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 effect 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-grained aggregates is the main factor in determining the strength of concrete. In general, smaller coarse-grained 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.
The permitted percentage of harmful substances for fine and coarse-grained aggregates are listed in Tables 1 and 3 of ASTM C 33, respectively. The most common classification of aggregates based on specific gravity in bulk is light, normal weight and heavy weight aggregates. 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. Optimized grading based on aggregate availability and project requirements will result in an economical concrete with good workability and finishability. 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. Note that removing sand from a mix transforms conventional concrete into a fineless mix, also known as permeable concrete (see Permeable Concrete Pavements).
Crushed stone is preferred in pavement mixes since the higher paste-aggregate bond produces greater flexural strengths (Photo courtesy of PCA). Since the weight of the aggregate depends on the moisture content of the aggregate, a constant moisture content is required. All aggregates contain some moisture depending on the porosity of the particles and the moisture status of the storage area. The absorption and surface moisture of aggregates are simple but critically important aspects of producing concrete that consistently achieves the specified or desired strength. Bulk density measures the volume that the graded aggregate will occupy in the concrete, including solid aggregate particles and the voids between them.
But a closer look reveals that aggregate plays an important role in influencing both fresh and hardened concrete properties. Although they are known to be inert fillers in concrete, different properties of aggregate have a major impact on strength, durability, workability and economy. In order to dose the mixture correctly, it is important to know how much space is occupied by aggregate particles including pores within them. To avoid post-placement problems in concrete that are difficult to rectify, verify that these limits are not exceeded at the time of sending material.
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