Behaviour of Concrete Confinement by PVC Tubes and Strengthened with Carbon Textile-Reinforced Concrete

To improve the properties of concrete, it is common to combine it with various materials to enhance its characteristics and durability. Traditionally, steel tubes have been used to reinforce concrete due to their strength and rigidity. When steel tubes are used and filled with concrete, these two elements combine the compressive strength of concrete with the tensile strength of steel, offering a wide range of applications in buildings, including long-span bridges and high-rise buildings [1, 2]. On the other hand, steel tubes incur high construction and maintenance costs and are susceptible to corrosion under harsh environmental conditions [3, 4]. Therefore, alternatives to steel tubes, including fiber-reinforced polymer (FRP) tubes, were used. While FRP tube is highly corrosion-resistant, its construction costs and brittleness limit its suitability for certain construction applications [5-9]. Conversely, polyvinyl chloride (PVC) tube is a superior alternative to steel and FRP tube because of its unique properties: a blend of polymer resins, stabilizers, and pigments, along with the ability to customize it to meet specific requirements [10, 11]. PVC is a universal option that outperforms other alternatives economically and is suitable for various infrastructure projects at different scales [12]. Polymer materials are widely used due to their exceptional properties, and these materials include tubes used for water supply and drainage, as well as materials used in the construction of cables and wires [13-15]. The construction sector has extensively exploited the properties of polymers in construction processes [16-26]. In many civil engineering applications, concrete-filled tubular columns are used because of their excellent seismic performance, characterized by high resistance to axial compression and stiffness [27]. These tubes delay the lateral expansion of the concrete and provide support to the tube to resist buckling by surrounding its outer core [28]. Confining the concrete within the tube places it under triaxial stress, thereby improving the pipe’s compressive strength [29, 30]. From a sustainability perspective, using tubes eliminates the need for formwork, as the tubes used are recycled, reducing construction costs and time [31].

Therefore, the demand for and use of these types of columns has increased due to their ease of construction [32]. Concrete-filled plastic tubes are a promising option in the field, offering an excellent alternative to steel columns for their superior durability [33]. PVC tubes have many features that qualify them for use as concrete columns, including their light weight, minimal need for maintenance, ease of manufacturing from recycled materials, which provides economical construction, excellent corrosion resistance, and, similarly, their resistance to chemicals, to which they have very high resistance [34]. Recent studies have addressed the use of PVC tubes to reinforce concrete, including [35]: Askari et al. investigated the behavior of unconfined and confined columns using unplasticized polyvinyl chloride (uPVC) tube reinforced with polypropylene fibers. Several parameters, including sample sizes, compressive strength, pipe type, and polypropylene fiber content in the concrete mix [36]. Woldemariam et al. evaluated confinement of concrete using a uPVC tube by studying several variables, including pipe wall thickness, concrete compressive strength, and length-to-diameter ratio (L/D) [37]. Hammadi et al. examined the effectiveness of full and partial confinement using PVC tube with varying pipe diameters, and the effect of changing the height-to-diameter ratio was investigated [38]. Chena et al. studied the strengthening of previously damaged columns with carbon fiber. The study focused on two factors: the degree of damage to the columns and the type of material used for strengthening, including engineered cementitious composite (ECC) in addition to polymer fabric, strengthening using carbon fibers embedded in textile-reinforced ECC (TRE), and strengthening using carbon fibers embedded in cement mortar textile-reinforced mortar (TRM) as a comparative sample [39]. Ling and Chin [39] examined the effect of concrete type on tube behavior by using plain concrete and glass-fiber-reinforced concrete (GFRC). It also investigated the effect of varying sample diameters (from 75 mm to 150 mm) and fiber content (from 0 to 0.15% of cement weight). Regarding confinement, the study considered both the use of PVC tubes and the absence of PVC tubes for the same parameters.

From this perspective, a deficiency was observed in some variables and types of strengthening that can be applied to PVC concrete-filled tubes. This study aims to bridge this gap by using a new strengthening method: Carbon textile-reinforced concrete (CTRC). It also investigates the effects of confinement and non-confinement, as well as the impact of variations in diameter, height, thickness of the PVC tube, and the number of CTRC layers for each case. CTRC technology relies on an inorganic cementitious binder (which offers fire resistance and durability compared to polymeric materials), unlike conventional carbon fiber-reinforced polymer (CFRP) technology, which utilizes organic polymer resins.

3.1 Confined concrete with polyvinyl chloride tubes

In general, confined concrete with a PVC tube significantly increases its Strength Enhancement Index (SEI) compared to an unconfined control sample (Eq. (1)). Specifically, the peak load capacity of samples in the group with a constant diameter of 100 mm (D1L1-2, D1L2-2, and D1L3-2) increased by 35.30%, 92.60%, and 89.60%, respectively, as shown in Figure 4(a). The lengths varied between 100 mm, 200 mm, and 300 mm, as shown in Figure 4(a). Similarly, in the group with a diameter of 150 mm (D2L1-2, D2L2-2, and D2L3-3), the increases were 18.40%, 30.50%, and 43.00%, respectively, as illustrated in Figure 4(b), when using the same lengths as the first group. The increases were 16.50%, 58.60%, and 38.10% for samples (D3L1-2, D3L2-2, and D3L3-3) with a diameter of 200 mm, as indicated in Figure 4(c), using the same lengths as the first group, compared with the unconfined samples, as shown in Table 7. Figure 5 shows the test of the specimens.

