Question 4 of 6: Hot Mix Asphalt — Mix Types, Design Objectives and Mixture Behaviour

Question 4: Hot Mix Asphalt — Mix Types, Design Objectives and Mixture Behaviour (3 + 4 + 3 + 4 + 3 + 3 = 20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Part (a) — The three types of hot mix asphalt (3 marks)

Hot mix asphalt is classified by the shape of its aggregate gradation, because the gradation determines how the load is carried and how much void space is available for binder and air.

Dense-graded mixes use a continuous, well-distributed gradation from the nominal maximum size down to the filler, so that the smaller particles progressively fill the voids between the larger ones. They are the general-purpose mix used for surface, binder and base courses. Advantages: high strength and stability, low permeability, good durability, well understood, and suitable for any layer in the structure. Disadvantages: a relatively smooth, comparatively impervious surface that produces spray and hydroplaning risk in wet weather and higher tyre-pavement noise; the load is carried partly through the fine matrix rather than by stone-on-stone contact, so heavy, slow, channelised loading can rut it.

Open-graded mixes (open-graded friction courses, and asphalt-treated permeable bases) use a nearly single-sized coarse aggregate with very little fine material, giving an air void content of 18 to 22 per cent. Advantages: water drains within the layer instead of running on the surface, which sharply reduces splash, spray, hydroplaning and night-time glare, improves wet skid resistance, and reduces noise; as a permeable base it drains the pavement structure. Disadvantages: low strength, so it is never a structural layer; the open structure allows air and water to reach the binder film so it ages and ravels faster and has a short service life; it can clog with detritus; and in a freeze-thaw climate it is vulnerable to ice retention and de-icer demand.

Gap-graded mixes omit one or more intermediate sizes, leaving a coarse skeleton whose voids are filled with a rich mortar of binder, filler and fine sand. Stone mastic asphalt is the modern engineered example. Advantages: stone-on-stone contact gives outstanding rutting resistance, and the high binder content in the mastic gives durability and fatigue resistance. Disadvantages: it is expensive, needs a stabilising additive to stop binder drain-down, is sensitive to production and placement tolerances, and requires an experienced contractor.

Part (b) — Seven specific mix design objectives (4 marks)

The Asphalt Institute states the purpose of mix design as selecting an aggregate gradation and a binder content that simultaneously achieve seven objectives. They conflict with one another, so mix design is an exercise in balance rather than optimisation of any one property.

1. Resistance to permanent deformation (stability). The mix must not distort, rut or shove under repeated heavy loads, particularly at high summer pavement temperatures and at intersections where loading is slow and channelised.
2. Fatigue resistance. The mix must withstand repeated flexural strain from traffic without cracking, over the design number of load repetitions and given the stiffness of the supporting layers.
3. Resistance to low-temperature (thermal) cracking. The binder must remain flexible enough at the lowest expected pavement temperature that thermally induced tensile stress does not exceed the mixture's strength — a governing criterion in most of Canada.
4. Durability. The mix must resist the hardening of the binder by oxidative ageing and volatilisation, and must resist ravelling and disintegration, over its service life.
5. Resistance to moisture damage (stripping). The bond between binder and aggregate must survive saturation and freeze-thaw cycling; where it will not, an anti-stripping agent or hydrated lime is required.
6. Skid resistance. The surface course must retain adequate micro- and macro-texture, and therefore adequate wet friction, throughout its life; this constrains both aggregate polish resistance and binder content.
7. Workability. The mix must be capable of being placed and compacted with normal equipment at practical temperatures, without segregating, tearing or requiring excessive rolling effort.

Part (c) — Effects of increasing the asphalt binder content (3 marks)

Adding binder thickens the film coating each aggregate particle and fills more of the voids in the mineral aggregate.

Three properties improved. (1) Durability — a thicker binder film oxidises more slowly through its depth, so the mix ages and ravels less. (2) Fatigue resistance and flexibility — the mix can accommodate more flexural strain before cracking, and low-temperature cracking resistance improves for the same reason. (3) Workability — the mix is easier to place and compact, and it is also less permeable, because the extra binder closes the interconnected air voids.

