Experimental and theoretical study of used GFRP I-profile composite columns

The study of Glass Fiber Reinforced Polymer (GFRP) composite columns under axial load is increasingly important in civil engineering due to the growing adoption of composite materials in construction. GFRP offers advantages such as a high strength-to-weight ratio, corrosion resistance, and durability. However, its limitations include poor fire resistance, lower ductility, and potential long-term deflection concerns. When combined with concrete, GFRP-reinforced columns can enhance load-bearing capacity and ductility compared to conventional reinforced concrete (RC) columns. The structural behavior of GFRP-concrete composite columns depends on factors such as the type and ratio of GFRP, the bond between GFRP and concrete, and the level of confinement provided. Understanding these interactions is crucial for designing safe, efficient structures. Further research is needed to refine design guidelines and optimize material usage, leading to cost-effective solutions for structural applications.
Several studies have investigated the axial load performance of GFRP-reinforced composite columns. Muhammad et al. [1] examined GFRP I-section and C-section composite columns under axial and eccentric loads. Their results showed that GFRP I-section-reinforced columns achieved higher ultimate loads but exhibited lower ductility compared to steel-reinforced columns. Jing et al. [2] studied concrete-filled square GFRP tubular columns under axial compression and found that reinforcement cages significantly improved load capacity and ductility. Muhammad et al. [3] further investigated GFRP tube-reinforced concrete columns, concluding that GFRP tubes enhance both ductility and strength. Ishaqian and Keramti [4] confirmed that pultruded GFRP I-shapes effectively improve concrete column performance. However, Zureick and Scott [5] identified global buckling as the primary failure mechanism in slender GFRP-reinforced columns. Hashem [6] conducted analytical and experimental research on short and long GFRP composite columns, while Lei [7] investigated slender GFRP square hollow columns under axial and eccentric loads and revealed that the compression performance of GFRP columns was significantly affected by the eccentricity and the moment capacity of bolted sleeve joint. Srinath T [8] conducted an experimental and numerical investigation on the bonding and buckling behavior of GFRP circular column under axial compression, and results showed more performance than similar conventional. Srinath et al. [8] studied the bonding and buckling behavior of GFRP circular columns under axial compression. Hiba et al. [9] found that composite columns with encased GFRP I-sections outperformed traditional columns under various loading conditions. Aydin [10] evaluated GFRP profile strength at different temperatures, reporting that tensile and compressive strengths decreased by $28\mathrm{\%}$$28\mathrm{\%}$28%28 \%$28\mathrm{\%}$ and $75\mathrm{\%}$$75\mathrm{\%}$75%75 \%$75\mathrm{\%}$, respectively, at ${100}^{\circ }\mathrm{C}$${100}^{\circ }\mathrm{C}$100^(@)C100^{\circ} \mathrm{C}${100}^{\circ }\mathrm{C}$ compared to ambient conditions. At ${200}^{\circ }\mathrm{C}$${200}^{\circ }\mathrm{C}$200^(@)C200^{\circ} \mathrm{C}${200}^{\circ }\mathrm{C}$, GFRP lost approximately $50\mathrm{\%}$$50\mathrm{\%}$50%50 \%$50\mathrm{\%}$ of its tensile strength and all its compressive strength due to the degradation and softening of the polymer matrix within the GFRP components at high temperatures, resulting in reduced load carrying capacity. In addition, exposure to fire causes thermal expansion mismatches between the GFRP and concrete layers, inducing cracking and delamination effects, which further reduce structural performance.
GFRP profiles exhibit high sensitivity to elevated temperatures but remain stable under lower temperature conditions. Therefore, fire resistance must be carefully considered when using GFRP in high-temperature environments. This study investigates the behavior of composite columns reinforced with GFRP I-section subjected to ${500}^{\circ }\mathrm{C}$${500}^{\circ }\mathrm{C}$500^(@)C500^{\circ} \mathrm{C}${500}^{\circ }\mathrm{C}$ for 90 minutes. Experimental, theoretical, and numerical analyses, including finite element modeling, are conducted to assess axial load performance. A practical design guideline for optimizing GFRP-reinforced composite columns is proposed
to enhance structural efficiency and cost-effectiveness. All specimens are tested under axial loading, with load-strain behavior and failure modes analyzed to provide insights into the structural performance of GFRP-concrete composite columns.

