Improvement of bond behavior of fiber reinforced polymer bars using coarse sand and steel fibers: experimental study

doi:10.21203/rs.3.rs-1819428/v1

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Improvement of bond behavior of fiber reinforced polymer bars using coarse sand and steel fibers: experimental study

https://doi.org/10.21203/rs.3.rs-1819428/v1

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The use of fiber reinforced polymer composites as alternative to conventional steel reinforcement have been increased during last decades for their high tensile strength, light weight, and durability. This paper focused on the behavior of CFRP bars embedded in high strength concrete. Thirteen beam spacemen of 100×180×650 mm clear span were cast, each beam consist of two concrete blocks linked together. The parameters studied are: concrete strength, CFRP bar diameter, and two enhancement schemes; coarse sand and steel fibers applied directly to the surface area of the CFRP bars using epoxy adhesive. All beams are tested using four points loading system. The results indicates that the maximum bond stress and the strength ratio of CFRP bars enhanced using coarse sand particles or steel fibers increased by are 9.6 to 24.4% and 9.7 to 24.3% if compared with undeveloped bars due to the improved surface area which provides additional adhesive, friction resistance, and mechanical interlock. However, the bond index decreased up to 24.8%. Also, increasing of concrete compressive strength 55.38 MPa to 78.34 MPa increased the maximum bond stress and the strength ratio by 21.1 to 31.2% and 3.1 to 9% while the decreasing of the relative bond stress and bond index is 6 to 7% and 5.7 to 6.7%. Furthermore, using of CFRP bars of 10 mm diameter instead of 6 mm decreased the bond stress and the strength ratio by 18 to 27% and 42 to 44% due to the larger surface area and higher ultimate tensile strength.

Bond performance

CFRP bars

High strength concrete

Coarse sand

Steel fibers

Recently, Bond behavior of fiber reinforced polymer FRP bars (carbon, glass, aramid, and hybrid) in concrete have been investigated by many researchers owing to the prevalence of FRP composites in some applications of civil engineering. An attempt to enhance bonding between smooth GFRP bars and concrete using sand coating performed by (Arias et al., 2012). Fifteen spacemen were cast to study the influence of sand, rebar diameter, embedded length, and concrete strength. Results of direct pull out tests indicated that finer sand led to stronger chemical adhesion while friction and interlocking forces gained from coarse sand were the main part of bond strength.

The effect of anchor head on bond behavior of sand coated glass fiber reinforced polymer bars embedded in geopolymer concrete was investigated by (Maranan et al., 2015) using direct pullout test, test results declared that anchor head enhanced significantly the bond capacity of GFRP. An effort was made by (Park et al., 2016) to study the bond performance of FRP bars embedded in concrete. A total of 63 specimens were conducted to investigate the influence of FRP types, locations and casting positions. It is concluded that bond performance of FRP bars was significantly affected by FRP type and surface condition. Using beam test, (Kotynia et al., 2017) studied the influence of bar diameter, concrete strength, and concrete cover on the bond behavior of GFRP bars. As a conclusion, the ultimate bond stress increased by 18 to 22% when the bar diameter increased from 16 mm to 18 mm. Also, higher concrete compressive strength produced better bond behavior while the bond stress decreased owing to the decrease of concrete cover.

In order to investigate bond performance of basalt fiber reinforced polymer bars in coral concrete, (Wang et al., 2019) carried out 30 pullout tests divided into 10 groups with different bar diameters, concrete strength and bonding lengths. Good bonding were achieved between BFRP bars and coral concrete. Also, it is found that concrete strength is the most effective way to improve the bond performance. Solyom and Bala^,zs (2020) performed an experimental study on bond behavior of FRP bars to investigate the effect of surface configuration using pull out and direct tension tests. It is found that sand coated FRP bars provides approximately similar bond strength as helically wrapped but with lowest slip value.

(DoostMohamadi et al., 2021) investigated the effect of concrete type on bond strength of GFRP bars. 18 cube specimens were fabricated and tested and tested using direct pull out. Normal weight, light weight, and light weight fiber reinforced concrete were used in the study. As a conclusion, LWC cased lower bond strength if compared with NWC due to the weakness of aggregates in mechanical interlocking. Also, LWFRC of 0.3 and 0.5% fiber volume fraction controlled cracks and increase the bond strength between GFRP bars and concrete. (Başaran et al., 2022) reviewed the physical parameters of bond behavior of FRP bars in concrete. Ten parameters based on FRP properties, concrete properties, and reinforcement arrangement and configuration. It is found that surface texture was the most effective parameter on FRP–concrete bond strength.

Despite of all the previous researches that dealt with the effect of FRP types, concrete types, bar diameter, surface condition, and type of bond test. Bond performance of FRP bars embedded in concrete still needs more investigations.

