Axial compressive behavior of green sustainable Water Hyacinth & Bio-Resin (WHBR) FRP composites-confined circular concrete

. This study examines the behavior of circular concretes externally confined by green sustainable WHBR FRP containing water hyacinth fiber ropes and bio-resin. This study principally purposes to discover the axial stress-strain relationship behavior of WHBR FRP-confined circular concrete. A total of 10 circular concretes with a dimension of 150 × 300 mm were cast, strengthened with one to four layers of WHBR FRP, and tested under compression. The stress and the deformability of WHBR FRP-confined circular concretes were observed to be increased along with the addition of WHBR FRP layers. The accurateness of existing ultimate stress and strain models of natural FRP-confined circular concrete was evaluated using the test results. Indicating the need for the newly developed models to precisely predict the ultimate stress and strain values of WHBR FRP-confined circular concrete, newly developed models were developed to be precise in estimating the ultimate stress and strain of WHBR FRP-confined circular concrete. Keywords concrete, confinement, disaster risk reduction, stress-strain, water hyacinth fiber.


Introduction
As existing concrete structures are usually deteriorated and damaged throughout their life services, improving the quality of materials and strengthening to improve structural performance is necessary to assure safety [1][2][3][4][5][6][7].Regarding the external concrete retrofitting technology, the most prevalent application to uplift both strength and ductility is Fibre-Reinforced Polymer (FRP) to strengthen concrete [8][9].
FRP has been widely used as external concrete strengthening in high-story buildings or long-span bridges as shown in Fig. 1.Most commercial FRPs are made from ultra-high tensile strength synthetic fibers such as carbon, glass, aramid, and basalt.However, the global warming rate assessment result for one kg production of some fibers is shown in Fig. 2 [10].That figure indicates a very significant global warming rate of 7.8 kg CO2 eq to produce carbon fibers, *Corresponding author: tavio@its.ac.id in contrast to 0.41 kg CO2 eq of jute fibers and 1.17 kg CO2 eq of hemp fibers.Fig. 2. Global warming rate of some fibers production [10].
To date, natural-based fibers such as jute, hemp, cotton, flax, and sisal, have been produced as natural FRP to strengthen concrete to improve sustainability.Regarding naturally FRP-strengthened concrete, this composite material has been successful to improve both the strength and ductility of concrete under axial and flexural loading.
In 2021, Jirawattanasomkul et al. conducted the strengthening effect of water hyacinth FRP-confined circular concrete [10].This investigation result showed an ultimate stress increment of more than 100% of initial concrete stress with three layers of water hyacinth FRP.
In 2019, Jirawattanasomkul et al. investigated the behavior of circular concrete confined with natural FRP such as hemp and cotton fibers under axial loading [11].Each type of FRP-confined circular concrete shows different axial stress-strain relationship behavior as shown in Fig. 3.
Hemp FRP-confined circular concrete shows a bilinear stress-strain response with a significant stress increase of up to 100% along with the addition of FRP layers.However, cotton FRP-confined circular concrete shows a trilinear stress-strain relationship behavior with the maximum drop in descending part before achieving ultimate strength.Cotton FRP-confined circular concrete shows an extra ultimate strain of 8% and lowstress enhancement of 45% with three layers of cotton FRP application.
Advanced research has been conducted to develop the analytical formula to precisely estimate the ultimate stress (  ) and strain (  ) value of natural FRPconfined circular concrete.The lateral confining pressure (  ) and initial concrete stress (  ) are related to the developed analytical models.
In 1928, Richart et al. initiated the equation for effectively confined concrete with confinement following the form of Equations 1-2 [12].
where  1 and  2 are the confining coefficient for stress and strain respectively,   is the lateral confining pressure of FRP as defined in Equation 3.Where   is FRP tensile strength,   is FRP thickness, and  is the concrete diameter.
In 1997, Karbhari and Gao proposed the equations for both ultimate stress-strain of composite jacketing material-confined circular concrete shown in Equations 4-5, respectively [13].
In 1997, Miyauchi et al. also created the equations to predict the value of the ultimate stress-strain of carbon FRP-confined circular concrete as shown in Equations 6-7 [14].
Besides that, Djafar-Henni and Kassoul proposed a power function equation to predict the ultimate stress and strain of jute FRP-confined circular concrete as shown in Equations 8-9 [15].
where  ℎ, is the rupture strain of jute FRP material.
Seeing the potential of natural FRP as an external concrete strengthening that has been investigated before, one of the plants that have high cellulose of almost 75% that can be developed into natural FRP is water hyacinth [16].It has also been widely spread in several main rivers in Indonesia with a total amount of 9.6 million tons per month according to the Ministry of Public Housing of Indonesia.As a result of the massive growth of water hyacinth, it has become crucially a major challenge causing irrigation problems, health, and social environment impact [17].
In accordance with the Ministry of Public Housing of Indonesia, some urban offices have spent a huge amount of national budgets each year to maintain these water hyacinth issues through incineration and landfill which left the wastes.So, this study redirects the new concept to manage this domestic water hyacinth issue by producing the water hyacinth fibres as base material and completing with bio-resin to develop WHBR FRP for concrete retrofitting to uplift sustainability.

