Study of Asphalt Binders using Lap Shear Bonds

by Gregg B. Babcock, Robert J. Statz, Doug S. Larson
E.I. DuPont de Nemours Inc., Wilmington, Delaware

Abstract | Introduction | Background | Purpose of Study | Experimental Design | Discussion of Results | Low Temperature Results | Future Work | Conclusions | References

Lap shear bonds are commonly used in the adhesives industry to evaluate different adhesives. Asphalt binder in a road can be thought of as an adhesive that binds rocks together. This project used Rolling Thin Film Oven Test asphalt residues to adhere inorganic substrates together in a reproducible manner to mimic the performance of asphalt binder in a road. A tensile tester was used to measure the cohesive and adhesive strength of the bond. An environmental chamber can be used to study the strength of the asphalt bonds at high and low temperatures.

The data demonstrated that some polymer-modified systems have superior cohesive tensile properties as compared to unmodified asphalt at room and higher temperatures. The temperature at which the bond fails with zero cohesive strength closely relates to the failure temperature as predicted by Dynamic Shear Rheometer measurements on the asphalt residue from Rolling Thin Film Oven test. Between 6°C and 0°C the failure mode switches from cohesive failure to adhesive failure, indicating that the bond to the inorganic substrate is not as strong as the internal bond in the asphalt at these colder temperatures.

A Dynamic Shear Rheometer (DSR) and a Bending Beam Rheometer (BBR) determine the high and low temperature properties, respectively, of asphalt. In these two Strategic Highway Research Program (SHRP) tests the asphalts have two separate geometric configurations. The authors investigated the possibility of measuring both high and low temperature properties in a test using the same geometric configuration. The method chosen was to use a tensile tester in an environmental chamber to evaluate Rolling Thin Film Oven (RTFO) asphalt residues in lap shear bonds.

Asphalt binders using five different polymer system were evaluated at temperatures from 0 to 64 °C, and tensile stress was measured at 6°C increments to mimic PG SHRP grades. The relative lap shear strength was then compared to determine the effect of each modifier on lap shear strength.

For thirty years people have used polymers to improve asphalt binders. It is believed that the polymer- modified asphalt binders perform better in road applications because they have higher strength, greater elasticity, and can perform well over a wider range of temperature extremes.

Mix tests done at two separate sites indicated that some polymers improve the split tensile properties of asphalt mixes. Measurements done by Prof. M. W. Witczak at the University of Maryland indicated that that a Reactive Elastomeric Terpolymer (RET) increases the split tensile strength of the mix by a factor of two over that of the unmodified control. Interestingly, the percent retained tensile strength after water exposure was also higher with increasing levels of the Reactive Elastomeric Terpolymer. See Figure 1 and Table 1.

Table 1. Effect of Polymer Content and Exposure to Water on Indirect Tensile Strength of an Asphalt Mix

Work done by Joe Goodrich,² Chevron Asphalt Research, indicates that 2% RET increases the split tensile strength by a factor of four when mixed with Watsonville granite. Also, RET gives the mix twice the indirect tensile strength when compared with the polymer styrene-butadiene-styrene (SBS). See Figure 2 and Table 2.

Table 2. AASHTO 283-89 Indirect Tensile Test on Mixes prepared with Watsonville Granite and Polymer-modified Asphalt Binders

Goodrich also studied the effect of RET when used in conjunction with lime on a Watsonville Granite aggregate. The test was to see if RET would have an additive effect on the anti-strip properties of the lime mix. As can be seen from Figure 3 and Table 3, lime and RET when used together do provide a additive benefit, especially in retained wet strength.

To study the observed improvement in split tensile strength, the authors used a simple lap shear test to determine if the improvements could be attributed to better cohesive or adhesive properties.

The authors thought that lap shear bonds could be used to test the strength of the binder, and to determine if failure was in an adhesive or cohesive manner. Adhesive failure is the loss of adhesion between the binder and the substrate. Cohesive failure is the loss of integrity within the asphalt itself. Adhesive failure is noted when part or all of the failed substrate is free of asphalt. Cohesive failure is observed when the binder is adhered to both substrates after failure. The lap shear test is a common test in the adhesive industry. Two glass slides are stuck together to give an overlapping area of 6.45 cm2. The slides can then be pulled apart in a tensile tester enclosed in an environmental chamber. (See Picture 1) With this equipment, the mode of failure and lap shear strength can be evaluated at different temperatures.

A Conoco asphalt (penetration 120/150) was formulated with four different polymers. They were then subjected to RTFO treatment and bonds were made between glass slides which were 8 cm long by 2.54 cm wide (See Picture 2). The bonds were produced by applying 1.0 to 1.2 grams of binder onto the slides pre-heated to 140 °C and shimmed to produce a 1.6 mm thick asphalt layer. Pressure of 1 kg per 6.54 cm² was applied to each bond (See Picture 3). The glass slides were cooled and aged for five days at room temperature.

Picture 2. Asphalt Binder applied to OverlappingGlass Slides	Picture 3. Pressure is applied to each Asphalt Bond

The glass slides were then placed in a tensile tester that was in an environmental chamber and pulled at a speed 0.51 cm/minute with a chart speed 2.54 cm per minute. Stress-strain curves were generated using this technique for all the modified asphalts and the control (See Picture 4). Measurements were made above and below room temperature in 6 °C increments. The maximum stress from the stress-strain curve was interpreted to be the adhesive or cohesive strength of the asphalt joint. By examining the glass slides, it could be determined whether the failure mode was adhesive or cohesive. The results reported are in Kg/cm², calculated as the force the tensile tester pulled on the slides at maximum strain, divided by the area of the lap shear bond.

