Introduction to Fly ash and Classification of Fly ash and Properties of Fly ash

Introduction to Fly ash and the Durability of Fly ash ?
       

Pulverized fuel ash commonly known as Fly ash, is comprised of the non-combustible mineral portion of coal. When coal is consumed in a power plant, it is first ground to the fineness of powder. Blown into the power plant’s boiler, the carbon is consumed — leaving molten particles rich in silica, alumina and calcium. These particles solidify as microscopic, glassy spheres that are collected from the power plant’s exhaust before they can “fly” away — hence the product’s name: Fly Ash.

Fly ash is a pozzolanic When mixed with lime (calcium hydroxide), pozzolanics combine to form cementitious compounds. Concrete containing fly ash becomes stronger, more durable, and more resistant to chemical attack.

Classification of Fly ash:

  ASTM C618 classifies the fly ash into two classes:

·          Class F

·          Class C

·         Class F Fly ash:

Fly ash normally produced from burning anthracite or bituminous coal falls in this category. This class of fly ash exhibits pozzolanic property but rarely if any, self-hardening property. In Class F fly ash, total calcium typically ranges from 1 to 12 percent, mostly in the form of calcium hydroxide, calcium sulphate, and glassy components in combination with silica and alumina.

·         Class C Fly ash:

Fly ash normally produced from lignite or sub- bituminous coal is the only material included in this category. This class of fly ash has both pozzolanic and varying degree of self cementitious properties. Most class C fly ashes contain more than 15 % CaO. But some class C fly ashes may contain as little as 10 % CaO.

NOTE: The chief difference between Class F and Class C fly ash is in the amount of calcium and the silica, alumina, and iron content in the ash.The chemical properties of the fly ash are largely influenced by the chemical content of the coal burned (i.e. bituminous, sub-bituminous, lignite). 

Quality of Fly ash:

The quality of Fly ash is governed by IS 3812-2003-Part1&2.In this Standard Pulverized Fuel Ash (PFA i.e., fly ash, bottom ash, pond ash or mound ash) has been divided into two categories:

·      Siliceous PFA normally produced by burning anthracite or bituminous coal and normally contains reactive CaO less than 10% by mass.

·         Calcareous PFA normally produced by burning lignite or sub-bituminous coal and reactive CaO not less than 10% by mass.

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Also check: Hot mix asphalt pavement technology

Hot mix asphalt pavement technology and overview on rigid pavements and flexible pavements

PAVEMENT DESIGN AND TYPES OF PAVEMENTS 

Definition of Asphalt:

Asphalt concrete is a composite material commonly used in construction projects such as road surfaces, parking lots, and airports. Asphalt concrete consists of asphalt (used as a binder) mixed with mineral aggregate and then laid down in layers and compacted.

Asphalt

What is Asphalt pavement ?

Asphalt pavement refers to any paved road surfaced with asphalt. Hot Mix Asphalt (HMA) is a combination of approximately 95% stone, sand, or gravel bound together by asphalt cement, a product of crude oil. Asphalt cement is heated aggregate, combined, and mixed with the aggregate at an HMA facility. The resulting Hot Mix Asphalt is loaded into trucks for transport to the paving site. The trucks dump the Hot Mix Asphalt into hoppers located at the front of paving machines. The asphalt is placed, and then compacted using a heavy roller, which is driven over the asphalt. Traffic is generally permitted on the pavement as soon as the pavement has cooled. 

Asphalt paving temperature:

Ambient (air) temperatures, base (aggregate and existing asphalt) temperatures, and hot mixed asphalt temperatures are very critical to obtaining compaction and longevity of the newly paved surfaces and patches.

Hot mixed asphalt is manufactured at temperatures between 270°F and 325°F. Depending on the environmental conditions and the distance from the hot mix plant to the project, hot mix asphalt can lose between 5°F and 25°F.

The temperature of the mix on the base after it has passed through the laydown machine – not the mix or manufacturing temperature – is the most important factor in determining the available time for compaction. Hot mix asphalt pavement arrives at a project at temperatures between 275°F and 300°F and is installed on the existing base by mechanical methods (laydown machines).

If the air and base temperatures are colder than required or specified, then the asphalt pavement will cool too fast, causing it to set up and making it very difficult to obtain the required or specified compacted density.

Thin pavement layers will cool quicker than thicker layers, and should the base or ambient temperature be low, the hot mix asphalt will cool quicker, density won’t be achieved, and the patch will ravel and fall apart. On paving and overlay projects, if the hot mix asphalt pavement cools too quickly, the entire surface will ravel leaving a rough, rocky surface in a short period of time. This not only results in a poor surface, it results in a surface that will retain water, reducing the life of the pavement or the patch by accelerating the raveling process. It is very important to monitor all temperatures (ambient, base, and hot mix asphalt) and wind velocity during the paving process.

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