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The incorporation of fiber-reinforced-polymer (FRP) bars in construction as a replacement to steel bars provides a superior material which is capable to overcome corrosion problems. However, serviceability requirements are important issues to be considered in the design of concrete elements reinforced with glass-FRP (GFRP) bars which are known to have larger deflections and wider crack widths as well as weaker bond compared with steel reinforced concrete. As a solution to this problem, square GFRP bars are proposed. This paper presents the results of an experimental investigation that was performed, in which newly developed square and circular GFRP bars were fabricated in the lab. Also, the GFRP bars were tested and used to reinforce concrete slabs. A total of nine full-scale GFRP-reinforced concrete (RC) one-way slabs were constructed, tested and analyzed, considering the most influencing parameters such as the cross sectional shape of GFRP bars, reinforcement ratio, the concrete characteristics strength, and adding polypropylene fibers to the concrete mixture. The test results were showed that, the tested slabs with GFRP square bars improved the deflection and cracking behavior as well as the ultimate load.

Durability of reinforced concrete (RC) structures is one of the most important points to be considered. In many cases steel RC structures are subjected to corrosive environments. In aggressive environments, the use of steel reinforcing bars stands out as a significant factor leading to significant limit of the life expectancy of reinforced concrete structures. The use of fiber reinforced polymers (FRP) reinforcement is particularly attractive for structures that operate in aggressive environments, such as in coastal regions. The magnetic transparency of FRP bars makes it a unique material that can be used to reinforce buildings that host magnetic resonance imaging (MRI) units or other equipments sensitive to electromagnetic fields. FRP composite bars in general offer many advantages over conventional steel, including one-quarter to one-fifth the density of steel, high fatigue resistance, no corrosion even in harsh chemical environments, greater tensile strength than steel, and ease of handling at job sites and cutting [

GFRP bars with square and circular cross sections are fabricated in the lab. The study presents stages of fabrication of the FRP bars as well as their mechanical and physical properties. The effect of using those bars was examined through testing the behavior of one way slabs with different concrete types. Therefore, better understanding to the effect of test parameters on improving the bond that resulted in the enhancement of the slab general behavior. The physical and mechanical characteristics of GFRP bars, considered fiber content of GFRP-bars, unit weight of the GFRP-bars, and tensile strength and modulus of elasticity of GFRP-bars with square and circular cross section area.

Fifteen 30 mm long samples of square GFRP bars were prepared and tested according to ASTM E1131 [

where,

F% = fiber content;

W_{L} = weight loss at 600˚C;

W_{T} = total weight of FRP sample.

The unit weight of the bars is an important parameter for comparing between the GFRP bars (square and circular cross section) and steel bars as reinforcement for the structural elements. The unit weight of the bars given an indication of strength to weight of the GFRP bar. The unit weight (γ) of the GFRP bars was determined by the following equation:

where,

W = Total weight of square GFRP sample;

B = Dimension of square GFRP-bar;

L = Measured length of square GFRP sample.

All square GFRP-bars of 14.1 × 14.1 mm and circular GFRP-bars 16 mm diameter specimens were tested under tension according to ASTM D7205. The GFRP specimen length as well as the length and diameter of the anchor to be used for the tensile test were calculated according to ASTM D7205 [

GFRP square (14.1 × 14.1 mm) and circular (16 mm diameter) cross-sections reinforcing bars were fabricated in the lab by mechanical pultrusion process. The pultrusion is a common technique for manufacturing continuous lengths of FRP bars of constant profile. As illustrated in

To ensure good bond with concrete, the surface of the bars was braided by fiber yarns by a pitch of 10 mm. The properties of the final product of the bar are shown in

Slab specimens were constructed using normal weight concrete mix. Polypropylene fibers were used in the concrete mix with a variable average compressive strength as 25, 35, and 45 MPa. The concrete compressive strength was determined from the average test results of 10 concrete cylinders (150 × 300 mm) at the same day of slabs tests.

Bar type | Bar cross section (mm) | Area (mm^{2}) | Elastic tensile modulus (GPa) | Tensile strength (MPa) | Ultimate strain (%) |
---|---|---|---|---|---|

Square GFRP bars | |||||

GFRP | Sq. 14.1 | 199 | 47.5 | 642 | 1.36 |

GFRP bars | |||||

GFRP | Cir. 16 | 200 | 45.1 | 630 | 1.42 |

Steel bars | |||||

M15 | Cir. 16 | 200 | 200 | 460 | 0.2 |

A total of nine full-size one way slabs reinforced with GFRP and steel bars were constructed and tested up to failure. The slabs measured 2100 mm long, 500 mm wide, and 140 mm depth. _{f} [0.857% (3 bars), 1.142% (4 bars), and 1.428% (5 bars)], bar cross section shape (square or circular), concrete compressive strength (25, 35, and 45 MPa), and polypropylene fiber ratio [0%, 1.5% (14.7 N/m^{3}) , and 2.5% (24.5 N/m^{3})] which were added to the concrete mix.

