1 PART 2 – PAVEMENTS

Different types of pavement are commonly used in the construction of roadways. There are three different types of pavement. These are: - Flexible Pavements - Rigid Pavements - Semi-rigid Pavements

Flexible Pavements

A flexible pavement structure consists of the following layers – the sub-base, base course, intermediate course, surface course, and where determined necessary, a friction course. In flexible pavements the top layers are made of asphalt concrete (also known as bituminous mixtures). The layers have generally the following characteristics: - The sub-base consists of granular material - gravel, crushed stone, reclaimed material or a combination of these materials. - A gravel base course can be designed and specified for depending traffic and subgrade strength. - The base layer is an asphalt concrete pavement layer placed upon the compacted sub-base. When it is required a binder layer is placed on top of the base layer. This layer is also made from asphalt concrete. - The surface course (or wearing course) is the top HMA pavement layer and is placed upon the base or binder layer. This layer provides the surface of the pavement. - A friction course is a specialized thin-lift wearing course which, when specified, is placed over the surface course. Friction courses provide improved vehicle skid resistance, but do not provide any structural value to the pavement. They are very common in maintenance operations.

2 Rigid Pavements

A rigid pavement is made by a Portland cement concrete (PCC) slab placed generally over a cement stabilized sub-base (soil or aggregate stabilized with cement). PCC pavements are jointed or continuously reinforced. The PCC pavements offer great rigidity and consequently a good distribution of the loads on the foundation and excellent fatigue behaviour. They are also not affected by oil and fuels spilled in the surface.

Semi-Rigid Pavements

A semi-rigid pavement consists of one or more asphalt concrete layers over a base made with cement stabilized aggregate.

FLEXIBLE PAVEMENT CONSTRUCTION

Layers in flexible pavements are made, generally, of unbound aggregate (the use of stabilized materials is also possible) and bituminous mixtures. A bituminous mixture in fact is a mixture of mineral aggregates (filler, sand, gravel or crushed stone) that are glued together by a bituminous binder (bitumen). The aggregate skeleton mainly takes compressive stresses while the bitumen takes tensile stresses.

Bitumen Bituminous bound materials can be applied in the base (asphalt base) and in the remaining part of the (asphalt) pavement structure. The mechanical properties of a bituminous mixture are dependent on the nature and the amount of the components. The bitumen plays an important role and therefore special attention is given to bituminous binders. Bitumen is obtained through destillation of crude oil in an oil refinery. At high temperatures bitumen is a liquid and at low temperatures bitumen is hard and brittle.

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At high temperatures and long loading times bitumen behaves as a liquid (viscous) while at low temperatures and short loading times it behaves as a solid material (elastic). In the intermediate area bitumen behaves visco-elastic. For example, to enable good mixing of the bitumen with the aggregates (filler, sand, gravel or crushed stone) the bitumen has to be heated. On the other hand, on hot summer days the temperature of an asphalt surface course can reach a temperature of 60°C and at such temperatures the bitumen should not behave too viscous as that would result in substantial rutting. At low temperatures the bitumen, that then is a solid material, should not exhibit brittle behavior. All this means that the behavior of bitumen has to be optimized. Other factor that is important is the ageing of the bitumen. The ageing of bitumen usually increases the viscosity resulting in hardening and embrittlement of bitumens, both in application and in service. The main ageing mechanism is an irreversible one,

4 characterized by chemical changes of the binder, which in turn has an impact on the rheological properties. The processes contributing to this type of ageing include oxidation, loss of volatile components and exudation (migration of oily components from the bitumen into the aggregate).

Common Tests for Bitumen Behavior Characterization Penetration test

This test measures the relative hardness or consistency of bitumen at 25ºC, representing an average in-service temperature. The value is used to classify the bitumen into standard penetration ranges (in accordance with European Standard EN 12591). The penetration value of bitumen is defined as the distance in tenths of a millimetre (dmm) that a standard needle will penetrate into the bitumen under a load of 100g applied for five seconds at 25ºC. A bitumen, which presented a penetration from 40 dmm to 60 dmm, is referred to as 40/60 pen bitumen. It will be noted that the higher the penetration, the softer the bitumen.

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Softening point test

This is another test of consistency, which determines the temperature at which the bitumen is transformed from a solid to liquid phase. For the majority of bitumens this viscosity value is in the region of 1200 Pa.s. The results of this test also indicate the capacity of the bitumen to perform adequately at high in-service temperatures. (For instance bitumen with a softening point too low may, in a particular environment of climate and traffic, lead to excessive bleeding in chip seals or rutting in asphalt layers). Also referred to as the Ring-and-Ball Softening Point test, this test determines the temperature at which a bitumen disc of controlled dimensions softens sufficiently to allow a steel ball, initially placed on the surface, to sink through the disc and to a further prescribed distance.

