1 Introduction

Concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) column with properties of higher bearing capacity, better ductility and corrosion resistance performance has attracted significant research attention in the field of civil engineering applications in recent years [1,2,3,4,5,6,7]. The CFFT column was proposed by Mirmiran and Shahawy [8] using FRP to enclose concrete piers to address the corrosion problem of bridge systems. Vincent and Ozbakkaloglu [9] investigated the effect of fiber angle on the axial compression behavior of the CFFT columns. Results showed that when fibers are arranged in a hoop direction, the CFFT column showed higher strength and ductility. Khan et al. [10] investigated the effect of the thickness of both carbon fiber reinforced polymer (CFRP) tubes and GFRP tubes on the axial compression behavior of CFFT columns and found that the axial stress and axial strain of CFRP-CFFT columns and GFRP-CFFT columns increased significantly with the thickness of FRP tubes, and observed a similar pattern of brittle damage in both types of columns. Ozbakkaloglu [11] tested the effect of type and manufacture method of FRP tube on compression performance of CFFT columns and found that the mechanical properties of the fibers significantly influenced the compression properties of CFFT column. In contrast, the influence of the wrapping technology of the FRP tube was not very significant. The high strength, ductility, fire resistance, and reduced weight make the CFFT columns more competitive as structural elements.

Teng et al. [12] proposed a novel FRP-concrete-steel hybrid double-skin tubular columns (DSTC), consisting of an internal steel tube and outer FRP tube with infilled concrete. The combination of the three different materials showed superior performance compared to traditional CFFT columns [13,14,15,16,17,18,19]. Fanggi and Ozbakkaloglu [20] studied the compressive behavior of DSTC under concentric compression. Results showed that FRP and steel tubes provided effective confinement to the concrete and exhibited high ductile compressive behavior. Han et al. [21] discovered that the increase of FRP layers could improve the ultimate strength of DSTC in constant axial load and cyclically increasing flexural loading tests but had no significant effect on ductility and energy dissipation. Idris and Ozbakkaloglu [22] conducted seismic performance tests on DSTC and found that filling the inner steel tube with concrete increased the lateral deformation capacity of the DSTC. In general, DSTC can further improve the structural performance compared to CFFT columns. However, the internal steel tube of DSTC is prone to local inward buckling and corrosion, reducing the structure's bearing capacity to a certain extent [16, 23, 24].

Timber, a natural and sustainable building material with important advantages such as lightweight, green, energy efficiency, low-carbon environmental protection, and renewable, is increasingly widely employed in civil engineering applications [25,26,27]. El-Salakawy and Gamal [28] conducted axial compression performance tests on timber concrete composite columns and found that the energy dissipation capacity of timber concrete composite columns was significantly enhanced compared to that of normal concrete columns. Nabati et al. [29] investigated the axial compression behavior of CFRP-reinforced concrete-filled steel tubes with timber core and found that using timber and CFRP can improve axial ductility, energy absorption and reduce the weight of components. In addition, CFRP can compensate for the loss of stiffness due to the incorporation of timber. Compared with CFRP material, GFRP has a low modulus of elasticity, higher elongation, better resin permeability, is more economical, and is widely used in civil engineering applications.

In order to make full use of the properties of GFRP, timber and concrete materials, this research proposed a novel composite column-GFRP-timber-reinforced concrete composite column (GTRC column) (Fig. 1), with a purpose to provide reference and ideas for structural design in the field of civil engineering infrastructures. The GFRP materials are wrapped outside the specimen, the timber core and steel bar is placed at the center, and the concrete material is poured between the GFRP and the timber core. Among them, the role of the steel bar is mainly to increase the compressive strength of this composite column. Thirteen specimens were tested to validate the effects of various timber core diameters, slenderness ratios, and GFRP laminate layers/angles on the mechanical properties of the composite columns according to axial compression tests. Moreover, a theoretical analysis formula is proposed for predicting the axial bearing capacity of the GTRC columns.

Fig. 1
figure 1

Typical GFRP-timber-reinforced concrete composite column

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2 Experimental program

2.1 Specimen design

To investigate the effect of various parameters on the compressive performance of composite columns, thirteen specimens were developed, fabricated, and tested using axial compression tests. The timber cross-section diameters (0, 30 mm, 40 mm, 50 mm, and 60 mm), GFRP laminate layers (0, 1, 2 and 3), lay-up/wrap-up angles (45°, 60°, and 90°), and the slenderness ratios (2.4, 4.0, and 5.6) are the key design characteristics of specimens. The specimens' details parameters are shown in Fig. 2 and Table 1. It should be noted that the specimen G0-T90-300 refers to the exterior surface of the specimen without the GFRP cloth wrapped around it, and the specimen G2-90-T0-300 refers to the internal surface of the specimen without the timber core.

