Quality and durability control of GFRP structures

Extended Abstract

João Luís Martins e Belo Martins Supervisor: Prof. Dr. Fernando António Baptista Branco (IST) Co-supervisor: Prof. Dr. João Pedro Ramôa Ribeiro Correia (IST)

Lisbon, May 2011

1

1. Introduction In recent years, the costs of maintenance of traditional material structures, such as concrete or steel, have been growing significantly. The increase in the development of new structural materials is related to the need of lighter materials that require lower maintenance and allow for faster construction which means higher profitability. A composite material is a combination of two or more materials. If the materials are used alone, they may not be suitable as building purposes. However, when they are combined, it is possible to create new materials, which increase the properties of each one [1, 2]. The glass fiber reinforced polymers (GFRP) profiles are made of a polymer matrix, which is usually polyester or vinylester, and are reinforced with glass fibers, mainly arranged in a unidirectional direction. These materials are obtained through a manufacturing process called pultrusion. Since the 1980s, GFRP began to be used extensively and their potential became more widely-known. The main advantages of these materials are:

- The high ratio between its strength and weight; - The very high ratio between its stiffness and weight; - Fatigue resistance; - Durability in aggressive environment; - Low self weight; - Electromagnetic transparency; - The ability to produce any kind of shape.

However, there are several factors that have slowed the acceptance of GFRP profiles. For example, the cost of production when compared to other concrete structures or steel is not very competitive. Apart from this, there are other issues, such as deformability, instability, technologies of the connections, lack of consistent information about the durability and lack of regulations [1]. Typically, the application of GFRP profiles in the construction industry is mainly centered in secondary elements such as floors, stairs and catwalks. However, there are projects where these profiles are used as structural elements, for example in bridges and buildings.

2. GFRP properties and manufacturing Fiber reinforced polymers have physical and chemical features that result from the combination of a polymeric matrix and fibers responsible for great part of its strength and stiffness. This polymeric matrix is like the “glue” for the composite, ensuring the load transfer between the fiber and the matrix [1, 2]. Beyond the matrix and the fibers, this type of material can include fillers and additives. The additives are added in order to improve the material features [1, 2].

2

2.1 Materials

Fibers

The main function of fiber reinforcement is to support the mechanical load applied to the elements, ensuring sufficient strength and stiffness along the direction in which they are developed [1, 2].

Polimeric matrix The polymeric matrix has four essential functions in the profiles performance, which are [1, 3]:

- Keeping the fibers in the correct position; - Ensuring the transference and the load distribution to fibers; - Avoiding the buckling of the fibers, under compression; - Protecting the fiber from environmental aggressive agents.

Filler The inorganic fillers are used in the matrix composition to reduce the cost of the final product. They also improve its performance, ensuring certain properties which are not usually obtained with resins and fibers [1, 2].

Additive

The variety of additives that can be introduced into the matrix serve to improve the material performance, processing or changing certain properties, such as [1, 2]:

- Reducing of the retraction; - Increasing of the hardness; - Changing the color; - Preventing loss of brightness, cracks and ageing due to the UV radiation.

2.2 Properties

GFRP properties depend on the characteristics of their constituent materials (type of polymer matrix and reinforcing fiber), fiber orientation, fiber content and also the interaction between fiber and matrix [1]. Due to the internal structure of the GFRP laminates profiles, the behavior of the material is isotropic, with higher mechanical properties in the direction of the rovings. For example, it is higher in the direction of pultrusion process than any other direction [1]. Some typical values from mechanical properties are shown in table 1.

3

Table 1 – GFRP Properties

Properties Unit Fiber parallel

direction Fiber transverse

direction

Tension Strength MPa 200 - 400 50 - 60

Compression Strength MPa 200 - 400 70 - 140

Sheer Strength MPa 25 - 30

Elastic modulus GPa 20 - 40 5 - 9

Distortional moulus GPa 3 - 4

In the Figure 2 and 2, GFRP profiles are compared with typical construction materials, such as wood, aluminum, PVC and steel, in terms of stress – strain tensile constitutive relationship, young’s modulus and tensile strenght.

