated slag as the only binder (specimens prepared in this study) shows the maximum strength increase after 1-year exposure. Except for the specimens in Wegian (2010) and parts of the specimens in Mohammed et al. (2004), the compressive strength after exposure is generally higher than the 28-day compressive strength. For concrete-filled FRP/stainless steel tubes, the strength degradation of concrete should not be a big issue since the concrete is further protected by the encasing tubes.

SWSSC-filled FRP tubular stub columns

FRP rings in solutions

Three types of filament wound FRP tubes (i.e. GFRP, CFRP and basalt fibre-reinforced polymer (BFRP)) were investigated in this study. Based on the manufacturer’s data, about 20%, 40% and 40% fibres were oriented in 15°, ±40° and ±75° with respect to the tube axis and the fibre volume ratio is 0.6. FRP fibres in various orientations can provide somewhat longitudinal strength when compared with the case where all the fibres are in the hoop direction. It is not the intention to replace internal longitudinal rebars. The longitudinal strength provided by FRP fibres in various orientations is much less than that achieved by longitudinal rebars. SWSSC-filled double-skin FRP tubes with a length of 250 mm were first prepared (outer tube: 158 mm × 3 mm, inner tube: 100 mm × 3 mm) and cured at ambient temperature for 28 days. Then, concrete-filled GFRP and BFRP tubes were cut into 20 mm wide discs by diamond saws. The width for CFRP discs was restricted to 8 mm due to the limited capacity of test apparatus. Ageing was conducted by fully immersing these specimens in 3.5% NaCl solution or distilled water in a 5-L capacity beaker (Figure 4). The beakers were then placed in a commercial available thermostatic water bath filled with tap water to maintain the target temperature. Three temperatures (room temperature 23°C, 40°C and 60°C) and three immersion durations (30, 90 and 180 days) were chosen to age the specimens. The maximum ageing temperature is much lower than the glass transition temperature of polymer matrix of FRP, and it is believed that the degradation mechanisms are the same for these three temperatures. After each period, three specimens were removed from the beakers and the sandwich concrete was removed. Split-disc test in accordance with ASTM D2290-16 (2016) was then conducted on the aged FRP rings to obtain the mechanical properties in hoop direction (same as the test setup in Li et al. (2016)). Unconditioned FRP rings with the same width as conditioned specimens were also tested to get the reference hoop strength.

Figure 4.

Immersion of FRP specimens in solution.

It is necessary to mention that the alkali ions in concrete could move to the solutions, which leads to an increase of the pH of the solution. The pH of the saltwater, in which the SWSSC-filled double-skin BFRP tubes were immersed, at 1-, 50-, 120- and 180-day is 9.8, 10.1, 11.3 and 12.5, respectively. Due to the influence of concrete, the saltwater and distilled water became alkaline instead of neutral. However, if the specimens are exposed to a real marine environment, then the pH of seawater ranges from 7.5 to 8.4, which is less alkaline than the laboratory environment produced in this study. Therefore, this laboratory study may overestimate the degradation of FRP if applied in a real marine environment.

Hoop strength of unexposed FRP outer rings (158 mm × 3 mm) is 368.8, 548.0 and 328.9 MPa for GFRP, CFRP and BFRP respectively, whereas the hoop strength of the inner rings (100 mm × 3 mm) is 321.3, 442.3 and 319.6 MPa, respectively. Hoop strength retentions of aged FRP rings (i.e. the ratio of the hoop strength of the aged FRP rings to that of the unconditioned FRP rings) are summarized in Figure 5. In general, the strength of FRP decreases with the increase of ageing temperature and ageing time. The degradation process of FRP tends to slow down with the increase of ageing time. No obvious difference is observed in terms of the hoop strength retention of the inner and outer rings. Therefore, the environment combinations do not affect the durability of FRP and the same design theory can be applied to both outer and inner tubes of SWSSC-filled double-skin tubular columns. Based on the strength retention data of GFRP and BFRP at 60°C, distilled water is slightly more aggressive than saltwater. It is probably because the NaCl in saltwater could block the pathway of water molecular movement in FRP to slow down the diffusion process. It can be concluded that NaCl in SWSSC or seawater does not worsen the durability behaviour of FRP and it even has some beneficial effects. Figure 5 shows that CFRP has much superior durability than GFRP or BFRP, which agrees with past studies (Wang et al., 2017). BFRP behaves similar to GFRP at 60°C, but a greater strength loss is observed for BFRP at 23°C and 40°C indicating that BFRP performs worst among the three FRP types.

