Lecture Hour -41 CONSTRUCTION OF WATER BOUND MACADAM ROADS

Transcription

1 Lecture Hour -41 Unit-7 10CV56 TE-I CONSTRUCTION OF WATER BOUND MACADAM ROADS General The Water bound macadam (WBM) is the construction known after the name of John also article 2.16 and The term macadam in the present day means, the pavement base course made of crushed or broken aggregate mechanically interlocked by rolling and the voids filled with screening and binding material with the assistance of water. Specifications of Materials for WBM Pavement Coarse Aggregates The coarse aggregate used in WBM generally consists of hard varieties of crushed aggregates or broken stones. However, soft aggregates like over burnt bricks metal or naturally occurring soft aggregates such as kankar or laterite may be used. Crushed slag obtained from blast furnace may also be used.

2 Property Requirements for Pavement layer Sub-base Base Course Surfacing course (i) Los Angeles abrasion (maximum value, percent) or (ii) Aggregate impact (maximum value, percent) (iii) Flakiness index (maximum value, percent) Properties of Coarse Aggregates The crushed stone aggregate should be generally hard, durable and free from flaky and elongated particles. The IRC specifies the following ph requirement of coarse aggregates for WBM construction, in terms of the test value the three pavement layers. Size and Grading Requirements of Coarse Aggregates The coarse aggregates for each layer of construction should, as far as possible conform to any one of the three gradings specified below. Grading No.1. consists of Coarse aggregates of size range 90 to 40 mm and is more suitable for sub-base course. Thickness of compacted layer is usually 100 mm. Grading No. 2 consists of aggregates

3 size range 63 to 40 mm and grading No. 3 of range 50 to 20 mm and compacted thickness of each layer is normally 75 mm. Binding Material: Binding material consisting of fine grained material is used in WBM construction to prevent raveling of the stones Kankar nodules or lime stone dust may also be utilized, if locally available. The binding material with plasticity index value 4 to 9% is used in WBM surface course construction; the plasticity index of binding course material should be less than 6.0% in the case of WBM layers used as base course or sub-base course, with bituminous surfacing. If the screenings used consist of crushable material like moorum or soft gravel, there is no need to apply binding material, unless the plasticity index value is low. Quantity of Materials a) The approximate loose quantities of materials required in m3 for 10 cm compacted thickness of WBM sub-base using coarse aggregate of grading no I per 10 m2 area are: a) Coarse aggregate size 90 to 40mm = 1.21 to 1.43 b) Stone screening type A, 12.5 mm size = 0.4 to 0.44 c) Crushable type screening =0.44 to 0.47 d) Binder material for sub-base course = 0.88 to 0.10 The approximate loose quantities of materials required in m3 for 7.5 cm compacted thickness of WBM base course or surfacing course using coarse aggregate of grading No. 2 per 10 m2 area are: (a) Coarse aggregate size 63 to 40mm = 0.91 to 1.07 (b) Stone screening type A, 12.5 mm size for base course = 0.18 to 0.21 Stone screenings for surfacing course = 0.15 to 0.17 Alternatively, Stone screenings type B,

4 9.0 mm size for base course = 0.30 to mm size for surfacing = 0.24 to 0.26 Alternatively, crushable type screenings = 0.33 to 0.35 (c) Binding material for base course = 0.06 to 0.09 Binding material for surfacing course = 0.10 to 0.15 (Note: Binding material is not required if crushable type of screening is used). Construction Procedure Preparation of Foundation for Receiving the WBM course The foundation for receiving the new layer of WBM may be either the subgrade or subbase or base course. This foundation layer is prepared to the required grade and camber and the dust and either loose materials are cleaned. On existing road surface, the depressions and potholes are filled and the corrugations are removed by scarifying and reshaping the surface to the required grade and camber as necessary. If the existing surface is a bituminous surfacing, for about of depth 50 mm and width 50 mm cut at 1.0 m intervals and at 45 degrees to the center line of the carriageway before laying the Coarse aggregate. Provision of Lateral confinement Lateral confinement is to be provided before starting WBM construction. This may be done by constructing the shoulders to advance, to a thickness equal to that of the compacted WBM layer and by trimming the inner sides vertically Spreading of Coarse Aggregates The coarse aggregates are spread uniformly to proper profile to even thickness upon the prepared foundation and checked by templates. The WBM course is normally constructed to

5 compacted thickness of 7.5 cm except in the case of WBM sub-base course using coarse aggregate grading no.1 which is of 10.0 cm compacted thickness. Rolling After spreading the coarse aggregates properly, compaction is done by a three wheeled power roller of capacity 6 to 10 tons or alternatively by an equivalent vibratory roi1q the weight of the roller depends on the type of coarse aggregates. Application of Screenings. After the coarse aggregates are rolled adequately, the dry screenings are gradually over the surface to fill the interstices in three or more applications. Dry rolling is continued as the screenings are being spread and brooming carried out. Sprinkling and Grouting After the application of screenings, the surface is sprinkled with water, swept rolled. Wet screenings are swept into the voids using hand brooms. Ad screenings are applied and rolled till the coarse aggregates are well bonded and firmly set. Application of Binding Material After the application of screening and rolling, binding material is applied at a uniform and slow rate at two or more successive thin layers. After each application of binding material, the surface is copiously sprinkled with water and wet slurry swept with brooms to fill the voids. Setting and Drying After final compaction, the WBM course is allowed to set over-night. On the next day the hungry spots are located and are filled with screenings or binding material, lightly sprinkled

6 with water if necessary and rolled. No traffic is allowed till the WBM layer sets and dries out. Checking of Surface Evenness and Rectification of Defects The surface evenness of longitudinal direction is checked by 3.0 m straight edge and the number of undulations exceeding 12mm in the case if WBM layer of grading no. 1 and 10mm in the case of grading nos. 2 and 3 are recorded in each completed length of 300m; the maximum number of undulations permitted in each case in 30. The spots with 15mm undulations are marked for rectification of defects.