Abstract
The axial capacity of light–gauge steel tube columns filled with concrete including recycled asphalt pavement (RAP) aggregates and recycled concrete aggregates (RCA) was investigated. A total of 51 specimens, including 6 bare steel tubes, 30 composite columns and 15 concrete-only columns were tested under uniaxial load. Fifteen concrete mixes were considered by replacing the weight of natural coarse aggregates (NA) with RCA and RAP at replacement levels of 0, 20, 40, 60, 80, and 100%. In addition, RAP and RCA were combined in the same mixes with replacement levels of (1) 20% RAP and 80% RCA; (2) 40% RAP and 60% RCA; (3) 60% RAP and 40% RCA; and (4) 80% RAP and 20% RCA. Experimental results were analyzed by reporting the ultimate capacities and the patterns of failure. Moreover, the predictions of EUROCODE 4 (EC4) and American Institute of Steel Construction (AISC) codes were checked. ABAQUS software was used to perform a finite element analysis (FEA) of the tested composite specimens. The results showed that using recycled aggregates decreased the carrying capacity of columns. Carrying capacity of light–gauge steel tubes filled with concrete including different combinations of RCA, NA and RAP aggregates can be conservatively predicted by the AISC and EC4 recommendations. Results of FEA showed a good agreement with the experimental results.
1 Introduction
The concrete-filled steel tube (CFST) columns are extensively used in structures subjected to large, applied moments, especially in high seismic risk zones. CFST columns are characterized by their high capacity and ductility. They have larger energy absorption compared to conventional steel and reinforced concrete columns. The enhancement in the structural characteristics of CFST columns is due to the composite action between the concrete core and the steel tube. The concrete core delays the bending and local buckling of the steel tube, while confinement of concrete through the steel tube enhances the strength and ductility of the concrete core. On the other hand, the steel tube works as column formwork in casting concrete and acts as longitudinal and transverse reinforcement, and thus it reduces the construction cost [1–4].
Extensive research has been conducted to study the behavior of CFST using conventional materials for concrete and steel under uniaxial load [1–4]. The research showed that concrete plays an important role in the capacity of CFST columns. Some researchers have tried to use new types of concrete other than conventional concrete to construct composite columns such as foamed and lightweight aggregates concrete [5, 6]. Recently, most of the research related to the study of improving properties of the concrete mix has been directed to investigate the effect of using recycled aggregates (RA) as an alternative to natural aggregates (NA) in the concrete mixture. The most used sources of RA are construction waste (crushed concrete, bricks…) and bituminous materials from roads maintenance.
Many researchers investigated whether recycled concrete aggregates (RCA) produced by crushing the construction waste could be used as a replacement of NA in concrete mixtures. The studies showed that using RCA reduces the mechanical properties of concrete. This reduction depends on the quantity of RCA, and the source of original concrete used to produce the RCA [7, 8]. Studies on the possibility of using recycled asphalt pavement (RAP) aggregates obtained by crashing the waste of roads maintenance in concrete mixtures have recently begun [9–13]. The studies showed that using RAP aggregates as an alternative to natural coarse aggregates in original concrete reduces the mechanical properties of concrete [9, 10]. But using RCA and RAP aggregates in the previous concrete improved the compressive, flexural, and splitting tensile strengths of the previous concrete [11]. Some research was conducted to study the possibility of improving the mechanical properties of concrete containing RAP aggregates by using different enhancing materials such as silica fume and class C fly ash (CFA). Using silica fume enhances the mechanical properties of concrete containing RAP aggregates, where the optimum content of silica fume was found to be 10% [12] while using CFA decreased compressive strength and indirect tensile strength of concrete significantly with the increase of RAP aggregates content [13].
The effect of using RA on the behavior of structural elements has been extensively investigated with RCA [14–17] while using RAP aggregates is limited in reinforced concrete elements and composite beams [18, 19]. In general, the behavior of structural elements including RA is similar to structural elements including normal aggregate, but there is a reduction in ultimate strength capacities.