Figure 5.

Hoop strength degradation of FRP rings: (a) GFRP, (b) CFRP and (c) BFRP.

Stub columns in solutions

In the authors’ previous study (Li et al., 2018b), strength reduction of SWSSC-filled G/C/BFRP tubular columns subjected to 3.5% NaCl solution at 40°C was investigated. After 6 months of exposure, the average hoop strength reduction for columns, which was derived from compressive capacity, reached 35% and 54% for SWSSC-filled GFRP and BFRP columns, respectively, and the reduction of SWSSC-filled CFRP columns was insignificant. It was found that the hoop strength reduction in columns was faster than that in FRP materials. SWSSC-filled GFRP and BFRP columns did not exhibit a superior durability as expected probably due to the high ageing temperature (i.e. 40°C) and full saturation of specimens.

Stub columns in air

This section investigates the long-term behaviour of SWSSC-filled GFRP stub columns exposed to an indoor environment for 1 and 2.5 years. After casting concrete, the stub columns were stored in a dry condition in the laboratory at the room temperature. Mixture of SWSSC was listed in Table 1 and the properties of GFRP tube were the same as those reported in Li et al. (2016) with hoop strength as 308.8 MPa and elastic modulus as 25.2 GPa. Axial compressive test was conducted on the stub columns after ageing and the experimental setup was described in Li et al. (2016).

A total of 16 specimens, including SWSSC fully filled GFRP tubes and SWSSC-filled double-skin tubes (stainless steel as inner and GFRP as outer tubes), were tested in this study. Dimensions of GFRP and stainless steel tubes used in this study are listed in Table 3, where D is the outer diameter and t is the thickness. Hoop strength of unexposed GFRP (f_uh) and yield stress of unexposed stainless steel (f_ys) were obtained by disc-split test and tensile coupon test, respectively, and their values are also listed in Table 3 (Li et al., 2016). Key test results of stub columns are presented in Table 4, where f_c’ is the unconfined concrete strength at test date and N_t is the experimental ultimate load-carrying capacity. The results of unexposed specimens (Li et al., 2016) and specimens immersed in 3.5% NaCl solution (Li et al., 2018b) are also given in Table 4 for a comparison purpose. The label of specimen consists of outer tube name, inner tube name (for double-skin tubes), concrete indicator (‘C’) and exposure time in month (‘0’ stands for unexposed specimen).

Table 3.

Dimensions and properties of GFRP and stainless steel.

Tube name	D (mm)	t (mm)	f_uh^a or f_ys^b (MPa)	Material type
G50	51	3.07	308.8	GFRP
G101	100	3.13	308.8	GFRP
G114	115	3.13	308.8	GFRP
G165	158	3.14	308.8	GFRP
S50	48	2.82	306.8	Stainless steel
S101	102	2.79	324.4	Stainless steel
S114	114	2.79	270.3	Stainless steel
S165	168	3.22	280.1	Stainless steel

GFRP: glass fibre-reinforced polymer.

f_uh = hoop strength of FRP.

f_ys = yield stress (i.e. 0.2% proof stress) of stainless steel.

Table 4.

Specimen table for SWSSC-filled GFRP tubes.

Cross-section	Unexposed^a			12-month in air			30-month in air			6-month in 40°C saltwater^b
Cross-section	Specimen	f_c’ (MPa)	N_t (kN)	Specimen	f_c’ (MPa)	N_t (kN)	Specimen	f_c’ (MPa)	N_t (kN)	Specimen	f_c’ (MPa)	N_t (kN)
Fully filled	G50-C-0	29.8	244	G50-C-12	41.7	272	G50-C-30	42.2	323	G50-C-6	63.9	274
	G101-C-0	29.8	670	G101-C-12	41.7	858	G101-C-30	42.2	878	G101-C-6	63.9	842
	G114-C-0	29.8	814	G114-C-12	41.7	982	G114-C-30	42.2	997	G114-C-6	63.9	842
	G165-C-0	29.8	1338	G165-C-12	41.7	1558	G165-C-30	42.2	1662	G165-C-6	63.9	842
Double-skin	G114-G50-C-0	32.9	797	G114-G50-C-12	38.2	824	G114-G50-C-30	40.2	926	G114-G50-C-6	60.0	870
	G165-G101-C-0	32.9	882	G165-G101-C-12	38.2	1004	G165-G101-C-30	40.2	1015	G165-G101-C-6	60.0	1005
	G114-S50-C-0	32.9	872	G114-S50-C-12	38.2	953	G114-S50-C-30	40.2	1019	G114-S50-C-6	60.0	952
	G165-S101-C-0	32.9	1301	G165-S101-C-12	38.2	1199	G165-S101-C-30	40.2	1223	G165-S101-C-6	60.0	1234

