Pavemnt analysis and design III

1.3. INNOVATIVE APPLICATIONS

Innovative applications may be construction method based or design principle based. Some of the relevant issues are discussed in the following.

1.3.1 Construction Method Based

A mixture of aggregate and binding material, at varied proportions, constitute various specifications for road construction, for example, bituminous concrete, built-up spray grout, wet mix macadam, lean cement concrete etc. Discussion on all these standard specifications have been skipped here, rather, some specific mixes and their construction methods are discussed.

Emulsified bituminous mix

Cold emulsified bituminous mix (EBM) is gaining more and more acceptance for its environmental friendliness and less hazardous construction process. A relative comparison between the EBM and hot bituminous mix (HBM) has been presented in Table 4. It may be noted that though the rate of strength gain in EBM is slower (refer Figure 7), the final strength of EBM is comparable to that of HBM.

Table 4. Comparison of hot bituminous mix (HBM) and emulsified bituminous mix (EBM)

Property	HBM	EBM
Heating	Strong heating required, oxidative hardening occurs	No heating required, so no oxidative hardening
Setting time	Low	High
Applicability	Clear weather with high ambient temperatures	All weather (wet surfaces, rainy seasons, cold)
Convenience	Relatively difficult construction than EBM	Relatively easy construction
Energy	Relatively higher requirement	Relatively lower requirement
Uniqueness	Modifiers needed	Inherent anti-stripping agents
Economy	Less costly	More costly
Safety	Hazards from fuming, fire and environmental pollution	Free from such hazards

Foamed bituminous mix Foamed bituminous mix (FBM) is a foamed mixture of air, water and bitumen. It is produced by injecting very small quantity of water into the hot bitumen, resulting in spontaneous foaming and temporary alteration of the physical properties of the bitumen. Figure-8 represents schematically the manufacture of FBM. Although the foamed bitumen technology was developed more than forty years ago, it is now gaining popularity owing to its good performance, ease of construction and compatibility with a wide range of aggregate types (Transportek 1998). Usage of FBM results in reduction in binder content and transportation costs, as it requires less binder and water than other types of cold mixing methods. FBM can be compacted immediately and can carry traffic almost immediately after compaction is completed ( Jenkins et al., 2003 ). The strength characteristics of FBMs are highly moisture dependent. This is because of the relatively low binder content and high void content of foamed bituminous mixes. FBMs are not as temperature susceptible as HBM. Since larger aggregates are not coated with binder, the friction between the aggregates is maintained at higher temperatures. Foamed bitumen can achieve stiffness comparable to those of cement-treated materials, with the added advantages of flexibility and fatigue resistance (Ramanujam and Kendall, 1999). FBMs usually lack resistance to abrasion and raveling and are not suitable for wearing/friction course applications. Figure - 8 Schematic presentation of FBM manufacture ( Romanoschi 2003 ) Some specific situations where use of foamed bitumen technology can be considered are: • A pavement which has been repeatedly patched to the extent that pavement repairs are no longer cost effective. • A weak granular base overlies a reasonably strong subgrade. • Granular base too thin to consider using cementations binders. • Can be effectively used in desert road stabilization etc. (Jenkins et al., 2003). Relatively high cost, requirement of specific equipment for mix production, sensitivity to aggregate grading and stripping risk are some of the disadvantages with the foamed bituminous mix ( Jenkins et al., 2003 ).

Fiber reinforced bituminous mix

Addition of various kinds of fibers to the binder and aggregates during mix preparation process results in fiber reinforced bituminous mix (FRBM). Fibers are generally blended with bitumen binder before mixing it with the aggregates to achieve complete coating and even distribution throughout the mix. Research shows that FRBMs develop good resistance to aging, fatigue cracking, moisture damage, bleeding, reflection cracking etc. (Serfass and Samanos, 1996; Maurer et al., 1989).

Ultra-thin white topping

Overlaying technique of pavement rehabilitation is well known and widely practiced. However, ultra thin whitetopping (UTW) of concrete over existing bituminous pavement is a relatively new concept. UTW can be designed for low-speed, low volume traffic areas such as street intersections, aviation taxiways and runways, bus stops and tollbooths.

