Factors Affecting Durability of Fiber Reinforced Concrete (FRC)
Madeh Izat Hamakareem
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There are various factors which affects the durability of fiber reinforced concrete such as temperature, weathering, corrosion, freezing and thawing etc. are discussed.
Durability of concrete element is the ability of the member to resist aggressive environment, accidental event, and impact effect and maintain the structural integrity.
In this article, the durability of fiber reinforced concrete (FRC) will be discussed.
Factors Affecting Durability of Fiber Reinforced Concrete (FRC)
Following are the factors which affects the durability of fiber reinforced concrete:
Extreme temperature and fire
Freezing and thawing
Degradation and embrittlement due to alkali attack and bundle affect
Weathering and scaling
Effect of Extreme Temperature and Fire on Durability of FRC
Generally, concrete has a reasonable resistance to severe temperature because of its low thermal conductivity, great heat capacity, and it is not burn easily while exposed to fire.
Concrete constituents for example specific aggregate types and cement clinker are not influenced by high temperature both chemically and physically. However, there are others concrete constituents that affected by temperature changes such as hydration product. It is influenced by loss of water, micro-cracking, and damage by differential expansion.
The addition of steel fiber, synthetic fiber, or combination of both to concrete enhances structural concrete elements resistant against substantial temperature and fire.
The strength of conventional concrete is decreased considerably if it exposed to fire for long time. Cement paste and aggregate bond in concrete is damaged at a temperature of 202oC and about half of the concrete strength is decline at temperature of 427 oC, and 90% of concrete strength is lost at temperature of larger than 927 oC.
Fiber provision do not impede concrete failure under this sever condition but it increases fire exposure safe time. The extension of fire exposure safe time provides more time during which evacuations and the fire extinguishment can be proceeded safely.
It is reported that, the application of hybrid combination of steel and polypropylene fibers in precast concrete fireplace hearths produced small or not explosive spalling.
Regarding concrete spalling, when concrete exposed to fire, excess water inside concrete, which used to provide workability during construction, changes to steam pressure. If the pressure inside concrete is not released and surpass concrete tensile strength, explosive spalling will occur.
The concrete spalling depends on the amount of free water and its distribution while concrete element is exposed to fire.
The damage caused by spalling may penetrate concrete to about 6 cm.
Spalling is a serious problem because it may expose steel reinforcement to high temperature. Hence, steel reinforcement is deteriorated quickly which in return ultimate load carrying capacity of concrete member is declined.
It is demonstrated that, when concrete reinforced with polypropylene fiber exposed to high temperature, the polypropylene fiber is melted and fine capillary pores will be emptied and this lead to release the accumulated steam pressure and the concrete maintain its strength.
The provision of steel fiber increases small concrete slab fire resistance to three to nine times that of the slab with no fibers.
Finally, fibers can be added to concrete to bridge cracks and keep structure integrity of the damaged structure.
Effect of Freezing and Thawing on Durability of FRC
In this section durability of three fiber reinforced concrete namely steel, synthetic, and cellulose FRC will be explained.
It is demonstrated that, among factors such as fiber content, air content, cement content, and water to cement ratio, the air content create significant effect on the steel fiber reinforced concrete resistance against freezing and thawing.
Moreover, the reduction of SFRC modulus of rupture due to freezing and thawing is smaller than that of concrete with zero fiber.
It is recommended by Rider and Heidersbach that, mix design of SFRC that is used in marine environment, need to have water content of no greater than 0.45, cement content should be at least 519 Kg/m3, and air content ranges from 6-7.5%.
Regarding synthetic fiber reinforced concrete, it is pointed out that, not only does the synthetic fibers improves freezing and thawing resistance of synthetic FRC but also enhances concrete ability to withstand deicer scaling.
Moreover, freezing and thawing cause reduction of flexural strength of concrete reinforced with polyolefin micro-fiber by about 15% whereas plain concrete flexural strength reduced by 30%.
As far as cellulose fiber is concerned, it is found that, fiber reinforced cement board (FRCB) which is laminated material and consist of cellulose fiber, cement, silica, and water, is vulnerable to freezing and thawing deterioration due to its high porosity, hydrophilic and tabular nature of cellulose fibers, and laminated nature of the composite.
FRC Degradation and Embrittlement due to Alkali Attack and Bundle Effect
Strength of various fibers for example glass, polymeric, and natural fibers are decreased in long term because of weathering. It is substantially important to know time-dependent reduction of durability and strength of those fibers in structurally related areas.
That is why deterioration mechanism of various fibers will be explained in this section.
Glass Fiber Concrete
Reinforced concrete commonly contains alkali resistance glass fibers between 3-5% of the whole composite weight. It is reported that, the corrosion of fiber is the major degradation mechanism.
However, it is claimed that, apart from the effect of corrosion, there are other factors that influence the durability of GFRC. Added to that, in most situations, calcium hydroxide, which is a product of cement hydration, is the agent that is to blame for decreasing GFRC durability.
That is why attempts made toward the reduction of calcium hydroxide in order t improve the durability of GFRC. Calcium hydroxide can be reduced by either adding admixtures for example fly ash, ground granulated blast furnace slag, and silica fume or avoid the use of conventional Portland cement especially those types which contain calcium aluminates or sulfo aluminates.
In summary, the glass fiber reinforced concrete damage mechanisms are chemical attack, mechanical attack, delayed fracture.
Cellulose Fiber Concrete
Cycles of wetting and drying lead to degrade cellulose fiber and this degradation occur in different mechanism includes change in degree of fiber cement bonding and fiber mineralization.
In the former mechanism, hydration product transportation specifically lime within the lumen of fibers and around the fibers lead to reduce interface porosity. This could be the cause of the increase of fiber cement bond and the decline of composite ductility.
In the latter mechanism, it is claimed that, the embrittlement of fiber occur as a result of the penetration of cement hydration product into the fiber.
Lastly, the durability of cellulose fiber may be increased by Fiber impregnation with blocking agents, and water repellent Agents Sealing of the matrix pore system; Reduction of Ca(OH)2 content in the matrix; and A combination of fiber impregnation and matrix modification.
Effect of Weathering and Scaling on Durability of FRC
The deicer salt scaling, which its mechanics is still not clear, is merely affect a thin layer of exposed concrete which not exceed few centimeters. It is reported that, the present fiber and the type of fiber do not influence dicer salt scaling resistance. Moreover, it is pointed out that, steel fibers that in contact with concrete which suffered scaling, rusts.