Flexural behaviour of glass fibre reinforced polymer (GFRP) laminated hybrid-fibre reinforced concrete beams
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The flexural behaviour of externally bonded glass fibre reinforced polymer (GFRP) reinforced concrete (RC) beams incorporating both ‘basalt’ and ‘polyolefin’ fibres at a constant ratio of 70:30 and in several combinations of fibre volume fractions (Vf) ranging from 0–2% (at a constant increment of 0.5%) were investigated, to highlight to role of strengthening and the hybrid fibres in beams. Three different types of beams, namely: a control beam (1 No.); GFRP laminated RC beam (1 No.) and laminated and hybrid fibre reinforced (HFRC) beams (4 Nos.) were cast, and tested under a four-point bending. The load-deflection response at: first crack, yield point, at initiation of debonding lamination and at ultimate stages were recorded by appropriate instrumentation. The results indicate that there is a ‘combined effect’ of lamination and incorporation of the above hybrid fibres in contributing to the very high load-carrying capacity and enhanced ductility of laminated HFRC beam, especially at a fibre volume content of 1.5%. Further, the maximum yield and ultimate load-carrying capacity of laminated HFRC beam is found to be 59% and 49% higher than the laminated RC beam and 125% and 98% higher than the control beam. However, the deflections are higher, and their permissibility have to be ascertained with respect to relevant codal provisions. All the laminated HFRC beams exhibited ‘gradual debonding’ and ‘ductile’ failure, whereas, the control beam exhibited ‘flexural mode’ of failure. The ‘combined effect’ can be used advantageously in structural applications, where both ‘strength’ and ‘ductility’ are important.
KeywordsFlexural strength GFRP Basalt and Polyolefin fibres Load-carrying capacity Ductility Modes of failure
Reinforced concrete (RC) structures need upgrading for several reasons such as the effect of deterioration, mistakes in design and construction, changing in the use of structure, the excessive applied load resulting from increase in both traffic volume and truck weight, corrosion of internal steel reinforcement and freeze-thaw action. Several novel methods have been established in order to strengthen the RC structures, among the number of materials that have been adopted for external strengthening, Fibre Reinforced Polymer (FRP) materials have drawn considerable attention in recent decades, due to their proven advantages over steel plates . External bonding using FRP laminates is most commonly used. There are many benefits of FRP like high strength to weight ratio, adaptable to curved surface, easy to handle, electrochemical corrosion resistant, available in any shape and length, resistance to fatigue, more ultimate strength and lesser density compared to steel, etc. are the few properties which make FRPs an ideal selection in terms of strengthening applications [2 , 3]. Among the most common types of FRP, GFRP (glass fibre reinforced polymer) is easily available, apart from less costly than CFRP (carbon fibre reinforced polymer), and hence studies on strengthening using GFRP are more relevant, especially in countries like India.
While strengthening enhances the load-carrying capacity, it has its own limitation/(s), such as, the ductility behaviour. The strengthening of beams using FRP laminates increase load carrying capacity and reduce the ductility of the beams. Galal and Amir  illustrated that the FRP strengthened beams have shown a load enhancement of 16% in the ultimate load when compared to that of control beam and deflection is 54% less than the control beam.
On the other hand, incorporation of fibres in cement/cementitious matrix enhances the ductility of concrete [5, 6, 7]. Therefore, several types of fibres were incorporated into concrete, such as steel, glass, carbon, polypropylene, natural, fibres from pre-and post-consumer wastes, basalt and polyolefin fibres. Among all these fibres, steel fibres are stiffer and has high elastic modulus and thus incorporation of steel fibre into concrete effectively improves the compressive, flexural strength and impact resistance [8, 9]. However, steel fibre poses several disadvantages, such as: easy rust formation, increased structural weight, fibre balling effect at high dosage, and thus reducing the workability . For these reasons, the better alternative is glass fibre. Glass fibre has been used to produce light weight architectural elements. The drawback of glass fibre is high sensitivity in alkaline environment conditions. Carbon fibre has stiff and chemically inactive nature, but it has a disadvantage of high cost.
