Proceedings of the 11th WSEAS International Conference on Sustainability in Science Engineering

The use of glass fibre reinforced polymer composites as reinforcement for tubular concrete poles NICOLAE TARANU, GABRIEL OPRISAN, MIHAI BUDESCU, ALEXANDRU SECU, IONEL GOSAV Department of Civil Engineering, Faculty of Civil Engineering Technical University of Iasi Bd. Dimitrie Mangeron 43 ROMANIA [email protected] http://www.ce.tuiasi.ro Abstract: - This paper presents the results of a theoretical and experimental study carried out on the possibility of using polymer composites reinforcing elements for a power supply tubular reinforced concrete (RC) column. The composite reinforcing elements are utilized both as longitudinal glass reinforced thermosetting polymer (vinyl-ester) bars and transverse reinforcement made of a glass fibre reinforced polypropylene (GFPP) spiral. The columns have been fabricated using the existing manufacturing facilities based on centrifuging procedure. An experimental program has been organized to determine the mechanical characteristics of the GFPP spiral and bonding of this composite element to concrete. The program has been extended to evaluate structural response of spun-cast element according to existing standard tests. A 4m long hollow tapered RC column has been tested to assess the load-deformation characteristics. A theoretical analysis has been performed to prove the suitability of existing calculation formulas for serviceability and ultimate limit states. The study confirms the possibility of using composite reinforcement for hollow tapered RC columns. Key-Words: Tapered columns, composite materials, hybrid structures, concrete-reinforcement bonding, structural response the composites. It exhibits a gradation of properties and it is a dominant factor in the resistance of the composite to corrosive environments. It also has a decisive role in the failure mechanisms and fracture toughness of the polymeric composites. Glass fibre reinforced composites provided the initial scientific and engineering understanding of FRP matrix composites. The main advantages that enabled the widespread of glass fibres in composites are competitive price, availability, good handleability, ease of processing, high strength and other convenient properties. Glass fibres are the most commonly used reinforcing fibres for polymeric matrix composites. The most common glass fibres are made of E-glass and Sglass. E-glass is the least expensive of all glass types and it has a wide application in fibre reinforced plastic industry. S-glass has higher tensile strength and higher modulus than E-glass. However, the higher cost of Sglass fibres makes them less popular than E-glass. Alkali-resistant glass fibres contain an amount of zirconium which helps prevent corrosion by alkali attacks in cement matrices. The main properties of E-glass and S-glass are summarized in Table 1. Glass fibre roving consist of up to 120 untwisted strands, usually supplied wound together on a spool and suitable for unidirectional (UD) fibre reinforcement of polymeric resins.

1 Glass fibre reinforced composite as a reinforcing materials Composites are materials consisting of two or more chemically distinct constituents on a macro-scale, having a distinct interface separating them and with properties which cannot be obtained by any constituent working individually. In fibrous polymeric composites, fibres with high strength and high stiffness are embedded in and bonded together by the low modulus continuous polymeric matrix. Each of the individual phases must perform certain functional requirements based on their mechanical properties so that a system containing them may perform satisfactorily as a composite. In the case of FRP composites the reinforcing fibres constitute the backbone of the material and they determine its strength and stiffness in the direction of fibres. The polymeric matrix is required to fulfill the following main functions: to bind together the fibres and protect their surfaces from damage during handling, fabrication and service life of the composite; to disperse the fibres and separate them and to transfer stresses to the fibres. The matrix should be chemically and thermally compatible with the reinforcing fibres. The interface region is small but it has an important role in controlling the overall stress-strain behavior of

ISSN: 1790-2769

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ISBN: 978-960-474-080-2

Proceedings of the 11th WSEAS International Conference on Sustainability in Science Engineering

strength and stiffness in the axial direction of the composite product. Fibre volume fractions of up to 70% are achievable with unidirectionally aligned fibres. The GFPP spiral was fabricated by extrusion, a process specific to thermoplastic polymers. To improve the bonding composite bar-concrete the GFRP bar has been provided with ribs fabricated with a cutting machine. The reinforcing products and the composite cage to reinforce the tubular RC column are illustrated in Fig.1.

