Shear Strength of Tack Coat on Flexible Pavement and Composite Pavement

Tack coat is a thin layer that spread on the surface of existing pavement or concrete pavement. The function of this layer is as an adhesive layer needed to give adhesive power between the existing pavement course and new pavement course or with the concrete pavement, therefore created one strong unity and would bond both, in order to withstand the sheer strength caused by the vehicle load on top of it. Strong adhesiveness of the tack coat course will add sheer strength between the interfaces of the pavement courses. Strong adhesiveness is gained from the asphalt mixed with solvent, therefore created the right viscosity level of the tack coat solution. This research was aimed to measure the sheer strength of tack coat by using modified direct shear asphalt samples. The research method used was by using gauge to measure the sheer strength of tack coat on asphalt sample which was the modified direct shear. While the samples were consisted of 2 types, the asphalt solvent sample as a model of flexible pavement course of AC-WC pavement type with optimum asphalt content of 6.1%, and concrete solvent sample as a model of rigid pavement. These samples were made in briquette form, made with Marshall Mould with 4 inches diameter. The tack coat solvent was made in three solvent variations, which was referring to the standard of the Directorate General of Highways. From this research, it could be concluded that the highest shear strength for flexible pavement was reached on the tack coal solvent mix composition of 30 pph and composite pavement of 15 pph. Tack coat spread rate of 0.25 l/m 2 gave highest shear strength on flexible pavement and composition pavement of 0.35 l/m 2 . Balance was reached on flexible pavement on tack coat variation of 30 pph and 50 pph; while on composite pavement, balance was reached on tack coat solvent variation of 15 pph and 30 pph. This modified direct shear sample could be a method to measure shear strength of tack coat on flexible pavement and composite pavement.


INTRODUCTION
Tack coat is thin layer that spread on the surface of existing pavement or concrete pavement, which functioned as adhesive layer needed to give adhesiveness between existing pavement course and new pavement course or with concrete pavement, to reach a strong uniformity that would withstand shear strength of vehicle load on top of it. The strong adhesiveness of the tack coat layer would add shear strength between the pavement courses interfaces. Strong adhesiveness is gained from asphalt mixed with solvent, therefore created the right viscosity level of the tack coat solution.
Tack coat could be made from rapid setting asphalt emulsion according to AASHTO M 140 (AASHTO, 2016) conditions or from cutback asphalt that fulfilled the conditions of AASHTO M 20 (AASHTO, 1970). Cutback asphalt tack coat is usually used on road construction, which is made from asphalt cement or refinery bitumen that is mixed with kerosene solvent (medium curing type). Asphalt cement is dissolved with 25-30 parts of kerosene per 100 part of asphalt, which usually called as 25-30 pph. Based on the general specification from Public Works Ministry, the Directorate General of Highways (2013) requires the using dose of tack coat is 0.15 l/m 2 -0.35 l/m 2 on porous and weather-exposed surface.
2 TACK COAT In general, tack coat is a thin adhesive layer sprayed in between the surface of existing asphalt pavement course and new asphalt pavement course, or between the composite pavements. Tack coat is smelting of liquid asphalt that rapidly evaporate on top of existing pavement course that already paved (Afrilianto, 2007). Tack coat is thin layer of asphalt that gives adhesiveness and strength between the existing pavement course and new pavement course. Tack coat could be made from emulsion asphalt of Rapid Setting type with water diluent and solvent, or from liquid asphalt (cutback asphalt) with solvent that easily evaporated. The distribution quantity of tack coat dose depends on the condition of the existing construction surface, started from 0.15 -0.35 l/m 2 with temperature of 110 ± 10˚C. The tack coat layer functioned as adhesive layer between the existing pavement course and the new pavement course, in order to give strong adhesiveness. Tack coat layering is conducted by using asphalt distributor or asphalt sprayer, by arranging the nozzle position and height to reach a well-distributed tack coat layer, according to the required distribution dose. The tack coat layer needed to be left for a while to give time for the diluent (water or oil) cutback asphalt or emulsion asphalt to evaporate. The duration for the diluent to evaporate (depends on the weather condition) known as the setting time, or sometimes the curing time. The shear resistance generated by the tack coat on the asphalt layer will increase with the length of the curing time up to a certain time limit. Longer curing time would decrease the resulting shear resistance, this tendency occurs because oil evaporation in the cutback asphalt would cause viscosity changes, thereby making the asphalt harder. If the curing time is too short, the shear resistance would also decrease because the oil in the cutback asphalt created weak bond between the layers interfaces (Rumkita & Yamin, 2006).

