There are various types of bitumen mixes with aggregates for pavement construction. Applications of these bitumen mixes for pavements is discussed in this article.
Based on the nature of gradation selected for the bitumen mixes, they can be classified into:
Fig.1: Gradation Chart for Dense, Open Graded and Gap Graded Bitumen Mixes Density
The bitumen mix that is densely graded has continuous gradation, say in the proximity of maximum density line. The bitumen mix with a large amount of fine aggregate i.e. sand will form open graded bitumen mix. When the mix lack materials of two or more sizes, it will form gap graded bitumen mix. The semi-graded mix will have a gradation lying in between the open graded and the gap graded. All the gradation variations are represented in figure-1.
This mix is mainly employed for base course and the binder course. This itself consist of two gradations; Grading-1 and Grading-2. Grading-1 has 37.5mm as the nominal maximum aggregate size (NMAS). While Grading-2 have NMAS of 25mm. The fine aggregate percentages in both the grading are same in a range of 28 to 42%. The main criteria that differ both the grading are that the grading 1 consist of large size particles i.e. 25mm to 45mm. The grading-1 with NMAS of 37.5mm has many disadvantages like segregation. This segregation will later result in honeycombing. At lower air-void levels, these mixes become permeable compared to the grading 2, with NMAS of 25mm. Hence, the grading-1 causes problems related to water exposure. With the increase of NMAS, the permeability will increase the multi-fold at a present void level condition.
Fig.2: Dense Bitumen Mixes Grade-1, NMAS mix of 37.5mm -Honeycombing Defect due to Segregation
The effect of the nominal maximum aggregate size on the permeability of the flexible pavement is represented in the graph shown in figure-3. The grade-1, have the advantage of rutting resistance compared to the grading-2. During the rainy season, the pavement made of NMAS 37.5MM, Grade 1 must be sealed or overlaid. This will avoid penetration of the water into the pavement and reach the Wet mix macadam. This is the WMM course also called as crushed stone base source.
Fig.3: Effect of Nominal Maximum Aggregate Size on Permeability of Flexible Pavement
The bituminous concrete mix is used in two forms: Grade-1 and Grade-2. Grade-1 with NMAS of 19mm. The Grade-2 of 13mm as the NMAS. From the above description about dense bitumen mixes, it was clear that dense bitumen mix grade-2 have larger use as the base course due to its fewer disadvantages compared to dense bitumen mix Grade-1. This hence was necessary to determine a grade for the binder and the surface course. The bituminous concrete grading-1 with nominal maximum aggregate size 19mm as the binder course binds the base course (dense bitumen mix Grade-1) as well as the wearing course bituminous concrete grade-2 of nominal maximum aggregate size 13mm, with NMAS 19 mm employed as a transition. The bituminous concrete grading-2 is good to be used as a wearing course. To facilitate medium and low traffic, a bituminous concrete grade of nominal maximum aggregate size 9.5 mm was necessary to considered for the construction of smooth and impermeable pavement in urban areas. To facilitate thin asphalt lifts, bituminous concrete grading-3 is more suitable than grade-2. The bituminous concrete grade-2 of NMAS 9.5mm has been efficiently used in the highway construction for the US. For higher traffic BC grade-2 are recommended.
Table-1: Recommended Bitumen Concrete(BC) Gradations
The semi-dense bituminous concrete mixes have neither dense or open graded characteristics. It consists of the so called pessimum voids when they are fully constructed. The word is an anonym of optimum. So, it is advised to make the mix get rid of pessimum voids. These tend to capture moisture or water that will later cause stripping. When the semi dense bituminous concrete is employed above the bitumen macadam (BM) layer, there is chances for the penetration of rainwater through the SDBC and reach the BM. This will create the separation of aggregate and the bitumen in the BM layer. This will cause stripping and the scaling of SDBC. The scaling later with time will result in the potholes on the road.
Fig.4: Semi-Dense Bituminous Concrete Highways with Shallow Potholes
Table-2: Aggregate Grading for Mixed Seal Surfacing Mix
The open graded Bitumen Mixes have fine aggregates in a minimum amount, hence they are very permeable to water. They are employed based on specific functions in the base and for surface mixes.
The Stone Matrix Asphalt (SMA) is the most commonly used gap graded bituminous mixes. With the increasing traffic and the high pressure of tires of the vehicles will give large stresses to the road pavement. The roads are subjected to overloading conditions in certain cases. The stone matrix asphalt mix is tough, highly stable in structure and rut resistant asphalt mix. These systems rely on the stone to stone contact that will facilitate in strength. The rich mortar used in the system will provide better durability.
Fig.5: As per NCAT, the rutting performance of the stone matrix asphalt mix pavement constructed in the US
The performance of the SMA pavements that are constructed in India has performed well. Compared to bitumen concrete, the cost for stone matrix asphalt mix has been found to be 25 to 30% higher. The increased cost of the SMA material is due to its composing materials like cellulose fiber, modified binder, and the binder content. In the US, the life cycle study conducted showed that the stone matrix asphalt mix is more cost effective in terms of performance and lesser maintenance. Read More: Why is Bitumen Used in Road Construction? Properties and Advantage of Bitumen for Pavements Different Types of Bitumen, their Properties and Uses Various Lab Tests on Bitumen for Pavement Construction Bituminous Materials – Types, Properties and Uses in Construction Processes in Bituminous Road Construction
