Gradation and Maximum size of Aggregate

Gradation describes the particle size distribution of the aggregate. The particle size distribution is an important attribute of the aggregates. Large aggregates are economically advantageous in portland cement and asphalt concrete, as they have less surface area and, therefore, require less binder. However, large aggregate mixes, whether asphalt or portland cement concrete, are harsher and more difficult to work into place. Hence, construction considerations, such as equipment capability, dimensions of construction members, clearance between reinforcing steel, and layer thickness, limit the maximum aggregate size.

Two definitions are used to describe the maximum particle size in an aggregate blend:Maximum aggregate size—the smallest sieve size through which 100% of the aggregates sample particles pass.Nominal maximum aggregate size—the largest sieve that retains any of the aggregate particles, but generally not more than 10%.

Some agencies define the maximum aggregate size as two sizes larger than the first sieve to retain more than 10% of the material, while the nominal maximum size is one size larger than the first sieve to retain more than10% of the material (The Asphalt Institute 1995; McGennis et al. 1995).

Sieve AnalysisGradation is evaluated by passing the aggregates through a series of sieves The sieve

retains particles larger than the opening, while smaller ones pass through. Metric sieve descriptions are based on the size of the openings measured in millimeters. Sieves smaller than 0.6 mm can be described in either millimeters or micrometers. Gradation results are described by the cumulative percentage of aggregates that either pass through or are retained by a specific sieve size. Percentages are reported to the nearest whole number, except that if the percentage passing the 0.075-mm (No. 200) sieve is less than 10%, it is reported to the nearest 0.1%.Gradation analysis results are generally plotted on a semilog chart.

Maximum Density Gradation The density of an aggregate mix is a function of the size distribution of the aggregates. In 1907 Fuller established the relationship for determining the distribution of aggregates that provides the maximum density or minimum amount of voids as

The value of the exponent n recommended by Fuller is 0.5. In the 1960s, the Federal Highway Administration recommended a value of 0.45 for n and introduced the “0.45 power” gradation chart, Figures 5.12 andA.22, designed to produce a straight line for maximum density gradations (Federal Highway Administration 1988).

Table 5.2 presents a sample calculation of the particle size distribution required for maximum density. Note that the gradation in Table 5.2 is plotted on both gradation charts in Figures 5.11 and 5.12. Frequently, a dense gradation, but not necessarily the maximum possible density, is desired in many construction applications, because of its highstability. Using a high-density gradation also means the aggregates occupy most of the volume of the material, limiting the binder content and thus reducing the cost. For example, aggregates for asphalt concrete must be dense,but must also have sufficient voids in the mineral aggregate to provide room for the binder, plus room for voids in the mixture.

Other Types of Gradation In addition to maximum density (i.e., well-graded), aggregates can have other characteristic distributions, as shown in Figure 5.13. A one-sized distribution has the majority of aggregates passing one sieve and being retained on the next smaller sieve. Hence, the majority of the aggregates have essentially the same diameter; their gradation curve is nearly vertical. One-sized graded aggregates will have good permeability, but poor stability, and are used in such applications as chip seals of pavements. Gapgraded aggregates are missing one or more sizes of material. Their gradation curve has a near horizontal section indicating that nearly the same portions of the aggregates pass two different sieve sizes. Open-graded aggregates are missing small aggregate sizes that would block the voids between the larger aggregate. Since there are a lot of voids, the material will be highly permeable, but may not have good stability.

Fig 5.13 Types of aggregate grain size distributions plotted on a 0.45 gradation chart

As shown in Table 5.3, the amount of fines has a major effect on the characteristics of aggregate base materials. Aggregates with the percentage of fines equal to the amount required for maximum density have excellent stabilityand density, but may have a problem with permeability, frost susceptibility, handling, and cohesion.