Table 1.Typical properties of glass fibres [4] Fibre

E-glass

S-glass

Density(kg/m3)

2500

2500

Tensile strength (MPa)

3450

4580

Young modulus (GPa)

72.4

85.5

Ultimate tensile strain (%)

2.4

3.3

5

2.9

0.22

0.22

Characteristics

Thermal exp. coefficient (10-6/ oC) Poisson’s coefficient

Vinyl-esters are resins based on methacrylate and acrylate. Some variations contain urethane and ester bridging groups. Due to their chemical structure these resins have fewer cross links and they are more flexible and have a higher fracture toughness than polyesters. They also have very good wet-out and good adhesion when reinforced with glass fibres. Vinyl esters properties are a good combination of those given by epoxy resins and polyesters. They exhibit good characteristics of epoxies such as chemical resistance and tensile strength, as well as those of polyesters such as viscosity and fast curing.

Fig. 1 The reinforcing composite products for the RC column: a. GFRP ribbed bar; b. GFPP strip; c. the assembly of the composite cage Tests have been performed to evaluate the typical required mechanical properties for each composite products Fig.2. A synthesis of the properties of all materials utilized to fabricate the tubular RC column is given in Table 3.

Table 2. Typical properties for vinyl-ester resin [4] Resin

Vinyl-ester

Property Density (kg/m3) Tensile strength (MPa)

1200 - 1400 55 - 130

Longitudinal modulus (GPa)

2.75 – 4.10

Poisson’s coefficient

0.38 – 0.40

Thermal expansion coefficient (10-6/oC) Moisture content (%) Service temperature (oC)

45 - 65 0.08 – 0.15 185

Fig.2 Tensile tests of GFPP strips (left) and GFRP ribbed bar (right)

The round GFRP bars are made by pultrusion which is a continuous fully automated manufacturing process, enabling the production of long, straight constant section structural shapes made of fibre reinforced polymeric composites. The process involves pulling these raw materials through a heated steel forming die using continuous forms such as rolls of roving or rolls of mats. In general pultrusion is dominated by the use of unidirectional reinforcement, which lends itself most appropriately to the process and gives maximum

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Table 3 Mechanical characteristics of the materials utilized for the experimental column Material properties

Tensile strength (MPa) -

Compressive strength (MPa) 37.5

Elastic modulus (GPa) 36

GFRP bars

1100

-

57.1

GFPP spiral

225

-

32

Concrete

509

Ultimate strain (%) 0.35 compr. 0.73 tensile 4.5 tensile

ISBN: 978-960-474-080-2

Proceedings of the 11th WSEAS International Conference on Sustainability in Science Engineering

in case of steel reinforced columns has been selected; about one hour after the centrifuging operation the formwork has then been kept in place; the formwork has been transported in the treatment chamber and kept for 2 days; the formwork was then removed and the column stored for 28 days until the complete curing of concrete.

An additional experimental program has been organized to determine the bond properties of GFPP spiral-concrete. Phases of this testing program are shown in Fig.3, where bond curve is also presented.

3 Structural response of the tubular column with composite reinforcement The column has been tested in horizontal position to bending at an age of 28 days, according to usual procedures, determining the maximum forces and displacements recorded at the free end of concrete pole reinforced with GFRP bars and GFPP strips. A specialized instrumentation has been designed and installed to characterize the structural response of the tested specimen. A dedicated testing stand was constructed to provide the adequate experimental conditions, Fig.4.

Fig. 4 The experimental set-up of the column

Fig. 3 Testing program on GFPP strip-concrete bonding: a. test specimen under tensile load; b. interface between strip and concrete; c. force-displacement curve

The locations of the linear voltage displacement transducers (LVDTs) to determine the lateral deflections are positioned as shown in Fig.4. The column was clamped at one end and the action was exerted by pulling the free end. A loading cell installed on the pulling device has been utilized to measure the corresponding applied load. Successive loading and unloading cycles were applied up to failure and the corresponding displacements for each applied load were measured. The RC specimen has been subjected to eight loading cycles reaching an ultimate peak value equal to 17000 N. In Fig. 5 the force-displacement envelope of all loading cycles is presented. It can be seen that the residual deformation after last loading cycle is relevant and confirms the type of behaviour typical for the materials utilized in the test specimen. It has been noticed that a first crack occurred at 4500 N, and a quite uniform cracking pattern has been obtained confirming a good bonding between concrete and the ribbed GFRP

