EFFECTS OF STEEL RATIO ON THE BEHAVIOR OF REINFORCED CONCRETE COLUMN CONFINED WITH CARBON FIBER REINFORCED POLYMER COMPOSTIES UNDER AXIAL LOAD

The present work deals with the analysis of experimental results, obtained from the tests on circular reinforced concrete (RC) columns, confinement with external sheets of carbon fiber reinforced polymer (CFRP). Two parameters was studied including the number of CFRP wrap strips and steel ratio of the columns ( , and ). Six circular RC columns with scale (100×900 mm) were exposed to pure axial compression load up to failure. Crack and failure load was recorded to estimate the load – displacement behavior, ultimate strength, and ductility of the specimens. The results show clearly that wrapping with (CFRP) can improve the structural performance of the RC columns by providing greater load carrying capacity and ductility than the unwrapped RC columns. The effects of test parameters are evidenced and compared.


Introduction
' Concrete as a building material have high compressive strength and small tensile strength.To reinforce a concrete structure, the most common way is to use steel reinforcing bars that are located in the structure before the cast of concrete [1].
RC columns as a main supporting component for any structure may, for several reasons, need strengthening.For these reasons, there is original rehabilitation and strengthening methods [2].One way that can be used to improve or strengthen structure, using fiber-reinforced polymer (FRP) materials which have been found to be an attractive choice as an alternative and highly effective retrofit technique in such cases by using composite wraps or jackets around a deteriorating column [3].They offer great advantages in stiffness and strength.No yielding is presented in FRP materials, but instead they are elastic up to failure.Fiber composites (FRP) takes the role of the principal load-bearing constituent, and the resin (matrix) is used to relate the fibers together, transfer the force between the fibers and to protect the fiber against external mechanical and environmental damage [4].
As for carbon fiber reinforced polymer composites (CFRP), one of the most promising applications for this composite is the confinement of concrete columns.It can be used to significantly increase the carrying capacity of the load and dissipate the energy of circular concrete columns.CFRP sheets has a remarkable properties, Such as high strength, easy to install and low weight, make it very suitable for the reinforced concrete (RC) columns confinement [5].
Some investigations on strengthened concrete columns with FRP under axial load have been done by researchers, Teng and Lam (2002) [6], showed that the axial compressive strength CFRP-confined concrete in elliptical specimens is measured by the amount of confining CFRP and the major-to-minor axis length ratio of the column section, So they found that confining CFRP becomes more effective as the section becomes more elliptical, Esfahani, M.R. and Kianoush, M.R. (2005) [7], study on the axial compressive strength of RC columns (square with sharp and rounded corners) strengthened with FRP wrap and establish that the square column with rounded corners showed a larger strength and ductility compared to those with sharp corners by FRP warp.
Similar results were recorded by Al-Salloum (2007) [8] but under uniaxial compression, Manuel Silva.(2011) [9], found that the efficacy of confinement RC columns with CFRP or AFRP is known to be more preponderant for specimens of circular cross-section compared to those of rectangular cross-section under axial load, because the uniqueness and stress concentration introduced on the edges and reduced confinement on the flat sides, Chikh, N., et al., (2011) [10], found that CFRP sheets can improve the structural act of RC columns under axial compression in terms of both maximum strength and ductility with changed in columns slenderness ratio, form of the section (circular and square) and the wrap layers number, Taghia, P. and Suhaimi Abu Bakar (2013) [11], They found that CFRP bonded sheets from outside were highly effective in the short concrete columns by enhancing the strength and ductility under pure axial static load by Nonlinear finite element analysis (NLFEA).Inspection of the results shows that, there is good agreement between the NLFEA and the experimental test results, A. Sulaiman, H. Almansour and H. Aoude (2016) [12], shows that parameters such as stacking sequence, fiber orientation, and number of confinement layers have a direct effect on the strength, stress-strain behavior and ductility of CFRP confined cylinder concrete column under pure axial compressive loading.The same observations were noted by Li andHadi (2003, 2004) [13,14] under eccentric load).
And some investigate the performance of strengthened RC column with FRP under eccentric load such as Hadi. (2006) [15], Study eccentrically plain concrete short circular columns warped with CFRP and GFRP strips.He obtainable that eccentric load could noticeably reduce the maximum failure load of confined concrete columns by FRP, compared to concrete columns under concentric load.However, compared to internally RC columns, the confined columns with FRP exhibited a greater load capacity and ductility, when tested in both concentrically and eccentrically; the same explanations were noted by Hadi.(2007) [16], using the high strength RC circular columns, Rahai, et al., (2008) [17], presented confirmed substantial improvement in compressive strength, ductility, and stiffness of the CFRP-confined concrete cylinders as compared to nonconfined concrete cylinders under uniaxial compression, A. P. D. Yarub Gatia Abtan, et al., (2015) [18], explore on extremely-high strength reactive concrete (RPC) columns before and after CFRP strengthening sheets indicated that CFRP jacketing rises the ultimate failure load of confined columns and let more ductile failure than the original columns.
This study explores the performance of circular RC columns having different steel ratio wrapped with CFRP composites under pure axial compression loading.The columns performance was estimated by the investigation of their load carrying capacity and ductility.

