The Dense Bituminous Macadam mix shall be prepared in a Hot Mix Plant (HMP) as per the approved Mix Design complying with Clause 507.2 and 507.3 of MORTH. The HMP shall be equipped with at least four cold bins, four-deck vibratory screen, hot bins for the storage of the heated aggregates. Precautions shall be taken to control the entrance of moisture.
Preparation of Dense Bituminous Macadam(DBM) Mix and Transportation:
The Dense Bituminous Macadam mix shall be prepared in a Hot Mix Plant (HMP) as per the approved Mix Design complying with Clause 507.2 and 507.3 of MORTH. The Hot Mix Plant(HMP) shall be equipped with at least four cold bins, four-deck vibratory screen, hot bins for the storage of the heated aggregates. Precautions shall be taken to control the entrance of moisture.
The mixing plant shall be of batch-mix type and shall be capable of regulating the composition of the mixture to within the tolerances as specified in table 501.3 of MORTH with respect to the approved job mix formula.
- The rated capacity of the mixing plant shall be 200 T/hr.
- The storage Bitumen tanks shall be capable of maintaining its contents at the specified temperature within a tolerance of ±5°C and shall be equipped with a thermostat to prevent the temperature rising above 170°C, and a fixed thermometer easily read from outside the tank. The dryer shall be of rotating drum type capable of heating the aggregates to the temperatures set forth in the specification. Bitumen shall be heated to the specified temperature in the tanks and delivered to the mixing plant via insulated pipes such that heat losses are minimized.
- The minimum mixing period shall be adopted to get a suitable mix. The mixing cycle times shall be in the range of 30 to 40 seconds.
- HMP shall have automatic controls that co-ordinate the proportioning, timing, and discharge from the mixer.
- The Bitumen shall be heated in the range of 150°C to 165°C so that it can be distributed uniformly.
- The Aggregates shall be heated so that they are mixed within the temperature range of 150°C to 170°C. The difference of temperature between the binder and the aggregate not exceeding 14°C
- The mixing shall be thorough to ensure that a homogeneous mixture is obtained
- Discharge temperature of mix shall be 165°C maximum
Bituminous Mixes shall be kept free of contamination and segregation during transportation. Dumpers used for the hauling of Asphalt mixture shall be adequately free of petroleum oils, solvents, or other materials that adversely affect Asphalt mixture. The Dumpers shall be covered with tarpaulins while carrying the mix to the site.
Laying of Dense Bituminous Macadam:
The DBM materials shall be laid by a self-propelled sensor paver finisher equipped with a hopper, delivery augers and capable of spreading, tamping and finishing the mix to specified grade, lines and cross-section. It shall be able of laying bituminous materials with no segregation, burning, dragging or other defects and within the defined level and surface regularity tolerances and shall satisfy the specification as per clause 507.6 of MORTH.
Immediately after receiving the transported materials from the Hot Mix Plant at the site, it shall be spread on the prepared and approved surface. The mix shall be spread to line and level by the sensor paver without segregation, tearing or dragging. The temperature of the mix at the time of laying shall not be less than 120°C, minimum. The mix shall be placed in full width of the pavement. The compacted thickness of any DBM layer shall be as per the specification. The paver shall be capable of working at forwarding speeds compatible with uniform and continuous laying of the DBM mix. The condition shall be seen to avoid continual stop and start fashion. The speed of the sensor paver shall be co-ordinated with the feed speed limiting to no drag condition.
The paver shall be equipped with accurate screed controls for maintaining the screed at the specified longitudinal grade and transverse slope. The operation shall be capable of maintaining the screed at the specified transverse slope within the allowable tolerance. Automatic controls in the paver shall be backed by manual controls that can be optionally activated, in case required.
If the automatic controls fail or malfunction, the equipment shall be operated manually for the remaining period of the normal working day, provided that the specified requirements are met with.
Compaction of the Dense Bituminous Macadam:
Sufficient rollers of appropriate types, sizes and weights shall be provided to achieve the specified compaction. Rollers shall be of self-propelled and of steel wheel, pneumatic tyre and vibratory types, including combinations thereof, capable of reversing without shoving or tearing the mix. The laid mix shall be rolled, and rolling shall be finished before the temperature of mix attains below 100°C, minimum.