Table 1 Specimen number and specific parameters
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Fig. 2
figure 2

Cross-section of different specimens

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2.2 Materials

The concrete grade utilized in the examples is C30, which comprises P.O 42.5 ordinary Portland cement and river sand with a fineness modulus of 2.9. The nominal maximum coarse aggregate size was 10 mm due to the smaller cross-section of the specimens. Table 2 shows the mix proportion design. According to ASTM C39 [30], the average compressive strength and elastic modulus of concrete test specimens were 30.1 MPa and 29.8 GPa, respectively.

Table 2 Concrete mix proportion
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Two groups of GFRP laminates bonded to epoxy resin with GFRP cloth (nominal thickness of 0.9 mm for each unidirectional layer of GFRP cloth) were tested for modulus of elasticity and ultimate tensile strength according to ASTM D3039 [31]. Six timber specimens were tested in compression according to ASTM-D143-14 [32], and the average compressive strength and modulus of elasticity were 25.1 MPa and 4.0 GPa, respectively. In this paper, the hoop reinforcement is made of thin wires, which only plays a certain supporting role and has no great influence on the bearing capacity of the specimen, so this paper only considers the influence of longitudinal steel bars on the composite column, and the tensile strength of steel bar with a diameter of 6 mm were obtained using ASTM A370-17a [33]. The above material properties are indicated in Table 3.

Table 3 Material properties
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2.3 Specimen preparation

The construction process of GTRC columns is shown in Fig. 3. Before casting the concrete, a formwork consisting of a PVC pipe, bottom plate, and triangle support was created, and steel bars and timber core were placed within the PVC tube (Fig. 3(a)). After the hardening of concrete, the specimens were cured for 28 days under conventional conditions. The specimens were then wrapped in GFRP, and the upper and lower ends of the specimens were strengthened using CFRP. The fiber cloth was given a 120 mm overlap to avoid early debonding (Fig. 3(b)). Finally, the ends of the specimens were smoothened using high-strength lime to have better contact between the samples and platens, and strain gauges were attached to the surface of the GFRP (Fig. 3(c)).

Fig. 3
figure 3

The fabrication process of a typical specimen

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2.4 Test setup and instruments

Compressive tests were performed utilizing a 5000 kN electrohydraulic servo pressure testing system in the Composite Material Structure Laboratory of Nanjing Tech University of Technology (Fig. 4(a)). Two linear variable differential transformers (LVDTs) were symmetrically installed at the loading plate of the compression test equipment to record axial deflections of the specimens. In the middle of the specimen, eight strain gauges were placed on the GFRP to quantify longitudinal and directional hoop strains (Fig. 4(b)). Before loading, the specimen was preloaded to ensure a tight fit between the top and lower plates of the testing equipment. The specimen was loaded at a rate of 3 mm/min until the specimen failed. Meanwhile, the test specimens' loads, stresses, and displacements were recorded using a static data acquisition system.

Fig. 4
figure 4

Loading device schematic illustrations and arrangement of measuring points

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3 Test results and discussion

3.1 Failure modes

Figure 5 depicts the various typical failure progress of specimens. The failure modes can be divided into three categories: (1) concrete crushed and completely separated from the timber core, which occurred in specimen G0-T50-300 without GFRP wrapping. (2) concrete crushed, GFRP ruptured, and axial compression failure, which occurred in GTRC column specimens with heights of 300 mm (specimens with wrapping GFRP and placing the timber core); (3) local buckling failure, which occurred in specimens G2-90-T50-500 and G2-90-T50-700 with the larger slenderness ratios.

Fig. 5
figure 5

Typical failure progress of specimens

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The failure mechanism can be illustrated as: (1) Specimen G0-T50-300 without GFRP constraint outside, with a weak bond between the timber core and the concrete, resulting in complete separation of the concrete matrix and the timber core during the compression process; (2) the failure process and morphology of GTRC column specimens with a height of 300 mm are similar during axial compression. At the initial loading stage, the concrete and GFRP were both subjected to axial compression, and the specimen was at the elastic stage, suffering slight deformation. With the continuous increase in the load, concrete cracked, and the restraint of GFRP was affected. When the load increases to peak load, the GFRP tensile strain exceeds its maximum value, the GFRP ruptures under hoop tension, and axial compression failure occurs. It is noted that the rupture location of the GFRP is mainly at the middle, reinforced end or overlap of GFRP, which forms along the laminated fiber direction; (3) damage to the concrete was seen to be concentrated within the middle region of specimens G2-90-T50-500 and G2-90-T50-700 due to increase of load, local buckling deformations were found on the middle region of specimens with large slenderness ratios.