Experimental Program
The experimental program of this study was divided into two groups were investigated the performance of circular RC columns strengthened by CFRP composite which conducted at the laboratories of the College of Engineering, Al-Mustansiriayah University, Iraq.Details of the main stages are given in the following.

Material Properties
Testing specimens has an average 28-days compressive strength of 34 MPa which was determined by conducting tests on 150 mm diameter cylinders.Normal strength concrete mix was used to make the test specimens.As shown in Table 1( The materials used through this study were ordinary Portland cement (type ɪ), Normal weight natural sand with 4.75mm maximum size and Crushed gravel with maximum size of 12mm.
Three types of steel reinforcement's bars were used, deformed steel bars Ø6, Ø10 and Ø12 for longitudinal reinforcement and deformed steel bars Ø4 for transversal Each Value is an Average of three Specimens (each 50 cm long) The fiber composite used for wrapping the column specimens was CFRP SikaWrap-301C product.The epoxy resin Sikadur-330 was used to bond the carbon fabrics over the columns.Table 1(

Columns Configuration
The column specimens were distributed into two groups: Group (A) includes four specimens with min.reinforcement, GAU#C0 unwrapped, GAS#C3 wrapped with three strips of CFRP having (50mm) width, GAS#C5 wrapped with five strips of CFRP having (50mm) width distributed equally and GAS# -fully wrapped with  CFRP.As for the other group: Group (B) includes two specimens with different steel ratio (GBS# -and GBS# -) wrapped all over the columns (full wrapped) with CFRP.The columns were labeled as shown in Table 2.

Strengthening Procedure
After preparing the surfaces and make sure they are clean and dry; The two parts of adhesive (gray and white) mixed together according to the manufacturer recommendations, then applied to the Specific place of RC column by using spoon putty between the lines that laid out for CFRP location.After that, CFRP was applied and handled carefully with a 20mm overlap.Once the relevant layer of fabric was wrapped, another coat of the epoxy resin was applied.After the application process had been completed.The specimens were kept in place for one week before being tested.After finishing the CFRP installation, and before the date of the test, all surfaces of concrete specimens were painted white to distinguish the crack propagation easily.The install procedure of CFRP on the RC column surface is shown in Plate 1.
Plate 1.The install procedure for CFRP

Details of Columns Specimens
In this study six normal strength RC columns with a circular cross section were cast and tested.All columns have the same dimension (100×900) mm.Four of these columns reinforced with minimum steel ratio (ρ=0.0113) which are equivalent (3Ø6mm) longitudinal steel bars to indicate the real effect of CFRP sheets, while the other two columns, one have mid steel ratio (ρ=0.0302) which is equivalent (3Ø10mm) longitudinal steel bars and one have maximum steel ratio (ρ=0.0720) which is equivalent (5Ø12mm) longitudinal steel bars in order to compared its results.As for the transverse steel reinforcement (ties) were used Ø4mm deformed bars for all columns at 100 mm from edge of longitudinal bars in top and bottom only.

Test Setup
Six columns were tested under pure axial compression loading up to failure by using 8551MFL SYSTEM of hydraulic universal testing machine type EPP300.According to the circumstances of this test, the upper and lower parts of the specimens are supported to be hinged.Plywood with steel plate in the ends of the column was first done to obtain a uniform distributed load applied to the entire face.Two dial gauges positions were marked in the middle of each column specimen at (450mm) height, from two sides (Longitudinal X and transverse Y) to measure the mid-height lateral displacement (deflection) of columns as shown in Plate 3. The resultant was taken from the two direction of displacement.

Test Results and Discussion
The ultimate loads and the corresponding lateral displacements of RC columns were recorded during the axial compression testing which based on the steel ratio with CFRP distribution.The test results for all specimens are given in Table 3(a) and 3(b).After every (10 second), load increment of the RC columns was checked for cracks and any probable failure marks.

