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CHAPTER 4

MECHANICAL BEHAVIOR of ROAD BUILDING MATERIALS

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4.1 Introduction: A great variety of materials are applied in modern road and railway construction such as:

clay sand crushed unbound (granular) stone for road base and railway ballastbed asphalt concrete

An important environmental aspect nowadays is the necessity to use as much as possible recycled materials. Road construction is especially suited to apply ‘upgraded’ waste materials because huge quantities can be used and because the processes required to upgrading the materials (and thus making them appropriate for application in road construction) are rather simple. This means that concrete granulate (crushed concrete rubble), mix granulate (a mixture of crushed concrete rubble and crushed masonry rubble) and various types of slags (steel slags, blast furnace slags) are frequently used as material for unbound road bases. Under certain conditions incinerator slags (the waste product of refuse incinerators) can very well be used as a fill material. Re-use of asphalt through ‘warm’ recycling (where old broken-up asphalt through certain handling techniques is upgraded to ‘new’ asphalt) is a very common and widely used technique in The Netherlands. Furthermore, old asphalt that is not suited for warm recycling can be recycled in a ‘cold’ way: the broken-up asphalt is then first crushed to asphalt granulate and next mixed with bitumen emulsion and/or cement. All in all this means that numerous road building materials are available. To enable a proper (re-)design of road pavement structures a sound knowledge about the behavior of these materials under different loading conditions is essential. If one wants to know the thickness of a pavement structure and the materials to be applied in this structure, knowledge is required about: a. the magnitude of the occurring stresses and strains b. the number of times that these stresses and strains will occur c. the stiffness, the (fatigue) strength and the resistance against permanent

deformation of the applied materials. In this paragraph attention is especially paid to the aspects mentioned above under item c. First the principles of the resilient (elastic) and permanent deformation behavior of sand and unbound base materials will be discussed. Triaxial tests are in fact required to determine the mechanical properties of this type of granular materials. Triaxial tests are however rather complex and quite time consuming and therefore they are not yet done on a wide scale in road engineering. Instead usually CBR-tests are done. The CBR-test was developed in the thirties and it is a well-known test worldwide. In this chapter the CBR-test will be explained and based on this test the relations between the moisture content, the compaction effort, the density and the bearing capacity will be discussed. Then the stabilization of road building materials will be briefly addressed and finally the behavior of bituminous mixtures is shortly described.

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4.2 Resilient deformation behavior of sand and unbound base materials: By nature sand and unbound base materials don’t exhibit any coherence. If one tries to stand on a bunch of loose sand one will sink into the sand. In other words, shear failure occurs. If however the same sand is put into a bucket, then the material has gained a substantial strength (coherence) and stiffness (see figure 4.1). Sinking away is out of the question and shear failure does not occur.

Figure 4.1: Confinement is essential for the bearing capacity of sand and other unbound materials.

So the strength of unbound materials is highly affected by the amount of horizontal confinement. In the absence of this horizontal confinement (e.g. bunch of loose sand) both the stiffness and the strength are very low (shear failure), while in the case of a high horizontal confinement (sand in bucket) also the stiffness and strength are high. This behavior complicates calculations on road pavement structures. Obviously the occurring stresses determine the stiffness and strength of unbound materials but to enable the calculation of the occurring stresses the

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stiffness has to be known. This implies that the calculations have to be done in an iterative way. In first instance certain stiffness characteristics have to be assumed which enables the calculation of the occurring stresses. Next the stiffness characteristics have to be adapted on the basis of the calculated stresses etc. etc. until the stresses and stiffnesses don’t change anymore. 4.2.1 Triaxial test: The so-called stress-dependent behavior of unbound road base materials and sands can be determined by means of the triaxial test (see figure 4.2).

Figure 4.2 Example of a triaxial test set-up.

The principle of the triaxial test is explained on the basis of the figures 4.3 and 4.4. Figure 4.3: Principle of the triaxial test. The deformation behavior observed during such a monotonic triaxial test is shown in figure 4.4.

∆h

h

σ1

σ3

There is an all-around cell pressure σ3 and an increasing axial stress σ1 is applied. The deviatoric stress σd = (σ1 + σ3) - σ3 = σ1 εv = ∆h/h

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Figure 4.4: Result of monotonic triaxial tests (schematic). This means that the “E-value” will increase with increasing confinement stress σ3. In general the stress dependency of the E-value of this type of unbound materials can be presented as shown in figure 4.5.

Figure 4.5: Stress dependent “E-value”. The E-value increases with increasing confinement stress σ3. An increasing deviatoric stress σd in first instance results in a small decrease of E; if however the deviatoric stress becomes so high that shear failure occurs, E approaches the value zero. In a triaxial test the confinement stress has a major effect on the sum of the principal stresses. Therefore the stress dependency of the E-value of granular materials is sometimes presented as a function of the sum of principal stresses (see figure 4.6).

Figure 4.6: Widely used representation of the stress dependent behavior of unbound (granular) materials.

εv

σ1

E

log E

log σ3

increasing σd

log θ = log (σ1 + 3σ3)

log E

increasing σ3

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One has however to realize that the representation of the stress dependent behavior in this way is in principle wrong. 4.2.2 Rules of the thumb: The foregoing implies that triaxial tests should be performed to determine the E-value as a function of the stress level. In the road engineering practice it is however felt that triaxial tests are rather complex and time consuming and therefore they are not done on a wide scale. The consequence is that the elastic modulus E has to be determined through rules of the thumb. For example, for sands is valid: E = 10 CBR with E in MPa and CBR in % (the CBR-value and the CBR-test are explained in paragraph 4.5). For many Dutch sands is valid: CBR = 10%, so E = 100 MPa The use of these relations is questionable as the CBR-value in fact represents the force required to obtain a certain deformation (0,1 or 0,2 inch). The greatest part of this deformation is however permanent deformation. Nevertheless the CBR-test yields the relation between force and displacement and for that reason the test gives information about the resilient deformation behavior. The boundary conditions are however difficult to quantify, e.g. what is the magnitude of the confinement stress resulting from the steel mould around the specimen? Therefore direct relationships between the E-values obtained by means of triaxial tests and the CBR-value should be used with care. For unbound road base materials applied on top of a sand sub-base the following rule of the thumb is sometimes used to obtain the E-value: Eb = 0.2 hb

0.45 Esb where hb = thickness of the base [mm],

Esb = E-value of the sand sub-base directly below the base [MPa],

Eb = E-value of the base [MPa]. The boundary condition is: Eb = 2 to 4 Esb One should realize that this rule of the thumb was developed in the sixties through experiments on unbound road base materials commonly used in that period of time (hard crushed natural stone). Use of this rule of the thumb for nowadays widely used unbound road base materials (such as blast furnace

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slags, lava and recycled granulates) may lead to substantial differences between the real E-value and the predicted E-value. As an example, for some types of lava is valid: Eb = 1.2 to 1.5 Esb while, due to hydraulic (cementing) action, for some types of blast furnace slags is valid: Eb = 5 to 10 Esb This illustrates again that one should be very careful in simply applying this kind of rules of the thumb. The experimental determination, through triaxial testing, is in fact the only correct way to determine the E-values of unbound road base materials. With triaxial tests again relationships are found as shown in figure 4.7.