2.1. Materials

Steel reinforcing bars of 6 mm 8 mm, and 10 mm diameters with yield stress of 634, 492, and 578 MPa, respectively, and two diameters of carbon fiber reinforced polymer bars (Ø 6 mm and Ø 10 mm) were used in this study. The CFRP bars are helically wrapped by multifilament yarn to improve the surface geometry. Table 1 presents the properties of CFRP bars.

Table 1

Mechanical properties of CFRP bars
Size (mm)	Nominal area (mm²)	Tensile strength (MPa)	Ultimate tensile load (kN)	Tensile modulus of elasticity (GPa)	Ultimate strain
6	31.67	2241	70.8	124	1.81%
10	71.26	2172	154.1	124	1.75%

Source: Supplied by the manufacturer

To investigate the effect of concrete strength, two concrete mixes were used in this study. Table 2 provides the details of each mix.

Table 2

Proportion of concrete mixes
Mix	Cement (kg/m³)	Fine aggregate (kg/m³)	Coarse aggregate (kg/m³)	Water (Liters)	Superplasticiser (Liter/100 kg cement)
M₁	450	560	740	160	0.8
M₂	550	880	950	180	1

Coarse sand (passing from 1.18 mm and retaining on 0.6 mm) and ordinary steel fibers were used at the surface area of CFRP bars through the bonded length to enhance the bond performance. The properties of steel fibers used are given in Table 3.

Table 3

Properties of the steel fiber
Specific Density (g/cm3)	Diameter (µm)	Length (mm)	Aspect Ratio, (l/d)	Tensile Strength (MPa)
7.8	160	12	75	2700

Source: Supplied by the manufacturer

2.2. Specimens

Test program involved thirteen rectangular concrete beams with of 100 x 180 mm cross sectional area and 800 mm total length as shown in Fig. 1 (Desnerck et al., 2010). All of the tested beams consisted of two concrete blocks linked together using CFRP bar in tension and steel hinge in compression, the unbonded length in front of each block was 50 mm to avoid failure of concrete at edges. The main parameters investigated in this study were: diameter of CFRP bars, concrete compressive strength, and bar surface area improvement scheme (coarse sand, steel fibers). Transverse reinforcement of Ø 6 mm spaced at 50 mm were used. Also, two steel reinforcing bars of 8 mm diameter were provided at top and bottom of each block. Table 4 provides the details of the tested beams.

Table 3

Details of the tested beams
Specimens	Bar type	Diameter (mm)	Concrete Mix	Coating Type
M1-C6-N	CFRP Bar	6	M1	Non
M1-C6-S	CFRP Bar	6	M1	Coarse Sand
M1-C6-F	CFRP Bar	6	M1	Steel fiber
M2-C6-N	CFRP Bar	6	M2	Non
M2-C6-S	CFRP Bar	6	M2	Sand
M2-C6-F	CFRP Bar	6	M2	Steel fiber
M1-R10	Steel Bar	10	M1	Non
M1-C10-N	CFRP Bar	10	M1	Non
M1-C10-S	CFRP Bar	10	M1	Sand
M1-C10-F	CFRP Bar	10	M1	Steel fiber
M2-R10	Steel Bar	10	M2	Non
M2-C10-N	CFRP Bar	10	M2	Non
M2-C10-S	CFRP Bar	10	M2	Coarse sand
M2-C10-F	CFRP Bar	10	M2	Steel fiber

Firstly, the CFRP bars were cut to the required length, the bonded length were the enhancement scheme will be applied was marked. Then, the coarse sand and steel fibers were used through the surface area of the required bonded length with epoxy. Each specimen was reinforced with steel and CFRP bars as illustrated in Fig. 2 and casted with concrete. During this process, cubes of 150 × 150 × 150 mm were used to evaluate the compressive strength of concrete.

2.3. Beam test setup

The system of testing consist from steel frame with a hydraulic jack of 100 kN capacity at the top and steel hinged supports at bottom. The beams were tested using four point load control system as shown in Fig. 3. The loads were applied monotonically with a velocity of 10 N/s. Two dial gauges fixed at each free end of the beam were used to measure the slip between the free part of the CFRP bar and the outer beam edge. All readings were video recorded during test.

3.1. General

The cubes for each concrete mix were tested at 7 and 28 days. Table 5 provides the compressive strength of concrete.

Table 5

Compressive strength of concrete
	Average cube compressive strength (MPa)
	7 days	28 days
M1	35.98	55.38
M2	52.12	78.34

The influence of the FRP bar diameter, compressive strength, and enhancement scheme have been investigated based on the results of the beam test interms of peak load, average bond strength, normalized bond strength, free end slip, and strength ratio. A summary of the beam test results are given in Table 6.

