Cement Concrete as a pavement material
Introduction

Cement concrete is a mixture of coarse aggregates, fine aggregates, cement and water in suitable proportions. Sometimes admixtures are also added to achieve specific behaviour/ property of the material. The components of cement concrete are briefly introduced in the following.

Components of cement concrete

Aggregates

Aggregates are naturally available pieces of rocks. The aggregates could be igneous, sedimentary and metamorphic type depending on its origin. Figure-1 shows a photograph of aggregates being manufactured from a stone query. The details about the physical properties of aggregates have discussed in the web-course on Transportation Engineering-1 .

Figure 2:  A typical stone quarry

Cement

Cement is manufactured by heating a mixture of limestone, iron ore, gypsum, clay and other ingredients. Two processes, namely dry process and wet process are followed while manufacturing cement. In the dry process, the raw materials are mixed in dry state, whereas in the wet process raw materials are mixed in presence of water to form slurry . After pre-heating, the raw material is passed through rotating kiln inclined with a small angle with the horizontal line. The kiln is progressively hotter towards its lower end, where the raw material gets molten. From this clinkers are formed when cooled, and after grinding the clinkers, cement is produced. An animated description of the whole process can be obtained elsewhere (cement.org 2006).

The Ordinary Portland Cement (OPC) is the most popular, all-purpose cement. There are various other types of cements (for example, natural cement, Portland pozzolanic cement, high alumina cement, expansive cement, quick setting cement, high performance cement, sulphate resistant cement, white cement etc.) and are manufactured to serve specialized purposes. For concrete pavement construction, OPC is most commonly used.

Water

Water participates in the hydration process; also it provides desirable level of workability. About one third of the water added is utilized in the hydration process, rest forms the pores of concrete, and thereby developing porosity to the concrete. Excess porosity reduces strength of the concrete, and however presence of porosity is good for the situations where there is a freeze-thaw problem.

Admixtures

Admixtures are generally of two types, chemical admixture, and mineral admixture. Air entrainer, retarder , accelerators are examples chemical admixture, and, fly ash, silica fume are the examples of mineral admixtures. One of the important concrete admixtures used in pavement construction is the air-entraining admixture. Air entraining admixtures are derived from natural wood resins, fats, sulfonated hydrocarbons and oils etc (Wright and Dixon 2004). Air-entraining admixtures provide durability against freeze-thaw situation. Plasticizers may be used for concrete pavement construction purposes which maintain workability without having increased the water-cement ratio. Calcium chloride is also used sometimes, as accelerating agent, which renders an early strength of concrete.