Mix Design

Constituent Materials

All constituent materials for concrete shall conform to BS 8500(2)/BS EN 206-1(3) and/or the clients specification.

Cement and cement additions

The performance of cement will vary from one source to another in terms of overall strength performance, rate of strength development, setting time, performance with admixtures, and colour.

A wide variety of cement combinations are available and widely used for various reasons. The vast majority of concrete is produced with a cement combination.

Combinations are formed by mixing Portland cement CEM I with a range of additions such as FA, GGBS or silica fume. These additions are generally added at the mixer as a separate powder but can also be supplied as a pre-blended product. The use of these materials will have an influence on the performance of the concrete, generally beneficial. The main benefits are economic, improved durability and reduced environmental impact.

Aggregates

Aggregate type will have a major influence on concrete performance. Crushed materials may produce concrete which is harsh with a higher water demand particularly if crushed fines are used. If water demand is not excessively increased strength performance will be improved. Gravel aggregates tend to produce lower water demand but due to often smooth surfaces texture may produce lower strength.

Particle size distribution is fundamental to the performance to ensure particle packing, poor particle size distributions will inhibit the paving process and produce poor surface texture and may encourage bleeding.

Aggregate type has a major impact of saw cutting joints

Admixtures

The use of admixtures is now commonplace and the use of multiple products is not unusual, for best practice technical advice should be sought from the admixture or ready mixed concrete supplier.

Air entraining admixtures are used to enhance the freeze thaw performance of concrete. Maintaining a stable and specific air content is crucial to the freeze thaw performance of concrete, whilst also assisting with the consistence of the concrete and aiding finish ability.

Water reducing admixtures are essential to help achieve the low water cement ratio’s and high strength requirements of PQ concrete while balancing the economics of the concrete mix design.

Performance of these admixtures is influenced by a number of factors such as variation in particle size distribution of the fine aggregate, cementitious type and ambient temperature. It is likely that dosage rates will need to be adjusted during concrete production.

Fibre reinforcement

Fibres for use in concrete have been produced from a wide variety of materials including organic polymers, and drawn steel.

Concrete Types

The traditional UK methods of specifying concrete have been retained in BS 8500(2) and one other concrete type added. The terminology adopted in the standard requires that one should refer to "concrete type" instead of "mix type". Therefore it is incorrect to use the term "Designed mix"; the correct term is now "Designed concrete".