Based on a review of the available literature, there is a lack of information about the validity of using RAP aggregates in the construction of CFST columns. Therefore, this research investigated the behavior of light–gauge steel tubes (LGST) columns filled with concrete including RCA and RAP aggregates. The concrete mixes were made by replacing NA with RCA and RAP with different replacement ratios. All columns were tested under uniaxial loading. The compressive strength of concrete was recorded, and the test results of RAP and RCA columns were compared with specimens containing only NA. The experimental capacities of the columns were compared with theoretical values calculated according to the American Institute of Steel Construction (AISC) [20] and the Eurocode 4 (EC4) [21]. In addition, a finite element analysis (FEA) was performed using ABAQUS software, and the results were compared with the experimental ones.
2 Materials and methods
2.1 Raw materials
2.1.1 Light–gauge steel tubes
Galvanized light–gauge steel sheets with 2 and 2.4 mm thickness (t) were used to fabricate the square steel tubes with 100 mm depth (D) and 1 m length. Steel thickness was chosen based on the availability of steel in the local market. Steel sheets were cut and formed to produce two pieces with shapes and dimensions shown in Fig. 1(a). The two pieces were welded using an Exx70 welding type with 3-mm thickness to construct the steel tubes as shown in Fig. 1(b). The tensile test of steel was performed according to ASTM A 37 [22] using two specimens for each thickness. The average values of the yield strength
Cross section details; (a) dimension of steel pieces; (b) cross-section of steel tube; (c) steel tube
Citation: International Review of Applied Sciences and Engineering 13, 2; 10.1556/1848.2021.00333
Light–gauge steel properties
Specimens | Yield strength (MPa) |
|
Tensile strength |
|
Elongation (%) | |
t = 2 mm | 1 | 221 | 220 | 357.2 | 355.3 | 12.90 |
2 | 219 | 353.4 | 12.50 | |||
t = 2.4 mm | 1 | 285 | 280 | 350 | 350 | 14.90 |
2 | 275 | 350 | 14.90 |
2.1.2 Aggregate and cement
NA, RCA, and RAP aggregates are the types of coarse aggregates used in producing the concrete mixes. RCA and RAP aggregates were obtained by crushing concrete cubes and road maintenance wastes, respectively. Concrete cubes were collected from different sites without knowing their compressive strength. NA was obtained from Al-Manaseer crushers located in Al-Karama City, West of Jordan. Particle aggregates sizes for the three types of aggregates range from 20 to 4.75 mm. Locally produced Silica-based sand and Portland-Pozzolana cement CEM II/A-P42.5N were used for all concrete mixes. The Portland-Pozzolana cement was produced according to Jordanian (JS) and European (EN) standards. The physical and chemical properties of cement are summarized in Tables 2 and 3 respectively.
Cement physical properties
Physical property | Unit | Result | Testing method | Specification |
Fineness by (Blaine) | cm2 g−1 | 4,000.0 | EN 196-6:1992 | |
Finene ss by (Residue)-45µm |
% | 2.5 | EN 196-6:1992 | |
Initial setting time | Min | 160 | JS 1470-3/05 EN 196-3:2005 | ≤60.0 |
Finial setting time | Min | 225 | JS 1470-3/05 EN 196-3:2005 | |
w/c | % | 29.6 | JS 1470-3/05 EN 196-3:2005 | |
Soundness | Mm | JS 1470-3/05 EN 196-3:2005 | ≤10.0 | |
2 days compressive strength | MPa | 24.0 | JS 1470-3/05 EN 196-1:2005 | ≥10.0 |
28 days compressive strength | MPa | 49.0 | JS 1470-3/05 EN 196-:12005 | ≥42.5 ≤ 62.5 |
Cement chemical properties
Chemical composition | Content |
SiO2 | 24.0 ± 0.5 |
Al2O3 | 6.5 ± 0.5 |
Fe2O3 | 6.5 ± 0.5 |
CaO | 52.5 ± 1.0 |
MgO | 3.2 ± 0.5 |
SO3 | 2.60 ± 0.2 |
K2O | 0.8 ± 0.1 |
Na2O | 0.95 ± 0.1 |
Sieve analysis for the three types of coarse aggregates was made according to ASTM C 136 [23]. Figure 2 shows the coarse aggregates grading chart for the three types with limits specified in ASTM C 33 for 25 mm (1 in.) coarse aggregates [23].