Figure 4.7: Examples of the stress dependency of the stiffness modulus of unbound road base materials.

The above-mentioned rule of the thumb is however certainly not based on nonsense. It can be shown that both the subgrade stiffness (‘sound board’ for compaction) and the base thickness have a pronounced effect on the ultimate E-modulus that is representative for the whole road base. The great disadvantage of the given rule of the thumb is that the effect of the type of unbound road base material is not taken into account at all. The type of material however appears to be a very relevant factor. 4.3 Permanent deformation behavior of sand and unbound

base materials: It is known from soil mechanics that shear failure will not occur if the stress condition is such that the Mohr’s stress circles do not intersect with Coulomb’s failure envelope (see figure 4.8). This shear failure behavior is however based on one single (monotonic) loading that does not represent the situation in a real pavement structure that is subjected to many millions of load repetitions. It is therefore important to know the effect of repeated loadings on the shear

concrete granulate mix granulate sand

log θ

log E

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failure behavior and on the development of permanent deformations in the sand sub-base and the unbound base.

Figure 4.8: Mohr-Coulomb shear failure criterion. Permanent deformations within the sand sub-base and the unbound base manifest themselves at the road surface as rutting and unevenness. The occurring permanent deformations depend on the stress conditions and the nature of the unbound material. Also in this case the triaxial test, carried out with dynamic (repeated) loadings, is the most appropriate test to determine the permanent deformation behavior. Then in most cases results are obtainedthat are schematically shown in figure 4.9.

Figure 4.9: Development of permanent deformations resulting from triaxial tests with dynamic loadings.

Performing triaxial tests with dynamic loadings is indeed time-consuming and complicated. This type of triaxial tests is therefore until now mainly done for research purposes despite the fact that they deliver very relevant information.

τ

σ

Mohr – Coulomb failure envelope

stress condition not leading to shear failure

stress condition leading to shear failure

increasing σd

masonry granulate

concrete granulate

log N

log εp

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Of course a variety of simplified models have been developed that enable a reasonable estimate of the development of permanent deformation under traffic loadings. One of such simplified models is: εp = εel * a * N

b In this model the permanent deformation εp is related to the elastic (resilient) deformation per load cycle (εel) and to the number of load repetitions (N). The problem again is that a and b will depend on the type of unbound material. As a first global estimate the following values can be used for a and b: a ≈ 2 b ≈ 0.2 to 0.3 An even greater simplification is a relationship that correlates the vertical elastic deformation at the top of the subgrade (or sand sub-base) εv directly to the number of load repetitions causing a certain amount of permanent deformation of the total pavement structure. This relationship was developed in the U.S.A. in the early sixties on the basis of the results from the AASHO Road Test. In this Road Test a great number of test pavements, with different structures and layer thicknesses, have been subjected to a great number of load repetitions. The development of the damage on the test pavements was analysed as well as the level of damage that still is acceptable for the road user (gives an acceptable ride-ability). The road user aspect was determined by regularly driving groups of people with different background (men/women, black/white, rich/arm etc.) over the test pavements and let them awarding the quality of the pavements. The mark of 5 was given for a pavement in a perfect condition and the mark of 0 for a pavement in an extremely bad condition. It appeared from this research that road maintenance should be done when this ride-ability mark (PSI, Present Serviceability Index) had dropped to a value between 2 and 2.5. Next the ride-ability mark was related to the damage visible at the pavement surface. The PSI appeared to be highly dependent on the longitudinal unevenness, less dependent on the rut depth and hardly dependent on cracking. It was also analysed after how many load repetitions the PSI had dropped to 2.5 (Npsi = 2.5). This number of load repetitions was finally correlated to the vertical elastic deformation at the top of the subgrade, and this has resulted in the so-called subgrade strain design criterion: εo = 2.8 x 10-2 x N-0.25 where: ε0 = elastic vertical compressive strain at the top of the subgrade [m/m], N = allowable number of strain repetitions until the deformations of the pavement structure become unacceptable. Because of lacking a better criterion, also today this criterion is still widely used for the structural design of asphalt pavements. Limiting the elastic deformation results in development of only limited permanent deformations

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and thus in an increased pavement life (the unevenness will develop more slowly). 4.4 Material properties influencing the elastic and

permanent deformation behavior: Sands and unbound (granular) base materials are rather simple materials. The behavior of this type of materials is mainly dependent on the grading, the degree of compaction, the particle shape and hardness. As a general statement one can say that a relative density as high as possible, which is obtained with a grading curve close to the Fuller-curve and with a high compaction effort, has a positive effect on the resistance against elastic and permanent deformation. Angularity of the particles also contributes to the resistance against elastic and permanent deformation. The hardness is directly related to the susceptibility for crushing of the particles. Crushing should not occur to prevent deviations from the optimal grading. 4.5 CBR-test: The California Bearing Ratio test (CBR-test) was developed at the end of the thirties by the California State Highway Department for determination of the strength of soils. In the forties the test was adopted by the U.S. Corps of Engineers to design flexible road pavements. Since then the CBR-test has been introduced and used almost worldwide as a simple method to determine the strength of soils, sands and unbound base materials. The principle of the CBR-test is shown in figure 4.10, while figure 4.11 presents an example of the force-displacement curve. It appears from figure 4.11 that the relation between force and displacement as measured on the material under investigation is compared to the relation obtained for a standard material of crushed stone.

Figure 4.10: Scheme of the laboratory CBR test device.

