Effects of Fly Ash on The Durability of Concrete

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Fly ash is used in concrete as an admixture as well as in cement. The effect of these fly ash on durability of concrete is discussed in this article. The use of concrete in aggressive and potentially aggressive environmental condition has been increased substantially. Concrete structures are employed to support machineries, staffs, and products of oil and gas exploration and productions. Concrete structures used to keep nuclear reactor and need to contain gases and vapors that released at high temperatures and pressure in emergency situations. In all aforementioned conditions, fly ash utilization like cementitious materials play significant role. So, studying and understanding the influence of fly ash on concrete durability is extremely important. In this article the effect of fly ash on the concrete durability will be explored.

Effects of Fly Ash on The Durability of Concrete

Following are the effects of fly ash on:

• Permeability of concrete
• Carbonation of concrete
• Durability of concrete subjected to repeated cycles of freezing and thawing
• Abrasion and erosion of fly ash concrete
• Sulfate resistance of concrete
• Alkali aggregate reactions in concrete
• The corrosion of steel reinforcement in concrete
• Concrete exposed to seawater

Fig.1: Typical Stockpile of Fly Ash

Effect of fly ash on permeability of concrete

Concrete permeability is directly related to the quantity of hydrated cementitious materials at any given time. It is claimed that, the permeability of fly ash was lower compared with the permeability of controlled concrete, after 28 days of curing. In contrary, after six months, fly ash concrete more impermeable and achieve substantial imperviousness. These differences in fly concrete at two different times might be due to pozzolanic activity of fly ash; pozzolanic reaction is low at early ages whereas it increases as the concrete fly ash is aging. Therefore, fly ash could produce better concrete durability which means better durability. Figure 2, which is provided by US department of transportation, illustrates the fly ash influence on concrete permeability.

Fig.2: Effect of Fly Ash on Concrete Permeability

Effect of fly ash on carbonation of concrete

Carbonation is a process in which carbon dioxide in air reacts with calcium hydroxide and sometimes calcium silicates and aluminates in hydrates cement and produce calcium carbonate. This process occurs in moist situations and the rate of carbonation of concrete is specified by concrete permeability, saturation degree, and quantity of calcium hydroxide ready for reaction. Carbonation could lead to decrease steel corrosion resistance. Finally, specific attention should be paid on proportions of concrete mixture, concrete cover, and period of moist curing when high amount of fly ash is used in concrete.

Influence of fly as on durability of concrete subjected to repeated cycles of freezing and thawing

Generally, air entrainment modified concrete to be frost resistant if all other criteria are met. Fly ash might lead to increase the quantity of admixtures which are necessary to obtain acceptable level of entrained air and occasionally influence entrained air stability in fresh concrete. Most of researches, which are carried out on the effect of fly ash on the durability of concrete subjected to repeated cycles of freezing and thawing, support the statement made by Larson “Fly ash has no apparent ill effects on the air voids in hardened concrete. When a proper volume of air is entrained, characteristics of the void system meet generally accepted criteria”.

Abrasion and erosion of fly ash concrete

There are various situations under which concrete is subjected to wear by scraping, attrition, sliding of cars, ice, and other objects. It is claimed that concrete resistance against abrasion is proportional to its compressive strength. Fly ash concrete with low abrasion resistant might be expected unless the concrete is adequately and thoroughly curried. It is demonstrated that, concrete with ASTM class F fly ash provide better abrasion resistance compare with those contained ASTM class C or not fly ash content. Concrete is likely to be eroded when water flow over its surfaces. At fixed slump value, concrete resistance against erosion can be enhanced with increasing strength and cement content.

Fly ash effect on sulfate resistance of concrete

It is reported by Dikeou, who carried out a research on the effect of fly ash on concrete sulfate resistance, that sulfate resistance of concrete can substantially be improved by using fly ash. The Concrete Manual, which is published by the U.S. Bureau of Reclamation, gives options for cementitious materials for producing sulfate-resistant concretes.

Effect of fly ash on alkali aggregate reactions in concrete

Fly ash is proven to be substantially influential in decreasing the detrimental effect of alkali aggregate reactions (AARs). This advantageous effect of fly ash is reduced or limited to reactions in which siliceous aggregates is involved. Additionally, alkali aggregate carbonation which is one type of AAR is approximately not responding to fly ash inclusion. However, the expansion that is resulted from alkali aggregate reactions can be decreased effectively when low calcium fly ash replaces about 25-30 percent concrete and under the condition that alkali content is less than four percent. The effect of high calcium fly ash is not focused on very much that is why there are no enough information about its utilization but there are some indications that effective replacement level may be higher compare with low calcium fly ash. Figure-3 shows and illustrates alkali aggregate reaction development.

Fig.3: Progresses of Alkali Aggregate Reactions in Concrete

Effect of fly ash on the corrosion of steel reinforcement in concrete

There is a concern about corrosion of steel reinforcement in fly ash concrete that is resulted from the effect of chloride ions from seawater or deicing. When carbonation depth in concrete reach steel reinforcement, the passivity could be reduced and consequently the steel bar will corrode if enough oxygen and moisture reach the surface of the reinforcements. Nonetheless, adequate protection can be obtained against corrosion if the fly ash concrete cover is sufficiently thick and impermeable.

Fly ash effect on concrete exposed to seawater

Concrete exposed to marine environment is seriously attacked and subjected to several extremely aggressive factors for example altering wetting and dying, action of waves, abrasion by debris and sand, freezing and thawing cycles, and reinforcement corrosion, these are occurred in chemical medium. However, entirely submerged concrete are less affected by these factors. It is shown that, fly ash concrete with 25 percent replacement by mass and under the condition of water to cementitious materials of less than 0.50, exhibit satisfactory performance under freezing and thawing and wetting and dying conditions. Read More: Effects of Fly Ash on Properties of Hardened Concrete