ENCI 579 41 Highway Materials, Soils, and Concrete Aggregates Soil Definition (Engineering) –“refers to all unconsolidated material in the earth’s crust,

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Highway Materials, Soils, and ConcreteAggregates

• Soil Definition (Engineering)– “refers to all unconsolidated material in the

earth’s crust, all material above the bedrock”

• mineral particles (gravel, sand, silt, clay)

• organic material (top soil, marshes)

• Aggregates– mineral particles of a soil

– specifically, granular soil group• gravel, sand, silt

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• Granular Soil Group (Aggregates)– Physical weathering

• action of frost, water, wind, glaciers, plant/animals

• particles transported by wind, water, ice

• soils formed are called granular soil type

• “grains are similar to the original bedrock”

– Larger grain sizes than clays– Particles tend to be more or less spheres/cubes– Bound water is small compared to overall mass

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• Granular Soil Group (Aggregates)– ability to achieve greater densities

• well graded granular material

– increased soil strength

– lower permeability

– reduced future settlement • These improvements dictate the use of

aggregates in pavement layers where wheel loads are greater

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• In combination with asphalt cement or portland cement to form asphalt concrete or cement concrete respectively

• In subbases and bases of a roadway structure

• drainage structures

• concrete blocks

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• Types of Aggregates

• Basic properties of these aggregates

• Tests used to evaluate these properties

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• Aggregate Sources– natural sand or gravel deposits *– crushed rock *– slag and mine refuse– rubble and refuse– artificial and processed materials– pulverized concrete and asphalt pavements– other recycled and waste materials

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• Natural sand and gravel deposits– sand and gravel pits– sand and gravel soils that have been

naturally sorted to eliminate most of the silt or clay sizes then deposited in:

• glacier formations (eskers, outwash plains)

• river deposits

• beaches of current and previous lakes and seas

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• Sand / Gravel Pit Development– Stripping of topsoil, vegetation… from surface

– Excavation of material• material is loose - front end loaders

– Crushing of the material• larger size aggregate is broke down to desired size

• crushed gravel is considered high quality aggregate

• washing of aggregate cleans dust removes silt/clay

– Type of material depends on bedrock source• Limestone, sandstone,granite,etc.

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• Crushed Rock– “Type of aggregates produced from

quarries depends on the type of bedrock”– Classes of Rocks

• Igneous rocks

• Sedimentary rocks

• Metamorphic rocks

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• Crushed Rock– Igneous Rocks

• Original bedrock formed from the cooling of molten material

• Coarse grained igneous rock (granite) cooled slowly

• Fine grained igneous rocks (basalt) cooled rapidly

– Sedimentary Rocks• Solidification of chemical or mineral sediments

deposited under ancient seas

• Layered since original material was deposited in this manner

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• Crushed Rock– Sedimentary Rocks

• Limestone Calcium Carbonate

• Dolomite Calcium/Magnesium Carbonate

• Shale Clay

• Sandstone Quartz

• Chert Fine sand

• Conglomerate Gravel

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• Crushed Rock– Metamorphic Rocks– Igneous or Sedimentary rocks that have been

metamorphosed due to intense heat and pressure

• Slate shale

• Marble limestone

• Quartzite sandstone

• Gneiss granite

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• Crushed Rock

– Igneous and Metamorphic rocks are very hard and make an excellent source for aggregates

– Limestone and Dolomite are common sedimentary rocks, are softer than igneous rocks but are acceptable for aggregates

– Shale primarily composed of clay grains is weak and disintegrates easily

– Chert also disintegrates easily

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Highway Materials, Soils, and Concrete Aggregates

• Quarry Pit Development– Opening and Stripping of the face of the quarry– Blasting of rock with dynamite into sizes that

can be transported– Crushing of rock into the required aggregate

sizes

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• Slag and Mine Refuse– Slag is a waste material resulting from the

treatment of ore to produce iron, steel, nickel, ..• Blast furnace slag from iron mills is a common

aggregate

– Mine tailings can also be used for aggregates

• Rubble and Refuse– recycling of pulverized concrete from structures

– recycled asphalt pavements in base courses

– recycled rubber, crushed glass in base courses

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• Types of Aggregates– Fine Aggregate

• aggregate particles mainly between the 4.75 mm size and the 75um sieve.

– Coarse Aggregate• aggregate particles mainly larger than 4.75 mm

– Pit Run• aggregate from a sand or gravel pit with no

processing

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• Types of Aggregates– Crushed Gravel

• pit gravel (or sand) that has been put through a crusher either to break the rounded gravel particles into smaller sizes or to produce rougher surfaces

– Crushed Rock• aggregate from the crushing of bedrock. All

particles are angular and not rounded as in gravel

– Screenings• chips, dust, powder that are produced from crushing

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• Types of Aggregates– Concrete Sand

• sand that has been washed to remove dust and fines

– Fines• silt, clay, or dust particles smaller than 75um usually

the undesirable impurities in aggregates

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• Types of Aggregates– Nominal Size

• Not economical to have 100% of the particles of an aggregate be within a specified size range.