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Figure 4.11: Determination of the CBR-value. The CBR-value of the material under investigation follows from the equation:

CBRPP st

0 10 1

0 1100%,

,

,*=

or from:

CBRPP st

0 20 2

0 2100%,

,

,*= if the CBR0,2-value appears to be greater than the

CBR0,1-value, then the CBR0,2-value is valid It might be clear from the nature of this plunger test that it is only possible to determine the CBR-value of unbound and rather fine-grained materials such as clay and sand. The CBR-test is not suited to determine the bearing capacity of bitumen-bound or cement-bound materials. As already mentioned the test is also done to determine the CBR-value of unbound base materials. These materials usually have particles with a diameter from 0 to 40 mm, so they are too coarse-grained for direct testing. In these cases all the particles with a diameter greater than 22.4 mm are sieved out. From materials applied in the embankment or sub-base, all the particles with a diameter greater than 4 mm are sieved out. In a CBR-test the resistance against permanent deformation of unbound materials is measured. If the test is continued until the maximum force is reached (figure 4.12a) the resistance against shear failure is measured and shear planes are found as illustrated in figure 4.12b.

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Figure 4.12a: Force-displacement curve at CBR-test.

Figure 4.12b: Shear planes in CBR-test at maximum force level.

4.6 In situ CBR-value: In paragraph 4.5 the CBR-test as done in the laboratory is described. The CBR-test can however also be done in situ, e.g. by mounting the force-actuator and the CBR-plunger at the rear side of a truck. Another method is to estimate the CBR-value from data of other tests such as the Dutch cone penetration test. Dutch cone penetration tests give an indication of the bearing capacity of the subsoil. These tests are also used to check the homogeneity of the layers in a subsoil. In this terrain-test a standard cone with an area of 1000 mm² and a top angle of 60° (figure 4.13a) is pushed with a certain constant speed into the subsoil-layer under investigation. The measured forces are the cone resistance and the wall-friction (around the pipe guiding the cone). For fine-grained soils (clay, clayey sands and fine embankment sands) there exists a global relation between the in situ CBR-value and the cone resistance Cs: CBR (in %) = 4 * Cs (in N/mm²)

force until shear failure

displacement

force

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Figure 4.13: Scheme of the Dutch cone penetration test device (a) and an

example of the measured cone resistance as a function of the depth within the subsoil (b).

There exist a great number of other tests for the in situ determination of the bearing capacity of soil, such as static plate bearing tests, vibration tests, hammer penetration tests and Clegg hammer tests. For these tests reference is made to the course CT4850 ‘Road building materials’. 4.7 Density – moisture content – bearing capacity: It will be clear that a greater density of a grain skeleton results in a greater bearing capacity. This can easily be explained because with increasing density the volume of the pores decreases and the number of contact points between the grains increases, leading to a greater friction between the grains and also to a greater resistance against shear failure. It already has been mentioned that also the moisture (water) content has quite an effect. This can be illustrated well by considering the traffic-ability on the beach. Close to the dunes it is rather impossible to bike because the sand is very dry and does not have any coherence. Also near the high-water mark the sand lacks bearing capacity because it is too wet; also here the bike sinks into the sand. However, in the drying area there is a zone where the capillary forces, the suction forces, are so high that the grain skeleton is to say prestressed. Through this confinement the sand gains certain strength and biking is not any problem at all. The above-mentioned problems are visualized in figure 4.14. To achieve a density as high as possible of course compaction effort is required but also a certain moisture content. The moisture acts as a lubricant between the grains. Figure 4.15a gives, for a number of Dutch sands, the relationship between the moisture content and the dry density as determined with the so-called Proctor test. The grain size distribution curves of the various sands are given in figure 4.15b.

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Figure 4.14: At a certain moisture content sand has an optimal bearing capacity (1).

4.7.1 Proctor test: The Proctor test is a worldwide known standard test for the determination of the relation moisture content – compaction effort – density. In fact it is a very simpel test. The material to be investigated is put into a steel mould, that has an internal diameter of 101.6 mm and a height of 126.4 mm, in three layers each about 40 mm thick. To enable this, an extension collar with a height of 60 mm and the same internal diameter is required on top of the mould. Each of the three layers is compacted by 25 blows of a cylindrical metal rammer that has a mass of 2.5 kg. The diameter of the rammer is 50.8 mm and the drop height is 305 mm. A test performed in this way is called the standard (normal) Proctor test. In a modified (heavy) Proctor test the specimen is compacted in five layers, each about 25 mm thick, with a metal rammer that has a mass of 4.54 kg while the drop height is 457 mm. The diameter of the steel mould is rather small, and therefore also in this test the coarse particles are sieved out. The procedure is described in (2). In general the Proctor curves for sands are rather flat. Other materials may exhibit a much more pronounced relationship between the moisture content and the density. This is, for example, the case for laterite that is a very widely available material in tropical areas (see figure 4.16a).

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Figure 4.15a: Proctor curves for a number of Dutch sands (1).

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Figure 4.15b: Grain size distribution curves of the sands from figure 4.15a (1).

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Figure 4.16a: Proctor curves for a Ghanaian laterite.

Figure 4.16b: Grain size distribution curve of the laterite from figure 4.16a. 4.7.2 CBR - density – moisture content: The effect of the density and the moisture content on the CBR-value is given in figure 4.17 (for the sands of figure 4.15) and in figure 4.18 (for the laterite of figure 4.16) respectively. It appears from all the information provided that with respect to both the density and the CBR-value there is an optimum moisture content at which the density or CBR-value is greatest. Generally the optimum moisture content to obtain the maximum density is somewhat higher than the optimum moisture content to obtain the maximum CBR-value.

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Figure 4.17: Relation CBR – density at different moisture contents for the sands from figure 4.15 (1).

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Figure 4.18: CBR-values for the laterite from figure 4.16. 4.8 Soil stabilization: Sometimes the materials for the embankment, the sub-base or the unbound base exhibit insufficient properties (e.g. too low bearing capacity, moisture and frost susceptibility) for their intended application. In those cases the material is stabilized in such a way that the desired properties are obtained. The following types of stabilization can be distinguished, dependent on the followed method: 1. Mechanical stabilization. This implies the optimal compaction of the

material. 2. Physical-mechanical stabilization. This includes the improvement of the

grain size distribution together with mechanical compaction. 3. Chemical-physical stabilization. This method includes the mixing of the

basic material with a binder material (mostly cement, lime or bitumen) together with mechanical compaction.

