Methodology of Rectification path in DBM

Dense Bituminous Macadam (hereinafter referred to as “DBM”) is a binder course used in the construction of all flexible roads. DBM is used for roads with more number of heavy commercial vehicles.

Machinery

1. Hot mix plant of suitable capacity
2. End Dumpers
3. Paver Finisher
4. Vibratory Rollers of capacity 80-100 KN static weight or pneumatic tyred roller of capacity 120-150KN weight.
5. Bitumen sprayer
6. Camber board and 3 m straight edge
7. Mechanical broom
8. Air Compressor
9. Water Tanker
10. Survey kit etc.,

Material

1. Paving Bitumen of penetration grade VG 30 shall be used generally and in accordance with clause 505.2.1 of MoRT&H.
2. Coarse aggregate shall be of clean, strong, durable, cubical shape and free from other deleterious matter and adherent coating as per clause 505.2.5 of MoRT&H.
3. Los Angeles Abrasion value for coarse aggregate shall not be more than 35% when tested as per IS: 2386 (Part 4).
4. Aggregate Impact Value shall not be more than 27% when tested as per IS: 2386 (Part 4).
5. Flakiness and Elongation Indices (Combined) shall not be more than 35% when tested as per IS: 2386 (Part 1).
6. Stripping value of bitumen aggregate mixtures shall be of minimum retained coating 95% when tested as per IS 6241.
7. Water absorption of the aggregate shall not be more than 2% when tested as per IS: 2386 (Part 5).
8. Plasticity Index of the filler shall not be greater than 4.
9. The fine aggregate shall have a sans equivalent value of not less than 50 when tested in accordance with the requirement of IS: 2720 (Part 37).
10. The filler shall be satisfy as per MoRT&H specification.

Preparation of Material

Dense Bituminous Macadam works shall be carried as per the MoRT&H Clause 505.

The aggregate for Dense Bituminous Macadam shall conform to requirements specified in the table 500.8 of MoRT&H specifications.

Bitumen: Viscosity grade paving bitumen confirming to IS: 73-2006. The grade shall be used as per requirement.

A mix of crushed stone and aggregates shall be so proportioned so as to achieve the grading specified as per MoRT&H table 500.10 of Grading-2.

The proportioning shall be done by determining the individual gradation of the individual ingredients and the blend determined by trail and error method so as to achieve the gradation specified. Dense Bituminous Macadam shall be constructed as per the MoRT&H specifications, Clause 505.

As per MS-2 sixth edition, VA (Air Voids), VMA (Voids in Mineral Aggregates), VFB (Voids filled with Bitumen), Stability and flow shall be determined for the material so blended.

The mixture shall meet the requirements as per MoRT&H table 500.11.

Mix Design with proportion of all materials grading/test reports shall be submitted to respective authorities and trail patch shall be constructed before actual commencement of Dense Bituminous Macadam work.

In case of variation of gradation in the course of work the proportion shall be suitably modifies so as to achieve the desired gradation.

Initial Procedure

1. When the existing surface to be overlaid is bituminous, any existing potholes and cracks shall be repaired and sealed in accordance with clause 3004.2 and 3004.3.
2. Marking shall be done by adjusting paver sensor to its lines, grades and cross sections.
3. Tack coat shall be applied over the prepared base by using approved material to the required quantity.
4. Mix produced from the hot mix plant shall be transported to the site by means of end dumpers. Proper arrangement be provided to cover the hot mix materials to prevent accumulation of dust reduce drop in temperature.
5. The mix shall be unloaded gently into the paver hopper and paving shall be started uniformly within the required temperature.
6. Care must be taken so that no segregation of hot mix material being laid at any place does occur.

Rectification Procedure

1. Any high spots in the existing black topped surface shall be removed by manual or other method, and all loose material shall be removed to the required specifications.
2. Where the maximum thickness of profile corrective course will be not more than 75 mm, the profile corrective course shall be constructed as an integral part of the overlay course. In other cases, the profile corrective course shall be constructed as a separate layer, adopting such construction procedures and using such equipment, to lay the specified type of material, to the thickness and tolerance as specified for the course to be provided.
3. The profile corrective course shall be laid to tolerance and densities as specified for wearing course if it is laid integral with the wearing course. The profile corrective course shall be laid to tolerance and densities as specified for base course, it it is to be covered with a wearing course layer.
4. The levels shall be checked before paving and after compaction with respect to the designated levels. If there is any variation in the levels, the same shall be adjusted in further paving.
5. Initial or break down rolling shall be carried out immediately following close to the paver in longitudinal direction from lower edge to upper edge with 80 – 100 KN static weight of smooth wheeled roller. The speed of the roller shall be maintained not more than 5 Km/hr.
6. Intermediate rolling shall be done with 80-100 KN static weight of vibratory roller or with a pneumatic tyred roller of 120-150 KN weight having Tyre pressure of atleast 0.56 Mpa and it should be continues till the required compaction is achieved.
7. Traffic shall be allowed after completion of the final rolling when the mix has cooled down to the surrounding temperature.

Quality Control

This shall be done as per table 900-3 of MOST (Ministry of Surface Transport).

1. The application of tack coat shall be in accordance with the specification.
2. The gradation of the mix shall be controlled at plant as per approved designed mix.
3. Mix temperature shall be controlled at plant within the range of 150-165°C and the temperature less than 140°C shall not be used for paving.
4. The compacted surface shall be checked for field density by taking cores at the frequency of 1 test for 700 Sqm. The degree of compaction shall have a minimum field density equal to or more than 92% of the average theoretical maximum specific gravity obtained on the day of compaction in accordance with ASTM-D-2041.
5. The surface shall be checked for levels at 5 m intervals longitudinally and transversely and the same shall be within +/-6 mm.
6. The thickness of the each layer shall be checked from cores and levels.
7. The requirements for DBM material and laid each day shall be confirmed by laboratory and field tests as per Clause 505.3.1 of MoRT&H.

All of the field testings included measuring mix temperature using a digital thermometer, measuring surface irregularity with a straight edge, measuring cross slope with a camber board, and measuring compaction by core cutting. Measure the layer’s depth using a thickness gauge, etc., In accordance with the specifications.

Environmental Control

1. While producing or crushing of aggregates in the crusher plant care should be taken in respect  of controlling and minimizing flow of duct, noise and use of blasting materials such as dynamite, safety aspects to workers are also vital and should be taken of.
2. Care will be taken to prevent spilling of material from the trucks during transportation.
3. Excess material, if any, after using in the pavement should be carefully disposed off in the designated area.

In conclusion, Dense Bituminous Macadam has ability to withstand the high shear stress that occurs below the bearing coarse.

Above all, this is the general procedure of rectification of Dense Bituminous Macadam layer. The procedure varies on type of road, design, pavement characteristics etc.

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Also Read:Construction Methodology of Dense Bituminous Macadam