Abstract
The compressive behavior of self-consolidating concrete columns strengthened by hybrid confinement of polypropylene fiber rope and glass fiber-reinforced polymer sheet was investigated experimentally. We cast and tested eight column specimens under axial compression load. (Six confined SCC columns and two conventional SCC columns.) The concrete grade utilized is SCC M40. For reinforcement, the SCC columns are enclosed with GFRP wrap and polypropylene fiber rope in various volumetric ratios. The compressive resistance of a confined SCC column, strength enhancement, stress-strain relationship, ductility ratio, and load deflection relationship are the parameters studied. The results are compared to establish the adequacy of the confinement. The wrapping of GFRP increases the load-carrying capacity and modulus of elasticity.
1 Introduction
Self-compacting concrete (SCC) is a type of concrete which can be consolidated by its own weight without being affected by external vibrations. SCC has achieved the most significant breakthroughs in concrete technology in recent years. The concrete's performance on-site may be impacted if there is inadequate homogeneity due to insufficient compaction and segregation [1]. To address this issue and enable concrete placement in congested reinforcing, the SCC was developed, ensuring proper compaction. In the late 1980s, Japan introduced SCC (self-compacting concrete) specifically for application in densely populated earthquake-prone areas with heavily reinforced structures. Although SCC shares similarities with conventional concrete in terms of composition, it stands apart due to the inclusion of mineral admixtures such as fly ash and glass granulated blast, which enable it to achieve its unique self-flowing properties [2].
The concept of filling ability pertains to concrete's capacity to flow freely into all areas within the formwork under its self-weight. By utilizing a combination of a high-range water reducer and a viscosity-modifying additive (VMA), it is possible to enhance the cohesiveness of fresh concrete [3]. This is accomplished by lowering the free-water content, increasing the paste volume, or a combination of the two [4].
The super plasticizer is essential for improving the flow ability of the SCGC. As the dosage of super plasticizer is raised, the workability likewise rises [5, 6]. In a 2017 study, Ashraf Mohamed examined the effects of super plasticizer at several dosages of 5, 6, and 7%. He found that for every 1% increase in super plasticizer, the slump value increased from 700 to 720 mm. The V funnel test is another way to assess filling capacity. With a higher super plasticizer dosage, the discharge duration is shortened from 11 to 8 s [7]. The flow ability (650 mm) is improved by employing 5% super plasticizer in fly ash-based SCC, according to the results reported by K. Mukilan et al. [8]. Concrete mixes become more compact with the addition of significant amounts of fine aggregate, which gives excellent flow ability to self-compacting concrete [9]. For SCC all common concreting sands are appropriate [10]. Both siliceous and calcareous sands are suitable. It is necessary to reach a minimal quantity of fines in order to prevent segregation [11].
2 Need for confinement
Utilizing FRP material for confining self-compacting concrete is a more reliable, efficient, and straightforward approach [12]. By incorporating inorganic binders instead of impregnation resins, current research in concrete confinement actively seeks to improve the concrete's strength, ductility, and energy dissipation capabilities. This promising approach aims to enhance the overall performance of concrete structures. As a result, fiber reinforcement in the form of appropriate textiles or fiber-reinforced sheets is generated. The use of inorganic binders provides reinforced structures with increased resistance to confinement strengthening, especially at high temperatures. Additionally, these binders exhibit exceptional durability when exposed to ultraviolet (UV) radiation and can adapt well to demanding environmental conditions [13].
Glass fiber sheets impart the necessary lateral restraint to convert concrete structures (strengthened by GFRP sheet) from a brittle material to a ductile one. The main advantage of introducing glass fibre sheet is not to significantly change the concrete's brittleness but rather to improve its strength and prevent microcracks from spreading and opening. Glass sheets stand out when compared to other reinforcing fibers due to their exceptional time-dependent performance and durability, accessibility and economical aspects. According to Rahul and Urmil (2013) when compared to GFRP and traditional steel reinforcement, the performance of CFRP straps was superior. FRP wraps were shown to boost the axial strength of the unretrofitted or base column [14]. The high modulus of elasticity in fiber reinforced polymers (FRP) like glass sheets contributes to their advantageous characteristics, but it also leads to a low failure strain [14]. The compressive strength was increased through efficient confinement using GFRP composite sheets. More GFRP wrap layers resulted in improved confinement and improved ductility of the column, as well as higher load carrying capacity of the column [15] specifically; traditional FRPs often experience failure through jacket fracture in confined columns. To address this issue, there has been limited focus on utilizing fiber ropes (FR) as a form of confining reinforcement to overcome this limitation. For exterior confinement applications, the FR does not necessitate the use of any impregnation resins or binders.
