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, N_f 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.