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3.4.1 Monotonic loading behaviour

Constant strain-rate tests were performed over a wide range of strain-rates at three different temperatures, 0^◦C, 10^◦C and 20^◦C. Figure 3.14 shows the nominal stress versus nominal strain response of Cariphalte TS at 0^◦C for three selected values of the applied strain-rate ˙ǫ (similar results were obtained at other temperatures). In each test, the tensile stress increases progressively until a maximum value is reached, as for pure bitumen. This value is defined as the steady-state stress σ_ss, following the procedure described for pure bitumen. With increasing applied strain-rate, the steady-state stress increases.

Constant stress creep tests were also performed over a range of stresses and tem-peratures and selected results at 0^◦ C are plotted in Fig. 3.15. The creep response

is seen to be similar to that of pure bitumen and therefore the creep strain-rate in the secondary creep region (in which the strain varies linearly with time) is defined as the steady-state strain-rate ˙ǫ_ss at the applied stress.

The steady-state behaviour of Cariphalte TS, is summarised in Fig. 3.16 for three test temperatures tested with the steady state strain-rate ˙ǫss plotted against the steady-state stress σ_ss. The modified Cross model (2.14) with constants listed in Table 3.2 along with the Arrhenius relation for temperature-dependence is seen to describe the steady-state response of the Cariphalte TS bitumen with reasonable accuracy over the range of temperatures, stresses and strain-rates tested.

Following a similar procedure to that followed for pure bitumen in section 3.3.1, calibration curves, ˙ǫoc versus ǫ, were obtained from constant strain-rate and creep tests at various test conditions. These calibration curves are shown in Fig. 3.17: all the curves for Cariphalte TS overlap to within experimental error suggesting that the model proposed for pure bitumen in section 3.3.1 is also applicable to this polymer-modified bitumen.

3.4.2 Creep recovery behaviour

Creep recovery experiments were performed at 0^◦C and 10^◦C at selected stress levels and for unloading from total strains ǫ^T in the range 0.02≤ǫ^T ≤0.9. Typical results at 0^◦C for an applied stress level σ = 0.64 MPa and ǫ^T ≈ 0.04 and 0.14 are shown in Fig. 3.18a. Similar to the case of the pure bitumen, the Cariphalte TS exhibits strain recovery with the recovered strainǫ^r increasing with total strainǫ^T, before load release.

The results from all the creep recovery tests performed are summarised in Fig. 3.18b where the recovered strain ǫ^r (defined in section 3.2.3) is plotted as a function of the total strain ǫ^T prior to unloading. The figure reveals that, to within experimental

error, ǫ^r =ψǫ^T with the slopeψ (0≤ψ ≤1) of the line in Fig. 3.18b independent of the stress and temperature as for pure bitumen. The “recovery constant” isψ ≈0.65 for the Cariphalte TS.

The recovery calibration curve ˙ǫuc(ˆǫ^r) for the Cariphalte TS bitumen was calculated following the same procedure described in section 3.3.2 for pure bitumen. For the sake of clarity only one recovery calibration curve is shown in Fig. 3.19 for the Cariphalte TS bitumen. The recovery calibration curves extracted from a series of tests were found to overlap this curve to within experimental error.

3.4.3 Continuous cyclic loading

The strain versus time response of the Cariphalte TS bitumen with R = 0.15 is shown in Fig. 3.20 at 0^◦C, for two selected values of the mean stress σm. The cyclic stress-controlled response is similar in form to the monotonic creep response (fig.

3.15), with primary, secondary and tertiary regimes of behaviour (the tertiary regime occurs for longer loading times than those shown in the figure). The cyclic steady-state strain-rate is defined as the mean gradient of the strain versus time history in the secondary regime of behaviour, as for pure bitumen. Fig. 3.20 shows that this steady-state strain-rate increases with increasing mean stress σm for a fixed R.

Next, consider the influence of the load ratioRand frequencyf on the cyclic stress controlled response. The strain versus time history at 0^◦C with σm= 0.36 MPa and f = 2 Hz is shown in Fig. 3.21a for three selected values of R. The response for loading with σm = 0.36 MPa and R = 0.3 is shown in Fig. 3.21b for four selected frequencies f. Both these figures demonstrate that the load ratio R and frequency f have a minimal effect on the strain versus time response for cyclic stress-controlled tests, as was the case for pure bitumen.

It is worth mentioning here that, similar to the pure bitumens studied, the mean

steady-state stress versus strain-rate responses of the Cariphalte TS bitumen mea-sured in cyclic tests is well represented by the Modified Cross model (2.14), with constants unchanged from the monotonic case (and listed in Table 3.2). This further confirms the observation that the continuous cyclic response is only a function of the mean stress and essentially independent of the stress amplitude or frequency.

3.4.4 Pulse train tests

Cyclic stress controlled pulse tests were performed for a range of temperatures, pulse stresses σp and time periods ∆p/∆g (see Fig. 3.2c). Representative results for tests at 0^◦C are shown in Fig. 3.22 for ∆p = 13 s and σp = 0.4 MPa at two selected values of the gap period ∆g for the Cariphalte TS bitumen. The results clearly show that for a fixed value of σp, the accumulated permanent strain decreases with increasing

∆g, as for pure bitumen.