Aggregates: sources, gradation and properties

Aggregates are the sand, gravel and crushed stone that make up most of the volume of concrete and asphalt and all of a granular road base. In normal concrete they fill roughly 60 to 75 percent of the volume; in asphalt concrete, more than 90 percent of the mass. Because there is so much of them, aggregates control the cost, the unit weight, much of the stiffness and a good deal of the durability of the finished material. A bad aggregate can ruin a perfect mix design, and understanding aggregates is the natural starting point for the rest of the course.

On the interchange project, the concrete supplier draws coarse aggregate from a glacial gravel pit northwest of Fargo and blends in crushed granite trucked from Minnesota, because the local gravel contains soft shale particles. The asphalt contractor uses crushed gravel from a different pit. Tamarack tests both sources before the work starts and at regular intervals during production.

Sources and types of aggregate

Natural aggregates come from sand and gravel deposits left by glaciers and rivers, or from quarried bedrock that is crushed and screened. Gravel particles are rounded and smooth, which makes fresh concrete easy to place but bonds less strongly to cement paste and gives less interlock in asphalt and base courses. Crushed stone is angular and rough, which interlocks well and bonds well but needs more paste or binder to coat and lubricate. Manufactured sand is crushed rock screened to sand sizes; it is increasingly used where natural sand is scarce.

Rock type matters. Granites, basalts and many limestones and dolostones make sound aggregate. Some rocks cause trouble: shale and clay ironstone particles soften and expand when wet and freeze, causing pop-outs on concrete surfaces; chert and some siliceous rocks can react with alkalis in cement; some argillaceous dolostones expand through alkali–carbonate reaction. Recycled concrete aggregate, made by crushing old pavements and buildings, and blast-furnace slag aggregate are also used, with extra testing.

Aggregates are divided by size. Coarse aggregate is retained on the 4.75 mm (No. 4) sieve; fine aggregate (sand) passes it. The nominal maximum size of a coarse aggregate, such as 20 mm or 28 mm (3/4 in or 1 in), is the smallest sieve on which some material is retained, by the usual definition, and it limits how thin a section or how closely spaced the reinforcement can be.

Gradation and the sieve analysis

Gradation is the distribution of particle sizes. It is measured by a sieve analysis: a dried sample is shaken through a nest of sieves with progressively smaller openings, the mass retained on each sieve is weighed, and the results are reported as cumulative percentage passing each sieve. Plotting percentage passing against sieve size on a logarithmic scale gives the gradation curve.

A well-graded (dense-graded) aggregate contains a smooth range of sizes, so smaller particles fill the voids between larger ones. This gives a dense packing that needs less cement paste in concrete or less binder in asphalt. A uniformly graded aggregate has most particles of similar size and leaves many voids; it is used deliberately in drainage layers and open-graded friction courses. A gap-graded aggregate is missing some intermediate sizes; it is used in stone matrix asphalt to produce stone-on-stone contact, but in concrete it can segregate.

A useful idealization is the maximum density gradation proposed by Fuller and Thompson and adapted for asphalt by the Federal Highway Administration's 0.45 power chart.

where \(P\) is the percentage passing sieve size \(d\) and \(D\) is the maximum size. A gradation close to this line packs densely; Superpave asphalt mixes are designed to pass near but not exactly on it, so that enough voids remain for binder and air (lesson 10).

Fineness modulus

For fine aggregate, gradation is summarized by the fineness modulus (FM): the sum of the cumulative percentages retained on the standard sieves 0.15, 0.3, 0.6, 1.18, 2.36 and 4.75 mm (No. 100, 50, 30, 16, 8 and 4), divided by 100. Sands for concrete usually have an FM between about 2.3 and 3.1. A higher FM means a coarser sand, which needs less water for the same workability but may make a harsh mix. Mix design methods use the FM to choose the volume of coarse aggregate (lesson 4).

