Análisis

In assessing the grading of a material, the first step is to determine particle size. This is established by sieving a soil sample through standard sieves or by using sedimentation methods and an hydrometer if the soil particles are too small to sieve. The coefficient of

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variation is usually below 5 percent and test results should be within 5 percentage points of the specification (Jameson, 1984).

Standard sieves are usually defined by their aperture size, which is the side dimension of the square space between each pair of equally-spaced precisely-woven wires. Common sieve sizes are based on the British (BS 410), U.S. (ASTM D422), or Australian (AS 1152) standards with openings typically ranging from 22 µm to 125 mm, over a series of some 60 or so sizes. The series is commonly geometric, providing an approximately constant ratio between successive sizes.

The test results from sieving are commonly plotted as the percentage of the soil sample passing a given size of sieve (related to the mass of material smaller than that diameter). This sieve size is the maximum particle diameter in millimetres and is plotted on a log scale to provide a particle-size distribution (or grain-size distribution or grading) curve. Typical curves are shown in Figure 8.5. Particle size distribution is often represented by the exponent, n, in the expression:

(% passing sieve size d)/(% passing sieve size D) = (d/D)n _(8.3)

which is also known as Talbot’s grading curve. N is usually found to be between 0.3 and 0.5. percent passing 100 50 0 0.0001 0.001 0.01 0.1 1 10 100 particle size, mm logarithm of (particle size in mm) clay silt uniformly graded sand well-graded sand basecourse 0 1 2 -1 -2 -3 -4

Figure 8.5 Typical particle-size distribution curves. Some of the terms are defined in Section 8.3.2.

Gradation is a term used to describe the particle size distribution of a soil. The region into which the data for a new material must fit when plotted on a particle size distribution curve is called a grading envelope. In a soil context, the word grain is often used as synonymous with ‘particle’. A further definition of grain for rock is given in Section 8.4.1.

In the sedimentation/hydrometer test, the rate at which the particles settle gives an indication of their respective sizes. Stokes’ Law is used to calculate the size of particle

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that has settled a particular distance in a particular time and the hydrometer measures the density of the water at these various times in order to determine the amount of material in that size range.

Roadmaking material is often referred to by its particle size as determined by sieving and the common terminology is shown in Table 8.2. The proportion of smaller sized particles can have major influences on performance, as the discussion on clays in Sections 8.3.3 & Section 8.4.2 will show. Whereas the properties of coarse-grained soils are largely influenced by their grading and particle shape (Section 8.3.2), and indirectly by gravity, the fine-grained soils are more influenced by their surface properties and, indirectly, by the electrical forces generated by charges on those surfaces (Section 8.4.2). In addition, the presence of a myriad of small interconnected voids, rather than a few large disconnected ones, will allow positive pore pressure to develop more readily and thus reduce shear strength (Section 9.2.3).

Table 8.2 Particle size terminology.

Term Definition Coarse fraction: Retained on a 4.75 mm sieve Fine fraction: Passing a 4.75 mm sieve Coarse aggregate: Retained on a 2.36 mm sieve

Fine aggregate: Passing a 2.36 mm sieve, retained on a 425 µm sieve

Note: some specifications have different sizes for the coarse and fine aggregate boundary.

Binder: Passing a 425 µm sieve

Note: material is called a binder when it forms a soil mortar that embeds the coarse aggregate and prevents its

movement. Thus it should be cohesive, impermeable, non- swelling, and – as a test – able to be formed into a shape with the hand and to retain that shape upon drying. Filler: Passing a 75 µm sieve (see Section 12.2.2)

Fillers are thus fine particles. They were defined in the context of Section 8.1 as < 60 µm, rather than < 75 µm.

8.3.2 Grading types

Classification of particulate material by grading is important in pavement engineering as the value of many relevant properties such as internal friction (Section 8.4.3), voids content, wear resistance, and permeability depend on the distribution of particle sizes. In addition, pavement materials can be in either of density categories (1) or (2) in Section 8.2.3, depending on their grading and the characteristics of their finer particles.

A well-graded (or dense-graded) particle size distribution is one which will permit each particle to fit into the voids created by inter-particle contact of the larger sizes, thus producing close-packing and maximum mix density. Grading changes can thus directly change the density of the placed material.

