Experiment No: 09
Determination of Bulk Density (“Unit Weight”) and Voids in Aggregate
Introduction
The theory surrounding Bulk Unit Weight and Voids
in Aggregate is crucial in understanding the behavior of aggregate
materials in construction, particularly in concrete and asphalt mixtures.
Bulk Unit Weight (also known as Bulk Density) is a measure of the weight of aggregate that fills a unit volume, including the solid particles and the voids between them. It is expressed in units of mass per unit volume (e.g., kg/m³). The Bulk Unit Weight depends on factors like:
- Aggregate
Size and Shape:
Well-graded aggregates with a range of particle sizes generally have a
higher bulk density because the smaller particles fill the voids between
the larger particles.
- Compaction: The degree of compaction
affects bulk density. More compaction leads to a higher bulk density as
particles are packed closer together, reducing the volume of voids.
- Moisture
Content:
The presence of water in the pores of the aggregate increases the bulk
density. However, if the aggregate is saturated, the bulk density will
reflect both the water and the aggregate mass.
Voids in Aggregate refer to the empty spaces within
the aggregate mass that are not occupied by solid particles. These voids are
important in determining the workability, strength, and durability of concrete
and asphalt. The volume of voids is typically expressed as a percentage of the
total volume.
- Voids Content Calculation: The volume of voids can be
calculated by subtracting the volume of the solid aggregate particles from
the total volume of the aggregate.
- Effect on Concrete Mix: High voids content indicates
more space for cement paste in concrete, which may affect the workability
and strength of the mix. Reducing voids in aggregate can lead to a denser,
stronger mix.
- Aggregate Grading: Well-graded aggregates (with a
mix of different sizes) tend to have fewer voids compared to poorly graded
aggregates (with uniform size), as smaller particles fill the gaps between
larger particles.
Relationship
Between Bulk Unit Weight and Voids
The bulk unit weight and voids are inversely related. As the
bulk unit weight increases, the void content decreases, indicating a denser and
more compact aggregate structure. Understanding this relationship helps in
optimizing the mix design for concrete or asphalt, ensuring the material
achieves the desired strength, durability, and workability.
Practical
Applications
- Concrete Mix Design: The bulk unit weight and voids
are considered when calculating the amount of cement, water, and aggregate
required for a concrete mix.
- Asphalt Pavements: In asphalt mixtures, controlling the voids content is
essential to prevent excessive deformation under load, improving the
pavement's longevity.
Scope
This test method covers the
determination of bulk density (unit weight) of aggregate in a compacted or
loose condition, and calculated voids between particles in fine, coarse, or
mixed aggregates based on the same determination. This test method is applicable
to aggregates not exceeding 125 mm [5 in.] in nominal maximum size.
Purpose
To determine
the bulk unit weight and voids in aggregate in either a compacted or loose
condition.
ASTM
Designation
ASTM C29—Bulk Density (Unit Weight) and Voids in
Aggregate.
Terminology
Bulk Density –
The
mass of a unit volume of bulk aggregate material, in which the volume includes
the volume of the individual particles and the volume of the voids between the
particles. Expressed in kg/m3 [lb/ft3].
Unit Weight —
Weight
(mass) per unit volume.
Voids —
In unit volume of aggregate, the space between particles in an aggregate mass not
occupied by solid mineral matter.
Significance and Use
This test method is used to determine bulk density values
for concrete mixture proportions and conversions in purchase agreements. Aggregates
in these units usually contain absorbed and surface moisture, which affects
bulking. The test method determines bulk density on a dry basis, and a
procedure is included to compute the percentage of voids between aggregate
particles based on the determined bulk density.
Apparatus
Balance:
Balance accurate to
0.05% of the sample weight or 0.5 g, whichever is greater.
Tamping
Rod
A round, straight steel
rod, 16 mm in diameter and approximately 600 mm in length, having the tamping
end, or both ends, rounded to a hemispherical tip, the diameter of which is 16
mm.
