Stephen Sebesta

Hydrated lime stabilization of sulfate-bearing vertisols in Texas

Sulfate-bearing subgrade soils treated with calcium-based stabilizers often experience heaving problems (three-dimensional swell) caused by chemical reactions with the sulfate or sulfide minerals. Two research questions were addressed: (a) what sulfate content results in deleterious chemical reactions using traditional (no mellowing) lime stabilization and (b) how effective mellowing, double lime application and increased moisture content are at reducing swell in high-sulfate soils. To determine what concentrations of sulfate are too high for stabilization with lime in Texas, a soil from the Vertisol order was selected for three-dimensional swell measurements. This soil did not contain any detectable sulfates. Two different compounds were added to the soil: sodium sulfate (Na2SO4) and gypsum (CaSO4.2H2O). The sulfates were added to individual samples at concentrations of 0; 1,000; 2,000; 3,000; 5,000; 7,000; and 12,000 parts per million (ppm). The samples were then subjected to a three-dimensional swell test for a minimum of 45 days. Results of systematic swell experiments revealed that sulfate contents up to 3,000 ppm could safely be treated with traditional lime stabilization. Coarse-grained sulfates take longer to swell, but still swell and form deleterious reaction products. Mellowing is effective at treating sulfate concentrations up to at least 7,000 ppm, higher molding moisture contents (2% above optimum moisture) reduce swell better than optimum moisture, and single application of lime reduces swell better than double application. With systematic laboratory experiments, empirical field observations of sulfate limits presented by other investigators were confirmed.

Use of microcracking to reduce shrinkage cracking in cement-treated bases

Shrinkage cracking occurs in cement-treated bases because of desiccation and cement hydration; eventually these cracks start to reflect through the pavement surfacing. Although initially considered cosmetic, these cracks open the pavement to water infiltration and increase the likelihood of accelerated pavement distress. Numerous options exist for minimizing the amount of reflective cracks that appear; microcracking is a promising approach. The microcracking concept can be defined as the application of several vibratory roller passes to the cement-treated base at a short curing stage, typically after 1 to 3 days, to create a fine network of cracks. In addition to the microcracked test sites, the contractor constructed moist-cured, dry-cured, and asphalt membrane–cured sites for comparison. Researchers used falling weight deflectometer (FWD) tests to control the microcracking process, periodic crack surveys to monitor crack performance, and FWD tests through time to track base moduli. Microcracking proved quite effective at reducing shrinkage cracking problems in the base; applying the procedure with three passes of the roller after 2 to 3 days of curing resulted in the best performance. In addition, researchers observed that, without microcracking, excessively high cement contents resulted in problematic cracking in the base even if they were cured according to good construction practice. Microcracking did not result in pavement damage or diminished inservice modulus; thus, microcracking should be considered a viable and inexpensive option to incorporate shrinkage crack control into the construction of cement-treated bases.

Recommendations for stabilization of high-sulfate soils in Texas

In an effort to construct roads more quickly, high-plasticity index soils stabilized with lime are now routinely compacted the day after mixing. With this practice has come an increasing number of heaves due to soluble sulfates reacting with the lime to form ettringite. Soils with sulfate concentrations below 7,000 to 8,000 parts per million (ppm) can generally be treated with lime. This research was performed to identify stabilizers that can be used with sulfate concentrations above 10,000 ppm. The effectiveness of the stabilizers was determined by the measurement of three-dimensional (3-D) swell reduction and unconfined compressive strength. The researchers evaluated 12 stabilizers, including enzymes, polymers, acids, emulsions, fly ash, and ground granulated blastfurnace slag (GGBFS). Three stabilizers significantly reduced volumetric swell. A polymer and an acid reduced swell by about 8%. GGBFS plus lime reduced swell by 10%. GGBFS plus lime was the only stabilizer that reduced swell, increased strength, and was cost-effective enough for the Texas Department of Transportation to consider as an alternative to lime in high-sulfate soils.

APPLICATION OF INFRARED IMAGING AND GROUND-PENETRATING RADAR TO DETECT SEGREGATION IN HOT-MIX ASPHALT OVERLAYS

Segregation is a serious problem in hot-mix asphalt and typically results in poor performance, poor durability, a shorter life, and higher maintenance costs for the pavement. A summary of the results and recommendations from three projects in Texas in which infrared imaging and ground-penetrating radar were used to examine the uniformity of the pavement mat is presented. Both techniques have significant advantages over currently used nuclear density techniques in that they provide virtually 100% coverage of the new surface. The effectiveness of both the infrared and radar techniques was evaluated by taking measurements on new overlays at the time of placement, coring, and then identifying relationships between changes in the infrared and radar data with changes in the measured volumetric and engineering properties of the cores. Analyses of the results showed that changes in both infrared and radar data are significantly related to changes in hot-mix asphalt properties such as air void content and gradation. On the basis of current Texas Department of Transportation specifications, significant changes in the hot-mix asphalt are expected if temperature differentials of greater than 25°F (13.9°C) are measured after placement but before rolling. If the surface dielectric of the in-place mat changes by more than 0.8 for coarse-graded mixes and 0.4 for dense-graded materials, significant changes in mix properties are expected. Given the promising results from this work, agencies should consider implementing both the infrared and ground-penetrating radar technologies.