3.2 Axial load–deflection behavior

The load–deflection curves of specimens with various parameters are shown in Fig. 6. As expected, specimens with a larger timber diameter or fewer GFRP laminates layers showed a lower peak load than those with a smaller diameter of timber or more GFRP laminates layers (Fig. 6(a) and (b)). A substantial load reduction is observed when the load–deflection curve reaches its peak value.

Fig. 6
figure 6

Comparison of load–deflection curves of specimens with different parameters

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All load–deflection curves were fairly similar at the start, and the displacement increased slowly as the load increased, showing that the specimens were in the elastic stage at the time. The nonlinear behavior of the initial phase of the curve is due to the lack of complete contact between the specimen and the loading device. Therefore, this part of the curve is ignored in the calculation of the structural stiffness. After increasing the yield load, some white stripes can be visible on the GFRP's outer face, the displacement growth rate is greater than the load growth rate (nonlinearity occurs), and the state where the specimens change from elastic stage to elastic–plastic stage. When the load hits its apex, the GFRP ruptures in a random position of white stripes, and the specimen's bearing capacity rapidly decreases until it is destroyed.

3.3 Axial load-strain behavior

Under axial compression force, each specimen’s variation curves of axial and hoop strain are shown in Fig. 7. The axial and hoop strains reported in Fig. 7 were averaged from four strain gauges pasted outside the GFRP surface. The load-strain curves of GTRC columns all followed a similar pattern. The load-strain curves of specimens wrapped in GFRP cloth had an obvious inflection point, which is a typical bilinear curve (Fig. 7(a)-(d)), but the load-strain curve of specimen G0-T50-300 without wrapping GFRP showed a clear monotonic linear relationship (Fig. 7(b)). The data revealed that the hoop and axial strain of GTRC columns had an obvious linear connection with the load at the initial loading stage, with the hoop strain growing slower than the axial strain. At this time, there is no apparent interaction between concrete and GFRP, and the specimen was under uniaxial compression at the elastic stage. Because of the restraint property of GFRP on concrete, the growth rate of hoop strain increased with the load, and the specimen followed multi-axial constraint under the elastic–plastic stage. Meanwhile, the density of cracks within the specimen grew along with the energy. When the GFRP reached its maximum tensile strength, there was a quick release of energy inside the specimen, and the specimen failed instantaneously. The material’s considerable hoop and axial deformation caused the strain gauges to debond or fail before the test was completed.

Fig. 7
figure 7

Load-strain curves of specimens with different parameters

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4 Discussion

Table 4 shows the main test results of the GTRC columns, where K is the axial stiffness of the specimen, P0.75 is the load value when the ultimate bearing capacity of the specimen reaches the 75% of peak load (Pu), a turning point where the slope of the load–deflection curve begins to decrease, and P0.85 is the post-peak-load drops to 85% of the peak-load (Pu), δu, δ0.75 and δ0.85 are the corresponding displacements of Pu, P0.75 and P0.85, respectively.

Table 4 The test results of different specimens
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4.1 Ultimate bearing capacity analysis

Figure 8 shows the comparison of the ultimate load-bearing capacity of specimens with different parameters obtained from the test results in Table 4, in which Pu is the ultimate load-bearing capacity and Pu/W is the load-to-weight ratio of specimens.

Fig. 8
figure 8

The influence of different parameters on the ultimate bearing capacity of the specimen

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With the increase in the timber core diameters, specimens' ultimate load-bearing capacity decreases. Compared to the specimen G2-90-T0-300 without the timber core, the ultimate load-bearing capacity of specimens with timber core diameters from 30 to 60 mm was reduced by 12.1% and 40.4%, respectively, but the load-to-weight ratio was reduced by 8.8% and 25.1%, respectively (Fig. 8(a)). The ultimate load-bearing capacity of the specimen G0-T50-300 with increasing GFRP layers from 1 to 4 increased by 91.0% and 212.1%, respectively, than the specimen G0-T50-300 without GFRP outside the specimen (Fig. 8(b)). The findings illustrate that replacing concrete with timber core decreased the specimen capacity to a certain extent, but the decreased bearing capacity due to reduced weight was compensated by the GFRP wrapped outside specimen.