_{f} = reinforcement ratio of

Group | Slab^{*} | P.P (N/m^{3}) | Parameters of studying | |||
---|---|---|---|---|---|---|

I | S-3C-25-0 | 25 | 0.857 | 3 bars | 0 | Reference |

II | G-3C-25-1.5 | 25 | 0.857 | 3 bars | 14.7 | Bar shape |

G-3S-25-1.5 | 25 | 0.857 | 3 bars | 14.7 | ||

III | G-3S-35-1.5 | 35 | 0.857 | 3 bars | 14.7 | |

G-3S-45-1.5 | 45 | 0.857 | 3 bars | 14.7 | ||

IV | G-4S-25-1.5 | 25 | 1.142 | 4 bars | 14.7 | |

G-5S-25-1.5 | 25 | 1.428 | 5 bars | 14.7 | ||

V | G-3S-25-0 | 25 | 0.857 | 3 bars | 0 | p.p |

G-3S-25-2.5 | 25 | 0.857 | 3 bars | 24.5 |

^{*}In first letter, S and G denote the longitudinal reinforcement type (Steel versus glass FRP). In second letter, C and S denote the cross section of the bars (C = circular bar, S = square bar).

longitudinal reinforcement.

GFRP and steel cages were assembled for the slab specimens, as shown in

All slabs were tested under four-point bending over a clear span of 1950 mm and a shear span of 650 mm, as shown in

In this study, the test results are presented in terms of the physical and mechanical properties of GFRP bars of square and circular cross sections. In addition, the results of cracking, deflections, strains in reinforcing bars and concrete, ultimate capacity and modes of failure in RC one-way slabs reinforced with GFRP and steel bars are presented.

The test results indicated that the glass-fiber content by weight for square-GFRP bars and circular-GFRP bars with the same number of yarns were 72.19% and 71.57%, respectively which is accepted according to [^{3} according to the number of yarns which were used between 80 and 144 yarns per bar.

Specimen No. | Bar dimension (mm) | Number of glass fibers yarns | Fiber content F (%) | Average fiber content F (%) | Unit weight (KN/m^{3}) |
---|---|---|---|---|---|

1 | 14.1 × 14.1 | 80 | 62.80 | 62.86 | 15.69 |

2 | 14.1 × 14.1 | 80 | 62.38 | ||

3 | 14.1 × 14.1 | 80 | 63.41 | ||

4 | 14.1 × 14.1 | 100 | 65.72 | 64.72 | 21.18 |

5 | 14.1 × 14.1 | 100 | 64.39 | ||

6 | 14.1 × 14.1 | 100 | 64.77 | ||

7 | 14.1 × 14.1 | 130 | 68.10 | 67.89 | 22.84 |

8 | 14.1 × 14.1 | 130 | 67.20 | ||

9 | 14.1 × 14.1 | 130 | 68.37 | ||

10 | 14.1 × 14.1 | 144 | 72.0 | 72.19 | 25.30 |

11 | 14.1 × 14.1 | 144 | 72.57 | ||

12 | 14.1 × 14.1 | 144 | 72.0 | ||

13 | 16 mm (circular) | 144 | 71.04 | 71.57 | 25.60 |

14 | 16 mm (circular) | 144 | 72.02 | ||

15 | 16 mm (circular) | 144 | 71.64 |

_{f}A_{f}.

Group | Slab | Cracking load (KN) | Ultimate load (KN) | Max. deflection (mm) | Max. strain (μe) | Mode of failure | ||
---|---|---|---|---|---|---|---|---|

Bars | Concrete | |||||||

I | Steel | S-3C-25-0 | 52 | 78.9 | 44.14 | 10470 | 3820 | A |

II | Circular GFRP | G-3C-25-1.5 | 22 | 63.0 | 45.92 | 5650 | 3705 | B |

Square GFRP | G-3S-25-1.5 | 27.8 | 70.3 | 32.18 | 7370 | 3420 | B | |

III | Square GFRP | G-3S-35-1.5 | 38.5 | 74.4 | 32.98 | 5120 | 3562 | B |

Square GFRP | G-3S-45-1.5 | 41.5 | 80.0 | 36.40 | 8528 | 3730 | B | |

IV | Square GFRP | G-4S-25-1.5 | 37.5 | 77.0 | 28.82 | 5911 | 3501 | B |

Square GFRP | G-5S-25-1.5 | 48 | 90.0 | 23.62 | 5131 | 3800 | B | |

V | Square GFRP | G-3S-25-0 | 19.2 | 64.4 | 28.62 | 4967 | 3201 | B |

Square GFRP | G-3S-25-2.5 | 32.2 | 75.2 | 31.74 | 6208 | 3430 | B |

a) Yielding of the steel followed by crushing of concrete, b) Crushing of concrete followed by the rupture.

the surface area of GFRP-bars by about 12% at the same cross section area. This increase appeared as a difference in the load deflection relationship. As a result, increasing the surface area of the bars improved the bond between these bars with concrete, and consequently increased the stiffness and decreased the deflection of slab (G-3S-25-1.5) up to failure.

