Aggregate functions as a filler in the concrete mixture. Aggregates fill 60-80% of the volume of concrete. Therefore, the chemical, physical and mechanical characteristics of the aggregates used in the mixing greatly affect the properties of the resulting concrete (such as compressive strength, strength, durability, weight, production costs and others). The aggregate weight used will determine the weight of the concrete produced. In general, a good aggregate must be an aggregate that has a cuboid or round shape, clean, hard, strong, well graded and chemically stable. The aggregate is divided into two groups, namely as follows.
The characteristics of the shape and outer texture of the exposed aggregate play an important role in the properties of concrete. Particles with a high surface area to volume ratio can reduce the workability of the concrete mixture. Flaky aggregates can be detrimental to the durability of the concrete because it tends to be oriented to one plane, so that water and air bubbles can form at the bottom. The surface texture of the aggregate greatly affects the properties of fresh concrete, such as chewiness. The shape and surface texture of fine aggregates can affect the water requirements of the concrete mixture. In addition, the aggregate must be chemically stable, so it will not damage the results of the concrete hydration reaction.
Because aggregate is the material with the most content in concrete, the more the percentage of aggregate content in the concrete mixture, the cheaper the price of concrete, provided that the mixture is still quite easy to work with (good workability) for structural elements that use the concrete.
Coarse aggregate is a filler for concrete mixture whose size has exceeded ¼ inch (6 mm). The properties of coarse aggregates affect the final strength of hard concrete and its resistance to concrete disintegration, weather and other damaging effects. This crude mineral aggregate must be free of organic matter and must have good bonds with cement cells.
Fine aggregate is a filler in the form of sand. The sizes vary under sieve no. 4 (0.075 mm) according to ASTM standards. Good fine aggregate must be free of organic matter, clay, or other materials that can damage the concrete mixture. The size variation in a mixture must have a good gradation, which is in accordance with the filter analysis standards of ASTM (American Society of Testing and Materials). For radiation resistant concrete, fine steel powder and crushed iron powder are used as fine aggregate.
Some of the mechanical properties of aggregates include
1) The force of attachment
The shape and texture of the aggregate surface affect the strength of the concrete, especially for high strength concrete. Flexural strength is influenced more by the shapes of the texture of the aggregate than by compressive strength. The coarser the texture, the greater the adhesion between the particles and the cement matrix. Usually, aggregates with good adhesion will find a lot of broken aggregate particles in the concrete that is tested to its capacity.
The compressive strength of the aggregate required for concrete is generally higher than that of the concrete itself. This is because the actual stress acting on the contact point of each aggregate particle is usually much higher than the compressive stress acting on the concrete.
Toughness can be defined as the resistance of the aggregate to destruction due to impact loads (impact).
Hardness, or resistance to aggregate wear, is an important property for concrete used for roads or floor surfaces which must carry heavy traffic.