Flexural behaviour of geopolymer concrete beams reinforced with BFRP and GFRP polymer composites

Abstract

The paper presents the experimental investigations on the flexural behaviour of geopolymer concrete beams reinforced with Basalt Fibre Reinforced Polymer (BFRP)/Glass Fibre Reinforced Polymer (GFRP) rebars and the effect of inclusion of the new adhesively bonded BFRP/GFRP stirrups. M30 grade geopolymer and conventional concrete beams with the dimension of 100 × 160 × 1700 mm were cast to investigae the flexural behaviour of BFRP/GFRP and steel bars. This study also examined the mode of failure, deflection behaviour, curvature moment capacity, crack width, pattern, propagation, strains and average crack width of the BFRP/GFRP bars with stirrups in the geopolymer concretes using a four-point static bending test. The results were compared to that of conventional steel-reinforced concrete, and it was found that the Basalt and Glass reinforced polymer beams demonstrated premature failure and sudden shear failure. Further, the FRP bars exhibited higher mid-span deflection, crack width and crack propagation than steel bars. Crack spacing of the FRP bars decreased with an increase in the number of cracks. The correlation between the load and the deflection behaviour of the beams was determined using statistical analysis of multi variables regression.

Keywords

GFRP BFRP geopolymer aluminosilicate deflection crack statistical analysis

Introduction

Reinforced concrete is mainly utilised in the construction of buildings, water retaining structures, roadways and bridges, and a slew of other things. As a result, the annual concrete production is estimated to be 25 billion tonnes. Ordinary Portland Cement (OPC) is used as a binding material in concrete construction, and the world market for cement production was estimated at 4.3 billion in 2014 (Zhang et al., 2014). The cement industry contributes to 5%–7% of the global carbon dioxide emissions (Miller et al., 2018).

Concrete industry requires an environment-friendly binding material to replace the portland cement concrete (Nwankwo et al., 2020). Geopolymer concrete is widely used in the construction industry since it has a lower embodied energy and lower carbon footprint with 80% less CO₂ emission than OPC concrete (Amran et al., 2020). The geopolymerization method, which is different from the cement hydration process, is used to manufacture geopolymer concrete from industrial waste materials activated by alkali activator solutions (Singh and Middendorf, 2020). Several studies have shown that geopolymer concrete is a suitable building material (Ma et al., 2018). Data from the industry point out that about 200 million tonnes of ground granulated blast furnace slag (GGBS) and 176 million tonnes of fly ash have been left unutilised thus ending up as landfills (Karthik et al., 2019). These waste materials could be used as potential aluminosilicate binders in geopolymer concrete. Severe ecological imbalances have resulted from river bed mining leading to an increase in the demand for sand in the construction industry. Many researchers use sea sand (Xiao et al., 2017) and manufactured sand (M-sand) (Mane et al., 2021) as alternative to river sand. Steel reinforcement corrodes quickly in adverse conditions due to the presence of chloride ions. Early strength degradation and loss of serviceability in reinforced concrete structures are caused mainly by steel corrosion. The cost of maintaining, repairing, rehabilitating and retrofitting steel-reinforced concrete structures is relatively high. Hence, in the recent years, steel reinforcement is being replaced with a non-corrosive fibre reinforced polymer to alleviate the corrosion problem (Provis et al., 2015). Because of the differences in the physical and mechanical qualities, FRP reinforced concrete structures behave differently than traditional steel-reinforced concrete buildings (El Ghadioui et al., 2020). Aramid (AFRP), glass (GFRP) and carbon are three common forms of fibre reinforced polymer (CFRP). In the recent years, Basalt (BFRP) fibres have sparked increased scientific attention.

