Cubic aggregate has the highest strength in concrete, since it has good packing and strength in all directions. Rounded aggregate is not suitable for concrete. The shape and texture of the aggregate affect the properties of fresh concrete more than hardened concrete. Concrete is more workable when a smooth, rounded aggregate is used instead of a rough or elongated angular aggregate.
Most natural sands and gravel in riverbeds or seashores are smooth and rounded and make excellent aggregates. Crushed stone produces much more angular and elongated aggregates, which have a higher surface-to-volume ratio, better bonding characteristics, but require more cement paste to produce a workable mix. The size of coarse-grated aggregates is the primary factor in determining the strength of concrete. In general, you'll want smaller coarse-grated aggregates for stronger concrete, with 20mm aggregates meeting the threshold for strong concrete and 40mm aggregates for normal strength concrete.
Since fine aggregates are used to fill the voids of coarse-grained aggregates, the smaller the coarse-grained aggregates, the finer the fine aggregates should be. They increase the workability of concrete. The aggregate is also very important for the strength, thermal and elastic properties of concrete, dimensional stability and volume stability. Cement is more likely to be affected by shrinkage.
The inclusion of aggregates in the mixture can control the level of shrinkage and prevent cracking. In your contract, construction or purchase agreement, the term “maximum aggregate size” refers to the smallest sieve that 100% of your aggregate can pass through. Concrete Mix Design and Control, Portland Cement Association (see Chapter 5, Aggregates for Concrete). The density of the aggregates is necessary in the dosing of the mixture to establish weight-volume ratios.
Most stacked coarse-grained aggregates are in the AD state with an uptake of less than one percent, but most fine aggregates are often in the wet state with a surface moisture of up to five percent. Each region has its own deficiencies in aggregates, but once a combined aggregate gradation (percent retained vs. Photo 7) is plotted, the size distribution of fine to coarse-grained aggregates plays an important role in concrete workability and performance. Using larger coarse-grated aggregates generally reduces the cost of a concrete mix by reducing requirements for cement, the most expensive ingredient.
Aggregate is commonly considered inert filler, accounting for 60 to 80 percent of the volume and 70 to 85 percent of the weight of concrete. The shape of your aggregates is important to the workability of the concrete, the quantity you want to place, and the strength of the concrete you want to make. A track record of good performance of a local aggregate also provides an indication of the performance of the material in service. The absorption and surface moisture of aggregates are simple but critically important aspects of producing concrete that consistently achieves the specified or desired strength.
Optimized grading based on aggregate availability and project requirements will result in cost-effective concrete with good workability and finishability. As a natural material, the aggregate will sometimes include worn or unstable particles in the supplied product.
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