Aggregate Grading

Objectives:

To explain the following:

o Definition and benefits of grading

o How to do grading

o Grading curves

o Well-graded aggregate

o Fineness modulus

o Fine aggregate grading

o Coarse aggregate grading

o Maximum size of aggregate 0 Gap-graded aggregates

o Effects of aggregate size and grading on reinforcement corrosion

(V) Aggregate Grading

The grading is the particle-size distribution of an aggregate. Grading is one of the most important characteristics of the aggregates

The grading and maximum size of aggregate affect the following:

o Relative aggregate proportions (i.e. FA/CA and FA/TA ratios)

o Cement and water requirements

o Workability and pumpability of fresh concrete: very coarse sands and coarse aggregate can produce harsh, unworkable mixes

o Uniformity of concrete from batch to batch

o Porosity, shrinkage, and durability of hardened concrete

o Economy in concrete production: very fine sands are often uneconomical

In general, aggregates that do not have a large deficiency or excess of any size and give a smooth grading curve, are found to be satisfactory

How to do the grading

Grading of the aggregates is determined by a sieves analysis (ASTM C 136)

The sieves used for grading the aggregates are made of wire-mesh with square openings, as shown in Fig. 4-2

Figure 4.2

The grading and grading limits are usually expressed as the percentage material (by weight) passing each sieve (Design Control of Concrete Mixture by Kosmatka).

The seven standard ASTM C 33 sieves for fine aggregate grading and as follows:

Sieve size from top to bottom3/8 in. (9.5 mm)No.4 (4.75 mm)No.8 (2.36 mm)

No. 16 (1.18 mm) No. 30 (600 μm) No. 50 (300 μm)

No. 100 (150 μm)

The thirteen standard ASTM C 33 sieves for coarse aggregate grading are as follows:

Sieve size from top to bottom 100 mm (4 in.)

90 mm (31/2 in.) 75 mm (3 in.)

63 mm (21/2 in.) 50 mm (2 in.)

37.5 mm (11/2 in.) 25 mm (1 in.)

19 mm ( 3/4in.) 12.5 mm ( 1/2 in.) 9.5 mm ( 3/8 in.) 4.75 mm (No. 4) 2.36 mm (No. 8)

1.18 mm (No. 16)

The grading of coarse aggregates is done for 13 size numbers (grading sizes), as follows:

Size number Nominal size (sieves with square openings)

1 3½ to 1½ in.

2 2½ to 1½ in.

3 2 to 1 in.

357 2 in. to No. 4

4 1½ to ¾ in.

467 1½ in. to No.4

5 1 to 1½ in.

56 1 to 3/8 in.

57 1 to No.4

6 ¾ to 3/8 in.

67 ¾ in. to No. 4

7 ½ in. to No. 4

8 3/8 in. to No. 8

Size numbers (grading sizes) for coarse aggregate apply to the amounts of aggregate (by weight) in percentages that pass through a set of sieves arranged from top to bottom “in the different size ranges”, as indicated in the above table

For example: A coarse aggregate is said to be graded with size number 57 if it is sieved through a set of sieves in the size range of 1 in. to No.4 (1 in., % in., 1/2 in., % in., and No.4)

The sieving results (i.e. the percentages of materials by weight passing through the various sieves) are plotted to obtain the grading curves

Grading curves indicate the limits of particle size distribution of the aggregates

Standard curves as developed by the ASTM C 33 for fine aggregates and coarse aggregates should be used to compare with the original grading curves obtained for the aggregates in hand

Standard grading curves as given by the ASTM C 33 for fine aggregate and c oarse aggregate of grading size no. 67 are shown in Fig.4-3

Fig. 4-3. Curves indicate the limits specified in ASTM C 33for fine aggregate and for one typically used size number (Design Control of Concrete Mixture by Kosmatka).

The aggregates having uniform size and shape contain more volume of voids as compared to the aggregates having different size and shape, as illustrated below with the help of Fig. 4-4

Fig. 4-4. The level of liquid in the graduates, representing voids, is constant for equal absolute volumes of aggregates of uniform but different size. When different sizes are combined, the void-content decreases. The illustration is not to scale. (Design Control of Concrete Mixture by Kosmatka).

Therefore, an aggregate is said to be well graded when it has particles of different shapes and sizes because in this case the total volume of voids is less

The amount of paste required to fill the voids in aggregate is more than the volume of voids between the aggregates, as illustrated in Fig. 4-5

Fig. 4-5. Illustration of the dispersion of aggregates in cohesive concrete mixtures. (Design Control of Concrete Mixture by Kosmatka).

The extra amount of paste required for providing workability to concrete depends on the desired degree of workability and the cohesiveness of the paste

Fineness Modulus (FM)

The fineness modulus (FM) of either fine or coarse aggregate according to ASTM C 125 is obtained by adding the cumulative percentages by weight retained on each of a specified series of sieves and dividing the sum by 100.

The specified sieves for determining FM are: No.100, No.50, No.30, No.16, No.8, No.4, 3/8 in. ¾ in. 1½ in. 3 in., and 6 in.

An example of determining the FM of a fine aggregate with an assumed sieve analysis, as follows:

FM is an index of the fineness of an aggregate - the higher the FM, the coarser the aggregate

The FM can be looked upon as a weighted average size of a sieve on which the material is retained, the sieves being counted from the finest.