Column Specimens (Group A)
The general behavior of the tested control columns is discussed in the following:-

Mode of Failure
This group consisted of four columns were tested, the specimen GAU#C0 is without strengthening for comparing with the other strengthened columns.The ultimate load equal to (195 kN, 291 kN, 372 KN, 562.5 kN) for (GAU#C0, GAS#C3, GAS#C5 and GAS# -) respectively "Table 3(a)".The control specimen failed due to the crushing of concrete (compression failure) were cracks initiation occurred at the top edge at the early stages of loading and propagated in the longitudinal direction of the member with increasing of loading until failure.
As for specimen GAS#C3 strengthened by three strips with 50mm width which represents (17% from surface area of column) installed on the top, bottom and center of column perpendicular to its axis, was tested and failed by (Compression failure) between the center and bottom strips of CFRP due to the bond of CFRP strips.And for specimen GAS#C5 strengthened by five strips with 50mm width which represents (28% from surface area of column) installed on the top, bottom, center of column and between exactly at 22.5mm from height at each side was tested and failed by (compression failure) at top edge under the first strip of CFRP and reached near the second CFRP strip due to the bond of CFRP also.
While the specimen GAS# -that have min reinforcement strengthened by strips which are installed on full face of column (100% from surface area of column).This concrete column unlike the other columns in group (A), which is failed by fracture the wrap of the composite in an explosive and sudden way with creeping sounds causing buckling with shear failure at the middle height of column due to the strength of CFRP and slenderness ratio as presented in Plate 4. The buckling in the steel bar and maximum lateral displacement was seen in the middle height of the specimen.After testing, the specimens of this group seemed to have nonlinear performance until the moment of failure was reached and failed at ultimate load (291 kN, 372 kN, 562.5 kN) for (GAS#C3, GAS#C5 and GAS#Cfull) with an increasing in load (49.2%, 90.8%, 188.5%) as compared with control column respectively as result of presence of CFRP sheets.Plate 4 shows the failure region of group (A).

Load-Displacement Behavior
It is shown clearly that the specimens performed in group (A) have a similar nonlinear behavior at load-displacement curves before reaching the failure load.The point of the first crack load was indicated by the initial change of slope of the loaddisplacement curves.Displacement continued to increase under increasing loads until failure.At ultimate load The deflection of column GAU#C0 was equal to 1.63mm and the deflection of columns(GAS#C3, GAS#C5, GAS# -) was equal to (1.59mm, 1.35mm, 7.32mm) respectively.
The differences between these columns and control column was (-2.5%, -17.2%, 349.1%) respectively as shown in Fig. 3(a) and 3(b).The decreasing in the displacement of strengthened columns due to effectiveness of CFRP make the columns less deflection with a high load, except column (GAS# -) had increasing in deflection due to the failure mode which was (buckling failure) and CFRP rupture at the mid-height of column.Confining the columns with CFRP improved the columns performance by increasing their displacement at failure and improved the stiffness with increasing the FRP longitudinal stiffness.The maximum load carrying of every specimen was achieving at the failure point of the FRP, up to the FRP was broken.

Mode of Failure
The RC columns with dimension (100×900) mm was designed in different steel ratio ( according to ACI 318M-14 [22] with the case of full CFRP to compare it results with the best results of the strengthened columns with CFRP sheet and find the best economical way form this compared results. This group content two columns (GBS-, GBS-strengthened by strips which are installed on full face of each column (100% from surface area of columns) and compare it with column (GAS-) which is within group (A).These strengthened columns were tested which have a nonlinear performance until the moment of failure was reached.The behavior of cracking did not directly observe due to the surfaces of concrete were covered by CFRP.It failed suddenly because of CFRP rupture at the end as a result of stress concentration at interfacial zone between concrete and CFRP.
The column (GBS-) failed by compression and shear failure initiated at the first top quarter of the column with rupture of CFRP due to poison ratio and causing buckling in the longitudinal reinforcement while (GBS-) failed by compression failure only with rupture of CFRP also in the lower part at the bottom edge of the column due to the strength of CFRP with the maximum of steel reinforcement.These two strengthened columns failed at ultimate load (625kN, 745kN) and with an increasing in load (11.1%, 32.4%) respectively as compared with column (GAS-) as listed in the Table 3(

load-displacement behavior
It is shown clearly that the columns performed a similar nonlinear behavior before reaching the maximum load and it was especially observed near the failure where crushing, shear and buckling of concrete take place.Displacement continued to increase under increasing loads until failure.In group (B) (GBS-, GBS-) the maximum displacement was (2.1mm, 1.86mm).The load-displacement behavior shows that increasing main reinforcement ratio from 1.08% to 3.02% and 7.2% clearly reduced mid-height displacement under certain load for fully strengthening columns by (-71.3% and -74.6%) for (GBS-and GBS-respectively comparing with GASas in Table 4, due to the increase in columns stiffness even with CFRP rupture with buckling in longitudinal reinforcement.Fig. 4(a) and 4(b) shows the load-displacement behavior for the three columns fully strengthened that have different steel ratio.

