Structural behavior of concrete ﬁ lled double-skin PVC tubular columns con ﬁ ned by plain PVC sockets

: The behavior of concrete-ﬁ lled double-skin poly vinyl chloride (PVC) tubular columns con ﬁ ned by plain PVC sockets is studied in this research. The plastic pipes have been used with concrete as columns. PVC has several advantages as it is inexpensive, light weight for construction, and decrease in the time taken for construction. Thus it can be used as an alternative to metal in various applications in which the maintenance cost is increased and the corrosion is reduced. This study is an experimental work on four groups of composite hollow PVC columns, each group containing three di ﬀ erent samples. The e ﬀ ects of several parameters are studied, including slenderness ratio, compactness ratio, compressive strength, and con ﬁ nement ratio. The experimental results show that sti ﬀ ness, strength, as well as the ductility of composite columns of di ﬀ erent loadings were considerably in ﬂ uenced by all these parameters. An increase in loading carrying capacity by about 40.4% was noticed as slenderness ratio decreased from 20 to 12 and about 21.1% as compactness ratio decreased from 50 to 25. Besides, the rate of increase in load capacity was about 96.6% as compressive strength increased from 33 to 54 MPa. For the case of variation of con ﬁ nement ratio from 25 to 75%, the increase in the axial load capacity was about 56.7%. The main bene ﬁ t of this type of column is the interaction between PVC tube and concrete. Besides, restriction of concrete by tube results in the delay of local buckling so that the concrete strength is enlarged by the e ﬀ ect of con ﬁ ning provided by the tube.


Introduction
Columns consisting of more than one material are typically named as composite columns. Various materials effort together to attack the strains as well as stresses induced by external applied loads in these columns.
Actually, the conventional reinforced concrete columns might be denoted as composite columns as they consist of steel in addition to concrete, though the term "composite columns" is typically used to state applications like sections filled with or encased in concrete as seen in Figure 1.
Various materials are usually used with concrete like wood, steel, aluminum, plastic tubes as well as the fiberreinforced polymer.
Basically, unplasticized poly vinyl chloride (PVC) pipes are resistant to destruction and remain for long time without the necessity for standby and are impermeable to fluids and gases in addition to being durable. These types of pipes are extensively used in the construction industry [1].
The plastic pipes have been used with concrete as columns. These kinds of columns are usually denoted to as concrete-filled plastic tube.
This type of column is used since it is inexpensive and economical kind of column for its light weight structure. In addition, plastic tube is stay-in-place formwork which is used as a substitute to the conventional formwork system.
The systems are chiefly gathered on site leading to simplify the construction process in addition to reducing the time of construction.
The interaction between concrete and plastic tube is regarded as one of the chief advantages, so the tube local buckling is delayed by restriction of concrete and the concrete strength is improved through the restraining effect of the tube [2].

Numerical and experimental research
The experimental and theoretical analyses on composite confined PVC columns filled with concrete have been studied by a number of researchers. An experimental investigations and theoretical analysis on commercially presented plastic pipe filled with a concrete core were conducted [3]. The theoretical analysis inferred a corresponding rising in the concrete strength and an interaction between the plastic pipe and concrete core. The structural behavior of the plastic pipe was like the performance of spiral reinforcement under a column loading. Besides, the plastic pipe causes an increase in the concrete core strength about 3.2 times the pipe burst pressure.
Marzouck and Sennah [4] assessed the use of the commercially available PVC tubes as compression members filled with concrete. Four concrete-filled PVC columns were tested experimentally up-to-failure. The diameter of whole samples was 100 mm and of several heights. Two reference plain concrete samples were tested for providing the involvement of the PVC tube in the concrete confinement. All specimens were exposed to monotonically increase the axial load until compression failure happened and were tested vertically. The relationship between axial load and displacement is documented for all samples. This research offered a latitude for additional study to conduct experiments on concrete-filled PVC tube having different concrete properties as well as different slenderness ratios to further recognize the behavior and then, suggest a design approach for using this type of composite columns in light construction.
The experimental research on columns of concretefilled unplasticized PVC tubes in marine environment was studied [5] by filling reinforced concrete to the tubes subjected to artificial sea water equipped in the laboratory. Totally, 72 specimens of length equal to 800 mm were casted by filling the reinforced concrete in the tubes with different diameters of 160, 200, and 225 mm. The diameter-to-thickness ratio of the specimen varied from 22.48 to 40.14 and length-to-diameter ratio of the specimens also varied from and 3.56 to 5. All the specimens were subjected to axial load on the core of the concrete in order to attain load-displacement curves and failure modes. No degradation in the ductility and strength of reinforced concrete-filled unplasticized PVC tubular specimens was detected after submerge in the sea water. It can be established that unplasticized PVC tube offers a safety jacketing to encase the core of concrete, and consequently develop ductility and strength.
Naftary et al. [6] studied the influence of using unplasticized poly vinyl tubes in restraining short concrete columns on the compressive strength by using an experimental program in which plastic tubeswith various diameters and heightswere used to restrict concrete of various strengths. The composite columns were applied to concentric axial compressive loads till failure. A typical shear failure was the principal failure mode. The results showed that compressive strength of the column samples increases through rising in concrete strength and reduced with the rising in column height.
Many experimental studies have been conducted on concrete-filled PVC tubes [7,14].
The concrete strength is increased by adding steel fibers. The steel fibers have a reinforcement effect on cement components that improves the brittle nature of cementitious materials [15].
Raheemah et al. [16] investigated experimental study with several variables such as concrete filling compressive strength, PVC section compactness ratio, and column slenderness ratio. The samples were exposed to uniaxial compression in two loading modes. For the first mode, a PVC tube was employed as a composite element to improve the concrete core, while a PVC tube in the second mode was employed to restrain the concrete core alone.
Nine sections of PVC-concrete with various characteristics of PVC tube and various compressive strengths of filled concrete were regarded in a number of fabricated slender PVC-concrete columns. The column lengths were changed in order to examine the total column buckling. The results presented that the columns composite mode displayed further strength enhancement than the confined mode. Experiments established that the effective flexural stiffness was accordingly normalized in the term of PVC tube stiffness as well as filling concrete stiffness, and the normalized results showed that effective flexural stiffness based on the mode whichever being composite or confining mode, PVC as well as strength of filling concrete, in addition to column slenderness ratio.
It is obvious from this review that it is essential to study the behavior of confined composite hollow PVC columns filled with concrete. However, it is inexpensive, more economical kind of column for light weight construction, and decrease in the time of construction as the plastic tube substitute the formwork. Also, the tube local buckling is delayed by restriction of concrete and the confining effect of the tube led to an increase in the concrete strength.