Gradation Specifications Gradation specifications define maximum and minimum cumulative percentages of material passing each sieve. Aggregates are commonly described as being either coarse or fine, depending on whether the material is predominantly retained on or passes through a 4.75mm (No. 4) sieve. Portland cement concrete requires separate specifications for coarse and fine aggregates. The IRC specifications for fine and coarse aggregates for dry lean concrete and lean bituminous macadam is shown in the table 1 and 2 respectively.

Table 1. Table 2.

Sieve size (Designation)

Percent by weight passing the sieve

(%)

45 mm 10026.5 mm 75-10022.4 mm 60-9511.2 mm 30-555.6 mm 15-352.36 mm 4-19

Sieve size (Designation)

Percent by weight passing the sieve

(%)31.5 mm 10026.5 mm 90-9519 mm 80-909.5 mm 55-754.75 mm 35-60

600 μ 10-3575 μ 0-8

75 μ 0-5

Fineness Modulus The fineness modulus is a measure of the fine aggregates’ gradation and is used primarily for portland cement concrete mix design. It can also be used as a daily quality control check in the production of concrete. When the fineness modulus is determined for fine aggregates, sieves larger than 9.5 mm (3/8 in.) are not used. The fineness modulus should be in the range of 2.3 to 3.1, with a higher number being a coarser aggregate.

Blending Aggregates to Meet Specifications:Generally, a single aggregate source is unlikely to meet gradation requirements for portland cement or asphalt concrete mixes. Thus, blending of aggregates from two or more sources would be required to satisfy the specifications. Figure 5.14 shows a graphical method for selecting the combination of two aggregates to meet a specification. Table 5.8 presents the data used for Figure 5.14. Determining a satisfactory aggregate blend with the graphical method entails the following steps (The Asphalt Institute 1995):1. Plot the percentages passing through each sieve on the right axis for aggregate A and on the left axis for aggregate B, shown as open circles in Figure 5.14.2. For each sieve size, connect the left and right axes.3. Plot the specification limits of each sieve on the corresponding sieve lines; that is, a mark is placed on the 9.5-mm (3/8 in.) sieve line corresponding to 70% and 90% on the vertical axis, shown as closed circles in Figure 5.14.4. Connect the upper- and lower-limit points on each sieve line. 5. Draw vertical lines through the rightmost point of the upper-limit line and the leftmost point of the lower-limit line. If the upper- and lower-limit lines overlap, no combination of the aggregates will meet specifications.6. Any vertical line drawn between these two vertical lines identifies an aggregate blend that will meet the specification. The intersection with the upper axis defines the percentage of aggregate B required for the blend. The projection to the lower axis defines the percentage of aggregate A required.7. Projecting intersections of the blend line and the sieve lines horizontally gives an estimate of the gradation of the blended aggregate. Figure 5.14 shows that a 50-50 blend of aggregates A and B will result in a blend with 43% passing through the 2.36-mm (No. 8) sieve. The gradation of the blend is shown in the last line of Table 5.8.

When more than two aggregates are required, the graphical procedure can be repeated in an iterative manner. However, a trial and error process is generally used to determine the proportions. The basic equation for blending is

Pi = Aa + Bb + Cc +…(5.16)

where, Pi = blend material passing sieve size i A, B, C, …= percent of aggregates A, B, C, passing sieve i a, b, c, … = decimal fractions by weight of aggregates A, B, and C used in the blend, where the total is 1.00

Table 5.9 demonstrates these calculations for two aggregate sources. The table shows the required specification range and the desired (or target) gradation, usually the midpoint of the specification. A trial percentage of each aggregate source is assumed and is multiplied by the percentage passing each sieve. These gradations are added to get the composite percentage passing each sieve for the blend. The gradation of the blend is compared to the specification range to determine if the blend is acceptable. With practice, blends of four aggregates can readily be resolved.

Properties of Blended Aggregates When two or more aggregates from different sources are blended, some of the properties of the blend can be calculated from the properties of the individual components. With the exception of specific gravity and density, the properties of the blend are the simple weighted averages of the properties of the components. This relationship can be expressed as

X = P1X1 + P2X2 + P3X3 +….