Lucie automates the calculation so the lab can process a day's samples quickly and flag any that fall outside the specification band.

sieves = [4.75, 2.36, 1.18, 0.60, 0.30, 0.15]          # mm
retained = [10, 60, 95, 110, 120, 80]                    # g on each sieve
pan, total = 25, 500.0
lower = [95, 80, 50, 25, 5, 0]                          # a typical sand band (% passing)
upper = [100, 100, 85, 60, 30, 10]

assert abs(sum(retained) + pan - total) < 0.5, "mass not conserved"
cum, passing = 0, []
for m in retained:
    cum += m
    passing.append(100 * (1 - cum / total))
fm = sum(100 - p for p in passing) / 100
for d, p, lo, hi in zip(sieves, passing, lower, upper):
    flag = "" if lo <= p <= hi else "  <-- outside band"
    print(f"{d:5.2f} mm  passing {p:5.1f} %  (band {lo}-{hi}){flag}")
print(f"Fineness modulus = {fm:.2f}")

 4.75 mm  passing  98.0 %  (band 95-100)
 2.36 mm  passing  86.0 %  (band 80-100)
 1.18 mm  passing  67.0 %  (band 50-85)
 0.60 mm  passing  45.0 %  (band 25-60)
 0.30 mm  passing  21.0 %  (band 5-30)
 0.15 mm  passing   5.0 %  (band 0-10)
Fineness modulus = 2.78

Material finer than 75 µm

The fraction passing the 75 µm (No. 200) sieve is found by washing the sample over that sieve. In concrete aggregate, too much fine dust or clay coats particles, raises the water demand and weakens the bond, so specifications limit it to a few percent. In asphalt mixes the same fraction, called filler or dust, stiffens the binder and is controlled by a dust-to-binder ratio. In granular base, some fines help compaction but too many hold water and lose strength when wet or frozen.

Specific gravity, absorption and moisture states

To design a mix by volume, you need to know how much space each kilogram of aggregate occupies. Aggregate particles contain pores, some of which absorb water. The relative density (specific gravity) is therefore defined in several ways, depending on which pores are counted.

• Bulk relative density, oven-dry (\(G_{sb}\) or OD): the oven-dry mass divided by the mass of water occupying the particle volume including permeable pores.
• Bulk relative density, saturated surface-dry (SSD): uses the SSD mass, with the permeable pores filled with water but no water on the surface. Concrete mix designs are usually done on an SSD basis.
• Apparent relative density: counts only the solid material and impermeable pores.
• Absorption: the mass of water in the permeable pores at SSD, as a percentage of the oven-dry mass. Typical values are 0.5 to 2 percent for sound gravel and stone, higher for some limestones and much higher for lightweight and recycled aggregates.

Stockpiled aggregate is rarely at SSD. If it is drier than SSD, it will absorb some of the mixing water, stiffening the mix; if it is wetter, its surface water adds to the mixing water, raising the water–cement ratio and lowering strength. Batch plants measure the moisture content of each aggregate and adjust the batch masses: they add the aggregate's free (surface) moisture to its mass and subtract the same water from the batch water.

where \(w\) is the total moisture content and \(a\) the absorption, both as fractions of oven-dry mass.

Bulk density and voids

The bulk density (unit weight) of an aggregate is the mass of a measured volume of particles, including the voids between them, usually found by rodding the aggregate into a container of known volume. Comparing bulk density with particle density gives the void content. Mix design methods use the dry-rodded bulk density of the coarse aggregate to decide how much coarse aggregate fits in a cubic meter of concrete.

For example, a gravel with a dry-rodded bulk density of 1650 kg/m³ and a bulk relative density of 2.65 has voids of \(1 - 1650/2650 = 0.377\), so about 38 percent of the rodded volume is space to be filled by mortar.

Durability and mechanical tests

Several tests screen aggregates for strength and durability.

• Abrasion and degradation: the Los Angeles abrasion test tumbles aggregate with steel balls in a rotating drum and measures the percentage broken down; the micro-Deval test tumbles it wet with small steel balls and is better at identifying soft, absorptive rock. Highway agencies set maximum losses for base, asphalt and concrete aggregate.
• Soundness: freeze–thaw testing of aggregate in water or brine, or the older sulfate soundness test, which simulates frost damage by repeated crystal growth in the pores.
• Deleterious particles: petrographic examination counts shale, clay lumps, chert, coal and other harmful particles, and a petrographic number is used in some provinces and states to rate gravel quality.
• Particle shape: flat and elongated particles break during compaction and make concrete harsh; crushed asphalt aggregate must have a minimum percentage of fractured faces so the mix interlocks and resists rutting.
• Alkali reactivity: the accelerated mortar bar test and the longer concrete prism test show whether an aggregate expands when exposed to alkalis (lesson 6).

Aggregates in granular base

Under pavements, layers of compacted crushed aggregate spread wheel loads down to the subgrade. Their performance depends on gradation, fractured faces, fines content and compaction. Field density is checked with a nuclear gauge or by sand-cone or other methods and compared with the maximum dry density from a laboratory Proctor test. A well-graded, well-compacted, free-draining base is the cheapest insurance a pavement has.