Viscosity test (With Brookfield Viscosimeter)

Viscosity, i.e. the resistance to flow or shear, is a fundamental characteristic of bitumen as it describes the behaviour of the material at a particular temperature or over a temperature range. The resistance to flow or shear stress is governed by the internal friction, and can be measured and expressed in units of stress required to overcome this friction. The ratio of applied shear stress and the rate of shear is called the coefficient of viscosity, dynamic viscosity or more often simply viscosity. The dynamic viscosity, or resistance to shear, of penetration grade bitumen can be determined by measuring the torque required to rotate a spindle which is immersed in bitumen. By varying the spindle size, the viscosity can be determined over a large range of bitumen grades from very viscous to very liquid materials. Viscosity can be measured over a wide range of temperatures, including maximum bitumen application and operating temperatures, enabling the susceptibility of viscosity to temperature to be assessed.

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Rolling Thin-Film Oven Test (RTFOT)

There is no direct measure for bitumen’s ageing. Rather, ageing effects are accounted

for by subjecting bitumen samples to simulated ageing then conducting other standard

physical tests. One of the methods to simulate bitumen ageing is the RTFOT. This test gauges the resistance of bitumen to ageing and hardening due to the effect of heat and oxidisation in the presence of air as would occur in a hot mix asphalt manufacturing plant. It does not, however, purport to simulate long term in-service ageing. In the RTFOT a series of glass containers rotates in a vertical plane so that a fresh surface of bitumen is continuously being exposed to air. This exposure (at 163ºC) is continued for 75 minutes and a controlled flow of air is blown over the surface of the bitumen from a single nozzle. At the end of the test, the change in mass, viscosity, softening point and penetration is assessed in terms of the requirements.

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The Bitumen Test Data Chart (BTDC)

Developed by Heukelom, the BTDC provides a system whereby penetration, softening point and viscosity can be jointly described as a function of temperature. During manufacture and construction of hot mix asphalt, there are optimal bitumen viscosities for coating of aggregate and compaction. The BTDC enables the selection of appropriate temperatures to achieve the optimum viscosity for any grade of bitumen.

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Bitumen Grades (EN 12591)

Bitumen Stiffness The nature of the bitumen depends on its chemical composition. Especially the presence or absence of asphaltenes (long hydrocarbon chains with a high molecular weight) is relevant. The nature of the bitumen is described with the Penetration Index PI that

9 represents the temperature susceptibility of the bitumen. The PI-value can be determined from the penetration test (pen) and the temperature ring and ball (Tr&b). When loaded the behavior of bitumen is strongly dependent on the temperature (T) and the loading time (t). Bitumen has visco-elastic properties and it can be characterized with the so-called stiffness modulus S(t,T). Especially Shell has carried out a lot of research into the stiffness behavior of bitumen. One of the achievements of this research is the nomograph given. This nomograph enables the determination of the bitumen stiffness Sbit as a function of the loading time, the temperature and the bitumen properties (PI-value).

Bituminous Emulsions Bituminous emulsions are emulsions of bitumen in water. Emulsification of bitumen is a means of reducing the viscosity of a binder. Thus is possible that the bitumen behaves as a fluid during application without heating it. Bitumen emulsions are two-phase systems consisting of a dispersion of bitumen droplets in water which contains an emulsifier. The emulsifiers are added to assist in the formation of the emulsion, to render it stable, and to modify its properties. In an emulsion, bitumen is dispersed throughout the water as discrete globules, typically of 0.1 to 50 μm in diameter,

10 held in suspension by electrical charges. Commonly bitumen emulsions are available in two classes: - Cationic - Anionic. The terms cationic and anionic derive from the electrical charges on the bitumen globules. In an anionic emulsion the bitumen particles are negatively charged (they would adhere to the anode). In a cationic emulsion the bitumen particles are positively charged (they would adhere to the cathode). Cationic emulsions are more widely used as they have superior adhesive properties to a range of mineral aggregates. Cationic emulsions break via a physical-chemical reaction, through the evaporation of the water phase and through mechanical action such as rolling. Anionic emulsions break predominantly when the bitumen particles agglomerate with the evaporation of the water and through mechanical action such as rolling. Another type of emulsion, termed "invert" is distinct from normal oil in water emulsions like cationic and anionic types in that the water is dispersed in the binder phase. Bitumen emulsions are normally manufactured in a continuous process using a colloidal mill. This equipment consists of a high speed rotor revolving at 1000-6000 rpm in a stator. The clearance between the rotor and stator can usually be adjusted between 0.25 and 0.5 μm.

