Behaviors of recycled aggregate concrete-filled steel tubular columns under eccentric loadings

Abstract

The paper investigates the behaviors of recycled aggregate concrete-filled steel tubular (RACFST) columns under eccentric loadings with the incorporation of expansive agents. A total of 16 RACFST columns were tested in this study. The main parameters varied in this study are recycled coarse aggregate replacement percentages (0%, 30%, 50%, 70%, and 100%), expansive agent dosages (0%, 8%, and 15%) and an eccentric distance of compressive load from the center of the column (0 and 40 mm). Experimental results showed that the ultimate stresses of RACFST columns decreased with increasing recycled coarse aggregate replacement percentages but appropriate expansive agent dosages can reduce the decrement; the incorporation of expansive agent decreased the ultimate stresses of RACFST columns but an appropriate dosage can increase the deformation ability. The recycled coarse aggregate replacement percentages have limited influence on the ultimate stresses of the RACFST columns and has more effect than that of the normal aggregate concrete-filled steel tubular columns.

Keywords

concrete filled steel tubes / recycled aggregate concrete / columns / expansive agent / eccentric load

Introduction

Recycled aggregate concrete (RAC) produced with the incorporation of recycled aggregate from wasted concrete is widely recognized as green concrete. RAC applications are effective ways to reuse construction waste and to protect natural resources. But the drawbacks of RAC, such as low strength and elastic modulus [¹–⁶], high shrinkage and creep [⁷,⁸] and inferior durability [⁹,¹⁰], hinder its development.

Nowadays, concrete-filled steel tubular (CFST) columns are widely applied for their excellent proprieties such as high strength and high stiffness. When confined by steel tubes, core concrete has no moisture exchange with the outside environment, its shrinkage and creep performances are therefore smaller than those of ordinary concrete [¹¹–¹³]. Confinement can also restrict the extension of internal micro-cracks, which improves compressive strength and deformation performance of the core concrete [¹⁴,¹⁵]. Confinement also seems to be a way to improve RAC strength and stiffness.

In recent years, a number of studies were carried out on RACFST columns to improve RAC strength, stiffness and also to reduce its shrinkage and creep performances. Yang and Han [¹⁶,¹⁷] studied the experimental behavior of RACFST columns under short-term static loads. In the study, 1) the influence of section shapes, including circular and square, 2) concrete types, including normal aggregate concrete and RAC, and 3) load eccentricity ratios, varied from 0 to 0.53 were considered. It was found that the failure modes of RACFST columns are similar to normal CFST columns which are both total buckling failure. The ultimate stresses of RACFST columns decrease with increasing load eccentricity ratios. The ultimate stresses of RACFST columns are lower than those of normal CFST columns which vary from 1.4% to 13.5%. Yang et al. [¹⁸] and Yang [¹⁹] studied the experimental behaviors of RACFST columns under long-term sustained loadings. It was found that the concrete shrinkage and creep strains of RACFST columns are 6% to 23% respectively which are higher than those of normal CFST columns. Yang and Zhu [²⁰] and Yang et al. [²¹] studied the behaviors of RACFST columns subjected to cyclic loads and found that the capacity and stiffness of specimens are lower than those of normal CFST specimens. Xiao et al. [²²] tested a total of 12 steel tubular columns confined with RAC short-columns subjected to axially loads and found that the ultimate stresses of columns with RAC had an average decrease of about 13.2% compared with normal aggregate concrete columns. Huang et al. [²³] made a theoretical study of the mechanical response of RAC confined by steel tubes under axial compression and find that the recycled coarse aggregate replacement percentage has a moderate effect on the mechanical response of confined concrete.

Being different from the previous findings that the ultimate stresses of RACFST specimens are lower than those of normal CFST columns, Chen et al. [²⁴,²⁵] found that the strengths of RACFST columns are higher than those of normal CFST columns. Mohanraj et al. [²⁶] similarly reported that the ultimate stresses of RACFST columns had an average of 3.9% increment compared with those of normal CFST columns in a study using 4 RACFST columns subjected to axially loads.