Research significance
This research project helped develop a green sustainable WHBR FRP material for concrete strengthening and retrofitting based on water hyacinth fiber material and bio-resin to confine circular concrete.This newly developed retrofitting material could be able to enhance the concrete properties and be beneficial for decreasing the environmental impact due to the application of synthetic FRP materials.At the next level, this WHBR could prospect as an affordable external concrete strengthening for low-rise buildings.

Fibre production and measurement
This research obtained raw water hyacinth from the local river in Semarang Regency, Central Java, Indonesia.The stems were cleaned and tied together and soaked in water.The part used is a stem with a length of 50 cm.The fiber production of water hyacinth was through mechanical treatment using a decorticating machine.The extracted fibers were air-dried at room temperature for 24 hours as shown in Fig. 4.
The produced water hyacinth fibers were tested to obtain the physical properties listed in Table 1.

WHBR FRP
To gain the mechanical properties of WHBR FRP, the water hyacinth fibres were developed into a twisted rope with a diameter of 5 mm as shown in Fig. 5.
The water hyacinth fiber ropes were impregnated with bio-resin to produce the WHBR FRP composite.The properties of bio-resin used in this study are listed in Table 2.
The tensile test was directed to observe the tensile behaviour of WHBR FRP materials using a universal testing machine (UTM) with a constant displacement rate of 2.0 mm/minute.The average tensile stress-strain relationship behaviour of WHBR FRP is shown in Fig. 6.
Based on Fig. 6, the bio-resin is crucial to improve the response and behavior of WHBR FRP.Using bioresin uplifts the tensile strength of water hyacinth fiber ropes significantly.The stiffness of WHBR FRP is increased compared to water hyacinth fiber ropes only.

Circular concrete
A single concrete mixing process was carried out to produce 10 circular concretes.The details of the concrete mix design are listed in Table 3. Portland cement and 19 mm crushed aggregates were used to produce the low-strength concrete of 16.7 MPa for typical low-rise buildings with a dimension of 150 x 300 mm.Table 3. Concrete mix design.

WHBR FRP application
Concrete with the age of 28 days was ready to be applied to the WHBR FRP as shown in Fig. 7. WHBR FRP was wrapped around the circular specimen section by manually winding the rope.At the beginning and the end of the rope application, super glue to fix the rope with a concrete surface was used.During the application, it was necessary to ensure that there were no gaps or spaces between the ropes.Next, the epoxy resin was applied to the surface of the water hyacinth fiber ropes until reaching their saturation.WHBR FRP-confined circular concrete specimens were cured in the room ambiance for 12 hours.the number of layer variations used in this study were one, two, three, and four WHBR FRP layers.

Instrumentation and axial loading setup
The loading setup consists of Linear Variable Displacement Transducers (LVDTs) placed 180 degrees apart on the sides of the specimen to record axial deformations and load cell to record axial loading during the loading process as shown in Fig. 8.A monotonic axial compression loading was increased at 4 kN/s to each specimen by Universal Testing Machine (UTM).

Results and discussions 4.1 Axial stress-strain behaviour
The axial compression test results of WHBR FRPconfined circular concrete are shown in Table 4.Where the number is the total of layers and the letter is the specimen.Based on the test results presented in Table 4, shows the results of increasing ultimate stress and strain of circular concrete reinforced with WHBR FRP.Significantly increased ultimate stress of concrete value up to 100% by using two layers of WHBR FRP.The stress-strain behavior of WHBR FRP-confined circular concrete shows a bilinear response as shown in Fig. 9.The kind of bilinear stress-strain relationship behavior was observed for the WHBR FRP-confined circular concrete because the tensile stress-strain behavior of WHBR FRP is essentially linear.