Shovel
or Scoop
A shovel or scoop of
convenient size for filling the measure with aggregate.
Plate
Glass
A piece of plate glass, preferably at least 6 mm thick and at
least 25 mm larger than the diameter of the measure to be calibrated.
Grease
A supply of water-pump,
chassis, or similar grease.
Thermometer
A thermometer having a
range of at least 10 to 32 °C and that is readable to at least 0.5 °C.
Measure
A
cylindrical metal measure with handles is required, preferably watertight, true
and even, and rigid enough to maintain its form under rough usage. The height
should be approximately equal to the diameter, with no height less than 80% or
more than 15% of the diameter. The measure's capacity must conform to the
limits in Table 1 for the aggregate size to be tested.
Table 1 Capacity of measures
|
Nominal Maximum Size of Aggregate |
Capacity of Measure |
||
|
mm |
in |
M3 (L) |
Ft3 |
|
12.5 |
½ |
0.0028[2.8] |
1/10 |
|
25.0 |
1 |
0.0093 [9.3] |
1/3 |
|
37.5 |
1 ½ |
0.014 [14] |
½ |
|
75 |
3 |
0.028 [28] |
1 |
|
100 |
4 |
0.070 [70] |
2 ½ |
|
125 |
5 |
0.100 [100] |
3 ½ |
Figure: Cylindrical metal
measure
Test Sample
The size of
the sample shall be approximately 125 to 200 % of the quantity required to fill
the measure, and shall be handled in a manner to avoid segregation. Dry the
aggregate sample to essentially constant mass, preferably in an oven at 110 ±5
°C.
Calibration of the Measure
1. Apply a thin layer of
grease on the rim of the measure to prevent water leakage.
2. Fill the measure with
room temperature water and cover it with plate glass to remove bubbles and
excess water. Remove water that overflowed onto the measure and plate glass.
3. Determine the mass of
the water, plate glass, and measure to the nearest 0.05 kg.
4. Measure the water
temperature to the nearest 0.5°C.
5. Determine its density
from table below and interpolating if necessary.
6. Calculate the volume (V)
of the measure or factor (F) of the measure.
Table 2 Density of Water
|
Temperature |
Kg/m3 |
Lb/ft3 |
|
oC |
|
|
|
15.6 |
999.01 |
62.366 |
|
18.3 |
998.54 |
62.336 |
|
21.1 |
997.97 |
62.301 |
|
23 |
997.54 |
62.274 |
|
23.9 |
997.32 |
62.261 |
|
26.7 |
997.59 |
62.216 |
|
29.4 |
995.83 |
62.166 |
Selection of
Procedure
The shoveling method described for
loose bulk density should only be used when explicitly specified. The compact
bulk density of aggregates with a nominal maximum size of 37.5 mm or less shall
be determined using the rodding process. For aggregates with a nominal maximum
size more than 37.5 mm but not exceeding 125 mm, the jigging procedure shall be
used.
Procedure:
Rodding Procedure
· Fill a measure one-third full and level the surface with fingers. Use 25 strokes of a tamping rod to evenly distribute the aggregate layer.
· Fill the measure two-thirds full and level again.
· Fill the measure to overflow and level the surface with fingers or a straightedge.
· Ensure that slight projections of larger coarse aggregate pieces balance larger voids in the surface below the top of the measure.
· To ensure proper rodding, avoid forcibly striking the bottom of the measure in the first layer, and use vigorous effort in the second and third layers, but not more than allowing the tamping rod to penetrate.
·
Determine the mass of the measure
plus its contents, and the mass of the measure itself.
Jigging
Procedure
·
Fill
a measure with three equal layers and compact them by placing it on a firm
base, raising the opposite sides alternately about 50 mm, and dropping it with
a sharp blow. This will arrange the aggregate particles in a densely compacted
condition.
