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Ian Ryabov
Ian Ryabov

Concrete Column Interaction Diagram Software.epub

Abstract:In Pakistan, raw material from several sources is utilized in the production of steel bars; consequently, the chemical and mechanical properties of locally manufactured bars differ drastically. According to the reviewed literature, there is a significant amount of variation in the data on rebar yield strength. This unintentionally higher yield strength might have serious consequences on a reinforced concrete (RC) column, as the failure mode could shift from ductile to brittle. The purpose of this study is to investigate the repercussions of an unintentionally higher rebar yield strength on an RC column. In order to mitigate the effects of an unintentionally higher rebar yield strength on the behaviour of the RC column, some modifications to the design approach are recommended.Keywords: uncertain yield strength; higher yield strength; RC column; production flaws; RC design; failure mode

Concrete Column Interaction Diagram Software.epub

Reinforced Concrete (RC) columns are structural members used mainly to carry compression loads. They have relatively small cross-sectional dimensions as compared with their height. RC columns are conventionally composed of steel reinforcing cages embedded in concrete (Abdualrahman and Al-Zuhairi, 2020a, Abdualrahman and Al-Zuhairi, 2020b). Nowadays, many researchers studied several types of nonconventional RC columns that used in structures such as composite columns, hollow columns and hybrid columns, as shown in Figure 1. These types of columns can be executed from one or two types of concrete. Structural combining of these types of materials can result in highly efficient and lightweight columns. This technique also offers benefits in terms of construction time-saving. However, some researchers studied the structural behaviour of tubular RC columns such as (Han et al., 2009; Yuan et al., 2018; Alshimmeri, 2016). In Figure 1(a), the concrete in the steel tube differs from the outside concrete without attachment between them. As well, many researchers studied the behaviour of RC hollow columns as displayed in Figure 1(b) with or without infill materials (Hadi and Le, 2014). Hybrid Columns (HC) are made from low strength concrete core jacketed with an outer skin made of high strength concrete. These columns can play a featured role in sustainability issues. The columns can be utilized effectively to recycle the crushed concrete wastes as the coarse aggregate of the inner concrete core. In this position, the problem of concrete carbonation that surely is faced in recycled concrete usage will be eliminated by exclusion this concrete from the area of existence of steel reinforcement, the most affected component by this problem. Thus, steel reinforcing rebars will remain protected from corrosion by the passive protection provided by the outer concrete despite the use of recycled concrete in the inner core.

Based on available previous studies, two methods were observed to produce hybrid concrete columns: the normal (or low) strength concrete is made as inner core either confined by fibre reinforced concrete or high strength concrete without fibre materials, i.e., the column is made with two different concrete types (Wu et al., 2018; Resheq; 2018, Hamid et al., 2020; Ali, 2020). In both methods, the outer skin and inner core are interacted either by the full bond between the two concrete layers or by a partially bond between them. If the column is constructed continuously at one time, the chemical-physical interaction that happened at the interface of the two concretes leads to mutual bonding between them (Resheq, 2018; Hamid et al., 2020). Three primary factors contribute to the bond strength; natural adhesion, friction between different layers and the use of reinforcements (Dybet and Watach, 2017). On the other hand, the behaviour of HC with partial bonding between concrete layers was also studied by other researchers. In these columns, the concrete layers are connected either by resin materials or by shear keys (Wu et al., 2018; Ali, 2020; Abdulhameed and Said, 2019). These studies are closer to the topic of strengthening or repairing existing columns.

Hybrid columns can play an important role in the issue of sustainability in which they can be utilized effectively to recycle concrete waste after crushing as a coarse aggregate of the inner concrete core. In this position, the concrete carbonation problem that surely will be faced in recycled concrete usage can be eliminated by excluding this concrete out from the area of existence of reinforcing steel rebars, which is the most affected component by this problematic phenomenon. Thus, steel reinforcing rebars will remain protected from corrosion by the passive protection provided by the outer concrete despite the use of recycled concrete in the inner core.