Aggregates sieve analysis
Citation: International Review of Applied Sciences and Engineering 13, 2; 10.1556/1848.2021.00333
The coarse aggregates properties were determined according to ASTM C 127 [23] and ASTM C29/C29M [23] and shown in Table 4. It can be seen that RA has less specific gravity and bulk density than the NA. According to water absorption, the RCA has the highest value due to the remained mortar layer that coats this type of aggregates, followed by RAP then NA. In some cases, RAP aggregates have the lowest absorption depends on the amount of asphalt that covers the aggregates and prevents them from absorbing water.
Coarse aggregates properties
Property | NA | RCA | RAP aggregates |
Apparent specific gravity | 2.71 | 2.65 | 2.32 |
Bulk dry specific gravity (OD) | 2.63 | 2.24 | 2.20 |
Bulk specific gravity saturated surface dry (SSD) | 2.66 | 2.4 | 2.25 |
Water absorption (%) | 1.23 | 6.92 | 2.42 |
Bulk density (unit weight) (kg m−3) | 1,670 | 1,371 | 1,316 |
2.2 Experimental program
2.2.1 Concrete mix proportions
Fifteen concrete mixes were produced in the structural engineering laboratory by replacing NA with RCA and RAP aggregates with replacement levels of (0, 20, 40, 60, 80, and 100%). In addition, NA was totally replaced by a combination of RAP aggregates and RCA using replacement levels of (20, 40, 60 and 80%). All these replacements were carried out by the weight of the coarse aggregates. NA-100 mix was designed to achieve a compressive cube strength of 30 MPa at 28 days. Table 5 shows the mix proportions and the slump test results for all concrete mixes. Water absorption amount for coarse aggregates was added to the mixes to maintain the amount of free water. Concrete mixes including RCA recorded the highest slump test results due to its high-water absorption. Six cubes with 150 mm depth were cast from each mix to determine the actual compressive strength in which all cubes were cured in water until the day of testing.
Details of mix proportions
Mix type | NA (kg m−3) | RAP (kg m−3) | RCA (kg m−3) | Unit weight (kg m−3) | Slump test results (mm) |
NA-100 | 1,145 | – | – | 2,500 | 70 |
RAP-20 & NA-80 | 916 | 229 | – | 2,383 | 75 |
RAP-40 & NA-60 | 687 | 458 | – | 2,324 | 77 |
RAP-60 & NA-40 | 458 | 687 | – | 2,314 | 83 |
RAP-80 & NA-20 | 229 | 916 | – | 2,297 | 85 |
RAP-100 | – | 1,145 | – | 2,226 | 105 |
RCA-20 & NA-80 | 916 | – | 229 | 2,382 | 85 |
RCA-40 & NA-60 | 687 | – | 458 | 2,323 | 105 |
RCA-60 & NA-40 | 458 | – | 687 | 2,311 | 112 |
RCA-80 & NA-20 | 229 | – | 916 | 2,331 | 135 |
RCA-100 | – | – | 1,145 | 2,248 | 150 |
RAP-80 & RCA-20 | – | 916 | 229 | 2,242 | 110 |
RAP-40 & RCA-60 | – | 687 | 458 | 2,275 | 124 |
RAP-40 & RCA-60 | – | 458 | 687 | 2,266 | 135 |
RAP-20 & RCA-80 | – | 229 | 916 | 2,279 | 145 |
For all concrete mixes, Cement = 375 kg m−3, Free water = 180 kg m−3, Sand = 638 kg m−3.