Employing the FR wrapping method proves to be the quickest option, as it omits the need for resin or mortar. By opting for resin-free FR materials, the retrofitting process ensures enhanced temperature resistance and creates a safer and healthier working environment for both workers and engineers [16]. Figure 1 shows the confined (GFRP + FR) SCC columns. After precise hand wrapping, one or more rope layers are applied while maintaining a sufficient and constant strain on the rope. Wrapping is only allowed if the confinement is sufficiently tight. The effectiveness of FR wrapping is greatly diminished by loose wrapping. Nevertheless, careful manual wrapping combined with the tight mechanical anchoring could effectively prevent concrete's lateral expansion. This approach is suitable for challenging climates and environments with high humidity and varying temperatures. The ability to reuse and recycle PPFRs enhances the structure's capacity to withstand significant deformations, while constrictions also enhance compressive strength.
Confined SCC column
Citation: International Review of Applied Sciences and Engineering 15, 2; 10.1556/1848.2023.00721
3 Admixtures used
Silica Fume: Silica fume is a byproduct of electric arc furnaces used to produce elemental silicon or silicon alloys. The reduction of high-purity quartz to silicon at around 2,000 °C produces silicon dioxide vapor, which oxidizes and condenses at low temperatures to produce silica fume. It possesses a specific gravity of 2.19 and a bulk density of 722 kg m−3.
Super plasticizer: Glenium Sky 8233 serves as a super plasticizer, with a recommended dosage of 2% by weight of cement. Color of super plasticizer is reddish brown and the density is 1.02–1.07 g cm−3
Viscosity modifying Agent: Glenium stream 2 is used as VMA and the dosage is 50–500 L m−3 of concrete.
4 Mix design for M40 grade
The mix design is given by using Modified Nan-Su Method [17–20]; the values are satisfying the EFNARC guidelines.
Maximum size of aggregate = 10 mm
Specific gravity of coarse aggregate = 2.77
Bulk density of coarse aggregate (loosely packed) = 1,309 kg m−3
Specific gravity of fine aggregate = 2.475
Bulk density of fine aggregate = 1,040 kg m−3
Specific gravity of binder (cement) = 3.148
Fine aggregate volume ratio = 55%
Coarse aggregate volume ratio = 45%
Super plasticizer (Masterglenium) specific gravity = 1.09
Assume air content in SCC = 2.1%
Design strength of self compacting concrete = 40 N mm−2
Step 1: Calculation of fine aggregate and coarse aggregate in volume of concrete [7]:
Assumed packing factor (PF) by EFNARC = 1.89
Quantity of fine aggregate needed per unit volume of SCC, Ws = Packing Factor *WsL* S/a = 676 kg m−3
Quantity of coarse aggregate needed per unit volume of SCC, Wg = Packing Factor *WgL* (1-S/a) = 722 kg m−3
Step 2: Calculation of cement quantity
Consider one kg of cement provides a compressive strength of 0.12 N mm−2 in 28 days curing
Quantity of cement required per unit volume of SCC = CF*(fc/0.12) = 460 kg m−3
(As per the guide lines of EFNARC: the cement value ranges from 350 kg m−3 to 450 kg m−3)
Step 3: Determination of quantity of water needed by cement
Assume water/cement (W/C) ratio = 0.38
Quantity of water needed = 174.8 L m−3
(As per guide lines of EFNARC: Water to powder ratio (W/P) ranges from 0.8 to 1.1 by volume, the quantity of water should not exceed 200 L m−3)
Step 4: Mix proportion
To increase the strength 10% of cement is replaced by silica fume [2].
5 Fresh properties of SCC
5.1 Slump flow test
To determine the horizontal free flow of concrete without reinforcement, the slump flow test is employed which helps to determine the consistency of fresh concrete before it sets, enabling the evaluation of its workability. Figure 2 shows the slump flow for the mix proportion given in Table 1. The slump flow values and the acceptable range are given in Table 2.
Slump flow test
Citation: International Review of Applied Sciences and Engineering 15, 2; 10.1556/1848.2023.00721
Proportion of mix
Cement (binder) | Fine aggregate | Coarse aggregate | SP (master glenium) | Water |
460 kg m−3 | 675 kg m−3 | 723 kg m−3 | 9.2 kg m−3 | 174.8 L m−3 |
1 | 1.46 | 1.57 | 0.02 | 0.38 |
Slump flow test (by EFNARC)
S. No | Tests | Values obtained for fresh SCC | Acceptable range of values by EFNARC | |
Min | Max | |||
1. | Flow ability by slump Flow test (mm) | 675 | 650 | 800 |
5.2 V-funnel test
The flow time of fresh self-compacting concrete is measured using the V-funnel test. Blocking is indicated by a prolonged flow time. The reduced flow time implies that the system is very workable. The V-funnel test apparatus is shown in Fig. 3. The values obtained for fresh concrete and the acceptable ranges are given in Table 3.