Objective of research
The objective of this research is to study the structural behavior of confined composite hollow PVC columns filled with concrete. This kind of columns is commonly known as concrete-filled PVC tube.
This type of columns is used since it is inexpensive, more economical due to light weight construction, and decrease in the time of construction as the removal. Also, the local buckling of the tube is delayed through restriction of concrete and the confining effect of the tube led to an increase in the concrete strength.
The main advantage of this type of column is the interaction between PVC tube and concrete. Besides, restriction of concrete by tube results in delay of the local buckling, so that the concrete strength is enlarged by the effect of confining provided by the tube.

Experimental program
Twelve samples in addition to control sample are tested by subjecting to a compression test by applying pressure load up to failure to determine the influence of these parameters on strength and the local buckling of composite PVC columns.

Specimens details
Twelve samples are divided into four groups. In addition, the control sample has a length equal to 600 mm, diameter of 100 mm, and the diameter of inner hollow equal to 50 mm. Each group consists of three specimens. All columns in the first group have the same thickness, same compressive strength, and diameter but with different length and slenderness ratio (L/r), which is defined as the ratio between the length of the tube to its radius (L/r)the external confinement of ratio is 50%. In the second group, all columns have same length, same compressive strength, and diameter but with different thickness and compactness ratio (D/tp), which is defined as the ratio between the tube diameters to its thickness (D/t) and having the same confinement ratio which is equal to 50%. In case of the third group, all parameters are constant except the compressive strength which is varied into three targets (33, 36, 54 MPa). For the fourth group, only confinement ratio varied which represents the ratio of the total length of PVC socket to the total length of PVC pipe and the other parameters remained constant. PVC pipe normally bonds to concrete. The details of all groups are displayed in Table 1

Material properties
The materials used in concrete mixes are ordinary Portland cement and crushed gravel having a maximum size of 10 mm particles. The bulk specific gravity of this aggregate is 2.8 and sulfate content is 0.08% for the case of natural sand, its sulfate content SO 3 is (0.11%) by sand weight and a specific gravity is 2.6. The grading of both coarse and fine aggregate is shown in Tables 2 and 3, respectively. Besides, discrete steel fiber as well as water reducer, which is special chemical products, is added to the concrete mixture before it is poured to reduce the water content. It reduces the porosity of concrete and increases the concrete strength. Different percent of steel fibers ranging from 1 to 1.9% is added by making several trail mixes to reach the required target of concrete strength. The properties of steel fiber are shown in Table 4. The concrete compressive strength depended on the average values of 150 mm × 150 mm × 150 mm cubes. The stress-strain curve of the concrete used is shown in Figure 3. In addition, a concrete rupture is calculated through splitting test. Table 5 shows the mixing proportions of the material used for concrete mix. The inner diameter of plain sockets is 110 mm and

Preparation stages 4.3.1 Cutting stage
The models were cut at the required lengths as mentioned previously and as shown in Figure 4.

Casting stage
At this stage, all models were poured according to the previously mentioned mixing ratios. First, the outer pipe is put        over a plate on the floor of the lab. Then the inner tube is centered in the middle of outer tube and the casting process started. A mechanical vibrator was used throughout the casting process to avoid nesting in concrete. A perforated cap was placed to measure the diameter of the inner tube, in order to keep the centrality of the inner tube which will be in the middle of composite column. The casting stage is shown in Figure 5.

