Key Factors Affecting Durability of Fiber Reinforced Concrete (FRC)

This article focuses on the durability of fiber reinforced concrete (FRC) and its resistance to various factors that can affect its longevity. These factors include temperature variations, weathering, corrosion, and freezing and thawing. Durability refers to the ability of a concrete element to withstand harsh environmental conditions, accidental events, and impact forces while maintaining its structural integrity.

What Factors are Affecting Durability of Fiber Reinforced Concrete (FRC)

The durability of fiber reinforced concrete is influenced by several factors. Extreme temperatures and exposure to fire can significantly impact its longevity. Freezing and thawing cycles can cause damage to the concrete structure over time. Additionally, degradation and embrittlement can occur due to alkali attack and bundle effect, leading to reduced durability. Weathering and scaling also play a role in affecting the longevity of fiber reinforced concrete. Lastly, corrosion resistance is another important factor to consider in order to ensure the durability of the concrete.

1. Extreme Temperature and Fire Effect on Durability of FRC

Concrete exhibits reasonable resistance to severe temperatures due to its low thermal conductivity, high heat capacity, and non-combustible nature when exposed to fire. While certain concrete constituents like specific aggregate types and cement clinker remain unaffected by high temperatures chemically and physically, other constituents, such as hydration products, are influenced by temperature changes, leading to water loss, micro-cracking, and damage from differential expansion.

Enhancing structural elements of concrete with steel fibers, synthetic fibers, or a combination of both can improve resistance to substantial temperatures and fire. However, prolonged exposure to fire significantly reduces the strength of conventional concrete. Cement paste and the bond between aggregates are damaged at temperatures around 202°C, resulting in a 50% strength decline at 427°C and a 90% strength loss at temperatures above 927°C.

While fiber reinforcement does not prevent concrete failure under such severe conditions, it does increase the time the concrete can withstand fire exposure, allowing for safer evacuations and firefighting efforts. By using a hybrid combination of steel and polypropylene fibers in precast concrete fireplace hearths, the occurrence of spalling, which is the explosive disintegration of concrete, can be minimized or eliminated. Spalling is caused by the conversion of excess water within the concrete into steam pressure, which, if not adequately released and exceeds the tensile strength of the concrete, leads to explosive spalling.

The severity of spalling depends on the amount and distribution of free water within the concrete when exposed to fire, and it can penetrate the concrete up to a depth of approximately 6 cm. Spalling is a critical issue as it exposes the steel reinforcement to high temperatures, causing rapid deterioration and a decrease in the ultimate load-carrying capacity of the concrete member.

However, when concrete reinforced with polypropylene fibers is exposed to high temperatures, the fibers melt, emptying fine capillary pores and effectively releasing accumulated steam pressure, thereby preserving the concrete’s strength. The inclusion of steel fibers significantly enhances the fire resistance of small concrete slabs by three to nine times compared to slabs without fibers. Ultimately, the addition of fibers to concrete can help bridge cracks and maintain the structural integrity of damaged structures.

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2. Freezing and Thawing Effect on Durability of FRC

In this section, we will discuss the durability of three types of fiber reinforced concrete: steel, synthetic, and cellulose FRC. It has been observed that various factors such as fiber content, air content, cement content, and water to cement ratio play a crucial role in determining the durability of steel fiber reinforced concrete against freezing and thawing. Among these factors, air content has been found to have a significant impact. Additionally, when compared to concrete without fibers, the reduction in modulus of rupture (a measure of the material’s ability to resist bending) is smaller in steel fiber reinforced concrete subjected to freezing and thawing.

Rider and Heidersbach recommend specific mix design requirements for steel fiber reinforced concrete used in marine environments. These include a maximum water content of 0.45, a minimum cement content of 519 Kg/m3, and an air content ranging from 6-7.5%.

In the case of synthetic fiber reinforced concrete, it has been noted that synthetic fibers not only improve the resistance to freezing and thawing but also enhance the concrete’s ability to withstand deicer scaling. Furthermore, freezing and thawing result in a reduction of flexural strength (the ability to withstand bending) in concrete reinforced with polyolefin micro-fibers by approximately 15%, whereas plain concrete experiences a larger reduction of 30%.

Cellulose fiber, on the other hand, presents certain challenges in terms of durability. Fiber reinforced cement board (FRCB), which is a laminated material consisting of cellulose fiber, cement, silica, and water, is particularly susceptible to deterioration caused by freezing and thawing. This vulnerability can be attributed to the high porosity, hydrophilic nature, tabular shape of cellulose fibers, and the laminated structure of the composite.

