There has been a lot of research done on whether it is better to rout cracks or not. To make a long story short it is not clear cut. The long story is that it depends on whether you are taking a short- or long-term view. Like many things a higher up-front investment gives you a longer term outcome. A lot of research shows that cracks that are routed last longer than cracks that aren’t routed.
The trade-off is that it is more expensive to rout cracks for several reasons. First of all you have a higher labor, time, and equipment cost. It simply takes more time and equipment to rout a crack than to simply clean it. The other factor is as you in effect make the crack larger you will need more overall sealant for the crack. So your upfront costs include labor/time, equipment, and material.
So what are the actual differences? How much longer do routed cracks last? How much more expensive is it to rout rather than clean? We’ve surveyed a field research study done in Texas to look a little more closely at the issue.
Before we dig into the issue we want to make another important point. When you zoom into a crack what is the difference between a routed crack and a non-routed crack? A routed crack will have vertical walls so the interface between the sealant and the crack is flat. When you have a flat interface between the sealant and the crack your seal is only as good as the chemical bonding strength of your sealant.
For a non-routed crack the story is not so simple. The contours of the crack will have underhangs and overhangs. So now your sealant is seeping into the crevices of your crack and not only do you have a chemical bond, you also have a mechanical bond. This is a subtle point but an important one. From a purely intuitive perspective it would seem that a non-routed crack gives you an additional level of mechanical bonding beyond the chemical bonding that a routed crack does not. It would seem that the most important factor is to clean your crack. Routing is a brute force way to clean your crack, but the downside is that you are giving up mechanical bonding for a clean chemical bond. By thoroughly cleaning a non-routed crack you will remove any loose material that would deteriorate your mechanical bond. But importantly you are maintaining the ability for the sealant to interlock with the crack interface. So let’s get to the research and see what the actual differences are.
The Findings
A study done by Texas State University showed that the initial cost of routing increased costs by 45%. Overall when looking at a 35 year time frame they found that routing reduced costs by 24%. This clearly shows that routing has a higher upfront cost with a long-term benefit. When comparing routed sealing and non-routed sealing to other types of surface treatments the benefits are clear. These two types of treatment can be considered as the logical alternatives.
Compared to other maintenance programs due to its economic benefits and vital role in extending the life of pavement, studies have shown that chip seal treatment cost 3-14 times more than crack sealing and an overlay costs 8-26 times as much as crack sealing. It is common to refer to routed crack repair as crack sealing and non-routed crack repair as crack filling. Crack sealing is defined as using a router to create a reservoir or routed channel in a crack. After that the routed channel is filled with a sealant material. On the other hand, crack filling is defined as minor crack preparation, such as using an air gun to blow debris out of cracks, prior to installation of the sealant. There is no pavement removed with crack filling.
With crack sealing treatment, cracks are routed to a predefined geometry, cleaned, and materials are placed into it in order to prevent the intrusion of water into the pavement surface through the upper surface. Routes are generally given with a width to depth ratio of one or greater than one that can enhance the sealant performance. Crack cleaning is the most important phase of a successful installation as it reduces the adhesion failures between the sealer material and the sidewall of the crack. High pressure air blasting is effective and efficient for removing dust, debris and some loosened asphalt concrete fragments. Wire brushing or hot lancing can also be effective ways to clean and prepare cracks.
For the study we are reviewing a survey of crack sealing and filling procedures was developed and distributed in Texas. Texas does not currently require routing, but decides on a case by case basis whether it is needed. Questionnaires were sent to 25 districts, responses were received from 19. In Texas, no district practice crack sealing however one of the responses (Amarillo) provided that they have routing practice in the past but leave it due to time consumption and equipment issues. Durability of crack sealant application varied from 3 to 5 years. Of all those districts that responded, six of them stated that blowing out the debris from cracks with air has seemed to get them clean enough to seal and ensure good success with the current method. Five districts responded that crack sealing is a costly practice. Other districts mentioned that this practice is uncommon and do not have proper guidelines. Also, one district stated that they do not have any idea about its benefit.
In the survey they were asked to estimate the typical lifespan for crack sealing and crack filling on both major and minor roads. The majority of the respondents think crack sealing on both major and minor roads can perform for 5-10 years, but that crack filling will only last 1-4 years. Other studies have reported that crack sealing without routing configuration using hot-pour sealant materials have a typical life cycle of 3-5 years. Additional studies reported that their prediction model indicated a life span of 3-6 years for crack filling treatment. According to literature review and survey from the Texas districts, the research team considered the pavement could withstand crack filling for 3 years and crack sealing for 5 years.
After looking at this research it’s hard to draw a hard and fast conclusion. The evidence does seem to suggest that routed crack sealing does last longer on a longer time-frame of 5-35 years. The evidence suggests that non-routed crack filling on average lasts 5 years. But here’s the thing no crack repair procedure is permanent. Some percentage of your cracks will fail every year. One of the largest factors in cracking is freeze-thaw cycles. This may be a larger factor than whether your cracks are routed or not. The important issue with crack filling may not be whether you save 45% of your upfront cost to then spend an added 24% in the long-term. The more important issue is to prevent water from getting into your cracks to begin with. Water getting in leads to much greater long-term costly damage than the difference between routed and non-routed cracks. Don’t let the perfect be the enemy of the good. But then again if you are going to do it do it right. Here’s a link to the study we surveyed so you can dig deeper if you like.