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Thursday, January 28, 2010

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 (εt) 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.

Pavemnt analysis and design II

Analysis and design of concrete pavements

(Thanks to professor Animesh das and Professor Partha Chakroborty.)

(http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/transport_e/TransportationII) Materials

Objectives

The lectures in this module propose to introduce the modern materials in pavement construction. It discusses about the scope, application potential, evaluation, and performance expectation of the new highway materials. The second part of the lectures focus on the innovative application concepts of the conventional or the modern materials. Usage of modern materials in highway construction and their innovative application is expected to bring economy in terms of material cost as well as better reliability in performance.

Bitumen as a pavement material

The characterization of bitumen and bituminous mix has been discussed in detail in the web-course Transportation Engineering - I

Bitumen is a complex material, its property ranges from viscous liquid to brittle solid. While bitumen shows linear viscoelastic behavior at small strains, the nonlinear behaviour becomes more prominent at large strains (Monismith and Secor 1962, Pagen 1968, Cheung and Cebon 1997). The deformation of bitumen is loading rate and temperature dependent (Van der Poel, 1955, Deshpande and Cebon 1997).

The bituminous mix is manufactured by mixing bitumen and aggregates of specified size distribution at some specified elevated temperature. Then, the mix is transported to the site, laid and subsequently compacted to pack the aggregate particles together. During the compaction process the air voids are brought down to its desired level. The compacted mix, thus, achieves its strength when it cools down and becomes serviceable asbituminous road . Figure-1 shows a typical cross-section of a bituminous mix sample.

The mechanical behavior of bituminous mix has been studied extensively through various tests, and empirical relationships have been developed for mix design and prediction of the performance of the mix. However, prediction of response of bituminous mix through mechanics based models is a difficult task. Various attempts have been made by the researchers, for example based on, linear viscoelastic principle (Lee and Kim 1998, Kim and Little 2004), elastic visco plastic principle (Uzan 2005), discrete element analysis (Sadd 2004, Abbas et al. 2005) etc., so as to capture the complex mechanical behavior of bituminous mixture.

Cement Concrete as a pavement material
Introduction

Cement concrete is a mixture of coarse aggregates, fine aggregates, cement and water in suitable proportions. Sometimes admixtures are also added to achieve specific behaviour/ property of the material. The components of cement concrete are briefly introduced in the following.

Components of cement concrete

Aggregates

Aggregates are naturally available pieces of rocks. The aggregates could be igneous, sedimentary and metamorphic type depending on its origin. Figure-1 shows a photograph of aggregates being manufactured from a stone query. The details about the physical properties of aggregates have discussed in the web-course on Transportation Engineering-1 .

Cement

Cement is manufactured by heating a mixture of limestone, iron ore, gypsum, clay and other ingredients. Two processes, namely dry process and wet process are followed while manufacturing cement. In the dry process, the raw materials are mixed in dry state, whereas in the wet process raw materials are mixed in presence of water to form slurry . After pre-heating, the raw material is passed through rotating kiln inclined with a small angle with the horizontal line. The kiln is progressively hotter towards its lower end, where the raw material gets molten. From this clinkers are formed when cooled, and after grinding the clinkers, cement is produced. An animated description of the whole process can be obtained elsewhere (cement.org 2006).

The Ordinary Portland Cement (OPC) is the most popular, all-purpose cement. There are various other types of cements (for example, natural cement, Portland pozzolanic cement, high alumina cement, expansive cement, quick setting cement, high performance cement, sulphate resistant cement, white cement etc.) and are manufactured to serve specialized purposes. For concrete pavement construction, OPC is most commonly used.

Water

Water participates in the hydration process; also it provides desirable level of workability. About one third of the water added is utilized in the hydration process, rest forms the pores of concrete, and thereby developing porosity to the concrete. Excess porosity reduces strength of the concrete, and however presence of porosity is good for the situations where there is a freeze-thaw problem.

Admixtures

Admixtures are generally of two types, chemical admixture, and mineral admixture. Air entrainer, retarder ,accelerators are examples chemical admixture, and, fly ash, silica fume are the examples of mineral admixtures. One of the important concrete admixtures used in pavement construction is the air-entraining admixture. Air entraining admixtures are derived from natural wood resins, fats, sulfonated hydrocarbons and oils etc (Wright and Dixon 2004). Air-entraining admixtures provide durability against freeze-thaw situation.Plasticizers may be used for concrete pavement construction purposes which maintain workability without having increased the water-cement ratio. Calcium chloride is also used sometimes, as accelerating agent, which renders an early strength of concrete.

