In order to make a concrete mix design that works, you should master all the concrete theories in combination with experiences of concreting at work. Here is some important things you need to know when design concrete mixes.
A. What do you need to know before designing concrete?
1. What are the strength requirements?
– Compressive (on cube or cylinder specimen) strength
– Flexural strength
– Tensile strength
2. What is the placing method? By pump or direct pouring?
3. How far is the job site from the batching plant?
4. What is the structure for casting? Pavement, foundation, elevated slab, etc.
5. What are the projects specification?
– Maximum or minimum cement contents
– Maximum water/cement ratio
– Slump or consistency limit
– Minimum Strength requirement @28 days
– Material specifications (what is the maximum size of aggregate?)
6. Latest testing results of materials is needed in the preliminary selection of materials and design calculation
B. What are Design Precautions and Things to Remember when design concrete mixes?
1. Increasing the sand/total aggregate ratio, increases the water requirement at the same consistency.
2. Increasing the water/cement ratio decreases the strength of concrete at the same cement content.
3. Remember that adding 5 liters of water per cubic meter increases the slump by 2.5cm.
4. Remember that adding 5 liters of water per cubic meter decreases strength by approximately 4%.
5. Always follow recommended admixture dosage.
6. Always have “control” when performing trial mixes, always perform trial mixes with another mix using the same materials. This data can be useful in diagnostics if a problem occurs.
7. Always adjust batching quantities to the actual moisture condition of the aggregates.
8. Volume tolerance for 1m3 concrete is 1 ± 0.2 m3.
9. Range of normal weight concrete is from 2,200 kg/m3 to 2,400 kg/m3