V-funnel test
Citation: International Review of Applied Sciences and Engineering 15, 2; 10.1556/1848.2023.00721
V-funnel test (by EFNARC)
S. No | Tests | Values obtained for fresh SCC | Acceptable range of values by EFNARC | |
Min | Max | |||
1. | V-Funnel test (sec) | 9 | 6 | 12 |
5.3 U-box test
The U-box test, developed by Taisei Corporation's Technology Research Center in Japan, is also referred to as the “box-shaped” test. Its primary purpose is to evaluate the filling and passing characteristics of self-compacting concrete (SCC). Figure 4 explains the U box test and values are entered in Table 4.
U-box test
Citation: International Review of Applied Sciences and Engineering 15, 2; 10.1556/1848.2023.00721
U-box test (by EFNARC)
S. No | Tests | Values obtained for fresh SCC | Acceptable range of values by EFNARC | |
Min | Max | |||
1. | U-Box test (mm) | 20 | 0 | 30 |
5.4 L-box test
The L-box test is utilized to evaluate the passing ability of fresh self-compacting concrete when it encounters obstructions due to reinforcement. Figure 5 clearly shows the L-box apparatus and the blocking reinforcement. The acceptable range of values from EFNARC and the obtained value for the given mix are noted in Table 5.
L-box test
Citation: International Review of Applied Sciences and Engineering 15, 2; 10.1556/1848.2023.00721
L-box test (by EFNARC)
S. No | Tests | Fresh properties of SCC | Typical range of values by EFNARC | |
Min | Max | |||
1. | L-Box test (mm) | 0.91 | 0.8 | 1.0 |
6 Experimental program
Eight self-compacting concrete columns were exposed to axial compression in this study. Two columns are unconfined and serve as control specimens. Two columns were strengthened with a single layer of GFRP sheet with a 150 mm overlap to prevent anchorage de-bonding failure. Two as hybrid confinement, with one layer of GFRP and one layer of polypropylene fiber rope. The remaining two columns will be reinforced with an increased volume of fiber rope. The mechanical properties of GFRP sheet and the Fiber rope are given in Table 6.
Mechanical properties of glass fiber reinforced polymer sheet and two strand fiber rope
Label | Cross sectional area, AFR (mm2) | Tensile strength (N mm−2) | Tensile modulus of elasticity, EFR (GPa) | Strain failure εFRu (%) |
GFRP | 0.155 mm m−1 of width | 2,044 | 73 | 2.8 |
PPFR | 12.089 | 406 | 2.1 | 20.28 |
Stretching fiber ropes requires no complex mechanical techniques. Instead, a continuous hand tensile tension is applied during wrapping to prevent any looseness. These fiber ropes can be reused multiple times before reaching their breaking point. To facilitate adjustments for various column dimensions, a straightforward mechanical device with a threaded bolt is utilized to connect the ends of the collar [11]. This uncomplicated mechanism enables easy anchorage by reducing the diameter of the top and bottom collars, streamlining the process for authentic columns. In non-critical areas, fiber ropes can be fixed without a steel collar, as multiple layers wrapped outside the crucial zone provide self-anchorage. The fiber rope is then wound around the column to ensure containment within the designated area. Ultimately, the fiber ropes are securely fastened using a straightforward knot tied onto a fixed bolt. The PPFR's unique characteristic of not requiring resin impregnation makes it a crucial component in the strengthening procedure. All columns have a circular cross section with a diameter of 130 mm and a height of 750 mm, and are reinforced with 6 numbers of 10# as primary reinforcement and 8# as ties spaced at 130 mm c/c spacing. 3.5% is the reinforcing ratio.
7 Result and discussion
The experimental examination comprises two parts: the first focuses on the physical aspects of SCC, and the second on the mechanical behavior of confined SCC columns. In the second stage, hybrid confinement is employed to strengthen the SCC columns. The study involves six columns with a height of 750 mm and a diameter of 130 mm, undergoing axial compression with varying degrees of confinement. Figure 6 illustrates the failure pattern of a hybrid confined SCC column, while Fig. 7 demonstrates that GFRP remains intact when confined with PPFR. By plotting the load displacement curve and strain ductility, the specimen's energy-absorbing capacity and ductility can be determined. Tables 7 and 8 shows that the load bearing capacity and modulus of elasticity of C4(increased volume of PPFR) is greater than C3. Tables 9 and 10 demonstrate the ductility ratio and the energy absorption capacity respectively.
