The chapter delves into the effects of hybridizing Basalt Fiber Reinforced Polymer (BFRP) bars with carbon fibers, focusing on enhancing their microstructure and mechanical properties. It begins by highlighting the growing importance of high-performance materials in construction, particularly FRP composites, and the advantages they offer, such as high tensile strength, corrosion resistance, and low carbon footprint. The study then examines the specific benefits of basalt fibers compared to glass and carbon fibers, noting their superior chemical resistance and cost-effectiveness. The main challenge addressed is the lower stiffness of BFRP bars, which affects structural performance. To overcome this, the chapter explores the hybridization process, discussing different fiber volume fraction ratios and optimizing the configuration using the Rule of Mixtures (ROM). The analysis includes both theoretical calculations and experimental results, showing that hybridization significantly increases the elasticity modulus by about 70% and improves tensile and shear strength. The chapter also presents Scanning Electron Microscopy (SEM) observations, revealing the nonhomogeneous distribution of fibers and their impact on mechanical properties. The study concludes with practical insights into the production process and the importance of ensuring a more random distribution of carbon fibers for better performance. This comprehensive approach makes the chapter a valuable resource for professionals seeking to optimize FRP composites for construction applications.
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Abstract
The FRP (Fiber Reinforced Polymer) bars are increasingly used as the main reinforcement of concrete structures, replacing traditional steel reinforcement. In this paper results of Basalt Fiber Reinforced Polymer bars (BFRP) modification by partial replacement of basalt fibers with carbon fibers were presented. The analysis of an effect of hybridization on a microstructure and mechanical properties of BFRP bars were performed. This analysis was thought out based on tests performed: tensile strength and shear strength and a microstructure observation with scanning electron microscope. The results obtained indicate that the hybridization effectively increases elasticity modulus compared to unmodified BFRP and the tensile strength and shear strength increase in lower extant. The nonhomogeneous distribution of carbon fiber in the cross-section of HFRP bars has relatively small effect of mechanical properties and their scattering.
1 Introduction
The development of building materials market and increasing importance of use of high-performance materials construction is conducive to the increasing use of FRP composites in Civil Engineering [1‐3]. The main advantages of FRP composites are: high tensile strength to weight ratio, corrosion resistance, electromagnetic transparency, fatigue resistance, easy production of unique shapes, flexible aesthetics and a low carbon footprint in the perspective of the life cycle [4]. Currently, there are more and more applications regarding use of FRP composites as the main reinforcement in concrete structures [5]. Among others, basalt fibers seem to be a compromise, between new plant fibers and the most used in construction glass and carbon fibers. Basalt Fiber Reinforced Polymer (BFRP) bars characterize very good mechanical properties and are more stable chemical resistance compared with Glass Fiber Reinforced Polymer (GFRP) bars. BFRP also demonstrates a wider range of working temperatures and provide significantly better cost-effectiveness compared to Carbon Fiber Reinforced Polymer (CFRP) bars [6‐8]. The main drawback of BFRP is lower stiffness, which affects the fulfillment of the SLS conditions of the structure, such as deflection and cracking. The increasing importance of rational material usage promotes materials modification. Combining fibers with different properties allows to use the advantages of individual fibers, their mechanical and physical properties, and allows for a more rational use of the material in terms of cost [9‐11]. To obtained high properties of Hybrid Fiber Reinforced Polymer (HFRP) bars many factors should be control during manufacturing; among others, the fiber placements in cross-sectional, which can affect their properties.
In this paper the effect of hybridization of BFRP bars by substitution of the part of basalt fibers with carbon fiber on microstructure and mechanical properties was discussed.
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2 General Concept
In order to improve the mechanical parameters (especially stiffness), the configurations of HFRP bars consisting of basalt or glass and carbon fibers were parametrically analyzed and optimized using rules of mixture (ROM). Initially, two types of hybrids were considered: carbon-basalt or carbon-glass in the fiber volume fraction ratio: 1:1; 1:2; 1:3, 1:4 or 1:9. [12, 13]. Then, it was decided to use a mix of carbon and basalt fibers, due to the better mechanical properties of basalt fibers compared to glass fibers. Carbon fibers are much stiffer than basalt fibers but at the same time several times more expensive. Moreover, the tensile strain of the carbon fibers should be close to the strain of the basalt fibers to avoid shear lag.
The location of the fibers in the cross-section of the bar is also important. The analyzes carried out indicated that it was more advantageous to arrange carbon fibers in the surface layer of the bar and basalt fibers in its core (Fig. 1a) due to the increase in stiffness. After starting the trial production of HFRP bars, it was noticed that in the surface layer of the HFRP bar, rovings consisting of carbon fibers were degraded as a result of scorching caused by excessive temperature (Fig. 1b).