Adapted from Li et al. (2016).

Adapted from Li et al. (2018b).

Both unexposed and exposed specimens failed by the hoop rupture of outer GFRP tubes and the typical failure modes of specimen group G165-C with and without ageing are shown in Figure 6. After a certain time of ageing, the colour of GFRP tube changed from green to slight brown. As shown in Figure 6, colour change of specimens in air is slight but it is more significant for stub columns in solutions due to a higher deterioration of GFRP tube. Load–axial strain curves of fully filled and double-skin tubes are plotted in Figures 7 and 8, respectively, where axial strain is the end shortening divided by column height and the unconfined concrete capacity is equal to unconfined concrete strength multiplied by concrete area. During a loading process, buckling of GFRP tube in longitudinal direction occurred for multiple times accompanied with a slight load drop. The amplitude of load drop for exposed columns is generally lower than that of unexposed columns probably due to a better bonding between GFRP tube and SWSSC for columns with longer curing time. Shapes of load–axial strain curves of unexposed and exposed columns are similar except for specimen G114-C-6 and G165-C-6. Strain hardening of these two specimens was insignificant as the hoop strength of GFRP was greatly degraded due to the harsh environment (i.e. 40°C saltwater). The capacity of columns aged in air (i.e. exposed to in-door environment) is higher than that of unexposed columns due to their higher concrete strength. Ageing time (12 months vs 30 months) does not obviously affect the load–axial strain curves of aged specimens, which indicates a negligible degradation caused by the indoor environment.

Figure 6.

Typical failure modes of SWSSC-filled GFRP tubes (G165-C).

Figure 7.

Load–axial strain curves of SWSSC fully filled GFRP tubes.

Figure 8.

Load–axial strain curves of SWSSC-filled double-skin tubes (GFRP as outer and SS as inner tubes).

As discussed in section ‘Mechanical properties’ and evidenced in Table 4, concrete did not deteriorate and even showed a strength gain within the ageing period. In order to exclude the effect of concrete strength change on assessing the durability performance, rupture stress of outer GFRP tube in stub columns (σ_uh) was derived from the experimental ultimate capacity (N_t)

σ_{uh} = \frac{f_{l} (D_{o} - 2 t_{o})}{2 t_{o}}

(1)

f_{l} = \frac{f'_{cc} - f'_{c}}{3.5}

(2)

f'_{cc} = \frac{N_{t} - f_{ys} A_{s}}{A_{c}}

(3)

where f_l is the confining pressure, D_o is the outer diameter of outer tube, t_o is the thickness of outer tube, f_cc’ is the confined concrete strength, f_c’ is the unconfined concrete strength, A_c is the concrete area, f_ys is the yield stress of stainless steel and A_s is the area of stainless steel tube. Equation (2) is a commonly used equation to specify the strength enhancement of concrete due to confining pressure provided by FRP wrap/tube (Li et al., 2018a; Teng et al., 2007). Equation (3) is based on the assumption that at ultimate condition, load carried by GFRP tube is negligible and load resisted by stainless steel tube is its yield capacity. If there is no stainless steel tube in columns, then the term f_ysA_s in equation (3) is zero.

σ_uh of stub columns and the exposed-to-unexposed ratio of σ_uh are presented in Figure 9, where each marker in the upper graph represents a specimen and the bar value in the lower graph is an average ratio of a group of specimens. As shown in Figure 9, specimens aged in air do not show any reduction of rupture stress indicating that no degradation occurred in stub columns. There is even a slight increase of σ_uh of aged specimens, which is likely caused by a further curing process of GFRP. However, obvious degradation is found for specimens exposed to 40°C saltwater water (Li et al., 2018b). It is clear that a wet (hydrothermal) environment is much more aggressive than a dry environment.

