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Su, Zhipeng
Edil, Tuncer; Tinjum, James M.
May 20, 2012
The ability to retain stability and integrity over years of exposure to the destructive forces of weathering is one of the most important aspects of cementitiously stabilized layers (CSL) in pavement design. However, the changes of the CSL properties over time and their distress models have not been adequately addressed in the Mechanistic-Empirical Pavement Design Guide (MEPDG). The objectives of this study were to identify mechanical properties changes of CSLs which are undergoing exposure to weathering, such as freeze-thaw (F-T) and wet-dry (W-D) cycling, and also to validate and develop durability performance models. Unconfined compressive strength (UCS) and vacuum saturation were also examined for correlation with the durability performance of the CSLs. Ultrasonic pulse velocity measurement was conducted as a non-destructive test to monitor the effect of F-T and W-D cycling on base and subgrade soils with cement, fly ash, and lime stabilization. Laboratory durability tests involved F-T cycling, W-D cycling, and vacuum saturation on nine cementitiously stabilized mixtures (CSM). P-wave velocity measurement after each F-T and W-D cycle and UCS at the end of F-T and W-D cycling were conducted. F-T cycling was an aggressive test, which resulted in degradation in the constrained modulus (D) (up to 71%) and UCS (up to 69%) of all CSMs. The F-T durability performance of the mixtures was greatly affected by the binder content, binder type, and also soil type. Reduction in normalized constrained modulus after the first F-T cycle indicated the frost susceptibility of CSM. Some mixtures showed high resistance to degradation during the first F-T cycle (constrained modulus reduction <10%), including silt-lime-fly ash, sand-cement, gravel-cement, gravel-fly ash, and sand-fly ash. Mixtures with higher initial UCS had less reduction in constrained modulus after the first F-T cycle and less reduction after the W-D cycling as well. Cement-stabilized soils were more durable to F-T cycling with generally higher residual constrained modulus and UCS. Class C fly ash was less effective than lime-fly ash (Class F) with respect to improvement in the durability, strength, and stiffness of silt. Binder content influenced F-T durability significantly; the greater the cement content, the stronger the F-T durability. An exponential modulus decay model is proposed to account for the effect of F-T cycling. A regression constant, k, can be used as a quantitative index of F-T susceptibility to characterize the performance of CSM. The smaller the k value, the more susceptible the mixtures were to F-T cycling. The effect of initial constrained modulus and UCS on the k value was investigated: mixtures with higher initial constrained modulus or UCS had smaller k, with the exception of clay-cement and gravel-cement mixtures. The F-T susceptibility was divided into four categories: ?high?, ?moderate?, ?low?, and ?negligible? based on several criteria. W-D cycling was not as detrimental as F-T cycling to CSM except for clay-lime and sand-fly ash mixtures. Some CSM had continuing cementation in cement mixtures and pozzolanic reactions in fly ash or lime mixtures, reflected in the enhancement in constrained modulus and UCS after the first W-D cycle. Cement-stabilized soils were the most durable to W-D cycling, having the largest residual constrained modulus and UCS among all CSM except gravel, for which fly ash was more effective. The combinations of silt with cement or lime-fly ash significantly improved the stiffness and W-D durability, but not with Class C fly ash.A basic W-D durability model involving a reduction factor (fR) for D is proposed for CSM subjected to W-D cycling. A larger fR means more reduction in D when subject to W-D cycling. Generally, fR for stabilized fine-grained soil (silt and clay) are larger than that for stabilized coarse-grained soil (sand and gravel), except for gravel-cement with 3% cement content. Generally, fR increases with increasing void ratio for CSM. The vacuum saturation procedure fully saturated the specimens. The vacuum-saturated specimens generally had lower UCS compared to that of unsaturated specimens. Although the correlation between residual UCS after F-T cycling versus UCS after vacuum saturation was not very strong, the vacuum saturation test can still serve as a fast way to predict the sensitivity to F-T of CSM when the complete F-T cycling test is not available. Effect of curing progress, moisture content, and compaction characteristics on the P-wave velocity for CSM was investigated as an extension of this study. In general, the P-wave velocity or constrained modulus of the mixtures increased with curing time. The increase in P-wave velocity and constrained modulus from the first to seventh curing days was more pronounced than the increase from the seventh to twenty-eighth curing days for cement-stabilized soil. A slow pozzolanic reaction in lime and fly ash stabilization was also detectable with the ultrasonic wave testing. Unsaturated specimens with higher P-wave velocity had a much smaller increase in P-wave velocity after saturation. The saturation-stiffening effect on the P-wave velocity tended to be larger for soft CSM specimens with low P-wave velocity. The trend between UCS versus dry density or compaction water content was the same as the trend of the P-wave velocity versus dry density. Therefore, the specimens with higher P-wave velocity possessed higher strength (i.e., UCS) for each CSM mixture. For stabilized fine-grained soils, there was a peak in P-wave velocity coinciding with the maximum dry density, whereas for stabilized sand or granular base materials, this trend was not present.The P-wave velocity and constrained modulus were strongly correlated to the UCS and elastic modulus (e.g., E0 and E50). The strength of CSM increased with increasing P-wave velocity. The predicted UCS was less proportional using prediction model determined from P-wave velocity than constrained modulus. The prediction model was also verified by the unsaturated UCS. Linear relationships between VP and E0, VP and E50, D and E0, and D and E50 were observed. Constrained modulus correlated best with E0, with R2 =0.71.In this study, the durability performance of CSM was studied and a distress model corresponding to freeze-thaw cycling and wet-dry cycling was proposed. Vacuum saturation techniques were used to correlate to F-T cycling. The characteristics of ultrasonic P-wave in CSM were investigated.
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