Use of Fly Ash in Concrete

Use of Fly Ash in Concrete

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Fly ash is used as a supplementary cementitious material  in the production of portland cement concrete. A supplementary cementitious material, when used in conjunction with portland cement, contributes to the properties of the hardened concrete through hydraulic or pozzolanic activity, or both.

Fly ash,

a powder resembling cement,

has been used in concrete since the 1930s

Fly ash has been used in concrete at levels ranging from 15% to 25% by mass of the cementitious material component. The actual amount used varies widely depending on the application, the properties of the fly ash, specification limits, and the geographic location and climate. Higher levels (30% to 50%) have been used in massive structures (for example, foundations and dams) to control temperature rise.

Increasing the amount of fly ash in concrete is not without shortcomings. At high levels problems may be encountered with extended set times and slow strength development, leading to low early-age strengths and delays in the rate of construction.

For any given situation there will be an optimum amount of fly ash that can be used in a concrete mixture which will maximize the technical, environmental, and economic benefits of fly ash use without significantly impacting the rate of construction or impairing the longterm performance of the finished product.

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Concrete Admixtures: Corrosion-Inhibiting

 concrete admixtures: 

corrosion-inhibiting

Corrosion-inhibiting admixtures fall into the specialty admixture category and are used to slow corrosion of reinforcing steel in concrete. Corrosion inhibitors can significantly reduce maintenance costs of reinforced concrete structures throughout a typical service life of 30 – 40 years. Other specialty admixtures include shrinkage-reducing admixtures and alkali-silica reactivity inhibitors. Corrosion-inhibiting admixtures have little effect on strength at later ages but may accelerate early strength development. Calcium nitrite based corrosion inhibitors do accelerate the setting times of concretes over a range of curing temperatures unless they are formulated with a set retarder to offset the accelerating effect.

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Concrete Admixtures: Super plasticizers

concrete admixtures: 

super plasticizers

The main purpose of using super plasticizers is to produce flowing concrete with a high slump in the range of 7-9 inches to be used in heavily reinforced structures and in placements where adequate consolidation by vibration cannot be readily achieved. The other major application is the production of high-strength concrete at w/c's ranging from 0.3 to 0.4. It has been found that for most types of cement, super plasticizer improves the workability of concrete. One problem associated with using a high range water reducer in concrete is slump loss. High workability concrete containing super plasticizer can be made with a high freeze-thaw resistance, but air content must be increased relative to concrete without super plasticizer.

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Concrete Admixtures: Shrinkage Reducing

concrete admixtures: 

Shrinkage Reducing

Shrinkage reducing concrete admixtures are added to concrete during initial mixing. This type of admixture could reduce early and long-term drying shrinkage. Shrinkage reducing admixtures can be used in situations where shrinkage crackingcould lead to durability problems or where large numbers of shrinkage joints are undesirable for economic or technical reasons. Shrinkage reducing admixtures can, in some cases, reduce strength development both at early and later ages.

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Concrete Admixtures: Accelerating

concrete admixtures: 

accelerating

Accelerating concrete admixtures are used to increase the rate of concrete strength development, or to reduce concrete setting time. Calcium chloride could be named as the most common accelerator component; however, it could promote corrosion activity of steel reinforcement. Nonetheless, concrete best practices such as proper consolidation, adequate cover and proper concrete mix design could prevent these corrosion issues. Accelerating admixtures are especially useful for modifying the properties of concrete in cold weather.

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Water-Reducing Concrete Admixtures

concrete admixtures:

Water-Reducing Concrete Admixtures

Water-reducing admixtures are chemical products that when added to concrete can create a desired slump at a lower water-cement ratio than what it is normally designed. Water-reducing admixtures are used to obtain specific concrete strength using lower cement content. ​Lower cement contents result in lower CO2 emissions and energy usage per volume of concrete produced. With this type of admixture, concrete properties are improved and help place concrete under difficult conditions. Water reducers have been used primarily in bridge decks, low-slump concrete overlays, and patching concrete. Recent advancements in admixture technology have led to the development of mid-range water reducers.

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Concrete Admixtures: Set-Retarding

concrete admixtures: 

Set-Retarding

Set retarding concrete admixtures are used to delay the chemical reaction that takes place when the concrete starts the setting process. These types of concrete admixtures are commonly used to reduce the effect of high temperatures that could produce a faster initial setting of concrete. Set retarding admixtures are used in concrete pavement construction, allowing more time for finishing ​concrete pavements, reducing additional costs to place a new concrete batch plant on the job site and helps eliminate cold joints in concrete. Retarders can also be used to resist cracking due to form deflection that can occur when horizontal slabs are placed in sections. Most retarders also function as water reducers and may entrain some air in concrete.

Concrete Admixtures: Air-Entrainment

concrete admixtures : 

Air-Entrainment

Air entrained concrete can increase the freeze-thaw durability of concrete. This type of admixture produces a more workable concrete than non-entrained concrete while reducing bleeding and segregation of fresh concrete. Improved resistance of concrete to severe frost action or freeze/thaw cycles. Other benefits from this admixture are:

• High resistance to cycles of wetting and drying
• High degree of workability
• High degree of durability

The entrained air bubbles act as a physical buffer against the cracking caused by the stresses due to water volume augmentation in freezing temperatures. Air entrainers are compatible with almost all the concrete admixtures. Typically for every 1% of entrained air, compressive strength will be reduced by about 5%.

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Cement hydration

By the process of hydration (reaction with water) Portland cement mixed with sand, gravel and water produces the synthetic rock we call concrete. Concrete is as essential a part of the modern world as are electricity or computers.

Clinker is anhydrous (without water) having come from a hot kiln. Cement powder is also anhydrous if we ignore the small amount of water in any gypsum added at the clinker grinding stage.

The reaction with water is termed "hydration". This involves many different reactions, often occurring at the same time. As the reactions proceed, the products of the hydration process gradually bond together the individual sand and gravel particles, and other components of the concrete, to form a solid mass.

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