Concrete Composition: Materials, Dosage, and Properties

Concrete Components

• Concrete Components

• Fine Aggregate Cement
• Coarse Aggregate
• Water
• Additives

These are substances or products incorporated into the concrete before or during mixing at a rate not exceeding 5% of the fresh concrete and/or to modify some of its characteristics, common properties, or behavior.

Common additives include water reducers, superplasticizers or plasticizers, retarders or accelerators for curing, air-entraining agents, dyes, and air occluders for frost resistance, etc.

• Additions

These are inorganic materials, pozzolanic or latent hydraulic, finely divided, that can be added to concrete to improve some of its properties or confer special properties.

An example includes the use of fly ash as an addenda. Fly ash is obtained by electrostatic precipitation or mechanical capture of combustion gases in thermal power plants that use coal.

The use of fly ash is not recommended for prestressed concrete due to its lower durability.

The maximum amount of fly ash shall not exceed 35% by weight of cement. The minimum quantity of cement should be no less than 200 kg/m³ in mass concrete and 250 kg/m³ in reinforced concrete.

Dosage of Concrete Components

• Dispensing of Concrete: "The process of selecting appropriate ingredients and determining their relative amounts to produce the most economical concrete of a certain minimum property, especially consistency, strength, and durability."
  • It is mandatory that the cement is dosed by weight, and water and aggregates by weight or volume, but dosage by weight is always preferred for greater accuracy.
  • Additives and supplements should be weighed if they are solid; volumetric dosing is permissible if they are liquid.
  • The control of the concrete manufacturing process is the factor that most influences the variability of concrete quality. The tolerances in the composition of centrally manufactured concrete are as follows:

Cement and aggregates: ± 3%

Water: ± 1%

Additives and supplements: ± 5%

• The decisions that the engineer should consider before dosing are:

What must be the specified characteristic strength?

Aggressiveness of the environment

Method of compaction

• To do this, you must specify the type of cement, aggregate type, maximum size of aggregates, and concrete consistency.
  • To obtain the final color of exposed concrete, the sand will be important. It must be river sand, with continuous particle size, a significant content of fine grains (60% less than 0.64 mm), and a contribution of particles less than 0.16 mm, supplementing the cement dosage.
  • For concrete, the dosage should be high in cement, and the values in the table are recommended.