Mix design

Through mix design, suitable proportions of the ingredients (coarse aggregates, fine aggregates, cement, water and admixture, if any) are estimated, keeping in view the strength, workability, durability and economic considerations. These proportions are achieved through iterative experimental procedure in the laboratory. There are number of methods for mix design of cement concrete, and a detailed discussion can be obtained elsewhere (Neville and Brooks 1999).

Water-cement ratio is an important consideration in the mix design process. As water cement ratio is increased in concrete, the durability and strength decreases, however, the workability enhances. Depending on the type of construction, workability requirements are different.

For large scale production of cement concrete, the proportioning operation is performed in the batch mixing plant . Figure 3 shows a photograph of a typical concrete batch mixing plant.

Properties of fresh concrete

Ideally a fresh concrete should be workable, should not segregate or bleed during construction. Constituent properties, their proportions, aggregate shape and sizes, temperature affect the performance of fresh mix. The tests that are conducted on fresh concrete include workability test and air-content test. Some of tests through which workability of can be estimated are Kelly ball penetration test, slump test, compacting factor test, Vee bee test and flow table test etc.

Curing of concrete

Presence of adequate amount of moisture, at some requisite temperature and for a suitable period of time, is necessary to complete the hydration process of cement. This process is called curing. The curing conditions significantly affect the final strength achieved by the concrete. For pavement construction, only in-situ curing methods are applicable. Curing compounds are sometimes applied to retain the moisture against evaporation. For final curing of concrete pavements continuous ponding or moistened hessain/ gunny bags are used .

Properties of hardened concrete

Tests are conducted on hardened concrete to estimate properties like, compressive strength, tensile strength, modulus of rupture, elastic modulus, Poisson's ratio, creep and shrinkage performance, durability, thermal expansion coefficient etc. These parameters are of functions of aggregate type, shape and size, type and quantity of cement and admixtures incorporated, water cement ratio, curing, age etc.

Compressive strength of concrete is the failure compressive stress on cubical or cylindrical samples of concrete. Compressive strength of concrete is related to the combined effect of temperature and time, a parameter called maturity. Maturity of concrete is calculated as the time of curing (in hours), multiplied by the temperature, (in degrees) above some specified reference temperature. Various empirical relationships are suggested to obtain the various strength parameters of concrete (elastic modulus, tensile strength, bending strength etc.) from the compressive strength of concrete.

Direct tension test on concrete is performed by applying tension to the cylindrical or dumble shaped samples of concrete. Indirect tension is applied to concrete samples by split cylinder test.

Modulus of rupture of concrete is estimated by measuring the maximum bending stress on concrete beam subjected to pure bending in static condition.

Fatigue test is generally performed subjecting the concrete beams with repetitive flexural loading. The more is the stress ratio (defined as the ratio between the bending stress applied to the modulus of rupture) the less is the fatigue life. The empirically derived fatigue equation by PCA (1974) is the following:

(1)

and

(2)

Where, Nf is the number of load applications to failure, SR is the stress ratio with reference to 90 days modulus of rupture.
The equation suggested by
AASHTO (1993) is the following:

(3)

Transportation of concrete

The transportation of concrete is to be done in such a way that segregation and premature setting is avoided.Wheel barrow, truck mixer, dumper truck, belt conveyor, pipe-line etc. are the various ways concrete is transported to the construction site. Figure 4 shows a typical truck concrete mixer.


A typical truck concrete mixer

Introduction

Road is a costly infrastructure to build and maintain. Thus there is always a need ofr development of (i) new road materials as well as (ii) innovative applications of existing/new materials. These issues are discussed here.

EMERGING ROAD MATERIALS

Modification of Existing Materials

Existing materials may require modifications so as to

  • improve engineering properties of material
  • satisfy general specification requirement of locally available material which in turn would prove to be cost effective
  • meet the demand of special purpose materials having specific properties. Example: high or low permeability, enhanced shear strength etc

These have been discussed further under two sections as,

  • binder (bitumen) modification
  • aggregate modification

Binder (bitumen) modification

Binder (bitumen) modification is done with the help of additives which may or may not react chemically with bitumen. Table 1 presents a partial list of various types of binder modifiers, their purpose and examples. Binder modification results improvement of one or more properties of the binder (and hence the mix) viz. fatigue resistance, stiffness modulus, rutting resistance, stripping potential, temperature susceptibility, oxidation potential etc.