• Reduce as much reject as possible from a pit in order to efficiently use the material resources of a pit

• Usually 5% - 10% of the aggregate particles can be allowed to be larger or smaller than specs

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• Types of Aggregates– Nominal Size

• Coarse aggregates 19-4.75 mm nominal aggregate

• Fine aggregate4.75 mm nominal aggregate

– Clear• a single size coarse aggregate is called clear. Most of

the particles are between the specified maximum size and a minimum size which is defined as one-half of the maximum

• 19 mm clear aggregate

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• Aggregate Properties– Gradation (grain size analysis)

• grain size distribution for highway bases and asphalt mixes that will provide a dense strong mixture

• ensure that the voids between the larger particles are filled with medium particles. The remaining voids are filled with still smaller particles until the smallest voids are filled with a small amount of fines.

• Ensure maximum density and strength using a maximum density curve

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Theoretical Maximum Density Curves• Fuller Maximum Density Curve

P = (d/D)0.5

P = % passing sieve size ‘d’ and ‘D’ represents the maximum sieve size (100% passing)

• Federal Highway Administration

P = (d/D)0.45

– plotted on semi-log paper where sieve sizes are raised to power 0.45

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Highway Materials, Soils, and ConcreteAggregate Gradations

• Strength or resistance to shear failure in road bases and pavements is increased greatly if the mixture is dense graded

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a) larger particles are in contact with each other developing frictional resistances to shearing failure. Particles are tightly bond together due to the inter-locking effect of smaller particles. This effect is higher in (a) than (b)

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b) Must limit the amount of fines– silt and clay particles are relatively weak– dust on larger aggregates will interface with the

aggregate/asphalt bond– Excessive fines in a base or subbase may lead to

drainage on frost leaving problems– Excessive fines (smaller aggregates) results in weak

structures because larger particles are not in contact with each other strength

– Sn (smaller particles) are weaker. Therefore managing % of fines is important.

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Highway Materials, Soils, and ConcreteAggregatesWashed Sieve Analysis• as a result it is important in determining the amount

passing the 75m sieve• sample is dried and washed, wash water poured out

over the 75 um sieve• material retained is returned to the sample for sieve

analysis• total amount passing 75 um is equal to the amount

lost in washing and % passing 75 um sieve

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Highway Materials, Soils, and ConcreteAggregatesExample 4-1

Mass of sample 446.7 gMass after washing 414.1 g

Results of dry sieving:Retained in 4.75 mm 0.0g

1.18 mm 205.3g300 mm 127.9g75 mm 76.4gPan 3.8g

Find the grain-size distribution:Lost in washing over 75 mm= 446.7 g - 414.1 g= 32.6 gPassing 75 mm in sieving 3.8 gTotal finer than 75 mm 36.4 g

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Note: Only 0.7 g was lost during sieving, which is an acceptable loss.) If a washed sieve analysis is not required, usually for coarse aggregates the procedure for grain-size analysis of soils (see Section 1-3.3) is used

Calculations:

SieveRetained (g)

Percentage Retained

Cumulative Passing

4.75 mm 0 0 100%1.18 mm 205.3 46.0 54300 m 127.9 28.7 25.375 m 76.4 17.1 8.2Pan 36.4 8.2

446.0 100%

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• Aggregate Properties– Gradation (grain size analysis)

• High density mixtures are important in terms of density and asphalt cement required. Asphalt must coat each particle and fill in most of the void space. If you fill in void space with cheaper material such as aggregates you save asphalt

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• The relative density (specific gravity) and absorption of aggregates are important properties especially in asphalt cement mixtures– In the mix designs, it is important to measure

accurately the volumes occupied by the aggregate and any water that may have seeped into the pores in the particles. Therefore voids must be considered in the aggregate.

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• For aggregates– Dry Mass = MD

– Total Mass = MSSD

• (dry mass MD + absorbed water MWA)

– Bulk Volume = VB

• (includes volume of absorbed water)

– Net Volume = VN

• VN = VB - volume of absorbed water

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• Relative density calculations are made as follows:Apparent RDA = MD/(VN x

W)

Bulk RDB = MD/(VB x W)

Saturated, surface-dry RDSSD = MSSD/(VB x W)

Percentage absorption % Abs = MWA/MD

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• Saturated Surface Dry– all permeable pores filled with water

Bulk Volume = VNET + VABSORBED WATER

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is 2268.4 g. The net volume is 835.4 cm3. Find the relative density values.