Nagajothi and Elavenil (2018) investigated the compressive strength of fly ash–based geopolymer concrete by varying different paramters such as sodium silicate to sodium hydroxide ratio, sodium hydroxide concentration and curing temperature, among others. The effect of Ground Granulated Blast Furnace Slag (GGBS) with fly ash and activator on the workability and strength qualities of geopolymer concrete under ambient curing conditions has been examined to avoid slow reactivity of fly ash (Deb et al., 2014). The durability properties of oil palm shell concrete containing GGBS partially replaced with fly ash and M-sand entirely replaced with river were investigated (Mo et al., 2016). Tang et al. (2020) recycled the municipal and construction solid wastes to be used as geopolymer composite materials. Tran et al. (2021) observed that steel-reinforced geopolymer concrete beams resembled the typical reinforced concrete beams (Tran et al., 2021) and hence can be used in buildings and bridges (Hassan et al., 2020). El-Nemr et al. (2018) studied the load-carrying capacity, deflection, strain and crack width of GFRP bar reinforced beams. The cracking behaviour of the tested beams confirmed that sand coated GFRP bars exhibited good bond performance in the concrete when compared with other GFRP bars. Goldston et al. (2016) examined the failure mode and energy absorbing capacities of GFRP reinforced concrete beams subjected to impact and static loading. The main influencing factors were the reinforcement ratio and strength of the concrete. FRP reinforced concrete beams have large deflections, wider crack and less stiffness at service conditions and a lower modulus of elasticity (Ruan et al., 2020). Comparison of the performance of GFRP bars in regular concrete (NC) and self-consolidating concrete (SCC) in terms of load-deflection, failure mode, load-carrying capacity and crack pattern revealed that the responses of SCC were similar to that of NC (Zemour et al., 2018). Hajiloo et al. (2018) developed a new concept for GFRP bars at critical temperatures. The effects of varying span/depth and thickness on the structural behaviour of GFRP circular bars in beams and columns have been investigated (Ullah et al., 2021).

Pawłowski and Szumigała (2015) determined the failure mechanism, deflection and ductility of the BFRP reinforced concrete beam and reported that both the peak load and the beam stiffness increased with the reinforcement ratio. Gao et al. (2020) investigated the flexural behaviour of concrete reinforced with AFRP prestressed tendons. Du et al. (2011) studied the flexural capacity of reinforced concrete with CFRP prestressed tendons. The authors discovered that combining unbonded and bonded FRP tendons improved the ductility of the prestressed concrete beams. Recent studies have focused on using glass and basalt fibre reinforced polymer bars in concrete since they are less expensive than carbon and aramid FRP. Li et al. (2019) studies that deal with fibre reinforced polymer in reinforced concrete with river sand and steel in geopolymer concrete with river sand are available in the literature.

The present work mainly focuses on the use of fly ash and GGBS, M-sand, and FRP which are considered as alternative materials for cement, river sand and steel respectively. The utilisation of these alternative materials helps to prevent CO₂ emission during cement production, overcome the scarcity of river sand and reduce the corrosion of steel. This work aims to evaluate the structural behaviour of geopolymer concrete beam elements reinforced with BFRP/GFRP bars using new adhesively bonded BFRP/GFRP stirrups in terms of crack pattern, failure mode, load-deflection relationship, crack width, crack propagation and spacing of cracks under static load. The results of the study are compared with that of control concrete reinforced beams with steel using M-sand. Further, regression analysis has been employed to formulate equations to determine the load properties of the geopolymer concrete and its deflection.

Experimental study

Materials

Geopolymer concrete

Geopolymer concrete was prepared by combining low calcium class F fly ash with an additive of GGBS as an aluminosilicate binder, coarse aggregate, fine aggregate and alkaline activator solution (AAS). Low calcium fly ash was collected from the Thermal power plant and GGBS from Astra chemicals, Chennai. Manufactured sand in the form of fine aggregates was obtained from KMC blue metals, Theni. Coarse aggregate of crushed granite with nominal overall size of 8 mm, 12 mm and 20 mm were obtained from the local market. The aggregates in the saturated surface dry (SSD) condition were used for preparing the geopolymer concrete. Ordinary portland cement of grade 53 was used for the preparation of standard concrete test specimens. The chemical composition of the aluminosilicate materials and the physical properties of the concrete materials used for making the concrete are presented in Tables 1 and 2, respectively.

Table 1.

Chemical composition of fly ash and GGBS.

Materials	SiO₂	CaO	MgO	Al₂O₃	Fe₂O₃	K₂O	SO₄	Na₂O	LOI
Fly ash (%)	63.32	2.49	0.29	26.76	5.55	0.0002	0.36	0.0004	0.97
GGBS (%)	35.05	34.64	6.34	12.5	0.3	0.6	0.38	0.9	0.26

Table 2.

