1. Introduction
Natural resources have been excessively consumed and large amounts of construction wastes have been produced owing to the fast development of the civil industry, resulting in crucial issues regarding the environment [
1,
2,
3,
4]. Therefore, releasing the heavy burden on the environment and establishing a sustainable society is an urgent challenge. Thus, reusing construction wastes to prepare new concrete offers a good strategy for solving this issue [
5,
6,
7]. After construction and demolition wastes undergo crushing, grading, and cleaning processes, recycled aggregate can be obtained; it presents good and stable mechanical behaviors [
8]. Utilizing the recycled aggregate can ease the release of pollution into the environment and diminish natural aggregate consumption, as well as decrease the carbon footprint of civil industries [
9,
10]. Natural coarse aggregate can be substituted with recycled coarse aggregate and can be applicable in non-structural positions [
11]. Moreover, recycled coarse aggregate also presents potential applications in bridges and roadways [
10,
12,
13]. Additionally, concrete members containing recycled coarse aggregate present good durability [
14].
Self-compacting concrete possesses good fluidity and filling capacity, and is also non-segregating. It can spread into complicated positions without vibration. In addition, the hardened properties of self-compacting concrete are similar to traditional vibrated concrete [
15,
16]. Therefore, self-compacting concrete can be applied in complicated conditions [
17,
18]. This concrete was first developed in Japan in the 1980s. Subsequently, it has been developed quickly and spread all over the word [
17,
19,
20,
21].
In recent years, making use of recycled aggregate to prepare self-compacting concrete has been appreciated as a good strategy to minimize construction and demolition wastes and protect the environment [
15,
22,
23,
24,
25,
26,
27]. The mechanical behavior of self-compacting concrete containing recycled aggregates is close to that of traditional concrete [
23]. Majeed et al. pointed out that self-compacting concretes with natural and recycled aggregate present the same workability. However, compared with the compressive strength of concrete containing natural aggregate, the compressive strength of concrete containing recycled aggregate is reduced [
28]. The hardened performances of self-compacting concrete are reduced when the replacement rate of recycled aggregate is within the scope of 10 wt.%~40 wt.%[
29]. However, it has also been demonstrated that self-compacting concrete containing 40 wt.% recycled aggregate displays the same mechanical properties as normal self-compacting concrete [
30]. In addition, the mechanical behaviors of self-compacting concrete containing 100 wt.% recycled aggregate is close to that of normal concrete; however, flexural toughness and stiffness is decreased [
31]. Additionally, the density of self-compacting concrete containing recycled aggregate is lower than that of normal self-compacting concrete [
29].
Concrete-filled steel tube columns present high strength, favorable ductility, and fire resistance as structural members. Moreover, steel tube columns can serve as formwork when concreting. This can save costs and construction time [
32]. Thin-walled steel tube columns are attractive in civil engineering due to the fast development of steel with excellent performance. High-performance steel can maintain outstanding performance while reducing cost when applied in concrete-filled steel tube columns [
33,
34]. Regarding the advantages, many investigations on concrete-filled thin-walled steel tube columns have been conducted [
33,
35,
36,
37,
38,
39,
40,
41,
42]. Because of their higher slenderness ratios, thin-walled steel tube columns filled with concrete are more prone to buckling, and ductility is poor [
39,
43,
44,
45,
46]. Le et al. demonstrated that the collaboration effect between steel tube columns and infill concrete is intensive [
47]. Wang et al. pointed out that the decrease rate of the descent phase between the relation of the load and displacement decreases upon increasing the wall thickness of the steel tube column [
48]. Wang et al. indicated that the ductility coefficient of thin-walled tubes filled with concrete is higher than 4.0 [
49]. Yu et al. researched eccentric compression behaviors on steel tube columns filled with self-compacting concrete containing recycled coarse aggregate columns. They demonstrated that the strain increased with the increase in recycled coarse aggregate [
50].
Previous studies have concentrated on using recycled aggregate in producing self-compacting concrete [
19,
51,
52,
53] and thin-walled steel tube columns filled with self-compacting concrete [
48]. These studies mainly concentrated on topics such as (1) steel tube columns filled with concrete [
54,
55,
56,
57]; (2) steel tube columns filled with self-compacting concrete [
58]; (3) steel tube columns with thin wall thickness filled with concrete [
34,
47,
49,
59,
60,
61,
62]; (4) steel tube columns with thin wall thickness filled with self-compacting concrete [
48,
63,
64]; (5) steel tube columns filled with recycled concrete [
65,
66,
67,
68,
69]; (6) steel tube columns filled with self-compacting concrete containing recycled aggregate [
50]; (7) steel tube columns with thin wall thickness filled with recycled concrete [
70,
71]. However, little comprehensive investigation has been conducted concerning the two aspects on the behavior of recycled aggregate in applications in self-compacting concrete as well as steel tube columns with thin wall thickness filled with self-compacting concrete. Therefore, the aim of this research is to evaluate the feasibility and promote design and application of steel tube columns with thin wall thickness filled with concrete containing recycled coarse aggregate in stub columns. That is the primary novel contribution of this paper.
