Laying of Dense Bituminous Macadam DBM – Road Construction

This method statement covers the laying of Dense Bituminous Macadam (DBM) in 50 to 100 mm thick layers as profile correction course (PCC) on a previously prepared existing concrete surface as per specified lines & grades.

The road construction works shall comply the contract agreement, project specifications, relevant drawings and applicable local and international standards.

Below is list of necessary machinery and tools etc.

• Hot Mix Plant
• Paver
• Vibratory Rollers
• Pneumatic Tyre Roller
• Bitumen Sprayer
• Dumpers
• Compressor

Materials

Source and test reports of all materials used in road construction will be submitted to the consultant/client before using the materials on the project site.

Bitumen shall be paving bitumen of Viscosity grade confirming to IS-73.

Coarse aggregates shall be obtained from the approved quarry from VSI crusher. This crusher shall produce aggregates cubical / angular type so that flakiness and elongation indices are within the specified limits.

Fine aggregates shall be of the fraction passing 2.36 mm sieve and retained on 75µ sieve, consisting of crushed stone aggregate.

Filler shall consist of finely divided hydrated lime powder and shall be graded within the limits indicated in specifications. Filler shall be free from organic impurities.

Apart from conformity with grading and quality requirements of individual ingredients, the mix shall meet the requirements set out in project specifications and any other applicable requirements.

The binder content shall be determined to achieve the requirements set out in specifications and any other requirements using Marshal Method for determining the optimum binder content as described in the Asphalt Institute Manual.

The binder content shall be selected to obtain 4 % air voids in the mix design.

Plant Trial

Plant trials shall be carried out after completion of Laboratory Job Mix Formula to establish that the plant can be set up to produce a uniform mix conforming to the job mix formula.

Laying Trial

A minimum 70m X 9m of DBM Profile correction course shall be laid after successfully completion of the plant trial to demonstrate that the proposed mix can be successfully laid and compacted approved by the client before undertaking layer construction at site.

Loose thickness of the layer, number of roller passes to achieve the 98% density of lab density of the mix, and line, level & cross fall shall be established during the trial.

A 200mm thick concrete base (nominal mix 1:2:4) having a smooth surface and resting on a well compacted foundation consisting of WMM/GSB, cement stabilized subgrade & earthen subgrade shall be prepared and fully cured in advance to simulate the existing concrete pavement

Dense Bituminous Macadam DBM Laying Method

Weather and Seasonal Limitations

The work of BDM laying shall not be taken up:

• during rainy or foggy weather
• the base course is damp or wet
• during dust storm or when the air temperature on the surface on which it is to be laid is less than 10 deg.
• and also wind speed at any temperature exceeds 40 km/hour at 2 meter height for conventional bitumen mixes.

Preparation of Base

The existing concrete surface already prepared shall be swept clean free from dust and foreign matters using mechanical broom or blown off by compressed air and applied with tack coat as per the specification.

Details of preparation of existing concrete surface are given under a separate method statement.

Tack Coat of VG 10 bitumen shall be used as per the standard, the rate of spraying of tack coat on cleaned concrete surface will be 0.40kg/m².

Layer Arrangement for PCC

The existing surface level data and proposed PCC top level data shall be analyzed to determine the layering arrangement required to achieve the profile correction.

Entire existing pavement area under PCC shall be divided into smaller areas requiring one, two (2) or three (3) layers of PCC depending on the total thickness of PCC.

Each layer of DBM PCC shall vary in thickness from 50mm to 75mm as per specifications.

Contractor shall prepare and submit Isopachytes drawings (using Civil 3D software) showing different thickness bands of DBM PCC to client for his review before start of construction.

Preparation and Transportation of Mix

DBM shall be prepared in automatic Batch type hot mix plant of adequate capacity capable to yield a mix of proper and uniform quality conforming to the specifications.

The mixture shall be transported from the plant in tippers covered with tarpaulin so as to maintain the temperature.

Spreading

The mix shall be transported to the site by the dumpers covered with Tarpaulin, which shall slowly discharge the mix in the Self-Propelled Paving Machine hopper, while both the tipper and paver will move forward steadily and slowly.

Automatic sensor shall ensure that the mix is being laid to the proper line and grade and level.

Direction of the paver will be guided by the string lines fixed with the stack & arms on both sides of the edges 0.5 m away from the edge.

The paving will be done in one go for the full width or in another lanes running forward simultaneously.

The temperature of the mix at the time of laying shall be 125°C to 160°C in compliance to the roadwork specification standards.

Longitudinal Joints

Longitudinal joints and edges shall be constructed true to the delineating line parallel to the Centre line of pavement.

The longitudinal joints shall be truly vertical in straight lines which are continuous for the full length of the pavement or in smooth curves around bends.

Joints in successive asphaltic concrete layers above shall be staggered by 500mm from the underlying layers.

The exposed vertical edges of the longitudinal lane joints shall be carefully cut back and trimmed to a dense vertical face in the compacted lane or for a minimum of 100mm, whichever is greater.