Rolling shall continue as far as possible after laying of the mix so that the mixture is neither too workable such that rolling causes undue displacement or shoving, nor too stiff, offering resistance to the desired degree of compaction.
- Initial or breakdown rolling shall be done with 8-10 tones static Tandem Rollers. The rollers shall be worked with the drive roll nearest to the laying equipment, at a uniform speed not exceeding 5 km/hr.
- Intermediate rolling shall follow immediately upon initial rolling with 8- 10 tonnes deadweight vibratory roller.
- It shall be followed by intermediate rolling with a pneumatic tyred roller of 12-15 tones weight having a minimum tyre pressure of 5.6 kg/sq cm.
- The finish rolling shall be done with 8-10 tones static Tandem roller to eliminate marks of the previous rolling.
a) Initial rolling -2 static passes with Vibratory roller
b) Intermediate rolling – 2 vibratory passes
c) Final rolling -2 passes with PTR
The roller wheels and drums shall be kept lightly moistened with water to prevent adhesion of the mixture to the rollers. Rolling shall start from the outer edge and move longitudinally parallel to the centerline, each pass overlapping the previous pass by one-third of the roller width. On super elevated portions, rolling shall begin at the low side and progress towards the high side. Rolling shall be continued till the air voids and density.
Achieved satisfies the requirement of clause 5.15 of MS 2 for the particular day of production and all roller marks are eliminated.
Finishing of Edges after Bituminous Macadam Laying:
Any mix that becomes loose or broken, mixed with dirt or foreign matter, or defective shall be withdrawn, and shall be replaced with new hot mix and compacted. The edges of the DBM course shall be rolled concurrently with or shortly after the finishing. For edges, roller wheels shall extend 50.00 mm to 100.00 mm beyond the edge. Any fresh mix on the existing work shall be removed carefully sweeping it back on to uncompacted work, so as to avoid the formation of irregularities on the previous surface.
Surface Finish and Quality Control of DBM:
The surface finish of construction shall be conforming to the requirements of clause 902 of MORTH, and all materials and workmanship shall comply with the requirements as mentioned in section 900 of MORTH. The finished level DBM shall be within the tolerance limits specified in Table 900.1. (i.e. +6 mm to – 6mm).
Arrangement for Traffic Flow after DBM:
During the construction period, arrangements for traffic flow shall be made as per clause 112 of MORTH technical specification.
Limitations to DBM Construction:
Laying shall be suspended while free-standing water is present on the surface to be covered, or during rain, fog and dust storms. After the rain, the bituminous surface, or surface coated with prime or tack coat, shall be blown off with a high-pressure air jet to remove excess moisture, or the surface shall be left to dry before laying. Laying of DBM mix shall not be carried out when the air temperature at the surface on which it is to be laid in below 100°C or when the wind speed at any temperature exceeds 40 km/hr at 2 m height unless specifically approved by the SC.
Quality Control Tests for DBM:
The Quality control tests shall be conducted on the material at the source prior transportation and at the DBM location after the completion of compaction operation. Following tests are pertinent to DBM.
|Quality of binder||IS: 73||Penetration between 60 and 70|
|Aggregate Impact Value||IS: 2386(part 4)||30% Maximum|
|Flakiness and Elongation Index||IS: 2386(part 1)||30% Maximum|
|Stripping value||AASHTO T 182||Minimum Retained coating 95%|
|Water absorption of aggregate||IS: 2386(part 3)||2% Maximum|
|sand equivalent test||IS: 2720(Part-37)||Not less than 50|
|Mix grading||Table 500.9 of specification clause 507||1 test for each 400 tonnes of mix subject to a minimum of 2 tests per day|
|Stability of Mix||ASTM D-1559||1set of 3 Marshal specimens to be tested for stability, flow, voids every 400 tonnes of mix, subject to a minimum of 2 sets per day.|
|Control of temperature||At regular close intervals|
|Control of binder content and gradation in the mix||1 test for each 400 tonnes of mix subject to a minimum of 2 tests per day|
|Rate of spread of mixed material||Regular control and through checks on the weight of mix and layer thickness|
|Density of compacted layer||1 test per 250 Sq.m area|