2.2.2 Test specimens
Fifty-one columns were tested in this study. The columns were arranged in three sets, the first set contained six steel tubes: three tubes from each steel thickness. The second set contained fifteen composite columns from each steel thickness filled with the concrete mixes mentioned in Table 5. Finally, the third set contained fifteen concrete only square columns of 100 mm side and 1 m length. All columns were cast and left without curing in the prevailing weather conditions until the day of testing.
2.2.3 Test setup
All columns were tested under an increasing axial load after 28 days of age. The test was performed using a 700 kN capacity MFL Prüf-systeme, Universal Testing Machine (Fig. 3(a)) at The University of Jordan laboratories.
(a) Compressive testing machine and (b) Test setup
Citation: International Review of Applied Sciences and Engineering 13, 2; 10.1556/1848.2021.00333
2.3 Theoretical capacity
Design codes evaluation of the carrying capacity of CFST columns is based upon the concrete and steel contributions to the resistance of the column. In this study, two codes were used to evaluate the carrying capacity of the CFST column, which are the American Institute of Steel Construction (AISC) [20] code and the EUROCODE 4 (EC4) [21]. Nominal axial load (Pn) for bare steel tubes was calculated according to the American Iron and Steel Institute (AISI) [24]. The ultimate capacity (Pun) of concrete only column was calculated according to American Concrete Institute ACI 318-14 [25]. The average cubes strength at 28 days of age was converted to concrete cylinder strength
2.4 Finite element analysis
Finite element analysis (FEA) was conducted to calculate the capacity of composite columns using the finite analysis program ABAQUS. The model was calibrated to predict the closest capacity of the composite columns, and the final model is described in this paper.
2.4.1 Part, meshing and assembly
This model contains two main parts, which are the steel tube and the concrete core meshed with an element size of 30 mm. The body of the composite column was assembled by placing the concrete core part inside the steel tube part. Figure 4(a) shows the body of composite column with meshing.
(a) Meshing (b) boundary condition
Citation: International Review of Applied Sciences and Engineering 13, 2; 10.1556/1848.2021.00333
2.4.2 Material definition
Plasticity parameters
Dilation angle | Eccentricity | Fb0/Fc0 | K | Viscosity parameter |
37° | 0.1 | 1.16 | 0.667 | 0.01 |
The compressive and tensile stress-strain relationships of concrete were defined based on Tsai’s equation for unconfined concrete [27].
2.4.3 Interaction
Interactions must be created between the entire model parts to allow them to act as a unit when any loads or boundary conditions are applied. Surface to surface interaction has been implemented between the inner surfaces of the steel tube and the outer surfaces of the concrete core with 0.3 friction coefficient and hard contact property.
2.4.4 Boundary condition
The boundary condition for the bottom end of the composite column was chosen as fixed support. The top end was modeled by restraining all the translational and rotational degrees of freedom, except the translational degree of freedom in the axial direction (Fig. 4(b)). The dynamic explicit step was defined in order to perform FEA. The axial deflection was applied to the column, until failure occurred, to simulate the actual test, and the failure loads were obtained and compared with the experimental results.
3 Results and discussion
3.1 Compressive strength of concrete
The compressive strength of each concrete mix was determined by testing six 150 mm cubes. Two cubes were examined at the age of 7 days; other cubes were tested at the age of 28 days. Average compressive strength and standard deviation (
Average compressive cubes strength and standard deviation
Citation: International Review of Applied Sciences and Engineering 13, 2; 10.1556/1848.2021.00333
3.2 General behavior and failure modes
Failure pattern of bare steel tubes and concrete only columns are shown in Fig. 6. The outward and inward deformation occurred in bare steel tubes at both ends, while concrete-only columns failed by crushing the top of columns.