The results also showed that the specimens' ultimate load-bearing capacity increased with the increase in the GFRP layering angles. Specimen G2-90-T50-300 has a nearly two-fold higher ultimate load-bearing capacity than specimen G2-45-T50-300 (Fig. 8(c)). The ultimate load-bearing capacity of specimen G2-90-T50-300 (slenderness ratio: 2.4) is higher than that of specimens G2-90-T5-500 (slenderness ratio: 4.0) and G2-90-T5-700 (slenderness ratio: 5.6), as shown in Fig. 8(d), but the latter exhibited nearly identical ultimate load-bearing capacities of 770.8 kN and 772.2 kN, respectively (Fig. 8(d)). When comparing the load–deflection curves of both specimens, it can be seen that the G2-90-T50-500 entered the elastic–plastic stage when the load reached around 400 kN, whereas the G2-90-T50-700 did not enter the elastic–plastic stage until the load reached 500 kN. Furthermore, during the test, the buckling of specimen G2-90-T50-500 was more noticeable due to the unequal internal pores of the concrete. As a result, the specimen G2-90-T50-500's low ultimate load-bearing capacity is attributable to premature buckling of the specimen, resulting in GFRP fracture.

4.2 Stiffness analysis

Figure 9 shows the comparison of the axial stiffness of specimens with different parameters obtained from the test results in Table 4, where K is axial stiffness, and K/W is the stiffness to weight ratio of specimens.

Fig. 9
figure 9

The influence of different parameters on the axial stiffness of the specimen

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Compared to specimen G2-90-T0-300, when timber core diameters increased from 30 to 60 mm, the axial stiffness of specimens decreased by 14.0% and 34.7%, respectively, and the stiffness to weight ratio was reduced by 10.8% and 17.8%, respectively. The stiffness of specimen G2-90-T40-300, on the other hand, revealed a noticeable anomalous drop, which was mostly attributed to the loss of epoxy resin colloid at the GFRP cloth overlap and insufficient bonding force at the overlapping, resulting in the specimen peeling pre-maturely (Fig. 9(a)). The axial stiffness of the specimens with wrapped GFRP increased by 17.0% and 26.6%, respectively, compared to those without wrapping GFRP materials. However, the axial stiffness of the specimens with wrapping GFRP appears to be slightly impacted by the GFRP laminates layers (Fig. 9(b)). These findings showed that the wrapped GFRP compensates for the loss of stiffness caused by the positioning of the timber core hence improving the axial stiffness properly. The maximum stiffness was attained when the GFRP lay-up angle was 90 degrees (Fig. 9(c)). The axial stiffness increases when the GFRP lay-up angle is increased. The maximum axial stiffness was obtained when the slenderness ratio was 2.4 (specimen G2-90-T50-300), while the lowest stiffness was obtained when the slenderness ratio was 4.0 (specimen G2-90-T50-500), which is caused by premature buckling of the specimen (Fig. 9(d)).

4.3 Ductility analysis

The ductility index (DI) has been proposed to evaluate the effect of the different parameters on the axial ductility of specimens, which can be expressed as: [29]

$$DI=\frac{{\delta }_{0.85}}{{1.33\times \delta }_{0.75}}$$
(1)

Here δ0.85 is the axial displacement when the post-peak-load drops to 85% of the peak load (P0.85), and δ0.75 is the axial displacement when the load reaches 75% of the pre-peak load (P0.75). Figure 10 shows the comparison of ductility of specimens under different parameters.

