A Field Test to Determine Tack Coat Adequacy

    Vivek Tandon, PhD, PE Associate Professor, The University of Texas at El Paso, El Paso, Texas, USA.

    Tack coat is a light application of emulsion or bituminous binder between existing roadway surface and the new overlay. The main purpose of the tack coat is to bond the two layers to prevent slippage and efficiently transfer traffic loads from overlay to the existing surface layer. In the case of poor quality or improper application of the tack coat, the bond between the layers diminishes and the top layer slips away (figure 1). The end result is the development of various types of distresses in the pavement and the significant reduction of the life of the overlaid pavement system. State roadway agencies have traced failure of overlays to tack coat.

    Tack Coat
    Figure 1
    Tack Coat
    Figure 2

    Although various laboratory devices exist for the evaluation of tack coat quality, an affordable device to evaluate quality of tack coat at the time of application in the field is not available. A study was conducted by the University of Texas at El Paso to develop the same under the guidance of Texas Department of Transportation (TxDOT). Based on the study, a UTEP Pull-Off-Device (UPOD) was developed (figure 2) to evaluate tack coat quality in the field. The device weighs about 16.6 lb (7.53 kg) and levels itself with the help of pivoting feet. It has a weight key on the top, which provides stability while placement of loads. A 3/8th in. (9.53 mm) nut fits a 3/8th inch drive torque wrench, which is used to pull the plate up from the tack-coated surface. The device consists of a 5-in. (127mm) diameter aluminum contact test plate.
    Tack Coat
    Figure 3

    To add flexibility to the contact plate, a 5 in2 3M double-sided tape is used and attached to the aluminum contact plate and 5 in2 moisture bearing foam is placed over the tape. The advantage of the moisture bearing foam is that it can be easily peeled off the double sided tape and four to five tests can be performed before the adhesive layer (double sided tape) needs replacement. The 5 in. aluminum contact test plate and placement of adhesive layer as well as moisture bearing foam is shown in figure 3. A 6-150 in.-lb (152-68 mm-Kg) torque wrench is used for measuring pull-off torque. This torque is converted into load by using calibration factor of UPOD.

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