Failure modes; (a) bare steel tubes; (b) concrete only columns
Citation: International Review of Applied Sciences and Engineering 13, 2; 10.1556/1848.2021.00333
All composite columns failed by crushing concrete cores. The outward deformation (local buckling) appeared in the steel section for specimens including NA and RAP aggregates, while local buckling did not appear in specimens including RCA and a combination of RCA & RAP aggregates. Figures 7 and 8 shown the failure modes of composite columns.
Failure modes of composite specimens; (a) NA-100; (b) RAP & NA combinations; (c) local buckling of different specimens
Citation: International Review of Applied Sciences and Engineering 13, 2; 10.1556/1848.2021.00333
Failure modes of composite specimens; (a) (RCA & NA) combinations; (b) (RAP & RCA) combinations
Citation: International Review of Applied Sciences and Engineering 13, 2; 10.1556/1848.2021.00333
3.3 Ultimate load test
Test results of composite and concrete-only columns are given in Table 7. Results revealed that the axial capacity of composite and concrete only columns decreased with the increase of the amount of RCA and RAP aggregates in concrete mixes, this outcome agrees with previous research [14–19]. Composite specimens of RAP-60 & NA-40 and RCA-80 & NA-20 mixes, in addition to concrete-only specimens with RCA-80 & NA-20 mix and composite specimens with 2.4 mm steel thickness and including RAP-40 & RCA-60 mix were discarded from the discussion because their capacity did not match the capacity pattern of other specimens.
Test results of composite and concrete-only columns
Concrete mix type | Experimental capacity |
||
Composite columns | Concrete only columns | ||
2 mm | 2.4 mm | ||
NA-100 | 443.60 | 554.30 | 305.47 |
RAP-20 & NA-80 | 447.27 | 559.47 | 245.90 |
RAP-40 & NA-60 | 439.27 | 490.67 | 238.12 |
RAP-60 & NA-40 | 539.47 | 510.27 | 227.87 |
RAP-80 & NA-20 | 369.27 | 420.07 | 183.07 |
RAP-100 | 380.50 | 451.97 | 158.57 |
RCA-20 & NA-80 | 434.17 | 545.37 | 250.57 |
RCA-40 & NA-60 | 423.17 | 485.87 | 269.87 |
RCA-60 & NA-40 | 414.07 | 471.77 | 238.67 |
RCA-80 & NA-20 | 461.07 | 329.27 | 254.87 |
RCA-100 | 352.27 | 461.77 | 190.67 |
RAP-80 & RCA-20 | 376.77 | 477.37 | 196.27 |
RAP-60 & RCA-40 | 354.57 | 467.97 | 179.27 |
RAP-40 & RCA-60 | 347.47 | 330.27 | 182.77 |
RAP-20 & RCA-80 | 389.37 | 482.17 | 204.14 |
According to the results of composite specimens with RAP & NA combination, composite specimens of RAP-80 & NA-20 concrete mix recorded the lowest capacity with a reduction percent of 14 and 18% for 2 and 2.4 mm steel thickness, respectively. The highest capacity was recorded for RAP-20 & NA-80 concrete, with values approximately the same as the composite specimens with NA-100. Reduction percent was recorded as 1 and 11% for 40% RAP composite specimens with 2 and 2.4 mm steel respectively and 14 and 18% for 100% RAP composite specimens with 2 and 2.4 mm steel thickness, respectively. The reduction in capacities of RAP specimens is due to weak bending between RAP aggregates coated with asphalt and concrete mix [18].
The capacity reduction of RCA & NA composite specimens relative to composite specimens including NA-100 was recorded as 2, 5, 7, and 21% for specimens with 2 mm steel thickness including 20, 40, 60 and 100% of RCA respectively, and as 2, 12, 15 and 17% for specimens with 2.4 mm steel thickness and including 20, 40, 60 and 100% of RCA respectively. The highest capacity recorded was for RCA-20 and NA-80 concrete. The capacity decreased with the increase of the RCA amount, this agrees with the conclusions of the research made by Yang and Han [14].
