Most of the mass of an asphalt pavement is aggregate; bitumen is only a thin film binding the particles together. Whether the pavement carries the wheel loads, stays skid-resistant in the rain, holds together in winter and does not rut in the summer heat depends largely on how those particles are chosen. The properties that matter most in concrete aggregate change places in asphalt: here hardness, particle shape, polishing resistance and adhesion to bitumen take the lead. This guide explains, from a field perspective, what every item on an asphalt aggregate test report measures, the reasoning behind it, and why the expectations for the wearing course and the base course differ.
Why is asphalt aggregate assessed differently from concrete aggregate?
In concrete the aggregate is locked inside a rigid cement paste matrix; the matrix distributes the load, and the aggregate acts largely as filler and skeleton. In asphalt, the binder is bitumen, a viscoelastic material that softens with heat, ages over time and can flow under load. In an asphalt pavement, therefore, the structure that actually carries the load is the aggregate skeleton formed by the interlocking of the particles. If the particles are not hard enough they break down under traffic; if their shape is poor they cannot interlock and the pavement flows into wheel ruts; if the surface polishes, the road becomes slippery when wet; and if they do not bond well with bitumen, water gets in between and the particles strip.
These four questions sum up the entire test programme for asphalt aggregate: durability, shape, surface and adhesion. In Europe and Türkiye, aggregate requirements for bituminous mixtures are defined by TS EN 13043; the standard ties each property to a test method and expresses the results as categories. Which category is required in which layer is set by the road authority's technical specification. This is why the first job of an asphalt aggregate producer is to ask which layer and which traffic class the product is intended for; the same quarry may comfortably suit the base course yet fall short for the wearing course.
The properties that stand out in concrete aggregate, such as alkali-silica reactivity and clay content, do not lose their importance in asphalt, but they move into the background. Conversely, polishing and bitumen affinity, which are secondary in concrete, become decisive in asphalt. For the concrete perspective, see the article on how aggregate quality affects concrete.
The Los Angeles abrasion test: what it measures and why
The Los Angeles test is the most widely used aggregate durability test in the world, measuring the aggregate's combined resistance to impact and abrasion. A dry aggregate sample of a specified fraction is placed in a horizontal drum containing steel balls. The drum is rotated for a set number of revolutions; on each revolution, a shelf on the inner wall of the drum lifts the aggregate and the balls and drops them. The particles are subjected both to the impact of the balls and to rubbing against one another. At the end of the test the sample is passed through a specified sieve, and the percentage passing, that is, the material that has broken down, is reported as the Los Angeles abrasion loss. The lower the value, the more durable the aggregate.
The logic of the test is that it mimics together the two stresses the pavement actually experiences. During laying and compaction the roller knocks the particles against one another; under traffic the wheel load both compresses the particles and rubs them against each other. A soft or internally cracked particle breaks under this stress; as it breaks, the grading changes, the void content of the mix is disturbed, and the bitumen film cannot coat the newly broken surfaces. A high Los Angeles loss therefore signals not only that the stone is weak but that the mix design will deteriorate in the field.
For limestone, this is the test item that must be read most carefully. Because of the relative softness of the calcite mineral, the Los Angeles loss of limestones is generally higher than that of basalt and granite; but the difference between recrystallised, dense-textured limestones and loose, shelly limestones is very large. Some of the limestones around Milas, thanks to a dense structure close to metamorphic, can comfortably give suitable results for base and binder courses and, at suitable levels, for the wearing course too. That is determined by regular testing at quarry level, not by the name of the rock. For the general character of limestone, see the article on what limestone is.
Flakiness index and crushed faces: how the skeleton is built
The load-bearing skeleton of an asphalt mix is formed by the interlocking of the particles. This interlock is best achieved with cubical particles, whose three dimensions are close to one another. Flat and elongated particles create two problems: during compaction they stack on their flat planes and take on a preferred orientation, which weakens the interlock; and because of their thin cross-section they break easily under the roller and under traffic. A broken flaky particle sets off the chain described in the Los Angeles section.
The flakiness index puts this shape problem into numbers. Each particle size group is first separated on a square-mesh sieve, then passed through the bar sieve corresponding to that size. Particles that can pass through the bar sieve, that is, those whose thickness is small relative to their size, count as flaky; their percentage of the total mass is the flakiness index. A low index means cubical, well-shaped aggregate. Shape depends less on the nature of the rock than on the set-up of the crushing line: bedded limestone is prone to producing flaky particles in a jaw crusher, but a closed-circuit line using cone or impact crushers in the secondary and tertiary stages largely eliminates this tendency.
The crushed-face ratio measures another aspect of shape. In aggregates produced from natural gravel, the particles are rounded and smooth; they generate less friction with bitumen and do not build a skeleton. In the test, particles are examined one by one and sorted into fully crushed, partly crushed and rounded, and their mass percentages are calculated. In limestone aggregate produced as crushed rock from a quarry, every face is a broken face, so this criterion is met automatically; the test is mainly meaningful for aggregates sourced from river gravel. In the wearing course and on heavily trafficked roads a high proportion of fully crushed faces is required, because rutting resistance depends directly on the particles rubbing against one another.
Polished stone value (PSV) and skid resistance
The wearing course is the surface the tyre touches directly, and safe braking on a wet road depends on the micro-texture of that surface. With every pass, the tyre flattens the upper surface of the particles a little further through friction; particles made of soft minerals gradually acquire a glassy, shiny surface on which the tyre cannot grip. This process is called polishing, and the aggregate's ability to resist it is the polished stone value (PSV).
