Concrete Design Mix in industrial projects

What is a design mix? 

Concrete design mix refers to selecting the ingredients and proportioning them to get the desired technical specifications in the finished product. Civil design consultants recommend the design mix based on the project requirements. The proportions are determined based on the strength, durability, and workability requirements of the mix for a specific application. Such design mixes are used for flatwork, underground pipes, bridges, and walls. 

As the design mix depends on each ingredient, it is necessary to determine their physical properties. Testing ingredients like soil, water, coarse aggregate, sand, cement, chemicals, and reinforcement is essential. The report’s values of the physical properties are a basis for all concrete mix design considerations.

Types of concrete mix

There are two types of concrete mix: Normal performance concrete and High-performance concrete. The compressive strength helps differentiate them. 

Normal Performance Concrete

A normal-performance concrete mix has a performance strength ranging between 20 and 40 MPa. This mix has good workability if the mix ingredients are in accurate proportions. Freshly mixed concrete must be plastic or semi-fluid to ensure proper moulding. 

High-Performance Concrete

A high-performance concrete design mix has a performance strength above 40 MPa. The main objective of using high-performance concrete is to reduce the weight, creep or permeability issues and improve the structures’ durability. This mix must be plastic or semi-fluid when freshly mixed to ensure proper moulding.

Because high-performance concrete has a higher cement content, handling it is difficult due to its stickyness. However, this ensures that the cement does not bleed, which is common in normal-performance concrete.

To compensate for the reduced workability, civil consultants recommend the addition of superplasticisers to high-strength mixtures. It is also essential to select aggregate carefully, especially for high-strength mixes. The weaker aggregates may not be able to resist the loads imposed on the concrete.

What factors do civil consultants consider for design mix?

1. Strength Requirements: The design mix must have the necessary strength of the concrete needed. Compressive strength is an essential property of concrete. Moreover, the mean compressive strength, usually taken at 28 days, determines the water-cement ratio of the mix. Additionally, the concrete strength at a given age, curing at a specified temperature, and compaction are factors when deciding the concrete.
2. Workability: The concrete workability is important in deciding the concrete mix. It includes ease of handling and placing the concrete. Some factors affect the degree of workability: size of the section, reinforcement quantity, and compaction method. Narrow areas like corners and inaccessible parts need concrete with high workability so that full compaction can be achieved reasonably. 
3. Durability: Resistance to environmental factors like weathering, chemical attacks, etc, is also a factor. This property is often control-tested at the site. 
4. Economy: Balancing the cost of materials to achieve the desired properties in a cost-effective design mix.

Factors

The strength and durability of the concrete mix design are dependent upon the following factors:

1. Grade designation: The strength is measured in N/mm2 after curing in a curing medium.
2. Choice of cement: The civil consultants recommend testing the cement for performance before being tested in the design mix.
3. Choice of aggregate size: The size of aggregates for each mix depends on the physical properties required. All aggregates must be quality-sized before use. Typically, the workability of concrete increases with an increase in the maximum size of the aggregate. However, the compressive strength tends to increase with the decrease in the size of the aggregate. Civil consultants design the mix to ensure the required properties of the design mix are achieved.
4. Water:  The water to be used for concrete mix design has to be tested before concreting. 
5. Water-to-cement ratio: Civil consultants recommend checking this ratio for consistency, initial and final setting, slump, soundness of the cement, workability and compacting factor.
6. Quality Control: Regular testing of the design mix ensures that the concrete is within the specified requirements. There may be variations in strength due to variations in the properties of the mix ingredients and lack of accuracy control. 
7. Documentation: A design mix report is necessary. This report includes all the materials used and the selection, proportioning and adherence to code and standards. 

Why do we need a design mix? 

1. Concrete mix design is a cost-efficient way of proportioning concrete ingredients for better strength and durability. An optimised and well-designed concrete mix ensures the structure is strong and can bear the loads it has been designed for. It also optimises the efficiency of ingredients by getting the most concrete out of the wet and dry ingredients, thus reducing wastage.
2. A design mix ensures a higher workability for a longer period. 
3. A lower water content is sufficient despite the higher workability properties. 
4. Reduced permeability
5. Reduced shrinkage
6. Improved resistance to atmospheric conditions
7. It needs a lower pumping pressure.
8. Best use of locally available materials

Conclusion

The industrial civil consultants must ensure that the tender gives the methodology to maintain the concrete design mix proportions throughout. A slight change in the proportion may result in a change in properties, thus affecting the structure‘s strength.