Base Construction.pptx

Base Construction

1. HIGHWAY CONSTRUCTION METHODS ROAD BASE ENG. S.A.S.T SALAWAVIDANA CEng FIESL, MCIHT, MEng (Highway & Traffic), BSc.Eng (Hons)

2. INTRODUCTION The base course is immediately beneath the surface course.  It provides additional load distribution and contributes to drainage and frost resistance.  Base courses are usually constructed out of: Aggregate .  Base courses are most typically constructed from durable aggregates that will not be damaged by moisture or frost action.  Aggregates can be either stabilized or unstabilized .  HMA .  In certain situations where high base stiffness is desired, base courses can be constructed using a variety of HMA mixes.  In relation to surface course HMA mixes, base course mixes usually contain larger maximum aggregate sizes, are more open graded and are subject to more lenient specifications Eng. Suneth Thushara Salawavidana 2

3. General Requirements for the Aggregate for the Construction of Bases The coarse aggregate (aggregate substantially retained on 4.75mm sieve) used for road bases and surfacing must be crushed rock from an approved quarry. It must consist of clean hard sound durable particles of angular shape and rough surface texture. They must be free from weathered, soft, laminated or elongated pieces, deleterious matter. It must also be free from clay and excess dust. Eng. Suneth Thushara Salawavidana 3

4. General Requirements for the Aggregate for the Construction of Bases The coarse aggregate must also conform to the following requirements. The Aggregate Impact Value (AIV) must not be greater than 30%. The Flakiness Index must not be greater than 35%. The lower layers of the base and for bases of roads carrying low traffic (total of less 300 vehicles per day), these requirements may be relaxed at the discretion of the Engineer. Eng. Suneth Thushara Salawavidana 4

5. General Requirements for the Aggregate for the Construction of Bases However, aggregates of AIV greater than 35% must normally not be used in road bases. Fine aggregates (aggregates substantially passing the 4.75mm sieve) used for road bases and surfaces must either be crusher fines or river sand. Where crusher fines are used, they must be derived from rock meeting the requirements of coarse aggregate for road bases and surfacing. Eng. Suneth Thushara Salawavidana 5

6. SINGLE SIZED AGGREGATE BASES The aggregates used must be of nominal sizes 50mm or 37.5mm and 19mm or 12.5mm respectively and nominal single size 100mm. All must conform to the requirements given above. The size requirements of 100mm aggregate must be such that the maximum dimensions of the pieces of aggregate must not exceed 150mm and their minimum dimensions must not be less than 75mm. In addition, the aggregates must be well graded between these two limiting dimensions. The grading requirements of the other aggregates must be according to the following table. Eng. Suneth Thushara Salawavidana 6

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8. Thickness of base course The nominal compacted thickness of the base course must be 75mm or 150mm depending on whether only two spreads of 50mm aggregate or whether one spread of 100mm aggregate and two spreads of 50mm aggregate are used in the construction respectively. Where 37.5mm aggregate is used, instead of the 50mm aggregate, the nominal compacted thickness must be reduced to 55mm and 130mm respectively. For higher thicknesses of bases, the composition of the aggregate courses must be specified. Eng. Suneth Thushara Salawavidana 8

9. Preparation of existing surface Where the base course is laid over a prepared subgrade or subbase, the surface must be brushed free from dust and extraneous matter and if required wetted before laying the base course. Where the base course is laid over an existing gravel road any unevenness of the surface and places of failure must be corrected and surface must be well prepared and wetted as above, prior to laying operations.  Where the base course is laid over an existing bituminous pavement, the surface must be scarified lightly and cleaned of any scarified material or in the alternative, a tack coat must be applied prior to laying operations. Eng. Suneth Thushara Salawavidana 9

10. Base Construction Before spreading the aggregate base, construct a partial width of the shoulder (not less than 0.6m wide) to the elevation of the top of each compacted layer. Trim the inside edge to near vertical face. Eng. Suneth Thushara Salawavidana 10

11. With 50mm and 37.5mm aggregate Spread aggregate evenly on the prepared surface at the specified rate to form a uniform layer of two stones thick. Eg : 75mm (3”) nominal thickness  50mm aggregate 8.5-10m 3 /100m 2 Eng. Suneth Thushara Salawavidana 11

12. With 100mm aggregate Spread aggregate (hand pack where necessary) to one stone thickness at the given rate. Use a 10 tonne roller to achieve sufficient embedment (avoid excessive rounding off). Fill surface voids with crusher dust if directed. Then spread 50mm or 37.5mm aggregates to two stones thickness. Eng. Suneth Thushara Salawavidana 12

13. Compaction Compact using 8-10 tonne roller. Rolling should be done by a series of overlapping longitudinal passes. Start from the edge and proceed towards the centre . While rolling, add or remove aggregates where necessary. Continue rolling till the aggregate does not shift or wave in front of the roller. (Avoid over rolling). The surface can be watered to help the bedding in of the aggregate except where the base is laid over an existing bituminous pavement. Eng. Suneth Thushara Salawavidana 13

14. Compaction Then spread 19mm or 12.5mm choker stone evenly at the given rate. Roll to ensure that the stones are well embedded in the surface interstices. While rolling, wet the surface and brush lightly to ensure that fine aggregate is worked into the crevices. The surface of the base should be maintained in its finished condition until a surfacing is placed. Eng. Suneth Thushara Salawavidana 14

15. Quality Control Aggregate Impact Value and Sieve Analysis one test per 400m 3 . Rate of spread-one test per day. Eng. Suneth Thushara Salawavidana 15

16. Eng. S.A.S.T Salawavidana Thank you