COMPARATIVE PERFORMANCE TO ICT R27-081 STUDY

Even though the primary objective of the current study varies considerably from that of the earlier project (ICT R27-081), related findings from that study might be useful when reviewing the benefits of using large rocks for working platform applications. Table 5-1 summarizes the design thicknesses, relative compaction, in-place moisture contents, and number of passes to failure from the ICT R27- 081 study (Mishra and Tutumluer 2013). Test sections in Cell 1 through Cell 4 were constructed over

an engineered subgrade with a CBR value of 3%. Each cell had three pavement test sections. Design thicknesses in Cell 5 varied because test sections in that cell had an engineered subgrade strength of CBR = 6%. Test sections in Cell 6 were constructed with three different sizes of large aggregates over a subgrade engineered to CBR strength of 1%. In Cell 6, the effect of geosynthetics was investigated.

Table 5-1: Summary of Performance Observed from the Accelerated Pavement Testing in the ICT R27-081 Study (after Mishra and Tutumluer 2013)

The pavement test section constructed with the 35.6-cm (14-in.) thick crushed dolomite survived the largest number of passes in the ICT R27-081 study. Despite having a more-tolerant rutting criterion of 10.2 cm (4 in.), none of the sections performed as good as the sections in the current study.

Uncrushed gravel material performed the worst among all the sections, surviving only 47 passes with a 35.6-cm (14-in.) aggregate cover. Having low angularity and surface texture properties, the gravel layer underwent internal material failure exhibiting the highest amount of heaving near the wheel paths. A similar failure pattern was also observed in the RAP aggregate subgrade sections in the current study. Further, the effect of subgrade support was also evident in the ICT R27-081 study by judging from the number of passes that Cell 5 sections withstood. Overall from the ICT R27-081 study, test sections with higher engineered subgrade strength (CBR = 6% in Cell 5) performed better than the sections in Cell 1 through Cell 4 even under flooded condition.

Compared with design thicknesses used in the current study, test sections in Cell 6 of the ICT R27- 081 study had 30.5-cm (12-in.) thick aggregate subgrade and 15.2-cm (6-in.) thick aggregate capping layers. This was reflected in the rutting performance of the sections with the lower number of passes until the rutting failure criterion of 10.2 cm (4 in.). Inclusion of a separation layer with geosynthetics proved to be beneficial because the sections with reinforcement commonly survived a higher number of passes. The influence of geosynthetic reinforcement should be studied in future because the

current study did not use any geosynthetics at the aggregate subgrade interface. The largest D6 aggregate subgrade performed the best among the sections in Cell 6, whereas the sections with intermediate-size aggregate performed the worst in the ICT R27-081 study. The riprap-size aggregate subgrade section survived 159 passes, while in the current study, the Type A RR01 section with dolomite capping withstood 4000 passes. Conversely, Section CP-II with Type E RAP capping and Type A aggregate subgrade failed at 100 passes as a result of a shallow water table.

5. 3 CONCLUSIONS

In light of the rutting performances observed in the construction platforms and flexible pavement test sections, the following conclusive remarks can be offered.

• Penetration of large rocks (i.e., aggregate subgrade) into very soft subgrade was

demonstrated to be effective in improving the weak subgrade and preparing a fairly stable working platform layer in pavement construction.

• Because RAP-capped sections accumulated higher permanent deformation, use of recycled asphalt pavement materials as capping materials should be further evaluated and likely minimized.

• Uniformly graded materials such as railway ballast–size RR01 aggregates (Type A) or primary crusher run aggregates (Type C: CS02) may exhibit wider variation in rutting performance because of the presence of inherent voids. Without the presence of smaller-size particles, aggregate interlock is minimal at the interface of aggregate subgrade, which eventually affects the subgrade strength as well.

• The blended recycled materials (e.g., Type D: CS01+40%RAP) appeared to perform the best out of the test sections in this study. Thus, blending RAP with the large-size CS01 crushed concrete appears to be a viable use of aggregate subgrade in construction platforms.

However, RAP aggregate subgrade (Type E) sections performed poorly compared with Type D aggregate subgrade sections. Therefore, a limitation to the percentage of RAP, such as the 40% in this study, should be considered in the design mix of RAP materials.

• Because both the construction platform and flexible pavement test sections constructed with Type B CS01 crushed concrete performed consistently well, no significant modification in the gradation band is needed.

Considering the different factors that had significant bearing on the material performance and evaluation strategy, the following discussion entails a brief list of recommended practices.

• Because the thin [76-mm (3-in.)] aggregate capping affected aggregate subgrade performance, special attention might be given during construction to ensure optimized

performance. A device such as the variable-energy PANDA penetration tester, along with geo- endoscopic imaging, may be useful for conducting quality control on the finished aggregate surface so that any as-constructed thickness variability could be detected before placing additional layers—and would allow preventive modifications to the corresponding layers to be applied.

• Human error in the use of grade control led to large variation in HMA thicknesses in this study. Toward that end, a more advanced methodology such as string lining or use of paver screed beam during HMA placement and compaction should be considered to ensure a smooth HMA

surface after construction. On the basis of the implications discussed in this report, a grade check on the aggregate layers should also be carried out more frequently and precisely. • The early failure in Section CP-II proved that better drainage is crucial for optimized material

performance. A drainage design in soils with wet of optimum condition may be extremely difficult to maintain (especially without knowing the type of aggregate material to be used in construction). The best practice would be to use a typical underdrain system and carefully follow IDOT’s standard guidelines for drainage design practices.