16501 14050 highway materials

8/2/2019 16501 14050 Highway Materials

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Subgrade soil, Stone aggregates, bituminous materials andcement concrete

Subgrade soil it is an integral part of road pavementstructure which provides support to the pavement

Stone aggregates form the major portion of pavementstructure and bears stresses occurring due to the wheelloads on the pavement and also resists wear due to abrasiveaction of traffic

Bituminous materials include bitumen and tar, used asbinders in pavement construction works

Cement concrete cement used in the construction ofcement concrete pavements CC pavements withstand heavytraffic even under adverse subgrade and climate conditions


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Integral part of pavement

The main function of subgrade soil is to provideadequate support to the pavement hence subgrade soil

properties are important in the design of pavementstructure

Pavement failures like formation waves , corrugations ,rutting , mud pumping etc. are because of poor

subgrade conditions Moisture and dry density influence the engineering

behaviour of a soil mass


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Stability resistance to permanent deformation

Incompressibility

Permanency of strength

Minimum changes in volume and stability underadverse conditions of weather and ground water differential expansion and differential strength values

Good drainage, and to reduce the potential frost

actionEase of compaction ensures higher dry density and

strength


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The wide range of soil types available as highwayconstruction materials

The soil properties on which their identification and

classification are based are known as index propertiesIndex properties grain size distribution, liquid limit

and plasticity index. Further some times shrinkage limit,compacted dry density and field moisture equivalent

centrifuge moisture equivalent are usedGrain size analysis seive analysis (gravel, sand, silt and

clay)


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Consistency limits atterberg limits and indices, liquid limit,plastic limit, plastic index and shrinkage limit

Soil classification systems: Burmister descriptive Casagrande Unified U.S public roads administration (PRA) HRB or AASHO or revised PRA Federal aviation agency (FAA) Civil aeronautic administration (CAA) Compaction

Unified system widely accepted for civil engineering purposes

HRB adopted for classification of subgrade spoils in highwayengineering


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Soil type

Moisture content

Dry density

Internal structural of the soil, andThe type and mode of stress application

Shearing resistance in soil represents strength

Sr

= C +

where cis cohesion per unit area, is the internalfriction and is the normal stress


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The tests used to evaluate the strength properties of soilsdivided into three groups shear tests direct shear test, triaxial compression test and

unconfined compression test (all these are in laboratory). Vaneshear test (in field)

Bearing tests carried out in situ with a load bearing area Penetration tests can be carried out in the laboratory or field,

CBR and cone penetration tests

Size and shape of specimen, method of loading, rate of

loading and drainage conditions affect the results ofstrength tests

Soil type , dry density , moisture content, permeability andother properties of soil affect the results


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Aggregates form major portion of the pavementstructure

Prime materials used in pavement construction

They have bear the stresses occurring due to wheelloads

They have to resist the wear duo to abrasive actionof traffic

Most of the road aggregates are prepared fromnatural rock (igneous, sedimentary andmetamorphic rocks)


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These are specified based on their grain size, shape,texture and gradation

Specification done by various agencies like the ASTM,BSI, ISI and IRC

Based on strength aggregates divided as hardaggregates and soft aggregates

Soft aggregates moorum, kankar, laterite, brick

aggregates and slagSoft aggregates can be used in lower layers in case of

low cost road construction


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Strength Should be sufficiently strong to with stand stresses due

to the traffic wheel load (particularly wearing coarse)

Should possess sufficient strength resistance to crushing

Hardness Should be hard enough to resist the wear due to abrasive

action of traffic

Abrasive action increased ?? by sand between tyre androad surface

The mutual rubbing of stones is called attrition


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Toughness Aggregates subjected to impact due to moving wheel

loads, severe impact quite common due to heavily loadedsteel tyred vehicles (WBM)

Impact increases with roughness of the surface, speedand other vehicular characteristics Hence resistance to impact or toughness is another

desirable property of aggregates

Durability Should durable to resist the disintegration due to action

of weather (this property is called soundness) Aggregates should be sound enough to with stand the

weathering action


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Shape of aggregates Rounded, cubical, angular flaky or elongated particles

Flaky and elongated particles will have less strength

Rounded particles have better workability in the case ofcement concrete mix

Angular particles preferred for WBM and bituminousconstruction

Too flaky and too elongated particles should be avoided

as far as possible

Adhesion with bitumen Aggregates should have less affinity with water when

compared with bituminous materials


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In order to decide the suitability of road aggregate forroad construction the following tests are conducted Crushing test

Abrasion test

Impact test

Soundness test

Shape test

Specific gravity and water absorption

Bitumen adhesion test


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Bitumen materials used as binders in pavement construction

If the bitumen contains some inert or minerals then it is called as asphaltbitumen

Bitumen and tar same??

