5.2:
Shape and Texture of Coarse Aggregate
The shape of the coarse aggregate is defined by two characteristics: roundness, which indicates the sharpness or angularity of its edges and corners, and sphericity, which measures how closely it resembles a sphere.
Using simple gauges, the shape of the coarse aggregate can be identified as elongated if its length is considerably larger than other dimensions, flaky if its thickness is smaller relative to the other dimensions, or both elongated and flaky.
The surface texture of coarse aggregate is generally categorized as rough or smooth based on the texture of its parent material and the extent to which forces act on it.
Freshly crushed rocks are angular with sharp corners and a rough texture. Over time, through weathering, the corners break down, forming subangular particles.
Further, as these particles are transported by weathering agents, their corners wear off, resulting in rounded particles with smooth texture.
Lastly, the shape and surface texture of coarse aggregates influence the ease of packing them into a dense configuration.
Angular and rough-textured aggregates are more difficult to compact than rounded, smooth textures.
5.2:
Shape and Texture of Coarse Aggregate
Aggregate shape is classified based on the relative sharpness or roundness of the edges and corners. This classification includes categories like rounded, angular, elongated, and flaky, each with specific characteristics. Rounded aggregates, fully shaped by attrition, are typical of river or seashore gravel, while angular aggregates, such as crushed rock, have well-defined edges. Aggregates that are elongated and flaky are less desirable, as they can reduce the workability and strength of concrete.
The texture of aggregates refers to their surface characteristics, which can range from smooth to rough. The texture affects the bond between aggregate and cement. Smooth textures, resulting from weathering by water or the breakage of fine-grained rocks, may require less water for concrete mixing, whereas rough textures, like those of crushed basalt or limestone, provide better bonding with cement but may increase water demand. The surface textures in aggregates can also include glassy, granular, honeycombed, and crystalline forms.
The packing of aggregates is also influenced by their shape and texture. Aggregates that are angular and rough offer less efficient packing but potentially stronger mechanical interlock in concrete compared to rounded and smooth aggregates.