In most cases, aggregates are considered to be an inert material within a concrete mixture. The totals can have a significant impact on the properties of concrete, both when it is fresh and after it has hardened. Character & performance of concrete are solely dependent on the size, weight, and moisture of aggregates.
The cost of concrete can be reduced significantly by selecting the maximum aggregate size. The use of coarser aggregate minimizes the amount of cement required in a mix. The water-cement ratio (w/c), when kept constant, will need less water for a combination with less glue. Less water will reduce shrinkage and cracking, which are related to restrained volume changes.
Below are some important conditions to consider when selecting and proportioning aggregate for concrete:
Average Aggregate Proportions
Aggregates can make up 60% to 80% of a standard mix of concrete. To ensure that concrete is durable, blend it for maximum efficiency and control its quality to maintain its strength, durability, finishability, and workability.
For a good mix of concrete, the proportions should be:
Ingredients Range
- Cement: 7% to 15%
- Aggregate: 60% to 80%
- Water: 14 – 18%
- Air: 2 – 8 %
Qualities of good quality aggregate
The aggregate should be solid, clean, and strong, with durable particles. It must not contain any absorbed damaging chemicals or coatings of clay. These types of aggregates are not suitable for use:
- Brittle or prone to breaking.
- Material containing a large amount of porous and soft materials.
- Popouts are surface defects that can occur on certain types of “chert” due to their poor resistance to weathering.
Gradations
The concrete will be of good quality, inexpensive, and have good workability and finish. The ratios between coarse and fine aggregates can be altered based on the characteristics of each total, the method of placement, and the desired finish.
The 3/8-inch sieve should be used to divide fine aggregate from coarse aggregate. The fineness module (FM) is an index that measures the fineness of aggregates. For FM calculation, you add the cumulative percentages with the mass retained in each sieve and divide the total by 100. Fine aggregate FM should be between 2.3 and 3.1. If the FM changes more than 0.2, then mix adjustments will be required. A mix that contains too many fine materials may need more water and result in a sticky mixture. A combination with too much coarse material can be difficult to arrange, solidify, or finish.
Once a grade is selected for a project, it should be maintained within a tight range to achieve superior batch-to-batch consistency. To do this, the aggregated must be properly accumulated and reworked in order to avoid too much segregation.
Following are the maximum sizes for coarse aggregates to be used in a mixture based on the size, shape, and reinforcement of a member:
- Cover distance: 3/4 of the space between rebars or rebars and forms.
- The thickness of the slab is 1/3 its depth
- 1/5 of the narrowest dimensions of a member
It is common to examine aggregates with a combined grading, combining fine and coarse materials in proportions intended for the concrete mix. This allows an estimation of the concrete mix’s performance. When the combined aggregate gradation (percentage retained against sieve size) is plotted, these deficiencies can be easily identified and corrected. To achieve the “ideal” aggregate gradation, alternate aggregate sources or supplementary blending can be used.