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Designed concrete are those for which the producer is responsible for selecting the mix proportions to meet the required performance as communicated by the specifier. Therefore it is essential that the specifier, in compiling the specification, takes account of:
- The uses of the fresh and hardened concrete
- The curing conditions
- The dimensions of the structure, this effects heat development
- The environmental exposure conditions
- Surface finish
- Maximum nominal aggregate size
- Restrictions on suitability of materials
The most common form of designed concrete is that defined by the characteristic compressive strength at 28 days and identified by the strength class. For example, class M30 (C25/30 as per British Standards) is the concrete having a characteristic compressive cube strength of 30 N/mm2. The same concrete would have a characteristic strength of 25 N/mm2 at 28 days if cylinders were used for testing, as in certain European countries.
However, strength alone does not necessarily define the required durability, and for structural concrete BS8500 indicates minimum strength class, the maximum free water/cement ratio and minimum cement content that are required for different degrees of exposure. The maximum free water/cement ratio, minimum cement content and types of constituent materials are the main factors influencing durability.
If a specification for designed concrete is to be complied correctly the following details need to be included:
- A requirement to conform to BS 5328, or BS EN 206-1 and BS 8500-2
- The compressive strength class
- The limiting values of composition e.g. maximum free water/cement ratio, minimum cement content or the deign chemical class where appropriate
- Type of cement or carbonation
- The maximum aggregate size
- The chloride class
- The consistence class.
Optional items may be included such as the target density of lightweight concrete, heat development or other technical requirements listed in BS 8500: Part-1.
At the time of publication, the use of Form A in BS 5328: Part -2 is recommended when specifying designed concrete.
Conformity of designed concrete is usually determined by strength testing of 100mm or 150mm cubes and in BS 8500, this is the responsibility of the producer. Recommendations about the required rate of sampling are given in BS 5328 and BS EN 206 -1. The producer will respond to the specification by producing a mix design that satisfies all of the specified requirements.