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Developing test method and specification limits to evaluate trackless tack and tack coats for different paving applications


Bonding of different layers within a pavement structure is critical to ensure its durability. The quality of this interfacial bond is dictated by the quality and durability of the tack coat used during pavement construction, maintenance or rehabilitation. Over the last few years, there have been several developments (some that are proprietary) in terms of the equipment to apply the tack coat as well as in terms of the tack coat material itself. Specifically, over the last few years the use of trackless tack has gained a lot of attention in the pavement industry. One of the main advantages of trackless tack is that it allows for the tack coat to remain in place on the surface even with construction traffic until the mix is placed on it. The ability to retain the desired amount of tack on the pavement surface is strongly tied to the overall durability of the bond and the pavement surface.

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Several producers have developed some variation of a trackless tack coat and have approached different roadway agencies for product approval. Agencies and in some cases producers of these products have developed several different laboratory tests to evaluate the trackless character of the emulsion and its bond strength to qualify the use of this material. There is a need to revisit these methods that have been developed, particularly over the last decade, and develop a standard test method(s), relevant parameters that are indicative of the trackless character and bond strength derived from these method(s), and tentative specification limits based on field performance and/or criteria.


The research study is intended to develop standard test method(s), parameters and tentative specification limits to evaluate the durability of tack coats and also the trackless characteristics of trackless tacks. These tests are intended to evaluate the tack coat material itself and supplement testing with mixture specimens as developed in previous studies.

Specifically, this study must consider, but not be limited to, the following aspects:

  1. Review existing literature for (i) available methods from NCHRP Project 09-40 and other studies to evaluate the quality of tack coats in the field and laboratory, and identify specific methods (field and/or lab) that can be used as the basis to validate the findings from this study, (ii) methods to evaluate the quality of the tack coats as a material including trackless tack using devices such as the DSR or pull-off tester, and (iii) methods to evaluate the trackless quality of a trackless tack.
  2. Develop and/or modify test method(s) to evaluate the performance related characteristics (e.g. bond strength, ductility while taking into account factors such as temperature sensitivity and aging) of tack coats including trackless tacks, and trackless characteristics of a trackless tack. Identify and validate parameter(s) from such test(s) using performance of the tack coat in laboratory and/or field conditions.
  3. Analyze performance requirements for different pavement structure configurations (e.g. tack coat shear strength or bonding requirements will be different when used with an ultrathin overlay), and propose specification limits for the tack coat properties measured above for different pavement configurations.
  4. Provide draft AASHTO specifications related to the methods developed above.

Tack coats dictate the efficiency of any pavement structure to function as an integral unit and perform for its intended design life. Over the past few years there has been increase in the use of preservation technologies such as thin and ultrathin overlays that induce very high stresses on the tack coat. In addition there has been an increase in the use of trackless tack to facilitate the construction of pavements and a number of producers have emerged with competing technologies. These emerging products and changes in construction can benefit the industry but there is also an urgent need to ensure that these technologies deliver what is promised. The findings from this study will help bring together and perhaps improve on the several disparate studies that are already being conducted by different states to address this issue.

Related Research:

Most of the research relevant to this topic can be classified into three categories.

The first broad category is related to the development of a mixture-on-mixture specimen test for use in the laboratory or field to evaluate the efficiency of a tack coat (1-6). For example, such tests involve the use of a composite lab or field specimen that is subjected to a direct shear, torsion, pull off or other mode of loading to induce either adhesive failure in the tack coat that binds the two specimens or cohesive failure in the specimen itself when the tack coat is exceptionally strong. This category also includes studies that have used the aforementioned test method to evaluate the optimal application rate of tack coat for different tack coat types and mixture types (e.g. optimal tack coat rate for fine vs coarse mixes). Note that these studies have focused on the use of tack coats in general and are not specific to trackless tack. The most recent large scale study on this topic is documented in NCHRP Report 712. In the context of this problem statement, it is important to review studies in this category for potential use as a validation tool in addition to field validation.

The second category of tests focus on the evaluation of the tack coat material itself without using the tack coat material between two mixture specimens. This is important from a standardization and purchase specification point of view because there is no standard asphalt mixture specimen that can be used to evaluate the bonding characteristics of a tack coat. For example, in one study the mixture-on-mixture testing format was used to evaluate the tack coat but using a polymer specimen instead of a mixture specimen to standardize the substrate type (8). Other studies have measured rheological properties of the tack coat (e.g. using a Dynamic Shear Rheometer or a pull off tester) and attempted to correlate these to the performance of the tack coat in laboratory or field (9-10). Again, these studies are not necessarily focused on trackless tack but are related to tack coats in general. Based on a preliminary review of the literature, there appears to be a need to identify such tests that can be used to screen and specify the tack coat.

The third category of tests is specific to trackless tack and relates to the ability of the trackless tack coat to retain its trackless characteristic under different conditions. A few different tests have emerged in this category. For example the ASTM D711 test with rubber gaskets and a heavy roller intended for paints has been adapted by Virginia to evaluate the trackless tacking characteristic (7). Another study reports the development and use of a DSR as a pull-off device to evaluate the trackless characteristic of the tack coat (11). This study also reports that this method was originally developed by some of the trackless tack producers to evaluate the efficacy of their products. This category is also one of main focus areas of the proposed study.


State highway agencies

Sponsoring Committee:AFK20, Asphalt Binders
Research Period:24 - 36 months
Research Priority:High
RNS Developer:Darren Hazlett, Amit Bhasin with input from AFK20 and AASHTO Committee COMP
Source Info:Members and friends of TRB AFK20 Committee and AASHTO committee on materials and pavements.
Date Posted:07/24/2018
Date Modified:07/31/2018
Index Terms:Test procedures, Tack coats, Performance based specifications, Pavements, Bond strength (Materials),
Cosponsoring Committees: 

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