رفتار سازه ستون های لوله فولادی پر شده بتن مصالح ریزدانه بازیافت شده تقویت شده با CFRP

This paper presents experimental research on structural behaviour of normal and recycled aggregate concrete filled steel tube (RACFT) columns externally strengthened with carbon fibre reinforced polymer (CFRP) sheets and subjected to axial loading. A total of 22 specimens were tested to investigate the influence of the following variables: (1) tube configurations, circular or square, solid or hollow; (2) types of concrete, normal or recycled aggregate concrete; and (3) strengthening arrangements, full wrapping or partial wrapping. The results show that the recycled aggregate concrete may degrade the concrete strength during the curing period for the first 28 days. However, the ultimate strength and stiffness (90-day curing period) for the square and circular solid steel tubes filled with recycled aggregate concrete show slightly higher values than the ones filled with normal concrete, but this is reversed for the steel hollow section tubes. The test results indicate that both reinforcing arrangements enhance the ultimate strength greatly in comparison to the control column, and that the full wrapping arrangement is much more effective than the partial wrapping one. Moreover, stiffness of the strengthened columns is increased significantly due to the restraint offered by the external CFRP wrapping on hoop deformations during compressive loading. In addition, theoretical calculations of the bearing capacity of the composite columns are presented and compared with the experimental results.

Composite construction encompassing the use of steel and concrete has been widely utilised in structural engineering. Currently, normal concrete filled steel tube (CFT) columns have increasingly found their applications in high-rise buildings, offshore structures, bridges and warehouses, particularly in the region with a high risk of seismic loading, due to the combined advantages of steel and concrete. Such columns possess a high strength-to-weight ratio, better seismic performance, high fire resistance and increased deformability [1], [2] and [3]. Through experimental work, theoretical approaches and the finite element analysis, a number of studies has been conducted to investigate the effects of the cross section shape [4] and [5], the concrete strength [2], [6], [7], [8], [9], [10] and [11], the depth-to-thickness ratio [2], [10], [11], [12] and [13], the loading conditions [14] and [15] and the types of filling materials [16] on the failure mode, load carrying capacity and ductility of CFT columns. In recent years, with the increasing demand for infrastructure development, the consumption of concrete as a main construction material keeps growing. However, the emission of carbon dioxide from the production processes is estimated about 1.35 billion tons annually or about 7% of the total greenhouse gas emissions to the earth’s atmosphere. Therefore, the recycled aggregate concrete filled steel tube (RACFT) columns have increasingly attracted structural engineers, as this is beneficial for the saving of natural resources and the environmental preservation and is also the need for a low carbon economy. The earthquake on 12 May 2008 in Sichuan was one of the most destructive earthquakes in modern Chinese history, which generated approximately 382 million tons of construction wastes from the collapsed buildings [17]. Some studies have been undertaken to investigate the structural behaviour of the recycled aggregate concrete columns made with different proportions of recycled aggregate [18] and [19] and other types of waste materials [20] and [21]. However, limited research has been conducted on the structural behaviour of steel tube columns filled with recycled aggregate concrete to produce RACFT columns. Yang and Han [22] studied the structural performance of steel tube columns with circular and square sections filled with normal and recycled aggregate concrete. They found that the typical failure modes of RACFT columns were similar to those of CFT columns, but the ultimate load carrying capacities and ductility of RACFT columns were slightly lower than those of CFT columns. However, RACFT columns can be strengthened by fibre reinforced polymer (FRP) jackets. Confinement of CFT columns by means of FRP jackets is a technique being frequently used to seek enhancement of load carrying capacity of such compressive members [23]. There have been a number of theoretical and experimental studies related to strengthening of CFT columns with FRP, which have shown a particular enhancement on the load carrying capacity due to the restriction and postponement of buckling failure offered by FRP. Research has been undertaken on the effect of cross-sectional types, i.e. square hollow [24], [25] and [26], circular hollow [27] and [28], square solid and circular solid [29] and the other section types [30], as well as the effect of loading conditions (cyclic or eccentric load) on square and circular solid [31] and [32] CFT columns strengthened with carbon and/or glass fibre reinforced polymer (CFRP/GFRP) composites. The results have also shown that there is a significant enhancement on both the load carrying capacity and the stiffness of CFT columns with externally bonded FRP. Moreover, Zhao and Zhang [33] gave a comprehensive review of research on FRP strengthened steel structures and suggested that the future research would likely be focused on the stability of the FRP strengthened steel hollow section members. There has been extensive experimental research on either the behaviour of RACFT columns or the FRP confined CFT columns, as indicated above. Also some attempts have been made to study the structural behaviour of the columns filled with recycled aggregate concrete [17], [18], [34], [35] and [36]. However, such the research together with external strengthening on the columns by FRP materials is limited. This paper presents experimental study of RACFT columns strengthened with CFRP sheets subjected to axial loading. Twenty two specimens of circular and square steel tube columns were manufactured and tested, which are comprised of 11 solid and 11 hollow ones, and filled with natural or recycled aggregate concrete and externally strengthened with two layers of CFRP sheets along the circumferential direction. The main parameters studied in the tests are (1) tube configurations, circular or square, solid or hollow; (2) types of concrete, natural or recycled aggregate concrete; and (3) strengthening arrangements, full wrapping or partial wrapping. Deformation and failure modes of the RACFT columns are also presented and discussed. In addition theoretical calculations of the load bearing capacity of the composite columns are undertaken and compared with the experimental results.

Structural performance of short steel tube columns filled with normal and recycled aggregate concrete and externally bonded with CFRP sheets has been investigated. A series of compressive tests of steel columns with different cross sections, i.e. circular solid and hollow, and square solid and hollow, has been undertaken to obtain the failure mode and load carrying capacity of the columns. The compressive strength of the normal concrete is slightly lower than that of the recycled aggregate concrete at 90 day curing period. This indicates that the longer curing period for RAC is crucial to gain the necessary compressive strength when it is used to fill steel tubes. The results show that the ultimate bearing load of the square and circular solid steel tubes filled with recycled aggregate concrete give comparable values to the ones filled with normal concrete after the 90-day curing period. The stiffness of the columns with a solid cross-section and filled with RAC is significantly higher than that of the similar columns filled with normal concrete, however, the stiffness of the columns with a hollow cross-section and filled with RAC is significantly lower than that of the similar columns filled with normal concrete. The results also indicate that the use of CFRP wrapping for strengthening recycled aggregate concrete filled steel tubes gives significant enhancement on the load carrying capacity and the ultimate displacement, and the structural performance of the columns with circular cross-section profile (either solid or hollow) is better than that of square ones. Furthermore, the full wrapping arrangement of CFRP is more effective than the partial wrapping arrangement in terms of enhancing the load carrying capacity and stiffness of the steel tube columns. In addition, regression equations with key physical parameters have been developed to estimate the load carrying capacity of the steel tube columns with and without CFRP reinforcement, and comparable results have been obtained between the theoretical calculations and experimental results.