Table 6

Summary of beam test results
Specimens	Bond stress (MPa)	Relative bond stress	Slip (mm)	Strength ratio
M1-C6-N	31.24	5.21	2.72	0.416
M1-C6-S	38.76	6.46	0.28	0.516
M1-C6-F	37.03	6.18	0.74	0.493
M2-C6-N	40.99	5.68	3.17	0.545
M2-C6-S	48.07	6.66	2.43	0.639
M2-C6-F	44.92	6.22	0.35	0.598
M1-R10	46.08	7.69	0.45	-
M1-C10-N	25.42	4.24	1.85	0.259
M1-C10-S	31.62	5.28	2.67	0.322
M1-C10-F	30.06	5.02	0.72	0.306
M2-R10	60.01	8.31	0.45	-
M2-C10-N	30.64	4.24	1.73	0.312
M2-C10-S	35.46	4.91	2.5	0.361
M2-C10-F	34.00	4.71	1.55	0.346

3.2. Bond stress – slip curves

The bond stress - slip relationship was linear with a sharp increase of the stress through a small range of slip, slope softening was noticed due to interface degradation directly around the embedded bar up to the maximum bond stress. Beyond the first peak, the stress - slip curves showed different characteristics based on bar diameter and strengthening scheme. Figure 4 shows bond stress – slip curves of the tested specimens.

3.3. Bond strength index

The ratio of bond stress at a slip of 5 or 10 mm to the maximum bond strength is defined as bond index (Kim et al., 2013). Table 7 provides the value of bond strength index of the tested specimens.

Table 7

Bond strength index for the tested specimens
Specimens	Bond stress (MPa)	Bond stress at slip (5 mm)	Bond index
M1-C6-N	31.24	27.61	0.884
M1-C6-S	38.76	23.46	0.605
M1-C6-F	37.03	24.93	0.673
M2-C6-N	40.99	35.74	0.872
M2-C6-S	48.07	36.37	0.757
M2-C6-F	44.92	39.23	0.873
M1-R10	46.08	29.91	0.649
M1-C10-N	25.42	21.63	0.851
M1-C10-S	31.62	27.84	0.880
M1-C10-F	30.06	21.45	0.714
M2-R10	60.01	37.86	0.631
M2-C10-N	30.64	27.41	0.895
M2-C10-S	35.46	29.11	0.821
M2-C10-F	34.00	22.89	0.673

3.4. Effect of the studied parameters

3.4.1. Effect of compressive strength

As shown in the results obtained from the tests, higher compressive strength produced improved bond performance. For CFRP bars of 6 mm diameter, increasing of compressive strength of concrete increased the bond and relative bond stress by 31.2 and 9% for bars without enhancement in the surface area, by 24 and 3.1% for bars enhanced with coarse sand, and by 21.3 and 6.5% for bars with surface area enhanced using steel fibers. Also, the strength ratio of CFRP bars increased by 31, 23.8, and 21.3%, respectively. Furthermore, the bond index for CFRP bars with surface area enhanced by coarse sand and steel fibers increased by 25.1 and 29.7%, respectively.

For 10 mm size CFRP bars, the bond stress increased by 20.5, 12.1, and 13.1% for bars without enhancement in the surface area, enhanced with coarse sand, and surface area enhanced using steel fibers, respectively while the relative bond stress decreased by 6 to 7% that the increase in compressive strength is higher than the increase in bond strength especially when the surface area of the CFRP bars increased. In addition, the strength ratio of CFRP bars increased by 20.4, 12.1, and 13%, respectively. However, the bond index decreased by 6.7 and 5.7% for bars with surface area enhanced by coarse sand and steel fibers due to the exhaustion through loading up to the increased maximum bond stress.

3.4.2. Effect of CFRP bar size

When the CFRP bar diameter increased, higher bond forces with lower bond stresses produced. For 55.38 MPa concrete grade the bond stresses decreased by 18 to 19% for Ø 10 mm CFRP bars if compared with bars of 6 mm size because the bond forces will be distributed over larger bar surface area. Similarly, the strength ratio decreased by 42 to 44% despite of the increased bond forces that the CFRP bars of 10 mm size have very high tensile ultimate load.

On the other hand, the bond index increased by 45.5 and 6.1% for CFRP bars enhanced through the bonded length with coarse sand and steel fibers, respectively that the bars still have moderated bond stresses after maximum values.

The phenomenon still almost the same for concrete mix of 78.34 MPa compressive strength. The bond stresses and strength ratios decreased by 24 to 27% and 42 to 44% while the bond index increased by 8.5% for CFRP bars of surface with coarse sand. However, the bond index decreased by 22.9% for CFRP bars of 10 mm diameter if compared with Ø 6 mm size that the steel fibers was not uniformly distributed as coarse sand so, the bond stress will be lost suddenly especially in high strength concrete.