Type of columns
Steel ratio Disp.at ultimate load (mm) % differences of Fig. 4(c) shows the load-displacement curves for three columns fully wrapped (GAS-, GBS-and GBS-) to obtain the effect of steel reinforcement ratio on the displacement behavior.5(d) show the effect of steel ratio and CFRP distribution (surface area of CFRP) on load and displacement behavior.For the behavior of loadsteel ratio was almost linear in crack and failure load, so the load increase when the steel ratio increase.While the behavior of displacement-steel ratio was nonlinear in both crack and failure load, because the increase in steel ratio led to decrease in displacement significantly as a result of the greater ductility and stiffness were indicated when using high main steel ratios.As for load-surface area of CFRP, the columns have a similar behavior at both crack and failure load, this indicate that increasing the CFRP wrapping lead to increase in the load caring capacity.Also at displacement-surface area at cracking load have deferent behavior from failure load due to the maximum effectiveness of CFRP which appeared at final stages of loading especially in fully wrapped that have large displacement due to CFRP rupture.

Ductility of columns
The ductility was also analyzed to describe the columns performance.The area under load-axial displacement curves can be an important method to express the ductility that obtained by using CFRP strengthening.Table 5 shows the calculated values of the columns ductility.It is clear from table that the columns which were strengthened with CFRP performed better than control columns.The increasing in ductility is of more important for fully strengthened columns.Also the strengthening by five layers gives more ductility than strengthening by three layers.

Conclusions
According to the analysis of the experimental results obtained in this study that conclude Studying the effect of various strengthened methods and steel ratio on the circular RC column.The main findings of this research can be summarized as follows:  The pure axial compressive loading test results of the circular RC columns show that wrapping columns with CFRP increases both the load-carrying capacity and the ductility to the same cross-section. A significant advantage was attained that wrapping with CFRP improved the columns performance by delaying the rupture of the reinforcement and concrete. Increasing the CFRP sheets number shows an increase in the compressive strength and the confined columns performance.It was proven that wrapping with a minimum of three-layers of CFRP sheets would be proposed to achieve important results. CFRP strengthened specimens shown a typical nonlinear behavior.Also individual post behavior is observed as the steel ratio increases. All tested columns were loaded up to their ultimate loads.It was noted that: The ultimate load of the strengthened columns is higher than the control column. The effect of increasing the steel ratio for the fully wrapped columns generally results in a small increase in the load carrying capacity and a reasonable reduction in the axial deformation, when the main reinforcement ratio increases from 1.08% to 7.2% (about 32.4% maximum increase in ultimate load) by the effect of steel ratio only, while the ultimate load for the same section, fully wrapped with reinforcement ratio (1.08%) increased up to (188.5%) comparing with the control column without strengthening, due to the effect of CFRP wrap only, so this equivalent to 6 times the Table 5. Area under the load-axial displacement curves ultimate load in increasing the steel ratio.It should be noted that max steel ratio with full carbon fiber together will increase the ultimate load up to (282%) for the same cross section.and this is a great percentage to get a higher strength and ductility but not economic, from this results we can use the strengthened column of minimum steel ratio (1.08%) instead of strengthened column of steel ratio (3.02%, 7.2%) and this will reduce the cost and time required for the installation of longitudinal steel bar and give a decent results. Bond strength can be increased by decreasing bar diameter.The bond strength for bar diameter 6mm is greater than that for bar diameter 12mm. The column GBS# -failed by compression and shear failure while the column GBS# -failed by compression failure only, in spite of the fact that they had the same surface area of CFRP.This may be due to local anchorage failure of the longitudinal reinforcement in GBS# -.
 The load-displacement reaction is not affected by the steel reinforcement grading at the stage of cracking.But after the stage of cracking and with the expectation of a significant contribution from tension reinforcement, we noticed that column (GDR50#Cfull-) have difference in displacement and load carrying capacity comparing with column (GDR50#Cfull-), which is reduce in ultimate displacement and increase in ultimate load capacity (-74.6 %) and (32.4%) respectively by (GDR50#Cfull- ).This indicates that increasing in reinforcement steel ratio lead to increase in load caring capacity and decrease in displacement of the tested column. Increasing the steel ratio of the fully strengthening columns lead to increasing in ductility.
c) and 1(d) shows the properties of CFRP sheets and epoxy that used in the experimental work of the present study.

Fig. 1 Figure 1 .
Figure 1.Details of loading and specimens All dimensions in (mm)

Plate 3 .
Test set-up of column specimen

Plate 4 .
the failure region of group (A)

Plate 5 .
The failure region of group (B)

Figure 5 (
Figure 5(a).Load-steel ratio behaviors at crack and failure load

Table 1 (
a). Properties of Mixture Details for (1  3 ) Of Concrete Trail Mix reinforcement (ties) at top and bottom only.The tensile strength of the reinforcement steel bars was conducted as shown in Table 1(b).

Table 1 (
b). Specification and Test Results of Steel Reinforcing Bar

Table 2 .
Details of Test Specimens The

Table 3 (
a). Test Results and Details Of columns up to failure Table 3(b).Test results and details of columns with the effect of steel ratio