X = composite property of the blendX1, X2, X3 = properties of fractions 1, 2, 3P1, P2, P3 = decimal fractions by weight of aggregates 1, 2, 3 used in the blend, where the total is 1.00This equation applies to properties such as angularity, absorption, strength, and modulus.

Destructive distillation is the chemical process involving the decomposition of feedstock by heating to a high temperature; the term generally applies to processing of organic material in the absence of air or in the presence of limited amounts of oxygen or other reagents, catalysts, or solvents, such as steam or phenols. It is an application of pyrolysis. The process breaks up or 'cracks' large molecules. Coke, coal gas, gas carbon, coal tar, ammonia liquor, and "coal oil" historically, are examples of commercial products of the destructive distillation of coal.

Destructive distillation of any particular inorganic feedstock produces only a small range of products as a rule, but destructive distillation of organic materials commonly produces many compounds, often hundreds, though not all chemical products of any particular process are of commercial importance. The molecules distilled off generally are smaller and more volatile than the feedstock molecules, but some reactions polymerise or condense small molecules into larger molecules, including heat-stable tarry substances and chars. Cracking into liquid and volatile compounds, and polymerisation or the formation of chars and solids may all occur in the same process, and any class of the products might be of commercial interest.

Currently the major industrial application of destructive distillation is to coal.[1][2]

Historically the process of destructive distillation and other forms of pyrolysis led to the discovery of many chemical compounds or elucidation of their structures before contemporary organic chemists had developed the processes to synthesise or specifically investigate the parent molecules. It was especially in early days that investigation of the products of destructive distillation, like those of other destructive processes, played parts in enabling chemists to deduce the chemical nature of many natural materials.[3] Well known examples include the deduction of the structures of pyranoses and furanoses.[4]

Bitumen emulsions are usually made using a colloid mill, although other dispersion devices are possible. In the colloid mill energy is applied to the system by passing the mixture of hot bitumen and water phase between a rotating disc, cone or flywheel and a stator. The rotor as well as stator may be grooved or have teeth in order to create a turbulent flow.

Bitumen emulsion can be produced either in a batch or an in-line process plant. The batch process involves at least two process stepswater phase (soap) preparation and the actual emulsion production. The water phase is prepared in a tank into which heated water, emulsifier and other emulsion chemicals are metered and the solution properly mixed. In the emulsion production process the bitumen and the pre-made water phase are dosed to the colloid mill. If solvent is to be added to the bitumen, then a batch tank is needed for bitumen as well, or the solvent must be dosed in-line.

In the batch plant the emulsion production itself involves only a few material flows, which allows manual process control. However, proper metering of the various components are decisive for the quality of the emulsion and automatic or semi-automatic control will make the manufacturing more efficient and reduce human error. Furthermore, the chemicals used may be hazardous as well as corrosive, which means closed dosage systems rather than open tanks and portable pumps are preferable in order to ensure safe work and environmental conditions. In the in-line process the water heating and all material dosage are done continuously using individual dosage pumps for each material. No batch tanks are used. Instead, the water phase system must

further be designed to provide sufficient reaction time for the chemicals so that adequate neutralization and solution take place before the water phase meets the bitumen. The process needs to be automatically controlled using flow meters for all material dosageexcept acid, which should be controlled by the pH in the water phase.

Various special additives such as latex, SBS or bitumen dope may be used and will then require special components and technical solutions. Latex for example is shear sensitive and may coagulate in pumps and lines. SBS modified bitumens usually require the emulsion to be produced above the boiling point of water, which requires production under pressure and cooling before release to atmospheric pressure in the storage tank.

MECHANISMS OF STRIPPING

A general definition of stripping is "the breaking of the adhesive bond between the aggregate surface and the asphalt cement" in an asphaltic pavement or mixture. Stripping is a complex problem dependent on many variables, including the type and use of a mix, asphalt characteristics, aggregate characteristics, environment, traffic, construction practice, and the use of anti-strip additives. However, the presence of moisture at the aggregate/asphalt interface is a common factor to all stripping related problem.

