7.6 Surface Dressing, Otta Seal Field Construction Procedure

1. Surface Dressing

Surface dressing is application of thin film of binder, generally bitumen or tar, which is sprayed onto the road surface and the covered with a layer of stone chippings. Surface dressing construction procedure are discussed below

A. Material collection

• Binder: The binder shall be either bitumen or rapid setting cationic bitumen emulsion.
• Aggregate/chippings: The stone chips shall be clean, hard and durable or cubical shape free from dust and organic or other deleterious substances.

B. Preparation of surface: All loose material and foreign matter shall be removed by through brushing with mechanical brooms or washing by use of compressor. The debris shall be deposited well clear of the surface to be sprayed. Road furniture shall be covered with adhesive paper or similar materials.

C. Application binder: The specified bitumen binder cut back (Cutback bitumen is a petroleum product produced by diluting bitumen in petroleum hydrocarbon (Kerosene), its application rate and spraying temperature shall be as per design on site after design based on traffic, existing surface, climatic condition and type of chippings. The range of spraying temperature for binder is normally within the range of 1400C to 1700C. The binder for second coat will be started only after 21 days of completion of first coat.

D. Application of cover aggregate: The cover aggregate shall be applied at the rate as per design, on site in square meters of coverage per cubic meter of loose aggregate after design of the surface dressing. The cover aggregate shall be applied immediately after the binder is applied and the approval of the Engineer. Aggregate applied to sprayed bitumen emulsion shall however be dry. The cover aggregate shall be applied using approved mechanical spreaders. They shall be capable of uniformly spreading the same speed as the binder distributors during spraying. The bitumen binder shall be covered with cover aggregate closely packed in one layer so that no bitumen binder is left uncovered.

E. Compacting the cover aggregates: The cover aggregate shall be rolled with pneumatic multi-tired power rollers. The roller shall follow directly behind the spreader and shall continue to roll at speed of approximately 8-10 km/hr. Tyre shall be in good condition and be kept clean and smooth to avoid pick up of bitumen and chippings.

F. Aftercare and traffic control: The road shall be open to traffic until the binder has attained sufficient viscosity to prevent the stones being wiped off. The contractor shall erect temporary traffic restriction signs, barrier and removal bumps or any other device as instructed by the Engineer to prevent vehicles travelling to fast over the newly laid surface dressing. Vehicle speed shall be restricted to a maximum of 30km/h. After traffic has been permitted to run on the surface dressing for a period of at least two weeks end when instructed by the Engineer all loose chipping shall be swept and taken away.

G. Rectification of defects: If any defect in surface dressing work is found, the reasons of the defect shall be established and keeping them in view the defect shall be rectified as per direction of the supervisory Engineer. 

2. Otta Seal

Otta seal is a surface dressing which comprises of a layer of binder followed by a layer of aggregate that is rolled into the binder using a pneumatic tyred roller or loaded trucks.  Otta seal construction procedure

A. Collection of materials

• Aggregate: The grading of the aggregate material is based on the level of traffic expected. Generally, for road carrying light traffic less than 100 vehicles per day a coarse grading should be chosen, while a dense grading should be applied to roads carrying greater than 100 vehicles per day.

• Binder: The binder shall be 150/200 penetration grade bitumen or MC 300 cutback bitumen or MC800 cutback bitumen as specified in the bill of quantities from a source approved by Engineer.

• Anti-stripping agent: When anti-stripping agent is required it shall be added to the bitumen immedi
• ately before the start of spraying operations and shall be circulated for a minimum period of 30 minutes prior to spraying.

B. Preparation for otta seal: An otta seal will not add to structural strength of the road and therefore the surface to be treated previously have been prepared.  Preparation of the road base may include re gravelling, reshaping and compaction. The construction of otta seal the road base must be broomed free of sand and excess dust, mud or any other material and might hinder bonding between the seal and road base.

C. Construction of otta seal:  Otta seal is formed in the following procedure:

• Remove dust and foreign objects from the base.
• Add small amounts of water to the base to suppress dust.
• Apply a hot binder at a rate of 1.6 to 1.9 liters/m2.
• Apply aggregate at a rate of 1.3 to 2.0 m3 to 100m2 (depending on the grading of the aggregate) of road surface.
• Flatten the Otta seal using a tired roller until binder is pressing up between the aggregate particles. This usually has to be done multiple times, typically several days after the first flattening.
• For the first two to three weeks, allow traffic at low speeds to assist with further kneading. Aggregates will be dislodged by traffic and they should be brushed back onto the seal.