Until the preparation of this paper, no obtainable information regarding the behaviour of hybrid concrete columns under the effect of biaxial loading is found available. Consequently, there is a lack of knowledge regarding the behaviour of biaxially loaded hybrid concrete columns with a full bond between concrete layers. This study aims to present experimental, analytical and numerical results of biaxially loaded HC with full interaction between inner and outer concrete layers. From these results, the general behaviour of such columns can be realized. The analytical study that was conducted on two conventional reinforced concrete and three hybrid concrete columns subjected to biaxial loading was starting from simple hypotheses of linear strain distribution and ending with developing a computer program to evaluate the strength and deformation behaviour of hybrid columns. Besides, all RC columns were analyzed by the finite element method via the ABAQUS program.

Ordinary Portland cement, river sand with fineness modulus of 2.84, crushed gravel with a maximum size of 9.5 mm and tap water were used for LSC mix production. In addition to these materials, a high-performance concrete superplasticizer was used to produce an HSC mix. The two concrete mixes were designed and prepared to produce the experimental RC column specimens. LSC mix was proportioned according to the ACI 211.1-2017. While the HSC mix was designed according to the ACI 211.4R-2017. Table 2 shows the quantities of materials per unit volume (kg/m3) for the two mixes.

A steel mould was fabricated to cast RC column specimens. The mould consists of three parts, the first part is the mould base which is a square steel plate with 8 mm thickness and 800 mm side dimensions. Three square grooves were centrically created on the steel base to receive the hollow steel sections that will be responsible for forming the inner concrete part. The other two parts were manufactured as two square halves made from a 6 mm steel plate that was connected by bolts to form a 200 mm square steel tube that is going to be connected to the steel base utilizing bolts. The details of the steel mould, as well as the reinforcement cage, is displayed in Figure 4.

All column specimens were cast vertically. The conventional two RC columns (HSC & LSC) were cast at one time without using a central steel tube. While, the other hybrid RC columns were produced by inserting a square hollow steel tube with a side dimension of 80, 100 or 120 mm inside the mould so that its lower end will be received by the corresponding groove of the steel base as shown in Figure 5. In addition, the top of the inner hollow steel section is laterally supported to the top sides of the outer steel mould using four adjusting bolts to prevent any lateral movement and providing the ability to pull out the inner steel section easily. The HSC layer was cast firstly in the space between the inner steel tube and the outer mould to about one-third of the mould height, then the LSC was cast inside the inner hollow steel section to the approximate same level of outer HSC. After that, the inner steel tube was moved carefully up to a quarter of steel mould height. This process was repeated twice so that the whole HC was constructed and the inner steel tube was drawn completely. By this construction procedure, the two types of concrete in HC have fully interacted. The steps of HC production were schematically drawn as shown in Figure 5.

The goal of this part is to develop a computer program able to evaluate the strength capacity of HC based on the discrete element method. This method has been used by some researchers such as (Hsu, 1989) for the analysis of biaxially loaded conventional RC columns. In this study, the proposed computer program was written via MATLAB coding R2015b.In the present model, the effect of longitudinal bars, distribution of lateral reinforcement and concrete confinement were taken into consideration with adopting the stress-strain curve of confined concrete proposed by (Mander et al., 1988; Bouafia et al., 2014). The present method is based on strain compatibility, forces equilibrium and assuming plane section before bending remains plane after bending and full bond between concrete and steel reinforcement.

Figure 9 Stress-strain curves for materials: (a) Uniaxial stress-strain diagram of unconfined plain concrete in compression; (b) Tensile stress-strain diagram of plain concrete; (c) Theoretical stress-strain curve of reinforcement (Bilinear Model).

A three-dimensional 8-noded hexahedral element (brick element) with 3 degrees of freedom in each node (C3D8R) was used for concrete modelling. This element has a 1-integration point. Besides, steel reinforcement was modelled with a 2-noded truss element (T3D2) having 3 degrees of freedom in each node assuming a full bond between concrete and steel bars by embedding the reinforcement element into the concrete elements. Two reference points (RP) were created at the top and bottom surfaces for each RC column with eccentricity equal to 50 mm (e/h = 0.4). A coupling constraint was used between each reference point and the top and bottom surface. The load was applied as a velocity equal to -80 mm/s at the top reference point. Besides, all displacements in the x, y, and z-plane were constrained except the axial displacement at the top reference point. However, rotation around these axes was allowed. Finally, in hybrid column specimens, the two types of concrete were constrained at the interface surface by tie constraint (master-slave type). Figure 10 displayed the loading, boundary conditions, meshing, and finite elements used in the hybrid column specimen. Table 3 shows the types of finite elements used in this study.


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