Strength Assessment of Soil-Cement Base

The Alabama Department of Transportation (ALDOT) uses soil cement as base material where the availability of crushed limestone has become limited. Soil cement is essentially a mixture of soil, water, and portland cement. Soil-cement base can be mixed in-place using on-site materials or mixed in a central plant using local or borrow material. The strength of the soil-cement base is an important property. A soil-cement base that is too weak will not provide adequate support for traffic. Whereas soil cement base that has excessive cement and is too strong may develop wide shrinkage cracks that could lead to reflective cracking in the hot-mixed asphalt surface course. A difficulty of using soil-cement base is finding a suitable method to reliably assess its strength for quality assurance purposes. Currently, ALDOT requires a 7-day compressive strength of 250 to 600 psi as determined by testing cores recovered from the roadbed. Note that the strength of soil cement is low and is approximately 6 to 12 percent of that of conventional 4,000 psi concrete. Results from ALDOT soil-cement base projects have revealed very high variability in core strength data. Cores taken a few feet apart on U.S. 84 had strengths that differed by more than 200 percent. The low strength desired for soil cement may be one reason why high variability in results are obtained when using core drilling and testing techniques originally intended for conventional concrete. It is also plausible that the variability in core results could be due to the materials, construction methods, or strength testing protocol used. The primary goal of this proposed work is to develop a method to reliably assess the strength of soil-cement base. For this research project, it is proposed to perform various tests during an ongoing ALDOT soil-cement base project on the Elba Bypass. By performing these tests, the cause of the high variability in core strength results can be identified.