Mixes which are not well-graded have either an excess of one size in an otherwise well-graded mix, or are gap-graded. A gap-(or skip-)graded mix has at least one size range of particles missing. One form of gap-graded mix is the uniformly-graded mix that has a preponderance of particles of a single size (Figure 8.5). Another gap-graded mix is the open-graded mix that lacks particles of one intermediate size. Although the term

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open-graded conventionally also excludes particles below 1 mm in size, some fine particles may be needed to make handling practical. Open-gradings are not frost- susceptible and have advantages for surfacings where noise reduction and permeability are required (Section 12.2.2). Gap-graded mixes — containing mainly coarse particles — will be below their potential strength, permeable, and prone to raveling (Section 12.1.2).

Mixes that are not well-graded are limited in their compaction potential as it is impossible to produce a close-packed geometric arrangement. During construction, such mixes will be difficult to work, shape, and compact and are often described as harsh. The finished layer will therefore be permeable and prone to excessively high interparticle- stresses, leading to crushing. Densities will be below that of the well-graded mix. As some of the sizes needed for close-packing will inevitably be missing, gaps and voids will always exist within the pavement layer.

If the deviation from a well-graded mix is caused by an excess of fine material, this will diminish the potential for direct contact and mechanical interlock between the larger particles (Sections 11.1.1&2), and hence reduce the strength and stiffness of any placed course. Such mixes will also be of low density due to a lack of close packing at a microscale and will have a surface that will be slippery when wet and dusty when dry. Section 8.4.2 shows that these mixes can also be very sensitive to the presence of water — becoming weak and unstable — particularly when the fine material is composed of clay particles. As little as 3 mm of rain can render unsurfaced clay roads impassable.

In terms of Equation 8.3, values of n between 0.45 to 0.50 indicate well-graded mixes. Open-graded mixes have a low n and mixes with an excessive proportion of fine material have a high n. A grading with n = 0.5 is known as Fuller’s maximum density curve and stems from the 1907 work of Fuller and Thompson. It is a useful approximation to the maximum density (i.e. minimum porosity) grading for a large range of materials although other workers have advocated that exponents as low as 0.4 must be used to produce a maximum density, and 0.6 is often used in asphalt (Section 12.2.3).

As discussed in Section 8.2, strength and stiffness will decrease as density decreases. Thus lower density mixes, such as open-graded and macadam (Section 11.1.2) mixes, are less stiff than well-graded mixes and are often termed unstable (thus perpetuating the incorrect use of stability for ‘stiffness’ which pervades much pavement technology (Section 10.1.1).

In order to achieve a desired distribution of particle sizes, it may be necessary to mix various particle fractions together. The crushing process (Section 8.5.9) usually produces some fine material automatically and this crusher-run mix will usually be the cheapest of the available gradings. Nevertheless, it is quite common to supplement the crushed rock with fine material from earlier crushings, or from other rock sources, in order to meet particular grading requirements. Fine crushed rock is a term used to describe many such (usually well-graded) mixtures. In addition, some breakdown of particle size will occur during compaction (Section 8.2.1).

The coefficient of uniformity of a grading is defined as d60/d30 where dn is the sieve

dimension at which n percent by mass of the soil is passed during sieving. A coefficient of uniformity of about 5 is the border between good and poor grading from the viewpoint of a well-graded mix. However, a gap-graded mix can have a deceptively high coefficient of uniformity. The d30 diameter fraction has a major influence on permeability (Section

9.2.2) and cohesion (Section 8.2.3) and is known as the effective size of the soil. Nevertheless, some definitions of the coefficient of uniformity are based on d60/d10. The

Pavement Materials 111 Segregation is a problem that occurs with a uniform mix when various particle sizes segregate together during the handling process, thus destroying the intent of the grading selection.

Some soil components such as mica (Section 8.4.2) meet nominal grading requirements but, because of their flat, plate-like (lamella) shape, interfere with the compaction process and achieve little interlock (Section 11.1.3). They thus deleteriously effect the in situ properties achieved. Angular, cubical shapes are to be preferred and some control on particle shape as well as on particle size is therefore necessary. This is provided by test a in Section 8.6.