Table 1. Some examples of binder (bitumen) modifiers

(RILEM 1998; ETM 1999; Asphalt Handbook 2000; Widyatmoko 2002, SEAM 2004 )

Purpose

Examples

Polymers

Fillers

to improve bitumen durability and check rutting

Lime, carbon black, fly ash

Anti-oxidants

to check oxidative hardening

Zinc anti-oxidants, lead anti-oxidant, phenolics, amines

Anti-stripping additives

to achieve better adhesion of bitumen to aggregates

Organic compounds (like arnines, andamides)

Extenders

to act as bitumen substitute and to improve fatigue resistance

Lignin, sulphur

Others

-

Shale oil, gilsonite, silicon, inorganic fibers, Trinidad lake asphalt (TLA)

Non Polymers

Fibers

to reduce viscosity, as filler material,

Polyester fibers, Polypropylene fibers

Plastics

-Thermoplastics


-Thermosets

to increase the viscosity and stiffness of bitumen at normal service temperatures without compromising with fatigue performance

to obtain insoluble, infusible material that do not flow on heating

Polyethylene (PE), Polypropylene (PP), Polyvinyl chloride (PVC), Polystyrene (PS) Ethylene vinyl acetate (EVA).

Epoxy resins

3. Elastomers

- Natural

- Synthetic

- Reclaimed rubbers

to reduce temperature susceptibility and temperature distresses, age-hardening, bleeding and binder-aggregate stripping.

Rubber

Styrene-butadiene copolymer (SBR), Styrene-butadiene-styrene copolymer (SBS), Ethylene-propylene-diene terpolymer (EPDM), Isobutene-isoprene copolymer (IIR)

For conventional binders, it is generally observed that the mixes with high stiffness modulus (E)show low fatigue life, and vice versa. However, for an economical pavement design, both high elastic modulus as well as high fatigue life is desirable. Through binder modification, this particular disadvantage can be avoided. Figure 5 presents this concept schematically.

As can be seen in Figure 5, for mixes with ordinary binder, although elastic modulus E value is higher initially at low temperatures, at high E value the fatigue performance generally becomes poor. On the other hand, at high temperature the E value becomes too low and the mix becomes soft. The bituminous mixes with modified binder does not allow the mix to be too hard (high E value) or too soft (low E value) at low and high temperatures respectively. Thus the stiffness versus temperature curve takes a 'S-shape' as shown in Figure 5.

Aggregate modification

  • The marginal or poor quality aggregates can be improved by using some cementing material such as cement, lime, pozzolanic substance etc.
  • The proportions of the cementing material and other ingredients (like water) can be suitably estimated in the laboratory

DEVELOPMENT OF ALTERNATIVE MATERIALS

  • Given the fact that good quality aggregates are depleting and cost of material extraction is increasing, researchers are looking for suitable alternative materials.
  • The tests and specifications, which are applicable for conventional materials, may be inappropriate for evaluation of non-conventional materials ( i.e. alternative materials).
  • This is because the material properties, for example, particle sizes, grading and chemical structure, may differ substantially from those of the conventional materials.
  • Thus, for an appropriate assessment of these materials, new tests are to be devised and new acceptability criteria are to be formed.
  • However, with the advent of performance-based tests, it is expected that the performances of the conventional as well as new materials can be tested on a same set-up and be compared.

Industrial and Domestic Wastes

  • Industrial and domestic waste products provide a prospective source of alternative materials.
  • These materials are cheaply available.
  • Also, their use in road construction provides an efficient solution to the associated problems of pollution and disposal of these wastes.

Table 2 presents a partial list of industrial waste materials that can be used in road construction. Table 3summarizes the advantages and disadvantages of using specific industrial wastes in road construction.