Figure 9.

Ultimate hoop stress of SWSSC-filled GFRP tubes.

Based on the accelerated degradation test data in section ‘FRP rings in solutions’, hoop strength of GFRP at room temperature could be predicted by Arrhenius’s law (Litherland et al., 1981). Following the procedures documented in Bank et al. (2003), strength retention of GFRP at 23°C could be estimated by

Y = - 24.8 \lg (t_{ageing}) + 142.8

(4)

where Y is the strength retention in percentage and t_ageing is the ageing time in day. After 2.5 years of ageing, the predicted strength reduction of GFRP is 30.6%. However, as shown in Figure 9, no degradation was observed from experimental results. It seems that the existing degradation model is too conservative when predicting the actual long-term performance of FRP in a dry environment. The unreliability is mainly caused by two reasons: (a) current prediction models have not been validated on the basis of demonstrations from field test data and (b) accelerated degradation test generally uses a solution environment for an easy control of temperatures but it cannot represent the real environment that FRP is subjected to (e.g. ageing in air in this study). Therefore, more studies are needed in this research field to reasonably assess the long-term performance of FRP-related structures.

Besides rupture stress, another indicator of the durability of concrete-filled FRP tubes is strengthening ratio (f_cc’/f_c’). The exposed-to-unexposed ratio of f_cc’/f_c’ was 0.85 and 0.90 for fully filled and double-skin columns aged in air for 1 year, whereas the ratio was 0.90 and 0.87 when they were aged in air for 2.5 years. There is an obvious reduction of strengthening ratios, which seems contradictory to the observed trend of rupture stresses (Figure 9). It is because of the fact that a significant increase of f_c’ of aged concrete could lead to a decrease of f_cc’/f_c’ although hoop strength of FRP does not degrade (see equations (1) and (2)). Therefore, when using strengthening ratio to assess the durability of concrete-filled FRP tubes, the influence of unconfined concrete strength should be carefully examined. Otherwise, the conclusion drawn from f_cc’/f_c’ may not be correct.

SWSSC-filled stainless steel tubular stub columns

Stainless steel coupons in solutions

In order to assess the durability of stainless steel (AISI 316 grade in this study), tensile coupon tests were conducted on unexposed and exposed stainless steel coupons cut for tube S165. Edges and surfaces of coupons were not treated such as polishing or sealing. Exposed coupons were fully immersed in 3.5% NaCl solution at temperatures of 23°C, 40°C and 60°C for durations of 1, 3 and 6 months. After exposure, surface of coupons did not show any changes. As expected, no change of strength (i.e. yield stress and ultimate strength) was observed for all the stainless steel coupons.

Stub columns in air and solutions

Three groups of SWSSC fully filled stainless steel tubular stub columns, which were aged in air for 12 and 30 months, and in 3.5% NaCl solution for 18 months (6 months at 40°C and followed by 12 months at 23°C) respectively, were studied for their durability. Details of the specimens are listed in Table 5, in which f_c’ is the unconfined concrete strength at test date and N_t is the experimental ultimate capacity (defined as the maximum load within 0.05 axial strain). Dimensions of stainless steel tube and its yield stress (without exposure) are reported in Table 3. After each duration, axial compressive test was carried out and the test setup is same as that for SWSSC-filled GFRP tubes.

Table 5.

Specimen table for SWSSC-filled SS tubes.

Cross-section	Unexposed^a			12-month in air			30-month in air			18-month in saltwater
Cross-section	Specimen	f_c’ (MPa)	N_t (kN)	Specimen	f_c’ (MPa)	N_t (kN)	Specimen	f_c’ (MPa)	N_t (kN)	Specimen	f_c’ (MPa)	N_t (kN)
Fully filled	S101-C-0	31.4	729	S101-C-12	41.7	797	S101-C-30	42.2	858	S101-C-18	56.8	915
	S114-C-0	31.4	800	S114-C-12	41.7	926	S114-C-30	42.2	967	S114-C-18	56.8	1055
	S165-C-0	31.4	1522	S165-C-12	41.7	1832	S165-C-30	42.2	1917	S165-C-18	56.8	2161

Adapted from Li et al. (2016).

