Prolonging concrete pavement life

December 7, 2016

joints_opener
All images courtesy Seal/No Seal Group

by Scott Eilken
A concrete pavement’s longevity can be affected by several factors, including the design and construction of the transverse joint. Research was recently conducted to determine what conditions will influence the performance and life cycle cost of a sealed pavement joint, and to understand how the initial conditions affect the joint and sealant.

This research involved evaluating and developing the best practices related to joint sealing with the intent of extending the life of concrete pavement. To accomplish this, the Seal/No Seal Group (an industry-formed coalition committed to the long-term effectiveness of sealants used in concrete pavement) began working with Dan Zollinger at the Texas Transportation Institute (TTI) to study the various factors over an extended period.

Zollinger’s research focused on sealant effectiveness on jointed concrete pavement performance. The newly released outcome will likely be useful to pavement designers, contractors, state departments of transportation (DOTs), and maintenance agencies making critical design-related decisions on using sealants with respect to long-term performances. (To learn more about this research and its results, read a complete version of Zollinger’s report, Qualification of Joint Sealant Effectiveness Regarding Jointed Concrete Pavement Performance, visit sealnoseal.org/PDF/TechBrief/Tech_Brief_SNS_TTI_Report.pdf[1].) Such decisions include striking a balance between traffic, climate, and base erodibility to minimize costs, streamline maintenance, and improve performance and pavement serviceability.

Why is this important?
As cost pressures continue to increase, there has been an increased interest in eliminating joint sealants as a means of lowering the initial cost of concrete pavements. However, there has also been a lack of data in the industry to instruct owners on sealant effectiveness and the long-term impact of using or forgoing them.

The primary purpose of sealing joints in rigid pavement is to reduce the amount of water and incompressible infiltration in the structure. Both issues contribute to various distress types that eventually deteriorate the structure and result in decreased service life. For instance, an inevitable consequence of water infiltration through joints in concrete pavement is the erosion at the slab/subbase interface. Subbase erosion directly contributes to joint faulting.

Faulting is a major distress type in seen jointed concrete pavements, and a key feature in designing them, as its effects have structural and serviceability implications. The effectiveness of joint sealants in protecting jointed concrete pavement against water-related distresses has been a focus of interest.

Sealant challenges
For more than a decade, both formal and informal studies on the effects of joint sealing—funded by state agencies, Federal Highway Administration (FHWA), and National Cooperative Highway Research Program (NCHRP)—have focused on the question of ‘to seal or not to seal’ joints in concrete pavements. These studies variously involved performance data, field cores, field observations, personal opinions, drainage modeling, and statistical analysis. Their results have largely lacked evidence supporting the idea using sealed joints in concrete pavements is beneficial.

Out of concern this lack of compelling beneficial evidence has caused several agencies to elect not to seal pavements, the Seal/No Seal Group joined Zollinger in conducting a study on sealing effectiveness. However, this was not an attempt to research sealant effectiveness through traditional approaches (such as characterizing sealant performance in terms of joint seal properties). Instead, a more rigorous and fundamental approach was implemented, focusing on the amount of water infiltration through the joint and its consequential impacts on subbase erosion and pavement distress.

A field-testing program was carried out at the Riverside Campus of Texas A&M University on SR-59 near Joliet, Illinois, and on the site of the Specific Pavement Studies-2 (SPS-2) experiment, Strategic Study of Structural Factors for Rigid Pavements, on I-10 in Goodyear, Arizona. The goal was to study the effectiveness of different sealant types several years beyond installation—after failure conditions begin to manifest and limit surface drainage-related infiltration of the joint—under different degrees of failure, as represented by various joint-openings and bonding conditions.

Testing approach
This study characterized joint infiltration as a function of construction quality as well as environmental factors. The impact of construction was assessed as a function of joint cleanliness, sealant damage, joint movement, and sealant type, and the environmental factor as the predicted infiltration resulting from rainfall intensity, joint geometry, and cross slope.

Three sealant types were evaluated: silicone, hot pour, and compression seals. The impact of joint infiltration on pavement performance was determined by conducting laboratory tests to quantify subbase erosion, and by developing a predictive model and supporting software. With these tools, an owner can determine the impact of sealing joints, as well as joint conditions on pavement performance. Another aspect of the study evaluated the use of ground-penetrating radar (GPR) to detect the existence of moisture under the slabs in the vicinity of the joint.

One age-old question on this matter is: ‘When is it all right to reseal?’ This was resolved by applying weighting factors to a sealant condition, such as amount of adhesive and cohesive failure, missing sealant, etc. However, none of these surrogates addresses fundamental properties related to actual performance. With the use of GPR, it seems feasible to detect the existence of moisture under the slabs in the vicinity of the joint—from a water infiltration standpoint—and more importantly, to assess when a sealant is no longer effective.

Onsite testing
The study consisted of both controlled field experiments and testing of in-service pavements. The former were conducted at TTI’s Riverside Campus (Figure 1), and designed to evaluate the effects of sealant damage and joint cleanliness on infiltration rates. After the first round of testing was conducted using this setup, a unique approach was developed. This approach allowed the width of the joint opening to be varied, then taken into account along with the joint seal damage.