The choice between these types of stabilization is dependent on the nature of the basic material to be stabilized (e.g. mixing of cement and bitumen with heavy clay is nearly impossible) and the function of the stabilized layer in the pavement structure (working platform, protection layer against climatic effects and/or structural layer). For the various stabilization techniques and the mechanical behavior of stabilized materials reference is made to the lecture note ‘Soil Stabilization’ of the course CT4850 ‘Road building materials’. Here only some theoretical background is discussed. It is known from soil mechanics that according to Coulomb’s failure curve the relationship between the shear strength (τ) and the normal stress (σn) is as follows: τ σ= +c f n

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The term c is usually called the cohesion and the term f the friction coefficient with f = tg ϕ, where ϕ is the angle of internal friction. When stabilizing a material either c or ϕ is affected. For instance, improvement of the grain size distribution through mixing with another material mainly results in an increase of the angle of internal friction ϕ. Mixing cement through an unbound material mainly leads to an increase of the cohesion c. 4.9 Bitumen: Bituminous bound materials can be applied in the base (asphalt base) and in the remaining part of the (asphalt) pavement structure. 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. The mechanical properties of a bituminous mixture are dependent on the nature and the amount of both components. The bitumen plays an important role and therefore special attention is given to bituminous binders. 4.9.1 Bituminous binders: Bitumen is obtained through destillation of crude oil in an oil refinery. This process is schematically depicted in figure 4.19. At high temperatures bitumen is a liquid and at low temperatures bitumen is hard and glass-like.

Figure 4.19: Manufacturing of bitumen out of crude oil. 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). This complex visco-elastic behavior of bitumen is discussed in the next paragraph. 4.9.2 Visco-elastic behavior of bitumen in relation to temperature and

loading time: Similar to every other oil product, the behavior of bitumen is dependent on the temperature and the loading time. At high temperatures and long loading

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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. To enable good mixing of the bitumen with the aggregates (filler, sand, gravel or crushed stone) at the lowest energy costs (heating) the bitumen should behave as a liquid at rather low temperatures. On the other hand, on hot summer days the temperature of an asphalt wearing course can reach a temperature of 60°C (in The Netherlands) 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 a bitumen has to be optimized. Let’s first analyze how the various phases of the behavior of bitumen (elastic, viscous and delayed elastic) can be modeled. In figure 4.20 it is depicted that the elastic behavior can be modeled with a spring. When a tensile load is applied an instantaneous extension of the spring occurs while the spring immediately returns to its original condition if the load is removed. The viscous behavior can be modeled with a dashpot or shock absorber. When the load is applied the deformation gradually increases. The longer the loading time the greater the deformation. If the load is removed the dashpot remains in its deformed condition. The delayed elastic behavior can be modeled as a parallel system of a spring and a dashpot. When the load is applied the spring likes to deform immediately but that is obstructed by the dashpot. If the load is removed the dashpot likes to maintain its deformed condition but the spring (that would like to return to its original condition immediately) will ‘pull back’ the dashpot to the original condition; this however takes some time. The total deformation behavior of bitumen now can be described with a system of springs and dashpots that represent the elastic, delayed elastic and viscous behavior respectively. This model, that is called the Burgers model, is also shown in figure 4.20. It depends on the temperature and the loading time and also on the type (nature) of the bitumen to what extent the springs and dashpots determine the bitumen behavior. 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 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). Figure 4.21 shows to what extent the elastic, the delayed elastic and the viscous behavior determine the total response of the bitumen, in relation to the bitumen stiffness Sbit and the PI-value. As a compromise between handling and behavior on the in service road, usually bitumen with a PI-value between –1 and +1 is chosen for applications in road construction. It will be clear that the behavior of the bitumen, as depicted in figure 4.21, directly reflects in the behavior of the bituminous mixture.

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Figure 4.20: Schematization of the behavior of bitumen.

It appears from figure 4.21 that the ratio between the elastic, the delayed elastic and the viscous part of the deformation changes as a function of the loading time and the temperature, that jointly are represented by the stiffness modulus Sbit. With decreasing temperature and decreasing loading time (Sbit increases) the share of the elastic deformation in the total deformation increases while at high temperatures and long loading times (Sbit decreases) the material behaves nearly purely viscous.

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Figure 4.21: Behavior of bitumen in relation to Sbit and PI.

The stiffness parameter S is obtained by dividing the applied stress by the total strain (that is calculated from the total deformation). In this way not a real but an apparent elastic modulus is calculated as the value is dependent on the loading time and temperature. This parameter therefore is not denoted as E but as the stiffness modulus S. The stiffness modulus S thus is:

S t Tt T

( , )( , )

=σε

Stiffness as a function of = applied stress loading time and temperature strain as a function of loading time and temperature An example of such a relationship is given in figure 4.22.

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Figure 4.22: Effect of temperature and loading time on the behavior of two

strongly different types of bitumen.

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 in figure 4.23 (3). This nomograph enables the determination of the bitumen stiffness Sbit as a function of the loading time, the temperature and the bitumen properties. 4.10 Asphalt mixes: Asphalt mixtures (bituminous mixtures) are important road building materials. In The Netherlands every year some 7 million tons of asphalt are applied. The costs of an asphalt mix depend on various factors but amount roughly between € 50 and €70 per ton (including construction costs). Various types of asphalt mixes can be distinguished: a. gravel asphalt concrete (gac) for the bottom layers of lightly loaded roads, b. stone asphalt concrete (stac) for the bottom layers of heavily loaded roads, c. open asphalt concrete (oac) for the intermediate (‘binder’) layers, d. dense asphalt concrete (dac) for wearing courses, e. stone mastic asphalt (sma) for wearing courses, f. porous (very open) asphalt concrete (zoac, in Dutch ‘zoab’) for wearing

courses. These asphalt mixes are all so-called hot mixes, which means that the mixing of sand, gravel or crushed stone, filler and bitumen occurs at a temperature of about 180°C. This high temperature is required to give the bitumen the correct viscosity (liquidity) to enable the mixing process. Later the above-mentioned types of asphalt mixes are discussed in more detail. First however attention is paid to some basic principles. After that the mechanical properties of asphalt mixes are briefly discussed. Finally a short and global description is given of the various asphalt mixes.

