BS EN 13877-1 limits the maximum aggregate size to one-third of the slab depth, and this is consistent with U.S. standards and general practice around the world, although some agencies may have slightly different restrictions. In practice, there is no maximum size for the aggregate. But usually the highest you'll want to go for structural applications is 40mm, unless you're running a mass concreting project, such as a dam.
So you can go up to 150 mm. Surface of a flat concrete work where a detachment occurred on a chert particle. Although not a structural concern, popouts are a persistent defect in the surface that can be a source of aesthetic problems and, if excessive, can compromise the quality of the roof in reinforced slabs. Photo courtesy of the Portland Cement Association) Aggregates are often analyzed by the combined classification of fine and coarse-grained materials, provided as intended in the proposed concrete mix (Photo.
This provides an estimate of how the concrete mix will behave. Each region has its own deficiencies in aggregates, but once a combined aggregate gradation is plotted (percent withheld vs. sieve size), these deficiencies can be more easily identified and remedied. Alternative sources of aggregates or additional aggregate mixtures can be considered to approximate the elusive ideal gradation that provides the best workability, pumpability, shrinkage reduction and economy (Figure.
Note that removing sand from a mix transforms conventional concrete into a fineless mix, also known as permeable concrete (see Permeable Concrete Pavements). Permeable Concrete Qualifies for LEED Points, a Green Building Rating System Developed by the U.S. UU. Green Building Council (USGBC), because it allows runoff to seep directly into the subgrade, recharging the water table.
Photo 4 - To address excessive particle size segregation, batching is usually preceded by reworking the stacks around the circumference to remix the sizes, rather than working directly on the stack. Photo courtesy of PCA) Photo 5: Recommended maximum size of the aggregate to traverse the clearance between the reinforcing steel and the roof dimension. Photo courtesy of PCA) Photo 6: Maximum aggregate size depending on laying method and slab thickness. (Photo courtesy of PCA) Photo 7 - Size distribution of fine to coarse-grained aggregates plays an important role in concrete workability and performance.
Photo courtesy of PCA) Figure 1: Optimal Combined Aggregate Leveling for Conventional Concrete. Courtesy of PCA) Both gravel and crushed stone produce quality concrete. Gravel will have lower water demand compared to crushed stone. Gravel is preferred for concrete with exposed aggregate in walkways and decorative applications.
Crushed stone will generally exhibit greater paste-aggregate bonding. Crushed stone is preferred in pavement mixes, since the higher paste-aggregate bond produces higher flexural strengths (Photo courtesy of PCA) Figure 2 - The moisture content of the aggregate plays a role in the workability of concrete. If the aggregates are too dry, they will absorb (steal) the water from the mixture. If the aggregates are too humid, excess moisture should be subtracted from the expected amount of mixing water.
Photo courtesy of PCA) Concrete Mix Design and Control, Portland Cement Association (see Chapter 5, Concrete Aggregates). The stone is mined, crushed and ground to produce a variety of aggregate sizes that meet both “coarse-grained” and “fine” specifications. Aggregates must be solid, clean, hard, durable and free of excessive fines or contaminants that could affect cement hydration or disrupt the paste-aggregate bond. Aggregates are extracted from natural sand or sand and gravel pits, hard rock quarries, dredging submerged deposits, or mining underground sediment.
Changes in gradation, maximum size, unit weight, and moisture content can alter the character and performance of your concrete mix. A track record of good performance of a local aggregate also provides an indication of the performance of the material in service. If their absorption is not satisfied, these sponges steal water from the designated amount of mixing water, reducing concrete settling. Optimized grading based on aggregate availability and project requirements will result in an economical concrete with good workability and finishability.
This is usually achieved through proper storage of aggregates and reprocessing of stocks to counteract excessive segregation (Photo. Ensure your concrete producer purchases good quality aggregates, as verified by regular aggregate test results in accordance with ASTM C 33, Standard Specifications for Concrete Aggregates. The fundamental relationship between the water-cement ratio and strength begins with the correction of the contribution or absorption of moisture by aggregates. Recycled concrete is created by breaking, removing and crushing existing concrete to a preferred size.
Concrete Mix Design and Control, Portland Cement Association (see Chapter 5, Concrete Aggregates). Both gravel and crushed stone are generally acceptable for manufacturing quality concrete (Photo, although gravel is generally preferred for exposed aggregate). Photo 7 - The size distribution of fine to coarse-grained aggregates plays an important role in the workability and performance of concrete. .
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