Chapter I INTRODUCTION

Chapter I

INTRODUCTION

1.1    Introduction

The term "surface dressing relates to the application of a thin film of bitumen followed by a covering of stone chipping, usually less than one inch thick, to any kind of road surface.

Properly constructed, asphalt surface treatments are economical, easy to place & long lasting. A surface dressing performs the following functions.

1)          It provides a dust free surface for a previously untreated road.

2)          It provides a water proof seal thus preventing surface water from getting into the road structure & foundation

3)          It stops the disintegration of an old bituminous surface that is beginning to show sign of wear.

4)          It restores non-skid qualities to an old bituminous surface that has become smooth or slippery.

1.2    PREPARATION OF EXISTING SURFACE

Unsurfaced road:  If the surface treatment is to be done on a non-asphaltic base, the existing surface should be shaped to the desired section & compacted properly. It is essential that it should be dry, clean & as free from dust as possible so as to ensure good adhesion between the new bitumen films and the surface being treated.

Bituminous Surfaced road: If the surface is to be treated on an old asphaltic road, the existing surface should be examined carefully & any defects should be repaired properly before the surface dressing is applied.

Prime Coat: It is used to prime on untreated base for an asphalted surface. The object of the prime coat is to penetrate into the road surface & to bind any surface dust that may be left after cleaning.

The depth of penetration of the primer is important & should be between 5-10 mm. It is often sound practice to moisten slightly the road surface before applying the prime as this will help lay the dust & allow the primer more easily to spread over the surface & to penetrate. This can be done when the surface of the base is too dry.

For priming work low viscosity cut backs, usually the medium curing such as Mc30 and Mc70 are used (in rear case Mc250) but in the case of very opened textured surface, a rapid curing cut back is to be preferred. In general, the denser the texture of the surface the less viscous the primer should be in order to get adequate penetration. N.D. Lea & associates has recommended Mc30-Mc70 for Hetauda - Narayanghat Highway Project. N.C.C.N. is using Mc-30 for priming work.

The rate of application of the primer will largely depend on the texture of the road surface. There should be at least five trial sections of 100-meter length with various rates. The engineer in charge of the project should select the best one for the application rate for the whole project. N.D Lea & Associates has recommended from 0.4-1 Liter/m² for this project. The rate of application for this project has been fixed 0.75 Lit. /m² as the trial section of this rate has shown the best result.

The road should be closed to traffic until the primer has soaked into the surface, which may take up to 24 hours. If the road cannot be closed to traffic for all this period, pools of excess primer on the surface should be brushed out & the whole prime area then lightly sanded before traffic is allowed on it. The traffic should not be left for many days because it may deteriorate the surface before the actual surface dressing is carried out.