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Figure 4.23: Nomograph for the determination of the stiffness modulus Sbit of bitumen.

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4.10.1 Mix composition and requirements for the mixes Asphalt mixes are composed of: a. bitumen, b. filler, this is mineral powder with a particle diameter < 63 µm, c. sand, d. gravel or crushed stone. As already described, bitumen is a residue of the refinery process of crude oil. The behavior of bitumen is dependent on the loading time and the temperature. The bitumen acts as the glue in the asphalt mix and therefore also the behavior of asphalt mixes depends on the loading time and temperature. Table 4.1 contains the requirements with respect to the composition of some of the Dutch asphalt mixes. It is remarked that the composition is given in percentages by mass and that the amount of bitumen is expressed as the (mass) percentage compared to 100% of mineral aggregate. Table 4.2 contains the requirements for asphalt mixes for traffic class 4. It is striking that the mass percentage of bitumen varies from 4% (minimum percentage for stone asphalt concrete) to 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%. Porous asphalt (‘zoab’) thus is a very open asphalt mix with a high draining action (no splash and spray) and besides it yields a strong reduction of traffic noise. Stone asphalt

concrete 0/22 Open asphalt concrete 0/16 type 3

Dense asphalt concrete 0/16

Porous asphalt concrete 0/16

C22.4 C16 C11.2 C8 C5.6 2 mm 63 µm

0 - 6 -

15 - 40 - -

54 - 60 92 - 94

- 0 - 6

10 - 20 -

35 - 50 64 - 70 93 - 95

- 0 - 6

5 - 25 -

30 - 55 57 - 63

y-0.5 - y+1.0

- 0 - 7

15 - 30 50 - 65 70 - 85

85 95.5

Percentage of bitumen 4 - 5 4.8 - 5.8 6.0 - 6.4 4.5 N.B. - y = 100 - 7 x density filler/2700

- for traffic class 4 always 40/60 penetration bitumen must be applied, for porous asphalt however 70/100 penetration bitumen should be used

Table 4.1: Requirements (mass percentages) for the composition of various

asphalt mixes for traffic class 4 (2). Stone asphalt

concrete Open asphalt concrete type 3

Dense asphalt concrete

Porous asphalt concrete

Marshall stability (N) Marshall flow (mm) Marshall quotient (N/mm) Percentage of air (% v/v) Degree of filling (% v/v) Degree of compaction (%)

> 6000 1.5 – 3 > 3000

< 7 50 – 68

> 98

> 7000 2 – 4

> 3000 < 7

< 72 > 98

> 7500 2 – 4

> 3000 < 6

< 80 > 98

> 97

Table 4.2: Requirements for various asphalt mixes for traffic class 4 (2).

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The requirements with respect to the composition and the properties of the asphalt mixes are dependent on the traffic class. The traffic class is related to the amount of the expected daily heavy traffic on the road. Table 4.3 gives an overview of the traffic classes distinguished in The Netherlands.

Traffic class SAL100 Explanation 2 3 4 5

< 500 500 – 4000

> 4000 > 5000

Lightly loaded pavements Moderately loaded pavements Heavily loaded pavements Heavily loaded pavements with slow driving (vehicle speed < 15 km/h) and standing heavy traffic

SAL100 = Iv . VSF100 with: Iv = intensity of truck traffic on most heavily loaded traffic lane (annual working day average in one direction) VSF100 = truck damage factor (number of equivalent 100 kN standard axle loads per truck)

Type of truck traffic VSF100 Light: on average < 2.5 axles per truck Moderate: on average 2.5 – 3 axles per truck Heavy: on average > 3 axles per truck

0.2 – 0.5 0.5-1.0

1.0 – 2.0

Table 4.3: Traffic classes (2).

4.10.2 Marshall test: The tables 4.1 and 4.2 learn that in The Netherlands the requirements to asphalt mixes deal with the composition and the Marshall properties. The Marshall properties are determined by means of the so-called Marshall test. For Marshall testing asphalt specimens with a diameter of 100 mm and a height of 50 mm are produced. These specimens are compacted according to the Marshall compaction method; this is an exactly prescribed hammer compaction. Marshall tests are done at 60°C with a loading speed of 0.83 mm/sec. The load is applied through exactly defined testing heads (see figure 4.24). During the test the load – displacement curve is recorded (figure 4.25). From this curve the Marshall stability Pm and the Marshall flow Fm are determined. The Marshall quotient is the ratio of Pm and Fm. The Marshall test is done to gain insight in the stability of the asphalt mix at high temperatures. A great Marshall stability in general implies a high resistance against permanent deformation (rutting). Although the requirements (specifications) to asphalt mixes are focused on their composition by mass, research has learned that especially their composition by volume is relevant. Starting with the density of the mineral aggregates (around 27 kN/m3) and that of bitumen (somewhat greater than 10 kN/m3) the volumetric composition can easily be calculated from the composition by mass. The volume percentage of bitumen (Vb) is roughly between 9% and 13%, while the volume percentage of mineral aggregate (Vg) is between 80% and 85%; the mineral aggregate includes the sand, gravel or crushed stone, and the filler. Besides of that, every asphalt mix should contain a certain percentage of air (Va), also called ‘empty space’; except for porous asphalt Va usually is between 3% and 7%.

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Figure 4.24: Cross section of Marshall testing heads; dimensions in mm at room temperature, tolerance ± 0,1 mm.

Figure 4.25: Marshall load – displacement curve (schematic). 4.10.3 The aggregate skeleton: It already has been mentioned that an asphalt mix is composed of bitumen, filler, sand and gravel or crushed stone. The properties of the bitumen have been discussed in paragraph 4.9. In this paragraph the aggregate skeleton is shortly addressed.

