The chapter delves into the serviceability limit state of fiber reinforced concrete beams using basalt fiber polymer (BFRP) bars and stirrups. It begins by discussing the importance of durability in reinforced concrete structures, particularly in harsh environments. The research focuses on the behavior of BFRP-reinforced concrete beams under bending, comparing them to traditional steel-reinforced concrete beams. The study includes detailed material properties, beam dimensions, and reinforcement details. Notably, the research reveals that BFRP-reinforced beams exhibit different deformation and failure mechanisms, with higher tensile stress values and a significantly reduced compression zone. The addition of basalt fibers to the concrete also reduces deformation values. The chapter concludes by emphasizing the lower deformability and rapid failure of BFRP-reinforced beams, suggesting that their use can meet serviceability limit state requirements for deflection and cracking control.
AI Generated
This summary of the content was generated with the help of AI.
Abstract
The work analyzes the serviceability ultimate limit state of 4.5 m long fiber-reinforced concrete beams with basalt bars and stirrups (BFRP). On the basis of previous tests, deformations in beams with composite reinforcement are above acceptable values. Beams were made of concrete with basalt fibers to improve deformability, cracks resistance and deflection. The tests showed that the load capacity of beams reinforced with BFRP bars was lower than that of beams with steel reinforcement, resulting from different failure mechanisms of both beams. The failure of beams with BFRP reinforcement was rapid. Deformations in the concrete were reduced by using basalt fibers in the concrete. Increasing the stiffness of the structure with reinforcement with BFRP bars and stirrups using concrete with basalt fibers can meet the SLS requirements for limiting the deflection and cracking of concrete elements reinforced with them.
1 Introduction
Over the years, various attempts have been made to reduce the effects of corrosion in reinforced concrete structures. The durability of reinforced concrete structures holds utmost significance when considering industrial facilities and various elements exposed to coastal environments, bridge pavements, ground contact, thin-walled components, or situations where achieving high-quality concrete is impractical [1‐3]. The solution may be to use non-metallic reinforcement. An important feature is the good corrosion resistance of FRP fibers [4]. The most commonly used composite materials currently include: composites with glass fibers GFRP (Glass Fiber Reinforced Polymer), with carbon fibers CFRP (Carbon Fiber Reinforced Polymer) and with basalt fibers BFRP (Basalt Fiber Reinforced Polymer) [5].
The research carried out so far shows that the modulus of elasticity of composite bars is about five times smaller than the modulus of elasticity of steel reinforcement. The results presented in [4, 6] shown a much greater reduction of the cross-section stiffness of the FRP reinforced concrete element after its cracking than in the case of a concrete element with steel reinforcement. The moment of inertia of the cross-section after cracking in beams with composite reinforcement is about four times lower than in beams with steel reinforcement [7]. As a result, in the serviceability limit state (SLS), much greater values of deflections and crack widths are observed, comparing to reinforced concrete elements (beams and slabs). Tests of concrete with basalt fibers used in two-span beams show the possibility of reducing the deflection and deformation values on the concrete surface [8].
Advertisement
The tests presented in the paper aimed to evaluate the behavior of concrete beams with BFRP bars and stirrups made of concrete with basalt fibers. Beams were subjected to bending. During the tests, the development of cracking and deformation of beam elements was analyzed.
2 Materials and Research Methods
The concrete was made of Portland cement CEM I 42.5R and natural aggregate with a grain size of 0-16mm. The cement content was 320 kg/m3. Concretes with w/c = 0.5 ratio were selected for the test. Basalt fibers were added to the concrete in an amount of 8 kg/m3. Basalt fibers are made of thin chopped basalt fibers with a fiber elementary diameter of 20 µm, tensile strength of 750 MPa and Young's modulus of 89 GPa. The concrete recipe is given in Table 1.
The concrete had an average compressive strength fck = 43,78 MPa, tensile strength fctm = 5,55 MPa and a modulus of elasticity Ecm = 40,64 GPa. Basalt fiber reinforced concrete, respectively: fck = 44,52 MPa, fctm = 6,11 MPa and Ecm = 42,02 GPa.
The subject of the tests were beams with dimensions of 120x300x4500 mm. The reinforcement of the beam was steel and basalt bars and stirrups. Ribbed bars with a diameter of ∅6mm and ∅14mm, made of BSt500s steel with a yield strength of fyk = 500MPa. BFRP reinforcement with a diameter of ∅6 mm and ∅14 mm and guaranteed tensile strength equal to fu,ave = 1180 MPa, guaranteed modulus of elasticity Ef = 47,6 GPa and guaranteed deformation at break ε*fu = 2,0%.
Advertisement
Two series of test elements were made, differentiated due to the longitudinal reinforcement A - steel and B - basalt. In series A, beams of concrete with basalt fibers in the amount of 8 kg/m3 (WB) and reference beams of concrete without fibers were also made (W0). In series B, concrete with basalt fibers in the amount of 8 kg/m3 was used in all beams (WB).
In the program of testing single-span beams, three series were assumed, differing in the distribution of transverse reinforcement. Series I beams were shear reinforced with stirrups with spacing determined according to PN-EN 1992-1-1 [9] due to the maximum spacing. In series II, the stirrups had a spacing twice as large as that established in PN-EN 1992-1-1 [9]. Series III beams have not stirrups. The Fig. 1 shows a diagram of the research program.
Fig. 1.
