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How to determine the structure of a road

Roads in Malta are designed according to the directive entitled the Standardisation of Pavements for traffic areas a directive written in the year 2000, by Profs. Dr. Ing. Klaus Muller while consulting the Government of Malta on road construction matters. You may download a pdf document of the directive from the official Transport Malta Website.

This directive is based on the German Code of Practice RStO 2000, and takes into consideration Maltese conditions such as climate, available construction materials, sub-grade conditions, and pavement performance.

The main feature of this directive is a chart, which I refer to as the Road Structure Catalog, from where engineers can select the type of road to construct depending on various factors such as:

  1. Sub-grade conditions and bearing capacity
  2. Construction method
  3. Condition of the traffic area
  4. Present and future traffic loading etc.

Organisation of Road Structure Catalog

The road structure catalog is divided into a series of columns indicating the construction class of a road. The construction class is based on the amount of traffic the road is expected to receive, calculated in millions of equivalent 10 ton axle passes. In the chart this value is indicated by the letter B.

Extracted from Directive for Standardisation of Pavements in Traffic area Edition 2000 by Profs. Dr. Ing. Klaus Muller.

Extracted from Directive for Standardisation of Pavements in Traffic area Edition 2000 by Profs. Dr. Ing. Klaus Muller.

Construction Classes based on Traffic loading

The catalog caters for the following construction classes:

  • Class HD for traffic >32 million equivalent 10t axle passes. This type of road is usually constructed for highly industrious areas.
  • Class 01 for traffic >10 but <32 million equivalent 10t axle passes.
  • Class 02 for traffic > 03 but <10 million equivalent 10t axle passes.
  • Class 03 for traffic >0.8 but <10 million equivalent 10t axle passes.
  • Class 04 for traffic >0.3 but <0.8 million equivalent 10t axle passes.
  • Class 05 for traffic >0.1 but <0.3 million equivalent 10t axle passes.
  • Class 06 for traffic <0.1 million equivalent 10t axle passes. This type of road is typically used for a typical road in a village, where traffic is light.

Pavement Construction classes

On the left hand side of the chart, there are 5 rows of different types of pavement construction based on available thickness and sub-grade conditions.

Each pavement construction type in a particular construction class can withstand the traffic loading of its construction class. However, the pavement construction has to be adapted according to the bearing capacity of the sub grade.

Each Pavement type in the highlighted category is capable of withstanding <0.1 million equivalent ten ton axle passes.

Each Pavement type in the highlighted category is capable of withstanding <0.1 million equivalent ten ton axle passes.

The bearing capacity of the sub grade is determined by a plate bearing test. The plate bearing test consists of a steel plate which is loaded at a designated area at the sub grade level. Deformation of the sub grade below the steel plate is recorded when the plate is loaded. The load is then released and deformation under the steel plate is re-measured at regular intervals during the 2nd loading.

This test produces 2 numbers, EV1 and Ev2 from which the ratio Ev2 / Ev1 can be calculated. According to these numbers we may select the type of pavement construction. Ev2 / Ev1 shall always be less than 2.2 in every case.

Interpreting Plate Bearing Test Results

When Ev2 / Ev1 < 2.2, this indicates that the sub grade is well compacted. If Ev2/ Ev1 > 2.2 then it means that the sub grade needs further compaction.

  • If Ev2 is greater than 120, then one may choose any of the first three pavement construction classes, depending on the available depth.
  • If Ev2 is less than 45, then the sub grade has week bearing capacity. One may choose either pavement construction class 4 or 5, which include the laying of a cement stabilisation layer to improve the subgrade.

How is the selection of the road structure done?