For composite specimens including RAP & RCA combination, the reduction was recorded as 15, 20, 21 and 12% for specimens with 2 mm steel thickness, including RAP-80 & RCA-20, RAP-60 & RCA-40, RAP-40 & RCA-60, and RAP-20 & RCA-80 mixes, respectively, while it was recorded as 14, 16 and 13% for specimens with 2.4 mm steel thickness including RAP-80 & RCA-20, RAP-60 & RCA-40, and RAP-20 & RCA-80 mixes respectively.
For concrete only columns, RCA & NA specimens recorded the lowest reduction in capacity compared with results of RAP & NA specimens and RAP & RCA specimens. Capacity Reductions were 20–48%, 12–38% and 33–41% for groups RAP & NA, RCA & NA, and RAP & RCA respectively.
The composite specimens with RAP gave slightly higher axial loads than those for the RCA composite column specimens at the same NA replacement levels. This might be attributed to the confinement of concrete made using RAP aggregates, as the effect of confinement by the steel tube can be greater compared with the concrete made with RCA aggregates; thus, showed better enhancement that compensated the difference in the compressive strength.
Table 8 shows the test results of bare steel tubes. The average capacity for bare steel tubes with 2 mm thickness was calculated using the results of all specimens. For 2.4 mm steel, the specimen with 208 kN capacity was not within the range of standard deviation, thus it was discarded.
Test results of bare steel tubes
Thickness (mm) | Experimental capacity |
Standard deviation ( |
Average capacity (kN) |
2 | 173.07 | 7.42 | 163 |
155.50 | |||
160.27 | |||
2.4 | 277.47 | 35.24 | 265.17 |
207.97 | |||
252.87 |
It can be seen that concrete including RA enhances the capacity of steel section in composite columns. The contribution of concrete to the capacity of composite columns with 2 mm was recorded as 56–64% for RAP & NA combination, 54–62% for RCA & NA combination and 53–58% for RAP & RCA combination. For specimens with 2.4 mm, the contribution of concrete was recorded as 37–52%, 43–51% and 43–45% for RAP & NA, RCA & NA, and RAP & RCA combinations respectively.
The results of this study indicate that RAC and RAP are feasible to be used in concrete filled LGST columns as a replacement of NA with low replacement levels, which did not introduce a high reduction in the capacity. This agrees well with the conclusion of the previous literature done by Yang and Han (2006) [14, 17] and El-Nimri et al. (2020) [19].
3.4 Theoretical capacity
The carrying capacities of CFST columns were evaluated by using AISC and EC4 codes are shown in Table 9. Comparing the theoretically predicted values with the experimental results shows that AISC and EC4 methods are conservative for predicting the ultimate capacity of the composite specimens. AISC method gives an ultimate capacity about 19% lower than the experimental results. The ultimate capacity predicted by EC4 is in close agreement with the experimental results for all composite columns except specimens with 2.4 mm steel thickness including RCA-40 & NA-60, RCA-60 & NA-40, and PAP-80 & NA-20 mixes. The predicted capacities of these specimens were about 5, 7, and 13% larger than the experimental results, respectively. For specimens with 2 mm thickness including NA-100 mix and specimens with 2.4 mm thickness including RAP-100 and RAP-20 & RCA-80 mixes, the calculated capacity according to EC 4 was only 2% larger than the experimental results, which lies within an acceptable range of overestimation [1].