The research program
×
Single-span beams were loaded in a the 4-point bending with a span length of leff = 4500mm. The support and load diagram of the beams is shown in Fig. 2. The distances between the supports were 4200 mm. The measured values were the displacements of the beams on the supports (Fig. 3) and in the span, deformations at various points of the height of the support section and the measurement of span deformations using the DIC method. The test load increment was 5.0 kN. At each stage, the values of deflections and deformations were recorded. At the same time, crack propagation was mapped. Deformation measurement was made using a contact extensometer with a measurement base of 250 mm and an accuracy of 0.001 mm. The deflections were measured using inductive sensors with a measurement base of 50 mm and an accuracy of 0.01 mm. The deflections were measured using inductive sensors with a measurement base of 50 mm and an accuracy of 0.01 mm.
Fig. 2.
Geometry, details of reinforcement, loads and instrumentation
Fig. 3.
Scheme of the distribution of deformation measurement points on the beam surface
×
×
3 Analysis of the Serviceability Limit State of BFRP Beams
3.1 The State of Deformation
Figure 4 shows comparative diagrams of deformation values on the concrete surface in the support zones in particular series in the failure phase.
Fig. 4.
Comparative diagram of the deformation values on the concrete surface in the support zones in the series a)I b II) c)III (black-A-W0; red-A-WB; green B-WB)
×
The extent of the compression zone was significantly reduced in the case of beams with composite reinforcement. The highest tensile stress values were also recorded for them.
The different character of deformations of cracked beams reinforced with BFRP bars, compared to reinforced concrete beams, results from the way of deformation of both types of reinforcement. Steel bars deform elastically until they reach their elastic limit. BFRP rods, on the other hand, deform elastically until they break. The addition of basalt fibers in the amount of 8 kg/m3 significantly reduced the deformation of the concrete element. Higher deformation values are typical for elements with BFRP reinforcement, which results from the similar values of the modulus of elasticity of the composite reinforcement and concrete.
Figure 5 shows the maps of the main deformations, obtained using DIC (Digital Image Correlation System) and diagrams of deformations on the concrete surface at the level of the longitudinal compression and tension reinforcement. The analysis of the deformation maps of the individual series corresponds to the recorded cracking, as shown in Fig. 5.
Fig. 5.
Strains in concrete for series a) I, b) II c) III
×
Beams reinforced with BFRP bars and stirrups were characterized by smaller ranges of the compressed zone in the middle of the span than the beams with steel reinforcement. In Fig. 6, one can observe the change in the position of the neutral axis of the cross-section in individual series and the difference in the height of the perpendicular cracks. The height of the compressed zone decreased with the use of composite reinforcement. The multiplicity of tensile stresses in all series turned out to be the highest in BFRP beams. In the case of the addition of basalt fibers, the neutral axis of the cross-section and the size of the deformations in the failure phase were smaller with the deformations in the reference beams.
Fig. 6.
Strains recorded using DIC in span area
×
3.2 Failure Mode
Figure 7 shows the image of failure of the example beams of the series with composite reinforcement.
Fig. 7.
Model of failure of beams of series a) A-I-W0 b) A-I-WB c) B-I-WB
×
In series I and II with steel reinforcement, the beams failed by bending. Most of the cracks were perpendicular to the axis of the element, single diagonal cracks appeared at the effort of approx. 75% Pult. In beams with composite reinforcement, the failure was determined by the transverse force, causing failure by shear in the support sections. By far the largest number of cracks with the largest crack width were characteristic of beams with composite reinforcement. The failure of the BFRP beams was sudden, caused by brittle fracture of the stirrups. In the case of beams with steel reinforcement, the failure process was much slower. The number of cracks in the BFRP beams was greater, mainly perpendicular cracks in the middle part of the element caused by bending.
The failure of series III beams resulted from the shearing of the support zone in all series. The destructive crack started from the place where the load was applied and ended at the support. In the beams of the A-III-W0 and A-III-WB series, cracks along the reinforcement caused by slippage were observed. The steel reinforcement under destructive loads was subject to local adhesion loss, while the composite reinforcement was subject to brittle fracture.
Figure 8 shows a model of failure by brittle cracking of the longitudinal or transverse reinforcement typical for BFRP structures. The BFRP reinforcement fractured suddenly and unexpectedly, or in the case of beams without transverse reinforcement, the longitudinal members buckled and completely failed the compression zone. The destruction of the stirrups BY high shear force caused the stirrups to open or break in the vicinity of the bend.
Fig. 8.
Selected details of failure of beams with composite reinforcement
×
4 Conclusions
Based on the tests, it was found that the deformability of the tested beams reinforced with BFRP bars was lower than that of beams with traditional reinforcement, which resulted from different failure mechanisms of both types of beams.
Beams reinforced with BFRP experienced rapid failure. In both Series I and II, BFRP stirrups exhibited brittle cracking. Beams without transverse reinforcement showed signs of slippage along the reinforcement.
Thanks to the use of basalt fibers in concrete, the size of deformations at the same levels of the beams was reduced without the addition of fibers. Increasing the stiffness of the structure with reinforcement with BFRP bars and stirrups using concrete with basalt fibers can meet the SLS requirements for limiting deflection and cracking of concrete elements reinforced with them.
Acknowledgements
The work was carried out at the Białystok University of Technology as part of a research project financed by the National Center for Research and Development entitled “Innovative hybrid FRP reinforcement for infrastructural structures with increased durability” project number PBS3 / A2 / 20/2015 (ID 245084) and as part of financing by the Ministry of Science and Higher Education of the Republic of Poland; project number WZ/WB-IIL/ 4/2020.
Open Access This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.
The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.
