What is aggregates?
Aggregate is an aggregation of non-metallic minerals to form a solid particle. Aggregates can be natural or artificial. Natural aggregates are formed by weathering, disintegration on parent rocks.
Pebbles, sand, gravel, are examples of natural aggregates. Natural aggregates are round, flaky, and platy in shape. Similarly, an artificial aggregate is obtained by crushing the parent rock.
They are mostly angular in shape. Besides, artificial aggregates can be made by fly ash also.
Aggregates are mostly used in the construction industry. It holds about 60-70% part in normal concrete. Aggregate in concrete should chemically stable or inert. Aggregates in the concrete are responsible for unit weight, elastic modulus, and thermal properties of the concrete.
Aggregates can be classified on the basis of their size, shape, surface moisture, surface texture, and bulk density of aggregates.
Lightweight Aggregates: The aggregates having a bulk density of less than 1100kg/m3. Aggregates are supposed to have lightweight due to the presence of pores in them.
Normal weight aggregate:– these aggregate’s weight has a density of more than 1100kg/m3 but less than 2080 kg/m3.
Heavyweight Aggregates:-The aggregates those having a bulk density of more than 2080-4485 kg/m3. These are used where a high mass to volume ratio is demanded. E.g. barite, limonite, barite, goethite
Grading of Aggregates
Grading of aggregate is the particle size distribution of a sample of aggregates on the basis of sieve analysis and sedimentation analysis. The grading of aggregate is plotted in a semi-logarithmic graph to get the particle size distribution curve which is in form of ‘S’.
Sieve analysis or sedimentation analysis is performed to find out the grading of the aggregates. Sieve analysis is a procedure in which aggregates are allowed to pass through a set of sieves arranged in descending order.
The weight of aggregates retained in each sieve is weighed to perform further calculations. Similarly, sedimentation analysis is performed for silt like particles.
In this curve, the cumulative percentage of material passing down is plotted inordinate, and the size of the sieve opening is plotted in abscissa which is a logarithmic scale.
Types of aggregates on basis of gradation curve
1) Uniformly graded Aggregates
This refers to the sample which consists of a particular size of aggregates predominately. There will be high chances of voids formation and paste requirement is high.
The nature of the s- curve is steep for uniformly graded aggregates. Concretes with uniformly graded aggregates are hard to compact and are likely to have high permeability and low stability.
2) Well graded Aggregates
This consists of all sizes of particles. Voids are almost filled by smaller aggregates giving the consistent unit weight of the compacted mass. It is the most preferred grading of an aggregate sample. This sample has high stability and less permeability.
3) Gap graded Aggregates
In this grading system, a certain mid-size of aggregate is missing. They have moderate voids, permeability, and low stability. The curve is flat in the middle portion of the curve.
The gap graded aggregate is used to provide a more economical mix since less sand can be used for a given workability. Though sometimes it is used to improve the aesthetic view.
4) Open Graded Aggregates
In this sample, small size aggregates are present in only small percentage resulting to more air voids due to absence of fillers. The curve is near vertical in the middle and flat in the ends.
Fineness Modulus of Aggregate
It is an indication of average size of particle in the aggregate.
FM = Sum of all cumulative percentage retained/100
If FM=2-3.5, it is fine
If FM=5-5.5, it is coarse
Significance of grading of aggregates
- If the aggregates are of uniform size, there will form voids. Therefore, the grading of aggregate is done to lessen those voids by accommodating smaller size aggregates in those voids.
- Grading of aggregates gives uniformity in the specific weight of concrete.
- Grading of aggregates reduces the chances of segregation in concrete.
- The graduation curve is also used to estimate the type of soil.
- The graduation curve is used to calculate the coefficient of uniformity and coefficient of curvature.
- It helps to determine parameters like D30, D60, and D10 which can be used as design parameters.
Where, D10 is diameter of particle, then which 10% are finer and 90% are coarser.
D30 is diameter of particle, then which 30% are finer and 70% are coarser.
D60 is diameter of particle, then which 60% are finer and 40% are coarser.
I hope this article on “Grading of aggregate” remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Rukshana Shrestha