The sieves used for making a sieve analysis should conform to BS 410 or BS EN 933-2. The tests should be carried out in accordance with the procedure given in BS 812 or BS EN 933-1
An aggregate containing a high proportion of large particles is referred to as being 'coarsely' graded and one containing a high proportion of small particles as 'finely' graded. Grading envelopes for sand of grading M and for 20 mm graded coarse aggregate are shown in Figure 2.
Grading limits for 'all-in' aggregates are also given in BS 882/BS EN 12620. All-in aggregate, composed of both fine and coarse
Table :
Grading limits for coarse aggregate
(from
BS 882 : 1992).
Sieve size (mm )
|
Percentage by mass passing BS sieves
for nominal sizes
|
||||||
Graded aggregate
|
Single-sized aggregate
|
||||||
4
0 m m t o 5 m m
|
2
0 m m t o 5 m m
|
14 m m t o 5 m m
|
4
0 m m
|
2
0 m m
|
14 m m
|
10 m m
|
|
50
37.5
20
14
10
5
2.36
|
100
90 - 100
35
- 70
25
- 55
10
- 40
0
- 5
-
|
-
100
90 - 100
40 - 80
30 - 60
0 - 10
-
|
-
-
100
90 - 100
50 - 85
0 - 10
-
|
100
85 - 100
0 - 25
-
0 - 5
-
-
|
-
100
85
- 100
0 - 70
0 - 25
0 - 5
-
|
-
-
100
85
- 100
0 - 50
0 - 10
-
|
-
-
-
100
85 - 100
0 - 25
0 - 5
|
aggregate, should not be used for structural reinforced concrete work because the grading will vary considerably from time to time and hence from batch to batch, resulting in excessive variation in the consistence and the strength. To ensure that the proper amount of sand is present, the separate delivery, storage and batching of coarse and fine aggregates is essential.
Coarse aggregates
For a high degree of control over the production of concrete, and particularly where high-quality surface finishes are required, it is necessary for the coarse aggregate to be delivered, stored, and batched using separate single sizes rather than a graded coarse aggregate.
The sieve sizes in general use are 50, 37.5, 20, 14, 10, 5 and 2.36 mm for coarse aggregate. The grading limits are shown in Table 5.
Graded coarse aggregates which have been produced by layer loading - (filling a lorry with, say, two grabs of 20 - 10 mm and one grab of 10 - 5 mm) - are seldom satisfactory because the materials are unmixed and will not be uniformly graded. Graded coarse aggregate should be mixed efficiently by the producer before loading lorries.
Sand
The sieve sizes in general use are 10, 5, 2.36, 1.18 mm and 600,
300 and 150 μm. The fines content is determined by wet sieving through a 75 μm sieve.
The C-M-F system of classification for sand in Table 6 is useful for selecting appropriate proportions of fine and coarse aggregates in a mix, because the optimum proportion of sand is partly related to its fineness. Good concrete can be made with sand within the overall limits shown in Table 6. Where the variability of grading needs to be restricted further for the design of particular mixes or for the adjustment of sand content of prescribed concrete mixes, this can be achieved by reference to one or more of the three additional grading limits C, M or F. Sand with grading F should normally be used only after trial mixes have been made with the proposed combination of sand and coarse aggregates and cement to determine their suitability for the particular purpose. There may be occasions, such as when a high degree of control is required, and when high-quality surface finishes have to be achieved, when it is necessary to specify sand gradings to closer limits than those permitted in BS 882/BS EN 12620 and shown in Table 6. On the other hand, sand whose gradings fall outside the Standard limits may produce perfectly satisfactory concrete. It is not so much the grading limits themselves that are important, but that the grading is maintained reasonably uniform.
*Increased to 20% for crushed rock sand except when they are used for heavy-duty floors NOTE Sand not conforming to this table may also be used provided that the supplier can satisfy the purchaser that such materials can produce concrete of the required quality. For heavy-duty concrete floor finishes, the sand should conform to gradings CorM . | ||||||||||||||||||||||