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Filler Filler is a fine-grained mineral powder; the particles have a diameter smaller than 63 µm. Examples of fillers are limestone flour and fly ash. The filler contributes to the mineral skeleton. Besides of that, together with the bitumen the filler forms the mortar gluing the greater aggregate particles. Not every filler acts in the same way: one type of filler can bind considerably more bitumen than another type. This implies that the type of filler not only affects the handling of the asphalt mix but also the mechanical properties. Sand and gravel/crushed stone The sand and the gravel or crushed stone are the ‘bearing components’ within the asphalt mix. Important factors influencing the bond between the mortar and the aggregate are the particle shape (angular or round), the absorption capacity (porous or not) and the degree of acidity. The grain size distribution (grading) is especially important with respect to the resistance against rutting as will be explained later. 4.11 Mechanical behavior of asphalt mixes: Asphalt mixes must possess certain properties to be able to meet the requirements with respect to their resistance against deformation and cracking and with respect to their durability. These three aspects are discussed in the following paragraphs. 4.11.1 Deformation characteristics: Mix stiffness Asphalt mixes should preferably have a high mix stiffness (elastic modulus) to achieve a traffic load spreading as great as possible. A high mix stiffness is obtained if the asphalt mix:

has a volume percentage of air as small as possible; has a volume percentage of bitumen as small as possible; contains a type of bitumen that is affected as less as possible by the temperature and the loading time;

exhibits a good bond between the bituminous mortar and the mineral aggregates.

Shell also has done a lot of research into the stiffness of asphalt mixes. One of the results is a nomograph that enables to determine the stiffness modulus of the asphalt mix (Smix) from the stiffness modulus of the bitumen (Sbit) and the volumetric composition of the asphalt mix. This nomograph is shown in figure 4.26 (3). Figure 4.26 learns that the asphalt mix stiffness is determined by the stiffness of the bitumen and the volumetric composition of the mix. However, the mix composition is usually known in mass percentages. Therefore first a calculation from mass percentages into volume percentages must be done. This calculation procedure is described in Appendix I.

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Figure 4.26: Nomograph for the determination of the asphalt mix stiffness.

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Permanent deformation The asphalt mix should have a resistance against permanent deformation as high as possible. To this end the asphalt mix: a. should have such a grain size distribution that the shear resistance of the

aggregate skeleton is as high as possible; in general this calls for a dense packing,

b. should contain mineral aggregates which such a shape that a friction resistance as high as possible is obtained,

c. should have a volume percentage of bitumen as small as possible, d. should contain a type of bitumen that is rather unsusceptible for variations

in temperature and loading time. In conclusion it can be stated that in general an asphalt mix with a high elastic stiffness also exhibits a good resistance against permanent deformation. This conclusion is valid for the conventional, continuously graded asphalt mixes. A mix such as porous asphalt concrete (‘zoab’) has a good resistance against rutting despite the fact that it has a very high percentage of air. Also the stone mastic asphalt mix has a good resistance against rutting despite the fact that it contains a high amount of bitumen. In these two cases the high resistance against rutting is to be attributed to an optimized aggregate skeleton. The earlier mentioned conventional mixes with high stiffness are in general difficult to handle. Predicting the occurring rutting is very complex because of the great number of influencing factors. For this reason, in contrast to the asphalt mix stiffness modulus, no nomograph is available to predict the resistance against permanent deformation of asphalt mixes. To however obtain some insight in the way permanent deformation could be calculated, hereafter the model developed by Shell is discussed. In Shell’s rutting model it is assumed that the permanent deformation of asphalt mixes is determined by the viscous deformation of the mix. It already has been discussed earlier how the total permanent deformation is divided into a viscous, a delayed elastic and an elastic part. For a constant wheel loading and a constant temperature the viscous stiffness of bitumen can be calculated with the equation:

Sbit,visc = 3 / (N t/η)

where: Sbit,visc = viscous stiffness of the bitumen [Pa] N = number of load repetitions t = loading time of a wheel passage [s] η = viscosity [Pa.s] The viscosity η depends on the temperature, the loading time and the type of bitumen. Values for η can be obtained through viscosity measurements, they can however also be read in figure 4.27.

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Figure 4.27: Graph for the determination of the viscosity of bitumen.

Next the viscous stiffness of the asphalt mix Smix,visc has to be determined from Sbit,visc, preferably by performing tests such as the static creep test or the uni-axial compressive test with repeated loadings. By doing so one gets results such as presented in figure 4.28. If no test results are available this figure gives an indication of the viscous mix stiffness.

Figure 4.28: Determination of Smix,visc from Sbit,visc.

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The permanent deformation is then calculated by means of the equation:

∆h = C * h * σz,av / Smix,visc

where: ∆h = viscous deformation of the asphalt layer [mm], C = factor to take into account the dynamic effect of the wheel load; 1 ≤ C ≤ 2,

h = thickness of the asphalt layer [mm], σz,av = average vertical stress in the asphalt layer [MPa], Smix,visc = viscous stiffness of the asphalt mix [MPa]. As already stated, the temperature has a huge effect on the permanent deformation in an asphalt mix. Therefore figure 4.29 gives a relationship between the air temperature and the asphalt temperature. The term MMAT means “Mean Monthly Air Temperature”; in this course it is not further explained how to calculate MMAT.

Figure 4.29: Relationship between the average air temperature MMAT and

the asphalt temperature.

The above-discussed Shell method will be further explained by means of an example. Suppose that one wants to predict the rutting in a 150 mm thick asphalt layer that is placed on a sand sub-base with an elastic modulus Esand = 100 MPa. The rutting has to be calculated for a number of load repetitions N = 1000000. The vehicle speed is rather low, i.e. 36 km/h (10 m/s). The traffic loadings consist of heavy truck wheel loads of 75 kN with a tyre pressure p =

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1.05 MPa. It follows from these wheel load data that the radius of the contact area amounts 150 mm, so the diameter of the contact area is 300 mm. From the vehicle speed and the diameter of the contact area can easily be calculated that the loading time t of a wheel passage is 0.03 s. The total loading time N * t thus amounts 0.03 * 1000000 = 30000 s. The air temperature MMAT = 35°C and the temperature ring & ball of the bitumen Tr&k = 55°C; the Penetration Index of the bitumen PI = 0. With some extrapolation it follows from figure 4.29 that the asphalt temperature is about 45°C. This leads to T–Tr&k = –10°C; this value together with the PI input in figure 4.27 results in viscosity η = 1.7*104 Pa.s; for the viscous stiffness of the bitumen is then found Sbit,visc = 1.7 Pa. With figure 4.28 it is finally found that the viscous stiffness of gravel asphalt concrete Smix,visc = 10 MPa. Chapter 7 contains graphs to determine the vertical stresses in a two-layer system. From these graphs it follows that at the bottom of the asphalt layer the vertical stress σz = 0.8 p, where p is the contact pressure. At the top of the asphalt layer σz is equal to the contact pressure. This means that σz,av = 0.9 p = 0.945 MPa. Assuming that the dynamic factor C = 1.5 the rutting is calculated with: ∆h = 1.5 * 150 * 0.945/10 = 21 mm Some remarks must be made with respect to the application of the Shell method: a. Only viscous deformation is taken into account while in reality there is also

plastic deformation of the aggregate skeleton, b. The beneficial effect of horizontal confinement stresses on the

development of permanent deformation in the asphalt mix is not taken into account,

c. The method has originally been developed around the static creep test while in reality the traffic loading is dynamic by nature. The relationship between Sbit,visc and Smix,visc as determined with repeated load tests is different from the relationship based upon a static creep test. This means that the method in fact is only suited to investigate the effect of different types of bitumen on the permanent deformation behavior of the one and same asphalt mix.

