Environmental factors affecting GFRP products

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Chapter 2 LITERATURE REVIEW

General:

A major hindrance in using GFRP reinforcing bars in civil engineering applications is the susceptibility of their behavior to weathering conditions. The research done to date has shown that GFRP is prone to degradation when exposed to different environmental conditions. The scope of this literature review encompasses a brief overview of fibers and matrices used in FRP reinforcing bars and the various environmental conditions that cause the degradation.

Materials:

Fiber Reinforced Polymers are made up of fibers and matrix, where the fibers provide strength and stiffness while matrix holds the fibers together, protects them from abrasion and corrosion and also transfers stresses to the fibers.
Fibers:

Fiber Reinforced Polymer reinforcing bars are typically made up of one of three types of fibers –glass, carbon and aramid. Glass fibers are the most popular of all the fibers used for reinforcement due to their relatively lower costs. Table 2.1 presents some mechanical properties of the reinforcing bars made up of different fibers as compared to steel

Carbon Fibers:

Among all the fibers carbon fibers have the highest strength and stiffness. They are quite popular in the aerospace industry. But the high cost creates a major hindrance to their use in civil engineering applications.

Aramid Fibers:

Aramid Fibers are aromatic polyamide called poly paraphenylene terephthalamide (PPD-T). They have high strength and are rigid due to their chain alignment but fracture in a ductile manner.

Glass Fibers:

Glass fibers are of different types such as E-Glass, S-2 Glass, AR- Glass, A-Glass, C-Glass, D-glass, R-Glass and ECR-Glass depending on their properties and chemical composition [13]. Of the different types of glass fibers, E-Glass is mostly used for reinforcement due to its high strength and electrical resistivity. Glass fibers have high strength and temperature resistance, but it is the low cost that makes GFRP the most popular FRP reinforcement in civil engineering applications. However, glass fibers corrode in acidic as well as alkali environments and lose a significant percentage of their tensile strength when exposed to high temperature. Below is a brief review of work done by different researchers to study the strength degradation of different types of glass fibers when exposed to aggressive environment.
Hartman et al. (1994) observed that E-Glass fibers lose more strength than S-2 Glass fibers when exposed at 96°C to acidic environment (H2SO4 and HCl), alkali environment (Na2SO4) and water for a period of 24hrs. and 168 hrs.
Tannous and Saadatmanesh (1999) did a study on the durability of AR glass FRP bars when exposed to aggressive environment. They observed that for No.3 bars (vinyl ester matrix) when exposed to Ca (OH)2 with a pH of 12 and maintained at 25°C and 60°C, experienced strength loss of 13% and 23% respectively.
According to Fuji et al. (1993) there was a reduction of tensile strength to about 28% when E-Glass fibers were exposed to 5% HNO3 after 100 hrs.

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Matrix:

Polymeric matrices are of two types, thermosetting resins and thermoplastic resins. Vinyl ester, polyester and epoxy are all thermosetting resins while polysulfone, polycarbonate and polyphenylene oxide are all thermoplastic resins. The type of matrix used and also its toughness play an important role in the characteristics such as matrix cracking and debonding of the fiber/matrix interface as shown by Fuji et al.(1993)

Thermoplastic Resins:

The different types of resins mentioned above for thermoplastic resins are more prevalent in the aerospace industry. They have higher viscosity than thermosetting resins and they may be crystalline in nature. These resins are tough, less brittle and are used mostly with discontinuous fibers.

Thermosetting resins:

Among all the thermosetting resins mentioned above vinyl ester and polyester are used more in construction industry. Vinyl ester though costlier than polyester resin, has a higher tensile strength. Thermosetting resins are semi-permeable thus allowing water to pass through it but not the alkali ions (Dejke, 2001). However, alkali penetration was detected in the test results on GFRP done by Dejke (2001) after about 6 months of exposure at 60°C in alkaline solution. Below is a brief review of the study performed on the strength properties of vinyl ester and polyester resin after accelerated ageing.
Chin et al. (1997) observed that when vinyl ester and polyester were exposed to water, salt water and cement pore water at temperatures 23°C, 60°C and 90°C there was not much change in the glass transition temperature (Tg) but there was considerable change in their tensile strengths. The change in tensile strength of polyester resin was so much that they could not be tested after 10 weeks at 90°C as they were degraded [16].
Bakis et al. (1998) studied E-Glass fiber reinforced plastic composite reinforcement rods made with different proportions of resins-100% vinyl ester, 50% vinyl ester and 50% iso-polyester, 20% vinyl ester and 80% iso-polyester following accelerated ageing. They observed that rods made up of 100%vinyl ester had the smallest reduction in modulus of elasticity and the least degradation in tensile strength as compared to the rods made with the other proportions [17].
Gangarao and Vijay (1997) did a study on accelerated aging of GFRP bars with five different types of resins and under aging conditions like high pH, sustained stress as well as humidity and temperature variations. It was observed that the bars consisting of vinyl ester resin showed the least degradation of tensile strength.

 Manufacturing Process:

GFRP Reinforcing bars are manufactured by modified pultrusion process. The molding process used for the fabrication of fibers and thermosetting resins is called a pultrusion process and is used in the fabrication of composites having constant cross-sectional profile. The manufacturing process is as shown in figure 2.3 above, initially the glass fibers are passed through a resin bath containing thermosetting resin, where the fibers are thoroughly coated with the resin. After impregnating the fibers with resins, they are passed through a heated dye where they are given desired shape, after which they are helically wrapped and later sand-coated. The next step is to pass the composite through a curing tunnel, which is maintained at high temperature. After which they are passed through a pulling machine and later cut to desired sizes.
The higher the glass content the better is the end product. According to Gremel (1999) the maximum achievable and ideal ratio is about 75%of glass content and 25% resin.

Applications:

GFRP bars should be considered in the following cases:

  • In a concrete member where the reinforcement is exposed to chloride ion or chemical solution
  • In a concrete member exposed to electromagnetic waves
  • In zecondary load bearing member.
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Environmental factors affecting GFRP products:

The use of any material in structural applications needs a detailed study of the effect of environmental conditions on its properties. The environmental factors that cause the degradation of GFRP reinforcing bars and sheets are temperature, moisture, alkalinity, freeze-thaw, ultraviolet rays and others. Considering the environmental effect on the degradation of FRP, ACI 440 has recommended environmental reduction factors for different fibers depending on their exposure condition. The environmental factors for GFRP are 0.7-0.8 as per their exposure conditions. In this study, the evaluation of GFRP will be done only for conditions of temperature, moisture and alkalinity.

Chapter 1 Introduction 
1.1 Background
1.2 Objectives
1.2.1 Primary Goal
1.2.2 Secondary Goals
Chapter 2 Literature Review 
2.1 General
2.2 Materials
2.2.1 Fibers
2.2.1.1 Carbon Fibers
2.2.1.2 Aramid Fibers
2.2.1.3 Glass Fibers
2.2.2 Matrix
2.2.2.1Thermoplastic Resins
2.2.2.2 Thermosetting resins
2.3 Manufacturing Process
2.4 Applications
2.5 Environmental factors affecting GFRP products
2.5.1 Moisture and Temperature
2.5.3 Alkalinity
2.6 Moisture Absorption property of GFRP bars
2.6 Summary
Chapter 3 Experimental Methodology 
3.1 Introduction
3.2 Experimental Setup
3.2.1 Conditioning Tanks
3.2.2 GFRP Reinforcing Bar Specimens
3.2.2.1 Tensile Strength Specimen
3.2.2.2 Tensile Tests
3.2.2.3 Moisture Absorption study
Chapter 4 Results 
4.1 Introduction
4.2 Plain Specimens
4.3 Conditioned and Incased Specimens
4.3.1 Tensile Strength
4.3.2 Modulus of Elasticity
4.4 Unconditioned but Incased Specimens
4.5 Stress–Strain Behavior
4.6 Moisture Content
Chapter 5 Prediction of Strength Retention 
5.1 Introduction
5.2 Fickian Model
5.2.1 Conclusion
5.3 Time Shift Method
5.3.1 Conclusion
Chapter 6 Conclusion and Recommendations 
6.1 Introduction
6.2 Conclusion
6.3 Recommendations
References 
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