FIVE WAYS TO PREVENT ASPHALT RAVELLING
Ravelling is one of the most common, but easily preventable failure modes of asphalt pavements. Ravelling occurs as individual aggregate particles dislodge from the pavement surface downward. It usually starts with the loss of fine aggregate (fines) and advances to the loss of larger aggregate sizes.
A ravelling pavement will deteriorate in an exponential manner, meaning that deterioration will become progressively faster with time. A ravelling pavement will initially lose fines, leaving a pock-marked surface texture, followed by the loss of larger aggregate sizes leading to a rough surface texture with large, exposed aggregate, eventually resulting in the loss of the entire lift of asphalt pavement.
The various stages of ravelling are usually described as light (loss of surface fines), moderate (loss of fines and some larger aggregate exposed), and severe (loss of fine and coarse aggregate).
Ravelling can result from poor mix design, inadequate production practices, or poor construction practices. In many cases, ravelling is a combination of more than one contributing factor.
When placing hot-mix asphalt (HMA), ensure that the mix temperature is a minimum of 290°F at the mid-depth of the mat behind the screed when the compaction process begins. The ideal mix temperature will vary slightly depending on the binder type used in the mix design, lift thickness, the time available for compaction and ambient conditions.
The most efficient breakdown rolling should always occur at temperatures above 290°F. When breakdown rolling begins at temperatures lower than 290°F, the risk of not achieving sufficient compaction is greatly increased. It is almost impossible to achieve proper compaction when breakdown rolling begins with internal mat temperatures less than 260°F.
Mat temperatures should be measured at mid-depth of the mat. If a non-contact thermometer is being used to measure surface temperature, the user should be familiar with the difference in readings between the surface temperature and internal mat temperature. Non-contact thermometers are effective, provided a reasonable correlation exists between the surface temperature reading and the internal mat temperature.
Typically, non-contact thermometers will read approximately 20°F lower behind the screed than the mat temperature at mid-depth. For example, a surface temperature reading of 270°F would be optimal for the start of breakdown rolling on a mix where an internal mix temperature of 290°F is desired.
Compaction should be completed to a minimum of 92 percent of the theoretical maximum density (TMD) of the mix. Inadequate compaction results in too high an air void content in the placed mix. Cold weather paving presents a greater challenge to achieving proper compaction than fair weather conditions.
Employ best practices to minimize segregation of the asphalt mixture. This requires attention to plant storage (silos), truck loading and unloading techniques, and movement of the mix through the paver. Dense-graded asphalt mixes are designed for compaction to an optimal air void content. A segregated mix results in an excess of interconnected void space and two problems occur:
The inter-connected void space allows water to travel through the mixture, stripping asphalt cement from the aggregate, resulting in a loss of bond that leads to ravelling. The action of traffic on the pavement rapidly accelerates the ravelling process as it develops a pumping action that drives free water through the pavement and provides the physical forces necessary to remove the aggregates as the aggregate/asphalt bonds are weakened.
Placement in wet weather greatly increases the ravelling potential of a mix. As described above, moisture has a very damaging effect on HMA. When moisture is introduced during laydown by way of rain, fog or high humidity, a film of moisture forms on the coated aggregate particles of the HMA, preventing a strong bond from developing.
The potential for ravelling under these conditions can be accentuated by a mix that is too cool to rapidly evaporate any moisture it contacts (i.e. sitting too long in a truck or paver), and by lower compaction, which is more likely under the cooler ambient temperatures associated with wet weather. Lift thickness plays a critical role in preventing ravelling as it relates to compaction. A minimum lift thickness of two times the maximum aggregate size should always be maintained. This allows enough room for particle reorientation and proper compaction to occur.
If lifts are thinner than twice the maximum aggregate size, there is insufficient room for the aggregate to reorient itself in a dense configuration that is impermeable to water. Also, if the lift thickness is less than 2:1, aggregate fracture often occurs in the rolling process, leaving uncoated aggregate surfaces in the mix that will not bond.
Mix design/binder content
Mix design and binder content are critical to mix performance. Binder content that is too low for a given mix does not provide enough “glue” to bind the aggregate together. This can become a problem with a good mix design if the characteristics of the aggregate change, particularly if the aggregate becomes more absorptive. Asphalt cement that is absorbed into the aggregate is not effective.
Envision a sponge soaking up water. The water is present but cannot be used unless it is squeezed out of the sponge. Sufficient water needs to be added and the sponge must be saturated before water will move freely about the surface of the sponge.
A similar behaviour occurs when aggregate absorbs asphalt cement. A specific quantity of asphalt cement is absorbed by the aggregate and is not effective in bonding aggregate particles together. The quantity of binder in the mix must exceed the absorbed amount (saturate the aggregate) in sufficient quantity to provide free asphalt cement to bind the aggregate particles together.
It should be noted that mix quality problems arise with too high of a binder content as well. Care should be taken to stay within the recommended limits provided in the mix design.
The single most common cause of ravelling results from insufficient compaction of the asphalt pavement. The importance of achieving a minimum of 92 percent of TMD (theoretical maximum density) to mitigate ravelling and promote a durable pavement that will last for years cannot be overemphasized.
The cleanliness of the aggregate is very critical in mitigating ravelling. Dirty aggregates coated with dust or fines create a barrier to direct contact (bonding) between the asphalt cement and the aggregate.
The weakest portion of the resulting aggregate-dust-asphalt bond is the aggregate-dust adhesion. After several repetitions of the physical forces of traffic loading, the forces of adhesion holding the dust to the aggregate will break and dislodging of the aggregate from the pavement mass will occur. An excess amount of dust or fines also has the potential to absorb a high percentage of asphalt cement, lowering the effective asphalt content of the mix, leading to binder content problems outlined earlier.
The plant’s role
Finally, the plant mixing temperature is critical. Asphalt binders that are heated too high, typically over 330°F, will become age-hardened and lose their maximum effectiveness as a binder. The active bonding ingredients, or volatiles, in the asphalt cement are burned off, resulting in a much weaker binder. Extreme care should be taken by plant operators to maintain consistent mix temperatures that do not exceed the recommended mixing temperature for the grade of asphalt binder being used.