Bases, Subbases, & Low Cost Surfaces

1. Bases, Subbases, & Low Cost Surfaces Dr. TALEB M. AL-ROUSAN

2. 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.

3. 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.

4. Classes of Stabilization • Soil-Aggregate Roads and Granular Stabilization. • Bituminous Soil Stabilization. • Soil-Cement Roads • Stabilization by addition of Salt, lime, and various other chemicals.

5. 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: • Road mix construction • Traveling plant construction • Central plant construction

6. 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.

7. Bituminous Soil Stabilization • Bituminous materials are used in conjunction with aggregates and soil-aggregate mixture for two purposes: • Supply cohesion to the stabilized soil mixtures (Like in Sand stabilization). • 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.

8. Bituminous Soil Stabilization Cont. • Choice of bitumen depend on local experience and comparative cost. • Use 4 – 7% bitumen • Types of bitumen: • RC: use for sandy soil or soils with min. clay or silt. • MC : used for soils with higher clay or silt particles (give more homogenous mixes) • SC: used for soils with 30 -40% silt & clay. • MS & SS emulsified asphalt (Table 18-3).

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

14. 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).

15. Bituminous Macadam Construction • Spreading and rolling of coarse aggregates. • Initial application of bituminous material. • Spreading and rolling of key aggregates. • Application of seal coat

16. Prime Coats • Application of liquid bituminous material to previously untreated base or wearing surface. • Bituminous material penetrate surface and completely absorbed • Purpose: • Promote adhesion “bond” between base and wearing surface. • Consolidate the surface on which the new treatment is to be placed. • It may function as a deterrent to the rise of capillary moisture into the wearing surface.

17. 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).

18. 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).

19. 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.

20. Seal Coat Cont. • Purpose: • Waterproof or seal the surface. • Rejuvenate or revitalize old bituminous wearing surfaces. • To nonskid slippery surfaces. • Improve night visibility.

21. 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.