A Cold-Mix Performance Evaluation by City of Lexington, Ky.
Contributed by Streets & Roads Division in Lexington, Ky., directed by Sam Williams
Striving to improve the overall handling process for potholes, The City of Lexington (Kentucky) Streets and Roads Department initiated a cold mix evaluation program to identify possible advantages using alternative pothole repair material. Multiple parameters were considered in determining which candidate materials would be compared. The four primaries are as follows:
- Survivability – time materials stays in the repair
- Workability – ease of use during installation
- Total cost – initial, installation and re-repairs
- Availability in the Lexington area in bulk and bag quantities
The higher cost of the Unique Paving Materials’ (UPM)Permanent Pavement Repair Material was initially a detriment in considering premium cold mixes; however, if the labor and equipment commitment to street repair could be reduced through elimination of redundant repairs, the overall net gain would be substantial and far greater than the increased cost of material. The net result would be increased resources availability to the city.
This line of thought is echoed in the Strategic Highway Research Program (SHRP) report: SHRP-H-353, Innovative Materials by Thomas P. Wilson
“Utilize the best materials available to reduce re-patching,” Wilson wrote in the report. “The cost of patching the same potholes over and over because of poor-quality material quickly offsets the savings from purchasing a less expensive cold mix. In most cases, the poorer performance associated with inexpensive cold mixes will result in greater overall costs for patching because of increased costs for labor, equipment, traffic control,and user delay.”
Reviewing multiple commercial cold mixes, three products claiming superior performance the Lexington Streets and Roads Department selected the following:
- UPM bulk permanent road repair material
- Mago bulk cold mix
- QPR bulk cold mix
Field trials started in December 2011 and will continue for at least a year. Throughout the trial preliminary reports will be issued. Multiple installations both pothole and utility cuts will be photographed and rated.Findings from the field trial will be incorporated into performance specifications used to select the best-in-class cold mix products for the City of Lexington, Ky. The report summarizes the first 11 months from January through November 2012.
Early in the program it was determined that the UPM bulk material was superior to other candidate materials based on workability and survivability. Due to the observed performance differences and limited resources, it was decided to focus on the UPM material.
Overall the UPM material is outperforming previously used cold mix materials and candidate materials initially included in the evaluation based on survivability and overall lower costs. The greater overall value of this material was quickly obvious through elimination of frequent re-repairs.The summary graph following, highlights the number of potholes recorded for the nine year period from 2003-2011 (January through November) with an average of 17,006 per year.
Starting in 2012, the number of recorded potholes decreased to 5,852, a reduction of 66 percent, significantly below the previous nine year range.
Reference data provided by the City of Lexington Division of Streets and Roads:
The city’s centralized service and information contact center, LexCall, handles thousands of calls each winter from citizens reporting potholes.
The Division of Streets and Roads is extremely aggressive and pro-active in monitoring the city’s 2,124 lane miles it’s responsible for repairing and maintaining. This combination of documenting and reporting allows the city of maintain accurate records of repairs and costs attributed to them each year.During the months of January to November 2003-2011, the City of Lexington spent $544,203 repairing potholes. This is attributed to 17,006 potholes documented during this time period. Potholes were counted independent of being newly formed or a re-repairs.
It is common to re-repair potholes; repair frequency is typically related to repair material quality. It was determined in a fiscal review of costs attributed to pothole repairs in 2011 that the cost to repair an average size pothole was $32.
For comparison the $32.00 per pothole will be used to compare pothole repair costs.In 2012, during the 11-month period (January-November), the city repaired 5,852 potholes spending $187,264, significantly less than the nine-year average. This represents a $356,939 savings or a 66 percent annual savings for the city. The primary contributor to the reduction in potholes is a cold mix material that goes in the hole once and stays there.The absence of multiple re-repairs has resulted in the city saving two thirds of their average budget for the first three eleven months of the program.
Increased savings are expected in the upcoming years. In addition to these remarkable savings, the city reports this is the first time in 9 years they have not had a claim for vehicle damage or injury caused by a pothole on any of the 2,124 lane miles within the city’s jurisdiction.
