REQUIREMENTS OF BITUMINOUS MIXES

A good bituminous paving mix should exhibit stability, durability, flexibility, skid resistance, and workability properties, besides economy. 

1. Stability is defined as resistance of the paving mix to deformation under load and thus it is a stress which causes a specified strain depending upon anticipated field condition. Stability is a function of friction cohesion.
2. Durability is defined as the resistance of the mix against weathering and abrasive actions.
3. Flexibility is a property of the mix that measures the level the bending strength.
4. Skid resistance is defined as the resistance of the finished pavement against skidding and is function surface texture and bitumen content.
5. Workability is the ease with which the mix can be laid and compacted. It is function gradation of aggregates, their shape and texture, bitumen content and its type.   

RESISTANCE OF WATER ACTION ON BITUMINOUS MATERIAL

The durability of a bitumen pavement greatly depends upon the ability of the bitumen to adhere to the aggregate in the presence of water. Loss of adherence between aggregate and bitumen causes deterioration of the road in an early age. Stripping of bitumen from the aggregate has been limited to cold mixes only, using cut backs. In such cases anti-strip additives may be used with bitumen, to improve the bond between aggregate and bitumen.

DURABILITY OR RESISTANCE TO WEATHERING OF BITUMINOUS MATERIAL

To perform its function satisfactorily as binder, paving bitumen must remain plastic. But when bitumen is exposed to weather in a thin film it loses quite a lot of its plasticity and becomes brittle. This happens because of certain chemical and physical changes. This phenomenon of natural deterioration is called weathering. Progressive weathering may ultimately result in the formation of fine cracks which go on widening with time. Engineer in charge should adopt suitable ways and means to minimize the effect of weathering. Rate of curing of the liquid bitumen (cut back) is affected by following factors:

1. Evaporation rate of the solvent or volatility.
2. Amount of solvent in the cut-back. 
3. Penetration of the bitumen used as base.

PROPERTIES OF BITUMINOUS MATERIAL

According Mr. F.N. Hveem properties of paving bituminous materials can be grouped under following four heading:

1. Consistency
2. Durability or Resistance to weathering
3. Rate of curing
4. Resistance to water action.

 Quality of bitumen and its suitability for any given use are completely defined by these four properties.