SWSSC-filled stainless steel tubes before and after ageing are shown in Figure 10 and the stainless steel tube surface did not change obviously after exposure in air or solutions. Failure modes of the unexposed and exposed stub columns are the same. Load–axial strain curves of the stub columns are plotted in Figure 11 including both unexposed and exposed specimens. Exposed specimens sustained a higher load than unexposed specimens due to the increase of unconfined concrete strength after ageing. An obvious descending branch of load–axial strain curves is observed for columns in saltwater and it is also caused by the higher f_c’ of 56.8 MPa. Based on previous studies (Li et al., 2019), concrete-filled stainless steel tubes with higher concrete strength or lower confining pressure exhibited a more distinct descending post-peak curve.

Figure 10.

Typical failure modes of SWSSC-filled stainless steel tubes (S165-C).

Figure 11.

Load–axial strain curves of SWSSC fully filled SS tubes.

Due to the strength development of concrete, effect of environments on the durability behaviour of SWSSC-filled stainless steel columns cannot be clearly identified from the load–axial strain curves. Yield stress of stainless steel tube, which is derived from the ultimate capacity, is selected as the durability indicator for stub columns. In Li et al. (2018d), the ultimate compressive capacity of concrete-filled SS tubes can be estimated by

N_{u} = (A_{c} + A_{s}) f_{scy}

(5)

f_{scy} = (1 + 1.41 ξ) f_{c}'

(6)

ξ = \frac{f_{ys} A_{o}}{f_{c}' A_{c}}

(7)

where N_u is the predicted ultimate capacity, A_c is the concrete area, A_s is the stainless steel tube area, f_scy is the nominal yield strength of composite section, ξ is the confinement factor, f_ys is the yield stress of stainless steel and f_c’ is the unconfined concrete strength. If N_u is set as the experimental capacity N_t, then the yield stress of stainless steel tube in a column could be ‘back calculated’. This yield stress is denoted as σ_y to highlight that f_ys is an intrinsic property of stainless steel but σ_y is the derived yield stress in stainless steel tube at the ultimate condition of a column. σ_y of unexposed and exposed columns and the averaged exposed-to-unexposed ratio of σ_y are presented in Figure 12. Stub column dose not degrade after exposure as evidenced by the bar graph of exposed-to-unexposed ratio of σ_y in Figure 12. It can be concluded that SWSSC-filled stainless steel tubes does not deteriorate after 30-month ageing in air or 18-month ageing in saltwater.

Figure 12.

Yield stress of SWSSC fully filled SS tubes.

Conclusion

The study presented herein intended to investigate the effects of ageing on the mechanical properties of SWSSC and SWSSC-filled FRP/stainless steel tubular stub columns. The following conclusions could be drawn:

Sodium chloride in SWSSC and curing water was beneficial for the strength development of plain concrete (i.e. without any reinforcement). Relationship between elastic modulus and concrete strength of normal concrete (e.g. ACI 318) could be applied for SWSSC.

Durability of FRP in terms of hoop strength followed the order of CFRP > GFRP > BFRP. Environment combination with inner SWSSC and outer solution affected the durability of FRP to the same extent as the combination with inner solution and outer SWSSC. As compared to distilled water, NaCl in seawater does not worsen the durability of FRP.

SWSSC-filled GFRP tubes did not show obvious strength degradation after 2.5 years of ageing in air. A wet (hydrothermal) environment is much more aggressive for FRP than a dry environment. The existing degradation model was too conservative when it was used to predict the long-term performance of FRP in a dry environment.

After 2.5-year ageing in air or 1.5-year exposure to saltwater, SWSSC-filled stainless steel tubes exhibited no capacity reduction.

Footnotes

Acknowledgements

The authors thank the support from the Bayside City Council for collection of sea sand and seawater. Laboratory staffs, Mr Long Goh, Mr Saravanan Mani and Mr Jeff Doddrell, at Monash University are also acknowledged for their assistance.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship and/or publication of this article: The financial support provided by the ARC Discovery Grant (DP160100739) from the Australian Research Council (ARC) is greatly appreciated.

ORCID iD

Xiao-Ling Zhao

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