Figure 1 – Stress – strain tensile constitutive relationship [4]

Figure 2 – Comparison between tensile properties

In Figure 3, GFRP are compared with other materials in terms of specific weight, thermal conductivity and thermal expansion coefficient.

0 50 100 150 200

GFRP

PVC

ALU

STEEL

WOOD

405

65200

13

Yong's Modulus (GPa)

0 200 400

GFRP

PVC

ALU

STEEL

WOOD

40050

150400

80

Tensile Strength (MPa)

4

Figure 3 – Comparison between physical and thermal properties [4] When the GFRP mechanical properties are compared with the properties of other materials, such as wood, steel, aluminum and PVC, the conclusions of that comparison are the following:

- The relation tension-strain is a linear-elastic relation until failure, which contrast with the behavior of ductile steel;

- There is an higher tension failure when compared with structural steel; - The elastic section modulus is smaller; - The Young model is smaller; - GFRP is a lightweight material.

3. GFRP durability

3.1 Degradation Due to a lack of information about the durability of FRP, there is a gap in the knowledge of the main degradation agents of the material. Some authors identify the major environmental factors that affect the durability of FRP’s in structural applications. Some of those factors are [5]:

Moisture;

Alkaline environment;

0 5 10

GFRP

PVC

ALU

STEEL

WOOD

2,6

1,5

3

8

0,5

Specific Weight (g / cm3)

0 50 100 150

GFRP

PVC

ALU

STEEL

WOOD

0,2

0,15

150

50

0,15

Thermal condutivity(W/ºK.m)

0 20 40 60 80

GFRP

PVC

ALU

STEEL

WOOD

12

80

22

11

12

Thermal expansion Coefficient (K-1 X 10-6)

5

Temperature;

Creep;

Fatigue;

UV radiation;

Fire.

3.1.1 Moisture

The structures constructed with GFRP profiles, as any other structure, can be placed in water or water solutions. Water is one of the most aggressive elements to the construction and it is important to know how it influences the properties and durability of this material. Moisture, atmospheric or by direct contact, is considered to be one of the most important causes of long-term degradation of the composites [8]. For the structure, water solutions affect the composite by decreasing the glass transition temperature, stiffness and strength of the composite and increasing its volume [9].

3.1.2 Alkaline environment

FRP materials may have contact with alkaline environments. This contact causes fiber degradation. This environment causes the following problems for GFRP [10]:

- Tension failure decrease; - Increase of the size of the pores, causing cracks.

3.1.3 Temperature

Typically, the temperature alters the FRP properties, having a direct impact on their durability [7]. If the FRP matrix is not cured, high temperature may be beneficial because they contribute to the process of cure [7]. Materials expand when exposed to high temperature. In the case of FRP, the coefficient of thermal conductivity of the polymer is greater than the fibers. This means that there are different behaviors of the constituents of composite material, resulting in residual tensions [1, 16].

3.1.4 Creep

The creep behavior of the material depends on the orientation of the fibers, and the effects of that creep will be more or less important depending on the orientation of the applied load [2]. In the case of resins with an incomplete resin cure, especially resins cured at room temperature, the effect of creep can be quite significant and there is a tendency for small cracks to appear in the material [2].

6

3.1.5 Fatigue

The structure load can be mechanic (charge applied), chemical (humidity, oxidation), or can result of thermal variations (variation of temperature) [2].

Composite materials, especially GFRP profiles, show a higher fatigue resistance when compared with metallic materials.

3.1.6 UV radiation

Radiation from sunlight arrive the earth with a range of wavelengths larger enough to break bonds in polymeric materials [6]. If the composite material is not prepared to withstand this type of radiation, through painting or a protective coating material, the surface material will be damaged and in a long-term can expose the reinforced fiber. Tests show that after ageing caused by UV radiation, the GFRP does not present significant decrease in strength or young modulus [6].