The experimental study of this research is based on testing the behavior of GFRP-concrete composite columns under axial load. The column samples are shown schematically in Fig. 1, and they consist of the following:

Two main parameters are considered: the effect of GFRP I-section on the behavior of RC composite columns and the effect of fire. The concrete dimensions and reinforcement details of the tested columns are given in Tab. 1 and Fig. 1. A total of four specimens were fabricated and tested in this experimental study.
All tested columns had an overall height of 1540 mm and a cross section of 160 mm x 160 mm . They were reinforced with longitudinal four steel bars of 10 mm diameter and steel transverse stirrups of 8 mm diameter at 100 mm spacing. The GFRP I-section had a total height of 100 mm , a flange width of 80 mm and a web and flange thickness of 5.5 mm positioned centrally within the RC composite columns. All of the specimens had the same concrete cover of 15 mm .

Note that:
RCC: The tested conventional reinforced concrete column,
GI : The GRPR I-section
C : Center position
F : Fire

Table 1: Designations of tested columns.

The concrete used was cement (Sina Cement Company) ( $450\mathrm{kg}/{\mathrm{m}}^3$$450\mathrm{kg}/{\mathrm{m}}^3$450kg//m^(3)450 \mathrm{~kg} / \mathrm{m}^{3}$450\mathrm{kg}/{\mathrm{m}}^3$ ), natural siliceous sand, crushed dolomite coarse aggregate of maximum size 12.5 mm and super plasticizer's (Addicrete BVS type G) $2\mathrm{\%}$$2\mathrm{\%}$2%2 \%$2\mathrm{\%}$ from the weight of the cement. The target mean strength was 45 MPa . The mix had a water-cement ( $\mathrm{w}/\mathrm{c}$$\mathrm{w}/\mathrm{c}$w//c\mathrm{w} / \mathrm{c}$\mathrm{w}/\mathrm{c}$ ) ratio of 0.37 , cement content of $500\mathrm{kg}/{\mathrm{m}}^3$$500\mathrm{kg}/{\mathrm{m}}^3$500kg//m^(3)500 \mathrm{~kg} / \mathrm{m}^{3}$500\mathrm{kg}/{\mathrm{m}}^3$, fine aggregate content of $628\mathrm{kg}/{\mathrm{m}}^3$$628\mathrm{kg}/{\mathrm{m}}^3$628kg//m^(3)628 \mathrm{~kg} / \mathrm{m}^{3}$628\mathrm{kg}/{\mathrm{m}}^3$, coarse aggregate content of $1144\mathrm{kg}/{\mathrm{m}}^3$$1144\mathrm{kg}/{\mathrm{m}}^3$1144kg//m^(3)1144 \mathrm{~kg} / \mathrm{m}^{3}$1144\mathrm{kg}/{\mathrm{m}}^3$, and super- plasticizer content of $10\mathrm{kg}/{\mathrm{m}}^3$$10\mathrm{kg}/{\mathrm{m}}^3$10kg//m^(3)10 \mathrm{~kg} / \mathrm{m}^{3}$10\mathrm{kg}/{\mathrm{m}}^3$. Nine standard cubes with dimensions $150\mathrm{mm}\times 150\mathrm{mm}\times 150\mathrm{mm}$$150\mathrm{mm}\times 150\mathrm{mm}\times 150\mathrm{mm}$150mmxx150mmxx150mm150 \mathrm{~mm} \times 150 \mathrm{~mm} \times 150 \mathrm{~mm}$150\mathrm{mm}\times 150\mathrm{mm}\times 150\mathrm{mm}$ were cast to perform concrete quality control. The cubes were immersed in water until the day of testing. The $\mathbf{3}\mathbf{5}$$\mathbf{3}\mathbf{5}$35\mathbf{3 5}$\mathbf{3}\mathbf{5}$-day strength achieved was the average compressive strength of $\mathbf{4}\mathbf{6}\mathbf{M}\mathbf{P}\mathbf{a}$$\mathbf{4}\mathbf{6}\mathbf{M}\mathbf{P}\mathbf{a}$46MPa\mathbf{4 6 ~ M P a}$\mathbf{4}\mathbf{6}\mathbf{M}\mathbf{P}\mathbf{a}$ at the test day.
The yield strength and ultimate strength of smooth steel bars were 316 MPa and 431 MPa for 8 mm stirrups bars, and 470 MPa and 636 MPa for 10 mm respectively. The results of steel bars are summarized in Tab. 2.