8.3.3 The Atterberg Limits

Section 8.1 mentioned that pavement materials consisted of three phases: gaseous, liquid, and solid. As water is added to a particulate material, an increase in water pressure was seen to diminish both cohesion and interparticle interlock (Section 8.2.3). Eventually the mixture practically becomes a liquid with the fine material particles in suspension and no effective interlock. It can then flow under its own weight. The moisture content at which this extreme condition occurs is known as the liquid limit, LL (percent). From the reverse viewpoint, the LL can also be defined by removing water from the liquid mix. The LL is then the moisture content at which the mix changes form a liquid to a plastic condition. In this condition a mixture can be permanently deformed under load without losing its strength; e.g. it can be moulded and rolled into threads. The effect can be easily observed in clays which can readily be made plastic by moistening and kneading.

Note that plastic describes that property of a material that allows it to deform in a ductile, non-brittle, and permanent fashion under steady load. Plasticity is formally defined in Section 33.3.3. Commonly, soils are made more plastic by adding clay and less plastic by adding sand. A soil exhibits dilatancy when hand-shaking a sample horizontally brings water to the surface and when the water recedes again after the sample is pressed with the fingers.

The LL is determined by a standard test which has a coefficient of variation of about 6 percent (Jameson, 1984). It involves making a groove in a sample and observing whether the groove closes when the base of the sample container is struck 25 times. A maximum LL value of 25–35 is a typical specification limit to control the presence of particles that would deleteriously affect compaction and cohesion (e.g. see items 2 & 10 in Table 8.3). The test is only applied to fine particles likely to be suspended in water. Its application to layers of stones is thus almost irrelevant.

The moisture content at which a material becomes too dry and ‘solid’ to be plastic is called its plastic limit and is determined by a standard rolling test. It has a coefficient of variation of about 10 percent (Jameson, 1984). In many fine-grained soils the plastic limit is a little below OMC. Soils at the plastic limit are typically about a hundred times stronger than when near the wetter liquid limit.

For very fine-grained soils, the plastic limit can provide a more appropriate moisture content than OMC for reference purposes. In this case, a moisture ratio is defined as:

(moisture ratio) = (moisture content)/[(plastic limit)(proportion finer than 425 µm)] A moisture ratio of one represents an upper field limit for well-compacted, well-drained soils (Wallace, 1981).

The further removal of water will cause volume changes within the soil until a moisture content — called the shrinkage limit (or linear shrinkage) — is reached, below

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which no volume changes occur with the further removal of water. It has a coefficient of variation of about 15 percent. The shrinkage product is the product of the shrinkage limit and the proportion of the material finer than 425 µm (Section 8.3.1). A low shrinkage product indicates a material that may ravel and corrugate if used on unsurfaced roads, whereas a high product indicates material that could be slippery in service.

These three limits — shrinkage, plastic and liquid — are called Atterberg (or Consistency) Limits. They must be used with caution when the particles involved are porous. In many ways the limits define the clayiness of the fine fraction of a soil.

The plasticity index (PI) — also referred to as Ip, which is the international symbol

— is the difference between the plastic and liquid limits. It is thus a measure of the range of moisture contents over which the soil will remain plastic and derives mainly from the surface activity of the clay component of the soil. A PI above ten usually indicates a clayey soil; the higher the PI, the ‘heavier’ the clay (Section 8.4.2). For many clays:

PI = 0.7(LL − 20)

However, silts (Section 8.4.1) have low PIs, passing rapidly into the liquid stage once they have reached the plastic limit.

The plasticity index is basically a control on the moisture susceptibility of the material, with a high value indicating susceptibility. Empirical evidence indicates that materials with low plasticity indices make the best subgrades and basecourses and a maximum value of six is a typical specification limit for high quality material.

However, in an examination on Australian pavement materials, Jewell (1969) found that only six of his 20 samples met the Atterberg specification limits but 15 of the 20 were performing satisfactorily in pavements with an impervious surface. Indeed, only one of the 20 complied strictly with all the grading and Atterberg limits of a typical specification. In a detailed study of a further 102 arid Australian road sites, Brodie (1970) found no correlation between plasticity index and pavement performance.

Although the index is widely used, in addition to the above caution, Ingles and Noble (1975) note that it can also be one of the least useful of tests because of both its high variability, poor reproducibility and relatively high cost and time to complete. Although the within-sample coefficient of variation is about 20 percent (Jameson, 1984), Ingles and Noble suggest that it has a field coefficient of variation of about 75 percent. It is better to try to understand the material, than to rely on PI values.

8.4 SOILS