Test results of the work
The experimental tests were carried out on 12 composite doubly skinned hollow PVC composite columns with different lengths, thicknesses, compressive strength, and confinement ratios which are subjected to a compression test by applying pressure load up to failure using a hydraulically testing machine as shown in Figure 6. The experimental results have shown that the increase in length reduces the vulnerability of carrying columns as well as increase in thickness and compressive strength of concrete and the restriction ratios cause an increasing in concrete load carrying capacity. The variations of lateral strain on the applied load at the third and middle height of control column can be represented as Sc1 and Sc2, respectively, and as St1 and St2, respectively, for the case of PVC column. Besides, the axial strain at the mid-height can be represented as Sc3 for the case of control column and as St3 for the case of PVC column as shown in Figures 7-12.
The failure load and the corresponding axial and lateral strain are summarized in Table 8.

Failure modes
All composite PVC columns undergo global elastic buckling which is followed by substantial local buckling defor-   mation, causing a sudden reduction in loading carrying capacity. In addition, the outward expansion of the composite PVC columns leds to drum type failure.
In the case of concrete column, a longitudinal crack in the top region of column in addition to concrete crushing is noticed as shown in Figure 13.
As concerned to slender PVC concrete columns, stability failure controls the wholly tested samples that also undergo global elastic buckling in addition to local buckling deformation and considerable failure of concrete led to sudden reduction in the loading capacity with assigned sustainable lateral deformation.

Parametric study
Several parameters in this experimental test are studied, including slenderness ratio, compactness ratio, target of compressive strength, as well as confinement ratio, so as to study the effect of these parameters on the peak load of each composite PVC columns and the value of the lateral and axial strain.

Slenderness ratio
The failure modes of composite PVC column having different slenderness ratios which is defined as the ratio between the length of tube to its radius (L/r) by change the length of the sample pipe and fix all the retained parameters are shown in Figure 18. By making a comparison between them, it can be observed that stiffness and ultimate load of the composite column decrease with an increase in the slenderness ratio (i.e. increase in length). It can be noticed that there is an increase in peak load of about 17.97% as slenderness ratio decrease from 20 to 16 and about 19% as slenderness ratio decreased from 16 to 12. Subsequently, the total increase in peak load was about 40.4 as slenderness ratio decreased from 20 to 12. This variety in slenderness ratio is due to varying in the length of the column that made the global buckling could be examined the less resistance to the applied load will be, and it will lead to faster deformation as well as faster failure. Columns with a high slenderness ratio are more susceptible to buckling and are classified as "long" columns. The failure modes for composite column are shown in Figures 13-18. Figure 23 shows another comparison between composite PVC column with different compactness ratios which is defined as the ratio between column diameters to its thickness (D/t) by change the thickness of all sample columns   and fix all other parameters. From this comparison, it can be observed that the rate of increase in peak load was about 16.67% as the compactness ratio decreased from 50 to 33.3 and about 3.8% as it decreased from 33.3 to 25. Consequently, the total increase in peak load was about 21.1% as compactness ratio decreased from 50 to 25 as shown in Figure 8. The failure mode of these composite column is shown in Figures 19-23. The total increase in peak load was about 96.6% as compressive strength increased from 33 to 54.

Confinement ratio
Finally, the effect of confinement ratio was studied in the fourth group by studying the effect of three confinement ratios (25, 50, and 75%) on the behavior of hollow PVC composite column. It can be noted from Figure 33 that    columns with confinement equal to 75% increase the load carrying capacity as compared to other confinement ratio. The rate of increase was about 28.5% as confinement ratio increase from 25 to 50%, and about 22% as confinement ratio increase from 50 to 75%. The total increase in load was about 56.7% as confinement ratio increase from 25 to 75%. The failure modes of these columns are shown in Figures 29-33.

Conclusion
Based on the total results achieved from the experimental work using 12 composite columns of different slenderness ratios, compactness ratio, target of compressive strength, and confinement ratio, the following conclusions are drawn: 1. The loading resistance of PVC composites with concrete showed high strength development and same effects were detected in the latitude of ductility. 2. The varying of loading influence on load-strain response had considerable influence on strength improvement for    composite mode samples so that PVC could act like reinforcement. 3. Stiffness, strength, as well as the ductility of composite PVC concrete columns of different loadings were considerably influenced by the slenderness ratio (L/r) of the column, the compactness of PVC section, and the filling concrete strength. An increase in loading carrying capacity by about 40.4% was noticed as the slenderness ratio decreased from 20 to 12. In additrion, an increase in load carrying capacity by about 21.1% was observed as compactness ratio decreased from 50 to 25. For the case of compressive strength, it was noticed that the rate of increase in load capacity was about 96.6% as compressive strength increased from 33 to 54 MPa. For the case of variation of confinement ratio from 25 to75%, the increase in the axial load capacity was about 56.7%. 4. All the tested columns conquered stability failure; besides, the samples underwent global elastic buckling with substantial failure of concrete in addition to local buckling deformation causing a sudden decrease in loading capacity corresponding to lateral deformation.