The most misunderstood items in an aggregate test report are the ones concerning fines. One engineer sees the fines content, judges it high and rejects the material; another looks at the same figure and sees no problem. Both are seeing only half the picture, because the quantity of fines on its own says nothing; the real question is what those fines are. Stone dust and clay pass the same sieve, yet they behave in diametrically opposite ways in concrete and on the road. This guide explains, in order, the tests that measure fines, the water absorption and moisture values, and what all of this turns into on site.
The 0.063 mm boundary: what fines are and where they come from
In aggregate technology, fines are the material passing the 0.063 mm sieve. This limit is not arbitrary; below this size the total surface area of the particles grows rapidly and the material stops behaving like a particle and starts behaving like a powder. As the surface area grows, more water is needed to wet it, chemical admixtures attach themselves to that surface and the flow behaviour of the mix changes. In other words, 0.063 mm is the line that separates the particles building the skeleton from the dust filling the voids in that skeleton.
Fines come from two different sources. The first is stone dust: the powder created as the rock is crushed in the crusher, with the same mineralogy as the parent rock. At a limestone quarry this is calcium carbonate powder. The second is clay: red earth accumulated in karst fissures, clayey interbeds between strata, or overburden that has not been fully stripped. Clay is not the same mineral as the parent rock; it has a platy crystal structure, a very high specific surface and a charge balance that interacts with water.
These two materials behave in completely different ways. Stone dust does not absorb water, does not swell, is chemically inert and simply sits inside the mix as a filler. Clay, on the other hand, draws water, grows in volume, shrinks as it dries and coats the particle surfaces, preventing the binder from bonding. Since both pass the same sieve, sieve analysis cannot tell them apart. To assess fines correctly, tests that measure quantity and tests that measure nature have to be read together. We covered how sieve analysis is carried out in the article on sieve analysis and the grading curve.
Washing loss: measuring the quantity of fines correctly
The fines content is determined by wet sieving. The sample is dried to constant mass and weighed. It is then washed with water over the 0.063 mm sieve, with a protective sieve placed above it. Washing continues until the water running off is clear, so that all the material below 0.063 mm leaves the system with the water. The material retained on the sieve is dried and weighed again. The difference between the initial and the final mass is the washing loss, and it is expressed as a proportion of the total mass and reported as the percentage of fines.
The importance of this test lies in understanding why dry sieving is not enough. In aggregate coming off the crushing and screening line, the fine dust is stuck to the moist surface of the particles. In dry sieving that dust moves with the particle and never passes the 0.063 mm sieve. The result is that a fines content which is in fact high appears low. This is usually why material that feels dusty when rubbed between the fingers on site looks clean in the report; the report was based on dry sieving.
The acceptance limit for the fines content varies with the application and the type of product. TS EN 12620 for concrete aggregate and TS EN 13043 for bituminous mixtures define this proportion in categories; which category is required is set by the project specification. The general tendency is this: fines are not wanted in coarse aggregate, because there they mean contamination rather than filler and they disturb the contact between the particle and the binder. In fine aggregate a certain amount of fines is beneficial, and in some mixes even desirable; the limit depends on whether those fines contain clay. This is why the washing loss is read not on its own but together with the methylene blue value.
Methylene blue: it measures the harm, not the quantity
The methylene blue test measures not how much fine material there is but how active it is. The principle behind the test is simple: because of their platy structure and their surface charge, clay minerals hold methylene blue dye on their surfaces, whereas stone dust holds practically none. The amount of dye a sample consumes is therefore a measure of the presence and the type of clay it contains.
In practice the sample is made into a suspension with water and dye solution is added in steps while it is stirred. After each addition a drop of the suspension is placed on filter paper. As long as all the dye is being held by the clay, only a dark stain forms on the paper. The moment the clay is saturated, the free dye creates a light blue halo around the stain. The point at which the halo persists for a set period is the end point of the test, and the amount of dye consumed up to that moment is reported as a value per unit mass of sample.
The interpretation of the result depends on the type of clay. Swelling clay groups hold a great deal of dye even in small quantities because of their very high specific surface, and they raise the value quickly; these are the most harmful in concrete and in road layers. Non-swelling clays hold less dye and their effects are more limited. A high methylene blue value therefore does not only say that clay is present; it also says that the clay is likely to be of the active and dangerous kind.