Fig. 10
figure 10

The influence of different parameters on the ductility indexes of the specimen

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The DI of G2-90-T60-300 specimens was found to be the largest, reaching 1.83, and a noticeable increase in DI for specimens with the timber core was exhibited compared to specimen G2-90-T0-300. The DI of the specimens increased by 10.2% and 52.3%, respectively, as the diameter of the timber core increased from 30 to 60 mm (Fig. 10(a)). The experimental analysis discovered that most of the concrete inside the specimen was crushed, while the timber core was only cracked in a few places or was not broken at all. The DI of specimens with wrapped GFRP increased by 48.8% and 64.3%, respectively, compared to specimens without wrapped GFRP (Fig. 10(b)). The DI of specimen G2-60-T50-300 is the highest, with a value of 2.10, while the DI of specimen G2-90-T50-300 is the lowest, with a value of 1.56 (Fig. 10(c)). This can be interpreted as the inside concrete specimens with small layer-up angles are not completely cracked when the load reaches the peak load and can still bear a certain load, resulting in a higher value of δ0.85, and the effect is more noticeable with smaller GFRP layer-up angles, which can clearly explain the increase in ductility. The value of DI increased as the slenderness ratios increased for specimens with varying slenderness ratios. This can be attributed to the GFRP rupture in specimens G2-90-T50-500 and G2-90-T50-700 produced by local buckling of the specimens, and the rupture process was significantly slower than that of specimen G2-90-T50-300 caused by hoop stress. As a result, the results show that putting the timber core and by wrapping the GFRP can improve the post-yield deformation behavior of the GTRC columns.

4.4 Energy dissipation capability analysis

The transition area (TA) has been proposed to evaluate the energy dissipation capability of specimens [29]:

$$TA={S}_{ABUCD}$$
(2)

Here, SABUCD is the area under the load–deflection curve (Fig. 11). The comparisons of the energy dissipation capability of specimens under different parameters are shown in Fig. 12.

Fig. 11
figure 11

Schematic definition of TA

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Fig. 12
figure 12

The influence of different parameters on the energy dissipation capability of the specimen

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With increased timber core diameters, the energy dissipation capability of specimens increases. Compared to specimens without a timber core, the TA increased by 11.2% and 21.6% when the diameter of the timber core increased from 30 to 60 mm (Fig. 12(a)). When GFRP laminate layers are increased from 1 to 4, the TA of specimens increases by 800% and 3820% compared to specimens without wrapped GFRP (G0-T50-300). The results show that the GFRP laminate layers significantly impact the energy dissipation capability (Fig. 12(b)). The energy dissipation capacity of specimens G2-60-T50-300 and G2-90-T50-300 increased by 93.3% and 166.2%, respectively, compared to specimen G2-45-T50-300 (Fig. 12(c)). Furthermore, the longer elastic–plastic section of specimens G2-90-T50-500 and G2-90-T50-700 is why the energy dissipation capability increases with the slenderness ratio (Fig. 12(d)). As a result, the findings suggest that the timber core diameters and GFRP laminate layers significantly impact the energy dissipation capabilities of GTRC columns. By placing timber inside the specimen and GFRP wrapped outside the specimen can improve the seismic performance of GTRC columns.

5 Theoretical predictions and proposed equation

The stress state of the cross-section of the GTRC columns is shown in Fig. 13. Based on the principle of force equilibrium and superposition, as shown in Fig. 13(a) and (d), the axial compressive strength of GTRC columns can be given as follows:

$${N}_{u}={N}_{c}+{N}_{t}+{N}_{s}$$
(3)
Fig. 13
figure 13

The stress state of the cross-section of the GFRP-timber-reinforced concrete composite column

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Where Nu is the axial compressive strength of GTRC columns, Nc, Nt and Ns are the axial compressive strengths of the concrete, timber and steel bars, respectively. In the theoretical model of this paper, the axial stress of GFRP is neglected and only the tensile stress is considered. Using the relationships between stress and strength, Eq. (3) can be expressed as follows:

$${N}_{u}={\sigma }_{cz}{A}_{c}+{\sigma }_{tz}{A}_{t}+{\sigma }_{sz}{A}_{s}$$
(4)

Where σcz is the strength of the confined concrete; σtz is the compressive strength of the paulownia timber; σsz is the yield strength of steel bar; Ac, At and As are the section areas of the concrete, timber and steel bars, respectively.

The strength of confined concrete σcz is expressed by the compressive strength of the unconfined concrete (fco) as follows:

$${\sigma }_{cz}={f}_{co}+k\left({\sigma }_{fr}\right.+\left.{\sigma }_{tr}\right)$$
(5)

Where k is the confinement coefficient, equal to 3.3 [34]; σfr and σtr are lateral confining pressure imposed by the GFRP cloth and timber core which can be assumed to be uniformly distributed around the circumference (Fig. 13(f) and (j)), respectively; Since the timber core had an insignificant effect on the constraint of the internal concrete, the lateral confining pressure σtr is considered to be zero.