Fig. 1.
a) The modulus of elasticity of the HFRP bars calculated using RoM (Rule of Mixtures) and obtained by FEM (Finite Element Method) simulation for at two different ideal arrangement of fibers in bar; b) visible burn of the carbon roving of the HFRP bar
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Therefore, the concept of carbon fiber distribution was changed. It was decided to place the carbon fibers in the core of the bar, and the basalt fibers near its surface. Additional stiffness analyzes of the bar modified in this way indicated a minimal decrease in the stiffness of the bar in relation to its original configuration, which was compensated by an increase in the share of carbon fibers. At the same time, the phenomenon of carbonization of carbon fibers, which can only occur under ventilation conditions, has been eliminated. In the pultrusion process, some fiber properties influenced the final architecture of the bar structure. The problem with uneven distribution was caused by the difficulty of keeping the carbon fibers in the core of the bar during the pultrusion process - the tendency of the carbon fiber to float in the consolidation part. Some carbon fibers were also burned during the pultrusion process.
3 Materials and Methods
Table 1 presents properties of individual ingredients, collected on the basis of manufacturers data: Toho Tenax Europe GmbH for carbon fibers, Kamenny Vek for basalt fibers and Ciech Sarzyna S.A. company for resin matrix (four-component 1300 System®).
Table 1.
Properties of constituents used for preparing BFRP and HFRP bars.
Property
Epoxy resin
Carbon fiber
Basalt fiber
Density ρ (g/cm3)
1.16
1.79
2.89
Filament diameter (μm)
–
7.00
17.17
Young’s Modulus \({E}_{11}\) (GPa)
3.45
242.10
89.00
Poisson Number \({\upnu }_{12}\) (-)
0.35
0.28
0.26
Shear Modulus \({G}_{12}\) (MPa)
1.28
24.00
21.70
Tensile Strength \({\sigma }_{11}\) (MPa)
55
4240.8
3000
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Taking into account results of computer simulation and the trail production the compositions of BFRP and HFRP were selected (Tab.2). After an in-depth analysis of possible configurations, HFRP bars consisting of carbon and basalt fibers were recommended for production in a volume ratio of 1:4 in relation to the total volume of fibers. Thus, the weight ratio of carbon to basalt fibers in the HFRP bar was 1:3.
Table 2.
Types of tested FRP bars.
Bar type
In the total fibers by weight
In the total bar by weight
Basalt Fiber
Carbon fiber
Basalt fiber
Carbon fiber
BFRP
100%
–
70.3%
–
HFRP
75.3%
24.7%
49.4%
16.2%
In this research project two type of tests of mechanical properties were conducted in room temperature: transverse shear and longitudinal tensile (performed in accordance with the ACI 440.3R [14] guidelines and ASTM methods ASTM D7617/D7617M-11(2017) [15] and ASTM D7205/D7205M-21(2021) [16], respectively. Five specimens were tested for each measurement conditions. The observation with SEM electron microscopy were conducted on cross-section of samples before mechanical tests and taken nearby of failure place after shear test. Details about the tests carried out in the author's previous research works are in the literature [17, 18].
4 Results and Discussion
4.1 Mechanical Properties
The test results of longitudinal tensile test and transverse shear test of samples of BFRP and HFRP bars are presented the Table 3.
Table 3.
Results of longitudinal tensile test and transverse shear test
Property
BFRP
HFRP
Mean value
COV, %
Mean value
COV, %
Tensile strength, MPa
1103.33
2.07
1277.92
4.34
Tensile strain, %
2.52
2.09
1.73
4.33
Modulus of elasticity, GPa
43.87
1.95
73.89
4.15
Shear strength, MPa
205.37
5.63
229.44
1.77
Note: COV – coefficient of variation (ratio of standard deviation to mean value) in %
The results of mechanical tests indicates that hybridization with carbon fibers effectively increases elasticity modulus ~70% compared to unmodified BFRP. The tensile strength and transverse shear strength increase in lower extant: ~16% and ~11% respectively. The hybridization increase two times value of COV in the case of tensile properties, but they were still low below 5%. In the case of shear strength the COV was even three times lower.
4.2 SEM Observation of Microstructure
The examples of SEM microstructures of BFRP and HFRP bars (dimeter of 8mm) at magnification 60× are presented in Fig. 2. They confirmed nonhomogeneous distribution of fibers in the case BFRP and HFRP. The question is how it influences mechanical properties of both types of FRP bars. In the case of HFRP bars carbon fibers were located in the core of the bars. However, the SEM micrographs of the samples after shear tests showed uneven carbon fibers distribution in the HFRP bar, where they were located outside the core zone (Fig. 3). It was also observed that the distribution of the fibers in one bar varied along its entire length. The supervision of the constant distribution of the fibers in the hybridization process, can be a great challenge in the production process. The analysis of the SEM images also indicates that there is no one way of cracks propagation trough HFRP bar during shear tests. It seems that even voids in bar were not preferable way of cracks propagation. It was observed that some cracks were blocked on voids and interfaces between basalt fibers zone and carbon fibers one.