A bituminous mixture derives its strength from the cohesional resistance of the binder and grain interlock and frictional resistance of the aggregate. The cohesional resistance is only fully available if a good bond exists between the binder and the aggregate. If a good bond exists, failure of the mixture occurs within the binder. If the bond is poor, the failure may occur at the binder-aggregate interface and may result in premature failure of the mix.

DetachmentDetachment is the separation of an asphalt film from an aggregate surface by a thin layer of water, with no obvious break in the asphalt film (1,3). Where strippingby detachment has occurred, the asphalt film can be peeled cleanly from the aggregate, indicating a complete loss of adhesion (1). The theory of interracial energy provides the rationale for explaining the detachment mechanism. This widely accepted theory considers adhesion as a thermodynamic phenomenon related to the surface energies of the materials involved, namely, asphalt and mineral aggregates. The surface tension of water is much lower than that of asphalt. The wettability of an aggregate increases as the surface tension (or free surface energy) of the adhesive decreases (3). Thus, if a three-phase interface consisting of aggregate, asphalt, and water exists, water reduces the free surface energy of the system more than does asphalt to form a thermodynamically stable condition of minimum surface energy (3). The theory of interfacial energy emphasizes the effect of polarity of the molecules present at the surface of the two phases. Most aggregates have electrically charged surfaces. Asphalt, which is composed chiefly of high molecular weight hydrocarbons, exhibits little polar activity; therefore, the bond that develops between asphalt and an aggregate is primarily due to relatively weak dispersion forces (5). Water molecules, on the other hand, are highly polar and are attracted to aggregates by much stronger orientation forces (5).

DisplacementStripping by displacement results from the penetration of water to the aggregate surface through a break in the asphalt film (1,3,6,7). The break can be caused by incomplete coating of the

aggregate initially or by film rupture (1,3,5,6). Because the asphalt film at these locations is generally thinner and under tension, rupture of the asphalt film is probably at the sharp edges and corners of angular aggregate pieces as a result of traffic loading. Stripping by displacement can result from pinholes in the asphalt film, which can form soon after coating a dusty aggregate (6). The concept of stripping by displacement is congruent with the thermodynamic approach to adhesion; that is, water will displace asphalt from an aggregate surface when the three-phase interface exists. The chemical reaction theory of adhesion can also be used to explain stripping by displacement (7). Changes in the pH of the microscopic water accumulations at the mineral aggregate surface can alter the type of polar groups adsorbed, as well as their state of ionization/dissociation, leading to the build-up of opposing, negatively-charged, electrical double layers on the aggregate and asphalt surfaces (7). The drive to reach equilibrium attracts more water and leads to physical separation of the asphalt from the aggregate (7).

Spontaneous EmulsificationIn spontaneous emulsification, water and asphalt combine to form an inverted emulsion, where asphalt represents the continuous phase and water represents the discontinuous phase. The formation of such an emulsion leads to stripping and is further aggravated by the presence of emulsifiers such as mineral clays and some asphalt additives (1,6,7). Frornm observed that spontaneous emulsification occurs whenever asphalt films are immersed in water but that the rate of emulsion formation depends on the nature of the asphalt and the presence of additives (6). The fact that stripping has been observed to be reversible lends support to the spontaneous emulsification mechanism because evaporation of the water from the emulsion would restore the asphalt to its original condition (6).

Pore PressurePore pressure has been suggested as a mechanism of stripping in high void mixes where water may circulate freely through interconnected voids (1,3). Upon densification of the mix from traffic loading, water may become trapped in impermeable voids that previously permitted water circulation. Further traffic may induce high excess pore pressures in the trapped water causing stripping of the asphalt film from the aggregate (1,3).