Physical properties of the concrete materials.

Description	Fly ash	GGBS	Cement	River sand	M-sand	Coarse aggregate
Specific gravity	2.13	2.85	3.13	2.66	2.72	2.73
Fineness modulus	—	—	—	3.04	2.90	—
Water absorption (%)	—	—	—	1.33	1.52	0.64

Sodium silicate (SiO₂/Na₂O = 2.0) and sodium hydroxide (NaOH) in the form of flakes with a concentration of 8M were used as alkaline activator solutions (AAS) in the geopolymer concrete mixes. As sodium hydroxide flakes generate much heat on reacting with distilled water, NaOH solution was prepared a day before to bring down its temperature to the ambient temperature. The NaOH and Na₂SiO₃ solutions were then mixed together half an hour before casting. The AAS was used to activate silicon and aluminium source materials, available in FA and GGBS. A naphthalene-based superplasticiser was employed in the geopolymer concrete to achieve the desired workability.

Reinforcement bars

In this research, BFRP and GFRP were used as reinforcements in the geopolymer concrete beams, and steel was used as reinforcement in cement concrete for the purpose of comparison. The basalt fibre reinforced polymer rebar was purchased from NickunjEximp Pvt Ltd, Mumbai, and Glass fibre reinforced polymer rebar was purchased from Meena fibres, Puducherry. A locally available TMT steel rod was used for this study. BFRP, GFRP and steel rods of dimensions 12 mm, 10 mm and 8 mm rods were used. Table 3 shows the material properties of the BFRP, GFRP and steel bars. The images of BFRP and GFRP are shown in Figure 1.

Table 3.

Properties of the FRP, GFRP and Steel reinforcement bars.

Properties	BFRP	GFRP	Steel
Tensile strength, (MPa)	513	515	495
Tensile strain energy, (N/mm)	29	40	25
Elastic modulus, (GPa)	94	54	200
Poisson’s ratio	0.23	0.24	0.27
Strain of reinforcements	0.005	0.009	0.0025

Figure 1.

Images of BFRP and GFRP rebars.

Mix proportions

The mix proportion of the geopolymer concrete used for the investigation was 1: 2.22: 3.86: 6.95 by mass in the order of alkaline activator solutions, aluminosilicate binder, fine aggregate and coarse aggregates. The ratio between the alkaline activator solutions and the binder was 0.45, and the sodium silicate (Na₂SiO₃) to sodium hydroxide (NaOH) ratio in the alkaline activator solution was 2.5. The desired workability of the geopolymer concrete was achieved by using 1% of superplasticiser (Conplast SP430). Four geopolymer concrete beam specimens with BFRP and GFRP rebars were cast for the static load tests. For the purpose of comparison, two conventional concrete beams with steel bars were also cast. The mix constituents per cubic meter of the geopolymer and traditional concrete bars are shown in Table 4 (Nagajothi and Elavenil, 2018). Test specimens were prepared with both the geopolymer and the conventional concrete mixtures in order to evaluate their mechanical properties. Specimens in the shape of cubes of dimension 150 × 150 × 150 mm, cylinders 200 mm high and 100 mm in diameter and prisms of dimension 100 × 100 × 500 mm were prepared for compression, split tension and flexure tests, respectively. Specimens of the geopolymer concrete were cured at room temperature while specimens of the control concrete were subject to underwater curing for 28 days. Three identical specimens were tested individually after 28 days of curing as per IS 516-1959, IS 5816-1999 and IS 516-1959 (Nagajothi and Elavenil, 2020). Table 5 shows the mechanical properties of the G30 geopolymer concrete and M30 conventional concrete mix.

Table 4.

Constituents of the geopolymer concrete and conventional concrete.

Geopolymer concrete		Conventional concrete
Item	Quantity (Kg/m³)	Item	Quantity (Kg/m³)
Fly ash	304	Cement	380
GGBS	76	Cement	380
M-sand	660	River sand	660
Coarse aggregate	1189	Coarse aggregate	1189
AAS	171	Water	171
Superplasticizer	3.8	Water	171

Table 5.

Mechanical properties of the geopolymer and the conventional concrete mixtures.