In this technique, a thin layer of high-strength, fiber-reinforced concrete is placed over a clean, milled surface of distressed bituminous concrete pavement to achieve a full or partial bonding. Bonding makes the two layers behave as a monolithic unit and share the load. Due to bonding, the neutral axis in concrete shifts from the middle of concrete layer towards its bottom. This results in a lowering of stresses at the bottom of concrete layer. Thick composite section behavior causes the corner stresses to decrease. On the other hand, downward shifting of neutral axis may cause critical load location to shift from edges to corners thus increasing the corner stresses. Short joint spacing is used to decrease the slab area that can curl or warp thus minimizing the corresponding stresses (MTTP 2004). A schematic diagram of UTW have been presented inFigure-9.

Figure -9 Flexible composite pavement using UTW

UTW is an excellent resurfacing option for deteriorated bituminous pavements which otherwise require frequent repair or overlays.

Following are some of the advantages of a UTW system (CAC 2004, Murison 2002):

• It is beneficial for repair of roads and intersections having problems of rutting, cracks, and poor drainage.

• It provides improved skid resistance.

• Its light colour reflects more light than bituminous pavement.

• Its heat-reflecting property can help to lower the average city temperature.

• It is less costly to maintain, than conventional flexible pavements, and does not require frequent resurfacing and repairs.

• The UTW concrete resists bitumen aging.

• The UTW concrete prevents degradation of bituminous surface due to fuel spills.

• It causes minimal traffic disruption due to faster construction and repair procedure.

• Its small panels are ideal for utility maintenance.

Bituminous recycling

In recycling method, bitumen and aggregates are separated out (partly or fully) and used again. The specific benefits of recycling of bituminous pavement can be summarized as:

• Conservation of energy and construction material.
• Prevention of undesirable rise in height of finished surface and preservation of the existing road geometrics.
• Reuse of deteriorated road materials which in turn solves the disposal problem.
• Solution to the problem of scarcity of good quality material.
• Preservation of the environment.
• Reduction in susceptibility to reflection cracking.

Bitumen ages due to oxidation with atmospheric oxygen as a result of which resins get converted into asphaltenes (Petersen, 1984). By this process bitumen loses its ductility and becomes more brittle. Recycling is based on the fact that bitumen obtained from old deteriorated bituminous pavement, may still has its residual properties and recycling helps in restoring those residual properties of the bitumen.

To judge the suitability for use as a recycled material, aggregates are tested for their gradation and bitumen is tested for its engineering properties. The optimum quantity of reclaimed material to be mixed with fresh material is generally determined from mix design process. Fresh thin (soft grade) bitumen having low viscosity can be used to replenish the aged bitumen. Rejuvenators (like road oils and flux oils) are sometimes added for improvement in properties of reclaimed bitumen.

There are four major technologies exist for bituminous pavement recycling (NCHRP-452). They are

(i) Hot mix recycling

Here recycled asphalt pavement ( RAP) is combined with fresh aggregate and bituminous binder or recycling agent in a hot mix plant. Mix is transported to paving site, placed, and compacted.

(ii) Cold in-place recycling

In this the existing pavement is milled up to a depth of 75 to 100mm, RAP, if necessary and recycling agent in emulsion form is introduced, and then compacted.

(iii) Hot in-place recycling

In hot in-place recycling method the existing asphalt surface is heated, scarified to a depth from 20 to 40 mm, scarified material combined with aggregate and/or bituminous binder and/or recycling agent and compacted. New overlay may or may not be provided.

(iv) Full depth reclamation

Here all the bituminous layers and predetermined thickness of underlying material is pulverized, stabilized with additives, and compacted. A surface course is applied over it.

Semi flexible grouted macadam

• Grouted Macadam consists of a single sized porous bituminous layer whose voids can be filled with the selected fluid grout or cementations slurry.
• The porous bituminous skeleton is designed to achieve a high void content while maintaining a thick bitumen coating on the aggregate particles (Zoorob et al., 2002).
• Grouted macadam gives advantages of both flexible and rigid pavements namely,

• Flexibility and absence of joints by use of bitumen,

• High static bearing capacity and wear resistance (as for concrete) by use of cementations grout.