Hydraulic ScouringHydraulic scouring is a mechanism of stripping that is applicable only to surface courses. Stripping due to hydraulic scouring results from the action of vehicle tires on a saturated avement surface. This causes water to be pressed down into the pavement in front of the tire and immediately sucked away from the pavement behind the tire. This compressiontension cycle is believed to contribute to the stripping of the asphalt film from the aggregate. In addition to the mechanisms outlined above, which have gained varying degrees of acceptance among investigators of the stripping problem, other potential mechanisms for stripping have been proposed. Osmosis has been suggested as a possible mechanism of stripping, but no laboratory evidence for this mechanism has been found (6). It has been observed that asphalt will creep up an air-water interface, such as an air bubble on the pavement surface, as a result of surface tension (6). If the air-water interface is sufficiently large, this pulling of the asphalt film may result in film rupture or may result in a film that is so thin that spontaneous emulsification is rapid (6). Related to the mechanism of stripping is the initiation and progression of stripping in a typical asphalt pavement. Inspection of field specimens of stripped pavements has revealed that

stripping begins at the bottom of the layer and works its way up, stripping mostly the coarse aggregate (1,6). This behavior is not surprising because the asphalt at the bottom of a pavement layer is in tension upon the applications of load and is often subject to prolonged exposure to moisture from water trapped within a granular base course above the subgrade.

Anti-stripping Additives (Methods of improving adhesion):

The anti-stripping agent is added to prevent stripping in the mix, improving pavement performance by negating the effects of moisture damage on pavement rutting or fatigue. Anti-stripping additives are defined as substances that convert the aggregate surface to one that is more easily wetted with asphalt than water. There are two main types of anti-stripping additives currently available: liquid and hydrated lime. Both additives have been proven effective in field trials and various studies in resisting stripping .

Liquid anti-strip agents are surface active agents, meaning they reduce the surface tensionof asphalt cement, which promotes asphalt adhesion to aggregate. Most liquid anti-stripping additives contain amines as their active ingredient. In addition to improving the mix in regardsto stripping, the liquid anti-strip agents must also be heat stable so that they are able to maintain their effectiveness at high temperatures. The most common method of application in practice is to combine small volumes (0.5% by weight) of anti-stripping agent to the binder. This method isinefficient because not all of the agent reaches the surface of the aggregate. However, it is muchmore economical than the alternative, which involves full coating of the aggregates . The overall performance of the mixture is very dependent on the amount of agent added to the binder. The use of too much additive may be detrimental to the mix, weakening it’s resistance to permanent deformation.

The anti-stripping mechanism of lime additives is not well understood, however many studies have proven its effectiveness as an anti-stripping agent . The effectiveness of lime may be due to the fact that it is directly applied to the aggregate and therefore more of it has a chance to contribute to the stripping resistance of the mixture. Hydrated lime can be applied in one of two ways: either by wet application as a slurry or a completely dry application directly to the aggregates. Various projects have been conducted to evaluate the relative effectiveness of the dry addition process to a variety of wet processes. The results of the studies have been inconclusive; therefore the dry method of addition is preferred based on economic considerations.

Conceptually both of these additives achieve the same objective in reducing the amount of stripping realized in an asphalt mix. However, to be determined successful in a practical sense the reduction of stripping caused by each additive must correlate to improved pavement performance and decreased life cycle cost. In this regard the additives are drastically different. In

general, the long term effectiveness of liquid anti-stripping additives has not been fully established .

The use of polymer modified asphalts in pavements also reduces mixture moisture susceptibility; however, polymers are rarely added to the asphalt binder with the sole intent of improving mixture resistance to moisture damage. Polymer modification improves the performance of asphalt pavement to permanent deformation and fatigue resistance due to increased elasticity of the asphalt binder, allowing for faster recovery between loading cycles. Another effect of polymer modification is increased asphalt binder viscosity. The higher viscosity promotes better adhesion of asphalt to aggregate, thus improving resistance to failure in a pavement due to loss of asphalt aggregate adhesion caused by moisture damage.