3.1.7 Fire

The resistance to fire is of great concern to the application of the material in civil engineering. There are two concerns about the material depending on the type of use [2]:

- If they are to be used in confined spaces, such as tunnels, it is important to know the level of toxicity of gases released during a fire;

- If the composite is to be used for structural elements (bridges or buildings, for example), it is necessary to ascertain whether the reduced resistance resulting from the exposure to a fire is significant enought to cause a possible collapse.

4. Important GFRP structures

4.1 GFRP structures in Portugal

In Portugal, most of the GFRP applications are in secondary structures or in non-structural elements. Figure 4 shows an architectural structure in the Palácio Porto Hotel (right) and the catwalk in 25th April bridge (left).

7

Figure 4 – Catwalk in 25 April bridge (left) and architectural structure in Palácio Porto Hotel (right)

4.2 GFRP structures abroad

In other countries, bridges and buildings have been built in GFRP profiles, such as the Lerida Bridge in Spain (left picture), and the Bonds Mills Bridge in UK (right picture), in Figure 5. In these cases, GFRP profiles were chosen for their electromagnetic transparency and lightness.

Figure 5 – Lerida bridge (left) and Bonds Mills bridge (right)

5. Quality control of GFRP

5.1 Quality control in manufacturing

The quality control in production (resin and fiber) is very important to ensure the product’s final quality. On reception of raw material, the control parameters may change from profile to profile. During the manufacturing process some parameters must be inspected such as temperature, quantity and stacking of glass fiber and resin formulation, because the process is very delicate and the failure of some parameters causes the profile collapse.

8

5.2 Quality control before manufacturing After manufacturing the profile should be inspected. Geometric tolerances (based on EN 13706-2:2002) and various issues, such as: color, size and appearance should be checked. The Barcol test will is used to measure the level of resin cure. Tension and flexure destructive test will be carried out.

5.3 Quality control in aplication

During the application, a quality control must be carried out to guarantee the correct position of GFRP profiles. The pumping station in Olivais – Moscavide was monitored during the application. In Figure 6, the left picture shows the typical brightness of GFRP. In right picture is shown a ring in a bolt to avoid a profile crush.

Figure 6 – Catwalk and stairs in Olivais – Moscavide pump station As a result of some lack of care during the transporting and in the storage conditions, the profile has some cracks, as can be seen in Figure 7.

Figure 7 – Crack in floor

9

5.4 Quality control in service

Based on EN 13706-2:2002, some anomalies were identified. To evaluate the profile quality, the GFRP structures must be inspected, because the profile may need to be repaired or replaced. Some of the GFRP structures inspected were: Lisbon Oceanarium, Rossio Railway Station and Colombo Shopping Center. Some deficiencies are shown below.

Lisbon Oceanarium In Lisbon Oceanarium, some anomalies were found, such as the excessive deflection of the catwalk and a looseness in the bolted span, as shown in Figure 8.

Figure 8 – Catwalk in Lisbon Oceanarium

Rossio Railway Station

In Rossio Railway Station, as a result of the lack of maintenance, some profiles show fiber blooming as shown in Figure 9 (left).

The right picture of Figure 9 shows a wrong cut in the profile, because of the lack of quality control in application.

Figure 9 – Fiber blooming (left) and wrong profile cut (right) in Rossio Railway Station catwalk

10

Colombo Shopping Center

In Colombo Shopping Center GFRP composites are applied in a secondary structure, with an architecture purpose. In the left picture of Figure 10, corrosion in a bolt, due to the wrong choice of the material can be abserved. A beam collapse is shown in Figure 9 (right), possibly due to the overcharge.

Figure 10 – Bolted corrosion (left) and GFRP beam collapse (right) in Colombo Shopping Center

6. GFRP repair

Some anomalies described above can be considered for repair work, whereas only causes aesthetic problems, without affecting the structural security. In Portugal, typically, no companies carry out repairs. Whenever an anomaly is detected, the profile is replaced. However, some anomalies could be repaired, such as:

- Profile crash; - Fiber blooming; - Cracks; - Spot damage.