The mechanical properties of the used GFRP I-section are shown in Tab. 3 according to manufacturer's date [11] obtained from standard tests, the tensile strength of GFRP I-section was 336 MPa , and tensile modulus of elasticity 35300 MPa .

The test setup of axial compressive loading for the tested columns is presented in Fig. 2. The test columns were simply supported by steel plates at two ends. The column specimens were tested using the test frame as shown in Fig. 2 was fixed to the strong solid floor of the RC laboratory at the Faculty of Engineering of Helwan University. The supporting setup consisted of a horizontal frame made of four steel I-beams, supported by four vertical steel I-beam legs installed on the strong solid floor. The tested columns were rested on the frame. Loading was achieved by using a 1250 kN capacity cell, which transferred the axial load to a steel plate with high rigidity at the top of the columns, set up as shown in Fig. 2.
The load was measured by a 1250 kN capacity load cell, which was connected to a digital display unit. The horizontal sway and vertical displacement at the points shown in Fig. 3a were measured using two linear variable differential transducers (LVDTs) and two dial gauges (DGs) with an accuracy of 0.01 mm .
The (LVDTs) and (DGs) were placed on the side and top surface of the columns as shown in Fig. 3b. Four electrical strain gauges were used to measure the strain in the steel bars and GFRP I-section at the maximum tensile point of the steel bars and GFRP I-section as shown in Fig. 4. This measurement location was maintained for all tested columns.

The RC column specimens were placed in a circular steel frame with side holes through which the fire was ignited. The specimens were exposed to a fire at ${500}^{\circ }\mathrm{C}$${500}^{\circ }\mathrm{C}$500^(@)C500^{\circ} \mathrm{C}${500}^{\circ }\mathrm{C}$ for 90 -minutes as shown in Fig. 5. After exposure, the RC columns were allowed to cool at room temperature for more than 4 hours before being loaded to failure. A wired digital reader with thermal sensor was used to stabilize the temperature at ${500}^{\circ }\mathrm{C}$${500}^{\circ }\mathrm{C}$500^(@)C500^{\circ} \mathrm{C}${500}^{\circ }\mathrm{C}$ for 90 -minutes duration.

The failure modes of the columns of all tested specimens are clearly shown in Fig. 6. All the tested specimens exhibited concrete compression failure under axial load and splitting of the reinforcing steel bars, which occurred after the concrete, reached its ultimate stress value. In addition, collapse occurred in GFRP I-section flanges of the tested RC composite column, as shown in Fig. 6b. The concrete failed at the mid height of the column due to deterioration in the polymers when exposed to fire. This was caused by the flammability of the organic resin matrix used in the GFRP I-section components, leading to significant weakening, as observed in the fire-exposed RC composite column in Fig. 6d.

a) RCC

Cracking of the columns started the applied loads reached more than $83\mathrm{\%}$$83\mathrm{\%}$83%83 \%$83\mathrm{\%}$ of the maximum load for the reinforced concrete columns without fire and $94\mathrm{\%}$$94\mathrm{\%}$94%94 \%$94\mathrm{\%}$ of the maximum load for the columns exposed to fire, respectively.

The cracking and maximum loads are summarized in Tab. 2. The results show that the composite RC columns achieved $117\mathrm{\%}$$117\mathrm{\%}$117%117 \%$117\mathrm{\%}$ of the axial load bearing capacity of the conventional RC columns. While, the composite circular encased GFRP I-section columns achieved $9.7\mathrm{\%}$$9.7\mathrm{\%}$9.7%9.7 \%$9.7\mathrm{\%}$ higher maximum load than the conventional RC columns under a compression load of 25 mm eccentricity [9]. The conventional RC columns exposed to fire achieved $86\mathrm{\%}$$86\mathrm{\%}$86%86 \%$86\mathrm{\%}$ of the capacity compared to the RC columns without fire. Meanwhile, the RC composite columns exposed to fire achieved $61\mathrm{\%}$$61\mathrm{\%}$61%61 \%$61\mathrm{\%}$ of the capacity compared to RC composite columns without fire due to the flammability of the organic resin matrix used in the GFRP compounds.
The results shown in Fig. 7 indicate that the maximum bearing capacity was achieved in RC composite column. The maximum reduction in column bearing capacity was observed in the GFRP I-profile RC composite column exposed to fire.