Extensive studies on the flexural behaviour of fibre reinforced concrete (FRC) beams were carried out, in the past several decades. The addition of waste metalized plastic (WMP) fibres along with palm oil fuel ash in concrete yielded great strength properties was studied Hosseini et al. , moreover, the incorporation of WMP fibres resulted in lower the drying shrinkage .
Basalt fibre is a new kind of inorganic fibre, made from basalt rocks, which are melted at 1400 °C, basalt fibres are environmentally safe, non-hazardous and non-toxic nature . Basalt fibre is more economical in compare with glass and carbon fibres, because the raw materials required for basalt fibre are excessively available and the manufacturing process of basalt fibre include no additives . Addition of basalt fibres can improve the energy absorption and deformation capacity, flexural strength, reduced the brittleness and improve the toughness index and reduce drying shrinkage of FRC significantly . Resistance to high temperature . Self-repairing of concrete cracks . Dong et al.  studied the mechanical properties of concrete made with basalt fibre, the results showed that addition of basalt fibres enhanced concrete strength and ductility.
Recently macro synthetic fibres are being produced and promoted for use in structural applications. Specific advantages of these fibres over steel fibres include ease of handling, chemical inertness and enhance impact resistance. Among synthetic fibres, Polyolefin fibre is one of the most popular reinforcing material in concrete.Polyolefin fibers are produced from organic polymers formed by the chain growth polymerization of olefins (alkenes) which contain greater than 85% polymerized propylene or ethylene . Significant improvements in toughness and ductility with addition of polyolefin fibres in concrete . Polyolefin fibres are less cost, when compared to steel fibre . Bantia et al.  observed that, Polyolefin fibres acts to arrest the propagation of internal cracks, reduce the extent of shrinkage cracking, improve the ductility of concrete and less chance for segregation, balling and bleeding.
It is found from extensive literature review, that the studies on external strengthening of beams using FRP lamination. However, the combined effect of external strengthening and incorporation of fibres, more so, on the incorporation of hybrid fibres, comprising a combination of ‘high’ and ‘low’ modulus, is rather scarce . On the other hand, such studies are needed, where the both ‘strength’ and ‘ductility’ are needed for certain structural applications. Hence, in this study, the flexural behaviour of RC beams laminated with GFRP and incorporating ‘hybrid fibres’ consisting of ‘basalt’ (high modulus) and ‘polyolefin’ (low modulus) have been experimentally investigated, and their ‘combined effect’ highlighted.
2 Experimental program
2.1 Materials used
Ordinary Portland cement (OPC) 53 grade conforming to IS: 12269-1987 ; locally available clean river sand passing through 4.75 mm sieve with a specific gravity of 2.63 and conforming to zone II as per IS: 383-2016  as fine aggregate; crushed granite stone with specific gravity 2.75 having a maximum aggregate size of 20 mm as coarse aggregate, are the various materials used in concrete. Portable water was used for mixing and curing concrete. 10 mm and 8 mm diameter high yield strength deformed (HYSD) bars were used as main and secondary reinforcement in beams. The average ultimate tensile strength of the above bars are: 620 MPa and 601 MPa respectively, as determined by standard IS code.
