Base course modification through stabilization using cement and bitumen

American Journal of Applied Sciences, Jan, 2009 by Marandi S.M., Safapour P.

INTRODUCTION

A huge amount of investments are used for maintenance, construction and extending roads and freeways in many countries. Statistics shows that during last fifty years, the most investments are used on roads in compare to other general infrastructures (1).

The roads built with conventional methods do not satisfy the huge volume of loads no more. However, in undeveloped and developing countries such as Afghanistan, which are many limitations (e.g., limited time schedule because of internal civil war, lack of good quality material, over loading passing trucks and budget), the conventional methods may be cost and execution time effective. Thus, new methods must be adopted for new roads to increase the bearing capacity of the pavements.

The concept of soil modification through stabilization with additives has been around for several thousand years (2). Even today, stabilization with manufactured additives such as lime, lime fly ash, Portland cement, bituminous materials, fibers and geo-synthetics are gaining popularity in many parts of the world (3). Attention has been focused on substitute materials such as stabilized soils.

Chemically modified soils often change surface molecular properties of the soil grains and, in certain cases, the grains are cemented together, resulting in an increase in strength and improvement of other desired engineering properties (4).

In the cement stabilization, the cement and water, when combined, from cementations calcium silicate hydrate and aluminates hydrates, which bind soil particles together and make the matrix compact (4).

Stabilization of soils and aggregates with bitumen differs greatly from cement stabilization. The basic mechanism involved in bitumen stabilization is a waterproofing phenomenon. Soil particles are coated with bitumen that prevents or slows the penetration of water, which could normally result in a decrease in soil strength. In addition, bitumen stabilization can improve durability characteristics by making the soil resistant to the detrimental effects of water such as volume. In non-cohesive materials, such as sand and gravel, crushed gravel and crushed stone, two basic mechanisms are active: waterproofing and adhesion. The bitumen coating on the cohesionless materials provides a membrane, which prevents the penetration of water and thereby reduces the tendency of the material to lose strength in the presence of water (5).

The role of combined cement and bitumen emulsion is to increase the stiffness and elasticity of the stabilized layer. However, to find a correlation between the laboratory and field results, more data is needed to establish a pavement design specification for stabilized base layer using combined cement and bitumen emulsion.

The main objectives of this research was to analyze the use of combined cement and bitumen emulsion in base course stabilization in details and examine its replacement with conventional pavement in regions with low quality materials and limited construction period. The results believed to contribute a better insight into the combined cement and bitumen emulsion behavior and obtaining new data for future theoretical pavement analysis.

To conduct the objectives, a study was carried out to examine the potential of using a stabilized base using combined bitumen emulsion and cement for paving the various road projects in Heart-Afghanistan.

Herat is the largest city in western Afghanistan and is the main trade city for exports to Iran and now Turkmenistan and Middle East. The main road connecting Iran to Heart is one of the busiest roads in Afghanistan with most of the exports from Afghanistan and southwest Pakistan going through this route to Turkmenistan through border city of Torghundi.

Herat-Dogharoun main road project with 122 km in length, that had been intended primarily, using conventional method, was designed and executed with base stabilized method using cement and bitumen emulsion and compared with conventional pavement. New data was found for optimizing cement and emulsion. Furthermore, the constructed 122 km roadway was monitored for behavior of the layers and their effects on pavement bearing capacity.

In spite of many researches carried out on soil stabilization using various additives including lime, lime fly ash, Portland cement, bituminous materials, fibers and geo-synthetics in many parts of the world, the results have been mixed. To date, there has been no standard method for designing cement-bitumen emulsion stabilized mixes. However, more research must be carried out to obtain new data for standardizing the method. The new contributions may be using in future theoretical analysis.

OPERATION EQUIPMENTS

During the construction project on the stretch from Dogharoun to Heart a stabilized bearing course with a length of 122 km was constructed from natural gravel and sand material by in-place stabilization using cement and bitumen emulsion as binding agent. The bases of the project was the newly formulated Austrian RVS 85.05.13, it describes the technical and legal conditions for the construction of stabilized bearing courses when using different binding agents or combinations of binding agents. Only soil stabilizers with appropriate dosing facilities guaranteed a consistently mixing quality. A constant working depth and the requisite dosing exactitude of water and binding agent were considered for the implementation of this RVS and the applied construction procedure. Therefore, two stabilizers, two 19 tons single drum rollers for uniform and optimal compaction of the bitumen-cement soil mixture and a tandem roller was provided.