Comparing the maximum loads of the tested columns, it can be noted that the maximum load of the RC composite columns without fire was higher than that of the conventional RC columns. In the RC composite columns, the maximum load increased to $117\mathrm{\%}$$117\mathrm{\%}$117%117 \%$117\mathrm{\%}$ compared to conventional RC columns.

The load-vertical displacement relationships for the tested conventional columns and RC composite columns are shown in Fig. 8. The displacement values were measured at the top height point of the RC columns.
The axial stiffness of the tested columns represents their resistance of the columns to deflection or deformation under axial load. Stiffness is defined as the ratio of ultimate load to ultimate deflection [12]. The axial stiffness of conventional RC columns without fire was higher than that of RC composite columns, with values of $131.61\mathrm{kN}/\mathrm{mm}$$131.61\mathrm{kN}/\mathrm{mm}$131.61kN//mm131.61 \mathrm{kN} / \mathrm{mm}$131.61\mathrm{kN}/\mathrm{mm}$ and $85.94\mathrm{kN}/\mathrm{mm}$$85.94\mathrm{kN}/\mathrm{mm}$85.94kN//mm85.94 \mathrm{kN} / \mathrm{mm}$85.94\mathrm{kN}/\mathrm{mm}$, respectively. The axial stiffness of the tested columns without fire was also higher than that of RC columns exposed to fire. The axial stiffness of conventional RC columns was $131.61\mathrm{kN}/\mathrm{mm}$$131.61\mathrm{kN}/\mathrm{mm}$131.61kN//mm131.61 \mathrm{kN} / \mathrm{mm}$131.61\mathrm{kN}/\mathrm{mm}$ without fire and 80.27 $\mathrm{kN}/\mathrm{mm}$$\mathrm{kN}/\mathrm{mm}$kN//mm\mathrm{kN} / \mathrm{mm}$\mathrm{kN}/\mathrm{mm}$ with fire. For the RC composite columns, the axial stiffness was $85.94\mathrm{kN}/\mathrm{mm}$$85.94\mathrm{kN}/\mathrm{mm}$85.94kN//mm85.94 \mathrm{kN} / \mathrm{mm}$85.94\mathrm{kN}/\mathrm{mm}$ without fire and 59.52 $\mathrm{kN}/\mathrm{mm}$$\mathrm{kN}/\mathrm{mm}$kN//mm\mathrm{kN} / \mathrm{mm}$\mathrm{kN}/\mathrm{mm}$ with fire. The reduction in stiffness for the fire-exposed specimens was due to increased deformation.

The comparison between the conventional RC columns and the RC composite columns without fire shows that the RC composite columns are more ductile than the conventional RC columns.

The measured stress-strain curves of the tested RC composite column without fire are shown in Fig. 9. The axial strain in steel bars and GFRP I-section exhibited similar behavior at the same load level, recording low strains values of the tested column without fire. In contrast, the transverse strain in the GFRP I-section was significantly lower compared to the axial strain. In general, the maximum strain in the steel bars and GFRP I-section did not reach half of the yielding strain of the steel bars. Fig. 9 for the RC composite column shows strain compatibility in deflection, indicating that both the internal steel and GFRP materials within the columns experienced similar strains and deformations when subjected to axial load.

The ability of common structural analysis tools to predict the performance of the tested RC composite columns was investigated to provide practicing engineers with information on their reliability according to the different codes.
The ultimate strength of the conventional RC columns was calculated at the point of failure, the theoretical ultimate strength or nominal strength of a short axially loaded column can be determined accurately by the using Egyptian Code of Practice (ECP-203) [13] and British Standard BS 8110-97 [14]. The theoretical maximum load can be calculated by considering the contributions of both the concrete and reinforcement in bearing the axial load, as follows;

According to ACI 318-11 [15], the theoretical maximum load can be calculated by considering the contribution of both the concrete and the reinforcement in carrying the axial load;

The maximum nominal axial load on a column is calculated using the following equation according to Eurocode [16];

where;

The maximum nominal axial load on the column is calculated using the following equation according to CSA A23.30[17];

By comparing the experimental results with the values calculated using the theoretical equation of the different codes, it w found that the experimental and theoretical results calculated by the Egyptian Code of Practice (ECP-203) [13], British Standard BS 8110-97 [14], and ACI 318-11 [15] converge very closely for the axial load. This indicates the auracy and reliability of the findings as the theoretical results align well with the experimental observations, showing no significant differences in axial loads as presented in Tab. 3. Therefore, the following proposed equation is recommended to calculate the axial load of RC composite columns based on the Egyptian Code of Practice, and other Standard codes as following;