All debris/loose material arising from this operation shall be removed from the pavement and the underlying surface shall be cleaned.

Exposed joints face shall then be cleaned and painted with hot bitumen at the rate of 1 kg/10 sqm immediately before the laying of the adjacent lane.

Transverse Joints

Transverse joints shall be formed at right angles to the longitudinal joints and be truly vertical.

The transverse joints shall be staggered from each other and in the layers above at least 500mm.

At the end of day’s operations, the roller shall pass over the unprotected end of the freshly laid mix.

The exposed vertical edges of the transverse joints shall be cut back and trimmed to firm material or for a distance 100mm the thickness of layer or up to meeting a dense material of full thickness, whichever is greater.

All debris/loose material arising from this operation shall be removed from the pavement and the underlying surface to be cleaned.

The exposed joints shall then be cleaned and painted with hot bitumen immediately at the rate of 1 kg/10 sqm before resumption of the laying operation.

Compaction

As soon as the mix has been laid with the paver, compaction shall be done with the help of a set of rollers moving at a speed not exceeding 5 km/hour following close to the paver.

Rolling sequence will be as under or as established during laying trial stretch to achieve 98% of laboratory density of the mix.

i) Initial or breakdown rolling shall be done with 80-100 kN static weight smooth wheeled rollers. No vibration will be applied during the operation.

ii) Intermediate rolling shall be done with 80-100 kN static weight vibratory roller. Vibrations may be applied during this stage, the amplitude of vibration and the number of passes required shall be decided after laying a trial bed.

iii) The final and finish rolling shall be done with 80-100 kN weight smooth wheeled tandem roller as well as a pneumatic tyred roller of 100-250 kN weight having tyre-pressure of 0.7 MPa. No vibration will be applied in this stage.

During rolling, wheels of rollers shall be kept moist by sprinkling water from the water storage fitted with the roller to prevent the mix from adhering to the wheels.

The roller shall proceed on the fresh material with rear or fixed wheel leading so as to minimize the pushing of the mix.

Rolling of longitudinal joint

In portions having unidirectional cross fall / super-elevation, rolling shall commence from the lower edge and progress gradually towards the upper edge.

Thereafter, roller shall progress parallel to the center line of the road, uniformly over-lapping each preceding track by at least one third width until the entire surface has been rolled.

In Camber portions, rolling should begin at the edge with the roller forward and backward until the edges have been firmly compacted.

Roller shall then progress gradually towards the center parallel to the center line uniformly overlapping each of the preceding tracks by one-third width until the entire surface has been rolled.

Along forms, kerb and walls where rolling cannot be done by rollers, those portions shall be compacted with mechanical tampers or a plate compactor or rammers as may be feasible.

Any undulations / displacement found as a result of reversing of the direction of a roller or due to some other reasons shall be rectified as specified or removed and made good.

All the field tests such as checking of compaction by core cutting/sand replacement method, surface irregularity by using a straight edge and checking of cross slope using a template, rate of spray of tack coat using trays, temperature of mix using digital thermometer, depth of the layer using thickness gauge etc. shall be carried out as per the requirements.

Test frequency and surface finish shall conform to the requirements of the project specifications.

The works shall be so executed that no construction traffic moves on the finished DBM surface before it cools down to ambient temperature and gains full strength.

Safety procedures for handling Hot paving operations

Workmen engaged in handling hot mix paving operations such as plant operation, laying of hot mix etc. shall be provided with personnel protective equipment like gum shoes, hand gloves, close fitting uniforms, safety helmet etc.

A safety supervisor shall be posted on the laying side permanently to monitor the safety procedures during the laying.

For the other plants like crushing plant and asphalt plant dust control devices such as water sprinklers and vacuum suction devices shall be provided to safeguard the atmosphere’s pollution.

Personal equipment like gum shoes, ear plugs, goggles and safety hats shall be provided to workman working on the plant.

Safety supervisor shall monitor the safety precautions on these plants regularly.

Workers shall be provided with the personnel protection equipment PPE.

Safety at Construction site

During construction of DBM PCC, adequate safety & traffic signs shall be installed adjacent to the road shoulders to protect the ongoing works.

For locations where the paving of DBM is done a minimum two flagman with reflection vests holding red flags each shall be deployed to regulate the traffic.

The safety officer shall make frequent patrols along the highway to ensure that the safety equipment and signs are operational.

Note:- Safety plan for diversions, roads sign fixing and for other aspects of safety shall be as per the approved safety manual Safety Plan.

Vehicle tracks shall be kept moist to prevent flying of dust particles.

Water sprinklers or vacuum suction arrangements shall be employed on crushing, hot mix plants to prevent the dust particles from polluting the atmosphere.

Asphalt from trimming of edges or from demolition the old carriageway shall be disposed off at the designated sites.

Plantation shall be done around the plant to make the atmosphere free from heavy particles and access carbon dioxide.

Provisions given in the contract agreement, specifications, drawings, approved mix designs & ITP shall get precedence over this roadwork method statement.