Thursday, 3 June 2010

Sub-base and Roadbase Materials

Unbound materials:

Clause 803 Granular sub-base, Type 1

This has to be crushed rock, slag or other hard materials graded as follows;

Clause 804 Granular sub-base, Type 2

This is a much smaller sized material than type I, therefore natural sand and gravels may be used as this material. The Table below shows the grading requirements and variations between type 1 and type 2 stone.

Sieve size

Proportion passing sieve

Type 1

Type 2





85 - 100

85 - 100


40 – 70

45 – 100


25 – 45

25 – 85

600 μm

8 – 22

8 – 45

75 μm

0 – 10

0 - 10

μ = micrometer = one millionth of a metre

Clause 805 Wet-mix macadam

This is a ‘plant manufactured’ material using crushed rock or slag accurately graded and batched, and mixed with 2-6% water according to the nature of the aggregate. The purpose of the water is not so much to make a ‘wet’ mix as a ‘damp’ mix, with the result that segregation during transport and laying is minimized, while at the same time the material is more easily compacted. The material is usually laid in compacted layers not exceeding 200 mm, the aggregate grading being within the stated limits. Care should be taken to keep the moisture content within the optimum limits; drying out, or excess moisture will have a serious detrimental effect.

Bituminous-bound materials:

Dense Roadbase Macadam to BS 4987 (DOT Clauses 902/903)

The need for strong but truly flexible bases that will not crack has led to the use of these dense bituminous materials.

The main requirements in the composition of the dense macadam for use in road bases are that the materials have a fines content (aggregate passing 3.35 mm sieve) of 38% and are made with high-viscosity binders, i.e. 50,54 or 58°C evt tar, or 50 pen, 100 pen or 200 pen bitumen.

‘Evt’ is the abbreviation for ‘equi-viscous temperature’ which is the temperature in degrees Celsius at which a tar a has a viscosity of 50 seconds as determined by the British Standard Test (see Section 10.21). ‘Pen’ is the abbreviation for ‘penetration grade’ and is a measure of the hardness of a bitumen binder. It is obtained from the standard penetration test which is described in Section 10.20.

Rolled Asphalt to BS 594 (DOT Clause 904)

Rolled asphalt is the oldest established bituminous material used for road base construction and has load-spreading properties superior to those of other flexible road bases.

The composition of rolled asphalt for road base construction is a mixture to BS 594, containing 65% of coarse aggregate and normally made with 50 or 70 pen bitumen.

Concrete and cement-bound materials:

Clause 1030 Wet-lean concrete

Clause 1036 Cement-bound granular material, category 1 (CBM 1)

Clause 1037 Cement-bound granular material, category 2 (CBM 2)

Clause 1038 Cement-bound granular material, category 3 (CBM 3)

Clause 1039 Cement-bound granular material, category 4 (CBM 4)

The grades of concrete covered in the DOT Specification (Clause 1001) range from those suitable for unreinforced and reinforced slabs to Cement Bound Granular Material Category 4.

Concrete for road pavements will be covered under the Rigid Road construction.

Cement-bound Materials

Soil cement, cement-bound granular material and lean concrete are all categories of the group known as cement bound material (CBM).

The DOT has renamed these as cement-bound material categories 1, 2 and 3 (CBM1, CBM2 and CBM3). A new and stronger category, CBM4, has been introduced for roads of higher traffic category.

Cement-bound materials are mixtures of raw material and cement which have a moisture content compatible with compaction by rolling, and capable of meeting the requirements for surface level, regularity and finish.

CBM 1 is the finest and weakest of these materials and should have a minimum average crushing strength of 4.5 N/mm2 after 7 days.

CBM 2 is both coarser and stronger, being of 40 mm nominal size, with a strength of 7 N/mm2

CBM 3 and 4 may be either 40mm or 20 mm nominal size and must be made of natural aggregate or slag with a strength of 10 N/mm2 for CBM 3 or 15 N/mm2 for CBM 4.

The DOT Specification covers requirements for batching, mixing, laying, compaction and curing. The mix is determined from site trials and can include PFA (pulverized fuel ash) or ggbs (ground granular blast furnace slag), used in combination with ordinary Portland cement (OPC).

Mixing and Batching

CBMI and 2 can be mixed in place or mixed in a plant and can be batched either by weight or by volume.

CBM3 and 4 must be mixed in a plant and batched by mass.

The mix-in-plant method of construction requires the material, cement and water to be mixed in a central plant and then transported to the point of laying and spreading. The mixer should be capable of mixing evenly and have an output sufficient to meet the demands of the spreading and compacting operations.


Mixed material must be transported as quickly as possible to the point of laying and also be protected from the weather during this operation.


All cement-bound material must be placed and spread evenly in such a manner as to prevent segregation and drying. Roadbase cement-bound material must be spread by means of either a paving machine or a spreader box approved by the Engineer.

Cement-bound material must be spread in one layer so that after compaction the total thickness is as specified. Joints between each day’s work, or, where work has had to be interrupted, should be vertical.


Compaction must be carried out immediately after spreading. All loose segregated or otherwise defective areas must be removed and replaced to the full thickness of the layer.


Immediately after compaction the material should be cured for a minimum of 7 days. Curing may be by:

(1) covering with impermeable sheeting,

(2) bituminous spraying, or

(3) spraying with a curing compound.

Traffic should not be allowed on the surface until the 7 day strength has been attained.

Surface tolerance

All road bases should be laid to a surface tolerance of ±

15 mm.

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