Specification of a Concrete Mix
The object of concrete mix design is to find the proportions in which the concrete materials - cement, water, fine aggregate and coarse aggregate - should be combined in order to provide the specified strength, workability and durability and possibly meet other requirements, as listed in standards such as the British BS 5328.( see References 7).
The specification of a concrete mix must therefore define the materials and the strength, workability and durability to be attained. How Firstmix deals with these requirements, is discussed below:
In mix design the most important property of a type of cement is its influence on the strength of the concrete. Firstmix allows the properties of three standard cements to be used or for new types to be defined.
In mix design water is assumed not to contain impurities that affect its suitability for concrete and its specification is therefore irrelevant to mix design.
The type (crushed or uncrushed) and grading of the fine aggregate are important in mix design and should be specified. The density should be also be specified if known but Firstmix will use a typical value if it is not specified.
The type (crushed or uncrushed) and maximum size of the coarse aggregate are important in mix design and should be specified. The density should be also be specified if known but Firstmix will use a typical value if it is not specified.
The required concrete strength, the age at testing and the method of testing must be specified for a designed concrete mix. In mix design a target mean strength is aimed for. The target mean strength is generally derived from a characteristic strength or ‘grade of concrete’, to which a ‘margin’ is added to allow for statistical variation. In Firstmix a characteristic strength must be specified together with either a margin or statistical particulars from which a margin can be calculated.
The required workability and the method of testing must be specified for a designed concrete mix.
Tests for durability are not normally specified for concrete. Instead conditions are set for the mix, which are intended to provide the required durability. The conditions may include:
All these conditions can be set in Firstmix.
A maximum cement content can be set in Firstmix and the density of the concrete is output.