MEASURING SULFATE IN SUBGRADE SOIL: DIFFICULTIES AND TRIUMPHS

The accurate analysis of sulfate sulfur in subgrade soil is essential for road construction that involves calcium-based stabilizers (lime or cement). The objective was to determine if other tests give more reproducible results in a more timely fashion than Texas’s current sulfate test method (Tex-620-J), a gravimetric method. Literature review and interviews with commercial soil testing laboratories revealed three techniques to compare with Tex-620-J: ion chromatography, conductivity, and colorimetry/spectrophotometry. Soils were manufactured with known sulfate (gypsum and anhydrite) concentrations and sent to laboratories that performed gravimetric analysis and ion chromatography. Conductivity and colorimetric testing were performed in-house. Testing showed that Tex-620-J is not very precise, which creates the need for an unrealistic number of samples to obtain an accurate estimate of the sulfate concentration. To define the 95% confidence interval for true sulfate content to within ±10% of the true known value for a concentration of 5,000 ppm, Tex-620-J requires 43 tests; ion chromatography requires 14 tests. The colorimeter achieved the desired results in only one test, based on sulfate standard solutions. Results of this study revealed the difficulty with obtaining accurate sulfate measurements in the laboratory and indicated a few inexpensive pieces of equipment that can be used in both the field and laboratory settings that may yield better results.

Part 1: Cementitious, Chemical, and Mechanical Stabilization: Use of Microcracking to Reduce Shrinkage Cracking in Cement-Treated Bases

Shrinkage cracking occurs in cement-treated bases because of desiccation and cement hydration; eventually these cracks start to reflect through the pavement surfacing. Although initially considered cosmetic, these cracks open the pavement to water infiltration and increase the likelihood of accelerated pavement distress. Numerous options exist for minimizing the amount of reflective cracks that appear; microcracking is a promising approach. The microcracking concept can be defined as the application of several vibratory roller passes to the cement-treated base at a short curing stage, typically after 1 to 3 days, to create a fine network of cracks. In addition to the microcracked test sites, the contractor constructed moist-cured, dry-cured, and asphalt membrane-cured sites for comparison. Researchers used falling weight deflectometer (FWD) tests to control the microcracking process, periodic crack surveys to monitor crack performance, and FWD tests through time to track base moduli. Microcracking proved ...

Using Infrared and High-Speed Ground-Penetrating Radar for Uniformity Measurements on New HMA Layers

In this project researchers summarized the availability of infrared and radar systems suitable for testing essentially the entire surface area during new hot-mix asphalt construction, and then they demonstrated an infrared (IR) sensor bar system and two ground-penetrating radar (GPR) systems on construction projects in each of the four American Association of State Highway and Transportation Officials (AASHTO) regions. The thermal profiling system demonstrated provides a real-time view of thermal uniformity and in general correlated well with the constructed mat density. The IR system is now commercially available and should be considered for implementation into agency specifications. This report presents options for how agencies could implement the infrared technology. Demonstration of the GPR systems showed they are suitable for evaluating pavement uniformity and in general can detect low-density defect areas in new overlays. However, streamlining of the data collection and processing needs to take place before GPR can be widely implemented and incorporated into specifications. This report presents a framework for accomplishing the automation of the GPR process to achieve an implementable system.

Part 3: Cementitious, Chemical, and Mechanical Stabilization: Recommendations for Stabilization of High-Sulfate Soils in Texas

In an effort to construct roads more quickly, high-plasticity index soils stabilized with lime are now routinely compacted the day after mixing. With this practice has come an increasing number of heaves due to soluble sulfates reacting with the lime to form ettringite. Soils with sulfate concentrations below 7,000 to 8,000 parts per million (ppm) can generally be treated with lime. This research was performed to identify stabilizers that can be used with sulfate concentrations above 10,000 ppm. The effectiveness of the stabilizers was determined by the measurement of three-dimensional (3-D) swell reduction and unconfined compressive strength. The researchers evaluated 12 stabilizers, including enzymes, polymers, acids, emulsions, fly ash, and ground granulated blastfurnace slag (GGBFS). Three stabilizers significantly reduced volumetric swell. A polymer and an acid reduced swell by about 8%. GGBFS plus lime reduced swell by 10%. GGBFS plus lime was the only stabilizer that reduced swell, increased streng...

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.

Instrumented Rollers to Assess Construction Quality on Texas Projects

The concept of using a vibratory compactor as a tool for assessing construction quality by measuring the drum displacement with an accelerometer began decades ago. Initiated in Europe and correlated with plate bearing tests, this technology has advanced to “intelligent” compaction, in which feedback from the instrumentation can control the amplitude of vibration. With the national push to implement such methods in the United States, researchers at the Texas Transportation Institute sought to examine the relationship between the vibration amplitude of the roller drum and pavement layer properties on several Texas construction projects. Testing revealed that although the roller drum vibration amplitudes did relate to pavement layer properties, the correlation was with the stiffness of foundation layers, typically at least 12 in. below the surface. This finding presents at least one significant hurdle to implementing the instrumented–intelligent compaction concept: a change from density-based to stiffness-based specifications would be necessary. In addition, valid approaches to extracting properties of the surface layer are needed. The results from projects tested indicate that the most promising approaches to using this technology are for evaluating uniformity, compaction monitoring (determining when to stop rolling), and proof rolling. The proof rolling application requires validation to determine the depth of the weak spot.