Ratio of experimental and theoretical axial loads for AISC and EC 4 code
Concrete mix type | AISC | EC4 | ||||||
2 mm | 2.4 mm | 2 mm | 2.4 mm | |||||
|
|
|
|
|
|
|
|
|
NA-100 | 359.30 | 1.23 | 439.50 | 1.26 | 451.50 | 0.98 | 554.10 | 1 |
RAP-20 & NA-80 | 311.80 | 1.43 | 393.20 | 1.42 | 388.50 | 1.15 | 492.20 | 1.14 |
RAP-40 & NA-60 | 306.70 | 1.43 | 388.30 | 1.26 | 381.80 | 1.15 | 485.60 | 1.01 |
RAP-60 & NA-40 | – | – | – | – | – | – | – | – |
RAP-80 & NA-20 | 297.70 | 1.24 | 379.50 | 1.11 | 370.00 | 1 | 474.00 | 0.89 |
RAP-100 | 287.30 | 1.32 | 369.40 | 1.22 | 356.40 | 1.06 | 460.60 | 0.98 |
RCA-20 & NA-80 | 333.50 | 1.30 | 414.40 | 1.32 | 417.20 | 1.04 | 520.30 | 1.05 |
RCA-40 & NA-60 | 323.80 | 1.31 | 405.00 | 1.20 | 404.40 | 1.05 | 507.80 | 0.96 |
RCA-60 & NA-40 | 322.30 | 1.28 | 403.40 | 1.17 | 402.30 | 1.03 | 505.70 | 0.93 |
RCA-80 & NA-20 | – | – | – | – | – | – | – | – |
RCA-100 | 276.20 | 1.26 | 358.60 | 1.29 | 341.90 | 1.03 | 446.40 | 1.03 |
RAP-80 & RCA-20 | 288.30 | 1.29 | 370.30 | 1.30 | 357.60 | 1.05 | 461.90 | 1.03 |
RAP-60 & RCA-40 | 281.10 | 1.26 | 363.40 | 1.30 | 348.30 | 1.02 | 452.70 | 1.03 |
RAP-40 & RCA-60 | 281.50 | 1.23 | – | – | 348.90 | 0.99 | – | – |
RAP-20 & RCA-80 | 310.70 | 1.25 | 392.10 | 1.23 | 387.00 | 1.00 | 490.70 | 0.98 |
Mean Value | 1.30 | 1.26 | 1.044 | 1.003 | ||||
Standard Deviation | 0.064 | 0.077 | 0.0516 | 0.0607 | ||||
Coefficient of Variation | 0.049 | 0.061 | 0.049 | 0.0605 |
Tables 10 and 11 show the nominal axial load (Pn) for bare steel tubes and the ultimate compressive capacity (Pun) of concrete only columns. The nominal axial load (Pn) for bare steel tubes is higher than the average of experimental capacities, which means that specimens have failed before they reach local buckling. For concrete only columns, Pexp is greater than Pun except for the specimen with RAP-100.
Pn of bare steel tube
Thickness (mm) | P n (kN) | Average capacity/P n |
2 | 199.50 | 0.82 |
2.4 | 300.40 | 0.88 |
Pun of concrete only column
Concrete mix type | P un (kN) | P exp /P un |
NA-100 | 268.60 | 1.14 |
RAP-20 & NA-80 | 199.60 | 1.23 |
RAP-40 & NA-60 | 192.30 | 1.24 |
RAP-60 & NA-40 | 192.50 | 1.18 |
RAP-80 & NA-20 | 179.50 | 1.02 |
RAP-100 | 164.40 | 0.96 |
RCA-20 & NA-80 | 231.10 | 1.08 |
RCA-40 & NA-60 | 217.10 | 1.24 |
RCA-60 & NA-40 | 214.80 | 1.10 |
RCA-80 & NA-20 | – | – |
RCA-100 | 148.70 | 1.28 |
RAP-80 & RCA-20 | 165.90 | 1.18 |
RAP-60 & RCA-40 | 155.70 | 1.15 |
RAP-40 & RCA-60 | 156.30 | 1.17 |
RAP-20 & RCA-80 | 198.10 | 1.03 |
3.5 FEA results
Table 12 compares the experimental capacity (P exp ) for composite columns with results of finite element analysis (P FE ). The maximum axial load obtained from finite element model showed a close agreement with the experimental results, except NA-100 specimen with 2 mm steel thickness and RCA-40 & NA-60, RCA-60 & NA-40, and PAP-80 & NA-20 specimens with 2.4 mm steel thickness, which give about 4–13% higher than experimental results.
