When it comes to creating the strongest concrete, the size of the coarse-grained aggregate is key. Generally, a maximum size of 19 mm is used to obtain high-strength concrete, although additional cement may be required for the additional surface area. Recycled concrete can also be used as a viable source of aggregate, and has been successfully employed in granular subbases, soil cement and new concrete. Increasing the maximum grain size to 120-180 mm can reduce the tensile strength by 30 to 50% compared to concretes with a maximum aggregate size of 20 mm. To ensure that the aggregates are not degraded or contaminated, they should be handled and stored properly.
The use of flat, elongated aggregate particles should be avoided or at least limited to a minimum of 15 percent. ASTM C 33, Standard Specification for Concrete Aggregates, provides classification requirements for coarse-grained and fine aggregates in Tables (and (. Very coarse-grained sands can produce hard and impracticable concrete mixes, while very fine sands increase water and cement requirements and are not cost-effective. The maximum size of a well-graded coarse-grained aggregate of a given mineralogy can have two opposite effects on the strength of normal concrete. The alkali content and type of aggregate in the system are likely to be unknown, so mixing with unsuitable materials could lead to an alkali-silica reaction.
Aggregates can be extracted from natural sand or sand and gravel pits, hard rock quarries, dredging submerged deposits or mining underground sediment. Fine aggregate usually contains less material that passes through a 300 µm and 150 µm screen due to its higher cement content. The classification of an aggregate is determined by a sieve analysis, which is the distribution of particles of granular materials among various sizes. For railway ballast, the overall size used in the test is -19.0 mm +9.5 mm and an actual ballast sample can be crushed to provide the sample. In addition, with a given cement content, more mixing water is generally needed to obtain the desired workability in a concrete mix containing rough-textured aggregates. Sand and aggregate help reduce cost and also limit the amount of shrinkage that occurs in concrete as it cures.