Experimental Studies on Treated Sub-base Soil with Fly Ash and Cement for Sustainable Design Recommendations
Engineers in Malaysia often faced with problem of constructing roadbeds on problematic soils. These soils do not possess enough strength to support the wheel loads upon them either in construction or during the service life of the pavement. Thus, soil treatment must be conducted to provide a stable sub-base or a working platform for construction of pavement. One of the strategies to achieve this is soil stabilization. The soil stabilization includes both physical and chemical stabilization. Chemical stabilization involves mixing binding agents with natural soils to remove moisture and improve strength properties of the soil. Generally the role of stabilizing agent in the treatment process is either reinforcing of the bounds between the particles or filling of the pore spaces. There are two types of chemical stabilization depending to the depth of the problematic soil and the type of geotechnical application: surface or deep stabilization. The traditional surface stabilization begins by excavating and breaking up the clods of the soil followed by the addition of stabilizing agents. The known amount of water is mixed with the soil and additives and compacted. Approximately 150 to 250mm depth of soil can be strengthened by this surface method. Moreover, by using heavy equipment with appropriate modification, the depth of stabilized zone can be increased to 1m. These methods are used extensively to stabilized bases and sub-bases of highways and airfield pavements (3). The pavement soil qualities will be improved by thoroughly mixing and compacting with additives include portland cement, fly ash, bitumen, and combinations of any of the additives (4). The type of the additive and the amount required are dependent upon the soil classification and the degree of improvement desired (5). The pavement structure consists of a relatively thin wearing surface constructed over a base course and a sub-base course that rests upon a sub-grade course. Asphalt/concrete is primarily used for the wearing surface. The properties of all the pavement layers are considered in the design of the flexible pavement system (6). They suggested that construction of long-lasting, economical flexible pavement requires materials that exhibit good engineering properties. The sub-base should possess desirable properties to extend the service life of the roadway section and reduce thickness of the flexible pavement structure. Strength, drainage, and permanency of strength are the few desirable properties. The fly ash classified 'F' exhibits low self-cementing properties and however, with a addition of an activator such as lime the fly ash Class 'F' produces cementitious product (7). Even for very expansive clayey soil, unconfined compressive strength increased by 106% when fly ash content was increased from 0 to 25% (8).
Stabilization of Soil with Self-Cementing Coal Ashes
The combustion of subbituminous coals in electrical generating units produces a fly ash that has proven benefits for the construction industry. Fly ash consists of the inorganic material within the coal that has been fused during combustion, solidified while suspended in the exhaust gases and collected by electrostatic precipitators. Bituminous coals have low concentrations of calcium compounds and the ash produced (Class “F”) exhibits no self-cementing characteristics. The addition of activators such as lime yields cementitious products so that this material can be used for a wide range of soil stabilization applications. Subbituminous coals have higher concentrations of calcium carbonate (CaCO3); thus, the ash (Class “C”) produced during combustion is rich in calcium, resulting in the self-cementing characteristics. Since Class “C” fly ash is self-cementing; activators such as lime or Portland cement are not required. Upon exposure to water, Class “C” fly ash hydrates forming cementitious products similar to those produced during the hydration of Portland cement. This property makes the self-cementing fly ash a very effective and economical stabilization agent for use in variety of construction applications.
INCREASING CEMENTITIOUS PRODUCTS OF A CLASS C FLY ASH
The results of using minute quantities of secondary chemical additives with an Iowa Class C cementitious fly ash are presented in this paper. Soil stabilization with fly ash conventionally requires the addition of water and lime to initiate pozzolanic reactions with subsequent formation of cementitious products. The glossy (amorphous) phase of Iowa fly ashes accounts for about 75 percent of their composition. They are known to contain significant amounts of calcium, aluminum, and silicon, the building blocks of cementitious compounds. If these elements could be economically liberated, the need to use lime would be reduced or eliminated. Lime is a costly ingredient available only at a few locations in Iowa. Potential additives were selected on the basis of economical considerations and expected reactions, as agents that either initiate chemical attack and dissolution of the glassy phase and seed crystalline compounds. Ammonium phosphate appeared the most promising of the seven additives studied. At 3.0 percent by weight of fly ash, the average 28-day strength for ammonium phosphate-treated samples was 2.5 times that of untreated. Scanning electron microscopy indicated the glassy phase of treated fly ash was being attacked. X-ray diffraction showed formation of cementitious reaction products not present in untreated samples. A rapid means of evaluating early strength gain and setting time properties of fly ash paste is also presented.