Mass of absorbed water 2268.4 - 2239.1 = 29.3 g

Volume of absorbed water 29.3 g = 29.3 cm3

1 g/cm3

Bulk Volume 835.4 cm3 + 29.3 cm3 = 864.7 cm3

Therefore RDA = 2239.1 g = 2.68

835.4 cm3 x 1 g/cm3

RDB = 2239.1 g = 2.59

864.7 cm3 x 1 g/cm3

RDSSD = 2268.4 g = 2.62

864.7 cm3 x 1 g/cm3

% Abs = 29.3 g = 1.31%2239.1 g

Example 4-2 The dry mass of a sample is 2239.1 g. The mass in saturated surface-

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Highway Materials, Soils, and Concrete Aggregate Properties

• Aggregate Hardness (resistant to wear)– It is important that aggregates for pavement

surfaces not become rounded or polished thereby reducing skid resistance

– Load cycles in the pavement structure tend to break aggregates or fines will result changing the gradation (finer) resulting in reduced strength of the pavement structure

– Broken aggregates are not cemented into the structure, again reducing strength

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Highway Materials, Soils, and Concrete Aggregate Properties• Aggregate Hardness (cont’d)

– Resistance to degradation during mixing, transportation, placing and compacting is important as soft particles may break changing the gradation

– Los Angeles Abrasion Test measures the hardness of aggregates

– Deval Apparatus

– Aggregate Impact Value Test

– Polished Stone Value Test

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• Aggregate Durability– resistance to degradation due to cycles of wetting

and drying, heating and cooling, and freezing and thawing

– freezing and thawing• pore spaces in the aggregate are often saturated and on

freezing expands

• repeated cycles can cause the aggregate to break

• sedimentary rocks are vulnerable because of planes of weakness between layers

– Soundness Test Field Performance / Absorption Value

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• Aggregate Particle Shape/Surface Texture – bonding capability with asphalt cement

• particles with rough fractured faces allow a better bond with asphalt cements than rounded smooth particles

– resistance to one particle sliding over another– flat, thin, long aggregate particles break easier

than cubical particles• Specifications restrict the percentage of long thin

particles and require aggregates particles having at least one fractured face

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• Deleterious Substances– harmful or injurious materials including various

types of weak or low quality particles or coatings found on the surface of aggregates

• dust (material passing the 75 um sieve)

• clay lumps, shale, coal particles, friable particles, chert (weak in terms of freezing and thawing)

– These substances effect the bond between cements and aggregates and break easily

– Petrographic analysis Sand Equivalency Test

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• Aggregate Crushing Strength– crushing strength is the compressive load that

aggregate particles can carry before breaking– relatively unimportant for most aggregates

strength is higher than the strength of an asphalt mix

• Chemical Stability– refers to specific problems due to chemical

composition

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Highway Materials, Soils, and Concrete Aggregate Specifications

– Specifications by highway departments takes into account the aggregate properties we just discussed

– Requirements for aggregates to be used in bases and subbases differ from aggregates to be used in asphalt mixes

– specifications include local experience, availability of materials and type of project

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Abrasion test:

Original mass 5009g 5009-3267 = 34.8%

Final mass 3267 g 5009

Soundness test:

Original mass 2649g 2649-2115 = 20.2%

Final mass 2115 g 2649

-lower strength fines content

-drainage and frost heave potential

-durability question freeze/thaw resistance

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Highway Materials, Soils, and Concrete Aggregate Sampling and Testing

• The characterization of an aggregate source depends on how representative the samples are taken from the aggregates– size of samples are specified

– samples should be obtained from the final product if possible, after all the steps in processing and transportation have been completed

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– Samples taken from a production or discharge should be taken at various times and across the entire cross section of discharge and combined to form one sample

– Sampling from stockpiles requires care due to possible segregation of material. Three samples should be taken, one from the top third, middle third, bottom third and combined

– Sample tubes for fine aggregate stockpiles should be used, five tube samples combined

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– Samples taken from a truck or railway car should be done from a cross sectional trench, three combined to form one sample

– Combined samples should be done with equal size component samples

– Randomness should be used to determine location or time to eliminate personal bias

– Ensure proper identification of the sample

– Samples should be properly secured

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– Samples tested in the lab must be representative of the samples delivered to the lab

• Sample Splitting– testing aggregates composed of significant amounts

of both coarse and fine aggregates the sample must be split on the 4.75 mm sieve and the two fractions sieved separately

– If not, the amount of sample on the fine sieves may be too great for effective sieving

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Documents

ENCI 579 41 Highway Materials, Soils, and Concrete Aggregates Soil Definition (Engineering) –“refers to all unconsolidated material in the earth’s crust,