What is the Basis for Selection of Concrete Mix Proportions?
Madeh Izat Hamakareem
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The decision on selecting concrete mixture proportion is the process of optimizing number of favored properties based on the requirements of the project. In the following sections, the basis for selection of concrete mixture proportions will be discussed.
The characteristics that form basis for proportioning concrete mixture involves:
1. Economy in Producing Concrete
The major motivation to rigorously proportioning concrete mixture is the economy. The best economy can be obtained by declining cement quantity used or replacing concrete with low cost cement replacement materials like slag.
Additionally, the economy of mixture proportions can be further improved through the use of the largest maximum aggregate size available to the project and consistent with dimension requirements of the structure.
2. Strength of Concrete
There are several factors that influence the strength of concrete. For example, concrete strength decline with the increase of water to cement ration. Moreover, it should be known that other factors such as durability, wearing resistance, and permeability are also affect the concrete strength to a certain degree.
Furthermore, concrete strength cannot be accurately predicted based on water to cement ratio only, so it should be confirmed by testing concrete specimen on 28 days.
The concrete strength at the age of 28days is used as a parameter for concrete mixture proportioning in addition to structural design and concrete evaluation. However, strength of mass concrete cannot be known by testing specimen tested on 28days of age since sizable amount of cement replacement materials are used to lower internal heat.
Therefore, mass concrete need longer time like 90 days to achieve adequate strength, and hence such concrete requires to be proportioned to gain adequate early strength that may be required for the formwork removal.
3. Durability of Concrete
Concrete shall be able to resist damages and deteriorations due to environmental influences for instance freezing and thawing, chemical attacks, wetting and drying, and abrasion.
Durability of concrete is enhanced as water to cement ratio is reduced because low water to cement ratio would result in low permeability and consequently water penetration will be declined.
Additionally, concrete cannot withstand detrimental effects of freezing and thawing unless it is made with frost resistant aggregate and suitable air void system, and concrete shall gain sufficient strength before it is exposed to freezing and thawing.
Lastly, concrete needs to be proportioned properly to achieve satisfactory durability.
4. Placeability of Concrete
It is a property of concrete to be easily proportioned, mixed, delivered, placed, consolidated, and finished.
These characteristics are affected by aggregate grading, shape, proportioning, quantity and quality of cement materials used, presence and absence of entrained air and chemical admixture, and the consistency of the mixture.
So, careful concrete mixture proportioning is a major consideration through which placeable concrete can be produced.
The concrete shall be placeable by the conveying and placing equipment to be employed on project site without the addition of water at the placement site.
The need for careful concrete mixture proportioning in producing placeable concrete can be noticed more clearly in the case of mass concrete.
Such concrete would encounter placement problem unless it is proportioned with utmost cautions.