Fig. 2.
Examples of microstructures BFRP (a) and HFRP (b) bars of 8 mm diameters
Fig. 3.
The SEM micrographs of the HFRP bars cross-section previously subjected to a shear test at different magnification of the bar.
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4.3 Experimental Results vs Theoretical Estimation of Mechanical Properties
Based on the parameters provided for each of the components by the manufacturer, the values were calculated based on theoretical consideration. The composite will usually break when the stresses in the fibers reach their strength ffu. After the fibers break, the matrix is unable to carry to load. Therefore, the composite strain to failure εcu is equal to the fiber strain to failure εfu. At this strain level, the matrix has not failed yet because it is more compliant and can sustain larger strains. Under these conditions, it can be assumed that the longitudinal tensile force is controlled by the fiber tensile stress and is represented by Eq. (1):
where: \({\sigma }_{f}-\) tensile stress in the fiber; \({\sigma }_{m}-\) tensile stress in the matrix; \({V}_{f}-\) fiber volume in composite; \({V}_{m}-\) matrix volume in composite; \({E}_{m}-\) longitudinal tensile modulus of elasticity for matrix; \({E}_{f}-\) longitudinal tensile modulus of elasticity for fibres.
This equation assumes that the strain in the matrix and the fibers are the same, which is true if the fiber-matrix bond is perfect. The ultimate strain or stress of the matrix is not realized, because the fibers are more brittle (fail at a lower strain). The underlying assumption is that once the fibers break, the matrix is not capable of sustaining the load and the composite fails. The shear stress-strain law of the composite was assumed to be linear Eq. (2):
The maximum-shear-strain criterion limited the strains in the tension-compression quadrants to account for shear failure of the fibres. The shear strain is computed as Eq. (3):
where: \({\varepsilon }_{1t}\) – strain in the fiber at failure of the unidirectional composite in tension;
\({\nu }_{12}\) – fibre’s Poisson coefficient.
The performed calculations overestimate the measured values by about 25% (Tab.4).
Table 4.
Calculated values vs measured values of tensile and transverse shear strength.
Calculated values
Mean of measured values
Ratio of measured value/calculated value
Bar type
Tensile strength \({F}_{1t}\) [MPa]
Shear strength
\(\tau \) [MPa]
Tensile strength [MPa]
Shear
strength
[MPa]
Tensile
strength
[%]
Shear
strength
[%]
BFRP
1368.7
267.3
1103.3
205.4
80.6
76.8
HFRP
1549.5
261.8
1277.9
229.4
82.5
87.6
The predictions are higher than measured values. This is due to defects exist in the composites, which translates into an imperfect fiber-matrix bond. This shows that with ideal production process (reduced imperfection effect) the performance of composites can be higher. The measured to calculated shear strength ratio (87.6%) is relatively closer to the test values compared to the analogous tensile strength ratio (82.5%) for HFRP composites. In addition, test to calculated ratios are greater for HFRP compared to BFRP for both tensile strength and transverse shear strength. This indicates a better technological processing of HFRP bars compared to homogeneous BFRP bars. In the analysis of this case should be underlined that the above forms do not differentiate the distribution of fibers in the bars in any way, but only include the content of individual fractions.
5 Conclusions
On the basis of the results obtained the following main conclusions on hybridization of BFRP bars with carbon fibers can be drawn:
the hybridization effectively increases elasticity modulus ~70% compared to unmodified BFRP. The tensile strength and shear strength increase in lower extant: ~16% and ~11% respectively;
the hybridization increase two times value of coefficient of variation in the case of tensile properties, but they were still low below 5%. In the case of shear strength the COV was even three times lower;
the nonhomogeneous distribution of fibers in the cross-section of HFRP bars has relatively small effect of mechanical properties. The analysis of the SEM images also indicates that there is no one preferable way for cracks propagation trough HFRP bar during shear tests.
Additionally, it can be concluded that better situation is if the carbon fibers are more randomly dispersed across in the HFRP cross-section because of the fibers with higher elongation can safely withstand higher loads.
Acknowledgements
The article was prepared within the framework of the Internal Grant of the Faculty of Civil Engineering at Warsaw University of Technology no. 504/04740/1080/44.000000. The results of the project “Innovative Hybrid – FRP composites for infrastructure design with high durability” NCBR: PBS3/A2/20/2015 were partially used during preparation of this paper.
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