In the test, aggregate particles are set around the rim of a wheel and mounted in an accelerated polishing machine. A rubber wheel is run over the specimens for a set period together with abrasive powder and water, compressing years of traffic into hours. The surface friction of the polished specimens is then measured with a pendulum tester swinging across the wet surface. The less swing the pendulum loses, the more slippery the surface. The result is given as a number in which a higher value means better skid resistance.
Polishing resistance is directly related to the mineral composition of the rock. In rocks containing minerals of different hardness, the soft minerals wear away first, the hard minerals remain as protrusions and the surface constantly renews itself; such rocks give a high polished stone value. Rocks made of a single mineral that wear homogeneously, on the other hand, tend towards a smooth, slippery surface. Limestone is closer to this second group: because calcite dominates, many types of limestone give a medium or low polished stone value. This is why, in the wearing course of heavily trafficked roads, limestone is either blended with a polish-resistant rock or used only in the lower layers. On low- and medium-traffic roads, away from critical points such as junctions and ramps, a suitable limestone can also find a place in the wearing course on its own; the decision rests with the road authority's specification and the measured value.
Water absorption and freeze-thaw resistance
Water absorption is the amount of water an aggregate particle can take into its pores relative to its dry mass, and in asphalt it leads to two different consequences. First, a porous particle absorbs not only water but bitumen too. Part of the bitumen added to the mix is drawn into the particles and the effective film surrounding them thins; this leads to early ageing, embrittlement and susceptibility to stripping. Mix design allows for this loss, but if absorption is high it becomes unsustainable both economically and technically.
The second consequence is freeze-thaw. Water entering the pores expands when it freezes and breaks the particle apart from within; this cycle repeats dozens of times every winter. Dense rocks with low water absorption are unaffected by this cycle, while porous rocks crumble within a few winters. Freeze-thaw resistance can be measured directly, and it is also assessed with a test that repeatedly immerses the aggregate particles in a saturated magnesium sulfate solution and dries them, mimicking frost through crystal growth. Aggregate that shows high mass loss in this test is not accepted for the wearing course in cold climates.
For limestone, water absorption is one of the properties that varies most from one quarry level to another. Dense, recrystallised limestone gives low absorption, while shelly types and those with solution cavities show high absorption. That these two types can exist side by side in the same quarry shows once again why level selection and regular testing are indispensable. Because the Aegean coastal climate sees far fewer freeze-thaw cycles than the inland and eastern regions, the weight given to this criterion varies with project location; but the effect of water absorption on bitumen consumption applies in every climate.
Bitumen adhesion and stripping: the aggregate's chemistry comes into play
The most insidious form of asphalt deterioration is stripping: water seeps between the bitumen film and the aggregate surface, breaks the bond, and the particles separate from the bitumen. The result is first a grainy, ravelling surface and then potholes. The origin of this process lies in the chemical relationship between bitumen and the aggregate surface. Bitumen competes with water for the aggregate surface, and if the surface prefers water, that is, if it is hydrophilic, bitumen loses that competition.
Silica-rich rocks, especially quartzite, granite and some sandstones, have an acidic surface chemistry and a preference for water; asphalts made with these rocks are more prone to stripping. Limestone, on the other hand, is a rock with a basic surface. The calcium carbonate surface forms a stronger bond with the acidic components of bitumen and attracts less water. This is limestone's least known but perhaps most valuable property in asphalt: while it lags behind siliceous rocks in polishing and abrasion, it leads in adhesion. This is exactly why, in the field, a certain amount of limestone dust or hydrated lime is added to siliceous aggregate in some mixes; the lime acts as an adhesion-promoting additive.
Adhesion is measured in the laboratory by tumbling bitumen-coated aggregate particles in water for a set period and then assessing the proportion of surface still coated with bitumen; the more of the coated surface is retained, the better the affinity. It is also checked at mix level by comparing the strength of compacted mix specimens in the water-saturated and dry states. If adhesion is found to be inadequate, amine-based adhesion promoters or hydrated lime are used; but choosing the right aggregate reduces dependence on these additives from the outset.
Wearing course versus base course: why do the expectations differ?
An asphalt pavement is not a single material but a system of layers, each with a different job. At the bottom is the bituminous base, in the middle the binder course, and on top the wearing course. What each of these layers expects from the aggregate depends on the stresses it is exposed to; demanding the same criterion with the same rigour for every layer is therefore both an unnecessary cost and a waste of resources.
The wearing course takes wheel loads, tyre friction, water, sun and temperature swings directly. Here the polished stone value, Los Angeles loss, flakiness index, crushed-face ratio and water absorption are demanded at their strictest; the aggregate must be a material that stays both hard and rough while also bonding well to bitumen. Meeting these expectations with a single rock is not always possible, which is why designs that blend hard siliceous aggregate with basic limestone, one providing skid resistance and the other adhesion, are common.
The binder course takes the load from the wearing course and spreads it; since rutting resistance is critical here, particle shape and interlock come to the fore, while polishing is not an issue because the tyre never touches these particles. The bituminous base course carries the greatest load by volume; here more lenient categories are accepted for Los Angeles loss and shape, and cost-effectiveness is decisive. Limestone aggregate takes advantage of this latitude and becomes the natural choice in binder and base courses. In the granular base and sub-base layers beneath the asphalt, expectations change again; for that layer see the article on what sub-base aggregate is.
The right approach is to read the categories required for each layer of the project from the specification and match them against the measured values of each level in the quarry. That way, the material meeting the strictest requirements goes only to the layer that needs it, and the remaining layers are produced from more economical fractions. İTS Maden produces its fractions for asphalt and road layers at its limestone quarry in Milas with this distinction in mind and shares product test results layer by layer. You can review our quality control system and talk to us to determine the right fractions for the layers in your project.