In summary, the durability of fiber reinforced concrete varies depending on the type of fiber used. Steel fiber reinforced concrete is influenced significantly by air content in its resistance to freezing and thawing. Synthetic fiber reinforced concrete improves both freezing and thawing resistance and the ability to withstand deicer scaling. Cellulose fiber reinforced cement board faces challenges due to its porosity, hydrophilic nature, tabular fiber shape, and laminated structure when exposed to freezing and thawing conditions.

Read Also: What Factors are Affecting Rheological Properties of Fresh Concrete

3. Understanding Alkali Attack: Mitigating FRC Degradation and Embrittlement

Understanding the time-dependent reduction of durability and strength in various fibers is crucial for structurally related areas, as the strength of fibers such as glass, polymeric, and natural fibers tends to decrease over the long term due to weathering. In this section, we will delve into the deterioration mechanism of these fibers to shed light on the factors contributing to their weakening over time.

Cellulose Fiber Concrete

The degradation of cellulose fiber can occur due to repeated cycles of wetting and drying. This degradation involves various mechanisms, including changes in the degree of fiber cement bonding and fiber mineralization. In the former mechanism, the transportation of hydration products, particularly lime, within the lumen of fibers and around them, results in a reduction of interface porosity. Consequently, there is an increase in fiber cement bond strength while the composite’s ductility declines. The latter mechanism suggests that the embrittlement of fibers happens when cement hydration products penetrate the fiber structure. This penetration causes the fibers to become brittle.

To enhance the durability of cellulose fiber, certain measures can be taken. One approach involves impregnating the fiber with blocking agents and water repellent agents. This helps seal the pore system within the matrix, preventing the ingress of water and other harmful substances. Additionally, reducing the content of calcium hydroxide (Ca(OH)2) in the matrix contributes to improved durability. Furthermore, a combination of fiber impregnation and modification of the matrix can also enhance the overall durability of cellulose fiber. By implementing these strategies, the lifespan and performance of cellulose fiber-based materials can be significantly increased.

Glass Fiber Concrete

Reinforced concrete commonly incorporates alkali-resistant glass fibers, typically comprising 3-5% of the total composite weight. The degradation of these fibers primarily occurs due to corrosion. However, besides corrosion, there are other factors that significantly impact the durability of Glass Fiber Reinforced Concrete (GFRC).

One prominent factor is the presence of calcium hydroxide, a byproduct of cement hydration, which is known to decrease the durability of GFRC. Therefore, efforts have been made to reduce the amount of calcium hydroxide in order to enhance the durability of GFRC.

This reduction can be achieved by incorporating admixtures such as fly ash, ground granulated blast furnace slag, and silica fume, or by avoiding the use of conventional Portland cement, especially types that contain calcium aluminates or sulfo aluminates. In summary, the damage mechanisms affecting glass fiber reinforced concrete include chemical attack, mechanical attack, and delayed fracture.

Read Also: Effect of Water Impurities on Concrete Strength, Durability and Other Properties

4. Effect of Weathering and Scaling on Durability of FRC

The phenomenon of deicer salt scaling, although its mechanics remain unclear, primarily affects a thin layer of exposed concrete, typically no more than a few centimeters in depth. Surprisingly, it has been observed that the presence and type of fiber in the concrete do not have any significant impact on the resistance to deicer salt scaling. Additionally, it has been noted that steel fibers, when in contact with concrete that has experienced scaling, tend to rust. This suggests a potential correlation between the occurrence of scaling and the rusting of steel fibers within the concrete.

5. Corrosion Resistance of Fiber Reinforced Concrete

Fiber Reinforced Concrete (FRC) differs from regular reinforced concrete beams as it is dispersed within the concrete, with some fibers located near or at the surface. This positioning makes the fibers vulnerable to corrosion if they are not adequately protected by the surrounding concrete. The corrosion of these exposed fibers can be triggered by various factors, such as chloride-induced corrosion or a decrease in the pH level of the concrete mixture.

Research has shown that low carbon steel and galvanized steel fibers are resistant to corrosion even when exposed to chloride concentrations exceeding 2 percent by weight. Additionally, even in significantly higher chloride ion concentrations, melt-extracted fibers have been found to remain corrosion-free. These findings indicate that certain types of fibers have a greater capacity to withstand corrosion under challenging conditions.

FAQs about Fiber Reinforced Concrete (FRC)