·
Drop
the measure 50 times, 25 times on each side, and level the aggregate surface
with fingers or a straightedge to balance the larger voids below the top of the
measure.
·
Determine
the mass of the measure plus its contents, and the mass of the measure itself.
Shoveling
Procedure
·
Fill
the measure to overflowing with aggregate by means of a shovel
or scoop, from a height not exceeding 50mm above the top of the measure.
·
Avoid
segregation of particle sizes and level the aggregate surface with fingers or a
straightedge.
·
Projections
of larger coarse aggregate pieces should balance larger voids below the top of
the measure.
·
Determine
the mass of the measure plus its contents, and the mass of the measure itself.
Calculation
Bulk Density—Calculate the bulk density for the
rodding, jigging, or shoveling procedure as follows:
M
=
(G – T)/V
or
M
=
(G -T) x F
where:
M = bulk density of the aggregate, kg/m3 [lb/ft3],
G = mass of the aggregate plus the measure, kg [lb],
T = mass of the measure, kg [lb],
V = volume of the measure, m3 [ft3]
F = factor for measure, m-3 [ft-3]
The
bulk density determined by this test method is for aggregate in an oven-dry
condition. If the bulk density in terms of saturated-surface-dry (SSD)
condition is desired, use the exact procedure in this test method, and then
calculate the SSD bulk density using the following formula:
Mssd = M
[1+(A/100)]
Where,
Mssd =
bulk density in SSD condition, kg/m3 [lb/ft3]
A = % absorption
Void Content—Calculate the void content in the
aggregate using the bulk density determined by either the rodding, jigging, or
shoveling procedure, as follows:
%
Voids = 100[(S x W) - M] / (S x W)
Where,
M = bulk density of
the aggregate, kg/m3 [lb/ft3]
S = bulk specific
gravity (dry basis) (ASTM C127 or C128)
W = density of water,
998 kg/m3 [62.3 lb/ft3]
Volume of Measure—Calculate the volume of a measure as follows:
V = (W –
M)/D
F =
D/(W – M)
Where:
V =
volume of the measure, m3 [ft3]
W =
mass of the water, plate glass, and measure, kg [lb]
M =
mass of the plate glass and measure, kg [lb]
D =
density of the water for the measured temperature, kg/m3 [lb/ft3], and
F =
factor for the measure, 1/m3 [1/ft3]
Report
·
Bulk density
·
Voids in aggregate
LAB Assignment Questions:
Basic Understanding
1. What
is bulk density (unit weight) of aggregate?
2. How
do you define voids in aggregates?
3. Why
is it important to determine the bulk density and voids in aggregates?
Procedure and Equipment
4. What
are the standard methods used to determine bulk density?
5. Can
you explain the procedure for determining the bulk density of aggregates?
6. What
is the purpose of tamping the aggregate in the container during the bulk
density test?
7. What
type of container is used in this test?
8. Why
is it necessary to level the surface of the aggregate after filling the
container?
9. How
does the moisture content of the aggregate affect the bulk density?
Calculation and Interpretation
10. How
do you calculate bulk density from the test data?
11. How
do you calculate the percentage of voids in the aggregate?
12. What
factors can affect the bulk density of an aggregate?
13. Why
is the bulk density of an aggregate important in concrete mix design?
14. How
does the shape and size of aggregate particles affect the bulk density and
voids?
Standards and Specifications
15. Which
standards (e.g., ASTM, IS) are followed for determining bulk density and voids
in aggregates?
16. What
is the typical range of bulk density for coarse and fine aggregates?
17. What
are the acceptable limits of void content in aggregates for concrete?
Application and Practical Considerations
18. How
does the bulk density of aggregate influence the properties of fresh and
hardened concrete?
19. In
what scenarios might you want to maximize or minimize the void content in an
aggregate?
20. What
precautions should be taken while conducting the bulk density and voids test?
References:
ASTM C29: Standard Test Method for Bulk Density
(“Unit Weight”) and Voids in Aggregate.