Relative Density and Absorption of Aggregate

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Important Properties

Hardness (resistance to wear)

Durability (resistance to weathering)

Shape and surface texture

Relative density and absorption

Gradation

Deleterious substances

Crushing strength

Soft and lightweight particles

Chemical stability

Aggregate Moisture

Though they look solid, all aggregate particles havepervious pores that can absorb water.

Water can also be present on the aggregate surfaceand trapped in the void spaces between particles. Thelatter is called “free water.”

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Aggregate Moisture

When designing a portland cement concrete mix, it iscrucial to account for the aggregate moisture.

If the pores aren’t completely filled with water, theaggregate will absorb water from the cement paste. Ifthere is free water present, it will be incorporatedinto the cement paste.

Either way, the properties of the cement paste will bealtered from what the mix designer intended.

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Aggregate Moisture

Oven-Dry(no moisture)

SaturatedSurface

Dry (SSD)

Wet(some surface

moisture)

Air-Dry(some absorbed

moisture)

Cement pastewill losemoisture

Cement pastewill gainmoisture

Cement pasteis unaffected

Absorption

The amount of moisture needed to make an aggregateexactly SSD is called the absorption. It is generallyexpressed as a percentage of the dry aggregate mass.

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Absorption

AW

D

M% Abs 100%

M

MD = mass of dry aggregate

MAW = mass of absorbed water

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Absorption

Mass ofSSD

Aggregate

Mass ofOven-DryAggregate

Mass ofAbsorbed

Water

Achieving an SSD State

Coarse aggregate should be soaked in room temperature water for 24±4 hours then rolled in a large absorbent cloth (i.e., towel) until all visible surface moisture is removed.

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https://www.pavementinteractive.org/reference-desk/testing/aggregate-tests/coarse-aggregate-specific-gravity/


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https://www.slideshare.net/BSGLY/block-9-agg-specific-gravity-13


Fine aggregate should be brought to a watercontent of at least 6% and allowed to standfor 24±4 hours. The aggregate is then spreadon a nonabsorbent surface and a stream of airis blown over the sample (using a fan or a hairdryer on the lowest setting) until it attains afree-flowing condition.

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https://www.pavementinteractive.org/reference-desk/testing/aggregate-tests/fine-aggregate-specific-gravity/


The accuracy of this test depends on knowingthe exact point when the aggregate achieves afree-flowing condition. One way to do that isto place the aggregate into a small brass coneand lightly tamp it. If the aggregate retainsthe shape of the cone when it is removed,there is still free water present.

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Particle Density

A 1-ft3 bucket holds 100 lb. of aggregate. What fraction of the volume is occupied by the voids between the particles? (This is the void content.)

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Particle Density

First we need to find the volume of the aggregate particles, which means we need to know the particle density.

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Particle Density

Then we can subtract the aggregate volume from the volume of the bucket to get the volume of the voids between particles.

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Particle Density

Particle density is the ratio of the massof an aggregate particle to its own volume(also called mass density of solids).

3 3

M kg gor

V m cm

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Particle Density

Particle density can also be expressedas the ratio of the weight of an aggregateparticle to its own volume.

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3

W lbV ft

Relative Density

Relative density is the mass densityof an object relative to the mass densityof water (also called specific gravity).

w

RD w

M V

w

MV

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Relative Density

Relative density (specific gravity) canalso be expressed in weight-based units.

w

RD w

W V

w

WV

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Relative Density

Relative density depends on the volumeyou assume for the aggregate particles.

NetVolume

BulkVolume

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Relative Density

It also depends on the mass you assumefor the aggregate particles.

SSDMass

DryMass

Apparent Relative Density

D

NA

w

MR

VD

Net volume

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Bulk (OD) Relative Density

D

BB

w

MR

VD

Bulk volume

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SSD Relative Density

SSD

BSSD

wVM

RD

SSD aggregate

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Example

An aggregate sample has an oven-dry mass of3954.2 g, an SSD mass of 4006.8 g, and a netvolume of 1532.6 cm3.

