As the need grows for new rehabilitation methodologies to improve the performance of hot-mix asphalt (HMA) overlays on top of existing concrete pavement, saw and seal offers a cost-effective solution to the reflective cracking problem when properly constructed.
Reflection of cracks in HMA overlays represents a serious challenge associated with pavement rehabilitation. Reflective cracking is caused by discontinuities (cracks or joints) in underlying layers, which propagate through an HMA overlay due to continuous movement at the crack prompted by thermal and traffic loading. If the new overlay is bonded to the distressed layer, cracks in the existing pavement usually propagate to the surface within one to five years.
Trying not to crack
Since the early 1930s, considerable resources and efforts have been spent to find new and relatively inexpensive techniques to delay reflection cracking. Different methods, including the use of interlayer systems, have been suggested for enhancing pavement resistance to reflection cracking. Louisiana had experimented with various techniques and treatments to control reflection cracking since the 1970s; however, the performance and cost-effectiveness of these methods were not evaluated in many projects. Performance and economical assessments of these various treatment methods present a critical need to ensure successful control of this distress and effective use of available funds.
The objective of this research was to evaluate and to compare different reflection-cracking-control treatments by evaluating the performance, constructability and cost-effectiveness of pavements built with these methods across Louisiana. Results of this analysis assessed the benefits of these crack-control techniques in terms of performance, economic worthiness, constructability and long-term benefits. Based on the findings and the results of this project, recommendations for cost-effective control of reflective cracking were made.
Joint effort
State practices for control of reflective cracking were identified through district surveys and by reviewing the Louisiana Department of Transportation and Development (LADOTD) databases and Pavement Management System (PMS) data. Projects built with different crack-control-treatment methods were identified. The treatment methods that were evaluated in this research were saw and seal, asphaltic surface treatment (AST - chip seal) as a crack-relief interlayer, fiber-glass grid, stress-absorbing membrane interlayer (SAMI), fabrics and STRATA. However, there were an insufficient number of projects for SAMI, paving fabrics and STRATA interlayers to allow for drawing conclusions on the cost-effectiveness of these treatment methods.
The saw-and-seal method consists of sawing the HMA overlay to create transverse and longitudinal joints at the exact locations of the PCC joints followed by sealing of the constructed joints. Success of the saw-and-seal method depends on applying the treatment at the exact locations of the joints. Prior to the overlay, existing joints on the concrete pavement are located and marked. Joints are then re-established with chalk after the overlay. These joints are dry cut using a rideable concrete saw. The cuts are cleaned prior to placing the sealant. The cleaning process involves usage of hot compressed air to get rid of all the dust particles, loose debris and, most importantly, moisture that clings to the walls of the groove. For cleaner joints, a sand blaster may be used to remove any remaining debris. The final step is to seal the joints with a low-modulus rubberized sealant. Sealing the created joints prevents the infiltration of water and incompressible materials from getting into the underlying layers. Since water infiltration and the possible stripping of HMA accelerate pavement deterioration, sealing the overlay joints properly plays an instrumental role in extending pavement service life.
Projects in which different crack-treatment methods were used were identified. The basic requirement for a treatment to be considered as a reflective-crack-prevention technique is that it should be applied over an existing concrete layer and below an asphaltic overlay.
The performance and cost-effectiveness of the different treatment methods were assessed by analyzing performance data obtained from the LADOTD pavement-management system for the period ranging from 1995 to 2009. Digital videos, which are collected every two years for every state highway, were reviewed to identify the cracks at joints. The Reflective Cracking Index (RCI) and the Pavement Condition Index (PCI) were the two parameters used to assess the performance of the pavement sections.
A simplified economic evaluation was then performed on all the projects that were selected for detailed analysis. The adopted economic approach calculated the total annual cost (TAC) per mile for each pavement section by dividing the total cost of the project, obtained from bid items, by the performance service life in years and the length of the section. Comparison was then established between the total annual cost of the treated and untreated segments to determine cost effectiveness.
Showing improvement
For saw and seal, the majority of the sites showed a positive improvement—40% of the sections showed an improvement from one to three years and 47% of the evaluated sections showed an improvement from four to 12 years. The average level of improvement to the pavement service life due to the use of saw and seal was four years. The vast majority of the sections (80%) indicate that saw and seal was cost-effective as compared with HMA overlays without treatments. The effectiveness of the saw-and-seal treatment method depends on the success of the construction process to ensure that the treatment is applied at the exact locations of the joints.
The effect of traffic levels on the effectiveness of the saw-and-seal method was investigated. It was determined that the saw-and-seal method was more effective for low and medium traffic levels as compared with high traffic levels. In fact, the two sites in which the untreated sections outperformed the treated sections are in the high-traffic category. One possible reason for this trend is that the increase in traffic loading may result in minor rutting in the wheel paths, which may cause the sealant to come off with time and, therefore, gradually decrease the serviceability of the pavement structure. However, a survey of district engineers in Louisiana showed that most practitioners observed no difference in performance with traffic level.
For the use of chip seal as a crack-relief interlayer, the majority of the sites showed an improvement due to the use of chip seal—25% of the sections showed an improvement from one to three years and 33% of the evaluated sections showed an improvement from four to 10 years. The average level of improvement to the pavement service life due to the use of chip seal was four years. The vast majority of the sections (75%) indicate that chip seal was cost-effective as compared with HMA overlays without treatments. The increase in cost of overlay due to the use of chip-seal treatment ranged from 10 to 71%.
Sticking to a policy
Among the various treatments that were analyzed, saw and seal and chip seal as a crack-relief interlayer showed the most promising results in terms of performance and economic worthiness. Based on the findings and the results of this study, a reflective-crack-control policy was developed for Louisiana. A choice is recommended for the districts between two treatment methods that were determined to be cost-effective for the climatic and operating conditions encountered in the state:
System A: System A consists of sawing the overlaid asphaltic concrete pavement to create transverse and longitudinal joints at the exact locations of underlying PCC joints followed by sealing of those constructed joints; and
System B: System B consists of applying an asphaltic surface treatment (chip seal) as a crack-relief interlayer prior to the HMA overlay. Typical AST interlayers used in Louisiana are known as Type D and Type E. AT