Micro-surfacing

• Micro-surfacing is a fast and economical surface treatment technique used for preventive maintenance of bituminous and cement concrete pavements.
• Polymer modified emulsion, cold blended with fine graded aggregates, mineral fillers, additives and water, gives the high performing micro-surfacing mixture.
• Micro-surfacing is generally used to restore the top wearing surface of pavement as a maintenance measure, thereby extending the pavement life.
• Its thickness may be varied to achieve desired objective(s) such as rut-filling, skid resistance improvement, surface sealing, surface texturing, noise reduction, repairing abraded wheel path channels etc (ODOT 2004; Miller 2004).

Design Principle Based

This section discusses about the design principle based innovative applications of road materials. Discussion has been divided into two parts viz.

• Structural design considerations, and
• Mix design considerations

Optimum pavement design thickness

In Mechanistic-Empirical pavement design, generally sustainability of a pavement structure against fatigue and rutting failures is considered, for which the critical responses are: (a) the tensile strain at the bottom fiber of bituminous layer and (b) the vertical strain at the top of the subgrade. A number of design thickness combinations of bituminous and granular layers are possible which satisfies the above mentioned requirement.

Standard design charts developed by various organizations (Shell 1978; Austroads 1992; Asphalt Institute 1981; IRC:37-2001) are available; these design charts generally provide thickness composition of bituminous and granular layers, depending upon other input parameters viz. temperature, traffic, design life, subgrade strength, material type etc. A designer can choose any suitable granular layer thickness, and, corresponding thickness of bituminous layer can be read from these charts.

Figure 10. Typical pavement design chart

POINT A - Safe from rutting but over safe from fatigue considerations.

POINT B- Safe from rutting but unsafe from fatigue considerations.

POINT C- Safe from fatigue but insafe from rutting considerations.

POINT D- Safe from fatigue but oversafe from rutting considerations.

POINT E - Unsafe from both rutting and fatigue considerations.

Point F- Oversafe from both rutting and fatigue considerations

POINT O- Just safe from both rutting and fatigue considerations.

Figure 10 illustrates a typical design chart. The design chart consists of two curves: fatigue curve and rutting curve. The fatigue curve shown as COD in Figure 10 represents the points, which are just safe from fatigue consideration. Similarly, the rutting curve shown as AOB in Figure 10 represents those points which are just safe from rutting consideration. Figure 10 shows various points like A, B, C, D, etc. They are safe, oversafe or unsafe from fatigue or rutting considerations. The reader can point the cursor on the respective points to know about their status. In the design chart the fatigue curve and the rutting curve intersects at a point (point O in this case) that may be called as structurally balanced design point (Narasimham, et al., 2001). Thickness of pavement layers chosen according to this point will result in a pavement deign which would fail due to fatigue and rutting simultaneously. Similarly, there could be cost optimal point, where bituminous and granular layer thicknesses are selected such that the total cost of materials used is minimized, without compromising with the structural adequacy of the pavement. The cost optimal point may or may not coincide with the structurally optimal point (Narasimham et al. 2001, Das 2004 ).

Perpetual pavement

A perpetual bituminous pavement may be defined as a pavement designed and built to last longer than fifty years without requiring major structural rehabilitation or reconstruction (APA101 2001). This pavement may only require periodic replacement of top wearing surface and recycling of old pavement material (TRL 2001; AA-2 2001).

The concept of full depth bituminous pavement is in vogue from 1980s in USA. Nunn and his associates of Transport Research Laboratory, UK found (Nunn et al., 1997) that thick bituminous pavements tend to show long lasting performance and may require only minor surface repairs. California Department of Transportation in collaboration with University of California, Berkeley (Monismith et al., 2001) first implemented concept of perpetual pavement in a rehabilitation planning project.