The most valuable feature of the test in practice is that it discriminates. A crushed sand with a high fines content but a low methylene blue value most probably carries clean stone dust and will cause no trouble in the mix. A natural sand with the same fines content but a high methylene blue value carries active clay and will upset the behaviour of the cement and the admixture in the mix. The measured proportion is the same; the outcome is completely different.
Reading them together with the sand equivalent: quantity, nature and consistency
The sand equivalent test is the quickest and the most site-friendly of the three. The sample is placed in a transparent cylinder together with a flocculating solution that holds clay and silt in suspension, shaken in a set pattern and left to stand. The heavy sand particles settle quickly to the bottom, while the clay and the silt stay in suspension and form a cloudy layer above. After a set period the height of the sand column and the height of the total column are measured and the ratio is given as a percentage. A high value means clean sand, a low value a sand carrying a heavy load of fines.
The limitation of the sand equivalent is that it cannot separate clay from stone dust; both join the cloudy layer. A good crushed sand carrying clean stone dust can therefore give a low sand equivalent and be rejected unfairly. On the other hand the test is quick on site, its equipment is simple and it serves as a daily alarm for production monitoring; a sudden drop in the value reports that something somewhere on the line has changed.
Reading them correctly means using three values together. The washing loss tells you the quantity of fines. The methylene blue value tells you how harmful those fines are. The sand equivalent tracks day-to-day consistency. Seen together, the three let you know the material: if the fines content is high but the methylene blue value is low, the quantity can be adjusted by screening or washing and the material is sound. If the fines content is low but the methylene blue value is high, there is a small amount of active clay; this is the more dangerous case, because the figure in the report is reassuring while the material causes trouble. We also covered the place of these two tests in the comparison of crushed and natural sand in the article on choosing fine aggregate.
Water absorption and surface moisture: what upsets the mix water
Water absorption is the ratio of the water a particle can take into its pores to its dry mass. It is measured by soaking the sample for a set period until it is saturated, removing the free water from its surface, weighing it, and weighing it again after drying. The difference between the two weighings is the water that has entered the particle. The aggregate at this point is in the saturated surface-dry condition: pores full, surface dry. This is the state that mix calculations take as their reference.
Water absorption and surface moisture are often confused, but they are different things. Water absorption is the water that enters the particle and contributes nothing to the mix; it is spent saturating the particle. Surface moisture is the free water sitting on the outside of the particles and it is added directly to the mix water. Aggregate in the stockpile is usually wetter than saturated; after rain and in washed sand stockpiles this moisture is high. If the plant does not account for this free water, more water than planned enters the mix, the water-cement ratio rises and strength falls. Conversely, a dry and absorbent aggregate draws the mix water into itself; the concrete stiffens as though it were setting early and workability disappears.
This is why moisture measurement is an operation that has to be repeated several times a day for fine aggregate. The surface of the stockpile dries in the sun while the lower levels stay wet; the moisture in the morning is not the moisture in the afternoon. If a moisture probe is used at the plant, it must be verified periodically by the oven-drying method. When working with aggregates of high water absorption, pre-wetting the aggregate before it enters the mix markedly reduces both the loss of workability and the loss of slump.
Water absorption is also an indicator of durability. High absorption shows that the particle is porous, that it will therefore be stressed from within during freeze-thaw cycles and that in asphalt it will absorb bitumen and thin the effective film thickness. In limestone this is one of the properties that varies most between quarry levels; tight-textured levels give low absorption, while shelly levels and those with solution voids give high absorption. For the details see the article on the properties of limestone as an aggregate.
Consequences in concrete: water demand, admixtures, shrinkage and durability
The first effect of fines in concrete is on water demand. As the surface area increases, so does the surface that has to be wetted; reaching the same slump takes either more water or more plasticiser. If it is made up with water, the water-cement ratio rises and strength and impermeability fall together. The right course is to control the quantity of fines in production and to close the remaining gap with an admixture.
The picture becomes worse still when clay enters the equation. Clay minerals attract not only water but superplasticiser molecules as well. Part of the admixture is held on the clay surface instead of dispersing the cement particles, and this explains why increasing the dosage does not have the expected effect. The pattern often seen on site is this: the admixture dosage is raised, the slump holds for a while and then disappears rapidly. This behaviour is usually the result not of the admixture but of the clay in the aggregate.
