Understanding and addressing asphalt pavement segregation. By Peter Berton
According to the National Cooperative Highway Research Program (a US-based research organisation): “Segregation is defined as a lack of homogeneity in the hot-mix asphalt constituents of the in-place mat of such a magnitude that there is a reasonable expectation of accelerated pavement distress(es).
“Constituents should be interpreted to mean asphalt cement, aggregates, additives, and air voids.” (Source: NCHRP Report 441: Segregation in Hot-Mix Asphalt Pavements.)
In simpler language, segregation occurs when bigger aggregate particles in roading asphalt pavement are unevenly blended mixed with its smaller particles.
Such a ‘segregated mix’ risks being less strong than an evenly-mixed pavement, thanks to an unbalanced transfer of traffic load stress throughout the roadway.
This topic was recently explored in the Canadian magazine Asphaltopics (Summer 2018 edition, ‘Lessons Learned from Segregation’).
“A segregated asphalt mix can make the laid-down pavement prone to joint/edge issues, longitudinal cracking, and other forms of premature failure,” said Laikram Narsingh, manager of Commercial Support and Development with Wirtgen America in Antioch, Tennessee.
“The result is a roadway that fails sooner.”
There are many reasons for segregation to occur, starting with improper aggregate storage back at the mixing facility, through inadequate transportation to the job site, and non-optimal application by the paving machine and its operator.
To make matters worse, it can be difficult to diagnose the precise cause of segregation if the problem lacks a pattern. In such cases, the only solution may be to remove the roadbed and start again.
Here’s what can go wrong at each stage of asphalt pavement preparation and application, and how to fix it before segregated roadbeds occur.
Segregation can start with storage
The genesis of a segregated roadbed can begin in an asphalt plant’s storage yard. If a driver lets the aggregates pour from their dump truck too quickly and/or violently, the various pieces of rock can sift themselves by size – with the smaller particles falling through the larger ones and onto the ground. The result is a material pile that is already segregated before it is mixed into asphalt pavement.
A giveaway to a too-fast pour is seeing bigger pieces of gravel roll down the side of the pile, like an avalanche on a snowy mountain. (A too-steep pile can also cause this kind of rock avalanche.) If you see this happening, slow down your pour!
In general, if aggregates are dumped in a slow, steady fashion, then sifting won’t occur. This won’t prevent segregation from occurring due to other causes, but it will eliminate this particular possibility.
To minimise the chances of pile-based segregation occurring, consider sorting the aggregates by size and then storing them in separate piles. This allows the asphalt mixing technician to control the blend by drawing specific amounts of differently-sized rocks from each pile, giving them more control over the final mix.
The correct ‘mix design’ is critical to creating asphalt pavement that resists segregation. This means using a ‘recipe’ that combines the appropriate percentages of aggregates, asphalt binding agents, and RAP (recycled asphalt products) in a properly blended and balanced mix.
“Unfortunately, most people who design asphalt pavement mixes don’t spend time in the field,” said Narsingh in Asphaltopics magazine.
“They don’t get to understand the lay-down challenges associated with their mix designs; including how it can lead to unnecessary segregation.”
In the field, an asphalt mixing technician should look for issues such as segregation that can be fixed at the plant by adjusting the blending process. The technician can also make the mix being made too coarse – a problem that can occur with Super-pave blends – which can lead to poorer application of binders on the aggregates.
Less binder can result in a reduced level of adhesion between aggregates, and more air space between gravel pieces that can weaken the laid-down roadbed over time.
Other mixing hints
Hint #1: Using the correct temperature during asphalt production is a big part of preventing segregation. The heat should be constant throughout the mixing drum, from end-to-end and from start to finish.
This is why the drum must be in good condition, with intact insulation in place to keep any heat loss consistent. Otherwise, temperature variations will occur, resulting in an uneven mix.
Hint #2: The drum’s mixing flights should be checked regularly, to make sure that aggregates of differing sizes are being uniformly delivered in the drum’s interior. Bent or misaligned flights can prevent this from happening. If left unchecked, this can cause an uneven distribution of gravel that can literally cause ‘segregation in the drum’.
Hint #3: Once again, how the pour is loaded onto the asphalt truck can either prevent or cause segregation. Be sure to fill the truck slowly and at a consistent speed.
When unloading the asphalt from the truck, place the discharge conveyor belt at a right angle to the drum’s axis for best results. Then pour the asphalt fast enough to keep up the heat, but not so fast that the mix starts to cascade like an avalanche.
Heat loss due to transportation can cause segregation as well. This is because the adhesion qualities of asphalt binding agents change as the temperature drops. As well, cooler asphalt is harder to apply in consistent layers than properly-heated asphalt is.
To minimise heat loss, make sure that the asphalt plant is as close to the lay-down location as possible. Next, cover the asphalt with a tarp during transport to reduce heat loss due to wind.
Finally, use an insulated truck bed – or one with a heated bed if available – to keep the asphalt’s temperature as near to what it was when the asphalt left the plant, until it reaches the roadbed.
“A segregated asphalt mix can make the laid-down pavement prone to joint/edge issues, longitudinal cracking, and other forms of premature failure.”
On the work site
To keep segregation under control while moving asphalt at the work site, load it into material transfer vehicles (MTVs), for moving materials from dump trucks to pavers.
With enough capacity to hold more than one load, an MTV can keep a paver constantly supplied so that the paving process isn’t interrupted. Better yet, an MTV equipped with remixing augurs inside can keep the asphalt blended; spreading the heat around throughout the entire unit.
Be sure to warm up the MTV’s bed by adding and then removing the first load from the bed to transfer heat. The first load can go back in the MTV after two more loads have been added, and the entire contents blended together using the remixing augurs. This ensures that the overall mix is still hot.
The paver matters
The paver itself can be a potential cause of segregation. Poorly maintained and operated pavers can deliver uneven asphalt applications, leading to pavement segregation. The machine also needs to have the right attachments around the auger feeding system, and the correct inserts inside the paver hopper, for everything to work as it should.
Keep it going!
To reduce the chances of segregation occurring during paving, keep the process constant, consistent, and continuous. This minimises heat loss from the asphalt during paving, and the differences in temperature from one section to another.
Avoid raising the paver’s hopper wings during application. The reason: Larger unattached aggregates can often run into the corners of the hopper. If the wings are raised, these gravel pieces can end up being applied directly to the roadbed; causing segregation.
If the paver has to be stopped, be sure to leave the hopper at least half full to slow down heat loss. Also, make sure any loose rocks that have become separated from the mix don’t slip under the screed and get into the roadbed.
Finally, the paver operator has to monitor pavement application at all times for uniformity and consistency – and know how to react when things need adjusting.
This is why “the paver operator [must] have the experience and equipment knowledge to spot potential problems such as segregation early on, and adjust their paver equipment to minimise them,” Keveryga told Asphaltopics magazine.
“But most of all, they have to have an eye for it – because even with all the science available to us today, paving is an art.”
The bottom line
Segregation is an end-to-end risk in asphalt production and paving. This risk can be minimised – though never eliminated – through vigilance, regular inspections, and adherence to quality standards at every stage of the paving process.