Bryan T Wilson

Effects of Environmental Factors on Construction of Soil-Cement Pavement Layers

The specific objectives of this research were to quantify the effects of certain environmental factors on the relative strength loss of soil–cement subjected to compaction delay and to develop a numerical tool that can be easily used by engineers and contractors for determining a maximum compaction delay time for a given project. These objectives were addressed through extensive laboratory work and statistical analyses. The laboratory work involved testing an aggregate base material and a subgrade soil, each treated with two levels of cement. Environmental factors included in the experimentation were wind speed, air temperature, and relative humidity; three levels of each factor were evaluated in combination with three compaction delay times. The primary response variables in this research were relative compaction and relative strength. The findings indicate that relative strength is sensitive to variability among the selected independent variables within the ranges investigated in this research, while relative compaction is not. Inferring relative strength from relative density is therefore not a reliable approach on soil–cement projects. Consistent with theory, higher wind speed, higher air temperature, lower relative humidity, and higher compaction delay time generally result in lower relative strength. With the nomographs developed in this research, the maximum delay time permitted for compaction of either a base or subgrade material similar to those tested in this research can be calculated. Knowing in advance how much time is available for working the soil–cement will help contractors schedule their activities more appropriately and ultimately produce higher-quality roads.

Strength and Deformation Characteristics of Cement-Treated Reclaimed Pavement with a Chip Seal

The objective of this research was to analyze the strength and deformation characteristics of a cement-treated base (CTB) constructed with full-depth reclamation, microcracked, and then surfaced with a single chip seal. In this field study, strength characteristics of the CTB layer were determined at the time of construction, and both strength and deformation characteristics were evaluated after 9 months of low-volume, heavy truck traffic. Observed distresses at 9 months included transverse cracking, rutting, and chip seal joint failure. The loss of the chip seal was caused by poor chip seal construction practices and not a deficiency in the CTB layer. The average ride qualities in and out of the wheelpath were in the fair ride category; the roughness was not likely caused by trafficking but probably resulted from construction or climatic factors. Structural testing performed after 9 months of service indicated that CTB stiffness and modulus were greater than the values measured after microcracking at the time of construction, indicating continued strength gain. However, trafficking over the 9 months had caused significantly lower stiffness in the wheelpaths than between the wheelpaths. The average unconfined compressive strength (UCS) of the cores tested at 9 months did not differ significantly from the average UCS of the field-compacted specimens tested at 6 weeks. Recommendations for improved CTB performance include the use of a thicker, stiffer, or both, CTB layer to ensure a smooth CTB surface during construction and application of a double chip seal or equivalent.

Comparison of Density Tests for Thin Hot-Mix Asphalt Overlays

The thin overlay mix (TOM) and ultrathin overlay mix (UTOM) specifications in Texas test falling head water flow as a surrogate measure of density. Current flow time criterion has never been correlated to density; furthermore, there is no maximum flow time to prevent overcompaction. A rolling density meter (radar) and a circular track meter may also be used to measure thin lift density. These tests and traditional core testing were used on three projects, and the correlations between tests were analyzed. Correlations were strong on a project-by-project basis but generally poor when data sets were combined. Flow time and mean profile depth, flow time and core voids, and surface dielectric and core voids were the strongest correlations overall; measured voids were unusually high. The reliability of taking core measurements on such thin samples is questionable and may be overly influenced by the surface texture. The flow test should continue to be used as a surrogate measure of density. No minimum flow time was recommended because of unusually high void measurements. To avoid overcompaction in TOMs, flow time should be less than 6 min on higher-speed or critical sections and less than 10 min on lower-speed, noncritical sections. From the standpoint of macrotexture in the UTOM, no upper limit is recommended for flow time. The rolling density meter should be employed on project-specific issues when full-coverage density measurements are desired. Further testing should be conducted on the reliability of measuring voids on very thin cores.

Correlation of Results from Freeze-Thaw and Vacuum Saturation Testing of Cement-Treated Base and Subgrade Materials

The purpose of this research was to better establish a correlation between the results of freeze-thaw and vacuum saturation durability tests, with particular respect to mass loss in the freeze-thaw test, for cement-treated base and subgrade materials. Seven materials were used in the laboratory testing, including five aggregate base materials and two subgrade soils. Materials were stabilized with various cement contents and subjected to three different performance tests, including the standard freeze-thaw test, the standard 7-day unconfined compressive strength (UCS) test, and the vacuum saturation test. Correlations between the results of these tests were investigated, with the primary measure of durability being the rate of mass loss. The correlation between freeze-thaw mass loss rate and the baseline UCS had an R2 value of 0.87, and the correlation between freeze-thaw mass loss rate and the UCS after vacuum saturation had an R2 value of 0.89. In both cases, increasing UCS was associated with decreasing mass loss. Corresponding to the threshold of 14 percent mass loss in the freeze-thaw test, values for baseline UCS and UCS after vacuum saturation were determined at confidence levels of 70, 80, and 90 percent, and the resulting values are recommended for design purposes. The researchers recommend UCS testing after vacuum saturation as a viable alternative to the freeze-thaw test. The standard 7-day UCS test could also be used but may yield false positives for some materials, as shown in this study. The correlation between baseline UCS and UCS after vacuum saturation is strong, with an R2 value of 0.92; on average, the UCS after vacuum saturation was 90 percent of the standard UCS in this study. Other correlations with freeze-thaw mass loss rate were not as strong.