Two effects of clay stand out in hardened concrete. The first is shrinkage: clay loses volume as it dries and sets up tensile stress in the surrounding paste, so plastic and drying cracks increase. The second is bond: a film of clay coating the particle surface prevents the cement paste from adhering to the stone and weakens the aggregate-paste interface. That interface is already the weakest link in concrete; clay weakens it further. Alongside the loss of strength, the increase in permeability opens the door to reinforcement corrosion and durability problems in the long term.
Clean stone dust, by contrast, is generally good for concrete. It fills the voids, increases the cohesion of the mix, reduces segregation and bleeding and improves pumpability. In self-compacting and pumped concretes, filler is not a shortcoming but part of the design. The critical distinction is that the proportion is controlled and that the content is verified by testing to be free of clay.
Consequences on the road and in asphalt: plasticity, frost heave and stripping
The role of fines in road layers differs from their role in concrete, and it is not one-directional. In granular layers such as sub-base aggregate and plant-mix base a certain amount of fines is necessary: they fill the voids between the coarse particles, allow the layer to close up under compaction, bind the surface and give the layer bearing capacity. If the fine fraction is inadequate, the layer will not close under the roller, particles move at the surface and the target compaction density cannot be reached. For the function and the design of this layer, see the article on what sub-base aggregate is.
When the same fines are present in excess, the layer is left defenceless against water instead. The fine fraction containing clay turns plastic on contact with rainfall and groundwater; the layer turns to mud, loses its bearing capacity and settles under traffic. This is why road specifications cap the fines content of granular layers with an upper limit and restrict the plasticity of the fine fraction with a separate criterion. Plasticity shows whether the fine material changes shape with its water content; it is the most practical criterion for telling non-plastic stone dust from plastic clay on site.
In cold regions fines have a third effect: frost heave. Very fine-grained material draws water up from the water table by capillary action. When the temperature falls below zero, this water forms ice lenses within the layer, the layer heaves and the surface deforms. During the thaw the accumulated water is released, the layer becomes saturated and its bearing capacity drops suddenly. Along the Aegean coastal strip this risk is not as pronounced as it is inland, but it has to be taken into account at high elevations and on routes running towards the interior.
In asphalt, fines are two-faced. Filler forms a mastic together with the bitumen and increases the stiffness and the rutting resistance of the mix; stone dust is indispensable in a good asphalt mix. But clay stuck to the aggregate surface gets between the bitumen film and the stone, weakens the bond and starts stripping. For asphalt aggregate, therefore, a washed and clay-free product matters more than the filler content. For the details of the relationship between bitumen and aggregate, see the article on asphalt aggregate.
The role of washing: when it is needed and when it is not
Washing is the mechanical process that removes fines, and clay in particular, from an aggregate. Coarse aggregate is generally washed on a screen fitted with spray bars; the particles are washed as they are screened and the dust adhering to their surface is taken off with the water. For fine aggregate, blade or spiral washers are used together with dewatering screens. The wash water is directed to settling ponds where the fine material settles out and the water is recirculated as far as possible. We explained how this cycle is managed in the article on environmental management at the quarry.
The cases where washing really is necessary are clear. Washing is essential if there are clayey interbeds or karst fissure fillings in the quarry face, if the methylene blue value is rising, if a film of dust that would prevent the binder from bonding is forming on the surface of the coarse aggregate, and if drainage or filter material is being produced. In filter material, fines destroy the very reason the material exists; we set this subject out in detail in the article on choosing drainage and filter aggregate.
There are also cases where washing is unnecessary, or even harmful. In a crushed sand produced from clean rock whose fines are entirely stone dust, over-washing carries away the fine but useful fraction between 0.063 and 0.25 mm as well. The result is a sand emptied at the fine end of its grading curve and prone to segregation and bleeding. Concrete made with such a sand bleeds, dusts at the surface and segregates in the pump. Washing is therefore not an outcome but a process that has to be tuned.
The right approach is this: not to eliminate the fines but to control their quantity and their nature. Wash if there is clay, adjust by dry screening if there is too much stone dust, and leave well alone when the two are in balance. The basis for the decision is the washing loss, methylene blue and grading results of regularly taken samples. At its limestone quarry in Milas, İTS Maden runs its crushing, screening and washing plants under a system in which these three values are monitored together. To choose products that meet the fines and cleanliness requirements of your project, you can review our products, look at our approach to quality control and contact us.