Two properties adversely affected. (1) Stability, that is resistance to permanent deformation — beyond the optimum, the binder begins to act as a lubricant and force the aggregate particles apart, so the aggregate skeleton loses its interlock and the mix ruts and shoves. (2) Skid resistance — as the air voids fall below about 3 per cent, thermal expansion of the binder in summer has nowhere to go, and the excess bleeds to the surface (flushing), producing a slick, polished, low-friction surface. This is why the design binder content is chosen at a target air void content of 4 per cent: it is the highest binder content that still leaves enough void space for expansion.

Part (d) — Stone mastic (matrix) asphalt (4 marks)

What SMA is. Stone mastic asphalt is a gap-graded, high-stone-content hot mix developed in Germany in the 1960s to resist studded-tyre wear and now used across Canada on heavily trafficked highways. Typically 70 to 80 per cent of the aggregate is retained on the 4.75 mm sieve, so the coarse particles are in direct stone-on-stone contact and carry the load through the skeleton rather than through the mortar. The voids in that skeleton are filled with a rich mastic of 6 to 7.5 per cent binder (often polymer-modified), 8 to 12 per cent mineral filler passing 0.075 mm, and a stabilising additive — cellulose or mineral fibre at about 0.3 per cent — whose sole purpose is to hold the binder in suspension so it does not drain off the stone during storage and haul. Design air voids are about 4 per cent and the VMA is high, around 17 per cent.

Advantages. Excellent rutting resistance, because the load path is through the stone skeleton and not through a deformable sand-asphalt matrix; excellent durability and long service life, because the binder films are thick and the mix is nearly impermeable; good fatigue resistance and resistance to reflection and thermal cracking; a coarse surface macrotexture that gives good wet skid resistance and reduced spray; and lower tyre-pavement noise than a dense-graded surface. Life-cycle costing generally favours SMA on high-volume routes despite its first cost.

Disadvantages. Initial cost is roughly 20 to 25 per cent higher than a dense-graded surface because of the higher binder content, the fibre, the modified binder and the demand for hard, cubical, high-quality aggregate. Production requires tight control of the filler and fibre feed, and the mix is prone to binder drain-down if the fibre dose, mixing temperature or storage time is wrong. Placement demands experienced crews: it must not be over-rolled, and the surface can be slippery for the first few weeks until traffic wears the binder film off the surface stones. Finally, it is a surface mix only and does not replace the structural layers below.

Part (e) — Effects of excess binder, excess medium sand and rounded aggregate (3 marks)

All three of these defects act in the same direction: they weaken the aggregate skeleton and over-fill the voids. Excess binder lubricates and pushes particles apart; an excess of medium-size sand ("hump" in the gradation near the 0.60 to 0.30 mm sizes) produces a tender mix that shoves under the roller; and rounded, uncrushed aggregate has no angularity, so particles roll over one another instead of interlocking.

In short, this is a classic tender, rut-prone mix: it will look and place beautifully and will fail by deformation and bleeding within a few summers, especially at intersections and on grades. The remedies are to reduce the binder content, correct the gradation hump in the medium sand sizes, and specify a minimum percentage of crushed faces.

Part (f) — Effects of low binder content, high voids and water-susceptible aggregate (3 marks)

This mix has the opposite disease: it is dry, harsh and open, and its aggregate has a chemical affinity for water rather than for asphalt (typically siliceous, acidic aggregates such as granites and quartzites).

This mix will fail by ravelling, potholing and moisture-induced stripping, usually beginning in the first spring after construction. The remedies are to raise the binder content to the design optimum, achieve the specified field density so that in-place voids fall to 6 to 8 per cent, and treat the aggregate with hydrated lime or a liquid anti-stripping agent after confirming the problem by a tensile strength ratio test (AASHTO T283).