ABC & ASPHALTING

 

Aggregate Base Course (ABC)

  Normally construction of ABC shall be commenced after completing of Sub Base and the drainage works. Aggregate Base Course is a mixture of various sizes of aggregates and quarry dust in it. Normally aggregates of 37.5mm, 20mm, and 10mm… are included.

In this project, the thickness of the ABC layer is 225mm. The motor grader is used to laying ABC. Normally the road has a cross fall of 3% from the centre the motor grader can be used to blade the road as required.

After placing it shall be compacted well using the rollers and the compaction shall be tested. Degree of compaction should be 98% or more.

Where (0-40 mm) ABC material has to be spread on a sub-base, the surface of sub-Base should be cleared from any extraneous matter.

The ABC material was spread after checking all necessary testing like sieve analysis.

After checking the underlying layer for damages or deficiencies, ABC material layer was spread in layers not exceeding 225mm & the length of spreading was 500m or less at a time.

Before starting the compaction, required water content was added and mixed ABC with the help of a water bowser and compaction was started using suitable rollers.

The compacted layer was tested for degree of compaction & moisture content. In cases where an existing ABC layer is to be improved by adding a layer of material less than 100mm thick, the existing surface was scarified to a certain depth and mixed with the imported material. After that re-compaction was started. After compaction ABC layer final finish level was taken jointly with the allowable tolerance + or – 15mm.

Grading Requirements For ABC Materials

Sieve Size	Percentage Passing
Nominal Size (mm)
37.5
50.0	100
37.5	95-100
28.0	-
20.0	50-80
10.0	40-50
5.0	25-40
2.36	15-30
0.425	7-19
0.075	5-12

Aggregate Base Bound (ABB)

Aggregate base bound also used for instead of ABC in special occasions. Aggregate Base Bound is a mixture of various sizes of aggregates including bitumen content. ABB used in the place where ABC thickness to be less than 100mm up on existing road and highly traffic area like town.

Grading Requirements For ABB Materials

Sieve Size	Percentage (%)
(20-14)	40
(14-10)	20
(10-5)	20
(5-0)	20

Bitumen Content 3.5%

Prime coat

Generally Priming works were done when the existing surface is dry or sufficiently low in moisture and normally the atmospheric temperature is above the 15 oC .Reason to apply prime coat to provide a proper bond between the layers and also to serve as a protective measure for the base or sub base.

In this CSS1 (Cationic Slow Setting) is used to prime the ABC as tack coat

Before starting Priming, the area should be demarcated & safety signs should be properly arranged. Single lane traffic should be managed so that no vehicle commuters suffer unnecessary delay.

The surface to be primed was cleaned and removed loose and deleterious material from the surface using Tractor Broom.

Prime coat was applied at required rate and temperature as per the specification requirement by mean of a bitumen distributor for large areas and by hand lance and nozzles for small and inaccessible areas achieving even distribution of prime coat over the surface. The primed surface should be allowed to cure for 24 hours without being disturbed.

Primed surface was checked & corrected if any damaged area was found and corrected area was primed, before asphalt concrete surfacing.

Reasons to apply prime coat;

• Penetrates the surface
• Coats fine particles
• Seal surface pores
• Provides a waterproof membrane
• Strengthens the surface

Tack coat

This work shall consist of an application of a tack coat to an existing pavement prior to construction of surfacing, so an to provide a bond between the two layers. When preparing the surface, the surface will be dried , thoroughly swept and rendered clean of clay, dust and any other extraneous material. The tack coat shall be applied only when the surface dry. CRS 1 or MC 30 is used to prime on regulating course as tack coat. It was applied just before laying the wearing course.