In some instances emulsions have an advantage over hot binders because they can be used at lower temperatures, which is of the interests because: - Enhanced worker safety; - Lower energy consumption; - Reduced emissions; and

11 - Extended work periods during construction. - For handwork as no heating is required; - Where lower application rates are required through the dilution with water. Emulsions are used extensively in slurries, fog sprays, tack coats, stabilization, chip seals, prime coats, cold mix asphalt and crack sealants. A tack coat (also known as bond coat) is a light application of asphalt emulsion between hot mix asphalt layers designed to create a strong adhesive bond without slippage. Heavier applications may be used under porous layers or around patches where it also functions as a seal coat. Prime coats are applied over unbound sub-bases and are used for stabilizing or binding the surface fines together and promoting bond to the asphalt layer that will be place on top.

Aggregates Aggregates constitute major part of the pavement structure. Aggregates can be used in unbound layers or in the production of bituminous mixtures. The engineering properties of the aggregates, as well as its shape (i.e. form and angularity) and texture, substantially affect the overall performance of the pavement. A number of researchers reported that form and surface texture of aggregates have significant effect on the mechanical property of the bituminous mixes, for example, shear resistance, durability, stiffness, fatigue resistance, rutting resistance, workability, bitumen content etc. Aggregate Size Sizes are described in terms of a lower limiting sieve size, "small" d, and an upper limiting sieve size, "big" D. Written as d/D. The sizes of both small d and big D are selected from a list of prescribed sieve sizes.

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Aggregate Grading The aggregate grading is determined by a sieve analysis. In a sieve analysis, a sample of dry aggregate of known weight is separated through a series of sieves with progressively smaller openings. Once separated, the weight of particles retained on each sieve is measured and compared to the total sample weight. Particle size distribution is then expressed as a percent retained by weight on each sieve size. Results are usually expressed in tabular or graphical format.

Fines - Sand equivalent test The objective of the test is to quickly evaluate the relative portion of clay in the sand that is to be used in unbound and bituminous bound layers. A low value of sand equivalent characterize the fine aggregate as “dirty” and indicates that possibly the clay materials are harmful. The test is carried out on the 0/2mm fraction in fine aggregates. A sample of 120gr is taken and is poured into a plastic graduated cylinder. Then, a washing solution of calcium chloride is added into the cylinder, until it reaches the height of almost 100 mm. The cylinder is left for 10±1 min to soak the test sample. At the end of the 10 min period the cylinder is sealed and shaked for a period of 30±1 sec. The shaking can be done by hand or by a shaker. After the shaking the cylinder is replaced in an upright vertical position and more solution of calcium chloride is added. The adding stops when the level of the liquid reaches the upper mark of the 380±0.25 mm. The cylinder is left to settle, without disturbance and free from vibration for 20±0.25min. At the end of this period, the heights of the sand (hs) and clay (hc) are measured. The sand equivalent value is determined by the following equation:

Fines – Methylene blue test

The methylene blue test is executed in order to determine whether the clay minerals are

active and harmful. The active clay materials expand, depending on the moisture content. Methylene blue test is the only test that gives this information accurately and quickly. An appropriate quantity of the aggregate is dried (200g-210g of the 0/2mm fraction or 30±0.1g of the 0/0.125 mm fraction) and placed in a beaker with 500 ml of demineralized water. The mixture is stirred for 5 min. At the end of the 5 minute period, 5 ml of the methylene blue dye solution is added. The new mixture is stirred again for 1 min. Then, a “stain” test is executed. The stain test is carried out by dipping a glass rod into the mixture and then allowing a drop of the mixture to fall onto a filter paper. If the test portion has absorbed all the dye, the drop on the filter paper appears as a spot of blue stained grains surrounded by a colorless halo. The test is considered positive when the central spot is surrounded by a light blue halo, with a thickness of almost 1 mm. If after the addition of the 5 ml of the methylene blue dye solution a halo does not appear, 5 ml of the dye solution is added, the

13 mixture is stirred again and a test spot is carried out. If again, a halo does not appear the procedure is repeated, in exactly the same way, until the halo appears. The halo must remain visible for 5 min in order to consider that the test has been finished. If the halo disappears at the first 4 minutes another 5 ml of the dye solution is added. If it disappears during the fifth minute, only 2 ml of the dye solution is added. After the test is finished, the total volume Vi of the methylene blue dye solution which was used for the formation of the halo (retained visible for 5 minutes) is recorded to the nearest 1 ml. The Methylene blue value MB, recorded in grams of dye solution per kilograms of the aggregate of the 0/2mm fraction is given by the following equation:

Where Mi is the mass of the sample in grams and Vi is the total volume added in milliliters Aggregate Shape (flakiness index/Shape index)

Particle shape and surface texture are important for proper compaction, deformation resistance and workability. Rounded particles create less particle-to-particle interlock than angular particles and thus provide better workability and easier compaction. Flat or elongated particles tend to impede compaction or break during compaction and thus, may decrease strength. Particle shape can be described by flakiness index or shape index.