Mix	Compressive strength (MPa)	Split tensile strength (MPa)	Flexural strength (MPa)	Elastic modulus (GPa)
Geopolymer concrete	40.35	3.32	4.69	19.10
Conventional concrete	38.95	3.17	4.46	22.19

Note. Values given in the tables are the average values for three identical specimens.

Specimen details of the beam

Beam specimens 100 mm in width, 160 mm in depth and 1700 mm in length were made from reinforced geopolymer and conventional concrete beams and tested in simply support conditions over an effective span of 1500 mm. A 20 mm clear cover was used in the beam. The beams were designed with two 12 mm diameter main rebars at the bottom for tension and two 10 mm diameter rebars at the top for compression, and 8mm diameter stirrups at 100 mm intervals. Figure 2 shows the dimension and detailing of the reinforcement for the beam. The yield strength of the steel is f_y = 500 N/mm².

Figure 2.

Dimension and detailing of reinforcement for the beam (dimensions are in mm).

The reinforcement details for the beam specimens are shown in Table 6. Beam designation is denoted by XRyz, where X stands for the rebar type (B-Basalt, G-Glass and S-Steel), R-Reinforced and yz stands for the type of concrete (GC-Geopolymer concrete and CC-Conventional Concrete). The reinforcement ratio (ρ) for all the beam specimens was fixed at 1.5%.

Table 6.

Details of the reinforcement for the beam specimens.

Designation	Top reinforcement	Bottom reinforcement	Stirrups
BRGC	2#10 mm BFRP	2#12 mm BFRP	8 mm@100 mm-BFRP
GRGC	2#10 mm GFRP	2#12 mm GFRP	8 mm@100 mm-GFRP
SRCC	2#10 mm steel	2#12 mm steel	8 mm@100 mm-Steel

Making of stirrups

Several researchers have studied the behaviour of FRP rebars in conventional concrete and geopolymer concrete with steel stirrups. Hence, this research focuses on employing the FRP bar as the stirrup bar with a smaller diameter. Stirrup diameter (8 mm) FRP bars were hand made as they were not readily available in the market. The stirrups rod was cut to the required horizontal and vertical dimensions and were attached using epoxy resin. The joint was externally tied to the FRP mat with the help of epoxy resins after the stirrup rods were made. Araldite LY556 and Aradur HY951 were used as resin and hardener, respectively. Hardener and resin in the ratio of 1:10 were used to prepare the resins. The images of the prepared basalt and glass stirrups before being tied with the FRP mat is shown in Figure 3. As there are no data to quantify the minimum requirements for the joints, no testing was carried out for the joint strength at the material level.

Figure 3.

Prepared basalt and glass stirrups before being tied with FRP Mat.

Preparation of the specimen

The SSD aggregates and the binders of fly ash and GGBS were dry mixed in a pan mixer. This was followed by addition of the alkali activator solutions to the dry materials. The mixture was stirred for 5 min to obtain the geopolymer concrete. This process is important for the production of geopolymer concrete beams reinforced with BFRP/GFRP bars. The geopolymer concrete mixes in the fresh state before moulding is shown in Figure 4. The formwork for the beam mould before casting is shown in Figure 5 (Nagajothi and Elavenil, 2020).

Figure 4.

Fresh state of geopolymer concrete.

Figure 5.

Formwork for the beam mould.

The mould was coated with released oil before casting the concrete to prevent concrete adherence. The fabricated BFRP and GFRP reinforcements are shown in Figure 6(a) and (b). Fresh geopolymer concrete was poured into the beam mould in three equal layers and compacted with a vibrator. The beam was demoulded after 24 h and left to cure at ambient temperature for the geopolymer concrete while the control concrete beams were subjected to underwater curing for 28 days. The structural behaviour of the specimens was evaluated after they had been cured.

Figure 6.

Fabricated bars of (a) BFRP and (b) GFRP.

Experimental procedure

The four-point static bending tests were performed to study the geopolymer concrete beams reinforced with BFRP and GFRP and conventional concrete beams reinforced with steel. Figure 7(a) and (b) show the loading configuration for the static test and the flexural test setup for static loading conditions. The Universal Testing Machine (UTM) with a capacity of 1000 kN was used to test the specimens. The beams with effective span of 1500 mm were supported on a steel box girder of length 1700 mm. The beams were loaded at four points, each 250 mm away from the centre to the support, and loads were applied at 500 mm from each support.