C2 Column before testing, during testing, after testing
Citation: International Review of Applied Sciences and Engineering 15, 2; 10.1556/1848.2023.00721
Failure pattern of SCC column with GFRP sheet and PPFR
Citation: International Review of Applied Sciences and Engineering 15, 2; 10.1556/1848.2023.00721
Load bearing capacity
S. No. | Grade of concrete | Specimen | Load bearing capacity (KN) |
1 | M40 | C1 | 255.35 |
2 | M40 | C2 | 410.2 |
3 | M40 | C3 | 540.25 |
4 | M40 | C4 | 585.8 |
C1 Conventional SCC
C2 SCC + GFRP
C3 SCC + GFRP + PPFR
C4 SCC + GFRP + Increased volume of PPFR
Modulus of elasticity
S.No. | Grade of concrete | Specimen | Stress (N mm−2) | Strain in (%) | Modulus of elasticity (N mm−2) |
1 | M40 | C1 | 19 | 0.8 | 23,750 |
2 | M40 | C2 | 31 | 0.9 | 32,631.3 |
3 | M40 | C3 | 41 | 0.9 | 43,157.8 |
4 | M40 | C4 | 44 | 1.4 | 44,000 |
Ductility ratio
S. No. | Grade of concrete | Specimen | Ductility ratio |
1 | M40 | C1 | 1.2 |
2 | M40 | C2 | 1.42 |
3 | M40 | C3 | 1.5 |
4 | M40 | C4 | 1.64 |
Energy absorption capacity
S. No. | Grade of concrete | Specimen | Energy absorption capacity in KN mm |
1 | M40 | C1 | 970.33 |
2 | M40 | C2 | 1,538.25 |
3 | M40 | C3 | 2,025.94 |
4 | M40 | C4 | 2,460.36 |
8 Conclusion
The paper describes the physical and mechanical behavior of self-consolidating concrete columns constrained by GFRP sheet and PPFR with strength of M40.
The PPFR is an environmentally friendly material that can be reused and recycled. The main advantage of employing fiber rope is that it does not require any resin or mortar. The rope ends are mechanically fastened in an adjustable manner to secure them. As a result, depending on the size of the column, it might be enlarged or decreased. The restricted columns reveal a spring-like breakdown in the concrete core. Figure 8 denotes the spring like failure as well as the tested C4 and C5 columns after removal of GFRP sheet.
Tested C4 and C5 columns after removal of PPFR, spring like failure in concrete core
Citation: International Review of Applied Sciences and Engineering 15, 2; 10.1556/1848.2023.00721
Following the transient load drop, the load-regaining component increases at a faster pace. The PPFR regulates the unstable expansion of concrete even after a 32% load decrease.
When the GFRP sheet fractures in concrete with hybrid confinement, a brief load drop occurs. Sufficient PPFR offers the ability to withstand the energy produced following the GFRP fracture.
As a result, PPFR can prevent the loss of load capacity while also ensuring greater strain ductility of concrete and increased axial loads. Figures 9–12 illustrate the load bearing capacity, modulus of elasticity, ductility ratio and the elastic strain energy respectively.
Load bearing capacity vs specimen series
Citation: International Review of Applied Sciences and Engineering 15, 2; 10.1556/1848.2023.00721
Modulus of elasticity vs specimen series
Citation: International Review of Applied Sciences and Engineering 15, 2; 10.1556/1848.2023.00721
Ductility ratio vs specimen series
Citation: International Review of Applied Sciences and Engineering 15, 2; 10.1556/1848.2023.00721
Energy absorption capacity vs specimen series
Citation: International Review of Applied Sciences and Engineering 15, 2; 10.1556/1848.2023.00721
The ultimate load-carrying capacity of 0.6 mm diameter PPFR is higher than that of other diameters of fiber rope. In hybrid confinement scenarios, there is no concrete-spring-like failure. Even after several GFRP cracks, the columns with PPFR confinement show no fracture, and the PPFR can be reused.
Abbreviation
SCC | Self Compacting Concrete |
GFRP | Glass Fibre Reinforced Polymer |
PPFR | Polypropylene Fibre Rope |
VMA | Viscosity Modifying Agent |
SP | Super Plasticizer |
SCGC | Self Compacting Geopolymer Concrete |
Acknowledgement
First and foremost, I thank the Almighty for his invisible guidance and grace on me to complete this endeavor successfully.
I would like to convey my heartfelt gratitude to my project guide as well as all of the staff members and laboratory technicians at the Civil Engineering Department who provided assistance and suggestions to ensure the success of this project.
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