4.11.2 Fatigue: Fatigue means the development of cracks due to repeated loadings. It may be clear that the resistance against fatigue is positively affected if the bitumen content increases, the percentage of air decreases and the bond between the aggregates and the bitumen mortar improves. In the case of a very strong mortar and a very good bond also the strength of the gravel or crushed stone becomes important. Obviously weak gravel or crushed stone has a negative effect on the fatigue resistance. The determination of the fatigue characteristics requires expensive and time-consuming fatigue tests. Figure 4.30 shows an example of a fatigue test set-up.

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Figure 4.30: The four-point bending fatigue test set-up.

In a four-point bending test a beam is subjected to repeated bending. In principle such a test can be done in two ways. The first way is to repeatedly apply a constant force on the beam, and in this case the deflection (vertical displacement) of the beam slowly but consistently increases until failure occurs. This type of test is called the ‘constant force’ or ‘constant stress’ fatigue test. The second way is to repeatedly subject the beam to a constant deflection, and in this case the force required to obtain the set deflection level is slowly but consistently decreasing. This type of test is referred to as ‘constant displacement’ or ‘constant strain’ fatigue test. The fact that in the first case the deflection increases and that in the second case the force decreases directly follows from basic applied mechanics, as the equation for the deflection ∆ of the center of a beam is:

∆ = f( F/E*h3)

where F denotes the applied force, E the elastic modulus of the beam and h the height of the beam. The effective h decreases because of the development of cracks and that affects the displacement or force respectively. Shell has developed a nomograph that enables the easy determination of the fatigue characteristics as a function of the composition of the asphalt mix (figure 4.31). For the structural design of asphalt pavements normally the “constant strain” fatigue relationship is used. Figure 4.31 learns that the fatigue resistance of the asphalt mix depends on the volume percentage of bitumen, the stiffness modulus of the asphalt mix and the type of bitumen (PI).

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So also with respect to the fatigue characteristics the volumetric composition of the asphalt mix is relevant.

Figure 4.31: Nomograph for the determination of the fatigue resistance of

asphalt mixes.

It already has been argued that fatigue relationships resulting from laboratory testing cannot directly be applied in practice. The beneficial effects of lateral wander and healing should also be taken into account.

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Lateral wander means that not all the heavy vehicles drive in exactly the same track. This implies that the number of load repetitions at a certain point is not equal but smaller than the number of axle load repetitions on the traffic lane. Furthermore, in contrast to a laboratory fatigue test, in reality there are rest periods between the subsequent wheel loadings. If for instance the diameter of the contact area of the truck tyres amounts 0.2 m, the distance between the front and rear axle is 5 m and the speed of the truck is 80 km/h (22.22 m/s), then the duration of a load pulse is 0.009 s and the duration of the rest period between the front and rear axle 0.225 s. The ratio of the rest and load periods thus is 25. Obviously the rest period between the passage of a truck and the next truck even will be much greater. The duration of the rest period and especially the ratio of the rest and load periods have a very great effect on the behavior of asphalt mixes because the bituminous mortar (bitumen + filler + fine sand) has the capacity to recover, which means that part of the fatigue damage disappears during the rest period. This recovery capacity, also called ‘healing’, depends on the composition of the asphalt mix and the ratio of rest and load periods. A first estimate of the magnitude of the healing factor is obtained from figure 4.32.

Figure 4.32: Graph for the determination of the healing factor H.

The asphalt fatigue life to be used in practice (‘in the field’) then can be determined as follows: Nfield = H x V x Nlab where: Nlab = laboratory asphalt fatigue life according to figure 4.31, H = healing factor according to figure 4.32, V = lateral wander factor; for motorways: H ≈ 2.5, Nfield = asphalt fatigue life to be expected on the road (in the field).

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4.12 Durability: During time bitumen ages and then becomes brittle. The risk of failure within the film of bitumen around the aggregates thus increases, leading to loss of coherence within the asphalt mix. In principle there are two possibilities to make the asphalt mixes resistant against ageing. The first possibility is to reduce the effect of the factors that promote ageing. Ageing of the bitumen is primarily caused by sunlight and especially oxygen while water promotes the loss of coherence within the asphalt mix. Oxygen and water thus should be unable to penetrate within the asphalt mix and this requires an asphalt mix as dense (impermeable) as possible. The percentage of air in the asphalt mix thus must be low. The second possibility is to increase the resistance against ageing of the bitumen. Increasing the thickness of the bitumen film around the aggregates (so increasing the volume percentage of bitumen) is very effective in this respect. Also polymers can be added to the bitumen to limit the susceptibility of the asphalt mix for ageing, but this certainly is not yet common practice. From the discussion above it might be clear that with respect to durability porous asphalt (‘zoab’) in principle is a vulnerable asphalt mix. On the other hand, stone mastic asphalt (‘sma’) is a durable asphalt mix as it contains a high percentage of bitumen and a rather low percentage of air. In summary it can be stated that an asphalt mix is durable if: a. the film of bitumen around the aggregates is thick, b. the percentage of air is low, c. the bond between the bitumen mortar and the aggregates is good. 4.13 Summary: From the proceeding paragraphs it follows that in principle it is impossible to combine a good resistance against rutting and a good resistance against fatigue cracking! Dependent on the location (height) of the asphalt mix within the asphalt pavement structure emphasis is laid either on a higher resistance against rutting or on a higher resistance against fatigue cracking. In addition to that, asphalt wearing courses should possess a good durability and preferably also a good resistance against fatigue cracking. In general higher volume percentages of bitumen are beneficial with respect to fatigue resistance and durability, while on the other hand lower percentages of bitumen yield a better resistance against permanent deformation (rutting). 4.14 Global description of the function of the various asphalt

mixes: It already has been discussed that a distinction is made between asphalt mixes for the wearing course, mixes for the intermediate (‘binder’) asphalt layer and mixes for the lower asphalt layers. The stress conditions in these various asphalt layers are schematically depicted in figure 4.33.

