Presentation on theme: "Bases, Subbases, & Low Cost Surfaces Text Book Chapter 20 Dr. TALEB M. AL-ROUSAN."— Presentation transcript:
Bases, Subbases, & Low Cost Surfaces Text Book Chapter 20 Dr. TALEB M. AL-ROUSAN
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.
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).
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.
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
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.
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.
Classes of Stabilization 1. Soil-Aggregate Roads and Granular Stabilization. 2. Bituminous Stabilization. 3. Cement Stabilization 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 construction 2. Traveling plant construction 3. 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 Stabilization Bituminous materials are used in conjunction with aggregates and soil-aggregate mixture for two purposes: 1. Binding and Supply cohesion to the stabilized soil mixtures (Like in Sand stabilization). 2. 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.
Bituminous 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 % silt & clay. 4. MS & SS emulsified asphalt (Table 18-3 in Ref. 1).
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.
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.
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.
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.
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.
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).
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
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.
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
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.
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.
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.
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).
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.
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).
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.