Section III.

A series of experiments were done at room temperature to determine the reproducibility of the test. Figure 4 & Table 4 show the error bars associated with the test, run with 4.0% SBS and 1.75% and 2.25% of Polymer RET. The error bars indicate that there is not much experimental error associated with the test, and it is reproducible at room temperature. Table 4 shows that the test is reproducible, with very good standard deviations. Figure 4 shows the deviations graphically.

The same reproducibility tests were run with other polymers and temperatures. All the tests showed that lap shear testing is reproducible with various polymers and temperatures.

The lap shear tests were then run at 6 °C temperature increments starting at 0°C and up to 64°C. The 6°C increments were chosen to mimic SHRP data. Figure 5 shows the results of two levels of RET, and one level of SBS versus control on a semi-log scale. Figure 6 shows the same data on a standard scale. Looking at Figure 5, it is clear that RET and SBS increase the tensile strength of the binder at temperatures above 12 °C. The strength of binder modified with RET also increases as the polymer level is increased. As the test temperature is decreased to around 12 °C and below, the strength of all the asphalts becomes approximately equal, indicating that the polymers do not have as significant an effect on strength at lower temperatures. At temperatures below 6 °C the asphalt fails in an adhesive manner, or the binder looses adhesion to the glass substrate. At temperatures above 6 °C the asphalt fails cohesively, or the binder breaks prior to losing adhesion to the glass substrate. The data is summarized in Table 5.

The switch from cohesive to adhesive failure at temperatures below 6 °C implies that cold temperature failure is not caused by a lack of ductility within the asphalt. However cold cracking might be caused by a failure of the asphalt/aggregate bond. This can question the effectiveness of some cold temperature tests that are strictly run on binder, and test only the ductility of the binder.

In the stress-strain curves generated from lap shear experiments, it is possible not only to determine the maximum lap shear strength, but also to look at elongation of the sample at this maximum strength. This shows us that as we lower the temperature of the lap shear specimens, the elongation at which this maximum strength is observed is reduced. This indicates that the asphalt is becoming brittle at lower temperatures.

Interestingly the lap shear test indicates that the failure temperature of the unmodified control would be around 52 °C. This closely mimics the 58 °C predicted by SHRP DSR. The 2.25% RET sample had a predicted failure of 70 °C on the DSR, which is about what the lap shear test would predict.

The same tests were repeated with two other polymers, except that, to reduce the number of experiments, 12°C increments were chosen. Figure 7 shows a graphical representation of this data. Clearly, at temperatures at and above room temperature, 4% loadings of both styrene-butadiene rubber (SBR) and ethylene vinyl acetate (EVA) do not increase the cohesive strength of the asphalt.

At temperatures of 0 °C and below, the glass slides break prior to the asphalt failing in lap shear. For that reason, metal slides were used and run between -6 °C and +6 °C. The failure method at these cold temperatures continued to be an adhesive failure. As can be seen in Figure 8, the relative strength of the modified asphalt is essentially the same as unmodified between -6 °C and +6 °C.

Figure 8. Lap Shear Strength
of Asphalt Binder (with and without Polymer)
on Metal Slides at Cold Temperatures

Cont. = Control, Conoco 120/150 asphalt; RET = Reactive Elastomeric Terpolymer

The authors plan to evaluate the lap shear strength at colder temperatures using a substrate with a coefficient of expansion and contraction that is similar to that of asphalt, likely polypropylene. We feel that the thermal expansion of metal is sufficiently different from asphalt or aggregate that the cold temperature work should be re-evaluated. The authors also plan to evaluate a substrate with a rough surface over the entire temperature range.

Lap Shear tests could also be used to study the failure mechanism for stripping, and be an easy way to determine if a polymer affects the stripping performance of a given asphalt. We plan to prepare the slides, then condition them in warm water. Lap shear will then be used to determine the shear failure mode and the relative strength of the polymers.

The authors also believe that the elongation data from this test should provide meaningful data about the binder's performance in mix. The elongation at maximum lap shear strength decreases with decreasing temperature, possibly indicating the asphalt is becoming more brittle. We plan to study the relationship of elongation to the stress/strain curves.

1. Lap shear testing for asphalt properties is a reproducible test that can quickly be used to test high temperature binder properties.
2. The results of lap shear testing appear to agree with high temperature SHRP DSR measurements.
3. This test does show an increase in lap shear strength for binders modified with some polymers.
4. Indicated binder failure mode at temperatures above 6 °C is cohesive failure. Starting around 6 °C and colder, failure is adhesive, indicating that cold temperature failure of a road may be the result of loss of adhesion to the aggregate.

1. Witczak MW, Hafez I, Qi X. "Laboratory Characterization of DuPont Elvaloy^° Modified Asphaltic Mixtures", DuPont Company, January (1995).
2. Goodrich JL. "Asphaltic Binder Rheology, Asphalt Concrete Rheology, and Asphalt Concrete Mix Properties", Journal Association of Asphalt Paving Technologists 60 80-120 (1991).
3. STM D4896-95 "Standard Guide for Use of Adhesive-Bonded Single Lap-Joint Specimen Test Results".
4. ASTM D2872-88 "Standard Test Method for Effect of Heat and Air on a Moving Film of Asphalt (Rolling Thin-Film Oven Test)" (1995).
5. Deme IJ, Deserres MC, Lenters JR. "Effectiveness of Multigrade-Type Bitumens in Preventing Pavement Cracking and Rutting" Proceedings Canadian Technical Asphalt Association 40 100-136 (1995).