In general, concrete with larger aggregates tends to have greater compressive strength than concrete with smaller aggregates. This is because large aggregates can be intertwined within the mixture, providing strength depending on their internal properties and those of the cement. The shape and texture of the aggregate have a greater impact on the properties of fresh concrete than hardened concrete. Smooth, rounded aggregates are more workable than rough or elongated angular aggregates.
Most natural sands and gravel from 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 mixture. The size of coarse-grated aggregates is the main factor in determining the strength of concrete. In general, smaller coarse-grated aggregates are needed 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. This helps to increase the workability of concrete. To produce good quality, durable concrete containing a portion of recycled concrete aggregate, test concrete mixes and close control of the properties of old recycled concrete are often required, with mix adjustments made as needed. Therefore, it is desirable to minimize the amount of paste consistent with the production of concrete that can be handled, compacted and finished while providing the necessary strength and durability.
Keep in mind that the size of the aggregates is the size that most parts pass through a sieve of that size, not all, so there is no need to be too demanding. A track record of good performance of a local aggregate also provides an indication of the material's performance in service. The shape of your aggregates is important for the workability of the concrete, the quantity you want to place and the strength of the concrete you want to make. If there is a deficiency in a locally available fine aggregate, concrete may benefit from the addition of air entrainment, additional cement, or a supplemental cementitious material (SCM) to address these deficiencies.
In order to dose the mixture correctly, it is important to know the space occupied by the aggregate particles, including any pores within them. Good quality aggregate must be clean, hard, strong, have durable particles, and be free of absorbed harmful chemicals, clay coatings or other contaminants that may affect cement hydration or reduce paste-aggregate bonding. 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. It is important to use aggregates that meet ASTM C 33 Standard Specifications for Concrete Aggregates. To avoid post-placement problems in concrete that are difficult to rectify, verify that these limits are not exceeded at the time of sending the material.
The permitted percentage of harmful substances for fine and coarse-grained aggregates are listed in Tables 1 and 3 of ASTM C 33 respectively. If the aggregates are too humid, excess moisture should be subtracted from the expected amount of mixing water. Only 15-34% of zone 1 aggregates will pass a 0.6 mm screen; 35-59% of zone 2 aggregates will; 60-79% of zone 3 will; and 80-100% of zone 4 will. This is a summary of some key factors to consider when selecting and dosing concrete aggregate.
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