The RAC characteristics such as high shrinkage and creep performances may weaken the confining effect of RACFST specimens which leads to unsatisfactory performances of RACFST specimens. Incorporation of an expansive agent may reduce concrete shrinkage and creep performances, which may be an effective solution to improving the performances of RACFST specimens. In the past years, a series of studies on expansive agents [²⁷–³⁰] showed that they are useful for shrinkage reduction. Recently, José Oliveira et al. [³¹] reported that the autogenous shrinkage can be significantly reduced and even eliminated using appropriate expansive agent dosages.

Expansion agent has been fitted well with the purpose in the concrete-filled columns. Li et al. [³²] examined the combinations of expansive agent in mitigating shrinkage deformation of concrete under variable temperature condition. García Calvo et al. [³³] investigated the performance of expansive self-compacting concrete and expansive conventional concrete in different expansion and curing conditions. Limited studies were conducted in the use of expansive agent.

Based on the above literature with limited studies investigating the behavior of RACFST columns and particular limited studies related to expansion agents, this paper investigates the behavior of RACFST columns with different variables on recycled aggregate replacement ratios and expansive agents dosages under eccentric loads.

Experimental investigation

General

The main parameters varied in this study are: 1) recycled coarse aggregate replacement ratios, g (0%, 30%, 50%, 70%, 100%); 2) expansive agent dosages, b (0%, 8%, 15%); and 3) eccentric distance of compressive load from the center, e (0 mm, 40 mm). A total of 16 circular columns were investigated in this study with the typical combinations of the variables. Figure 1 shows the cross section of specimens, where D is the outside diameter of the circular steel tube and t is its wall thickness. All specimens have identical height L and diameter to wall thickness ratios.

Table 1 provides the details of specimens. The specimen designations start with RCFS referring to recycled aggregate concrete-filled steel tubes and followed by three values: 1) the first value denotes the recycled coarse aggregate replacement percentages; 2) the second value denotes the expansive agent dosages; and 3) the third value denotes the load eccentric distances. For example, the specimen RCFS-30-8-40 represents the RCFS column with 30% recycled coarse aggregate replacement percentage, 8% expansive agent dosage and tested under an eccentric distance of 40 mm.

Material properties

A steel plate was used to produce the tubes by machining and welding. The steel properties were obtained by randomly testing three tensile coupons taken from the plate. The experimental tensile stress-strain curve is shown in Fig. 2 and the measured tensile properties are given in Table 2.

The cement used was ordinary Portland cement 42.5R. A fourth-generation high performance concrete expansive agent was used and the mixing water was tap water. Both natural and recycled coarse aggregate were commercially available. The natural coarse aggregate was crushed stone with continues grading and the recycled coarse aggregate was composed of 5‒16.5 mm and 16.5‒31.5 mm in diameter with a mix ratio of 3:2. Table 3 provides the basic properties of the recycled coarse aggregate for the experiments. Equivalent mix proportions for concrete were used in this research: 1) cement: 430 kg/m³; 2) water: 185 kg/m³; 3) sand: 559 kg/m³; 4) coarse aggregate: 1118 kg/m³. Considering the influence of recycled coarse aggregate replacement percentages, g (0%, 30%, 50%, 70%, 100%), recycled coarse aggregate were used to replace natural coarse aggregate. Considering the influence of expansive agent dosages, b (0%, 8%, 15%), expansive agents were used to replace cement. When g and b are both reduced to zero, the concrete is virtually normal aggregate concrete.

Concrete cubes were prepared and tested to determine their concrete compressive strength f_cu. Concrete prisms were prepared and tested to determine their elastic modulus E_c as presented in Table 4. When b is unchanged, the concrete compressive strength and elastic modulus after 28 days dropped as g was increased. But when b is 0% and g is 100%, the concrete compressive strength is 6.2% higher than that obtained when g is 70%. When g is unchanged, the concrete compressive strength and elastic modulus after 28 days dropped as b was increased. But when g is 100% and b is 15%, the concrete compressive strength is 2.9% higher than that obtained when b is 8%.