Comparison between the Pexp of composite specimens and P FE
Concrete mix type | 2 mm | 2.4 mm | ||
|
|
|
|
|
NA-100 | 463.27 | 0.96 | 556.18 | 0.99 |
RAP-20 & NA-80 | 395.37 | 1.13 | 489.55 | 1.14 |
RAP-40 & NA-60 | 388.82 | 1.13 | 482.48 | 1.017 |
RAP-60 & NA-40 | – | – | – | – |
RAP-80 & NA-20 | 375.85 | 0.98 | 477.06 | 0.88 |
RAP-100 | 360.88 | 1.05 | 455.34 | 0.99 |
RCA-20 & NA-80 | 427.12 | 1.02 | 518.95 | 1.05 |
RCA-40 & NA-60 | 413.28 | 1.02 | 507.07 | 0.96 |
RCA-60 & NA-40 | 410.37 | 1.01 | 502.02 | 0.94 |
RCA-80 & NA-20 | – | – | – | – |
RCA-100 | 344.70 | 1.02 | 439.24 | 1.05 |
RAP-80 & RCA-20 | 362.19 | 1.03 | 452.07 | 1.06 |
RAP-60 & RCA-40 | 352.30 | 1.01 | 446.75 | 1.05 |
RAP-40 & RCA-60 | 354.30 | 0.98 | – | – |
RAP-20 & RCA-80 | 396.65 | 0.98 | 487.64 | 0.99 |
4 Conclusions
The compressive behavior of LGST columns filled with concrete including RA was investigated experimentally and theoretically in this study. Three types of coarse aggregates were used for the construction the columns with different combinations (RCA & NA, RAP & NA, and RAP & RCA). According to the results, the following conclusions are drawn from this study:
-
Increasing the amount of RCA or RAP aggregates in composite columns decrease the axial compressive capacity. The reduction ratio compared to NA-100 specimens was recorded as 1–14% and 11–18% for 2 and 2.4 mm steel thickness including 40–100% RAP aggregate respectively, and 2–21% and 2–17% for 2 and 2.4 mm steel thickness including 20–100% RCA, respectively.
-
The RAP & RCA composite columns showed a 12–21% reduction in capacities for specimens with 2 mm steel thickness and 13–16% for specimens with 2.4 mm steel thickness, in comparison with the specimen with NA-100.
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The optimum amount for using RCA or RAP as an alternative to NA has been reported at 20% of replacement. RAP-20 & NA-80 composite specimens recorded approximately the same capacity as the NA-100 composite specimens, while the reduction ratio was recorded as 2% for RCA-20 and NA-80 composite specimens.
-
Composite specimens with RAP gave slightly higher capacities than that for the RCA composite specimens at the same replacement levels. This indicates that the enhancement in the strength of concrete due to confinement is more effective in concrete including RAP than that including RCA, although compressive strength of concrete including RCA is higher than that containing RAP.
-
For concrete only columns, RCA & NA specimens recorded the lowest reduction in capacity compared with results of RAP & NA and RAP & RCA specimens. Capacity Reductions were 20–48%, 12–38%, and 33–41% for specimens including RAP, RCA, and RAP & RCA, respectively.
-
The carrying capacity of light–gauge steel tubes filled with concrete including RCA, RAP, and a combination of RCA and RAP aggregate can be conservatively predicted by AISC and EC4 recommendations, where the EC4 method gives closer predictions of the test results than the AISC method.
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The existing finite element model proposed in this study can predict the test results of light–gauge steel tubes filled with concrete including RCA, RAP aggregate reasonably well.
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The LGST columns filled with concrete including RA can be used in small to medium sized buildings with low to medium compressive strength.
Acknowledgments
The authors would like to gratefully acknowledge the financial support from Deanship of Scientific Research at The University of Jordan.
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