  1. Carry out traffic counts and determine the equivalent 10t axle passes. From this count, determine the construction class from Class HD to Class 6.
  2. Excavate pilot holes and check if there are physical hindrances, such as existing cables, water mains, sewer mains etc. Take note of level at which the services are located. The proposed pavement construction should preferably be shallower than the level of the highest service, so that the existing services do not need to be shifted.
  3. Once the pavement construction is determined scarify the existing surface down to subgrade level and compact it. Carry out plate bearing tests and check the value of Ev2 and the ratio of Ev2 / Ev1, which should be less than 2.2. Consult the structure catalog and check the value of EV2 required for the subgrade.
  4. If the sub grade is weak, it has to be improved using a 150mm thick cement stabilisation mix.
  5. If the bearing capacity of the sub grade is achieved then one can proceed with the laying of the type 1 and the asphalt layers according to the sections indicated in the structure catalog.

Components of the structure catalog

Wearing Course

Binder Course

Base Course

Type 1

Cement Stabilisation

Cost comparison of different road structures

Eventually, one must use the most economically feasible pavement structure of a particular construction class.

It would be interesting to come up with a standardised cost of a square metre of a particular road structure, so that the cost of the different road structures can be compared.

For this exercise, I will be using Transport Malta Framework Agreement rates according to CT2003/2013. The following assumptions are made:

  • Excavation will be carried out by planar equipment.
  • Foreign aggregate based wearing course is used.
  • Tack coat is used in between bituminous surfaces.

Transport Malta Framework Agreement Rates CT2003/2013

  1. Oversite excavation in any type of ground using planar equipment to reduce levels at 20.76 euros per cubic metre excl. VAT.
  2. Type 1 sub-base material as per series 800, clause 803. Nominal grain size 37.5mm, hard stone (>50kN on 10% fines value in wet condition) , in filling to make up levels, levelling and compacting in layers with a maximum thickness of 300mm as required. Material is to be compacted at the optimum moisture content at 15.88 euros per cubic metre excl. VAT.
  3. Cement Bound Material CBM2 as per series 1000 clause 1037. The CMB is to achieve a compressive strength between 7 and 12 Mpa after 28 days. This mix is to be used in filling to make up levels , levelling and compacting as required. The Cement Bound Material (also known as cement stabilisation) is used in weak sub grades and can be provided at a cost of 38.14 euros per cubic metre excl. VAT.
  4. Hot dense bituminous paving mixture in accordance to ASTM D3515 and approved mix designs, including for the necessary bitumen required as per ASTM D3515 and approved mix designs, and the painting of the vertical edges of any joints with hot bitumen:
  5. 0/25 mm for Base course 80mm thick in carriageway, hard shoulder and hard strip, compaction degree greater or equal to 97%; laid to comply to requirements in TM Standards Clause 901 and 903 @ 24.29 euro / sq.m. excl. VAT.
  6. 0/19 mm for Binder course 60mm thick in carriageway, hard shoulder and hard strip, compaction degree greater or equal to 97%; laid to comply to requirements in TM Standards Clause 901 and 904 @ 18.21 euro / sq.m. excl. VAT.
  7. 0/12.5 mm for Wearing course 40mm thick in carriageway, hard shoulder and hard strip, compaction degree greater or equal to 97%; laid to comply to requirements in TM Standards Clause 901 and 906, using aggregates with Los Angeles value less than or equal to 20 and Polished Stone Value greater or equal to 53 @ 15.53 euro / sq.m. excl. VAT.
  8. 0/19 mm for Combined Base Wearing course 80mm thick in carriageway, hard shoulder and hard strip, compaction degree greater or equal to 96%; laid to comply to requirements in TM Standards Clause 901 and 905 @ 26.06 euro / sq.m. excl. VAT.
  9. Tack Coat to surfaces to receive bituminous paving mixture @ 1.58 euro / sq.m. excl. VAT. Tack coat is used in between bituminous layers.

Example 1 : Determine cost per square metre of pavement construction type 1 of construction class HD

For easy reference, the road structure for this example is indicated in the image below.