Specimen preparation

The steel tubes were manufactured and cut to the required length. All tubes were placed on a horizontal steel plate and fixed before pouring. Sixteen types of concrete were first poured into half the tube height, then compacted using a vibrator. The remaining concrete was then poured into the other half of the tubes and compacted. All specimens were kept indoor and covered by a wet plastic sheet. As concrete cured, a small amount of longitudinal shrinkage occurred at the top of the specimens. High strength mortar was used to fill the gap before specimens were tested.

Column tests

The experiments were carried out on a 5000 kN capacity testing machine, including six columns under concentric compressive loads and ten columns under eccentric compressive loads as listed in Table 1. The specimens were placed on the lower platen of the machine and a steel roller was placed further to create eccentric loads, which require the eccentric distance from the center of the columns, as shown in Fig. 3(a). Three linear variable displacement transducers (LVDT) were placed to measure the lateral deflection as shown in Fig. 3(b). Nine strain gauges were attached at the mid-height of each column to measure deformation, including six (labeled 1‒6) for longitudinal deformation and three (labeled 7‒9) for transverse deformation, as shown in Fig. 3(c).

Prior to testing, a load level of 20‒30 kN was applied to ensure the machine’s platens being firmly attached to both ends of specimens and correct eccentric distances were applied. During the tests, compressive load was increased step by step before 80% of the ultimate stresses and the incremental step is 200 kN. When a step was finished, the load was maintained for 2 minutes and the next step was started. After the compressive load reached 80% of the ultimate stresses, a slow and continuous load was applied until the specimens failed. It was found that the failure modes of RACFST columns are generally buckling failure. For the columns subjected to concentric compressive loads, the steel tubes were yielded when the load reached about 90% of the ultimate stresses. The load was continuously increased until the welding seam failed, at which the columns reached the ultimate stresses as shown in Fig. 4(a). While the columns subjected to eccentric compressive load failed, local buckling was observed near the upper third height of the columns as shown in Fig. 4(b).

Results and discussion

Ultimate loads

In this paper, the ultimate loads N_ue are simply defined as the maximum load recorded during the tests. The ultimate loads of all composite columns are summarized in Table 1.

Influence of recycled coarse aggregate replacement percentages on ultimate loads

Figure 5 shows the influence of recycled coarse aggregate replacement percentages g on the ultimate loads N_ue of RACFS columns. In general, N_ue decreases with increasing g. When subjected to concentric loads, for columns with b of 0%, N_ue decreases 32.6% when g increases from 0% to 100%; for columns with b of 8%, N_ue decreases 23.5% when g increases from 0% to 100%; and for columns with b of 15%, N_ue decreases 30.6% when g increases from 0% to 100%. When subjected to eccentric loads with e of 40 mm, for columns with b of 0%, N_ue decreases 20.7% when g increases from 0% to 100%; and for columns with b of 8%, N_ue decreases 20.0% when g increases from 0% to 100%. But for columns subjected to eccentric loads, whether b is 0% or 8%, N_ue almost remains unchanged when g increases from 30% to 70%. These data indicate that the replacement of normal coarse aggregate with recycled coarse aggregate: (i) has relatively influence on the value of RCFS columns’ strength under concentric loads than under eccentric loads; (ii) has slight influence on the value of RCFS columns’ strength under concentric loads with appropriate expansive agent dosages which are more effective than an excessive dosage of expansive agent or without expansive agent; and (iii) has almost no influence on the value of RCFS columns’ strength under eccentric loads when the replacement percentages varies from 30% to 70%.

Influence of expansive agent dosages on ultimate loads

Figure 6 shows the influence of expansive agent dosages b on the ultimate loads N_ue of RACFS columns. In general, N_ue decreases with increasing b when columns were subjected to concentric loads. For columns with g of 0%, N_ue decreases 17.6% when b increases from 0% to 15%; and for columns with g of 100%, N_ue decreases 15.0% when b increases from 0% to 15%. For columns subjected to eccentric loads, when b increases from 0% to 8%, for columns with g of 0% or 100%, N_ue has a slightly increment of about 2.7% and 3.5%, respectively, while g varies from 30% to 70%, N_ue almost remains unchanged. These data indicate that, for columns subjected to concentric loads, N_ue decreases with the increasing of expansive agent dosages and has less decrement on recycled aggregate concrete columns than on the normal aggregate concrete columns. For columns subjected to eccentric loads, N_ue has a slightly increment with expansive agent dosages.

