Characteristics Of Aggregates That Are Desirable In The Production Of Concrete
What Are The Characteristics Of Aggregates That Are Desirable In The Production Of Concrete?
The selection of aggregates is a critical process in the production of concrete. The properties of aggregates strongly influence concrete’s freshly mixed and hardened properties, mixture proportions, and economy.
Consequently, it is important to choose aggregates that possess the desired characteristics. There are a number of characteristics that are considered when choosing aggregates, including grading, durability, particle shape, surface texture, abrasion and skid resistance, unit weights and voids, and absorption and surface moisture.
Grading.
Grading is the process of determining the particle-size distribution of aggregate. Grading limits and maximum aggregate size are defined because they influence the quantity of aggregate used, as well as the cement and water needs, workability, pumpability, and durability of concrete.
In general, provided the water-cement ratio is selected appropriately, a broad range of grading may be employed without affecting strength much. When gap-graded aggregate is provided, some aggregate particle sizes are removed from the size continuum.
Gap-graded aggregate is used in exposed aggregate concrete to provide homogeneous texturing. To avoid segregation, mixed proportions must be tightly controlled.
Shape And Size Matter.
The characteristics of newly mixed concrete are influenced more by particle form and surface texture than the qualities of cured concrete.
Rough-textured, angular, and elongated particles require more water than smooth, rounded compact aggregate to form workable concrete. As a result, the cement content must be increased in order to preserve the water-cement ratio.
Flat and elongated particles are often avoided or limited to roughly 15% of the total aggregate weight. The volume that graded aggregate and the spaces between them will occupy in concrete is measured in unit weight.
Voids And Unit Weights.
The amount of cement paste required for the mix is affected by the void content between particles. The void content is increased via angular aggregates. Increased grading and larger sizes of well-graded aggregate reduce void content.
Because aggregate’s interior structure is made up of solid material and spaces that may or may not hold water, absorption and surface moisture are monitored while selecting aggregate.
The amount of water in the concrete mixture must be changed to account for the aggregate’s moisture conditions.
Abrasion And Skid Resistance.
When an aggregate is used in concrete that is frequently subjected to abrasions, such as heavy-duty floors or pavements, abrasion and skid resistance are critical.
Different aggregate minerals wear and polish at different rates. In very abrasive situations, the harder aggregate might be chosen to reduce wear.
What Are The Typical Deleterious Substances In Aggregates That Affect Portland Cement Concrete Discuss These Effects Briefly?
Deleterious materials in aggregate are those that have a negative impact on the fresh and hardened qualities of concrete, such as strength, workability, and long-term performance of the concrete in which they are employed.
Organic Impurities.
Organic contaminants disrupt the hydration reaction. It is frequently found in sand and is composed of the byproducts of the breakdown of vegetative stuff. Washing can remove organic materials from sand. This strength must be compared to the strength of mortar containing washed sand.
Clay.
Clay may cover the surface of aggregates, weakening the connection between aggregate and cement paste. As a result, controlling the quantity of clay in aggregate is vital to improve the strength and durability of concrete.
Because no test exists to identify the clay content individually, fine material limitations are established in terms of the percentage of the material passing the sieve.
Crusher Dust And Silt.
Because of their fineness, silt and dust increase the surface area and hence the quantity of water required to moisten all of the particles in the mix.
Controlling the quantity of silt and fine particles in aggregate is essential.
Because no test exists to distinguish silt and dust, fine material limitations are given in terms of the percentage of the material passing sieve.
Alkali- Aggregate Reactions.
The “alkali-aggregate reaction” refers to the interaction between alkali from cement and silica and carbonate from aggregate.
The most typical reaction is that between the aggregate’s active silica components and the alkalis in cement, known as the “alkali-silica reaction.”
Another form of alkali-aggregate reaction is the “alkali-carbonate reaction,” which occurs between dolomitic limestone aggregates containing carbonate and alkalis in cement.
Both types of reactions produce concrete degradation, primarily cracking.
The reactive forms of silica opal (amorphous, i.e. shapeless), Chalcedony (cryptocrystalline fibrous), and tridymite (crystalline).
The production of gel on the surface of aggregate particles breaks the link between the aggregate and the cement paste.
The expanding nature of the gel creates internal pressure, which finally causes the hydrated cement paste to expand, split, and rupture.
In the case of the alkali-carbonate reaction, the gel is generated, which causes the concrete to expand as it swells.
Gel forms around the active aggregate particles, producing breaking inside the rims and eventually resulting in a network of fractures and a lack of a link between the aggregate and the cement paste.
Salts.
Salts can be found in sand from the coast, sand and coarse material dredged from the sea or a river estuary, and desert sand.
Salts passing through aggregates induce reinforcement corrosion as well as moisture absorption and efflorescence.
Unsound Particles.
Materials that lose their integrity and chemicals that expand violently when frozen or exposed to water are two prominent types of unsound particles.
Unsound particles include shale, low-density particles, clay lumps, wood, coal, mica, gypsum, and iron pyrites.
Unsound particles, if present in high amounts (more than 2 to 5% of the aggregate mass), may reduce the strength of concrete. These materials should not be permitted in abrasion-prone concrete.