Road Structure - Construction Class HD Pavement Construction Type 1

Road Structure – Construction Class HD Pavement Construction Type 1

  • The road structure is fully composed of asphalt layers, with an overall depth of 34cm which is divided as follows:
    • 40mm wearing course
    • 80mm binder course
    • 220mm base course.
  • The structure is laid over a sub grade with a bearing capacity of 120 N per sq.mm.

The cost of 1 square metre of this particular road structure is calculated as follows:

  1. Excavation (34cm deep)
    • Excavation of 0.34 cubic metres (1m x 1m x 0.34m deep) at 20.76 euros per cubic metre excl. VAT. This is equivalent to 7.06 euros per square metre excl. VAT.
  2. Base Course (22cm deep)
    • The rate for base course in the Transport Malta Framework agreement is 24.29 euros per sq.m. for an 80mm thick layer. However, this particular structure requires 22cm instead of 8cm.
    • The rate for 22cm of base course can be calculated proportionally as follows:
      • 8cm of base course = 24.29 euros per sq. m. excl. VAT
      • 22 cm of base course = ?
      • 24.29 euros x 22cm / 8cm
      • equivalent to 66.80 euros per sq.m. excl. VAT
  3. Binder Course (80mm)
    • The rate for binder course in the Transport Malta Framework agreement is 18.21 euros per sq.m. for a 60mm thick layer. However, this particular structure requires 8cm instead of 6cm.
    • The rate for 8cm of binder course can be calculated proportionally as follows:
      • 6cm of binder course = 18.21 euros per sq. m. excl. VAT
      • 8cm of binder course = ?
      • 18.21 euros x 8cm / 6cm
      • equivalent to 24.28 euros per sq.m. excl. VAT
  4. Wearing Course
    • The rate for wearing course in the Transport Malta Framework agreement is 15.53 euros per sq.m. for a 40mm thick layer.
  5. Tack Coat
    • Furthermore in between the asphalt layers a tack coat layer is sprayed so that the different layers of asphalt can bond together.
    • In this particular case, tack coat is used between base course and binder course, and between binder course and wearing course.
    • Hence 2 layers of tack coat are used.
    • Each layer costs 1.58 euros per sq.m excl. VAT.
    • Hence for both layers, the cost is 3.16 euros per sq. m excl. VAT (=2 x 1.58 euros per square m).

Adding all different components forming this road structure we have a cost of 116.83 euros per square metre excl. VAT. This can be summarised as follows:

  • 7.06 euros per sq.m. excavation
  • 66.80 euros per sq.m base course
  • 1.58 euros per sq.m. tack coat
  • 24.28 euros per sq.m. binder course
  • 1.58 euros per sq.m. tack coat
  • 15.53 euros per sq.m. wearing course

Example 2 : Determine cost per square metre of pavement construction type 1 of construction class VI

For easy reference I’m including an extract of the road structure catalog and marking the road structure in caption.

  • This road structure consists of a 10cm layer of a combined base wearing course on a sub grade having a bearing capacity of 120 N per sq. mm.
  • The cost of 1 sq.m. of such road structure is determined as follows:
  • Excavation of 0.1 cubic metres of material at a rate of 20.76 per cubic metre excl. VAT. This is equivalent to 2.08 euros per sq.m. excl. VAT (20.76 per cubic metre x 0.1m deep)
  • Base Wearing course 100mm thick at a rate of 26.06 euros per sq.m. for an 80mm thick layer. The cost of a 100mm layer can be calculated pro-rata as follows:

    = 26.06 x 100 / 80

    = 32.58 euros per sq.m. excl. VAT

  • No tack coat is required in this case since only 1 layer of asphalt is being used.
  • Hence total cost for this pavement construction is equivalent to 34.66 euros per sq.m. excl. VAT and is determined as follows:
    • 2.08 euros per sq.m. excl. VAT for excavation
    • 32.58 euros per sq.m. excl. VAT for combined base wearing course
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