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Using of gypsum waste as alternative to concrete in light gauge filled steel tube columns

	Abstract 1. Introduction 2. Methodology 3.Results and Discussion 4.Conclusions 5.References
Using of gypsum waste as alternative to concrete in light gauge filled steel tube columns



Alaa Hassoon Civil Engineering Department, University of Al-Qadisiyah, Diwaniyah, Iraq Alaa.Hassoon@qu.edu.iq



Abstract Resently, recycled materials are used in some of civil engineering applications owing to the enormous amounts of waste materials from industrial growth. This paper aims to investigate the behavior of hollow steel columns filled with gypsum waste instead of concrete. Six steel tubes of 1.5 mm thickness divided into two equal groups; three columns with 0.1 × 0.1 m cross section and three columns of 0.1 m diameter cross section were fabricated. Each group involved hollow steel column, steel tube column filled with concrete of (1:2:4) mix, and the last column filled with grinded gypsum waste. All columns were tested under axial loads with simply supported ends and 1 m effective length. The experimental results showed that the load capacity increased by 163 to 178% for columns filled with ordinary concrete and by 38 to 44% for gypsum waste if compared with hollow columns. In addition, the ductility index of columns filled with concrete increased by 78.3 to 84% and by 5.8 to 6.7% for gypsum waste. Additionally, using of ordinary concrete and gypsum waste as filling materials increased the stiffness of the CFST columns by 192 to 297.9 and 195.6 to 224.1% respectively. However, the capacity of columns filled with gypsum waste were lower than columns filled with concrete by about 48%. The filling materials enhanced the failure modes of the composite columns by prevention of local buckling and stability problems.

Keywords: CFST columns, Normal concrete, Gypsum waste, Axial loads, Failure mode




1. Introduction
Concrete filled steel tube CFST members are extensively used in multistory buildings, bridges, towers, and special moment resisting frames. They utilized the advantages for both steel and concrete. Recently, many researchers tried to investigate the behavior of CFST members filled with recycled materials instead of conventional concrete.
The behavior of CFST columns filled with recycled concrete has been investigated by Yang and Han [1]. Thirty specimens were tested to study the influence of section shape and eccentricity. As a conclusion, the capacity of recycled concrete filled columns is slightly less than those filled with normal concrete. Abdelgadir et al. [2] investigated the behavior of lightweight aggregate concrete filled steel tubes under eccentric loads. 54 specimens with different eccentricities, width thickness ratio, and height to width ratio were tested. The experimental outcomes declared that greater eccentricity produced less ultimate capacity while greater thickness increased the ultimate load capacity.
The effect of compressive strength on the behavior of CFST columns under concentric and eccentric loads has been studied by Hassoon [3]. As a conclusion, the confined concrete experienced axial capacity higher than bending. An experimental study was performed by AL Zaidi [4] to study the influence of recycled concrete. It is concluded that waste concrete filled steel columns provides more capacity than hollow columns that the filling materials controlled the local buckling and imparts an acceptable stiffness.
Sangeetha and Senthil [5] investigated the behavior of tubular columns filled with concrete of 20 MPa compressive strength, partially replaced concrete debris and quarry dust. The effect of slenderness ratio was also considered. The results indicated that using of concrete debris and quarry dust has improved the structural behavior of the composite columns in comparison with hollow columns. In addition, the use of recycled aggregate concrete instead of conventional concrete reduced the cost of the in filled concrete. The behavior of recycled aggregate concrete filled steel tube columns was evaluated by Tam et al. [6]. A total of 16 columns with replacement percentages (0%, 30%, 50%, 70%, and 100%) were tested under eccentric loadings. They concluded that increasing of replacement percentages decreased the ultimate capacity of columns.
Chen et al. [7] declared that recycled aggregate CFST frames could be utilized in multistory buildings and seismic areas due to the good seismic behavior experienced. Xu et al. [8] studied the behavior of hollow steel columns filled by recycled aggregate concrete under cyclic loads. It is concluded that the percentage of recycled concrete had a little influence on the hysteresis load displacement curves. Wang et al [9] declared that recycled CFST columns experienced higher fire resistance time when compared with normal concrete due to the lower thermal conductivity. Tao et al. [10] investigated the behavior of geopolymer CFST columns under elevated temperatures. Based on the results obtained, geopolymer CFST columns exhibited an improved fire resistance if compared with conventional concrete which could be applied to enhance the fire resistance of such columns.
The use of recycled asphalt pavement and recycled concrete in light gauge hollow steel columns was investigated by Sulaiman et al. [11], 51 specimens were tested. The parameters studied were: replacement percentage, combined replacement and loading type. It is concluded that using of both of recycled materials instead of conventional concrete led to a decrease in the load carrying capacity of the columns.
Although several previous researches on this topic, the behavior of steel tubes filled with recycled material still needs more investigation. So, this paper will focus on the behavior of the filled steel columns using recycled gypsum waste and compare the results obtained from experimental test with hollow and concrete filled columns. It is expected that the new filling material will improve the capacity and enhance the failure mode of the columns.

