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Precracking of soil-cement bases to reduce reflection cracking : Field investigation Scullion, Tom

By: Scullion, TomPublication details: Transportation Research Record, 2002Description: nr 1787, s. 22-32Subject(s): USA | Soil stabilization | Method | Soil | Cement | Roadbase | Cracking | Micro | Prevention | Shrinkage | Deflectograph | Stiffness | 62Bibl.nr: VTI P8169:2002 RefLocation: Abstract: A new approach to minimize the amount of shrinkage cracking in newly constructed soil-cement (S-C) bases is described. The approach, microcracking, is demonstrated on three city streets constructed in October 2000 in a new subdivision of College Station, Texas. The bases on these streets were designed to meet the Texas Department of Transportation's (DOT's) specification 272 requiring an unconfined compressive strength of 500 psi after 7 days moist cure. The base aggregate available in this area is a sandy gravel. Meeting the Texas DOT strength requirement requires a high cement content, typically 6% to 8%. However, with this cement content, the city has experienced substantial shrinkage cracking. To mitigate this problem, the microcracking approach was evaluated. Based on research from Austria, this requires a maximum of five passes of a steel wheel vibratory roller applied a few days after finishing. This application introduces a network of hairline cracks into the base early in its life with the idea that these microcracks will minimize the major shrinkage cracks associated with S-C bases. Furthermore, because this is performed only a few days after placement, the microcracking will not affect the pavement's overall structural capacity; the cracks will reheal and the base will continue to gain strength with time. During this study, the base stiffness was monitored with both the Humboldt Stiffness Gauge and a falling weight deflectometer (FWD). Large reductions in stiffness were monitored immediately after microcracking, but after 2 days of recovery, the base regained most of its initial stiffness. After 6 months, a visual inspection was made and the base stiffness was calculated from FWD data. The base was found to be very stiff, and only minor amounts of cracking were found in each of the three monitored sections. Consequently, draft specifications have been developed for incorporating microcracking in future projects.
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A new approach to minimize the amount of shrinkage cracking in newly constructed soil-cement (S-C) bases is described. The approach, microcracking, is demonstrated on three city streets constructed in October 2000 in a new subdivision of College Station, Texas. The bases on these streets were designed to meet the Texas Department of Transportation's (DOT's) specification 272 requiring an unconfined compressive strength of 500 psi after 7 days moist cure. The base aggregate available in this area is a sandy gravel. Meeting the Texas DOT strength requirement requires a high cement content, typically 6% to 8%. However, with this cement content, the city has experienced substantial shrinkage cracking. To mitigate this problem, the microcracking approach was evaluated. Based on research from Austria, this requires a maximum of five passes of a steel wheel vibratory roller applied a few days after finishing. This application introduces a network of hairline cracks into the base early in its life with the idea that these microcracks will minimize the major shrinkage cracks associated with S-C bases. Furthermore, because this is performed only a few days after placement, the microcracking will not affect the pavement's overall structural capacity; the cracks will reheal and the base will continue to gain strength with time. During this study, the base stiffness was monitored with both the Humboldt Stiffness Gauge and a falling weight deflectometer (FWD). Large reductions in stiffness were monitored immediately after microcracking, but after 2 days of recovery, the base regained most of its initial stiffness. After 6 months, a visual inspection was made and the base stiffness was calculated from FWD data. The base was found to be very stiff, and only minor amounts of cracking were found in each of the three monitored sections. Consequently, draft specifications have been developed for incorporating microcracking in future projects.

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