Adding polypropylene fibers to concrete in known to enhance the concrete microcracking and consequently the concrete tensile strength. ^{3} of the polypropylene fibers in its mix which had the minimum deflections and maximum stiffness compared to the corresponding slab (G-3S-25-0) without polypropylene fibers in its concrete mix.

For all tested slabs reinforced with GFRP-bars, the relationship between the cracking and ultimate loads versus different parameters (type of reinforcement bars, cross sectional shape of the bars, concrete compressive strength, reinforcement ratio, and polypropylene fibers ratio) are plotted. The results of cracking loads, ultimate loads are presented through Figures 11-15, which represent the effect of different parameters and their effect on the slab behavior which are explained through the following sections:

The change of the cross sectional shape of bars from circular to square, resulted in an increase in the surface area of the reinforcement by 12.2%. Consequently, this improved the bond strength of the GFRP-bars with concrete, and did not result in any bond slippage. This change resulted in difference in the surface area of bars, which changed the cracking loads and the failure loads of the slabs reinforced by GFRP-bars. By looking into the test results, it was found that slab G-3S-25-1.5 reinforced with GFRP-square bars, fell at a much higher load than the slab reinforced with 16 mm diameter GFRP-bars (about 10% of the failure load). The cracking load changed

between slabs G-3S-25-1.5 and G-3C-25-1.5 by about 26.3%. The slab reinforced with steel bars S-3C-25-0, had higher failure and cracking load compared to the cracking and failure load for slabs reinforced with GFRP- bars with the same concrete characteristics strength of 25 MPa and the same reinforcement ratio.

Another factor affected by the change of the bar shape are the behavior controlled by splitting cracks. Splitting cracks occur unavoidably under increasing loads depending on a variety of physical and mechanical factors, such as confining pressure, concrete cover, transverse reinforcement and concrete toughness. The last being the basis of crack cohesion. Of course longitudinal splitting may be limited to the concrete closest to the bars (Partial splitting) if one or more of the above mentioned, depending on the interaction between bar and the concrete. Two types of interaction have been traditionally acknowledged. Pull out failure and splitting failure. In the former case bond failure is mainly due to the shearing of the concrete keys cast between each pair of lugs and this type of bond may not vary a lot between rectangular and circular bars. The type that is possibly affected which results from failure due to local mechanism (interface collapse) even if the whole bar is involved. The later case, bond failure is mostly due to longitudinal splitting of the concrete surrounding the bar. Bond capacity vanishes once the radial cracks get to the outer surface of the structural element. The failure is related to a sort of structural collapse since structural parameters other than those pertaining to bond enter the scene. The concrete cover thickness as well as the stress concentration on cover from the bar changes by the change of the shape of reinforcing bars. Therefore, the rectangular shape of the bar resuts in slightly lower bar hight when the bars are placed at the same level like circular bars slightly higher concrete cover thickness (see

The cracking and failure loads were affected by changing the concret compressive strength of concrete mix. Where the slabs with higher concrete compressive strength had an improved material characteristics, the cracking and failure loads. The slab of 45 MPa concrete strength (G-3S-45-1.5) was higher than slabs of 35, and 25 MPa (G-3S-35-1.5 and G-3S-25-1.5) as shown in

The slab reinforced with the lower reinforcement ratio G-3S-25-1.5 (ρ_{f} = 0.857%) showed a decrease in the cracking and failure load with respect to the slab reinforced with high reinforcement ratio G-5S-25-1.5 (ρ_{f} = 1.428%). However, increasing the reinforcement ratio from 0.857% to 1.428% for slabs reinforced with GFRP- square bars increased the failure and cracking load by about 28% and 72.7%, respectively as shown in

Adding the polypropylene fibers to concrete mix, increased the cracking and failure loads of slabs reinforced with GFRP-square bars, because it improved the tensile characteristics of the concrete. While slab G-3S-25-0 reinforced with GFRP-square bars and without polypropylene fibers, showed lower cracking and failure loads compared to the slab G-3S-25-1.5 having polypropylene fibers by amount 14.7 N/m^{3}. Also, the slab G-3S- 25-2.5 having polypropylene fibers by amount 24.5 N/m^{3}, showed higher cracking and failure load than those slabs having amount 14.7 N/m^{3} as shown in ^{3}, increased the cracking and failure load by 44% and 10% respectively compared to the slab without polyproylene. In addition, the slab with the polypropylene fibers by amount 24.5 N/m^{3}, increased the cracking and failure load by 67.7% and 17% respectively.