Figure 7.

(a) Loading configuration for static test (Dimensions are in mm) and (b) flexural test setup for static load.

Test setup for static load

The Universal Testing Machine was used to apply loads up to 3 kN to the beams and obtain measurements. The measurement was taken at every 3kN loading increments. The vertical deflections were measured using three dial gauges with a minimum count of 0.01 mm. Two dial gauges were set at two load points, and one at the midpoint (middle), and the readings were recorded for varying loads. Brass pellets with an indentation on one side and a diameter of 6 mm were glued at the compression and tension zones for a gauge length of 200 mm from centre to centre. The tensile and compressive strains at the bottom and top zone were measured using a DEMEC strain gauge at different load intervals. The load, deflections and strain gauges were checked for initial crack, service load and ultimate load. At each load interval, the number of cracks in the Constant Bending Zone (CBZ), the total number of cracks, crack spacing, crack propagation and average crack width were observed for all the beams tested under flexure. Additionally, the mode of failure of the beams was also identified.

Test results and observations for static load

Failure mode and crack pattern

The failure mode and crack pattern for BRGC, GRGC and SRCC specimens are shown in Figure 8.

Figure 8.

Failure mode and crack pattern of BRGC, GRGC and SRCC specimens.

In the BRGC, GRGC and SRCC specimens, the flexural cracks were developed initially in the tension zone of the Constant Bending Moment (CBM) zone. The existing cracks were promulgated vertically towards the compression zone while new cracks developed at other places of the CBM zone as the load was increased. The crack spacing varied along the span. The crack patterns were similar in all the three beams at the initial load intervals. As the load was increased, no cracks were developed in the shear zone places in the SRCC specimens. However, in the BRGC and GRGC beams, the flexural cracks gave way to inclined cracks that were developed in the shear zone regions.

The failure load of the steel-reinforced conventional concrete beam deflected considerably. The failure of the SRCC beam was initiated by the development of tensile steel accompanied by concrete crushing in the compression region. Thus, the failure of the SRCC beams was attributed to both flexures and compression. In the case of BRGC beams, flexure failure was observed up to 30 kN, while sudden shear failure was observed for the ultimate load of 33.45 kN. For GRGC, flexure failure occurred at the ultimate load of 30 kN while sudden shear failure was observed at the ultimate load of 32.40 kN. The cracks propagated vertically in all the beams before the ultimate load was reached. Premature failure occurred due to the separation of the bond. Said et al. (2016) reported a similar bond failure between GFRP bars and concrete during their experimental tests. The sudden failure occurred in Basalt and Glass fibre reinforced polymer bar beams due to insufficient shear reinforcement. Hence, it was concluded that the mode of failure of BRGC and GRGC was pure shear failure. According to Goldston et al. (2016), there was no prior warning of the collapse, and the rupture of the GFRP reinforcing bars caused the failure.

Crack width, crack spacing, crack propagation and number of cracks

Number of cracks

Figure 9 shows the variation of the total number of cracks with change in the load. It was observed that the total number of cracks in BRGC, GRGC and SRCC increased as the load was increased up to the ultimate load level. The total numbers of cracks in BRGC were found to be similar to that of SRCC. When compared with SRCC, GRGC had lower number of cracks. It was reported that steel beams of different grade had larger number of cracks than the GFRP beam, with the crack depth remaining almost constant (Abed and Alhafiz, 2019). In this study, the number of cracks at 20 kN load level was 12, 12 and 6 for the BRGC, GRGC and SRCC beams, respectively.

Figure 9.

Load-Total number of cracks of beams.

Crack propagation

Figure 10 shows the variation of crack propagation with load for all beams. Cracks were developed vertically from the bottom of the beam to the top of the beam at different load intervals. The crack propagation was measured at each load interval. It was found that in all the beams, the crack propagation increased with increase in the load. The crack propagation was high in both, the BRGC and GRGC beams compared to the SRCC beam owing to the larger deformation of the FRP beams. Compared to BRGC and SRCC, the crack propagation in GRGC was high and remained constant for higher loads. For BRGC, GRGC and SRCC beams, the fracture propagation at 20 kN was determined to be 126, 135 and 75 mm, respectively.

Figure 10.