Most of the road stones have surfaces that are negatively charged. These aggregates which are electronegative are water-linking and are called hydrophillic (ex: most of the igneous rocks). Similarly there are some aggregates like lime-stones have a dislike for water and greater attraction to bitumen, as they have positive surface charge. These aggregates are called hydrophobic. It is important to know the type of surface charge of aggregates used in bituminous construction. Now a days, bitumen is also available as cationic (+ve) or anionic (-ve) and hence a suitable selection may be made depending on the aggregates available. Cationic bitumen may be selected for electronegative aggregate and anionic bitumen for electropositive aggregates. This selection of appropriate bitumen binder may help to improve the adhesion of bitumen with the binder.

Blending of various stone sizes to a given aggregate grading

The investigation of asphalt surfacing mixtures for design purposes should be based on an aggregate grading which can be realistically produced in the hot-mix asphalt plant to be used during construction. The first step in determining the jobmix grading is to combine the various stone fractions that will be available during construction to approximate as closely as possible the design or specification grading required. The best way of doing this is to obtain samples of the hot-bin fractions (in the case of a plant with hot screening) or to use the various fractions from the stockpile material. The proportions in which the various fractions should be combined to produce the desired grading can be investigated by various graphical or mathematical methods. The method described by Rothfuchs has been found most useful as it is reasonably quick and simple and can be applied to blends of any number of components. It consists essentially of the following stages:

1. The cumulative curve of the design aggregate's particle-size distribution is plotted, using the usual linear ordinates for the percentage passing, but choosing the scale of sieve sizes which allows the particle-size distribution to be plotted as a straight line. This is readily done by drawing an inclined straight line and marking on it the sizes corresponding to the various percentages passing.

2. The particle-size distribution curves of the stone fractions (including filler) to be mixed are plotted on this scale. It will generally be found that they are not straight lines.3. With the aid of a transparent straight-edge, the straight lines which most nearly approximate the particle-size distribution curves of each component are drawn. This is done by selecting for each curve a straight line such that the areas enclosed between it and the curve are minimal and are balanced about the straight line.4. The opposite ends of these straight lines are joined together and the proportions for mixing can be read from the points where these joining lines cross the diagonal straight line which represents the design grading.

The procedure will be apparent from the following example:

The particle-size distributions of three fractions of stone and filler available to produce the required design grading are given in Table A1. Note that for greater accuracy a wet particle-size analysis should be carried out on these components. The design or specification grading is also given in the Right-hand column of the table.

In Figure A1 the required grading of the blend is represented by the diagonal straight line 00I. The vertical ordinates of the grading sheet are graduated for percentages from 0 to 100 on a linear scale. The horizontal scale for sieve aperture size is graduated by drawing for each sieve size a vertical line which cuts the diagonal at a point where the ordinate equals the percentage passing that sieve, i.e. 100 % for 19,0 mm, 90 % for 13,2 mm, 78 % for 6,7 mm and so on.

The size distributions of the fractions to be mixed (A, B, C and D in Table A1) are plotted on this scale of sieve size giving lines EFOI (fraction A), GHI (fraction B), JKL (fraction C) and OPQ (fraction D) in Figure A1. The nearest straight lines to these size distributions are drawn with the aid of a transparent straight-edge, by the 'minimum balanced areas' method described above. They are the broken lines ROI, TS, VU and OW. The opposite ends of these lines are joined giving the chain lines RS, TU and VW. The points where these lines cross the required distribution line (diagonal 00I) are marked by the circles 1, 2 and 3. The proportions in which the four fractions should be mixed are obtained from the difference between the ordinates of these points and are shown on the right-hand side of Figure A1 (sections A, B, C and D). The theoretical particle-size distribution which will result from mixing the fractions in these proportions is given in Table A2. Although not identical to the design grading, it is close enough for practical purposes.