Factors Governing Concrete Mix Design
Grade Designation
Type And Grade Of Cement
Maximum Nominal Size Of Coarse Aggregate
Grading Of Combined Aggregate
Water-Cement Ratio
Workability
Durability
Grade Designation
The grade designation gives characteristic compressive strength requirements of the concrete. As Per IS: 456-2000, the characteristic compressive strength is defined as that value below which not more than 5 percent of the test results are expected to fall. Depending upon the degree of control available at the site, the concrete mix has to be designed for a target mean compressive strength which is somewhat higher than the characteristic strength.
Type And Grade Of Cement
This type of cement is important mainly through its influence on the rate of development of compressive strength of concrete. The choice of type of cement depends upon the requirements of performance at hand, where very high compressive strength is required e.g. in passed concrete railway sleepers high strength Portland cement of grade 43 and 53 conforming to IS: 8112-1989 and IS: 12269 - 1987, respectively, will be found suitable.
Maximum Nominal Size Of Coarse Aggregate
The maximum nominal size of the coarse aggregate is determined by sieve analysis and is designed by the sieve size higher than the target size on which 15 percent or more of the aggregate is retained. The maximum nominal size of the aggregate to be used in concrete is governed by the size of the aggregate to be used in concrete is governed by the size of the section and the spcaing of the reinforcement According to Is 456 - 20000 and IS : 1343- 1980, the minimum thickness of the member, and it should be restricted to 5 mm less than the minimum clear distance between the main bars or 5 mm less than the spacing between the prestressing cables. Within these limits, the nominal maximum size of aggregate may be as large as possible, because large the maximum size of aggregate smaller is the cement requirement for a particular water-cement ratio.
Grading of Combined Aggregate
The relative proportions of the fine and coarse aggregates in a concrete mix is one of the important factors affecting the strength of concrete. For dense concrete, it is essential that the course and fine aggregate be well graded.
Generally, the locally available aggregate do not conform to the standard gradings. In such cases, the aggregates need to be combined in suitable proportions so that the resultant (combined) grading approximates to a continuous grading close to the desired grading. the process of combining aggregates is aimed at obtaining a grading close to the coarsest grading of standard grading curves, the most economical mix having the highest permissible aggregate-cement ratio.
Water- Cement Ratio:
The compressive strength of concrete at a given age and under normal temperature depends primarily on the water. Cement ratio, lower the water-cement ratio greater is the compressive strength and vice- versa.
Workability
The workability of concrete for satisfactory placing and compaction is controlled by the size and shape to be concerted, the quantity and spacing of reinforcement, and the methods to be employed for transporting, placing and compaction fo concrete. The situation should be properly assessed to arrive at the desired workability.
Durability
The durability of concrete can be defined and interpreted to mean its resistance to deterioration influences which may reside inside the concrete itself, or to the aggressive environments. The requirements of durability are achieved by restricting the minimum cement content and the maximum water - cement ratio.
Grade Designation
Type And Grade Of Cement
Maximum Nominal Size Of Coarse Aggregate
Grading Of Combined Aggregate
Water-Cement Ratio
Workability
Durability
Grade Designation
The grade designation gives characteristic compressive strength requirements of the concrete. As Per IS: 456-2000, the characteristic compressive strength is defined as that value below which not more than 5 percent of the test results are expected to fall. Depending upon the degree of control available at the site, the concrete mix has to be designed for a target mean compressive strength which is somewhat higher than the characteristic strength.
Type And Grade Of Cement
This type of cement is important mainly through its influence on the rate of development of compressive strength of concrete. The choice of type of cement depends upon the requirements of performance at hand, where very high compressive strength is required e.g. in passed concrete railway sleepers high strength Portland cement of grade 43 and 53 conforming to IS: 8112-1989 and IS: 12269 - 1987, respectively, will be found suitable.
Maximum Nominal Size Of Coarse Aggregate
The maximum nominal size of the coarse aggregate is determined by sieve analysis and is designed by the sieve size higher than the target size on which 15 percent or more of the aggregate is retained. The maximum nominal size of the aggregate to be used in concrete is governed by the size of the aggregate to be used in concrete is governed by the size of the section and the spcaing of the reinforcement According to Is 456 - 20000 and IS : 1343- 1980, the minimum thickness of the member, and it should be restricted to 5 mm less than the minimum clear distance between the main bars or 5 mm less than the spacing between the prestressing cables. Within these limits, the nominal maximum size of aggregate may be as large as possible, because large the maximum size of aggregate smaller is the cement requirement for a particular water-cement ratio.
Grading of Combined Aggregate
The relative proportions of the fine and coarse aggregates in a concrete mix is one of the important factors affecting the strength of concrete. For dense concrete, it is essential that the course and fine aggregate be well graded.
Generally, the locally available aggregate do not conform to the standard gradings. In such cases, the aggregates need to be combined in suitable proportions so that the resultant (combined) grading approximates to a continuous grading close to the desired grading. the process of combining aggregates is aimed at obtaining a grading close to the coarsest grading of standard grading curves, the most economical mix having the highest permissible aggregate-cement ratio.
Water- Cement Ratio:
The compressive strength of concrete at a given age and under normal temperature depends primarily on the water. Cement ratio, lower the water-cement ratio greater is the compressive strength and vice- versa.
Workability
The workability of concrete for satisfactory placing and compaction is controlled by the size and shape to be concerted, the quantity and spacing of reinforcement, and the methods to be employed for transporting, placing and compaction fo concrete. The situation should be properly assessed to arrive at the desired workability.
Durability
The durability of concrete can be defined and interpreted to mean its resistance to deterioration influences which may reside inside the concrete itself, or to the aggressive environments. The requirements of durability are achieved by restricting the minimum cement content and the maximum water - cement ratio.