Aggregate

Aggregates

Introduction

aggregates occupy at least 3 parts of the volumeof the concrete.

1. Composition
2. Size & Shape
3. Surface Texture
4. Specific Gravity
5. Bulk Density
6. Voids
7. Porosity & Absorption
8. Bulking of Sand
9. Fineness Modulus of Aggregate
10. Surface Index of Aggregate
11. Deleterious Material
12. Crushing Value of Aggregate
13. Impact Value of Aggregate
14. Abrasion Value of Aggregate

1. COMPOSITION

Aggregates consisting of materials that can react with alkalies in cement and cause excessive expansion, cracking and deterioration of concrete mix should never be used. Therefore it is required to test aggregates to know whether there is presence of any such constituents in aggregate or not.

2. SIZE & SHAPE

The size and shape of the aggregate particles greatly influence the quantity of cement required in concrete mix and hence ultimately economy of concrete. For the preparation of economical concrete mix on should use largest coarse aggregates feasible for the structure. IS-456 suggests following recommendation to decide the maximum size of coarse aggregate to be used in P.C.C & R.C.C mix.

Maximum size of aggregate should be less than

• One-fourth of the minimum dimension of the concrete member.
• One-fifth of the minimum dimension of the reinforced concrete member.
• The minimum clear spacing between reinforced bars or 5 mm less than the minimum cover between the reinforced bars and form, whichever is smaller for heavily reinforced concrete members such as the ribs of the main bars.

Remember that the size & shape of aggregate particles influence the properties of freshly mixed concrete more as compared to those of hardened concrete.

3. SURFACE TEXTURE

The development of hard bond strength between aggregate particles and cement paste depends upon the surface texture, surface roughness and surface porosity of the aggregate particles.

If the surface is rough but porous, maximum bond strength develops. In porous surface aggregates, the bond strength increases due to setting of cement paste in the pores.

4. SPECIFIC GRAVITY

The ratio of weight of oven dried aggregates maintained for 24 hours at a temperature of 100 to 1100C, to the weight of equal volume of water displaced by saturated dry surface aggregate is known as specific gravity of aggregates.

Specific gravities are primarily of two types.

• Apparent specific gravity
• Bulk specific gravity

Specific gravity is a mean to decide the suitability of the aggregate. Low specific gravity generally indicates porous, weak and absorptive materials, whereas high specific gravity indicates materials of good quality. Specific gravity of major aggregates falls within the range of 2.6 to 2.9.

Specific gravity values are also used while designing concrete mix.

BULK DENSITY

It is defined as the weight of the aggregate required to fill a container of unit volume. It is generally expressed in kg/litre.

Bulk density of aggregates depends upon the following 3 factors.

• Degree of compaction
• Grading of aggregates
• Shape of aggregate particles

6. VOIDS

The empty spaces between the aggregate particles are known as voids. The volume of void equals the difference between the gross volume of the aggregate mass and the volume occupied by the particles alone.

 7. POROSITY & ABSORPTION

The minute holes formed in rocks during solidification of the molten magma, due to air bubbles, are known as pores. Rocks containing pores are called porous rocks.

Water absorption may be defined as the difference between the weight of very dry aggregates and the weight of the saturated aggregates with surface dry conditions.

Depending upon the amount of moisture content in aggregates, it can exist in any of the 4 conditions.

• Very dry aggregate ( having no moisture)
• Dry aggregate (contain some moisture in its pores)
• Saturated surface dry aggregate (pores completely filled with moisture but no moisture on surface)
• Moist or wet aggregates (pores are filled with moisture and also having moisture on surface)

8. BULKING OF SAND

It can be defined as in increase in the bulk volume of the quantity of sand (i.e. fine aggregate) in a moist condition over the volume of the same quantity of dry or completely saturated sand. The ratio of the volume of moist sand due to the volume of sand when dry, is called bulking factor.

Fine sands bulk more than coarse sand

When water is added to dry and loose sand, a thin film of water is formed around the sand particles. Interlocking of air in between the sand particles and the film of water tends to push the particles apart due to surface tension and thus increase the volume. But in case of fully saturated sand the water films are broken and the volume becomes equal to that of dry sand.

1. Aggregates: Bulk Density, Specific Gravity and Voids

Bulk Density
River sand
Fine	1.44
Medium	1.52
Coarse	1.60
Beach or river shingle	1.60
Broken stone	1.60
Stone screenings	1.44
Broken Granite	1.68

Specific Gravity
Trap	2.9
Granite	2.8
Gravel	2.66
Sand	2.65

Voids, Percent, Average
River sand
Fine	43
Coarse	35
Mixed and moist	38
Mixed and dry	30
Broken stone, graded
25 mm maximum size	46
50 mm maximum size	45
63 mm maximum size	41
Stone screenings	48

2. Aggregates: Limiting Values of Mechanical Properties

Properties	Limiting Values (Percentage)
	For Wearing Surfaces	Other Than for Wearing Surfaces
Crushing value	30	45
Impact value	30	45
Abrasion value (Los Angeles)	30	50
Soundness (Average loss of weight after 5 cycles)	When tested with Na2SO4	When tested with MgSO4
Fine aggregates	10	15
Coarse aggregates	12	18

3. Approximate Water Absorption of Aggregates, by Weight

Average sand	1.0 percent
Pebbles and crushed limestone	1.0 percent
Trap rock and granite	0.5 percent
Porous sandstone	0.5 percent
Very light and porous aggregates may absorb as much as	25 percent by weight

4. Limits of Deleterious Materials (Percentage by Weight, Maximum)

Deleterious Substances	Fine Aggregates		Coarse Aggregates
	Uncrushed	Crushed	Uncrushed	Crushed
Coal and lignite	1.00	1.00	1.00	1.00
Clay lumps	1.00	1.00	1.00	1.00
Materials finer than 75-micron sieve	2.00	15.00	3.00	3.00
Shale	1.00	–	–	–
Total of percentages of all deleterious materials **	5.00	2.00	5.00	5.00

5. Bulking of Sands for Various Moisture Contents

Moisture, percent	Percentage bulking in
	Fine sand	Medium sand	Coarse sand
1	16	8	6
2	26	16	12
3	32	22	15
4	36	27	17
5	38	29	18
6	37	28	18
8	35	26	16
10	32	22	12
12	28	19	8
15	22	12	2
17	18	7	0
20	9	0	0
27	0	0	0

6. Surface Water Carried by Aggregates

Aggregates	Approximate Qty of Surface Water
	Percent by mass	Ltr./cu.m
Very wet sand	7.5	120
Moderately wet sand	5.0	80
Moist sand	2.5	40
Moist gravel or crushed rock	1.25 – 2.5	20 – 40

Note: Coarser the aggregate, less the water it will carry. Source: Table 10 of IS: 456 – 2000.