Load-crack propagation of beams.

Crack spacing

The dependence of the crack spacing on the load for all the three beams is shown in Figure 11. It was evident that the crack spacing decreased with increase in the number of cracks. In addition, the crack spacing in the fibre reinforced geopolymer concrete was constant for a particular load level when compared with that of SRCC. It was also observed that the crack spacing in BRGC was similar to that of SRCC. At a load level of 20 kN, the crack spacing for BRGC, GRGC and steel-reinforced beams were 116, 121 and 127 mm, respectively.

Figure 11.

Load-spacing of cracks of beams.

Crack width

Figure 12 shows the average crack width of the beams as a function of the load. Due to the low modulus of elasticity, the average crack width of the fibre reinforced geopolymer concrete increased as the load level increased when compared to that of the steel-reinforced conventional concrete. The pattern of the variation of the average crack width of the BRGC and GRGC beams was similar (Hemalatha et al., 2020). However, the average crack width increased in GRGC when compared with BRGC. The average crack widths at 20 kN load for BRGC, GRGC and SRCC were found to be 0.457 mm, 0.775 mm and 0.132 mm, respectively.

Figure 12.

Load-Average crack width of beams.

Behaviour of load-deflection

Figure 13 shows that the first observable crack ensued on 9 kN, 6 kN and 15 kN (about 26.91%, 18.52% and 30% of the ultimate load) as a kink in the curve or first deviation for BRGC, GRGC and SRCC, respectively. The load-deflection pattern of BRGC and GRGC was similar to that of SRCC. However, the Glass and Basalt fibre reinforced polymer in geopolymer concrete exhibited significant mid-span deflection than the control steel reinforcement in conventional concrete. Comparison between the Glass and Basalt fibre reinforced polymer beams in the geopolymer concrete revealed that the Glass fibre rod beam yielded more deflection than the Basalt rod beam. Comparison of the mid-span deflections of BRGC and SRCC at the same load level showed that the BRGC deflection ranged between 9 and 5.1 times that of the SRCC control specimen.

Figure 13.

Load-central deflection of beams.

Similarly, the deflection of the GRGC was 8.6–11.5 times of the SRCC control specimen. Ali et al. (2014) found that the GFRP bar beams had larger deflections than that of the control concrete beams. Hence, it is evident that fibre reinforced polymer behaviour was different from that of the steel bars for the same area of reinforcement in the beam. At a load of 21 kN, the mid-span deflections of SRCC, BRGC and GRGC were 2.41 mm, 11.91 mm and 23.59 mm, respectively. The conventional steel has a lower deflection than Basalt and Glass FRP due to its higher Poisson’s ratio and modulus of elasticity.

The accurate energy values of all three beams were calculated as the area enclosed by the load-deflection curve using AutoCAD drawings. The energy values for the glass fibre reinforced polymer in geopolymer concrete were higher than that of the basalt fibre reinforced polymer in geopolymer concrete and steel in conventional concrete, as shown in Figure 14. This is attributed to the increased brittleness that causes the concrete to fail suddenly following the ultimate load imposed by a massive work (energy).

Figure 14.

Energy for materials.

Table 7 represents the deflections and span-to-deflection ratios for the SRCC, BRGC and GRGC beams. During ultimate loads, the span-to-deflection ratio received ranged from 30 to 88. The stiffness of the FRP beams was considerably low when compared to that of steel-reinforced control beams.

Table 7.

Load and deflection behaviour of the beams.

Beam	Ultimate load, P_u (kN)	δ_max (mm)	L/δ_max	Deflection δ = L/350		Stiffness (kN/mm)
Beam	Ultimate load, P_u (kN)	δ_max (mm)	L/δ_max	P _L/350(kN)	P _L/350/P_u	Stiffness (kN/mm)
BRGC	33.45	30.75	49	8.52	0.25	1.80
GRGC	32.40	49.90	30	5.47	0.17	1.09
SRCC	49.80	17.10	88	32.80	0.66	12.35

The ductility of the GFRP and BFRP bars was found to be higher than that of steel bars due to the presence of fibre in the FRP bars (Nagajothi and Elavenil, 2020). The beam stiffness of the FRP bars reduced due to their ductile nature thus leading to increase in the bar deformation with increase in the load. Cracks were formed at the bottom of the beam within the CBZ (Constant Bending Zone) as the stiffness of the beam decreased (Maranan et al., 2015). The load-deflection graph shows that on application of specific loads of 30 kN, 21 kN and 45 kN to BRGC, GRGC and SRCC, respectively, there was a dramatic increase in the deflection due to beam stiffness reduction (Madupalli et al., 2019).