2 Fabrication of tubular column reinforced with composite products A summary of the technological steps to fabricate the tapered columns includes: the reinforcement cage, has been set up on rigid fixing rings and this assembly has been placed in the formwork using distance-keepers; rigid fixing rings have been utilized to set-up the longitudinal reinforcement (8 GFRP bars); the formwork was steel-made, from two pieces joined by screws; on the formwork length there are transversal circular ribs, guided by the wheels of the centrifuging machine. The reinforcing cage has been placed in one half of the formwork and then the concrete has been poured; the amount of concrete has been determined so as to fill exactly the formwork; after that, the formwork was closed and placed on the centrifuging machine; a centrifuging time of 15 minutes, similar to that utilized

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Proceedings of the 11th WSEAS International Conference on Sustainability in Science Engineering

power supply installations weight is undertaken exclusively by the concrete area. The FRP considered reinforcement will be subject only to flexural elements design rules, in terms of strength requirements. The determination of bending moment capacity depends on the location of the neutral axis, whose position is determined by equation of equilibrium between the concrete compression and the FRP tension on the cross section. The post is idealized as a cantilever beam fixed at the bottom end, subjected to flexure caused by the transversely acting loads such as wind pressure or, accidentally, earthquakes. Since the GFRP bars resist only to tensile stresses occurring over the cross-section, on the tension side, their contribution in compressive loading resistance is negligible. In many cases it is considered that the design of such elements is controlled by stiffness, corresponding to serviceability limit state design, by checking the effective deflections versus allowable ones. As far as the ultimate limit state is concerned, there are two accepted failure modes in case of flexural FRP reinforced concrete elements: either concrete crushing or FRP rupture. Both concrete and FRP longitudinal reinforcement are brittle materials determining sudden failures. However, in case of concrete crushing there is a little plastic behaviour making this type a slightly more desirable one. Generally, if the design is performed to induce this failure mode, the serviceability requirement concerning the deflections is met. A special approach has been used to determine the stiffness of the cantilever column. It has been considered that both concrete and GFRP bars contribute to the total stiffness, according to (1): K = Kc + K f (1)

Force [N]

bars. A maximum horizontal deflection equal to 257 mm has been recorded, Fig.5. 1,80E+04 1,60E+04 1,40E+04 1,20E+04 1,00E+04 8,00E+03 6,00E+03 4,00E+03 2,00E+03 0,00E+00 0

50

100

150

200

250

300

Displacements [mm]

Fig. 5: The envelope of the force-displacement curves The cracking process developed progressively, Fig. 6, until the failure loads have been reached. It has been noticed that the post failed by concrete crushing, typical for over reinforced elements subjected to bending.

where, K is the total bending stiffness of the reinforced column, K c is the bending stiffness provided by the

Fig. 6: Crack development in the RC post

concrete gross section and K f is the bending stiffness

Cracks development in the tension part of concrete pole reinforced with GFRP bars due to bending depends on parameters such as crack spacing, bonding strength between composite bars and concrete and strain values in transverse and longitudinal reinforcements. The spuncast concrete pole show quite a large displacement from bending at the free end, with ultimate crack observed at around 1.4 m distance from the fixed end of pole, feature that is common in case of experimental pole tests

provided by GFRP bars. For uniformly tapered poles the moment of inertia of concrete section may be conservatively taken as the gross moment of inertia at a distance one-third of span from the smaller end of the column. The appropriate value of the pole concrete stiffness is also given in the above mentioned report:

Kc =

(2)

2.5

where, Ec is the concrete modulus of elasticity and I g is

4 Analysis of the RC column reinforced with composite elements

the gross moment of inertia of concrete as specified before. With geometrical and mechanical data already presented K c = 8.544 ⋅ 1011 Nmm 2 . The stiffness provided by the GFRP longitudinal bars

Power supply posts are special structural elements whose design although apparently simple raises specific problems. The axial load due to the self weight and

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Ec I g

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Proceedings of the 11th WSEAS International Conference on Sustainability in Science Engineering