According to Fig. 13(f) and (j), the equilibrium of σfr and σfθ is given as follows:

$$\sigma_{fr} { = }\frac{{2\sigma_{f\theta } t_{f} }}{{(D - 2t_{f} )}} = \frac{{2\sigma_{f\theta } t_{f} }}{{D_{c} }}$$
(6)

Where σfθ, D, and tf are the hoop tensile stress, diameter and thickness of the GFRP cloth, respectively. Considering the effect of timber core diameters, GFRP laminates layers reduction factor for timber core diameters (α) (Eqs. (7) and (8)) and reduction coefficient of constraint effect for multi-layer GFRP cloth (nf) (Eq. (9)) which are inspired by Nabati et al. [29] are developed to be used for GTRC column. Equation (4) can be rewritten as Eq. (10):

$$\left\{ {\begin{array}{*{20}c} {\alpha { = }1 {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} \mu \le 0.25}{\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} \\ {\alpha { = }1.05 - 0.6\mu {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} \mu \ge 0.25}{\kern 1pt} {\kern 1pt} \\ \end{array} } \right\}$$
(7)
$$\mu = D_{t} /D_{c}$$
(8)
$$\left\{ \begin{gathered} n_{f} = {1 }\, {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} n = {1} \hfill \\ n_{f} = 1 - 0.05 \times (n - 1) \, {\kern 1pt} {\kern 1pt}{\kern 1pt} {\kern 1pt} {\kern 1pt}{\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt}{\kern 1pt} {\kern 1pt} n > 1 \hfill \\ \end{gathered} \right\}$$
(9)
$$N_{u} = \alpha A_{c} (f_{co} + 3.3n_{f} \frac{{2\sigma_{f\theta } t_{f} }}{{D_{c} }}) + \sigma_{tz} A_{t} + \sigma_{sz} A_{s}$$
(10)

A comparison of experimental and theoretical data is shown in Table 5. The average theoretical to experimental result ratio is calculated to be 0.92. Equation (10) can accurately forecast the final axial capacity of the GTRC columns in this work, as observed. It should be noted that Eq. (10) is only applicable to circular GTRC columns with a slenderness ratio of less than 2.5 and a GFRP layer-up angle of 90°.

Table 5 Comparison between theoretical and experimental results
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6 Conclusions

This research proposed a new GFRP-timber-reinforced concrete composite column (GTRC column) made of GFRP, paulownia timber, concrete, and steel bars. The axial compression test and analysis were conducted to evaluate the compression behavior of GTRC columns. The following conclusions can be summarized:

  1. 1.

    The results indicated that the composite column's ductility and energy dissipation capability increased as the timber core diameter increased, with a maximum value of 52.3% and 21.6%, respectively. The effect of the timber core significantly reduced the weight, although the ultimate load-bearing capacity and initial stiffness showed a slight decrease as the timber core diameter increased.

  2. 2.

    The GFRP laminates considerably improved the ultimate load-bearing capacity, stiffness, ductility and energy dissipation capability by 212.1%, 26.6%, 64.3% and 3820%, respectively. Wrapping GFRP around the columns compensated for the loss of ultimate load and stiffness due to the timber core.

  3. 3.

    From the observation of the test results, the failure patterns are divided into three categories: concrete crushed and completely separated from the timber core, which occurred in the specimen without GFRP wrapping on the outside; crushed concrete, GFRP rupture, and axial compression failure, which occurred in GTRC column specimens with heights of 300 mm; and local buckling failure, which occurred in specimens with a larger slenderness ratio.

  4. 4.

    From the test result, the load–deflection curves of the GTRC columns are divided into three stages: linear elastic, elastic–plastic, and failure. Furthermore, the load-strain curves of the GTRC columns follow a similar pattern, with typical bilinear curves and a clear inflection point.

  5. 5.

    An ultimate load-bearing capacity adjustment coefficient was proposed, considering the influence of timber core diameter. It was also found that the introduction of timber core reduced the restraint effect of GFRP laminates. A formula was also established based on the force equilibrium and superposition for predicting the axial bearing capacity of the GTRC columns. The theoretical and experimental results showed a high degree of agreement.

  6. 6.

    In this paper, each parameter specimen is set up one, which does not fully take into account the errors existing in the test, based on which, the various conclusions obtained in this paper need more tests for verification. In the future, more tests are needed to further investigate the mechanical properties of GTRC columns under different loading schemes (e.g., combined axial loading and bending moment, combined axial loading and cyclic lateral loading), and it is necessary to consider the specimen stability and lateral load effects, as well as the effect of moisture-induced reduction of the composite structure on the compressive performance of the GTRC column.