Push-through shear test

The push-through shear test was performed to determine the bond-shear strength of the adhesive surface between the EPS and WPC. An adhesive with a high shear strength might cause rupture or cracks in the EPS, whereas a low shear strength might cause cracks or slippage between the EPS and the WPC (in the adhesive layer). However, if the adhesive has insignificant shear strength, the EPS and WPC can separate under load-bearing conditions, which are not desired. The Zwick Z250 was used to conduct this experiment.

7.7.1. Test methodology

This experiment was performed for the two adhesives as discussed in Chapter 6 and Section 7.1. For each adhesive, five specimens were tested at a displacement-rate of 1.3mm/minute. The test specimens have a depth of 100mm. A test specimen as well as the way in which the load was applied is shown in Figure 7.29.

The tests were conducted at a temperature of 22.4°C and a humidity of 70.9%. The adhesive was applied as described in Table 7.1. The Pekadur A663 polyurethane adhesive was cured for 20 hours before testing. The test-setups of the Pekadur A663 polyurethane adhesive and the GB685 spray grade rubber adhesive are shown in Figure 7.30 and Figure 7.31, respectively.

Figure 7.29: Schematic representation of push through shear test specimen with the applied load

Figure 7.30: Push-through shear test-setup for the Pekadur A663 polyurethane adhesive

Figure 7.31: Push-through shear test-setup for the GB685 spray grade rubber adhesive

7.7.2. Results

Half the applied load is transferred to each of the supports, which ensues equilibrium of the specimens.

Therefore, the shear force that passes through the adhesive at the supports is half of the applied load. The shear stress is calculated by dividing the shear force per bond surface by the shear area. The Zwick Z250 was used to determine the force as well as the deflection of the specimens. The two adhesives are discussed individually and then compared to each other.

The Pekadur A663 polyurethane adhesive

Figure 7.32 illustrates the results of the push-through shear test for the specimens tested with the Pekadur A663 polyurethane adhesive. At the peak shear stress, an initial rupture in the EPS occurred as shown in Figure 7.33. After this rupture, the specimen deforms. This resulted in the EPS of Specimens 1, 2 and 3 to move over the support. Therefore, in Figure 7.32, these specimens have different plastic behaviour in shear.

Specimens 4 and 5 represent pure shear, since after deformation of the specimen, the EPS remained unsupported.

Figure 7.32: Shear stress for Pekadur A663 polyurethane adhesive

Figure 7.33: Initial rupture in the EPS when Pekadur A663 polyurethane adhesive was used 0

10 20 30 40 50 60

0 5 10 15 20 25

Shear stress (kPa)

Deflection (mm)

Specimen 1 Specimen 2 Specimen 3 Specimen 4 Specimen 5

The results of this test for the five specimens are shown in Table 7.7. The results show a low standard deviation and a low coefficient of variation, which is desired.

Rupture occurred in the EPS, since the adhesive has a greater shear strength. This results in the EPS having an average yield shear stress of 46.662kPa. If Specimens 4 and 5 are considered (pure shear), the force decreases after initial rupture occurs, but some load is still sustained. This is due to the EPS pushing against the WPC after initial rupture has occurred. The initial crack propagates through the depth of the specimen parallel to the plane of the adhesive. This is shown in Figure 7.34.

Table 7.7: Results of the push-through shear test for the Pekadur A663 polyurethane adhesive Average maximum (yield) shear stress 46.662kPa

Standard deviation 1.186kPa

Coefficient of variation 2.542%

Characteristic shear stress (5%)

(Equation 7.3) 44.717kPa

Figure 7.34: Total failure of push-through shear test for Pekadur A663 polyurethane adhesive

GB685 spray grade rubber adhesive

The results of the push-through shear test for the GB685 spray grade rubber adhesive specimens are shown in Figure 7.36. The first neck of the curve illustrates the shear stress yield point. At this point, the EPS and WPC slide over each other in a vertical plane, but is still bonded by the adhesive as shown inFigure 7.35.

The specimen remains able to transfer the shear load to the supports, until total failure occurs. Total failure occurs when the applied load cannot be transferred and the specimen fails to transfer the load as shown in Figure 7.37.

Figure 7.35: EPS and WPC separate (push-through shear test, GB685 spray grade rubber adhesive)

Figure 7.36: Shear stress for the GB685 spray grade rubber adhesive

Figure 7.37: Total failure of push through shear test for the GB685 spray grade rubber adhesive

0 1 2 3 4 5 6 7 8 9

0 5 10 15 20 25 30 35 40 45 50 55 60 65

Shear stress (kPa)

Deflection (mm)

Specimen 1 Specimen 2 Specimen 3 Specimen 4 Specimen 5

Table 7.8 illustrates the results of the five test specimens. The results display a desired distribution of average maximum shear stress, since the results express a low standard deviation and coefficient of variation.

This shear stress of the adhesive is less than the shear stress of EPS (46.662kPa, determined from the push-though shear test of the Pekadur A663 polyurethane adhesive), which results in failure of the adhesive.

Table 7.8: Results of the push through shear test for the GB685 spray grade rubber adhesive Average shear ultimate stress 7.593kPa

Standard deviation 0.198kPa

Coefficient of variation 2.614%

Average shear yield stress 6.337kPa Characteristic shear ultimate stress 7.27kPa

Comparison of the two adhesives

The average shear stress obtained by the push-through test for both adhesives is shown in Figure 7.38. From this figure, it is clear that Pekadur A663 polyurethane adhesive can resist more shear stress than that of when the GB685 spray grade rubber adhesive is used. This is due to the failure of the EPS when Pekadur A663 polyurethane adhesive was used. In contrast, the adhesive failed when the GB685 spray grade rubber adhesive was used. Thus, according to these results, Pekadur A663 polyurethane adhesive shows better performance under shear loading.

Figure 7.38: Comparison of the average shear stress for both adhesives (push-through shear test)

46.662 46.662