2. Methodology
2.1 Specimens
Six specimens were prepared to study the influence of gypsum waste on the behavior of CFST columns, the details of all specimens are given in Table 1. Specimen nomenclature involves two parts, S or C, which illustrates the column cross sectional shape (square or circular) while the second part refers to the filling material (H for hollow section without fill, C for specimen filled with concrete, and G for those with gypsum waste).

Table 1 Specimens details


2.2. Materials
Ordinary Portland cement, natural sand, and crushed gravel of 14 mm maximum size was used in the concrete mixture. Also, six hollow steel section were used in this study. The height of all sections was 1 m. The yield strength of hollow steel columns (square and circular) were 378 and 392 MPa, respectively according to ASTM A6 [12]. Gypsum waste were brought and then grinded to a fine powder to prepare them for casting process as shown in Figure 1.


Fig.1 Gypsum waste

2.3. Casting of specimens
Concrete of (1:2:4) mix proportion with a water cement ratio of 0.45 was used to fill two of columns for comparison purposes as shown in Figure 2. Mechanical vibration is used to evict the air inside concrete to get high density for the compacted mass. During casting, three cubes of (150×150×150 mm) are sampled for compressive strength test of the concrete according to EN 12390-3 [13]. Table 2 gives the test results of these samples.


Fig.2 Casting of concrete
Table 2 Concrete compressive strength



Also, two columns were filled with gypsum waste. The powder was mixed with water and compacted as much as possible. All of filled columns are covered from sunlight as it is contained within an entire tube of iron and left for 28 days for curing. Finally, the columns are painted, labeled, and prepared for test as shown in Figure 3.


Fig.3 Filled columns after casting
2.4. Test procedure
All columns were tested under axial load with simply supported ends. The height of each specimen is 1 m. A loading frame with hydraulic actuator is used for test as illustrated in Figure 4. In order to measure the mid-height lateral displacements, A 25 mm dale gage of 0.01 mm accuracy is used, the ends of each specimen were welded with a plate to get a uniform stress distribution during test.

Fig.4 Test setup

3. Results and Discussion
3.1. General
The specimens are tested as illustrated before, the lateral displacements are recorded at mid-height for each load increment up to failure. the test results are given in Table 3.

Table 3 Summary of test results


The experimental results demonstrated that hollow steel columns without in filling material produced the lowest ultimate loads regarding to the unsupported light gauge of the section. The ultimate capacity of the tested specimens was provided by the columns filled with ordinary concrete raised by 163% for column with square cross section and by 178% for circular which is also concluded by Hassoon [3] and Sangeetha and Senthil [5] owing to the increased of cross sectional area and the lateral continuous support of concrete through the column height which prevent yielding and local bucking at service loads. Also, using of gypsum waste increased the ultimate load capacity of columns by 38% for square column and by 44% for column with circular cross section if compared with hollow sections but it still lower than columns filled with concrete by 38.1% for square columns and 48% for circular due to the difference between gypsum waste and standard concrete in the compressive strength and adhesive properties.
3.2. Load deflection relations
The lateral displacements at mid-height have been recorded during test for each load increment. Figure 5 showed the load deflection response for the tested columns.


Square columns

Circular columns

Fig.5 Specimens load deflection curves
The hollow steel section columns have the shortest linear parts because of the low stiffness of light gauge used which decreased significantly during the last stages of loading till failure. The stiffness provided by the normal concrete and gypsum waste is clear especially after the initial loading stages because of the additional area and the lateral support produced through the column height. This led to additional load capacity based on the strength of the filling materials and the confinement of steel.
3.3. Ductility
Ductility represents the plastic capacity of structural members under the applied loads. It could be simply found as the ratio of the displacement at ultimate loads to the corresponding at yield (approximately 80 to 85%) of the ultimate [8]. Table 4 provides the specimens, ductility index.

Table 4 Ductility index of the tested specimens.



The ductility index for square and circular columns filled with concrete increased by 84 and 78.3%, respectively if compared with hollow steel columns due to the ability of new composite section to absorb additional energy especially in plastic zone. Whereas the corresponding increase for specimens filled with gypsum waste is 5.8 and 6.7% that the strength of the recycled gypsum is significantly less than standard concrete.

3.4. Stiffness criteria
The slope of the secant drawn in load deflection polynomial at 75% of ultimate load is called stiffness criteria [14]. Specimens' stiffness criteria are given in Table 5.

Table 5 Stiffness criteria of the tested specimens.