The flakiness index (FI) is calculated as the mass of particles that pass the bar sieves with parallel slots, expressed as a percentage of the mass of the test portion. The test consists of two sieving operations. During the first operation test sieves are used to separate the sample into various particle size fractions. Each of the fractions is then sieved using bar sieves. These are sieves that consist of parallel cylindrical bars, set within a frame, with a specified width of slot in between. The flakiness index can be calculated for each fraction within the sample and for the whole sample. The overall index is calculated as the total mass of particles passing the bar sieves expressed as a percentage of the total dry mass of the whole sample. This test is used to determine the quantity of aggregate particles that are elongated, instead of cubicle, in shape

14 Shape index is determined only on the coarse aggregates. The principle of determination of shape index is to measure the thickness E and length L of each grain in a sample of several hundred stones and then to calculate the ratio L/E between the thickness and the length of each particle. If this ratio is higher than 3, than the particle is too long (non-cubic particle). Shape index SI is defined as a ratio between the weight of particles with L/E > 3 and weight of all measured particles.

Percentage of crushed and broken surfaces

This test procedure determines the amount (percent) of fracture faced rock particles, by visual inspection that meets specific requirements. Specifications contain requirements for percentage of crushed aggregate particles, with the purpose of maximizing shear strength in either bound or unbound aggregate mixtures. For coarse aggregate, a sample retained on the 4.75 mm (No. 4) sieve is collected and the weight of particles with fractured faces is compared to the weight of all the particles. A fractured face is defined as being caused either by mechanical means or by nature and should have sharp or slightly blunted edges. A broken surface constituting an area equal to at least 25% of the projected area of the particle, as viewed perpendicular to (looking directly at) the fractured face, is considered an acceptable fractured face.

Resistance to fragmentation (Los Angeles test)/Resistance to wear of coarse aggregate (Micro-Deval coefficient)

A common test used to characterize toughness and abrasion resistance is the Los Angeles (L.A.) abrasion test. For the L.A. abrasion test, the portion of an aggregate sample retained on the 1.70 mm (No. 12) sieve is placed in a large rotating drum that contains a shelf plate attached to the outer wall. A specified number of steel spheres are then placed in the machine and the drum is rotated for 500 revolutions at a speed of 30 - 33 revolutions per minute (RPM). The material is then extracted and separated into material passing the 1.70 mm (No. 12) sieve and material retained on the 1.70 mm (No. 12) sieve. The retained material (larger particles) is then weighed and compared to the original sample weight. The difference in weight is reported as a percent of the original weight and called the "percent loss".

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The Micro-Deval test, on the other hand, is a wet test of how aggregates degrade when tumbled in a rotating steel drum with water and steel balls. It is a better indication of aggregate's service when exposed to weather and moisture. Many aggregates are weaker when wet than when dry. The use of water in this test measures the reduction in resistance to degradation, in contrast to some other tests which are conducted on dry aggregate. A sample with standard grading is initially soaked in water for 15 to 19 hours. The sample is then placed in a jar mill with 2.0 liters of water and an abrasive charge consisting of 5000 grams of 9.5 mm diameter steel balls. The jar, aggregate, water, and charge are revolved at 100 rpm for 2 hours. The sample is then washed and oven dried. The loss is the amount of material passing the 1.18 mm sieve expressed as a percent by mass of the original sample. Resistance to polishing of coarse aggregate

The Polished Stone Value of aggregate gives a measure of resistance to the polishing action of vehicle tyres under conditions similar to those occurring on the surface of a road. The action of road vehicle tyres on road surfaces results in polishing of the top, exposed aggregate surface, and its state of polish is one of the main factors affecting the resistance to skidding. The PSV test is carried out in two stages - accelerated polishing of test specimens followed by measurement of their state of polish by a friction test.

A specimen is made by representative particles of aggregate. The Specimen is clamped around the periphery of the 'road wheel' and subjected to two phases of polishing by wheels with rubber tyres, first with corn emery for three hours, followed by three hours of polishing with an emery flour of. The degree of polish of the specimens is measured by means of the portable skid resistance tester

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The friction is measured by the Skid Resistance tester (British Pendulum). The machine is based on the hod principle. It has a pendulum consisting of a tubular arm rotating about a spindle attached to a vertical pillar. At the end of the tubular arm is a head of constant mass with a spring loaded rubber slider. The pendulum is released from a horizontal position so that it strikes the sample of aggregate with a constant velocity. The distance the head travels after striking the sample is determined by the friction of the surface of the sample,

Asphalt Mix Composition Asphalt mixes are composed of bitumen and aggregates (filler, this is mineral powder with a particle diameter < 63 μm, sand and gravel or crushed stone). The mass percentage of bitumen varies, generally, from 4% (minimum) percentage for stone asphalt concrete) to

17 6.4% (maximum percentage for dense asphalt concrete). The volume of air strongly depends on the type of asphalt mix: for dense asphalt concrete the volume of air is maximum 6% while in the pre-research on porous asphalt the volume of air has to be at least 20%.