The ratio of the balanced neutral axis depth c, to the effective depth of the section d1, can be expressed using strain compatibility, shown in fig.7:

has been evaluated considering an equivalent composite tubular section having a total area equal to the sum of the cross section of 8 composite bars. Using the characteristics of GFRP bars (Table 3) and the geometry of the reinforcing scheme (Fig. 7), the stiffness value of the composite equivalent tube has been evaluated, K f = 5.917 ⋅ 1011 Nmm 2 . Comparing the experimental

ε cu c

=

ε frp ,1

(5)

d1 − c

where, ε cu is the ultimate compressive strain in concrete, c is the depth of the neutral axis, d1 is the

and theoretical elastic deflections determined with total stiffness, K , an approximation of about 8% has been observed.

effective depth and ε frp ,1 is the corresponding value of the strain in FRP. It has been found by trials that the depth of the neutral axis exceeds the column wall thickness (c=60mm). In the next step the compressive and tensile resultants, Cc and Tfrp are determined. The compression force, Cc, on the concrete section is calculated with: Cc = α1φc f c' β1A c (6) where, Ac is the area of the concrete in compression. The coefficients α1 , β1 , φc are determined according to [2]: α1 is

the

stress-block

factor

for

concrete

α1 = 0.85 − 0.0015 f ≥ 0.67 , β1 is the stress-block factor for concrete β1 = 0.97 − 0.0025 f c' ≥ 0.67 and φc ' c

is the material resistance factor for concrete = 0.65. The area of concrete in compression Ac shown in Fig. 7, it is equal to the area of the annulus, Aa, calculated with [3]: 1 D2 1 d2 (7) Aa = ( θ1 − sin θ1 ) − ( θ2 − sin θ2 ) 2 4 2 4 where, D is the external diameter of concrete pole and d is the internal diameter of concrete pole. The values for θ1, θ2, the angles figured out in Fig.7, are calculated with:

Fig.7: The reinforcing pattern and the stress-strain distribution in concrete pole reinforced with GFRP Prediction of the most probably way to produce a failure, leading to the ultimate limit state, may be done by comparing the effective FRP reinforcement ratio to the balanced FRP reinforcement ratio. These two parameters are calculated with the following relations:

ρf =

Af

ρ fb = 0.85β1

⎛ D2 − y2 ⎜ −1 ⎜ 4 θ1 = 2 tan y ⎜ ⎜ ⎝

(3)

Ag

E f ε cu f c' f fu E f ε cu + f fu

(4)

⎞ ⎛ d2 − y2 ⎟ ⎜ ⎟ ;θ2 = 2 tan −1 ⎜ 4 y ⎟ ⎜ ⎟ ⎜ ⎠ ⎝

⎞ ⎟ ⎟ ⎟ ⎟ ⎠

(8)

The distance denoted by y, Fig.7, measured between the neutral axis and the horizontal axis of entire cross section is determined with:

where, ρ f is the FRP reinforcement ratio, ρ fb is the FRP reinforcement ratio producing balanced conditions, is the GFRP reinforcement area, Ag is the Af

y=

cross-sectional gross area, β1 is the reduction factor for concrete strength, or stress-block factor for concrete [2], f c' is the specified compressive strength of concrete

Q D3 3 ⎛ 1 ⎞ d 3 3 ⎛ 1 ⎞ ;Q = sin ⎜ θ1 ⎟ − sin ⎜ θ2 ⎟ Aa 12 ⎝ 2 ⎠ 12 ⎝2 ⎠

(9)

Using the mechanical and geometrical characteristics of the GFRP reinforced column a value was found for Cc = 93.85 kN. The total tensile force Tfrp of GFRP bars can be expressed as:

C45/50, (Table 3), f fu is the design tensile strength of

3

3

i =1

i =1

GFRP bars, (Table 3), E f is the guaranteed modulus of

Tfrp = ∑Tfrp,i = ∑φfrp A f ,iε frp,i E f

elasticity of GFRP, (Table 3) and is the ultimate strain in concrete, (Table 3).

where, φfrp is the resistance factor for GFRP, 0.4 [2],

(10)

