Mix Design Method: ACI Concrete Mix Design & British Method
Mix Design is the process of selecting the suitable ingredients of concrete and determining their relative quantities with the object of producing, as economically as possible, concrete of special minimum properties notably consistence, strength & durability. What engineers do is they ensure the proper design by selecting the ratio.
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Purposes Of Mix Design
The purposes of mix design is given below.
- Approximate design
- To find the cost
- To check the submitted design
- Preparing trial mixes
- Safe design for instant use.
- To achieve exact design for safety & economy
- For single-single specifications
- For multiple specification
- To achieve quality requirements
- Comparison of performance of materials or mixes.
Methods That Fullfill The Purposes Of Mix Design
Methods to fulfill the purposes of mix design are-
- Laboratory trial mix
- Full-scale trial mix
- Previous PRODUCTION DATA bank
- ‘Ready to use‘ standard data
- ‘Simplified‘ methods of design
- ‘Comprehensive methods’ of design
Basic Factors Of Mix Design
Basic factors of mix design are-
- Weight of Ingredient Per Batch
- Mix Proportions
- Capacity of mix
- Overall Grading
- W/C ratio
- Aggregate - Cement ratio
- Required workability
- Quality control
- Mean strength
- Minimum strength of concrete
- Max aggregate size
- Aggregate shape and texture
- Compaction method
- Type of cement
- Age at which strength is required
- Size of section & spacing of reinforcement
- Liability to chemical action
Mix Design Ratio
- Real ratios: On the basis of dry material ( by weight )
- Field ratio: Modified real ratio to be in field condition
- Nominal: Ratio expressed to distinguish between different types of works as 1:1.5:3, 1:3:6, 1:2:4 ( cement: FA: CA )
- w/c ratio
- cement concrete, aggregate, cement ratio
- Gradation of aggregate
Aspects of Mix Design
- Plastic strength
- Mix proportions
- Batch quantities
- Air content
- Special properties like surface finish
Importance Of Mix Design
Mix design is important in various ways. Structural concrete must resist external forces & internal stresses due to various types of loads. So concrete must possess certain minimum property such as strength, consistency, durability etc. For this reason some suitable materials should be selected & their relative quantities should be determined. Again concrete should be maid in the most economic process. Mix design provides all the requirements & so it is very important.
Methods Of Mix Design
- Old method
- Fineness modulus method
- Minimum voids method
- Trial mixes
- Arbitrary method
2. New method
- British method
- ACI method
Difference Between ACI Mix Design and British Mix Design MethodMethod
SSD condition is used
OD condition is used
No adjustment of aggregate mixture is required
Adjustment is required
Based on strength & durability
Based on balance between economy & requirements for strength, place ability, density, etc
Volume based calculation can’t be done. Only weight based calculation is possible.
Both volume & weight based calculation can be done
In mixing ingredients air content isn’t considered
Air content is considered
It’s used for producing only non-air entraining concrete
It is used for producing both air entraining & non-air entraining
Background Data For ACI Mix Design
To perform ACI Mix Design some background data is required.
- Sieve analysis of fine & coarse aggregate ( FA ∞ CA )
- Unit weight of CA
- Bulk specific gravity
- Absorption of gravity
- Sp. Gravity of OPC & other cementitious materials
- Mixing water requirement for concrete
- Relationship between strength and W/C ratio
- Blending of the aggregate
Mix Design Quality Control
The process of taking measures for minimizing the deflection in the resultant product of a construction material is termed as Quality Control.
Factors Affecting Mix Design Quality Control
The factors that affects the quality of the mix are given below.
- Change in the cement strength
- Change in the aggregate grading
- Insufficient batching & placing
- Occasional curing
- Poor compaction
- Deflection in testing
- Alteration in moisture content
Requirement For Good Quality Control
- Knowledge of variability
- Knowledge of factors of variability
- Knowledge of standard testing procedure
- Knowledge of statistical significance of result
Fresh Concrete Properties:
Aggregation: Aggregation refers to the separation of components of fresh concrete which converts to non-uniform mix. In general it means the segregation of coarse aggregate from mortar.
- Poor proportional mixture.
- Coarse aggregate with high specific gravity
- Change of particle shape away from smooth and rounded to odd
- Too wet or too dry mixer
- If we drop the concrete from heights
- If the design of the mixer isn’t good
- The proportion of the mixed is to be maintained properly.
- Transporting, placing, compacting & finishing should be done carefully
- If segregation is found remix the concrete
- By using air-entraining admixtures
Consistency is a general term which is used for indicating degree of fluidity or mobility or wetness. Wet concrete is much more workable to the dry one. But concretes of same consistency may differ in workability.
Bleeding: Bleeding of concrete may cause damage to the mix prepared. Bleeding should be kept under control.
It is the amount of work required to place or to compact the concrete completely. The concrete that satisfies those conditions are termed as workable concrete.