E-203 & BS

A

Eurocode

C

The reinforced concrete columns exposed to fire at 500 degrees, according to Eurocode 1991-1-2, section 3 [16], ruire the application of reduction factors $kc,\theta$$kc,\theta$kc,thetak c, \theta$kc,\theta$, and $ky,\theta$$ky,\theta$ky,thetak y, \theta$ky,\theta$, to to the compressive strength of concrete and the yield strength of steel, respectively. For conventional reinforced concrete columns, these reduction factors are 0.74 for concrete and 0.78 for steel. These reduction factors can also be applied to composite columns. However, the resistance of GFRP should be neglected due to the melting of the organic resin matrix used in GFRP compounds at high temperatures.

The theoretical calculations and the accuracy of the proposed equation based on different codes were able to accurately predict the load-carrying capacity of composite columns under vertical load.

The non-linear finite element analysis method, as well as, the finite element analysis by using ANSYS (R19) was considered. The columns exhibited non-linear behavior primarily due to the inherent nonlinear material, elastic-plastic behavior of the steel reinforcement bars and the linear-pseudo plastic and brittle behavior of the concrete. This analysis method is currently regarded as the state-of-the art for analyzing reinforced concrete structures. However, the use of this sophisticated tool is complicated by the need to select several control parameters, which are typically chosen based on prior experience in finite element results with experimental data from similar structures. In addition to replicating the behavior of the tested columns without fire exposure, the non-linear analyses were conducted to study the effects of the concrete compressive strength, steel yield strength, reinforcement ratio, and of the GFRP ratio in the composite columns. The analysis was performed using the finite element package ANSYS [18].

The solid element SOLID65 was used to model the concrete, while the 3D spar Link180 element was used to model the steel bars. The solid element SOLID185 was utilized to model the GFRP I-section and steel plates at the column supports. The model of the verification column is shown in Fig. 10.

Eqn. (1), proposed by MacGregor [19], is used to represent the uniaxial compressive stress-strain relationship for concrete.

This equation has been used to plot the multi-linear isotropic stress-strain curve for concrete from 0.3 fc to the ultimate compressive strength, $0.3f_c^{\prime }$$0.3f_c^{\prime }$0.3f_(c)^(')0.3 f_{c}^{\prime}$0.3f_c^{\prime }$.the Elastic modulus is defined as 0.3 fc , and calculated in the linear region. The elastic-plastic uniaxial stress-strain model was used to model reinforcing steel. Additionally, the modulus of elasticity and the tensile strength of concrete were calculated using the same specifications. The stress-strain curves of steel bars were presented according to the ECP 203 Code of Practice [13]. The GFRP was assumed to be linear, elastic and orthotropic material, with material properties provided by the manufacturer input into the model. The elastic-plastic uniaxial stress-strain model from the ECP 203 Code of Practice [13] was also used to model the steel.

For the validation of the ANSYS model for the RC composite columns, the parameters controlling the details of the concrete material model (SOLID65), such as tensile stiffening and shear retention across cracks of the reinforced concrete, were determined empirically by identifying the values that provided the best fit between the calculated stressstrain curve of the RC composite column and the experimental curve. The best-fit numerical curve and the experimental curve are shown in Fig. 11. In general, the numerical curve closely matches the experimental one. However, it is relatively less stiff than the experimental curve.

After the verification of the finite element model, four parameters were further investigated for both the conventional RC columns and the RC composite columns:
(i) Effect of the compressive strength of concrete,
(ii) Effect of the yield strength of steel,
(iii) Effect of reinforcement ratio, and
(iv) Effect the of GFRP I-section ratio in the RC composite columns.

Through this investigation, the proposed equation for calculating the axial load of RC composite columns can be verified.