Influence of eccentric distances on ultimate loads

Figure 7 shows the influence of eccentric distances e on the ultimate loads (N_ue) of RACFS columns. In general, N_ue decreases when e is increased from 0 to 40 mm. For the normal aggregate concrete columns, N_ue decreases 25.3% when b is 0% while N_ue decreases 10.3% when b is 8%. For 100% recycled aggregate replacement columns, N_ue decreases 12.0% when b is 0% while N_ue decreases 6.3% when b is 8%. These data indicate that increasing the eccentric distance has stronger influence on the ultimate stresses decrement of normal aggregate concrete columns than on the recycled aggregate concrete columns. An appropriate expansive agent dosage can reduce this influence which has more effects on recycled aggregate concrete columns than that on the normal aggregate concrete columns.

Axial strains

In this paper, positive strain denotes compression and negative strain denotes tension for axial strains. Figure 8 shows the load-axial strain curves of columns subjected to eccentric loadings. In Fig. 8, labels 1‒5 mean the axial strain measured by strain gauges 1‒5 as shown in Fig. 3(c).

Influence of recycled coarse aggregate replacement percentages on axial strains

The compression area of columns decrease with increasing recycled coarse aggregate replacement percentage. For columns with b of 0%, a tension zone appears when g reaches 50% and its area is broadened when g increases from 50% to 100%. While for columns with b of 8%, the tension zone does not appear until g reaches 100%. When g increases from 0% to 70%, axial strains in the compression zone increase for columns either with b of 0% or with b of 8%. When g increases from 70% to 100%, axial strains in the compression zone still increase for columns with b of 0% but slightly decrease for columns with b of 8%. Generally, the axial deformation capacity increases with increasing recycled coarse aggregate replacement percentage.

Figure 9 shows the load-axial strain curves of strain gauge 5 with different recycled coarse aggregate replacement percentages. In general, the deformation-resistant ability decreases with increasing g for columns subjected to eccentric loadings. When g increases from 0% to 30%, there is a significant decrement of deformation-resistant ability. When g varies from 30% to 70%, the deformation-resistant ability continues decreasing but the decrement is very small. While g increases from 70% to 100%, a significant decrement reappears. There was a similar decrement of deformation-resistant ability with increasing g when there is an 8% expansive agent dosage but the decrement becomes limited and no significant decrement appeared.

Influence of expansive agent dosages on axial strains

The strain 3, 4, and 5 of all columns reaches the yield strain of the steel tube when b is 8%, while only the strain 4 and 5 reaches the yield strain of the steel tube when b is 0%. This indicates that an expansive agent dosage can fully utilize material with limited voids and pores within the columns. Columns RCFS-50-0-40 and RCFS-70-0-40 appear as tension zones, while columns RCFS-50-8-40’s and RCFS-70-8-40’s full sections are under compression, which indicate that an expansive agent dosage may broaden the compression zone of columns when the replacement percentage of recycled coarse aggregate is kept unchanged. Using identical recycled coarse aggregate replacement percentages when it varies from 0% to 100%, the axial strains of columns with b of 8% are larger than those of columns with b of 0%, indicating that the axial deformation capacity increases using an expansive agent dosage. Figure 10 shows the typical load-axial strain curves of all axial strains for columns subjected to eccentric loadings with and without expansive agent dosages. It can be seen that columns clearly have effective deformation-resistant ability using an 8% expansive agent dosage.

Circumferential strains

In this paper, positive strain denotes tension and negative strain denotes compression for circumferential strains. Figure 11 shows the load-circumferential strain curves of columns subjected to eccentric loadings. In Fig. 11, labels 7‒9 mean the circumferential strain measured by strain gauges 7‒9 as shown in Fig. 3(c).

