Theory of sieve analysis
Aggregate is the inert, inexpensive materials dispersed throughout the cement paste so as the produce a large volume of concrete. They constitute more than three quarters of volume of concrete. They provide body to the concrete, reduce shrinkage and make it durable.
The aggregates are classified in two categories; fine aggregate and coarse aggregate. The size of fine aggregates is limited to a maximum of 4.75 mm, beyond which it is known as coarse aggregates. Many a time, fine aggregates are designated as coarse sand, medium sand and fine sand. These classifications do not give any precise meaning. What the supplier terms as fine sand may be really medium or even coarse sand. To avoid this ambiguity fineness modulus could be used as a yard stick to indicate the fineness of sand and in general aggregates.
Fineness modulus for a given aggregate is obtained by sieving known weight of it in a set of standard sieves and by adding the percent weight of material retained on all the sieves and dividing the total percentage by 100. It serves the purpose of comparing one aggregate with another in respect of fineness or coarseness. For classification of fine aggregates, the following limits may be taken as guidance:
Fine sand:Fineness modulus should lie in between 2.2 to 2.6
Medium sand:Fineness modulus should lie in between 2.6 to 2.9
Coarse sand: Fineness modulus should lie in between 2.9 to 3.2
Sand having a fineness modulus more than 3.2 is unsuitable for making satisfactory concrete.
The coarse aggregates have fineness modulus usually more than 5.
A heap of aggregate is classified as a single sized aggregate when the bulk of aggregate passes one sieve in normal concrete series and retained on next smaller size. Such aggregates are normally expressed by the maximum size of the aggregates present in considerable amount in it. For example, a heap of 20 mm size aggregate means that the heap contains maximum 20 mm size aggregate in a substantial amount.
A graded aggregate comprises of a proportion of all sizes in a normal concrete series.
When these sizes are so proportionated to provide a definite grading, it is known as well graded aggregate. Well graded aggregates are desirable for making concrete, as the space between larger particles is effectively filled by smaller particles to produce a well-packed structure. This minimizes the cement requirement.
All-in aggregates comprise a mixture of coarse aggregate and fine aggregates. Such aggregates may directly be used for low quality concreting. But in case of good quality concreting work; necessary adjustments may be made in the grading by the addition of single-sized aggregates.
IS 383:1970 specifies four grading zones for fine aggregates. These four grading zones become progressively finer from Grading Zone I to Grading Zone IV (see Table). The fine aggregates within each of these grading zones are suitable for making concrete. But, the ratio of ratio of fine to coarse aggregate reduces as the fine aggregate becomes finer from Grading Zones I to IV.
The grading of coarse aggregate may vary through wider limits than that of fine
aggregates. However, this variation does not much affect the workability, uniformity and finishing qualities of concrete mix. As per IS-383:1970 the grading limit of coarse aggregate, both for single size as well as graded should be as per the table given below.
Objective of sieve analysis
To determine fineness modulus and grade of fine and coarse aggregate.
Reference:
IS: 383-1970.
Apparatus:
Set of sieves; Balance; Gauging Trowel; Watch.
a) *For fine aggregates: 4.75 mm, 2.36 mm, 1.18 mm, 600 micron, 300 micron
& 150 micron, pan.
b) *For coarse aggregates: 80mm, 40mm, 20mm, 10mm, 4.75mm, pan.
Material:
a) Fine aggregates (1 Kg)
b) Coarse aggregates (5 Kg)
Procedure of sieve analysis
1. Take the aggregate from the sample by quartering.
2. Sieve the aggregate using the appropriate sieves.
3. Record the weight of aggregate retained on each sieve.
4. Calculate the cumulative weight of aggregate retained on each sieve.
5. Calculate the cumulative percentage of aggregate retained.
6. Add the cumulative weight of aggregate and calculate the fineness modulus
using formula:
a. Fineness modulus for fine aggregates =
∑ C/100
b. Fineness modulus for coarse aggregates = ∑ C/100+ 5 Where, C denotes the cumulative percentage of mass retained in a sieve.
7. Determine the grade of aggregates from the Table 1 and the Table 2.
8. Plot the gradation curves, in a semi-log graph, between percentage of
aggregates passed and size of sieve both for a) Fine aggregate and b) For
coarse aggregate.
(Note: A typical grading curve for fine aggregates looks like Figure 4. A
similar grading curve will be observed for coarse aggregates.)
Observation
Results and discussions:
The fineness modulus of given samples are:
a) For fine aggregates : __________.
b) For coarse aggregates : __________.
The grading to which the given samples belong are:
a) For fine aggregates : __________.
b) For coarse aggregates : __________.
Precautions: (Discuss about the precautions to be taken while conducting this experiment)
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Questions:
1. What are a) fine aggregate, b) coarse aggregate, and c) all-in aggregate?
2. What is the significance of measuring fineness modulus of aggregates?
3. What is a well graded aggregate? How grading of aggregates can be controlled?
4. What is the use of gradation curve?
5. Why well graded aggregate is most suitable for concreting purpose?
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