REINFORCED FLYASH IN RURAL ROAD SUB-BASES

Abstract

In aflexible pavement, purpose of sub-base course is to provide astress-distributing medium, which spread the load applied on the surface. Aflexible pavement consists of a compacted sub-base course of suitably graded crushed stone, slag or astabilized material. The major problem, the world is facing today is the scarcity of conventional construction material and increasing cost of construction by using these conventional materials. In recent years, use of various waste products in sub-base construction has gained considerable attention in view of shortage and high costs of suitable conventional aggregates due to depleting source of stones. Fly ash as such cannot be used in sub-base, because of its low strength. Therefore fly ash may be stabilized through traditional soil stabilization agents as lime, cement and chemicals. These materials are costly and inconvenient in handling with fly ash. Modern technique, reinforcement of pavement layers is simpler and more effective technique. Reinforced earth means the soil formed with the inclusion of strips, sheets, nets, mats and grids of metals, synthetic fiber to reduce the tensile strain. Reinforcement works as frictional and tension- resistance element, because it is afact that the soil is strong in compression and shear but weak in tension. The main advantages of the earth reinforcement are increase in the load carrying capacity, reduction in rut depth and reduction in cost and construction time. Although, good amount of research has been conducted in India on geosynthetic reinforced soil in the form of laboratory and field studies, yet very few studies have been reported on the behavior of reinforced fly ash in pavements. In pavements, maximum works have been reported on use of fly ash in embankments/ subgrade. Its use in subbases is still to be investigated thoroughly. There is much confusion about optimum percentage of fiber reinforced in fly ash and position of geosynthetic in layer to obtain maximum strength in layer. Various studies are conducted on various types of geosynthetics with fly ash from different sources. Therefore to understand complete behavior of reinforcement with fly ash, need is to compare all type of geosynthetics in one study. The influence of various reinforced fly ash parameters was planned to be investigated through static and dynamic load tests and semi field tests. It was planned to observe the effect of reinforcement such as polypropylene fibers, geogrids, geotextile and glass grids on conventional parameters of fly ash such as unconfined compressive strength, modulus of elasticity, shear strength and California bearing ratio and to study the effect of reinforcements and confinements on permanent strain, resilient strain and resilient modulus of fly ash. Fatigue tests have been carried out to study the effect of reinforcement on rut depth formation on model section with simulation of field conditions. Another aim of present study to develop mathematical model between fly ash strength (UCS, E-value, CBR etc.) and parameters of reinforcement (aspect ratio of fiber, percentage of fiber, height of reinforcement, thickness, tensile strength etc.). Finally, economic analysis has been carried out in using reinforced fly ash as road sub-base material. Since the objective was to use fly ash in bulk, so soil was mixed only up to 25%. In case of fiber reinforced fly ash, all tests were conducted for fiber fraction up to 0.5 %, because of considering the cost and economy of reinforcement in fly ash sub-base. Atotal of more than 500 tests were conducted to study the behavior of reinforced fly ash and soil-fly ash mixtures. Unconfined compressive strength (UCS) tests were conducted for four aspect ratios and five fiber contents of polypropylene fiber and without fiber. Aspect ratios (length/ diameter) were 60, 80, 100 and 120. Fiber contents were varied from 0.1% to 0.5% with 0.1% interval. These tests were conducted to obtain optimum aspect ratio or optimum length of polypropylene fiber to give maximum benefits. The results of triaxial tests were plotted in the form of stress- strain curves for three fly ash-soils combinations under different confining pressure and different fiber content. Atotal ofabout 270 plots were made for all tests conditions. It was observed that CBR values of fly ash, fly ash +15 %soil and fly ash +25 % soil were less than 15 %, which is required in rural road sub-bases (as per IRC-58, SP-20, 2002). Therefore reinforcement was must in fly ash. It was found that California Bearing Ratio (CBR) of fly ash increase with increase in percentage of fiber content at aparticular aspect ratio. But increments were more up to 0.3 %fiber content, after that gains in CBR values were reduced for both cases of soaked and unsoaked. Unconfined compressive strength, deviator stress at failure, modulus of elasticity and CBR value of fly ash and mixes for agiven fiber content (Fc), increase linearly with increase in aspect ratio (Ar) up to 100, thereafter the gain in strength is smaller. The stress-strain behavior of fly ash under static load condition improved considerably due to increase in fiber content (Fc) and increase in soil content. The modulus of elasticity (E) of fly ash increases with confining pressure (cr3) linearly and with fiber content (Fc) and aspect ratio (Ar) of fiber nonlinearly. The CBR value increases with increase in fiber content (Fc) in both cases of soaked and unsoaked conditions but CBR value increases more rapidly in case of soaked condition than unsoaked condition with fiber content (Fc). CBR value also increases with fiber aspect ratio (Ar) nonlinearly. Geosynthetics, i.e. geogrid, geotextile and glassgrid in reinforcements give maximum benefit, when they are placed at one-fifth height from top. Unconfined compressive strength (UCS), deviator stress at failure, modulus of elasticity (E) and CBR value of fly ash and mixes for a given height fraction (Ht), increase with reinforcement. Fly ash as such has CBR value of 8.5%, so it cannot be used in sub-base as such. After mixing 25% soil, CBR value becomes 14.6%, which is also not suitable for sub bases, as per SP-20; 2002 the sub base material should have minimum soaked CBR of 15% in case of rural roads. Fly ash with 0.2% fiber content has CBR value 16.6%. Therefore fly ash with 0.2% fiber content is suitable for rural road sub- bases. After mixing 25% soil with fly ash 0.1% fiber content is sufficient. In case of geosynthetic reinforced fly ash all geosynthetic used in his investigation are suitable rural road subbases. For rural roads with higher traffic IRC 37: 2001 is to be followed which states the CBR requirement of 20% and 30% depending upon the traffic fiber reinforcement of 0.3% and 0.4% of fiber will make the fly ash suitable for these conditions. Resilient modulus (Mr), modulus of subgrade reaction (k) and field CBR value of fly ash increase due to reinforcement and mixing of soil. Resilient modulus (Mr) increases with confining pressure, but decreases with the increase in both number ofload cycles and deviator stress. A saving of more than 25 % is observed in sub-base construction using reinforced fly ash in most of cases as compared to conventional granular sub-base. The models are developed to predict the unconfined compressive strength, modulus of elasticity and CBR value of reinforced fly ash. The validity of proposed model has been verified by comparing experimental test data. The findings of present study are useful to understand the complete behaviour ofreinforced fly ash as road sub-base.