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Figure 4.33: Traffic load stresses in two-layer asphalt pavement structure. It appears from figure 4.33 that the bottom asphalt layer (stone asphalt concrete (stac)) is mainly subjected to fatigue. The intermediate layer (open asphalt concrete (oac)) especially needs a good resistance against permanent deformation. The deviatoric stress (σv - σh) can be quite high in this layer. The wearing course (dense asphalt concrete (dac)) also needs a good resistance against permanent deformation and a high durability. This layer also should deliver sufficient skidding resistance to the vehicles driving over it and for this reason round aggregate with a polished surface is not acceptable in this layer. In summary it can be stated: dac • application of crushed stone aggregate for resistance against rutting and

skidding resistance • low percentage of air for durability • relativ high volume percentage of bitumen for durability oac • application of crushed stone aggregate for resistance against rutting • in comparison to dac a lower volume percentage of bitumen for resistance

against rutting stac • application of crushed gravel or crushed stone aggregate for resistance

against rutting; stac is not applied as a wearing course • relatively low volume percentage of bitumen; for an optimal resistance

against fatigue a higher bitumen content could be considered, however that leads to a greater susceptibility for rutting.

It can be concluded that the composition of dac and oac seems logical considering the function of these asphalt mixes in the asphalt pavement structure. Related to its function to some extent the composition of stac seems not logical. Compared to dac the fatigue resistance of stac is less. Therefore it is investigated whether in the bottom asphalt layer of heavily loaded roads a dac-type of mix can be applied.

asphalt sand

vertical stresses horizontal stresses

--

+

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As already stated, the wearing course materials porous asphalt (zoac) and stone mastic asphalt (sma) are based on a totally different concept. Zoac was first applied on airport pavements, especially to prevent aquaplaning on runways. The very high percentage of air (> 19%) not only results in a good drainage through the zoac layer but also in a substantial decrease of the traffic noise. The concept of zoac is a rather uniformly graded stone mixture (compare the grain size distribution curves of dac and zoac in table 4.1 with each other!) and a limited sand fraction. The bitumen content is relatively low. The composition of zoac is such that the mix has a good resistance against permanent deformation provided that there is enough horizontal confinement. Marshall tests, indirect tensile tests and three- or four-point bending tests don’t make much sense on this material as the obtained low values don’t reflect the good behavior observed in practice. The mix is however susceptible for ageing because of its open structure and its low bitumen content. The concept of sma is again totally different. This asphalt mix combines a good durability with a high resistance against permanent deformation and that is not an obvious combination. The high bitumen content is possible through the addition of matters (such as cellulose fibers) that limit the bitumen to run-off from the aggregates. The good resistance against permanent deformation has to be attributed to an optimized grain size distribution. The high bitumen content also leads to a relatively high resistance against fatigue. 4.15 Asphalt mix design: In this course only limited attention is paid to the design of asphalt mixes. Some basic knowledge must however be present and therefore some basic principles of the asphalt mix design will be discussed. The asphalt mixes obviously have to fulfill the requirements given in the tables 4.1 and 4.2. It is common practice to perform a Marshall pre-investigation on for instance a number of different asphalt mixes that contain a different type of aggregate (such as porphyry, moraine, grauwacke or crushed gravel (‘Dutch stone’)) and a different bitumen content. It is then determined to what extent the different mixes fulfill the requirements with respect to the Marshall properties, the percentage of air and the degree of filling. An example of such an investigation is shown in figure 4.34 (4); on the basis of these results an appropriate mix composition then can be chosen. One has however to realize that in this way one does not gain any insight into the mechanical properties required for the structural design of the asphalt pavement such as stiffness and fatigue resistance. Although these properties can be estimated by means of the nomographs included in this chapter, the mix is thus not designed with the aim to optimize these mechanical properties. It is however to be expected that in the near future the so-called functional mix design will be introduced. Such an asphalt mix design procedure aims to maximize the stiffness modulus, the resistance against fatigue, the resistance against rutting, the durability and the handling of the mix during construction. This functional asphalt mix design procedure will replace the current empirical Marshall asphalt mix design procedure.

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Figure 4.34: Example of results from a Marshall asphalt mix design pre-investigation.

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4.16 Concrete and other cement-bound materials: As stated earlier cement is frequently applied in road construction to stabilize materials. Well-known cement-stabilized base materials are sand-cement, lean concrete and cement-bound asphalt granulate. The mix design of such cement-bound materials always aims to obtain a certain compressive strength. For instance, cylinders cored out of a sand-cement base should have a compressive strength of at least 1.5 MPa at 28 days after construction. The required strength of e.g. lean concrete is always determined by the strength that is required for the structural design of the pavement. Nearly always this is the flexural tensile strength; this value is hardly ever directly measured but deducted from the compressive strength by means of available relationships between the flexural tensile strength and the compressive strength. In The Netherlands concrete roads are constructed only at a small scale, but concrete is widely applied on airport platforms because of the very heavy and static aircraft wheel loadings. Also in this case the design criterion is the flexural tensile strength while the constructed material is checked for its compressive strength. The design of cement-bound materials is not discussed here. Reference is made to courses on material science and concrete technology. The requirements with respect to the mix composition of cement-bound materials are included in (2). 4.17 References: 1. Various properties of natural sands for Netherlands highway engineering

Record 4; S.C.W.; Arnhem – 1978 (since 1985 S.C.W. is part of CROW, Ede)

2. Specifications for Road Construction 2000 (‘Standaard RAW

Bepalingen 2000’) (in Dutch) CROW; Ede - 2000

3. Shell bitumen handbook

Shell Bitumen U.K.; Chertsey – 1990 4. VBW-Asfalt

Guidelines for pre-investigation of asphalt mixes (in Dutch) Breukelen – 1985

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APPENDIX I

Calculation of the Volumetric Composition of an Asphalt Mix from the Composition by Mass

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159-a