Causes, culprits, and treatment
By Jeff Winke
Folklore and myths survive. Walk down a sidewalk or cross the street with a bunch of friends and observe how many avoid stepping on cracks. When asked, they’ll respond that they don’t want to break their momma’s back or release bad luck from demons that dwell in the underworld beneath.
Asphalt pavement specialists think differently, although there may be a few who avoid stepping on cracks just in case.
“The three main causes of cracks in asphalt pavement,” Tom Kelly, senior VP at Crafco, Inc., DeKalb, Illinois, stated, “are (1) thermal movement of the pavement — i.e., the expansion and contraction due to temperature changes, sometimes as much as 100 degrees F in a single day, (2) traffic loads – when cars and trucks pound the pavement, any weakness in the structure of the pavement will fracture or any flaw in the Hot Mix Asphalt (HMA) composition may cause premature cracking and pavement failure, and (3) age and oxidation –- as pavement ages, it loses flexibility and cohesion within the pavement structure, making it more susceptible to the first two causes of cracking.”
Causes for pavement cracks can be grouped in two categories: load-related causes and non-load (environmental) causes, reported Ben Thielbar, director of sales, with Cimline, Inc., Plymouth, Minnesota.
“Load-related cracks typically occur in the wheel paths in the form of fatigue cracks,” Thielbar said. “Non-load causes are from expansion and contraction of the pavement from temperatures (thermal cycling), made worse as the material oxidizes and becomes brittle. Damage is also accelerated when water can get into the asphalt and expand and strip or de-densify the material.
“In other words, in either load-related or non-load related, it’s when the stress applied to the layer exceeds the strength of that material.”
There are some interesting industry studies that focus on increasing education and awareness about road center line issues and cracking.
“We have found research that finds that the center line portion of the pavement typically fails first,” stated Jeffrey S. Ball, executive director of marketing and communications for Asphalt Materials, Inc., Indianapolis, Indiana. “The longitudinal joint is the Achille’s Heel of pavements.
“For example, asphalt (HMA) pavements constructed in Minnesota typically have air void contents of about 7% to 8% in the mat and often approach or exceed air void contents of 10% at the longitudinal joints. Higher air void contents at the longitudinal joints can expose the pavement to premature deterioration and, as a result, compromise pavement integrity and performance. In Minnesota, the primary concern during construction is the achievement of high density in the mixture, especially at the longitudinal joints.”
A blog published by NAC Supply, Inc., Ingleside, Illinois does a good job of capturing the culprits for asphalt pavement cracking: moisture, the sun, and ground movement.
With respect to moisture, the blog stated, “Asphalt is impressively waterproof, but that doesn’t mean it’s completely immune to moisture. Moisture will wash away the pavement’s gravel and sand base, which can lead the surface layer to shift and break. This shifting and breakage lead to the formation of cracks. Water can affect your pavement’s surface, too. It can make its way into crevices, creases, and dents and start exerting pressure on the pavement from above. Adequate drainage will divert water away from your pavement, but if you don’t have good drainage, a sealant will also help keep your pavement dry. Since water can pool in any dents or cracks, you’ll want to fill them up as soon as possible to avoid further damage.”
The sun, as described in the blog: “…the summer sun can cause heat damage. Heat can dry up the pavement and cause the materials in the aggregate to break down, resulting in cracks and other damage. Temperature changes that vary rapidly, like those in spring, can also wreak havoc on your pavement. The heat can melt snow, forming water—but if it’s cold again the next day, the water that just melted can freeze under the pavement’s surface, weakening the pavement’s sub-base or resulting in overexpansion. The easiest way to protect your pavement from extreme temperatures is to sealcoat it.”
And ground movement, as the blog delineated: “ The ground below us is continuously shifting, either due to seasonal conditions like frost and thaw or disasters like earthquakes and severe thunderstorms. When the ground moves, the asphalt’s gravel base will shift and put pressure on the asphalt, which can result in cracks or potholes. Unfortunately, you can’t stop the ground from shifting. You can, however, protect your asphalt pavement by keeping it in good condition. Pavement in good condition will hold up to the elements and other crack-causing factors much better than pavement that hasn’t had proper maintenance.”
Understanding why pavement cracks occur is helpful for achieving proper perspective.
“Cracks typically start to form at the center joint and gradually work their way out to the rest of the asphalt,” stated Michael Blake, director of marketing, KM International, North Branch, Michigan. When water penetrates into the cracks it expands and contracts due to the freeze-thaw cycle, which will cause the rest of the pavement to start to crack. This process is also compounded if/when you have base failure or shifting of the base.”
Preventative maintenance is always key for avoiding pavement cracking.
“Unfortunately, there is no way to completely eliminate cracks,” Blake said. “But most contractors will recommend sealcoating every 2-3 years and filling any cracks in the pavement immediately after they occur. The sooner you can seal a crack the better chance you are going to be able to prolong the overall life cycle of your pavement.”
Crack sealant is considered extremely effective if applied properly. Prior to filling, the crack needs to be completely cleaned out and dry before applying a sealant.
“Crack sealant is very effective, stated Girish C. Dubey, President, STAR, Inc., Norwalk, Connecticut. “There are research reports from many state DOTs, FHWA, Army Corp of Engineers, colleges, National Center for Asphalt Technology about the effectiveness of crack sealing. All the research has concluded that crack sealing adds years to the life cycle of the pavement. The reports that compare the effectiveness of crack sealing to other treatments show that crack sealing is the most cost-effective process you can apply to pavements and should be used in combination with any other maintenance treatment to enhance the performance results for other treatments such as sealcoating, micro surfacing, chip seals and hot mix overlays.”
Cracks in asphalt pavement will occur in time. Timely treatment is the solution for adding life to paved surfaces.
Jeff Winke is a business and construction writer based in Milwaukee, Wisconsin. He can be reached through firstname.lastname@example.org