LTTP back calculation results

Data obtained from FWD tests on the LTPP pavement structures was back calculated in order to obtain resilient modulus values for the different layers in each of these pavement structures. The resilient modulus values obtained from the back calculation were used as input to determine the deviator stress ratio in the BSM layers when subjected to loading, the results of which are discussed in Section 4.5.2. The method of back calculation used for the LTPP pavements is discussed in Section 3.4.1.

The process of back calculation has four requirements: deflection measurements, weight and dimensions of the loading plate, layer thicknesses and the Poisson’s ratio. This information was gathered for each of the LTPP pavements in order to perform the calculations. The layer thicknesses used for back calculation of layer stiffnesses are shown in Table 3.1 and 3.2. The deflection measurements and pressure applied by the falling load can be found on the attached CD. The Poisson’s ratio was set to 0.35 for all materials.

The resilient modulus was back calculated for all the available FWD data obtained for each of the LTPP pavements. Due to the large amount of data analysed, only a summary of the results are shown in this report. Once all the back calculations were done, the average resilient modulus for each layer was determined. The average resilient modulus of each layer for each pavement

was selected as the representative value. The LTPP pavements were analysed in both directions separately where possible.

A full account of each of the deflection bowls can be found on the CD submitted as part of this research. Table4.3provides a summary of the average back calculated resilient modulus results for each layer of each of the LTPP pavement structures. The grey squares in this table indicate that a four layer pavement structure was used for back calculation.

Table 4.3: Summary of back calculated layer stiffnesses for the LTPP pavements

Note that the N12-19(3), N12-19(4) and P243-1 are not included in Table 4.3. Deflection measurements were taken on the N12-19(3) in 2001 and 2005, but only at one location. It was therefore not deemed applicable to take one value as the average of the section. The deflection bowls for these two measurements are given in Figure G.1for 2001 andG.2 for 2005 in Appendix

G. Similarly, FWD data for the N12-19(4) was only available for two points in the BSM section. The deflection bowls for these three measurements are given in Figure G.3 and G.4 for 2001 and in Figure G.5for 2005 in AppendixG. No FWD data for the P243-1 was available for this study,

therefore, the resilient modulus of these layers were based on other layer stiffnesses with similar material classifications. The layer stiffnesses used for further analysis as well as the DSR analysis results for all the LTPP pavements are given in Table I.2toI.4in AppendixI.

Figure 4.17 and Figure 4.18 graphically illustrate the resilient modulus values of the BSM base layers obtained from the resilient modulus back calculations. The red lines in these figures indicate the maximum allowed resilient modulus for BSM 1 and BSM 2 layers according to the PN design method described in the TG2 (2009). This design method is known to be conservative, therefore it is expected that the back calculated stiffnesses would exceed these limits. In addition, it should be noted that the PN design method developed purpose-derived Mr values for its system, which is not necessarily directly comparable with the Mechanistic Empirical design method.

Figure 4.17: LTPP average back calculated stiffnesses for BSM base layers (1)

Figure 4.17and Figure 4.18shows that the resilient modulus of the BSM layer of the MR 27, MR 504 (A) and N1-1 are well above the maximum described by the TG2. The MR27 and N1-1 make use of a cement stabilised base layer and both have asphalt layers greater than 50 mm. The BSMs in these two pavements are also classified as BSM 1’s, therefore, the high resilient modulus was deemed representative of the in field conditions. The stiffnesses calculated for the MR 504 (A) are significantly higher than those fore the MR 504 (B). This is supported by the BSM classes: the MR 504 (A) implements a BSM 1 base, while the MR 504 (B) uses a BSM 2 base.

Figure 4.18: LTPP average back calculated stiffnesses for BSM base layers (2)

The N1-13 showed low resilient modulus values supported by the material classification specifying the base layer as a BSM 2. The N1-14 implements a BSM 1 base with a G4 subbase, however, this pavement showed a low resilient modulus compared to the other BSM 1 results. The resilient modulus of this layer correlates well with the high value for the BLI in Figure 4.5 and Dmax in

Figure 4.3. Therefore, the low resilient modulus was deemed a result of weak support from the deeper layers.

The BSM layer stiffnesses showed in Figure4.18indicate that the first four of these pavements have average resilient modulus values between 600 MPa and 1000 MPa. When compared to the BLI for these pavements, in Figure 4.6, a similar situation can be observed. The BLI of these pavements show that the top section of the pavements had a stiff to very stiff response to loading. The N11-8 showed a low resilient modulus value expected for a BSM 2. This value correlates well with the BLI value obtained for the same pavement.

Figure4.19shows the correlation between the BLI and the back calculated stiffnesses for the LTTP pavements. The inverse of BLI was used, rather than BLI itself, for visual effect. This graph gives an indication that the back calculation data represents the measured deflection data obtained from FWD testing. As the deflection in the base decreases (1/BLI increases) the layer stiffness should increase. The correlation coefficient (R2

= 0.8432) indicates that the back calculated resilient modulus values fairly represent the measured deflections and in turn the in field conditions.