In full depth bituminous pavement, the thickness is so designed that the fatigue and rutting strains developed are below the permissible limit (MS-1 1999 ). If the thickness is chosen to be sufficiently large so that the fatigue strain is close to the endurance limit, then the fatigue life becomes very long, and the pavement may be said to have attended 'perpetual life'. A perpetual pavement, in general, is made up of the following layers:

• The top wearing surface is designed in such a way that it is water-tight as well as removable and hence replaceable. Stone Matrix Asphalt (SMA) or Open Graded Friction Course (OGFC) are recommended. They also produce less noise due to tyre-pavement interaction.
• The intermediate layer is constituted with good quality aggregates and designed to be strongly resistive to rutting.
• The bottom part is made resistant to fatigue cracking by making it rich in bitumen and choosing a gradation that has less voids.

Figure 11 Layer composition of a perpetual pavement.

Figure-11 schematically represents the layer composition of a typical perpetual pavement.

A perpetual pavement is a full depth bituminous pavement in most of the cases. The principles based on which it is designed (mix design and structural thickness design) are the following:

• The pavement layers are chosen in such a way that they are rut resistive. The pavement is chosen to be adequately thick such that the vertical subgrade strain is low. Since subgrade contributes to the major part of rutting, low vertical subgrade strain would cause low level of rutting.
• The wearing surface should be adequately water-proof. The surface should be so designed that it can be repaired or recycled and the whole pavement will not require any major reconstruction (AA-2 2001).
• The thickness of the bituminous layer is chosen in such a way that the horizontal tensile strain (ε_t) developed is less than the endurance limit (refer Figure-12) of the bituminous mix, hence its laboratory fatigue life (N) becomes infinity (AA2-2001, Nunn et al. 1997). It is justifiable to design the pavement as 'bottom rich' (refer to next section), which shifts the endurance limit to higher level.
• The temperature gradient tends to be steeper towards the surface of the pavement (TRL 2001,Newcomb 2001) as shown schematically in Figure-12. Therefore the bituminous mixes with temperature susceptible binder should be avoided as surface course. Use of modified binder could be helpful in this regard.

Figure 12 Idealized diagram of fatigue characteristics of bituminous mixes.

Rich bottom bituminous pavement

Increased binder content above the optimum content can appreciably enhance the fatigue life.

Higher bitumen content increases the thickness of the binder film between aggregates resulting in lower stress in the binder film, and thus the fatigue life is improved (Sousa et al., 1998; Harvey et al., 1996).

However, with increased amount of binder content, the bituminous mix tends to be softer and thereby its stiffness modulus value may fall.

A mix designer's objective would be to achieve both high stiffness and high fatigue life.

This mutually contradictory situation can be handled by using a bituminous pavement layer where it is made richer in binder content towards the bottom layer(s).

Since fatigue cracks start from bottom of bituminous layer, higher bitumen content helps to give greater restraint against fatigue cracking.

This concept has been termed as 'rich bottom pavement' (Monismith et al., 2001; Harvey et al., 1997; Harvey and Tsai 1996).

Figures 13 and 14 provide two such options of achieving this condition. In Figure-13, quantity of bitumen is used more towards the bottom of the layer. In Figure-14, two different bituminous mixes are used in two layers. Out of three possible alternatives, alternative-II turns out to be the best alternative.