Aggregate blend Aggregates from more than one source or stockpile are used to obtain the final aggregate gradation used in a mix design. Trial blends of these different gradations are usually calculated until an acceptable final mix design gradation is achieved. The aggregates must satisfy the specifications (percentage of crushed stone, shape index, Resistance to fragmentation, etc). Typical specifications will require the gradation of the aggregate to be within a certain band.

Weight-Volume Terms and Relationships Basic bituminous mixture’s weight-volume relationships are important to understand for both mix design and construction purposes. Fundamentally, mix design is meant to determine the volume of asphalt binder and aggregates necessary to produce a mixture with the desired properties Bulk specific gravity of the mix Gm is the specific gravity considering air voids and is found out by:

where, Wm is the weight of mix in air, Ww is the weight of mix in water.

Theoretical specific gravity (Gt) is the specific gravity without considering air voids (as if th mixture was voidless, Va = 0).

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Volume of Air Voids (Vv) - The total volume of the small pockets of air between the coated aggregate particles throughout a compacted paving mixture, expressed as a percent of the bulk volume of the compacted paving mixture.

Volume of bitumen (Vb) is the percent of volume of bitumen to the total volume of the bituminous mixture. Voids in the Mineral Aggregate (VMA) - The volume of intergranular void space between the aggregate particles of a compacted paving mixture that includes the air voids and the effective asphalt content, expressed as a percent of the total volume of the specimen.

Voids Filled with Bitumen (VFB) - The portion of the voids in the mineral aggregate that contains bitumen. This represents the volume of the effective bitumen content. It can also be described as the percent of the volume of the VMA that is filled with bitumen.

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Marshall Method The Marshall Method, like other mix design methods (ex. Hveem Method), uses several trial aggregate-asphalt binder blends (typically 5 blends with 3 samples each for a total of 15 specimens), each with a different asphalt binder content. Then, by evaluating each trial blend's performance, optimum asphalt binder content can be selected. In order for this concept to work, the trial blends must contain a range of asphalt contents both above and below the optimum asphalt content. Therefore, the first step in sample preparation is to estimate optimum asphalt content, these could be achieved using mathematical expressions or by experience. Based on the results of the optimum asphalt binder content estimate, samples are typically prepared at 0,5 percent by weight of mix increments, with at least two samples above the estimated asphalt binder content and two below Each sample is then heated to the anticipated compaction temperature and compacted with a Marshall hammer. The number of blows is 35, 50 or 75 on each side depending upon anticipated traffic loading. The samples are cylindrical and have 102 mm in diameter and 64 mm in height. The compacted specimens are subjected to the following tests and analysis: bulk specific gravity test; stability and flow test; maximum theoretical gravity and voids analysis (determination of Vv, VMA and VFB). Prior to the stability and flow test, the specimens are immersed in a water bath at 60ºC for 30 to 40 min. The stability portion of the test measures the maximum load supported by the test specimen at a loading rate of 50.8 mm/minute. Basically, the load is increased until it reaches a maximum then when the load just begins to decrease, the loading is stopped and the maximum load is recorded, this load is taken as the Marshall Stability. The Marshall flow is the total vertical deformation of the specimen when it is loaded to the maximum load in the Marshall stability test.

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The average values of the above properties are determined for each mix with different bitumen content and the following graphical plots are prepared: - Binder content versus corrected Marshall Stability - Binder content versus Marshall flow - Binder content versus percentage of void (Vv) in the total mix - Binder content versus voids in the mineral aggregate (VMA) - Binder content versus voids filled with bitumen (VFA) - Binder content versus unit weight or bulk specific gravity

The optimum binder content for the mix is determined by taking average value of the following three bitumen contents found form the graphs obtained in the previous step, the binder content corresponding to maximum stability, the Binder content corresponding to maximum bulk specific gravity and the binder content corresponding to the median of designed limits of percent air voids in the total mix (generally 4%). The flow value, and VMA value (and VFB if defined) are checked with Marshall mix design specification.

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Example:

Property Specified value

Marshall Stability 8 KN minimum

Flow 15 (0,25 mm) maximum

Percent air voids Vv 3-5%

VMA 14 % minimum

Optimum Bitumen % = (5,2+4,3+4,8)/3

22 Flexible Pavement Design Flexible pavement design deals primarily with structural aspects (i.e., the selection of appropriate materials, characterization of strength or load-carrying properties, layer thickness determination). However, understanding pavement behaviour is a complex task. This complexity is due to the complex response of the individual pavement materials which is very difficult to predict. In a typical pavement a number of such materials are used together.

The concept of rational pavement design for design of bituminous pavement was conceived during the 1960s. Its upgraded version is known as Mechanistic-Empirical (M-E) pavement design, and is at present popularly being used for design of bituminous pavements in various countries.

