The most frequent aggregate size used in construction is 20 mm. In mass concrete, a greater size, 40 mm, is more typical. Larger aggregate sizes need less cement and water. Smaller aggregates are more reflective; they can be used to improve the appearance of dry mixes and of finished surfaces.
The other common aggregates used in construction are 50 mm and 90 mm. These sizes are used more often than 20 mm because they are easier to handle when pouring large or multiple concretes. Larger quantities of smaller aggregates can also be used to reduce the amount of cement required for a given project.
The type of aggregate used affects the strength and other properties of the concrete. For example, the density of concrete containing coarse aggregate is about 95 percent higher than that of plain concrete. Coarse aggregate makes up most of the weight of pavement, so this difference adds up quickly. As another example, concrete with sharp objects in it needs more time to cure than plain concrete, so it must go into a mold first to let the air out and allow any voids to be filled in before adding more ingredients. Finally, concrete containing steel fibers has better tensile strength and rigidity than ordinary concrete because the fibers act as reinforcements where they are placed.
The type of aggregate used in construction also affects how the concrete is made.
Fine aggregates are often sand or crushed stone with a diameter of less than 9.55mm. This is called the "agglomeration effect". The larger the particle, the longer it can be expected to stay in suspension in water.
Cement is the common name for calcium carbonate, which is the main ingredient in limestone. When calcium carbonate particles are mixed with water, calcium hydroxide forms immediately, causing the particles to clump together. As more calcium hydroxide is added, more calcium carbonate will become coated in a shell of hardened calcium hydroxide. Cement production involves heating limestone powder with sodium carbonate (saltwater) in a kiln. The heat decomposes the salt inside the limestone, leaving only pure calcium carbonate. This process yields 3 types of cement: gray, white, and red.
The amount of cement needed depends on several factors such as the type of mix required and the fineness of the aggregate. Limestone has a high calcium content (12%) so more cement is needed compared to other materials like soil (4-5%). Calcium oxide molecules from the cement react with water to form calcium hydroxide that causes the sand or gravel to agglomerate into a mass.
Why are aggregates used in concrete? Aggregates account for 60–80 percent of the volume and 70–85 percent of the mass of concrete. Aggregate is also critical for concrete strength, thermal and elastic characteristics, dimensional stability, and volume stability. Shrinkage is more likely to harm cement. Concrete that has high aggregate content tends to be stronger, while concrete with low aggregate content is more flexible.
The three main types of aggregate are sand, gravel, and crushed rock. Sand is silicon dioxide (silica) with small amounts of other elements such as iron, magnesium, aluminum, calcium, and potassium. It can be either natural or manufactured. Natural sand is mined from quarries and may have irregular shapes and sizes. Manufactured sand is produced by grinding up natural sands into a uniform size and shape suitable for use in concrete.
Gravel is defined as coarse-grained sediment consisting mainly of rounded stones or cobbles. It can be any color or shade of gray but is usually white due to the amount of clay contained within it. Gravel is commonly used as a base material for roads, parking lots, and walkways because of its ability to absorb traffic vibrations and noise pollution. It also adds weight to the concrete without adding much cost.
Crushed rock is a finely ground sediment composed of small pieces or rubble with some interstitial space between the pieces.
How to Select High-Quality Coarse Aggregate for Concrete Construction
Aggregates, which make up 60 to 75 percent of the entire volume of concrete, are classified into two types: fine and coarse. Fine aggregates are often made of natural sand or broken stone, with the majority of particles passing through a 3/8-inch screen. Coarse aggregates include gravel, crushed rock, and recycled materials such as glass and ceramic tiles.
The type of aggregate used in concrete has a significant effect on its performance. Concrete containing more than 50 percent coarse aggregate is better at resisting cracking caused by freezing and thawing. Sand tends to absorb water vapor from the atmosphere, while stone does not. This is why most concrete buildings are designed with at least some coarse aggregate to help prevent cracks from forming due to changes in humidity.
Concrete that is too soft may cause it to break down under its own weight or pressure. This can lead to crumbling and damage to any reinforcing steel bars that are included in the structure. If this happens, have your contractor check for voids in the concrete; if there are too many, have them filled with epoxy or other material. Also have them weigh the slab before it is poured to ensure that you do not have a surplus or shortage of concrete.
Concrete that is too hard may prevent it from drying out and curing properly, causing it to crack when it expands and contracts due to changes in temperature.
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