Figure 13 Rich bottom pavement

Figure 14 Two grades of bitumen used in two layers

Inverted pavements Inverted pavement system, or inverted base, is a high depth pavement whose supporting layers are thicker and stiffer than top layers. The system consists of a thin bituminous concrete (BC) layer provided on top of a graded aggregate base (GAB) layer. A Portland cement-treated stiff base layer is provided at the bottom. This arrangement causes the critical stress/strain plane to be located at the interface of the BC and GAB layers. Thus only the top portion of the inverted pavement structure absorbs the traffic loads as compared to conventional design where thick sections are required for load distribution. Research by South African Roads Board (SARB 2004) and Georgia Department of Transportation, has shown that an inverted base provides enough structural performance to support traffic loadings up to 100 million Equivalent Single-Axle Loads (ESAL s) with a maximum two inch bitumen riding course (Halsted, 2002). According to SARB, this type of system proves to be more cost effective for construction of long lasting pavements. Bituminous pavement with cemented base The cemented bases are derived from aggregates mixed with some binding material. Since it is bounded layer, it also has some fatigue life. Thus, unlike the unbound granular base, the cemented base layer contributes to some fatigue life, which may give rise to comparative reduction of design thickness of bituminous layer (Das and Pandey 1998). The stiffness modulus of cemented layer is generally found to be much higher than granular sub-base; however, due to shrinkage cracks, the stiffness modulus falls rapidly. This change in stiffness values at different stages of the design life has been schematically shown in Figure-15(a) and Figure-15(b) presents a typical design chart for design of bituminous pavement with cemented base made up of lime-soil mixture. 15 (a) Change of elastic modulus of cemented bases at different phases. 15 (b) A typical design chart of bituminous pavement with cemented base (LS º lime soil)

Mix design considerations

Non-standard gradation

The fatigue life of the mix can be increased by increasing the bitumen content.

But, Voids in Mineral Aggregates (VMA), being fixed for a given gradation and compaction level, increase in bitumen content will result in less Air Voids (VA), which is undesirable for a mix.

However one can deviate from the specified gradation in order to come up with a new gradation, which possibly can give rise to better fatigue performance, yet without compromising with the VMA and Marshall-stability requirements.

Stone matrix asphalt

Stone matrix asphalt (SMA) is a gap-graded bituminous mix with high percentage of coarse aggregates with high bitumen content. Gap gradation aims at maximizing stone-to-stone contact. This gives a structurally strong mix due to efficient load distribution through the stone-matrix skeleton. The drawback of this method is the absence of medium sized aggregates due to gap gradation. This may arise possibility of drain-down of low-grade penetration bitumen through the stone matrix. To check this possibility, modifiers, such as cellulose fibers, are used to hold the bitumen in place (Better Roads 2003; GDOT 1995; Decoene et al., 1990).

Porous pavement

• Porous pavement is a special type of pavement which allows surface water to pass through it, thereby keeping the road surface water-free, as well as providing drainage outlet to storm water.
• Porous pavement may be effectively used in light traffic areas like parking areas, airport taxiway and runway shoulders, footpaths, playgrounds etc. provided that the subsoil drainage, groundwater level and topography of the area is suitable (Michele, 2003; USEPA 1999; DEQ 1992).
• Pavement structure consists of a top porous bituminous layer placed over a filter layer below which a highly permeable open-graded stone layer (known as reservoir course).
• A geotextile layer is placed at the bottom to screen off fine soil particles. Porous bituminous layer consists of gap-graded aggregates (lower percentage fines), held together by a fiber-bitumen blend, giving a matrix structure which allows movement of water through its fine voids.
• Besides load bearing, the reservoir course stores the runoff water (in the void spaces in aggregate layers) until it can infiltrate into the soil beneath.
• Porous pavement has been found (RPL 2001) to be quite effective in reducing noise levels, splash and spray during rains, and aquaplaning tendency thereby improving the wet skid resistance.

CLOSING REMARKS

Certain standard methods are followed for road design and construction. They are modified from time-to-time to match with the technological advancements. Certain modifications in the mix design or structural design can give rise to substantial economy in terms of the longevity of the pavement or the cost of the material concerned.

Cement is manufactured by heating a mixture of limestone, iron ore, gypsum, clay and other ingredients. Cement concrete is a mixture of coarse aggregates, fine aggregates, cement and water, in suitable proportions. Through mix design, suitable proportions of the ingredients of concrete are estimated considering strength, workability, durability and economics. Workability test and air-content test are the tests generally conducted on fresh concrete. Compressive strength, tensile strength, modulus of rupture, elastic modulus, Poisson's ratio, creep and shrinkage, durability, thermal expansion coefficient etc are the tests conducted on hardened concrete.

Various modifications and innovatory applications of pavement materials and pavement design brings in better performance and economy.