The stiffness criteria for columns filled with concrete increased by 192 to 297.9% if compared with hollow steel columns while the increasing ratio for specimens filled with gypsum waste is 195.6 to 224.1% due to the additional stiffness provided by the filling material which reduced the lateral displacement and withstand more external loads.
3.5. Failure mode
The tested specimens exhibited different failure modes at ultimate loads involved local buckling or yielding of steel and crushing of the filling materials as shown in Figure 6. For hollow steel columns the failure occurred due to local buckling or yielding at ends, using of concrete enhanced the failure mode with higher loads up to concrete crushing while the low strength of the gypsum waste prevent the local buckling but crushed at the columns ends.


Fig.6 Failure mode of the tested specimens

4. Conclusions
An experimental axial load test is performed in this study to investigate the effect of gypsum waste on the behavior of CFST columns. The following conclusions can be summarized:
1. Hollow steel columns experienced low capacity and stiffness due to the small thickness and absence of any filling material.
2. As concluded in previous researches, using of ordinary concrete as filling material increased significantly the ultimate load and stiffness of the CFST columns by 163 to 178% and 192 to 297.9 respectively if compared with the corresponding hollow steel columns with an improved failure mode that the additional area provides more stiffness and support through column height.
3. The ductility index of columns filled with concrete increased by 78.3 to 84% that the confined concrete can withstand additional loads especially in plastic regions.
4. The load capacity and stiffness of columns filled with gypsum waste increased by 38 to 44% and 195.6 to 224.1% respectively with a slightly enhanced failure mode owing to the rather low strength of the recycled gypsum and the small thickness of steel section.
5. Using of gypsum waste instead of concrete increased slightly the ductility of columns by 5.8 to 6.7% that the strength of concrete is so high if compared with the recycled gypsum is significantly less than the standard concrete.
6. Columns filled with gypsum waste experienced load carrying capacity less by 38.1 to 48% and if compared with those filled with normal concrete due to the essential difference between concrete and gypsum waste.

References
1. Yang, Y. F., & Han, L. H. (2006). Experimental behaviour of recycled aggregate concrete filled steel tubular columns. Journal of Constructional Steel Research, 62(12), 1310-1324.
2. Abdelgadir, E., Bohai, J., Zhongqiu, F., & Zhengqing, H. (2011). The behavior of lightweight aggregate concrete filled steel tube columns under eccentric loading. Steel and Composite Structures, An International Journal, 11(6), 469-488.
3. Hassoon, A. H. H. (2016). Experimental study for the interaction curves of CFST columns subjected to a static centric or eccentric loads. University of Thi-Qar Journal, 11(2), 1-13.
4. Alzaidi, Z. A. K. (2017). Experimental study for behavior of waste concrete filled steel tubular subjected to a static axial loads. Al-Qadisiyah Journal for Engineering Sciences, 10(3), 291-298.
5. Sangeetha, P., & Senthil, R. (2017). Experimental behavior of steel tubular columns for varying in filled concrete. Archives of Civil Engineering, 63(4).
6. Tam, V. W., Xiao, J., Liu, S., & Chen, Z. (2019). Behaviors of recycled aggregate concrete-filled steel tubular columns under eccentric loadings. Frontiers of Structural and Civil Engineering, 13(3), 628-639.
7. Chen, Z., Zhou, J., Li, Z., Wang, X., & Zhou, X. (2020). Seismic Behavior of Concrete-Filled Circular Steel Tubular Column–Reinforced Concrete Beam Frames with Recycled Aggregate Concrete. Applied Sciences, 10(7), 2609.
8. Xu, D., Chen, Z., & Zhou, C. (2020). Seismic performance of recycled concrete filled circular steel tube columns. Frontiers in Materials, 7, 612059.
9. Wang, H., Zha, X., Liu, Y., Luo, C., & Iu, C. K. (2017). Study of recycled concrete–filled steel tubular columns on the compressive capacity and fire resistance. Advances in Mechanical Engineering, 9(6), 1687814017705064.
10. Tao, Z., Cao, Y. F., Pan, Z., & Hassan, M. K. (2018). Compressive behaviour of geopolymer concrete-filled steel columns at ambient and elevated temperatures. International Journal of High-Rise Buildings, 7(4), 327-342.
11. Sulaiman, A., Hunaiti, Y., Abdel-Jaber, M. T., & Abdel-Jaber, M. E. (2021). Compressive behavior of light–gauge steel tubes filled with concrete containing recycled aggregates. International Review of Applied Sciences and Engineering.
12. ASTM, A. (2011). Standard specification for general requirements for rolled structural steel bars, plates, shapes, and sheet piling.
13. EN 12390-3. (2019). Testing Hardened Concrete. Compressive Strength of Test Specimens.
14. Muthuswamy, K. R., & Thirugnanam, G. S. (2014). Structural behavior of hybrid fiber reinforced concrete exterior Beam-Column joint subjected to cyclic loading. International journal of civil and structural engineering, 4(3), 262.

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Using of gypsum waste as alternative to concrete in light gauge filled steel tube columns