Bases, Subbases, & Low Cost Surfaces

Dr. TALEB M. AL-ROUSAN

Soil Stabilized Roads

Soil stabilization: Combination and manipulation of soils, with or without admixtures, to produce a firm mass that is capable of supporting traffic in all weather conditions.

Stable stabilized soil must have sufficient shearing strength to withstand traffic loadings in all weather conditions without excessive deformations.

If stabilized soil is to be used as wearing surface, then it must be capable of withstanding the abrasive effects of traffic.

Soil Stabilized Roads Cont.

Purpose: Effective utilization of local materials in order to decrease construction cost.

Used when natural soil has unfavorable characteristics and require modification.

Primary use of stabilized soil is in base and subbase construction.

Stabilized base and subbase support thin wearing surfaces for light traffic.

Stabilized base and subbase may function as base for high type pavements with heavy traffic.

Classes of Stabilization

1. Soil-Aggregate Roads and Granular Stabilization.

2. Bituminous Soil Stabilization.

3. Soil-Cement Roads

4. Stabilization by addition of Salt, lime, and various other chemicals.

Soil-Aggregate Roads and Granular Stabilization

Consist of a substantial layer of a properly proportioned and blended mixture of soil and aggregates compacted to form a road.

AASHTO : Known as (dense-graded aggregates) It’s a natural or prepared mixture consisting predominantly of stone, gravel, or sand and containing silty clay.

Construction procedure:1. Road mix construction2. Traveling plant construction3. Central plant construction

Soil-Aggregate Roads and Granular Stabilization Cont.

Calcium Chloride is used some times to expedite the compaction process by slowing the rate of evaporation from the mixture.

Calcium Chloride might increase density and maintain moisture in wearing surfaces for desired max stability which will prevent raveling of the surface and reduce dust.

Bituminous Soil Stabilization

Bituminous materials are used in conjunction with aggregates and soil-aggregate mixture for two purposes:

1. Supply cohesion to the stabilized soil mixtures (Like in Sand stabilization).

2. Waterproofing Mostly used for base construction. Used with : well-graded soils, sands, and clays. Finer soils require more bitumen for waterproofing. Very fine plastic soils can not be stabilized

economically with bituminous materials because of difficulties of pulverization and mixing.

Bituminous Soil Stabilization Cont.

Choice of bitumen depend on local experience and comparative cost.

Use 4 – 7% bitumen Types of bitumen: 1. RC: use for sandy soil or soils with min. clay

or silt.2. MC : used for soils with higher clay or silt

particles (give more homogenous mixes)3. SC: used for soils with 30 -40% silt & clay.4. MS & SS emulsified asphalt (Table 18-3).

Bituminous Soil Stabilization Cont.

Oiled earth surfaces: Liquid bitumen is applied at properly prepared natural soil surface.

Prevent dust and provide a thin stabilized surface that support limited number of light vehicles.

See Construction method in Text.

Soil-Cement Roads

Incorporating Portland cement (7 – 14% volume of compacted mixture) with naturally occurring or artificially created soils or soil-aggregate mixtures.

Used for bases in city street constructions, driveways, shoulders, parking areas, and some airport runways.

Also known as cement-treated base.

Soil-Cement Roads Cont.

Nearly all subgrade soils may be stabilized with cement except those soils that contain high percentage of organic materials.

Silt and clay require large percentage of cement for successful stabilization.

Sandy and gravelly soils with 10 – 35% silt and clay have most favorable characteristics.

See construction method in Text.

Stabilization by Addition of Lime

Incorporating small amounts of hydrated lime improves certain plastic clay soils.

Used to reduce plasticity, shrinkage, and swell of clay soils and increasing their bearing capacity.

improve workability of plastic soil (easy to pulverize).

Waterproof soil to some extent and allow it to dry quickly when saturated to expedite construction.

Stabilization by Addition of Lime Cont.

Use 3 – 6%.Depth of treatment around 6 inches.Subgrade soil is scarified and pulverized,

then lime is spread mechanically, water is added (+ 5% above opt.), then allowed to cure for (1 -7days). Mixing and compaction is then applied.

Macadam Roads

Road surface or base in which crushed or broken stone was mechanically keyed or locked by rolling and cemented together by the application of stone screening and water.

Later, bitumen macadam roads were built using bitumen material as binder.

Water-bound macadam: broken stone bound together by stone dust and water applied during construction (Seldom Constructed).

Bituminous macadam: crushed stone base or wearing surface in which fragments are bound together by bituminoius material; the aggregate layer is compacted and bituminous material is applied to the surface (Known as penetration macadam).

Bituminous Macadam Construction

1. Spreading and rolling of coarse aggregates.

2. Initial application of bituminous material.

3. Spreading and rolling of key aggregates.

4. Application of seal coat

Prime Coats

Application of liquid bituminous material to previously untreated base or wearing surface.

Bituminous material penetrate surface and completely absorbed

Purpose:1. Promote adhesion “bond” between base and

wearing surface.2. Consolidate the surface on which the new

treatment is to be placed.3. It may function as a deterrent to the rise of

capillary moisture into the wearing surface.

Prime Coats Cont.

Use low viscous asphalt or tar. Asphalt preferred to have high penetrated

qualities and should leave high viscosity residue in the void spaces.

MC-30 and MC-70 widely used for dense tightly bond surfaces, and surface with more opened texture.

Slow setting emulsified asphalts are recommended for penetrable surfaces.

Recommended quantity (0.9 – 2.3 L/m^2).

Tack Coats

Single application of bituminous material to an existing bituminous, Portland cement concrete, brick, or block surface or base.

Purpose: Provide adhesion between the existing surface and the new bituminous wearing surface.

RC-70 through RC-250 are frequently used. Quantity required for application is small (0.19

to 0.38 L/m^2).

Seal Coat

Very thin single surface treatment that is usually less than (½ in) thick.

Single Surface treatment = single application of bituminous material that is covered by a light spreading of fine aggregate or sand (spread mechanically) then compacted with pneumatic tired rollers.

Applied as final step in many types of bituminous wearing surfaces.

Seal Coat Cont.

Purpose:

1.Waterproof or seal the surface.

2.Rejuvenate or revitalize old bituminous wearing surfaces.

3.To nonskid slippery surfaces.

4. Improve night visibility.

Slurry Seal Coat

Mixture of well-graded, fine aggregate, mineral filler (if needed), emulsified asphalt, and water.

Use principally for the resealing of old bituminous wearing surfaces.

Very thin (1/8” to ¼”).See Figure 18-6.