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road construction

Roads with flexible pavement – Part 5

Contd. from Part 4 ….
( https://civilconstructionresourcez.wordpress.com/2012/01/18/roads-with-flexible-pavement-part-4 )

For mixing purpose, bitumen for the bituminous base course is to be heated to a temperature between 135¤C and 170¤C. The aggregates are to be dried in hot bins at a max. temperature of 170C. In case the aggregates have enough moisture in it to cause foam in the mixture, the same are to be taken back to stockpiles.

Batching plant dry mixing time not to exceed 10 seconds and wet mixing time not to exceed 50 seconds. Mixing time to be counted from the moment of introduction of bitumen to pugmill. Temperature of bitumen not to be less than 14¤C below the temp of aggregates at the time of introduction.

The temperature of the mix at the point of delvery is to be maintained between 140 to 165¤C. Degree of compaction and bulk density of base course are to be maintained or achieved as per standards of AASHTO T230 and AASHTO T166 respectively. Density of asphalt to be at least 96% of max. density value in the Job-mix Formula.

Bituminous Wearing Course: Besides being used as the topmost layer in road pavement, this layer may also be applied in bridges. The overall grading of aggregates for this course is classified into 3 classes – A, B and C. Quality of bitumen and the mix design method is same as those of asphalt base course. Asphalt binder content should be between 3.5% to 5.5%.

Physical properties for Bituminous Wearing Course (AASHTO T245): Voids in total mix (3.5% to 5.5%), Voids filled with binder (63% to 75%), Voids in aggregates (min. 15%), Residual voids in total mix after 500 blows (2%), other properties are same as that of base course (refer previous part).

Tolerances for layer-thickness, surface levels, surface irregularities etc. are usually prescribed in project specifications of the particular project.

Some of the relevant codes are:

BS-1377 (1990), Part 1 to 4 and Part 9 — General requirements and sample preparation, classification tests, chemical and electro-chemical tests, compaction related tests, in situ tests respectively)

BS 812 — Flakiness Index, 10% fines value, Aggregates Impact Value, Soundness, Moisture Content etc.)

AASHTO T104 (Na & Mg sulphate soundness), AASHTO T96 (Loss by abrasion), AASHTO T112 (Friable particles), AASHTO T176 (Sand equivalent), AASHTO T193 (CBR), AASHTO T89 & T90 (LL and PI) etc.


About clazcons

B. Tech. (Civil Engineering)


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