Load-compressive and tensile strain

Load-strain behaviour of BRGC, GRGC and SRCC is shown in Figure 15. The maximum compressive strains observed for BRGC, GRGC and SRCC was 0.003, 0.005 and 0.002, respectively. Similarly, the ultimate tensile strains observed for BRGC, GRGC and SRCC was 0.009, 0.015 and 0.007, respectively. Mohamed et al. (2016) observed the GFRP bars tensile reinforcement stresses is 0.006-0.0125. The maximum compressive strain of the BRGC and GRGC was observed 1.5 times and 2.5 times the SRCC. Fibre-reinforced geopolymer concrete exposed higher tensile reinforcement strain than steel-reinforced conventional concrete. The tensile strain of BRGC and GRGC were 1.29, 2.14 times higher than SRCC.

Figure 15.

Load-strain of beams.

Statistical analysis

Figure 16 depicts the correlation analysis of load and deflection of geopolymer concrete with different bars. The results of the statistical analysis of the multi variables regression are given by equations (1–3) for BRGC, GRGC and SRCC, respectively. The equations were formulated to calculate the load properties of the geopolymer concrete and deflection using regression analysis

P = 0.84 + 2.019 δ - 0.029 δ^{2}

(1)

P = 1.567 + 0.986 δ - 0.008 δ^{2}

(2)

P = 2.6 + 7.851 δ - 0.298 δ^{2}

(3)

where P is the load of the geopolymer concrete beam and

δ

is the deflection of the geopolymer concrete for the applied load. Multiple factors 2.019 and −0.029 were examined along with different δ² values using multi variables regression equation (1) while 0.84 is the constant value based on BRGC experimental results.

Figure 16.

Correlation analysis between load and deflection for BRGC, GRGC and SRCC using multi variables regression.

For GRGC and SRCC, a similar procedure was employed to arrive at the prediction equations (2) and (3). The correlation coefficient (R2) values for equations (1–3) are 0.995, 0.984 and 0.994, respectively. The correlation quality between load and deflection for BRGC, GRGC and SRCC was found to be 100% accurate.

Conclusions

The following conclusions were drawn from experimental research and statistical analysis of the beams evaluated under four-point flexural stresses using BFRP/GFRP beams reinforced in geopolymer concrete and steel beams in control concrete.

• The geopolymer concrete showed better mechanical properties than the conventional concrete while replacing the percentage of GGBS in FA-based geopolymer concrete for the same mix proportion of conventional concrete.

• Compared to SRCC, the ultimate load-carrying capacity of the BRGC and GRGC beams reduced by 32.8% and 35%, respectively. However, compared to SRCC, the maximum deflection in BRGC and GRGC increased by 79.8% and 192%, respectively.

• Since the ductility of the FRP bars is higher than that of steel bars, BRGC and GRGC beams demonstrated higher crack propagation than steel-reinforced concrete beams.

• The failure of BRGC and GRGC beams occurred after reaching 95% of their ultimate load. A sudden shear cum flexure failure was noticed that resulted in premature failure. Since the premature failure of the FRP rods is caused by bond dissociation, FRP rods are capable of withstanding higher load than steel rods.

• As FRP reinforced bars with premature failure experienced sudden shear failure, the end anchorage could increase the ultimate load-carrying capacity. Also, premature failure can be avoided by providing a closer spacing of the shear reinforcement.