7. Conclusion

The quality control of profiles (during the manufacturing process, after manufacturing and application) is very low-level, due to the little relevance that GFRP structures have in Portugal. However, these profiles are an excellent choice when they are applied in aggressive environments, such as the case of the Lisbon Oceanarium where it was essential to avoid corrosion. Their light weight is significant when compared with similar materials. Consequently, the GFRP structures are never inspected nor do they have a quality control plan in its application in Portugal. During the manufacturing process there is quality control. However, companies do have work-sheets to use when profiles arrive from the factory.

11

In the application stage, due to the lack of quality control, a few anomalies were found in some structures, such as the catwalk in Rossio Railway Station, where wrong cuts in bolted connections were observed. However, before the inspection, no serious anomalies were found.

8. References

[1] Cabral-Fonseca, S., “Polymeric composite material reinforced with fibers used in Civil Engineering – Features and application” (in Portuguese), Scientific and technical information, LNEC, Lisbon, 2005. [2] Correia, J. P. R., “Pultruded glass fiber (GFRP). Application in GFRP composite beams GFRP-concrete in construction” (in Portuguese), Master thesis, Instituto Superior Técnico, March, 2004. [3] Keller, T., “Use of fiber reinforced polymers in bridge construction”, Structural Engineering Documents, Nº7, IABSE, Zurich, p. 131, 2003. [4] STEP, Catalog, Sociedade Técnica de Estruturas Pultrudidas Lda., 2010. [5] Karbhari, V.M., “Gap analysis for durability os fiber reinforced polymer composites in civil infrastructure – Chapter 1: Introduction”, CERF, 2001. [6] Ghorbel, I., Valentin, D., “Hydrotermal on the physic-chemical properties of pure glass fiber reinforced polyester and vinylester resins”, Polymer Composites, Vol. 14, No. 4, p. 324-334, 1993. [7] Pavlidou, S., Papaspyrides, C.D., “The effect of hydrothermal history on water sorption and interlaminar shear strength of glass/polyester composites with different interfacial strength”, Composites: Part A 34, p. 1117-1124, Elsevier, September, 2002. [8] Merdas, I., Thominette, F., Tcharkhtchi, A., Verdu, J., “Factors governing water absorption by composite matrice”, Composites Science and Technology, Vol. 62, p. 487-492, Elsevier, November, 2002. [9] Benmokrane, B., Faza, S., Ganga Rao, H.V.S., Karbhari. V.M., Porter, M., “Gap analysis for durability of fiber reinforced polymer composites in civil infraestructure – Chapter 4: Effects of alkaline environment”, CERF, 2001. [10] Won, J.P., Lee, S.J., Kim, Y.J., Jang, C.I., Lee, S.W., “The effect of exposure to alkaline solution and water on the strength-porosity relationship of GFRP rebar”, Composites: Part B 39, p. 764-772, Elsevier, August, 2007. [11] Braestruo, M., “Footbridge Constructed from Glass-Fiber-Reinforced Profiles, Denmark”, Structural Engineering International, Vol. 9, No. 9, p. 256-258, 1999 [12] Burgoyne, C., “Advanced Composites in Civil Engineering in Europe”, Structural Engineering International, Vol. 9, No. 9, p. 267-273, 1999.

12

[13] Keller, T., “Towards Structural Forms for Composite Fiber Materials”, Structural Engineering International, Vol. 9, No. 4, p. 297-300, 1999. [14] Sobrino, J.A., Pulido, M.D.G.,“Towards Advanced Composite Material Footbridge”, structural Engineering International, Vol. 12, No. 9, p. 84-86, 1999. [15] Keller, T., Bai, Y., Vallée, T., “Long-term Performance of a Glass Fiber-Reinforced Polymer Truss Bridge”, Journal of Composites for Construction, Vol. 11, No. 1, p. 99 – 108, ASCE, 1999. [16] Kharbari, V.M., Chin, J.W., Hunston, D., Benmokrane, B., Juska, T., Morgan, R., Lesko, J.J., Sorathia, U., Reynaud, D., “Durability Gap Analysis for Fiber Reinforced Polymer Composites in civil Infraestructure”, Journal of Composites for Construction, ASCE, Vol. 7, No. 3, p. 238-247, 2003.