To investigate the influence of normal and high compressive strength concrete on the bearing capacity of columns, nine analyses were performed for concrete strengths ranging from 20 MPa to 100 MPa for both conventional RC columns and RC composite columns.
The theoretical calculated maximum axial loads, based on the Egyptian Code of Practice (ECP-203) [13] and British Standard BS 8110-97 [14], along with the numerically calculated maximum axial loads from finite element analysis, are presented in Tab. 4 and Fig. 12. It is noted that the calculated maximum axial loads closely agree with the numerical values with deviations not exceeding $2\mathrm{\%}$$2\mathrm{\%}$2%2 \%$2\mathrm{\%}$ for conventional RC columns and $4\mathrm{\%}$$4\mathrm{\%}$4%4 \%$4\mathrm{\%}$ for RC composite columns, except when the compressive strength of concrete is equal to 20 MPa . In this case, the maximum axial loads calculated by finite element analysis shows an $8\mathrm{\%}$$8\mathrm{\%}$8%8 \%$8\mathrm{\%}$ increase compared to the maximum axial load calculated using the proposed equation for RC composite columns.

Fig. 13 shows the relationship between the compressive strength of concrete and the axial load for conventional RC columns and RC composite columns. It can be seen that the axial load increases linearly with the increase in the compressive strength of concrete.

The yield strength of steel does not significantly affect the axial load capacity of conventional RC columns, as shown in Tab. 5, and does not apply to RC composite columns. All stresses in the longitudinal reinforcement were monitored in theoretical specimens through finite element analysis based on the previous parameters. It is noted that the
maximum stress in steel bars is less than 300 MPa for concrete compressive strengths up to 50 MPa . For highstrength concrete ( 100 MPa ), the stress in the steel bars reaches 525.33 MPa . Therefore, Eurocode [16] assumes that the stress in the longitudinal steel is equal to the young's modulus of steel multiplied by the strain in concrete when it reaches its maximum strength.

To study the influence of the longitudinal reinforcement ratio ( $\varrho$$\varrho$ϱ\varrho$\varrho$ ) on the behavior of conventional RC columns and RC composite columns, analyses were performed for four different values: $0.79\mathrm{\%},1.23\mathrm{\%},1.77\mathrm{\%}$$0.79\mathrm{\%},1.23\mathrm{\%},1.77\mathrm{\%}$0.79%,1.23%,1.77%0.79 \%, 1.23 \%, 1.77 \%$0.79\mathrm{\%},1.23\mathrm{\%},1.77\mathrm{\%}$, and $3.14\mathrm{\%}$$3.14\mathrm{\%}$3.14%3.14 \%$3.14\mathrm{\%}$ using reinforcement diameters of $8\mathrm{mm},10\mathrm{mm},12\mathrm{mm}$$8\mathrm{mm},10\mathrm{mm},12\mathrm{mm}$8mm,10mm,12mm8 \mathrm{~mm}, 10 \mathrm{~mm}, 12 \mathrm{~mm}$8\mathrm{mm},10\mathrm{mm},12\mathrm{mm}$, and 16 mm , respectively. The analysis was conducted with a concrete compressive strength of 40 MPa and steel yield strength of 400 MPa .
The theoretical calculated maximum axial loads and the numerical maximum axial loads are presented in Tab. 6 and Fig. 14. It is noted that the calculated maximum axial loads closely match the numerical values, with deviations not exceeding $4.48\mathrm{\%}$$4.48\mathrm{\%}$4.48%4.48 \%$4.48\mathrm{\%}$ for RC composite columns. Additionally, the theoretical calculated maximum axial loads increase when the reinforcement ratio is less than $1.25\mathrm{\%}$$1.25\mathrm{\%}$1.25%1.25 \%$1.25\mathrm{\%}$ and decrease when the reinforcement ratio exceeds $2.00\mathrm{\%}$$2.00\mathrm{\%}$2.00%2.00 \%$2.00\mathrm{\%}$. Therefore, it may be necessary to reconsider the calculation of the value of axial load contribution resulting from the compressive stress in the reinforcing steel.

Fig. 15 shows the relationship between the reinforcement ratio and axial load for both conventional RC columns and RC composite columns. It can be seen that the numerically maximum axial load increases linearly with the reinforcement ratio up to approximately $3\mathrm{\%}$$3\mathrm{\%}$3%3 \%$3\mathrm{\%}$ and then decreases for conventional RC columns. In contrast, the rate of increase remains linear for RC composite columns.

To investigate the influence of the GFRP ratio on the behavior of RC composite columns, analyses were performed for twelve different values ranging from $2.03\mathrm{\%}$$2.03\mathrm{\%}$2.03%2.03 \%$2.03\mathrm{\%}$ to $17.18\mathrm{\%}$$17.18\mathrm{\%}$17.18%17.18 \%$17.18\mathrm{\%}$ using two different shapes of GFRP sections, flat plates and I-shaped sections. The concrete compressive strength was set at 40 MPa and steel yield strength was 400 MPa .