Firstly, six thin-walled steel tube stub columns filled with self-compacting concrete containing recycled and natural coarse aggregate are manufactured. In addition, four of the columns contain recycled coarse aggregate with a replacement rate of 100%. The other two columns are filled with self-compacting concrete containing natural coarse aggregate. Moreover, wall thicknesses of 1.2 mm and 3.0 mm are adopted. Subsequently, the compressive load behaviors of the columns are explored. The effect of the wall thickness on axial compressive behaviors and failure modes is analyzed in depth. This paper aims to create steel tubes filled with concrete as sustainable construction members and offer guidance for designing and facilitating the application of thin-walled steel tube stub columns containing recycled coarse aggregate in civil industry.
The differences between this work and the other existing studies are as follows. (1) the thin-walled steel tubes filled with self-compacting concrete containing recycled coarse aggregate. In this paper, we adopt a 100% replacement rate for the self-compacting concrete; (2) wall thicknesses of 1.2 mm and 3.0 mm are considered as the key parameters; (3) failure modes and axial compressive mechanical behaviors of the thin-walled steel tube columns filled with self-compacting concrete containing recycled aggregate are described; (4) theoretical models of the load–displacement/strain relationships of the columns containing recycled aggregate are proposed.
4. Conclusions
Thin-walled steel tube stub columns filled with self-compacting concrete containing recycled coarse aggregate are developed. The mechanical behaviors and failure modes of the columns undergoing axial compressive load to failure are investigated. The effects of wall thickness and types of self-compacting concrete on axial compressive behaviors are discussed. Theoretical models of the load–displacement/strain relationships are also proposed. Conclusions based on the results are as follows.
(1) The thin-walled steel tube columns with identical wall thickness filled with self-compacting concrete containing recycled and natural coarse aggregate display similar failure modes. In general, local buckling and rupture are the typical failure modes. However, the positions of the buckling and rupture are mainly distributed on the surface near the ends of the columns with a wall thickness of 1.2 mm, while they are distributed close to the middle height of the columns with a wall thickness of 3.0 mm. Moreover, the buckling and rupture are generated earlier and more extensively with the thinner wall thickness.
(2) The development trends of the relationships between load and displacement with identical wall thicknesses are similar to each other. Nevertheless, the maximum load and stiffness of the columns containing recycled coarse aggregate are lower than that of the maximum load and stiffness of the columns containing natural coarse aggregate. The load–displacement curves with thicker wall thickness present a longer plastic plateau after the maximum load, and better ductility.
(3) The maximum load of columns containing recycled coarse aggregate is correspondingly lower than the maximum load of the columns containing natural coarse aggregate. Compared with the maximum load of columns containing natural coarse aggregate, the maximum load of columns containing recycled coarse aggregate is decreased by 18.4% and 5.8%, corresponding to wall thicknesses of 1.2 mm and 3.0 mm, respectively. Similarly, the displacement corresponding to the maximum load of columns containing recycled coarse aggregate is increased by 19.8% and decreased by 9.6%, respectively.
(4) The models of the relationship between load and displacement/strain of the thin-walled steel tube columns containing recycled coarse aggregate subject to axial compression are established, and they present a good evaluation of load and displacement/strain.
(5) The columns with identical wall thickness filled with two different types of concrete display similar relationships between load and strain. However, the stiffness in the elastic stage of columns containing recycled coarse aggregate is lower than that of the stiffness in the elastic stage of columns containing natural coarse aggregate. Confinement by the steel tubes of the concrete is weaker before the maximum load, and it becomes more intensive after the maximum load.
This paper demonstrates that thin-walled steel tube columns containing recycled coarse aggregate present positive axial compressive behaviors and have great potential for applications in civil infrastructure, and promote the development of a renewable, sustainable, and carbon-neutral society. However, the results show that the maximum load, stiffness, and ductility of this column is a little lower than that of the corresponding values of the columns containing natural coarse aggregate. Moreover, the stability of the mechanical behaviors of the columns should also be explored comprehensively.