In M-E pavement design approach, the pavement is idealized as a layered structure (generally assumed as elastic for simplicity in analysis) consisting of three to four horizontal layers made up of bituminous surfacing, base, sub-base and the subgrade. Each layer is characterized by its elastic modulus, Poisson's ratio and the thickness. Fatigue cracking and rutting are generally considered as the important modes of failure of a bituminous pavement structure. To address these to modes of failure two major performance-related criteria are defined: (i) compressive vertical strain at the surface of the subgrade which controls the permanent deformation of the subgrade, (ii) horizontal tensile strain in the HMA layer, generally at the bottom, which controls the fatigue cracking of the layer.

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The developed critical initial strain values for each of the failure modes are computed from suitable pavement analysis routine. The allowable strain values are obtained from the input parameters (related to pavement material, environment, traffic and design life).

Finally, the process of pavement design involves adjusting and subsequently selecting the appropriate thickness values of various layers so that the critical strain parameters are within the allowable limits.

Failure Criterions The permanent deformation criterion is based on the analysis of the AASHO Road Test data, and is given by:

24 - 50% probability of rutting occurrence:

ε3 = 2,8*10-2

*N-0,25

- 15% probability of rutting occurrence:

ε3 = 2,1*10-2

*N-0,25

- 5% probability of rutting occurrence:

ε3 = 1,8*10-2

*N-0,25

The traditional approach to the fatigue cracking is through the 1978 Shell formula:

ε1 = (0,856*Vb + 1,08)*Smix-0,36

*N-0,2

Where N - Design number of cycles of pavement loading; Vb - Volume of bitumen in the mix; Smix – Modulus of the mix; ε1 - Flexural tensile strain at the bottom of the HMA layer; ε3 – Compressive strain at the top of the subgrade.

Soil and Aggregate Mechanic Behaviour In the pavement foundation design, multi-layered elastic analysis is commonly used to compute the strains in the pavement. This analysis requires the determination of the modulus of each layer. For subgrades, the resilient modulus Mr is normally estimated from an empirical relationship usually one that relates the stiffness to CBR. Mr = 10CBR (MPa) or Mr = 17,6*CBR0,64 (MPa)

A poison ratio of 0,35 or 0,40 is generally used for soils. For the unbound aggregate layers the modulus can be calculated using the expression: Eug= 0,2*h0,45*Mr

Where Mr – Modulus of the soil or the unbound aggregate layer underneath; h – Thickness of the layer in mm. A poison ratio of 0,30 or 0,35 is generally used for unbound aggregate layers. Conventional pavement design techniques employ “presumptive” values for modulus of bituminous mixtures. However, these are dependent on knowing certain properties of the bitumen along with the mix volumetric properties. The application of this methodology is fraught with difficulty when there are no presumptive values available. Such is the case for structural Stone Mastic Asphalt (SMA) mixes, and for the growing number of asphalt mixes utilising a variety of modifying agents in the binder.

25 Penetratrion for the aged bitumen: Pen25r = 0,65 * Pen25

Softwening point for the aged bitumen: SPr = 99,13-26,35 * log Pr

The predicted penetration index for the bitumen is computed using the relationship:

15,120)25log(*50

55,1955)25log(*500*20

rpenSpr

rpenSprIPen

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The bitumen stiffness is determined by the following expression (or the nomograph shown

previously):

Sb=1,157*10-7

*Lt-0.368

*2,718-Ipen

*(Spr-T)5

Where :

Lt – Load time (s) (equal to 1/v where v is the traffic speed)

T- Pavement in service temperature (ºC)

The Mixture Modulus is determined by the following expressions (SHELL Method):

For 5 (Mpa) < Sb < 1000 (Mpa)

Em = 10A

Where: 1088log*2

6889)8(log*

2

6889SmSb

SSSb

SSA

For 1000 (Mpa) < Sb < 3000 (Mpa)

Em = 10B

Where 891083log

9log*)891083109( SSm

SbSSmSmB

The parameters are givem by:

30log

)1083109(*12,189

SmSmS

1*33,1

1*37,1log*6,068

2

Vb

VbS

VbVa

VaSm

)100(*342,182,103109

243 *10*135,2**10*68,58108 VaVaSm

Where,

27 Va – Volume of aggregate in the mixture (%)

Vb – Volume of bitumen in the mixture (%)

Sb – Bitumen Stiffness (MPa)

The following monograph it’s a graphic representation of the previous mathematical

equations and can be used to estimate the modulus of the bituminous mixtures.

For bituminous mixture a Poisson ratio of 0,35 to 0,40 is generally used.

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Temperature

The environmental model allows two methods: (i) using the Mean Monthly Air Temperature (MMAT) for each of the twelve months of the year, the analysis can be affected in twelve increments, and (ii) using the Mean Annual Air Temperature, MAAT, a single analysis may be made. The computation of the MAAT is based on work by Witczak in 1972.