The theoretically calculated maximum axial loads and the numerically maximum axial loads are presented in Tab. 7 and Fig. 16.
It can be seen that the calculated maximum axial loads closely match the numerical values for RC composite columns with GFRP ratio up to $4\mathrm{\%}$$4\mathrm{\%}$4%4 \%$4\mathrm{\%}$ for flat plates and $8\mathrm{\%}$$8\mathrm{\%}$8%8 \%$8\mathrm{\%}$ I-shaped sections. However, the numerically maximum axial loads remain almost constant at GFRP ratios exceeding $4\mathrm{\%}$$4\mathrm{\%}$4%4 \%$4\mathrm{\%}$ for flat plates and $8\mathrm{\%}$$8\mathrm{\%}$8%8 \%$8\mathrm{\%}$ I-shaped sections. Therefore, the proposed equation for calculating of the axial load of RC composite columns is satisfactory at GFRP ratios less than $4\mathrm{\%}$$4\mathrm{\%}$4%4 \%$4\mathrm{\%}$ for flat plates and $8\mathrm{\%}$$8\mathrm{\%}$8%8 \%$8\mathrm{\%}$ I-shaped sections.

The resistance of the RC composite columns is increases as the surface area of the GFRP increases and as the homogeneity inside within the concrete section improves.

From studying the proposed model for composite concrete columns consisting of Glass Fiber Reinforced Polymer (GFRP) profiles and longitudinal steel bars under axial compressive load, and from theoretical calculations for experimental tested columns specimens and theoretical columns by different codes, and from studying the behavior of axial compressive for composite concrete columns under different parameters factors, the following conclusions can be drawn:

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$A_{ft}$$A_{ft}$A_(ft)A_{f t}$A_{ft}$ Total cross-sectional area of GFRP I-section
$Ag$$Ag$AgA g$Ag$ Gross area of the concrete column
$A_{st}$$A_{st}$A_(st)A_{s t}$A_{st}$ Total area of longitudinal reinforcement
$E_s$$E_s$E_(s)E_{s}$E_s$ Modulus of elasticity of steel
$f$$f$ff$f$ Stress at any strain $\epsilon$$\epsilon$epsi\varepsilon$\epsilon$
$f_c^{\prime }$$f_c^{\prime }$f_(c)^(')f_{c}^{\prime}$f_c^{\prime }$ Ultimate cylinder compressive strength for concrete
$f_{cu}$$f_{cu}$f_(cu)f_{c u}$f_{cu}$ Compressive strength of concrete
$f_s$$f_s$f_(s)f_{s}$f_s$ Stress in steel bars when the concrete reaches the maximum strength
${\mathit{f}}_f$${\mathit{f}}_f$f_(f)\boldsymbol{f}_{f}${\mathit{f}}_f$ Ultimate compressive strength of GFRP I-section
$f_y$$f_y$f_(y)f_{y}$f_y$ The yield strength of steel
$P_n$$P_n$P_(n)P_{n}$P_n$ Axial load at the maximum
a Ratio of average stress in rectangular compression block to the specified concrete strength
${\epsilon }_c$${\epsilon }_c$epsi_(c)\varepsilon_{c}${\epsilon }_c$ Strain in the concrete at reaching the maximum strength
$\eta$$\eta$eta\eta$\eta$ Reduction factor for the high strength concrete for the rectangular stress distribution
${\varphi }_c$${\varphi }_c$phi_(c)\phi_{c}${\varphi }_c$ Resistance factor for concrete and equal to 1.0 for laboratory conditions
${\varphi }_s$${\varphi }_s$phi_(s)\phi_{s}${\varphi }_s$ Resistance factor for steel and equal to 1.0 for laboratory conditions
${\epsilon }_o$${\epsilon }_o$epsi_(o)\varepsilon_{o}${\epsilon }_o$ Strain at the ultimate cylinder compressive strength $f_{\mathrm{c}}{}^{\prime }$$f_{\mathrm{c}}{}^{\prime }$f_(c)^(')f_{\mathrm{c}}{ }^{\prime}$f_{\mathrm{c}}{}^{\prime }$