Table 2. Industrial waste product usage in road construction (TFHRC 2004; Hamad et al., 2003;Hawkins et al., 2003; Mroueh et al., 2002; Okagbue et al. , 1999; Sherwood 1995; Javed et al., 1994)

Waste product

Source

Possible usage

Fly ash

Thermal power station

Bulk fill, filler in bituminous mix, artificial aggregates

Blast furnace slag

Steel industry

Base/ Sub-base material, Binder in soil stabilization (ground slag)

Construction and demolition waste

Construction industry

Base/ Sub-base material, bulk-fill, recycling

Colliery spoil

Coal mining

Bulk-fill

Spent oil shale

Petrochemical industry

Bulk-fill

Foundry sands

Foundry industry

Bulk-fill, filler for concrete, crack-relief layer

Mill tailings

Mineral processing industry

Granular base/sub-base, aggregates in bituminous mix, bulk fill

Cement kiln dust

Cement industry

Stabilization of base, binder in bituminous mix

Used engine oil

Automobile industry

Air entraining of concrete

Marble dust

Marble industry

Filler in bituminous mix

Waste tyres

Automobile industry

Rubber modfied bitumen, aggregate

Glass waste

Glass industry

Glass-fibre reinforcement, bulk fill

Nonferrous slags

Mineral processing industry

Bulk-fill, aggregates in bituminous mix

China clay

Bricks and tile industry

Bulk-fill, aggregates in bituminous mix

Table 3. Suitability of using industrial waste products in road construction

(TFHRC 2004; Hamad et al., 2003; Hawk ins et al., 2003; Nunes et al. 1996; Sherwood 1995; Javed et al., 1994)

Material

Advantages

Disadvantages

Fly ash

Lightweight, used as binder in stabilized base/ sub-base due to pozzolanic properties

Lack of homogeneity, presence of sulphates, slow strength development

Metallic slag

- Steel slag

- Nonferrous slag

Higher skid resistance

Light weight ( phosphorus slag)

Unsuitable for concrete and fill work beneath slabs.

May show inconsistent properties

Construction and

demolition waste

More strength, can be used as aggregates granular base

May show inconsistent properties

Blast furnace slag

Used in production of cement, granular fill

Ground water pollution due to leachate formation, used as unbound aggregates

Colliery spoil

-

Combustion of unburnt coal, sulphate attack in case of concrete roads

Spent oil shale

-

Burning of combustible materials

Foundry sands

Substitute for fine aggregate in bituminous mixes

Presence of heavy metals in non ferrous foundry origin, less affinity to bitumen

Mill tailings

Some are pozzolanic in nature

Presence of poisonous materials (e.g., cyanide from gold extraction)

Cement kiln dust

Hardens when exposed to moisture, can be used in soil stabilization

Corrosion of metals (used in concrete roads) in contact because of significant alkali percentage

Used engine oil

Good air entertainer, can be used in

concrete works

Requires well organized used oil collection system

Rubber tires

Enhances fatigue life

Requires special techniques for fine grinding and mixing with bitumen, sometimes segregation occurs

The incenerated municipal soild waste (MSW), after further processing, can be used as fines in bituminous mixes. Processing is done to remove ferrous and nonferrous metals and to achieve the required particle size gradation. Due to the presence of larger fraction of fines, MSW ash is primarily used as fine aggregate. It is also used as a fill material in road construction. The ash can also be stabilized with portland cement or lime to produce stabilized base/sub-base material (TFHRC 2004).

For conventional road materials, a number of tests are conducted and their acceptability is decided based on the test results and the specifications. This ensures the desirable level of performance of the chosen material, in terms of its permeability, volume stability, strength, hardness, toughness, fatigue, durability, shape,viscosity, specific gravity, purity, safety, temperature susceptibility etc., whichever are applicable.

There are a large number tests suggested by various guidelines/ specifications. Figure-6 presents a suggested flow chart to evaluate the suitability of industrial waste for potential usage in highway construction

Health and safety considerations should be given due importance handing industrial waste materials ( Mroueh and Wahlström 2002, Nunes et al. 1996).

1.2.2. Other alternative materials

Steel slag aggregate is a good example of synthetic aggregates obtained from by-products of industrial processes. It has good binding properties with bitumen due to its high calcium oxide content (NatSteel 1993). The angular shape of the aggregates helps to form strong interlocking structure. Road paving with steel slag aggregate show

  • good skid resistance
  • mechanical strength able to withstand heavy traffic and surface wearing.

Also, many industrial and other waste products like fly-ash, cement kiln dust, incenerated refuse etc. have been successfully used to produce synthetic aggregates.

Mixing bitumen with rubber (natural or crumb form) sometimes poses difficulty. As an alternative approach, tiny crumb rubber pieces can be mixed with aggregates - known as dry-process. Research shows improved fatigue performance for this kind of materials (Sibal et al. 2000), also, this process does not require any modification to the existing batch mixing plant.