Using the Mean Monthly Air Temperatures (MMAT) the mean annual air temperature is obtained as a weighted temperature. The weight factor (w-factor) is a function of the mean monthly air temperature (MMAT). Thus, the mean annual air temperature, as proposed by the Shell design method (w-MAAT), can be calculated using the following equations. wMAAT = 7,7068*Ln(wfactor)+20,257 wfactor = 0,0723*e0,1296MMAT where w-factor is the average of w-factors calculated for all 12 months of the year and MMAT is the mean monthly air temperature.

The pavement temperature or Monthly Mean Pavement Temperature (MMPT) at deepness of Z ( cm) can be determined using a mathematical model which developed by Witczak. It is possible to determine the design pavement temperature for each layer. The depth should be equal to one third of the corresponding depth of the layer. In a more simplistic approach one temperature can be used for the entire asphalt layers using the chart bellow. In this chart with the value wMAAT and the estimated thickness of the pavement a design temperature can be estimated.

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Traffic

The big variety of axle loads that act on the pavement structure determine the development of various strain and stress states, which are very difficult to analyse and consider in design. Still, fullscale tests performed by AASHO in the 50´s led to the conclusion (amongst other) that for each type of pavement, there is a relation between the destructive effects of axles with various loads, which can be expressed using the relationship:

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Where N1 is the number of applications of a single axle load P1 which determines on the pavement the same destructive effect as N2 applications of single axle load P2. The factor “f” is the “equivalence factor” between axles, which expresses the number of P1 single axle load applications which produces the same effect on the pavement as the application of P2 single axle load. From the AASHO full-scale tests, it was concluded that, for flexible pavements, the value of the exponent “x” is 4 (recent studies included in the COST 333 program show that there may slightly different values for the failure mode analysed, however this is the value that is mostly used). The double and triple axles must first be transformed in simple axles. Taking into account the equivalent effect on pavement structure, it is considered that a P load double axle corresponds to 1.4 P/2 load simple axles and a P load triple axle corresponds to 2.3 P/3 load simple axles. In this way, considering the various axle loads and number of loads applications during the pavement service life, it is possible to transform them into an equivalent number of applications of a reference axle load. Generally this reference axle load used in design is 80 kN for flexible pavements and 130 kN for rigid pavements. To determine the number of the reference axle loads is necessary to forecast the one-directional cumulative traffic flow for each category of heavy vehicles expected over the design life (which is half the value d the traffic in both directions) and projecting it at a selected growth rate, and cumulating the total over the design period. Growth rates will normally be in the range of 2 to 15 per cent per annum, and selected values should be based on all available indicators including historical data, and socio-economic trends. The following formula, using the average daily traffic flow for the first year (not the value at opening to traffic, but the projected average for the year), gives the cumulative totals:

DT = T * 365 * (1 + r/100)p - 1 )/(r/100) where DT is the cumulative design traffic in a vehicle category (or for the total of heavy vehicles), for one direction; T = average daily traffic in a vehicle category in the first year for the design lane; r = average assumed growth rate, per cent per annum p = design period in years (generally 20 years for flexible pavements)

The percent of trucks in the design lane is attained using a distribution factor, which is a lane distribution factor which accounts for the percentage of trucks in the design lane. On a multilane roadway, truck traffic will be found in all lanes, but only the lane with the majority of truck traffic is called the "design lane”. The design lane is the driving lane or the right lane (where most of the trucks tend to travel) and the pavement design for the other lanes is based on this lane.

31 Number Of Lanes In Each

Direction Percent Of Trucks in Design

Lane

1 100

2 90

3 or more 80

From the number of trucks in the design lane is possible to determine the number of reference axles using equivalent factors. These factors relate the pass of a truck to the number of passes of the reference axle that would induce the same damage in the pavement. The factors are defined generally by each country according to the maximum allowable weights by type of axle. These factor are obtained by studies in which several trucks in the road are weighted, this is necessary because not all the trucks travel at maximum load, some could be almost empty, and some could travel with overweight. In Portugal there are established factors for the total truck traffic (aggregated) or by category of truck traffic. In the table bellow are the equivalent factors for the aggregate truck traffic.

AADTT in the design lane Equivalent factor (80 KN axle)

50-150 2

150-300 3

300-500 4

500-800 4,5

800-1200 5

1200-2000 5,5

AADTT - Annual Average Daily Truck Traffic

The truck traffic can also be divided in different types of heavy vehicles. In Portugal the trucks can be divided in the following types.

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For each type of truck and axle configuration there are legal established maximum weights.

There are also equivalent factors for each category of heavy vehicle that can be used to determine the number of reference axle loads for pavement design.

