Bases, Subbases, & Low Cost Surfaces

1. Bases, Subbases, & Low Cost Surfaces Text Book Chapter 20 Dr. TALEB M. AL-ROUSAN

2. Subgrades • Is usually the natural material located along the horizontal alignment of the pavement. • It serves as the foundation of the pavement structure. • It may consist of a layer of selected borrow materials that is well compacted to some prescribed specification. • It may be necessary to treat the subgrade material to achieve certain strength properties required for the type of pavement being constructed.

3. Subbase Course • Located immediately above the subgrade. • It has better quality than subgrade. • When the quality of the subgrade materials meets the requirements of the subbase material, the subbase layer may be omitted. • When suitable subbase material is not available, the available materials can be treated to achieve necessary properties (Stabilization).

4. Base Course • Lies immediately above the subbase • Consists of granular materials (crushed stone, crushed slag, gravel and sand). • Specifications are stricter than those for subbase regarding (gradation, plasticity, and strength). • Materials that do not have the required properties can be used as base course material if the are properly stabilized with Portland cement, asphalt, or lime.

5. Surface Course • Upper course of the road pavement and is constructed immediately above the base course. • Consist of mixture of mineral aggregate and asphaltic materials. • It should be capable of : • withstanding high tire pressure, • Resisting traffic abrasive forces, • Providing a skid-resistance surface. • Preventing the penetration of surface water into the underlying layers

6. Soil Stabilization • 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. • Soil Stabilization: Is the treatment of natural soil to improve its engineering properties. • Soil Stabilization Methods: • Mechanical: Blending different grades of soil to obtain a required grade. • Chemical: is the blending of natural soil with chemical agents (most used are Portland cement, asphalt binders, and lime). • 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.

7. Soil Stabilization 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. • See Table 20.1 in Text for soil types and stabilization methods.

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

9. 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

10. 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.

11. Bituminous Stabilization • Bituminous materials are used in conjunction with aggregates and soil-aggregate mixture for two purposes: • Binding and Supply cohesion to the stabilized soil mixtures (Like in Sand stabilization). • Waterproofing • Mostly used for base construction. • Used for soils with PI < 10 • 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.

12. Bituminous 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 in Ref. 1).

13. Bituminous 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.

14. Cement Stabilization • Incorporating Portland cement (5 – 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.

15. Cement Stabilization 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.

16. Stabilization by Addition of Lime • Incorporating small amounts of hydrated lime improves certain plastic clay soils. • Most Commonly used Calcium Hydroxide Ca(OH)2 and dolomite [Ca(OH)2 + MgO] • To be most effective soils should have PI<10. • 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.

17. Stabilization by Addition of Lime Cont. • Use 3 – 10%. • 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.

18. 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 bituminous material; the aggregate layer is compacted and bituminous material is applied to the surface (Known as penetration macadam).

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

20. 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.

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

22. Seal Coats • Are usually single application of asphalt material that may or may not contain aggregates. • Three types of asphalt seal commonly used in asphalt maintenance: • Fog Seal • Slurry Seal • Aggregate Seal

23. Fog Seal Coat • Thin application of emulsified asphalt, usually with no aggregates added. • Slow setting emulsions are normally used. • After diluted , its usually sprayed at arate of 0.1 to 0.2 gal/yd2 • Fog seals are used mainly to: • Reduce infiltration of rain and water into the pavement. • Prevent the progressive separation of aggregate particles from the surfaceو • Bring the surface of the pavement to its original state.

24. Slurry Seal Coat • Mixture of well-graded, fine aggregate, mineral filler (if needed), slow setting emulsified asphalt, and water. • Use principally for the resealing of old bituminous wearing surfaces. • Considered as low cost maintenance material for light traffic • Very thin (1/16” to 1/8” and up to ¼”). • Cracks may appear shortly due to this surface. • See Figure 18-6 in Ref. 1.

25. Aggregate seal • Obtained by spraying asphalt , immediately covering it with aggregates, and then rolling the aggregate into the asphalt. • Asphalt used are usually the softer grades of paving asphalts and the heavier grades of liquid asphalts. • Used to restore the surface of old pavements.

26. 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.

27. 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).

28. Prime Coats • Obtained by spraying asphaltic Binder materials onto nonasphalt base courses. • Used to: • Provide a waterproof surface. • Fill capillary voids in the base. • Facilitate the bonding of loose mineral particles. • Facilitate the adhesion of the surface treatment to the base. • Medium curing are usually used. • The amount of asphalt used should be the maximum that could be absorbed by the base within 24 hrs of application. • The base course must contain a nominal amount of water to facilitate the penetration of the asphaltic material into the base.

29. 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).

30. Tack Coats • Is a thin layer of asphaltic material spayed over an old pavement to facilitate the bonding of the old layer and the new one. • Asphalt emulsions are usually used. • Amount spayed is less than in prime coat since no asphalt will be absorbed by the beneath layer.