Cracks in all of tested slabs were observed. Flexural cracks were initiated at the bottom surface of concrete slabs in the middle region between the two line loads, First crack appeared whenever the tensile stress exceeded the modulus of rupture of concrete. The crack appeared at middle of the slab and developed slowly across the width of the slab (i.e. parallel to the supports), and was accompanied by an increase in deflection due to stiffness reduction of the specimen. As the load increased, additional crack started to form along the length of the specimens between the two lines loads, and slowly propagated upward throughout the thickness of the slabs up to the failure. The crack distribution, width, and propagation were affected by the study parameters. The maximum crack width occurred in the specimen G-3C-25-1.5 and propagated fastest (reinforced with 3 circular GFRP-bars of 16 mm diameter) at the lowest level of loading. On other hand, the minimum crack width appeared and propagated at a slower rate in the specimen G-5S-25-1.5 (reinforced with 5 GFRP-square bars). For all slabs reinforced with GFRP bars the modes of failure started by crushing of concrete followed by the rupture of the GFRP reinforcements whereas the steel reinforced slab failed by yielding of the steel which was then followed by crushing of concrete. The propagation of crack pattern for all tested slabs is shown in

Figures 17-20 show typical load-strain relationships for reinforcement bars and concrete of RC slabs. The relationship consisted of a bilinear curve, low rate of strain increasing for a certain load increment up to the cracking load, and then higher rate of strain increasing for the same load increment when the concrete cracked but became also linear up to failure. In general, the strains of the slabs reinforced with GFRP-bars without enhancement were much higher than the steel sample due to the lower stiffness of the GFRP-bars. Enhancing parameters improved the strain values and became very close to the strain values of the steel sample.

This paper presented the fabrication and testing of new developed FRP bars. The physical and mechanical properties of a newly developed product of glass FRP-square and circular bars are presented. GFRP square bars were tested and compared with GFRP circular bars (16 mm diameter) and steel. The fabrication and testing of the bars and an experimental study of one-way concrete slabs reinforced with glass square and circular FRP bars and steel bars are presented. A total of 9 full-scale RC slabs were prepared to study the effect of five test parameters: type of reinforcement (steel versus GFRP), bar cross section shape, concrete compressive strength, reinforce- ment ration of longitudinal bars, and polypropylene fiber ratio. Based on the experimental test results presented in this paper, the following conclusions can be drawn:

1) The glass-fiber contents by weight of square-GFRP bars and circular-GFRP bars were 72.19% and 71.57%, respectively, which were made by 144 yarns. These results of fiber content are adopted with ACI 440.6M-08 and CSA S-807-10 (70% fiber content by weight or 55% as fiber content by volume).

2) The unit weight of square GFRP bars ranged between 15.69 to 25.6 KN/m^{3} according to the number of yarns which were used 80 - 144 yarns per bar.

3) The tensile strength, elastic tensile modulus, and ultimate strain of square-GFRP bars are 642 MPa, 47.5 GPa, and 1.36%, respectively. Moreover, the tensile strength, elastic tensile modulus, and ultimate strain of circular-GFRP bars (16 mm of diameter) are 630 MPa, 45.1 GPa, and 1.42%, respectively.

4) The behavior and ultimate flexural loads of concrete slabs reinforced with GFRP-square bars were improved compared with concrete slabs reinforced with GFRP-circular bars of 16 mm diameter.

5) Increasing the concrete compressive strength, resulted in improving the mechanical properties of the concrete mix for slab (G-3S-45-1.5) compared with slabs (G-3S-25-1.5, G-3S-35-1.5), consequently increasing the stiffness of slab and improved the behavior of concrete slabs with GFRP-square bars.

6) Slabs having the lowest reinforcement ratio gave the maximum deflections. This was very obvious, where in the slab having the minimum area of reinforcement of 3 bars (reinforcement ratio of 0.857%) had the maximum deflections. On the other hand, the slab having the maximum area of reinforcement of 5 bars (reinforcement ratio of 1.428%) had the minimum deflections.

7) The deflections of the slabs were inversely proportional to the amount of the polypropylene fibers added to the concrete mix. When the concrete mix had no polypropylene fibers, the deflections of the slabs were noticed. Adding the polypropylene fibers to the concrete mix decreased the deflections, and improved the behavior of concrete slabs reinforced with GFRP-square bars.

The authors would like to express their special thanks and gratitude to the technical staff of the structural lab of the Department of Civil Engineering, Faculty of Engineering at the Helwan University.