33 Cathegory EQ. FACTOR

F1 2,50

F2 3,38

F 2,74

G 6,92

H2 8,78

H3 4,75

H 6,89

I 2,50

The number of standard 80 KN axle (ESAL, Equivalent Single Axle Load) used for pavement design is given by: ESAL = DT* Equivalent Factor

Pavement Design Using Software (ex. Bisar)

The flexible pavement design procedure basically consists in determine the strains in the pavement when the reference single axle passes. This is made using specific software (Elsym5, Bisar, Kenlayer, etc.). The depths at which the strains should be determined are at the bottom of the bituminous mixtures and at the top of the foundation soil in order to control the fatigue and the permanent deformation failure criterions. In the bottom of the bituminous mixture the horizontal tensile strain is determined while that in the top of the foundation soil (sub-grade) is determined the vertical compressive strain.

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To perform the calculation only half of the 80 kN reference axle is used (two wheels with 40 KN; 20 kN each wheel). To account with stress overlap of the two wheels, which occurs at certain depth, two points are analysed for each layer, one exactly between the two wheels (for stress overlap) and one exactly bellow the centre of one of the wheels.

The reference 80 KN standard single axle has the following configuration.

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The Pavement structure and material characteristics are defined. For each layer the thickness, modulus and Poisson ratio should be determined (according to the expressions aforementioned).

Full friction between layers is generally assumed. This can be assumed because a tack coat is applied between asphalt layers during the pavement construction to bind them together. The positions at which the software should determine the stresses and strains are also imputed. The bottom of the bituminous layers and the top of the sub-grade soil are the critical points. In the Bisar software this is made automatically, for each layer two locations are defined, one between the wheels and one under the one of the wheels. For bituminous layers the locations are at the bottom of the layer and for the sub-grade soil at the top. After running the software the strains given for the critical locations are compared with the allowable strains given by the failure criterions. In the software results tensile strains are positive and compressive strains negative. At the bottom of the lower asphalt layer for the two locations the horizontal tensile strains, XX and YY, should be lower than the strain computed for the fatigue failure criterion using the calculated number of reference 80 kN axles for the pavement life. At the top of the sub-grade soil the vertical compressive train, ZZ, for the two locations should be lower than the strain computed for the deformation failure criterion using the calculated number of reference 80 kN axles for the pavement life.

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A parameter that can be used in the design is the damage and is defined as the damage introduced in the pavement by the number of reference axle when compared to the number of ESAL determined for the design life and the total number of ESALs that the pavement structure can resist until it reaches failure (a damage is defined for each of the failure criterions). Values of the damage between 65% to 85% are desirable.

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RIGID PAVEMENT CONSTRUCTION

PLAIN CONCRETE - SHORT PAVEMENT SLABS

This type of pavement consists of successive slabs whose length is limited to about 25 times the slab thickness. This type of pavements does not use any reinforcing steel but

does use dowel bars and tie bars. At present it is recommended that the paving slabs not be made longer than 5 m, even if the joints have dowels to transfer the loads. The movements as a result of fluctuations in temperature and humidity are concentrated in the joints. Normally, these joints are sealed to prevent water from penetrating the road structure. The width of the pavement slabs generally does not exceed 4.5 m. Dowel bars are typically used at transverse joints to assist in load transfer. Tie bars are typically used at longitudinal joints.

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Joints are needed in these pavements to avoid uncontrolled (“wild”) cracking of the concrete by shrinkage. Contraction joints have a crack onset which extends to a depth of one third of the slab thickness and are equipped with dowels. On main roads, the contraction joints are usually made by sawing. The saw cutting should occur as soon as possible, usually between 5 and 24 hours after placement of the concrete. To obtain even joints the concrete should have hardened sufficiently in order to prevent the edges of the joint from being damaged.

Transverse and longitudinal joints are usually sealed with a joint sealant to prevent water infiltration under the paving slabs in the future. To this end, hot or cold joint sealants or prefabricated joint strips are used.

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The pavements made with slabs can have reinforcement (Joint Reinforced Concrete Pavements). The use of a steel mesh in the slabs allows increasing in the distance between joints (from 7,0 m up to 15 m). Temperature and moisture stresses are expected to cause cracking between joints, hence reinforcing steel or a steel mesh is used to hold these cracks tightly together

REINFORCED CONCRETE

Continuously reinforced concrete

Continuously reinforced concrete pavements are characterised by the absence of transverse joints and are equipped with longitudinal steel reinforcement. The diameter of the reinforcing bars is calculated in such a way that cracking can be controlled and that the cracks are uniformly distributed (spacing at 1 to 3 m). The crackwidth has to remain very small, i.e. less than 0.3 mm.

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Steel fibre concrete

The use of steel fibre concrete pavements is mainly limited to industrial floors. However, in that sector they are used intensively. For road pavements steel fibre concrete can be used for thin or very thin paving slabs or for very specific applications.

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IMPORTANT NOTE These study sheets are a compilation of subject matters from several books, papers and web pages. It was not possible to include the references or acknowledge the authors. However, since these study sheets are just for classes support we appreciate the comprehension of the authors.