Influence of recycled coarse aggregate replacement percentages on circumferential strains

Circumferential strain 7 decreases with increasing recycled coarse aggregate replacement percentage, indicating that the confined concrete in a steel tube at strain gauge 7 declines because the shrinkage of concrete becomes larger with increasing recycled coarse aggregate replacement percentage. Circumferential strains 8 and 9 increase with increasing recycled coarse aggregate replacement percentage, indicating the confinement of concrete by steel tube at strain gauges 8 and 9 increase. For example, when the load is 1000 kN, the eccentricity is 40 mm and g is 0%, 50%, and 100%, the circumferential strain 9 of column with b of 8% is 0.0023, 00033, and 0.0038, respectively. When the load is 1500 kN, the eccentricity is 40 mm and g is 0%, 50%, and 100%, the circumferential strain 9 of column with b of 8% is 0.0048, 0.0082, and 0.0086, respectively. Figure 12 shows the load-circumferential strain curves of strain gauge 9 with different recycled coarse aggregate replacement percentages. The reason for this phenomenon is that the elastic modulus of concrete decreases, leading to a concrete larger deformation, then the confinement increases consistently, which is supported by literature [³⁴–³⁶].

Influence of expansive agent dosages on circumferential strains

When g is 30%, 70%, and 100%, circumferential strains 7, 8, and 9 of columns with b of 8% are larger than those of columns with b of 0%. When g is 0% and 50%, circumferential strain 7 of columns with b of 8% is smaller than that of columns with b of 0%, but circumferential strain 8 and 9 are on the contrary. Figure 13 shows the typical load-circumferential strain curves of all circumferential strains for columns subjected to eccentric loadings with and without expansive agent dosages. Generally, concrete confined in steel tubes increases with an expansive agent dosage using an identical recycled coarse aggregate replacement percentage. Especially, this influence is more effective on recycled concrete than on normal concrete.

Effect of confinement

The preceding parts of the research are based on an identical mix of concrete proportions, while the strength of each concrete is different because different recycled coarse aggregate replacement percentages and expansive agent dosages are employed. In order to compare the effect of confinement between different recycled coarse aggregate replacement percentages and expansive agent dosages, a strength index µ is defined as follows:

(1)\mu ={\displaystyle \frac{{{\displaystyle N}}_{ue}/{{\displaystyle A}}_c}{{{\displaystyle f}}_c}},

where {{\displaystyle N}}_{ue}/{{\displaystyle A}}_cand f_c are the ultimate stresses of RACFST columns and the strength of plain concrete respectively. As shown in Table 1, the strength index for columns subjected to concentric loadings ranges between 2.26 and 3.08, while for columns subjected to eccentric loadings, it ranges from 2.03 and 2.83. Figure 14 shows the strength indices of columns subjected to eccentric loadings. It can be seen that without expansive agent dosage, the strength index increases before g reaches 70% and then decreases. But when g is 100%, the strength index is still slightly higher than that obtained when g is 0%. With an 8% expansive agent dosage, a similar tendency can be found with increasing g, the strength index increases before g is 50% and then decreases. The strength index of g is 100% is also slightly higher than that obtained when g is 0%. These data indicates that the replacement percentages of recycled coarse aggregate are independent of confinement. However, an 8% expansive agent dosage can enhance the confinement effect as evidenced in Fig. 14.

Conclusions

A test program was carried out to study the behavior of RCFST columns under eccentric loadings with the influence of expansive agent dosages. The following conclusions were drawn from the study based on experimental observations and analytical results:

1) The replacement of normal coarse aggregate with recycled coarse aggregate has relatively limited influence on the ultimate loads of concrete-filled steel tubular columns subjected to eccentric loads than that subjected to concentric loads.

2) Eccentric distances have limited influence on the ultimate load of recycled aggregate concrete-filled steel tubular columns than that of normal aggregate concrete-filled steel tubular columns.

3) Expansive agent dosages decrease the ultimate load of columns under concentric loadings, but increase the ultimate load of columns under eccentric loadings.

4) For columns under eccentric loadings, axial and circumferential strains increase with increasing recycled coarse aggregate replacement percentage, an expansive agent dosage can increase the axial deformation capacity and the confinement of concrete by a steel tube using an identical recycled coarse aggregate replacement percentage.

5) Recycled coarse aggregate replacement percentages do not decrease the effect of confinement while appropriate expansive agent dosages can enhance the effect of confinement.

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