• Based on the experimental values, multi variables regression using statistical analysis and formulas were developed to describe the load-deflection behaviour of BRGC and GRGC in geopolymer concrete and SRCC in conventional concrete.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Nagajothi S

References

Abed

Alhafiz

(2019) Effect of basalt fibers on the flexural behavior of concrete beams reinforced with BFRP bars. Composite Structures 215: 23–34. DOI: 10.1016/j.compstruct.2019.02.050

Ali

Abdalla

Hawileh

, et al. (2014) CFRP mechanical anchorage for externally strengthened RC beams under flexure. Physics Procedia 55: 10–16. DOI: 10.1016/j.phpro.2014.07.002

, et al. (2020) Clean production and properties of geopolymer concrete; A review. Journal of Cleaner Production 251: 119679. DOI: 10.1016/j.jclepro.2019.119679

Deb

Nath

Sarker

(2014) The effects of ground granulated blast-furnace slag blending with fly ash and activator content on the workability and strength properties of geopolymer concrete cured at ambient temperature. Materials and Design 62: 32–39. DOI: 10.1016/j.matdes.2014.05.001

Wang

Liu

(2011) Flexural capacity of concrete beams prestressed with carbon fiber reinforced polymer (CFRP) tendons. Advanced Materials Research 168–170: 1353–1362. DOI: 10.4028/www.scientific.net/AMR.168-170.1353

El-Nemr

Ahmed

El-Safty

, et al. (2018) Evaluation of the flexural strength and serviceability of concrete beams reinforced with different types of GFRP bars. Engineering Structures 173: 606–619. DOI: 10.1016/j.engstruct.2018.06.089

El Ghadioui

Proske

Tran

, et al. (2020) Structural behaviour of CFRP reinforced concrete members under bending and shear loads. Materials and Structures/Materiaux et Constructions 53(3): 1–16. DOI: 10.1617/s11527-020-01496-7

Gao

Fang

You

, et al. (2020) Flexural behavior of reinforced concrete one-way slabs strengthened via external post-tensioned FRP tendons. Engineering Structures 216: 110718. DOI: 10.1016/j.engstruct.2020.110718

Goldston

Remennikov

Sheikh

(2016) Experimental investigation of the behaviour of concrete beams reinforced with GFRP bars under static and impact loading. Engineering Structures 113: 220–232. DOI: 10.1016/j.engstruct.2016.01.044

(2018) Mechanical properties of GFRP reinforcing bars at high temperatures. Construction and Building Materials 162: 142–154. DOI: 10.1016/j.conbuildmat.2017.12.025

(2020) A review of properties and behaviour of reinforced geopolymer concrete structural elements- A clean technology option for sustainable development. Journal of Cleaner Production 245: 118762. DOI: 10.1016/j.jclepro.2019.118762

, et al. (2020) Analysis of RCC T-beam and prestressed concrete box girder bridges super structure under different span conditions. Materials Today: Proceedings 37(2): 1507–1516. DOI: 10.1016/j.matpr.2020.07.119

, et al. (2019) Effect of bio-additives on physico-chemical properties of fly ash-ground granulated blast furnace slag based self cured geopolymer mortars. Journal of Hazardous Materials 361: 56–63. DOI: 10.1016/j.jhazmat.2018.08.078

, et al. (2019) A review on durability of alkali-activated system from sustainable construction materials to infrastructures. ES Materials & Manufacturing 4: 2–19. DOI: 10.30919/esmm5f204

15.

Awang

Omar

(2018) Structural and material performance of geopolymer concrete: a review. Construction and Building Materials 186: 90–102. DOI: 10.1016/j.conbuildmat.2018.07.111

, et al. (2019) Structural performance of non-linear analysis of turbo generator building using seismic protection techniques. International Journal of Recent Technology and Engineering 8(1): 1091–1095. https://www-scopus-com.web.bisu.edu.cn/inward/record.url?eid=2-s2.0-85067880441&partnerID=MN8TOARS

17.

Mane

Kulkarni

Prakash

(2021) Near-surface and chloride permeability of concrete using pozzolanic materials and manufactured sand as partial replacement of fine aggregate. Iranian Journal of Science and Technology - Transactions of Civil Engineering. 45: 1427–1439. DOI: 10.1007/s40996-020-00543-1

, et al. (2015) Evaluation of the flexural strength and serviceability of geopolymer concrete beams reinforced with glass-fibre-reinforced polymer (GFRP) bars. Engineering Structures 101: 529–541. DOI: 10.1016/j.engstruct.2015.08.003

, et al. (2018) Carbon dioxide reduction potential in the global cement industry by 2050. Cement and Concrete Research 114: 115–124. DOI: 10.1016/j.cemconres.2017.08.026

20.

Alengaram