$SE{{I}_{PVC}}=\frac{{{f}_{CC,pvC}}-{{f}_{C}}}{fC}$        (1)

SEI_PVC: Strength Enhancement Index

fc: The peak of unconfined concrete by PVC

fcc,_pvc: The peak of confined concrete by PVC

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(a)

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(b)

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(c)

Figure 4. The effect of polyvinyl chloride (PVC) confinement on the behavior of concrete: (a) diameter: 100 mm; (b) diameter: 150 mm; (c) diameter: 200 mm

Table 7. Examination results

Note: Strength Enhancement Index (SEI), polyvinyl chloride (PVC), carbon textile-reinforced concrete (CTRC), length-to-diameter ratio (L/D).

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Figure 5. Sample testing stages with and without polyvinyl chloride (PVC) confinement and carbon textile-reinforced concrete (CTRC) layers

3.2 Effect of carbon textile-reinforced concrete with polyvinyl chloride tube on the peak strength

The technique of adding a CTRC mesh network is a method for strengthening concrete-filled tubes to enhance their durability and resistance to applied loads when added in one, two, or three layers. Strength enhancement index (SEI) for a diameter of 100 mm and a length of 100 mm (Eq. (2)), the load-bearing strength of the samples was 35.50% and 57.90% when using one, two, and three layers of CTRC mesh, respectively, compared to the reference sample confinement with PVC; when the length is 200 mm, the results are 10.00%, 34.00%, and 35.80% when using one, two, and three layers consecutively. At a length of 300 mm, the results for the samples with different layers—one, two, and three are 23.40%, 45.30%, and 55.50%, respectively. As indicated in Table 7 and Figure 6. Also, Figure 7 shows the test results of the specimens.

$S E I_{C T R C}=\frac{f_{C C, C T R C}-f_{c c, p v c}}{f_{c c, p v c}}$          (2)

where,

$S E I_{C T R C}$: Strength Enhancement Index

f_cc,pvc: The peak of confined concrete by PVC

f_CC,CTRC: The peak of confined concrete by PVC and strengthening by CTRC

The effect of using the first, second, and third layers of a CTRC on samples with a diameter of 150 mm and lengths of 100, 200, and 300 mm resulted in 33.90%, 35.90%, and 40.80% and 16.00%, 43.50%, and 57.90% and 24.20%, 39.40%, and 45.80%, respectively, as shown in Table 7 and Figure 7.

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Figure 6. Effect of carbon textile-reinforced concrete (CTRC) with polyvinyl chloride (PVC) tube on the peak strength at D = 100 mm

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Figure 7. Effect of carbon textile-reinforced concrete (CTRC) with polyvinyl chloride (PVC) tube on the peak strength at D = 150 mm

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Figure 8. Effect of carbon textile-reinforced concrete (CTRC) with polyvinyl chloride (PVC) tube on the peak strength at D = 200 mm

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Figure 9. Sample testing stages with and without polyvinyl chloride (PVC) Confinement and carbon textile-reinforced concrete (CTRC) layers

In the case of using the diameter (200 mm) with the CTRC and lengths (100 mm, 200 mm, and 300 mm), with one layer and two and three layers, the samples gave results of (19.40%, 33.90%, and 46.10%) and (18.80%, 12.50%, and 44.10%) and (16.83%, 36.43%, and 65.56%), respectively, as shown in Figure 8 and Table 7. Figure 9 shows the test stages of the specimens.

3.3 Effect of the length-to-diameter ratio on confinement and carbon textile-reinforced concrete layers

The peak load capacity of cylinders (without confinement) is inversely proportional to the L/D up to a ratio of less than 1, where it is observed that the larger the diameter, the lower the load-bearing capacity of the samples, since increasing this ratio reduces the confinement that occurs to the samples during testing between them and the machine, thus reducing their load-bearing capacity. In cases where the ratio of height to diameter is the same, the superiority tends towards the samples with the larger diameter, and this is entirely consistent with what the researchers Woldemariam et al. [36] and Ling and Chin [39] concluded. The optimal height-to-diameter ratio, which provides both strength and restriction, is 1. If this ratio is present and repeated, the larger diameter will be the stronger limiter. Similarly, a thicker tube will have a higher load-bearing capacity than its counterparts, as detailed in Figure 10 and Table 7.

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Figure 10. Results of the sample without and with polyvinyl chloride (PVC) confinement and with carbon textile-reinforced concrete (CTRC) layers

3.4 Stress-strain relation

To determine the behavior and failure patterns of confined and unconfined concrete, a strain strip was installed on unrestrained concrete and on a PVC tube for confined concrete to determine the stress-strain behavior. The stress-strain behavior shown in Figure 11(a), (b), and (c) indicates that all unconfined concrete specimens exhibit nearly linear behavior during the elastic phase before brittle failure. The presence of the PVC tube led to a transformation in the concrete's behavior from brittle to almost ductile, as illustrated in Figure 11(d), (e), and (f). Initially, the constrained concrete exhibited linear behavior, but after the plasticization stage, the line continued straight due to the tube bearing the lateral pressure of the concrete before specimen failure. However, this failure stage was not abrupt. Furthermore, the constraining of the concrete increased its strain by 38%, 45%, and 30% for diameters of 100 mm and lengths of 100 mm, 200 mm, and 300 mm, respectively. Similarly, the strain of the constrained concrete with a diameter of 150 mm increased by 94%, 39%, and 37% for lengths of 100 mm, 200 mm, and 300 mm, respectively. Likewise, the constrained concrete with a diameter of 200 mm achieved increases of 57%, 34%, and 24% for lengths of 100 mm, 200 mm, and 300 mm, respectively.

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Figure 11. Stress-strain curves for (a) unconfined cote D100, (b) unconfined concrete D150, (c) unconfined concrete D200, (d) confined concrete D100, (e) confined concrete D150 and (f) confined concrete D200ncre

The authors would like to acknowledge the University of Diyala for providing the opportunity to conduct all research work in the structural lab of the Engineering Department in the College of Engineering. Gratitude is extended to the staff of the lab for collaborating and assisting.