Af ,i are the areas of the GFRP bars located in the

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Proceedings of the 11th WSEAS International Conference on Sustainability in Science Engineering

but the ultimate long term tensile strength of GFRP bars should be affected by a retention coefficient not exceeding 0.5 for a span life equal to 100 years [1]. The GFRP bars contribute to the stiffness of the member with 44% less than the steel bar, therefore the total deflection of the hollow column reinforced with composite bars is larger than that reinforced with steel bars. With a maximum 257 mm lateral deflection, representing a relative displacement equal to 6.67% the column shows a good flexibility under transverse loads. The force-deflection curves underline a good behaviour of the RC column with ribbed composite bars and GFPP strips. This behaviour can be also noticed in terms of compactness of loading/unloading curves and of residual deformations as well.

tension region, Fig.7, ε frp,i are the strains in the tension GFRP bars, Fig.7 and i is the current number of GFRP bars, Fig. 7. After all algebraic calculations, the tensile force was determined: Tfrp=94.69 kN. These values of Cc and Tfrp have been evaluated after a number of trials, selecting proper values of c. Since the column external and internal diameters are known, D=250mm and d=150mm, the neutral axis depth is also known and the bending moment capacity Mr can been evaluated as:

3 ⎛ β ⎞ M r = Af ,i ∑ f frp,i ei + Cc c ⎜ 1 − 1 ⎟ (11) 2⎠ ⎝ i =1 where, f frp,i is the tensile stress in the corresponding

GFRP bars, denoted 1, 2, 3 in Fig.7 and ei represents

e1 , e2 , e3 , respectively Fig.7.

5 Conclusion

Using the mechanical properties of GFRP bars and corresponding distances, the following effective stresses have been determined in the GFRP reinforcing elements, Table 4.

Hollow concrete columns reinforced with GFRP longitudinal bars and flexible composite spirals can be designed, installed and exploited in a similar manner with the columns reinforced with steel products. The centrifuged procedure can be utilized to fabricate GFRP reinforced hollow concrete columns. Ribbed GFRP bars provide a good bonding enabling the loading of the column to high values. Since the elastic modulus of GFRP bars is only slightly higher than that of concrete, the reinforcement contribution to the stiffness of the element is not very significant. Existing formulas in design norms and codes are suitable for this type of element, having in mind the particularities of composite bars as reinforcements. Finally all advantages of FRP composite bars (corrosion resistance, lower specific weight, convenient electric and magnetic properties) can be met and valorized in this structural element.

Table 4 Tensile stresses in GFRP bars Bar number Tensile strain Tensile stress (MPa) 1 0.00962 549.3 2 0.00793 452.8 3 0.00379 216.4 The bending moment capacity determined with formula (11) using the data from Table 4 is Mr=52.124 kNm. This value of the column flexural capacity satisfies the condition:

M r ≥ 1.5M cr =

fr It yt

(12)

required by [2]. In equation (12) the significances of notations are: M cr is the cracking moment, f r is the

References: [1] fib bulletin 40, FRP Reinforcement in RC Structures. International Federation for Structural Concrete, Sprint-Digital-Druck, Stuttgart, 2007. [2] ISIS Canada, Design Manual no. 3, Reinforcing Concrete Structures with Fibre Reinforced Polymers. Canadian Network of Centres of Excelence on Intelligent Sensing for Innovative Structures, Winnipeg, Manitoba, 2001. [3] Kocer, F.Y. and J.S. Arora, Design of Prestressed Concrete Transmission Poles: Optimization Approach. J. Struct. Engrg., ASCE, Vol.122, No. 7, 1996, pp. 804-814. [4] Taranu, N., Isopescu, D. Structures made of composite materials, Ed. Vesper, Iasi, 1996.

modulus of rupture of concrete equal to 0.6 f c' , I t is the transformed section moment of inertia and yt is the distance from the neutral axis to the extreme tension fibre. Using the mechanical and geometrical properties of the GFRP reinforced column, a cracking moment value Mcr =3.206 kNm has been calculated, that satisfies condition (12). Since the density of steel is 7850 kg/m3 and the density of GFRP bars 2200 kg/m3, a saving in weight of about 40% of the reinforcement has been obtained. The result is based on the same number of reinforcing bars utilized for centrifuged hollow columns fabricated with the same technology. The short term tensile strength of GFRP bars (1100 MPa) is much higher than that of steel bars (435 MPa),

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ISBN: 978-960-474-080-2