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Nominal maximum size of coarse aggregate

Concrete is a widely used material. Reinforced concrete structures can be found everywhere. The main reason behind the popularity of reinforced concrete structure is the high strength and the long life span of these structures. Concrete and steel are combined to produce reinforced concrete. Concrete is good at compression while steel good at tension. Concrete and steel form a perfect combination to resist loads imposed on the structure.

Concrete is a composite material composed of cement, coarse aggregate, fine aggregate, and water. Admixtures can be used to enhance concrete properties. A concrete mix consists of almost 65% aggregates. Aggregates are classified into fine and coarse aggregates. Since the aggregates form the majority of a concrete mix. The quality of aggregates can greatly affect the strength and durability of concrete. Aggregates can be natural or crushed aggregates. Used aggregates should be strong, hard, and should not contain a large amount of organic matter or harmful material that can adversely affect the concrete mix's strength. 
Figure 1

According to ACI-318/5[3.3.2], the nominal maximum size of coarse aggregate shall not be larger than:
  1. 1/5 the narrowest dimension between sides of forms, nor
  2. 1/3 the depth of slabs, nor
  3. 3/4 the minimum clear spacing between individual reinforcing bars or wires, bundles of bars, individual tendons, bundled tendons, or ducts.
The limitation of aggregate maximum size is to ensure the smooth placing of concrete. If the maximum size of aggregate is above the limit, the placing of concrete can be difficult, and there will be a possibility of concrete segregation. The aggregate will be separated from the concrete. If the maximum size of aggregate is above the limit, the aggregates will be trapped by the reinforcement, which will cause separation of concrete ingredients. 

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