Editing Concrete (section)

===Aggregates===
{{main|Construction aggregate}}
[[File:Gravel 03375C.JPG|thumbnail|Crushed stone [[Construction aggregate|aggregates]]]]

Fine and coarse aggregates make up the bulk of a concrete mixture. [[Sand]], natural gravel, and [[crushed stone]] are used mainly for this purpose. Recycled aggregates (from construction, demolition, and excavation waste) are increasingly used as partial replacements for natural aggregates, while a number of manufactured aggregates, including air-cooled [[blast furnace]] slag and [[bottom ash]] are also permitted.

The size distribution of the aggregate determines how much binder is required. Aggregate with a very even size distribution has the biggest gaps whereas adding aggregate with smaller particles tends to fill these gaps. The binder must fill the gaps between the aggregate as well as paste the surfaces of the aggregate together, and is typically the most expensive component. Thus, variation in sizes of the aggregate reduces the cost of concrete.<ref>{{Cite web |title=The Effect of Aggregate Properties on Concrete |url=https://www.engr.psu.edu/ce/courses/ce584/concrete/library/materials/Aggregate/Aggregatesmain.htm |access-date=2022-08-13 |website=www.engr.psu.edu|archive-url=https://web.archive.org/web/20121225184337/http://www.engr.psu.edu/ce/courses/ce584/concrete/library/materials/Aggregate/Aggregatesmain.htm |date=25 December 2012 |publisher=Engr.psu.edu|archive-date=25 December 2012 }}</ref> The aggregate is nearly always stronger than the binder, so its use does not negatively affect the strength of the concrete.

Redistribution of aggregates after compaction often creates non-homogeneity due to the influence of vibration. This can lead to strength gradients.<ref name="Veretennykov Yugov Dolmatov et al 2008">{{cite book |doi=10.1061/41002(328)17 |chapter=Concrete Inhomogeneity of Vertical Cast-in-Place Elements in Skeleton-Type Buildings |title=AEI 2008 |year=2008 |last1=Veretennykov |first1=Vitaliy I. |last2=Yugov |first2=Anatoliy M. |last3=Dolmatov |first3=Andriy O. |last4=Bulavytskyi |first4=Maksym S. |last5=Kukharev |first5=Dmytro I. |last6=Bulavytskyi |first6=Artem S. |pages=1–10 |isbn=978-0-7844-1002-8 }}</ref>

Decorative stones such as [[quartzite]], small river stones or crushed glass are sometimes added to the surface of concrete for a decorative "exposed aggregate" finish, popular among landscape designers.