3.4.2. Effect of the enhancement schemes

As illustrated before, two techniques for bond performance improvement were used. The results showed that applying of coarse sand and steel fibers directly to the surface area of the bonded length of CFRP bars of 6 mm diameter raised the maximum bond strength by 24.1 and 18.5% respectively for 55.38 MPa concrete grade and by 17.3 and 9.6% for concrete mix of 78.34 MPa compressive strength if compared with deprived bars due to the improved mechanical interlock provided by the enhancement schemes. Also, the strength ratios increased by 24 and 18.5% for the first concrete mix and by 17.2 and 9.7% for the second owing to the increase of the applied load. However, the bond index decreased by 13.2 to 31.6% because of the additional amount of maximum bond stress gained from the improved surface area.

For Ø 10 mm CFRP bars, similar behavior approximately happened, the maximum bond stress increased by 18.3 to 24.4% for the first concrete mix and by 10.9 to 15.7% for the second. In addition, the strength ratios raised by 10.9 to 24.3%. Anyhow, the bond index decreased by 8.3 to 24.8%.

On the other hand, the maximum bond stress between Ø 10 mm CFRP bars and the first concrete mix decreased by 44.8, 31.4, and 34.8% for bars without enhancement in the surface area, enhanced with coarse sand, and improved using steel fibers, respectively if compared with conventional steel reinforcement of 10 mm diameter. Also, the decreasing in the maximum bond stress for the second concrete mix is 48.9, 40.9, and 43.3% owing to the high disparity between the standard surface geometry of the steel bars and the improved surface area of the CFRP bars. Although, the bond index raised by 10 to 35.6% and by 6.7 to 41.8%.

In this paper, the effect of applying coarse sand and steel fibers to the surface area of the CFRP bars through all of the bonded length were investigated. Based on the experimental results, the following can be concluded:

Despite of the approximately similar bond slip response of CFRP and conventional steel bras embedded in concrete, the maximum bond stress of the CFRP bars decreased by 44.8 to 48.9% if compared with steel bars because of the clear gap between the planned standard surface geometry of the steel bars and the relatively modern FRP bars of different materials, surface geometry, and surface treatment.

Increasing the compressive strength of concrete by 41.5% raised the maximum CFRP bars concrete bond stress and the strength ratio by 21.3 to 31.2% and 21.3 to 31%, respectively for Ø 6 mm bars and by 12.1 to 20.5% and 12.1 to 20.4% for bars of 10 mm diameter. Also, the relative bond stress and bond index increased by 3.1 to 9% and 25.1 to 29.7%, respectively for bars of 6 mm diameter while the decreasing of the relative bond stress and bond index is 6 to 7% and 5.7 to 6.7%, respectively.

Increasing the area of the CFRP bars by 125% decreased the bond stress and the strength ratio by 18 to 27% and 42 to 44%, respectively owing to the increase of the normal or enhanced surface area and the ultimate tensile strength of the bar.

The maximum bond stress and the strength ratio of CFRP bars enhanced using coarse sand particles applied directly to the surface area with epoxy are 15.7 to 24.4% and 15.7 to 24.3% higher than bars without enhancement in the surface area due to the improved surface area which provides additional adhesive and friction resistance. However, the bond index decreased by 1.3 to 8.3%.

Using of steel fibers with epoxy to improve the surface area of the bonded length of CFRP bars increased the bond stress and the strength ratio by 9.6 to 18.5% and 9.7 to 18.5%, respectively if compared with bars without enhancement schemes because of the additional mechanical interlock gained from the steel fibers. Anyhow, the bond index decreased by 1 to 24.8%.

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript. The authors have no relevant financial or non-financial interests to disclose.

Acknowledgments

Special word of thanks to Mr. Ganim Deham for his effort in this paper throughout the experimental work.

Arias, J. P. M., Vazquez, A., & Escobar, M. M. (2012). Use of sand coating to improve bonding between GFRP bars and concrete. Journal of composite materials, 46(18), 2271-2278. https://doi.org/10.1177/0021998311431994
Başaran, B., Kalkan, İ., Beycioğlu, A., & Kasprzyk, I. (2022). A Review on the Physical Parameters Affecting the Bond Behavior of FRP Bars Embedded in Concrete. Polymers, 14(9), 1796. https://doi.org/ 10.3390/polym14091796
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No competing interests reported.

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You are reading this latest preprint version

Improvement of bond behavior of fiber reinforced polymer bars using coarse sand and steel fibers: experimental study

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental Work

2.1. Materials

2.2. Specimens

2.3. Beam test setup

3. Results And Discussion

3.1. General

3.2. Bond stress – slip curves

3.3. Bond strength index

3.4. Effect of the studied parameters

3.4.1. Effect of compressive strength

3.4.2. Effect of CFRP bar size

3.4.2. Effect of the enhancement schemes

4. Conclusions

Declarations

References

Additional Declarations

Status:

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