Forming Materials of Tack Coat
Asphalt is defined as black or dark brown coloured material, solid at room temperature, semi-solid and liquid if heated to a certain temperature. Hydrocarbon is the basic main ingredient of asphalt, which is generally called bitumen; therefore, asphalt is also known as bitumen. Composition of asphalt is consisted of asphaltenes and maltenes. Asphaltene is black or dark brown coloured material that does not dissolve in heptane, which is thick liquid that consists of resin and oil. Resin is a yellow or dark brown liquid that gives adhesive trait in asphalt, it is a part that easily lost or diminished during the road service lifetime.

Diluent/Solvent Material
The diluent/solvent material of cutback asphalt consisted of three types, which are: a) Rapid Curing Cutback (RC), in which the asphalt cement is dissolved with gasoline diluent material. RC is the fastest evaporating cutback asphalt. b) Medium Curing Cutback (MC), in which the asphalt cement is dissolved with a thicker diluent material, such as kerosene. This type is widely used, because its' relatively cheap price, and also because kerosene does not quickly evaporate, therefore giving ease of work (workability) and accuracy in researchs. c) Slow Curing Cutback (SC), in which the asphalt cement is dissolved with a more viscous material such as diesel. This type is the longest evaporating cutback asphalt; therefore it is rarely used in asphalt works.

Equipment
To measure the shear strength of tack coat on asphalt sample, modified direct shear test equipment was used. A Direct shear test equipment used was the same tool used to obtain soil shear parameters; modifications were only done on the proving ring base and the test sample base (shear box), which were adjusted to the test sample size which was 4 inches with height of ±7 cm.

Test sample
Test sample in this research consisted of two types, which were 135 pieces of asphalt mix test samples as the model for flexible pavement course, and 45 pieces of concrete mix test sample as the model for rigid pavement. The test sample was made into briquette with Marshall Mould in diameter of 4 inches. The test sample for flexible pavement was made from AC Wearing Course type with optimum asphalt content of 6.1%; and the test sample for flexible pavement was made with compressive strength on age of 28 days, not less than 350 kg/cm 2 . In order to provide test sample with condition that resembles the existing pavement condition, aging process was conducted. As quoted from Rumkita and Yamin (2006), the aging process was conducted by inserting the test sample into the oven for 5 days with temperature of 85 o C.

Tack Coat Solution
According to the standard of Directorate General of Highways, tack coat solution is of 25-30 pph with tack coat distribution of 0.15 -0.35 l/m 2 , therefore to obtain a good result, in this research three solution variations were made, which were 15 pph (one level below the standard), 30 pph (standard), and 50 pph (one level above the standard); and the tack coat distribution was 0.15-0.55 l/m 2 (two level above the standard dosage). The calculation for tack coat material composition on tack coat solution mix variation of 15 pph is as shown in Table 1 Test sample area of diameter 4 inch = 81.03 cm 2 Test load = 4 kg x 81.03 cm 2 = 324.12 kg Which could not be applied, so normal load (n) of 20 kg was used on the test (Rumkita & Yamin, 2006).
The shear box speed and the taken load could not comply with the real speed and load because of limitation of the equipment performance. By referring to previous research by (Hachiya & Sato, 1997), the speed of the shear box was set at 100 mm/min and 10 mm/min, whereas for the speed and normal load (N), Uzan et.al., (1978) set it as 2.5 mm/min with load variation of 0.05; 0.5; 1.0; 2.5; 5.0 kg/cm 2 .
The shift of the test sample on the interface layer was measured/read on the proving ring. The reading of tack coat shear strength was noticed when the test sample did not give shear resistance anymore, shown with the needle stopped on the proving ring.

Result of Shear Strength of Tack Coat on Flexible Pavement
From the reading result of shear strength value on the proving ring, the shear strength value of each tack coat variation on tack coat distribution, as in Table 2.  Table  3. It could be seen that the balance relation between tack coat shear strength with tack coat distribution was reached on 30 pph and 50 pph variations on portion of 0.3198 l/m 2 with shear strength of 372.196 kg. As for the 15 pph variation, the existing data did not show the balance relation with 30 pph and 50 pph variations. Graphic on balance relationship between shear strength and tack coat distribution is shown in Figure 2.

Result of Tack Coat Shear Strength on Composite Pavement
Test of shear strength on composite pavement resulted on shear strength for each tack coat variations on tack coat distribution, as shown in Table 4. From three of the tack coat variations for composite pavement, the result was the maximum shear strength value was 315.704 kg on tack coat distribution of 0.35 l/m 2 for 15 pph tack coat variation. Generally, it could be seen that the 15 pph and 50 pph tack coat solution variation were on portion ranged from 0.3 l/m 2 to 0.4 l/m 2 , as shown in Figure 3.  The balance between tack coat distribution and the composite pavement shear strength value could be seen in Table 5.