Designed concrete is a type of concrete where the producer is responsible for selecting the mix proportions to meet the required performance as communicated by the specifier. In order to ensure the desired performance, it is crucial that the specifier takes into account various factors when compiling the specification.
One important consideration is the intended use of the fresh and hardened concrete. The specifier needs to determine what the concrete will be used for and what specific performance requirements are needed for the structure or project.
Another factor to consider is the curing conditions. The specifier must take into account the curing time and temperature as it can affect the strength and durability of the concrete. The dimensions of the structure are also important to consider as they can impact the heat development during the curing process.
Environmental exposure conditions are also a significant factor in designing concrete. The specifier needs to be aware of the environment the concrete will be exposed to and make sure that the concrete can withstand any harsh conditions, such as extreme temperatures or exposure to chemicals.
Surface finish is another consideration. The specifier must determine the type of surface finish required and ensure that the mix proportions will achieve the desired finish.
The maximum nominal aggregate size is an important factor to consider as it can affect the strength and workability of the concrete. The specifier must determine the maximum size of aggregate that can be used while still achieving the desired performance.
Lastly, the specifier must be aware of any restrictions on the suitability of materials. Some materials may not be suitable for certain types of concrete, and it is essential that the specifier takes this into account when compiling the specification.
The most common form of designed concrete is identified by the strength class, which is defined by the characteristic compressive strength at 28 days. For example, class M30 (C25/30 as per British Standards) is the concrete having a characteristic compressive cube strength of 30 N/mm2. It is worth noting that 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.
The strength of structural concrete is not the only factor determining its durability. BS8500 specifies the minimum strength class, maximum free water/cement ratio, and minimum cement content necessary for various exposure levels. The durability of concrete is mainly influenced by the maximum free water/cement ratio, minimum cement content, and constituent materials used.
To ensure compliance with a concrete specification, several details need to be included. These include conformity to BS 5328 or BS EN 206-1 and BS 8500-2, compressive strength class, maximum aggregate size, chloride class, and consistence class. Optional items such as target density of lightweight concrete, heat development, or other technical requirements listed in BS 8500: Part-1 may also be included.
Form A in BS 5328: Part-2 is recommended when specifying designed concrete. To determine conformity of designed concrete, the producer is responsible for conducting strength testing on 100mm or 150mm cubes. Recommendations for the required rate of sampling are given in BS 5328 and BS EN 206-1. The producer will produce a mix design that satisfies all specified requirements.