Question

Write an account of the bituminous mixtures for the various layers of a flexible pavement. Compare the flexible pavement made of bituminous materials with the rigid pavement made of concrete. (min 150 words) (6 marks).

Answer 1

SOLUTION:-

A) BITUMINOUS MIXTURES FOR THE VARIOUS LAYERS OF FLEXIBLE PAVEMENT

A bituminous mixture is a combination of binders( bituminous material) , graded aggregates and additives/admixtures. NMAS stands for nominal maximum aggregate size .

Bituminous macadam (BM):- Highly permeable mix when compared to DBM. It has high structural strength and it is a good material to be used as a PCC because it resists reflection cracking. It is intended for low traffics only.

Dense bituminous macadam (DBM):- It uses two grading:- 1) NMAS of 37.5mm 2) NMAS of 25mm Grading 1 is highly permeable and should be covered during rainy season. Grading 2 suited for a base course. DBM has a low permeability as compared to BM

Semi dense bituminous concrete (SDBC):- It is neither dense graded nor open graded.

Bituminous concrete (BC):- It contains a binder course and two wearing course .Grading 1 with NMAS of 19mm which is suitable for binder course. Grading 2 having NMAS of 13mm which is suitable for wearing course on heavy traffic roads. Grading 3 having NMAS of 9.5mm is also needed.

B) COMPARISON BETWEEN FLEXIBLE PAVEMENT MADE OF BITUMINOUS MATERIALS AND RIGID PAVEMENT MADE OF CONCRETE

Flexible pavement:- The component layers of flexible pavement are sub grade, sub-base, base course, binder course and surfacing course. The deflection on the sub-base is reflected on the surfacing course. The load is distributed uniformly from top to bottom layer i.e. by grain to grain contact and this phenomenon is considered during its design. No thermal stresses occur as it can freely expand and contract due to presence of flexible layers. Expansion joints are not provided. Drainage layer is not necessary as the materials used in the layers are pervious or semi-pervious. The strength of the road is mostly dependent on the strength provided by the sub grade. Surfacing rolling is needed. Surfacing is laid on sub-base but not directly on sub grade. It has low flexural strength, low completion cost, short service life, low frictional force but high repair & maintenance cost. The road can be utilized for traffic within 24 hours.

Rigid pavement:- The component layers of flexible pavement are Sub grade, drainage layer, sub-base , CC (Cement concrete) slab. The deflection on the sub base does not harm the surfacing course. Thermal stresses are induced as it cannot freely expand and contract due to its rigidity. So the expansion joints are provided in this case. There is no such phenomenon of grain to grain transfer of load. Therefore its design based on flexural strength or slab action. Drainage layer is provided as the material used in the layers are mostly non-porous. Strength of the road is less dependent on the strength provided by the sub grade. Surfacing rolling is not needed. Surface can be laid directly on the sub grade. It has high flexural strength, high completion cost, longer service span, high frictional force but low repair and maintenance cost. The road cannot be used for traffic until the curing period i.e. 14 days