LTPP FWD results

FWD tests were performed on the LTPP pavements at various points in their design lives. The FWD data available for each of these 14 pavements is briefly summarized in Appendix B. The

information and test results were obtained from Long and Jooste (2007). The layer indices were calculated using the formulas stipulated in Section 3.3.

A large amount of FWD data was available for the LTPP pavements. This study uses only a portion of the available information for the calibration of a new transfer function for BSMs. The transfer function was initially calibrated to best fit the overall performance of BSMs. Therefore, averages were used as representative values to describe the response and characteristics of each of the LTPP pavements.

The data from each LTPP pavement was analysed to determine an average for dmax, BLI, MLI and

LLI for each direction and year. Table4.1provides a summary of the maximum deflection and the three layer indices (BLI, MLI, LLI) obtained from FWD testing. Only the LTPP pavements with sufficient data available to obtain representative results are discussed in this section.

The average maximum deflection measurements for each pavement are shown graphically in Figure

4.3 and Figure 4.4. The average BLI for each of these pavements is shown in Figure 4.5 and Figure 4.6. The MLI and LLI are shown with the BLI and Dmax for each of the LTPP pavements

individually is shown in Figure F.1 to Figure F.10in AppendixF.

Table4.1shows the average maximum deflection measurement and the average of each of the three layer indices for each of the LTPP pavements with sufficient deflection data. The averages were calculated per testing period, for example 1998 and 2002 for the MR 27. In the cases where the pavement implemented a BSM layer in two directions, averages were calculated separately for the two directions. This is shown for the MR 27 where the indices were calculated for the Northbound (NB) and Southbound (SB) carriageways at two points in time (1998 and 2002). The separation of direction was done as the traffic loads and volumes can vary for the two carriageways. The separation of data based on the time of testing was done as this allows for an indication of the change in the pavement’s response and overall condition.

Figures 4.3 and 4.4 graphically represent the values for Dmax, while Figures 4.5 and 4.6 show

the values for the BLI. The red horizontal lines on these figures indicate the behaviour state ranges defined in SAPEM(2014) for granular base pavements. The overall pavement response was considered very stiff if the Dmax values were below 300 µm, stiff between 300 µm and 500 µm,

flexible between 500 and 750 µm and very flexible above 750 µm.

attributed to these pavements all incorporating a cement stabilised subbase. This stiff supporting layer reduces the deflection observed during FWD testing. The other pavements in this figure implement either granular or a BSM 2 subbase, therefore, a higher deflection measurement is expected. The N1-14 is described as very flexible when considering the maximum deflection. The LLI for this pavement is much higher when compared to the other pavements indicating a weak subgrade layer, explaining the high maximum deflections measured on this pavement.

Table 4.1: Summary of layer indices for the LTPP pavements

Figure 4.4 shows that the N2-16, N4-1 and N4-5X have an overall very stiff response. These pavements also implement cement stabilised subbase layers, supporting the low deflection measurements. The deflection measurements taken on the N2-20 in 2001 indicated a flexible layer, making use of a granular subbase layer. However, deflection measurements taken in 2005 showed a significant decrease in maximum deflection and in turn resilient modulus. This may be due to residual compaction or settlement of the subgrade, providing better support to the upper layers over time. This conclusion is supported by the significant reduction in the LLI observed for this pavement.

Figure 4.3: Average maximum deflection measurements for LTTP pavements

Figure 4.4: Average maximum deflection measurements for LTTP pavements (continued)

The pavement temperature and moisture conditions can have a significant effect on its response to loading. Therefore, the time of year the FWD tests were performed may have an influence on the the results. Limited information was available regarding the time of year and pavement temperature during FWD testing. Due to the limited information, the time of year and pavement

temperatures were not investigated in this study.

The response of the BSM base layer was considered very stiff when the BLI values were below 80 µm, stiff between 80 µm and 250 µm, flexible between 250 and 500 µm and very flexible above 500 µm. Figure 4.5 shows that the MR 27 and N1-1 have stiff to very stiff base layers. The stiff response may be due to the stiff support provided by the cemented subbase. However, care should be taken when analysing these results as the BLI describes the response of the top 300 mm of the pavement structure. The top 300 mm of these pavements include the surfacing and a part of the cemented subbase. The other pavements shown in these figures showed reasonable responses when considering the granular support layers in these pavements.

Figure 4.5: Average BLI for LTTP pavements

Figure 4.6 indicates that the base layers of the N2-16, N4-1 and N4-5X have a very stiff response. Similar to the MR 27 and N1-1, the response of the cement stabilised subbase and asphalt may influence the BLI. The BLI of the N2-20 shows the same resilient modulus increasing trend that was observed for the maximum deflection of this pavement.

The results obtained from the FWD testing of the LTPP pavements were considered reasonable and a realistic representation of the condition of these pavements. The FWD was further analysed through the process of back calculation, the results of which are discussed in Section 4.3.1.

Figure 4.6: Average BLI for LTTP pavements (continued)