Testing of in-service pavements occurred on Route 59 (Plainfield, Illinois), and on I-10, just west of Phoenix, Arizona. The purpose of testing in-service pavements was to relate sealant effectiveness (i.e. infiltration rates) to actual pavement performance. The Plainfield project was approximately four years old and consisted of 10 test sections, including sealed and unsealed joints. The Phoenix project was a federally-funded, Long-term Pavement Performance (LTPP) Specific Pavement Study focused on Rigid Pavement (SPS-2), which was 20 years old and consisted of 20 test sections, including four different base types. The procedure consisted of conducting infiltration and falling weight deflectometer (FWD) testing (Figure 2).

Limited GPR testing was also conducted at each of the sites to evaluate the potential for using GPR to detect when the sealant was allowing water infiltration into the joint. A handheld, portable GPR unit was used for this testing (Figure 3). Subbase samples were also retrieved through core holes in the pavement to enable laboratory erosion testing using a Hamburg wheel tracking device (HWTD).

The HWTD test consisted of two component layers: a concrete cap on top, with another layer placed immediately under it. As a wheel passed on top of the two layers, the sensors recorded the deflection for each pass. The testing was conducted under wet conditions, in which erosion normally occurs due to mechanical and hydraulic shearing on the subbase layer generated by slab movement under an applied load. Erosion resistance (ER) is defined as the amount of erosion at 1,000,000 load applications under HWTD erosion testing. A greater ER indicates a subbase or subgrade material has less resistance against erosion.

Research findings
One of the most important effects of joint sealant effectiveness on concrete pavement performance is related to its potential for subbase erosion. The focus of this study is to link joint seal effectiveness at an age beyond the initial performance period to the time faulting would initiate through the use of a prediction model, addressing the potential for erosion before faulting occurs.

One Excel application demonstrates several aspects of sealant effectiveness on pavement performance. This spreadsheet, developed by Zollinger, uses a mechanistic-empirical approach to consider three main elements of subbase erosion:

Additional findings showed:

Mechanistic-empirical fault prediction model
A mechanistic-empirical fault prediction model—previously developed under National Ready Mixed Concrete Association (NRMCA)—was improved upon as part of this research. The impact of joint seal effectiveness was directly examined within the fault prediction model. One important factor addressed in the model was a means of evaluating the number of wet days based on water existing underneath the slab at the slab/subbase interface. The figure is not only a function of annual rainfall, but also surface inflow, sealant effectiveness, and subbase drainability.

The erosion resistance of materials, number of wet days, and traffic load were defined and coupled in this model to effectively analyze the potential for faulting and erosion in jointed concrete pavements. The model uses faulting/erosion as a function of the number of load repetitions with respect to wet days and the erosion resistance of the subbase.

The erosion model follows the Gumbel cumulative probability function, which refers to structural damage due to aging and loading overtime or traffic. The model calculates erosion in percentage equal to the ratio between the current faulting and total amount of faulting. To convert the daily traffic to an erosion-based equivalency, a traffic model was incorporated into the analysis process. The determination is expressed in terms of an equivalent single axle-load, having included factors such as lane distribution, equivalent load, equivalent axle, equivalent wander, and an estimated number of trucks. The model can be calibrated for local conditions as a function of distinct characteristics of the subbase or subgrade—an important capability in life cycle analysis. The model has been successfully implemented into a spreadsheet format. Results show the model fits well with the field data and can be implemented for design and maintenance management purposes.

Conclusion
Results confirm if joint seals are properly installed, and can therefore be very effective in preventing moisture infiltration and performance issues related to erosion damage. Thorough use of the mechanistic-empirical fault prediction model, one can determine the effectiveness of sealant in pavement sustainability. Unsealed joints had significantly higher flow rates compared to joints with varying degrees of damaged sealants. The results of this study also demonstrated the positive effects of proper sealant installation on joint seal drainage performance. Using this model, DOTs and contractors can determine how to effectively seal joints so the pavement can last longer.

Scott Eilken is a co-chair of the Seal/No Seal Group and owner of Quality Saw & Seal. He also provides information and makes presentations on best practices for sealant installation. Eilken is active in the International Grooving & Grinding Association (IGGA). He can be reached via e-mail at seilken@qualitysawandseal.com[4].

Endnotes:
  1. sealnoseal.org/PDF/TechBrief/Tech_Brief_SNS_TTI_Report.pdf: http://sealnoseal.org/PDF/TechBrief/Tech_Brief_SNS_TTI_Report.pdf
  2. [Image]: http://www.constructionspecifier.com/wp-content/uploads/2016/12/Figure-2-1.jpg
  3. [Image]: http://www.constructionspecifier.com/wp-content/uploads/2016/12/Figure-3-1.jpg
  4. seilken@qualitysawandseal.com: mailto:seilken@qualitysawandseal.com

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