Properties of tensile steel bar and GFRP laminates
Tensile steel ratio
No. of layers
Ultimate stress (MPa)
Modulus of elasticity (MPa)
2 × 105
Properties of basalt and polyolefin fibre
Size (mm) or dia
Young’s modulus (GPa)
Tensile strength (MPa)
2.2 Mix proportioning and preparation of specimens
Details of mix proportion
Fine aggregates (Kg/m3)
Coarse aggregate (Kg/m3)
Fibre volume fraction (%)
Proportion of fibres
GFRP laminate thickness (mm)
HB 0 L0a
HB 0 L5
2.3 Test set-up and testing
3 Results and discussion
3.1 Load-carrying capacity
Results of tested beams
First crack load (kN)
First crack deflection (mm)
Yield load (kN)
Yield load Deflection (mm)
Load at debonding initiation (kN)
Deflection at debonding initiation (mm)
Ultimate load (kN)
Ultimate load deflection (mm)
HB 0 L0
HB 0 L5
3.2 Effect of addition of hybrid (basalt and polyolefin) fibres
As proportion of hybrid fibres was maintained constant throughout the study, only the variation of actual fibre volume content (Vf) will have an effect on the load carrying capacity and other characteristics of the HFRC beams. As the Vf increases from 0.5% to 1.5%, the first crack load, yield load and ultimate load of ‘laminated and hybrid fibre-reinforced beams’ (HB0.5L5 to HB1.5L5) gradually increases, and reaches the maximum, when the Vf = 1.5% (Fig. 5). However, at the maximum Vf used (Vf = 2.0%), the load carrying capacity attained by the beam, at all the three stages are lower than the Vf = 1.5%. This shows the desured hybrid volume content is 1.5%, to achieve the maximum load-carrying capacity of HFRC beam (among the range of Vf considered). The maximum load carrying capacity of ‘laminated HFRC beam’ (HB1.5L5) at first-crack, yield and ultimate stages are respectively, about 54%, 59% and 49% higher than the ‘laminated RC beam’ (HB0L5) and about 101%, 125%, 98% higher than then ‘control beam’ (HB0L0) (Table 5). This shows that the hybrid fibres used in this study have contributed to ‘very high’ increase in the load carrying capacity of laminated HFRC beam (HB1.5L5).
Comparing the load-carrying behavior of ‘control beam’, ‘laminated RC beam’ and ‘laminated HFRC beam’, following inferences are drawn: (i) Lamination contributes to the increase in the load carrying capacity at all three stages; namely: at first crack,yield and ultimate. However, its contribution to the post-yield stage is less than at yield point, which may be attributed to the ‘debonding’ of lamination at or beyond yield point; (ii) Inclusion of hybrid fibres contribute to very high increase in the load-carrying at all three stages of ‘laminated beam’ and its post-crack behaviour due to ‘arresting or bridging’ the micro-cracking development due to loading, there- by, increasing the interfacial bond in concrete matrix and increasing the ductility of the beam; (iii) There is a ‘combined effect’ of lamination and ‘inclusion of hybrid fibres contributing to the very high load-carrying capacity and enhanced ductility of ‘laminated HFRC beams’ which is highly advantageous and can be used for structural applications where the twin objectives of higher load- carrying capacity and post-yield behaviour are to be fulfilled. Lijuan et al.  Yin and Wu  have also found the same observations on the improvement in the load carrying capacity of GFRP laminated beams by incorporating of steel and polypropylene fibres.
3.3 Deflection behaviour
Deflection of the laminated RC beam at ultimate stage is lower than the ‘control beam’. Deflection of laminated HFRC beams increase with the increase in Vf content up to 1.5% and after there decrease at Vf = 2.0%. The above two behaviour are similar to the behaviour of load-carrying capacity, from the stage of first-crack formation to the ultimate stage. But, the only difference is in the actual value of deflection at the three stages, especially, the post-yield stage. Deflection is maximum for the laminated HFRC beam, when Vf = 1.5% (HB1.5L5), at all the three stages (Table 5) and that the above maximum deflection (2.73 mm; 5.50 mm and 15.00 mm, respectively) is 118.4%, 52.8% and 74.4% higher than the ‘control beam’ at corresponding stages. Similarly, the maximum deflection of the above is 110%, 46.8% and 138.1% higher than the ‘laminated beam’. Comparing the above two sets of increase in deflection of respective beam/(s) over control beam reveal: (i) In general, the increase in deflection may be attributed to the increase in the load-carrying capacity of beam due to lamination and/or hybrid-fibre reinforcement. (ii) However, lamination of beam makes the beam stiffer, and that the role of fibres comes into play only after initiation of first crack and thus, contributing to the ductility. (iii) From an actual structural application point of view the maximum deflection exhibited by the ‘laminated HFRC beam’ is desirable or permissible, has to be ascertained with respect to the ‘relevant codal provision/(s)’. The findings of the current study are firmly comparable to that reported by Syeed et al.  for SFRC beams strengthened with GFRP laminates.