General observations by program participants: Pothole count is 66 percent lower using the UPM material relative to other cold mix materials, according to the Lexington Streets and Roads Department. Controlled performance evaluations like that conducted by Lexington are the best method to determine the value using a premium cold mix.
The process is difficult and can be time consuming; however, the benefits in developing the correct pothole repair strategy are significant.Director Sam Williams reported that in spite of the warmer temperatures, the city still experienced the freeze-thaw cycles normally expected during the winter months and attributes the cost savings to a high-performance pothole repair material that works. When considering candidate cold mix suppliers it’s important to note different business strategies.
When selecting a cold mix product, it is reasonable to assume there will be questions, training and general support issues.
Unlike hot mix asphalt specifications, which may be state regulated and application specific, design and performance specifications for cold mix do not exist. Without an effective quality control (QC) program, performance variability will be highly variable.In addition to the pothole count metrics maintained by the city, maintenance crews were requested to comment on overall handling and performance of the candidate cold mix products.
As crews patch potholes, it is difficult to record details of every installation as each is slightly different due to the severity of repair and quality of surrounding pavement. Completed repairs not requiring follow up re-repair quickly drop from the maintenance schedule. This creates the opportunity to focus on newly formed potholes or other maintenance projects.
After the five-month winter evaluation, all crews agreed the UPM material worked better than other cold mixes used through the previous nine years. Now more than ever, federal, state and local governments recognize the importance of spending their limited budgets on a product that works.
And, while they understand they may pay a little more upfront for a premium product, both the cost savings and fiscal responsibilities have provided enormous dividends for them and their departments in the eyes of the administration and the tax payers.
The total cost including pothole repair and re-repair should be considered in the cold mix selection process to maximize value to the city. As a result, a lower cost product may save initial dollars; however, it is the total cost impacting the budget. As an example, comparing two cold mix products: UPM at $100/ton and a competitor at $85/ton. The initial saving is $30,000 dollars based on 2,000 tons per year and difference in survivability rate between the two products of 45 points. (Survivability being the time material stays in the repair). However, the total impact to the budget is calculated at a $135,000 loss due to increased labor and equipment.
When selecting cold mix, the total cost of repair is the amount influencing budgets, not the initial difference in price. If the cheaper cold mix does not perform, its value is low at any cost.
Comparing the cold mixes using performance parameters known to correlate with field performance scores the QPR mix at 63.8 percent or a “D.” Cold mixes scoring less than 80 percent will experience early failure. Parameters are weighted based on comparative field-lab correlation; creating a balanced rating, if achieved will guarantee superior field performance. All parameters are interdependent; overall field performance deteriorates rapidly if any one parameter fails.
Cohesion & Adhesion: Measures the ability of the material to bond to itself and to the surrounding pavement. Performance related to the capability of the material to remain in place. Maintaining adequate adhesion and cohesion will determine the degree to which material ravels from the pothole. Workability & StabilityMeasures the force required to move and apply the material at greater than 72OF. Workability and stability must be balanced so that the material can be applied and yet stable enough to handle traffic loads. These are a function of gradation, viscosity, and application temperature. Cold WorkabilityMeasures the force required to move and apply the material in cold weather, less than 32OF.
Measures the thickness of the asphalt surrounding the aggregate. Testing for the effective film takes into account aggregate gradation and absorption characteristics. Film thickness determines shelf life, cohesion, and workability.
Sieve Analysis: Cold mix is 95 percent aggregate. Proper gradation is critical to achieve proper compaction, workability and load carrying strength. Surface characteristics, area and absorption are critical to control stripping and workability.
Stripping: Measures the separation of asphalt from the aggregate. Performance relates to water handling characteristics of material in wet environment (i.e. heavy rain).
The program identified a superior performing cold mix expected to provide significant annual saving to the city and drivers throughout Lexington.
Those individuals participating in the program experienced firsthand the performance advantages working with a premium cold mix engineered to perform.
Incorporating local weather, road condition and local maintenance crews yielded results that are directly applicable to maintenance practices in the area.
It is recommended that those interested in improving their overall maintenance strategy relative to pothole repair, design and implement a controlled performance evaluation with technically leading suppliers.
This article is contributed case history from Unique Paving Materials/the Streets & Roads Division in Lexington, Ky., directed by Sam Williams