Bitumen is obtained by thedistillation of crudepetroleum

Black in color

Soluble in carbon disulphide

and in carbon tetra chloride Less temperature susceptile

Less free carbon as seenfrom solubility test

Tar is obtained by destructivedistillation of coal or wood

Black in color

Soluble only in toluene

More temperature susceptile More free carbon

Chemical constituents arequite different from bitumen


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Requirements of bitumen The viscosity of bitumen at the time of mixing and

compaction should be adequate (heating or bitumen cutbacks)

It should be high temperature susceptile during hottestweather the bituminous mix should not become too softor unstable and during cold weather the mix should notbecome too hard and brittle

In the presence of water bitumen should not strip offfrom the aggregate adequate affinity between bitumenand aggregate


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Cutback bitumen is obtained by blending bitumen withvolatile diluent

Viscosity of bitumen reduced by the volatile diluent

After the cutback mix is used, the volatile gas gets

evaporated and the cutback develops bondingCharacteristics depends on both bitumen and volatile

oil in the diluent

Application fluidity of bitumen increased at low

temperaturesCutback bitumens are available as three types rapid

curing (RC), medium curing (MC), slow curing (SC).


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Bitumen is suspended in finely divided condition in anaqueous medium and stabilized by means of one or moresuitable materials

When the bitumen is applied on the road, it breaks down

and the binder starts binding with aggregates but fullbonding develops slowly as water gets evaporated

Used in road constructions, especially in maintenance andpatch repair works

The main advantage is that it can be used in wet weather

conditionsTypes rapid setting (RS), medium setting (MS), slow

setting (SS)


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To judge the suitability of bitumen binders various testsspecifies by agencies like ASTM, Asphalt institute, BSI,ISI Penetration tests Ductility tests

Viscosity tests Float test Specific gravity test Softening point test Flash and fire point test

Solubility test Spot test Loss on heat test Water content test


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The bituminous mix design aims to determine theproportion of bitumen, filler, fine aggregates, andcoarse aggregates to produce a mix which is workable,strong, durable and economical

The requirements of the mix design

The two major stages of the mix design, i.e. dry mixdesign and wet mix design


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sufficient bitumen to ensure a durable pavement,sufficient strength to resist shear deformation under

traffic at higher temperature,

sufficient air voids in the compacted bitumen to allow

for additional compaction by trafficsufficient workability to permit easy placement without

segregation,

sufficient flexibility to avoid premature cracking due to

repeated bending by traffic, andsufficient flexibility at low temperature to prevent

shrinkage cracks.


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Coarse aggregates: Offer compressive and shearstrength and shows good interlocking properties.E.g. Granite

Fine aggregates: Fills the voids in the coarseaggregate and stiffens the binder. E.g. Sand, Rockdust

Filler: Fills the voids, stiffens the binder and offers

permeability. E.g. Rock dust, cement, limeBinder: Fills the voids, cause particle adhesion and

gluing and offers impermeability. E.g. Bitumen,

Asphalt, Tar


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Stability - the resistance of the paving mix to deformationunder traffic load

Stability depend on the inter-particle friction, primarily of theaggregates and the cohesion offered by the bitumen

Sufficient binder must be available to coat all the particles at thesame time should offer enough liquid friction

stability decreases when the binder content is high and when theparticles are kept apart

Durability - resistance of the mix against weathering and

abrasive actions Weathering causes hardening due to loss of volatiles in the bitumen.

Abrasion is due to wheel loads which causes tensile strains

pot-holes, stripping


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Flexibility - level of bending strength needed tocounteract traffic load and prevent cracking of surface

shrinkage and brittleness Shrinkage cracks are due to volume change in the binder due

to aging. Brittleness is due to repeated bending of the surfacedue to traffic loads

Higher bitumen content will give better flexibility and less

fractureSkid resistance - resistance of the finished pavement

against skidding depends on the surface texture and bitumen content

Normally, an open graded coarse surface texture is desirableWorkability - the ease with which the mix can be laid

and compacted, and formed to the required conditionand shape

This depends on the gradation of aggregates, their shape and

texture, bitumen content and its type


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The desirable properties of a bituminous mix can besummarized as follows

Stability to meet traffic demand

Bitumen content to ensure proper binding and waterproofing

Voids to accommodate compaction due to traffic

Flexibility to meet traffic loads, esp. in cold season

Sufficient workability for constructionEconomical mix


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Bituminous mixes should be stable, durable,flexible, workable and should offer sufficient skidresistance.

The mix consists of coarse and fine aggregates,filler and binder. It may be well graded, opengraded, gap graded or unbounded as per the

requirements.As far as possible, it should be economical also.


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Portland cement used in the construction of cementconcrete pavements

Cement concrete are considered to the highestpavement type which withstand heavy traffic evenunder adverse subgrade and climatic conditions

Portland cement also used in soil-stabilization forconstruction of stabilized sun-base and base courses

The properties of cement , cement concrete mix designdetails of concrete already covered in BuildingConstruction Materials


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Well-graded mix: Dense mix, bituminous concrete has goodproportion of all constituents and are called densebituminous macadam, offers good compressive strengthand some tensile strength

Gap-graded mix: Some large coarse aggregates are missing

and has good fatigue and tensile strength.Open-graded mix: Fine aggregate and filler are missing, it is

porous and offers good friction, low strength and for highspeed.

Unbounded: Binder is absent and behaves under loads as if

its components were not linked together, though goodinterlocking exists. Very low tensile strength and needs kerbprotection.

16501 14050 highway materials

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