Find RDA, RDB, RDSSD, and %Abs

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Question

A 1-ft3 bucket holds 100 lb of the aggregate from the last example. How much volume is occupied by the voidsbetween the aggregate particles?

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Question

A 1-m3 bucket holds 1500 kg of the aggregate from the last example. How much volume is occupied by the voidsbetween the aggregate particles?

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Question

A 1-m3 bucket holds 1500 kg of aggregate from the last example. How much water can you add to the bucket without it overflowing?

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Measuring Relative Density

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Coarse Aggregate


w

MRD

Vmass of water displaced

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aggregate in air

water displaced

MRD

M

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To determine the relative density of a coarseaggregate sample, we can use the concept ofbuoyancy.

When a solid object is submerged in water, itweighs less because of the buoyant forceproduced by the water.

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The buoyant force acting on a submergedobject is equal to the weight of water displacedby the object.

That’s why we feel lighter in the water thanon land. It’s also why concrete canoes float!

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Buoyancy

Mwater displaced × g

Min water × g

= Min air × g

in water in air water displacedM M M

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aggregate in air aggregate in air

water displaced in air in water

M MRD

M M M

water displaced in air in waterM M M

Procedure

1. Soak aggregate in water for 24 ± 4 hours

2. Pour aggregate into wire basket to drain

3. Spread aggregate onto towel and dry to SSD

4. Weigh aggregate in air to obtain SSD mass

5. Place aggregate back in wire basket

6. Weigh aggregate suspended in water

7. Oven dry aggregate overnight

8. Weigh aggregate in air to obtain oven-dry mass


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https://www.pavementinteractive.org/reference-desk/testing/aggregate-tests/coarse-aggregate-specific-gravity/

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in air

Ain

OD

ODair in water

MRD

M M

Net volume

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in air

Bin aSSD

OD

ir in water

MRD

M M

Bulk volume

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in air

SSDi

SSD

SSDn air in water

MRD

M M

Bulk volume

Example

An SSD aggregate sample has a mass in air of4007 g and a mass suspended in water of 2426 g.After drying overnight, it has an oven-dry mass of3954 g.

Find RDA, RDB and RDSSD

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Fine Aggregate

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To determine the relative density of a fineaggregate sample, we indirectly measure themass of water displaced by the aggregate bycomparing the mass of a container filled withjust water with the mass of the same containerfilled with aggregate and water. The differenceis the mass of water that has been displacedby the aggregate.

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Mwater MblendMsand

~ 500 g

500 mlor

~ 500 g~ 500 g

< 500 g

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water displaced water sand blendM M M M

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aggregate in air

water sand blend

MRD

M M M

water displaced water sand blendM M M M

Procedure

1. Spread moist sand on counter and dry to SSD

2. Deposit 500 ± 10 g of sand into shallow pan

3. Weigh sand in air to obtain SSD mass

4. Weigh pycnometer filled with 500 ml clean water

5. Carefully pour sand into pycnometer

6. Weigh pycnometer with sand/water blend

7. Pour sand into metal pan and oven dry overnight

8. Weigh sand in air to obtain oven-dry mass


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Fill line (500 ml)“pycnometer”


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Fill lineFill line (500 ml)“pycnometer”

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sandA

water sand

OD

ODblend

MRD

M M M

Net volume

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sandB

waterSsandS

blend

D

D

OMRD

M M M

Bulk volume

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sandSSD

water sand

SSD

SSblen

Dd

MRD

M M M

Bulk volume

Example

A pycnometer filled with 500 ml of clean waterhas a mass of 660 g. After adding 495 g of SSDsand and refilling it to the 500-ml mark, it hasa mass of 965 g. After oven drying overnight,the sand has a mass of 489 g.

Find RDA, RDB and RDSSD

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Relative Density and Absorption of Aggregate

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