Different aggregate grading may have the same FM

FM of fine aggregate is useful in estimating proportions of fine and coarse aggregate (FA/CA ratio) in concrete mixtures

Fine-Aggregate Grading

Fine-aggregate grading depends on the type of work, the richness of mix, and the maximum size of coarse aggregate

Sieve size

Percentage of individualfraction

retained, byweight*

Cumulativepercentagepassing, by

weight

Cumulativepercentage retained, by

weight

3/8” No.4 No.8 No.16 No.30 No.50 No.100

Pan

Total

0213202024183

___________100

100988565452130

02

1535557997-

___________283

Fineness modulus= 283 ÷ 100 = 2.83

Fine-aggregate grading within the limits of ASTM C 33 is generally satisfactory for most concretes. The ASTM C 33 limits with respect to sieve size are as follows:

Sieve size Percent passing by weight3/8 in. (9.5 mm) 100

No.4 (4.45 mm) 95 to 100

No.8 (2.36 mm) 80 to 100

No.16 (1.18 mm) 50 to 85

No.30 (600 μm) 25 to 60

No.50 (300 μm) 10 to 30

No.100 (150 μm) 2 to 10

The fine aggregate must not have more than 45% retained between any two consecutive standard sieves

The FM of fine aggregate must be not less than 2.3 nor more than 3.1, nor vary more than 0.2 from the typical value of the aggregate source

The amounts of fine aggregate passing the No. 50 and No. 100 sieves affect workability, surface texture and bleeding of concrete

For hand-finished concrete floors or where a smooth surface texture is desired, fine aggregate with at least 15% passing the No.50 sieve and 3 % or more passing the No.100 sieve should be used

Coarse-Aggregate Grading

The coarse aggregate grading requirements of ASTM C 33 permit a wide range in grading and a variety of grading sizes, as shown in Table 7-5

Mixture proportions should be, changed to produce workable concrete if wide variations occur in the coarse-aggregate grading

SEE Table 7-5 (Handout- from Mehta text)

The maximum size of coarse aggregate used in concrete has a bearing on economy. Usually more water and cement is required for small-size aggregates than for large sizes

The water and cement required for a slump of approximately 3 in. and w/c ratio = 0.54 by wt. is shown in Fig. 4-6

Fig. 4-6. Cement and water contents in relation to maximum size of aggregates, for air-entrained and non-air-entrained concrete. Less cement and water are required in mixes having large, coarse aggregate. (Design Control of Concrete Mixture by Kosmatka).

Fig. 4-6 shows that, for a given w/c ratio, the amount of cement required decreases as the maximum size of coarse aggregate increases

In some instances, at the same w/c ratio, the concrete with the smaller maximum-size aggregate has higher compressive strength

The maximum size of coarse aggregate that can be used generally depends on the size and shape of the concrete member and the amount and distribution of reinforcing steel

The maximum size of aggregate particles generally should not exceed the following:

o One-fifth the narrowest dimension of a concrete member

o Three-fourths the clear spacing between reinforcing bars

o One-third the depth of slabs

Gap-Graded Aggregates

In gap-graded aggregates certain particle sizes are omitted

Gap-graded mixes are used to obtain uniform textures in exposed-aggregate concrete

For cast-in-place concrete, typical gap-graded aggregates consist of only one size of coarse aggregate with all the particles of fine aggregate able to pass through the voids in the compacted coarse aggregate

For an aggregate of ¾ in. maximum size, the No.4 to 3/8 .in. particles can be omitted without making the concrete unduly harsh or subject to segregation

In the case of 1½ in. aggregate, usually the No.4 to ¾ , in. sizes are omitted

Care must be taken in choosing the percentage of fine aggregate in a gap-grade mix. Fine aggregate is usually 25% to 35% by volume of the total aggregate. The lower percentage is used with rounded aggregates and the higher with crushed materials

In order to prevent the segregation of gap-graded mixes the slump should be restricted between zero and 3 in. depending on the thickness of the section, amount of reinforcement and height of casting

EFFECT OF AGGREGATE SIZE AND GRADING ON REINFORCEMENT CORROSION

It has been observed that for a given w/c ratio, the coefficient of permeability of concrete increases considerably with an increasing size of aggregate.

Keeping this in view, maximum size of aggregate is recommended as:

1½ in. for 5000 psi concrete ¾ in. for 6000 psi concrete 3/8 to ½ in. for > 6000 psi concrete

The proportioning of coarse and fine aggregates is important for production of a workable and durable concrete.

Enhanced workability on account of an optimum aggregate grading allows a reduction in the w/c ratio resulting in increased strength and durability of concrete.

The aggregate proportioning achieving good workability consists of fixing the optimum volume fraction of sand in the total aggregate content, as shown in the following Fig.:

Fig- Influence of aggregate proportioning on concrete workability, and thus durability. (Design Control of Concrete Mixture by Kosmatka).

The optimum proportioning for a maximum workability has been reported to be corresponding to a specific surface area of the combined aggregates in the range of 70 to 75 cm2/cm3 for concrete with cement content in the range of 300 to 390 Kg/m3, as shown in the following Fig.:

Fig- Effect of surface area of aggregate on the workability of concrete and thus durability. (Design Control of Concrete Mixture by Kosmatka).