Rupture mode
The ultimate rupture mode of WHBR FRP-confined circular concrete is shown in Fig. 10.According to the test result, it is known that all specimens of WHBR FRPconfined circular concrete have failure at the tensile rupture of WHBR FRP.circular concrete, the addition of WHBR FRP layers provides an increase in the stress and strain at ultimate and provides the ductile-stress-strain behavior of WHBR FRP-confined circular concrete.

Proposed ultimate stress-strain models
The existing models [13][14][15] versus experimental results from WHBR FRP-confined circular concrete investigation for each ultimate stress and strain are plotted as shown in Fig. 12.
(a) Existing models vs experimental result of strength.
(b) Existing models vs experimental result of strain.The models developed by Miyauchi.provide the best strain value on predicting the ultimate strain of WHBR FRP confined-circular concrete.Based on Fig. 12, the existing models are not eligible to estimate the ultimate stress and the corresponding strain of WHBR FRPconfined circular concrete.Therefore, it is necessary to recommend the new models to predict the ultimate stress and the corresponding strain of WHBR FRP-confined circular concretes.

Conclusions
The axial compressive behavior of WHBR-FRP confined circular concrete has been experimentally investigated in this study.The conclusions of this experimental study are listed in this section.1.It has been discovered that WHBR FRP is performing well.The axial stress-strain relationship behavior shows a bilinear trend.2. The addition of WHBR FRP layers considerably improved the axial deformability and ultimate stress Miyauchi [12] Djafar-Henni and Kassoul Miyauchi [12] Djafar-Henni and Kassoul [13] E3S

Fig. 10 .
Fig. 10.Rupture mode: (a) one layer, (b) two layers, (c) three layers, and (d) four layers.The rupture mode phases of WHBR FRP-confined circular concrete is demonstrated in Fig. 11.Fig. 11(a) indicates the unloading of WHBR FRP-confined circular concrete specimens.Figs.11(b) and (c) indicate the first and second part of ascending bilinear response of the stress-strain relationship until it reaches the rupture at the ultimate stress as shown in Fig. 11(d).

Fig. 11 .
Fig. 11.Rupture phase: (a) unloading, (b) first ascending, (c) second ascending, and (d) rupture.The lateral swelling of WHBR FPR-confined circular concrete was noticed and observed at the ultimate stress of confined circular concrete specimens.As a result, the crushing and bursting of WHBR FRPconfined circular concrete occurred.Based on the test result, stress-strain relationship behavior, and rupture mode of WHBR FRP-confined

Fig. 12 .
Fig. 12. Investigation of existing models to estimate ultimate stress-strain of WHBR FRP-confined circular concrete.
The models are developed from the initial formula by Richart et al for each ultimate stress model and ultimate strain model as shown in Fig.13.(a) The proposed ultimate stress model.(b) Proposed ultimate strain model.

Fig. 13 .
Fig. 13.Proposed ultimate stress-strain models of WHBR FRP-confined circular concrete.Based on Fig. 13, the newly developed models of ultimate stress and strain of WHBR FRP-confined circular concrete are established with linear formulas as shown in Equations 10-11.

Table 1 .
Physical characteristics and properties of water hyacinth fibers.
Web of Conferences 429, 05002 (2023) https://doi.org/10.1051/e3sconf/202342905002ICCIM 2023 was increased by 100% with 2 layers of WHBR FRP. 3. Based on the test result conducted, new models are successfully developed to estimate the ultimate stress and strain of WHBR FRP confined concrete as shown in Equations 10-11.This research was made possible due to financial support from the Directorate of Research, Technology, and Public Services, Ministry of Education and Culture, Research, and Technology of the Republic of Indonesia, and the Directorate of Research and Public Services, Institut Teknologi Sepuluh Nopember (ITS) through Research Grant PDD with Contract No: 1919/PKS/ITS/2023.The authors also gratefully acknowledge the financial support received from the Institut Teknologi Sepuluh Nopember for this work, under the project scheme of the Publication Writing and IPR Incentive Program (PPHKI) 2023.