3.4 Load-deflection response
Ductility and failure mode of tested beams
Mode of failure
HB 0 L0
HB 0 L5
Sudden debondig of GFRP laminates
Gradual debonding of GFRP laminates
Gradual debonding of GFRP laminates
Gradual debonding of GFRP laminates
Gradual debonding of GFRP laminates
3.6 Debonding behaviour and failure modes
Load at initiation of debonding of laminate and the corresponding deflection at that stage for all beams tested are summarized in Table 5. The load at initiation of debonding and the corresponding deflection for all strengthened HFRC beams (HB0.5L5 to HB1.5L5) increase with the increase in Vf up to Vf = 1.5% and thereafter decrease for Vf = 2.0% (HB2.0L5). The above trend is similar to the behaviour of load-carrying capacity and deflection at first crack, yield point and at ultimate stage of the above beams. However, the actual load at initiation of debonding and deflection at debonding lies between the yield point and ultimate stage, more specifically, slightly higher than the value of yield point. In other words, debonding (sudden/gradual) of the laminate occur immediately after the yield point for laminated and/or laminated HFRC beams. Any further increase in the load carrying capacity of the above beams is primarily due to the incorporation of hybrid fibres. On the other hand, the deflection of laminated HFRC beams at ultimate stage increases by more than two times than at the stage of initiation of debonding. This shows the ductility effect of hybrid fibres used and plays a very significant role, especially, after ‘debonding of laminate’. The above phenomenon correlates well with the actual values of ductility/ductility ratios (Table 6).
GFRP lamination has contributed to the increase in the load-carrying capacity at first crack, yield and ultimate stages of a RC beam than the ‘control beam’ (that is un-laminated RC beam).
There is a combined effect of lamination and inclusion of hybrid fibres, in contributing to the very high load-carrying capacity and enhanced ductility of laminated HFRC beam, especially, a fibre volume content of 1.5%,with the ratio of 70:30 (basalt: polyolefin). The above unique advantages can be used for structural applications, where, both load-carrying capacity and ductility are required.
The maximum yield and ultimate load- carrying capacity of laminated HFRC beam with Vf = 1.5% and 5 mm thick GFRP lamination (HB1.5L5), is 59% and 49% higher than laminated RC beam (HB0L5) and 125% and 98% higher than the control beam (HB0L0).
However, the deflection of the above beam 5.5 mm and 15.0 mm respectively at yield and ultimate stages and the above maximum deflections are 46.8% and 138.1% higher than the laminated RC beam. The permissibility of the above deflection values have to be ascertained with respect to relevant codal provisions for a particular structural application.
Laminated HFRC beam (HB1.5L5) has the highest ductility and is 62% higher than laminated RC beam (HB0L5), which is very highly advantageous from an application point of view.
All the laminated beams have failed by ‘debonding’ just after the ‘yield point’ and ductile failure thereafter, until ultimate stage.
The authors of this paper acknowledge the financial support provided by University Grant Commission (UGC), File no: MRP-6466/16(SERC/UGC), New Delhi, for carrying out this research work successfully. The authors also thank the Elasto-Plastic Concrete (Europe) Ltd for their support in providing polyolefin fibres to carry out the research work. The authors are also thankful to Pondicherry Engineering College for providing laboratory facilities for testing.
Compliance with ethical standards
Conflict of interest
The author(s) declares that they have no conflict of interest.
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