Below road works method statement covers the construction of Dry Lean Cement concrete sub-base for cement concrete pavement in accordance with requirement of roadwork specifications in conformity with the lines, grades and cross sections shown on the drawings and or as directed by the consulting engineer.
The road construction shall be done in compliance with contract agreement, Ministry of roads transport and highways specifications, relevant drawings and applicable international codes and standards.
Below is list of mandatory tools and equipment for the construction of dry lean concrete.
- Batching & Mixing plant
- Loader
- Electronic Sensor Paver
- Motor Grader
- Tandem Vibratory Roller
- Water tanker
- Tippers
- Survey Equipment’s
Concrete Mix Requirements
Coarse aggregate: Coarse aggregate shall be produced from approved quarry in required sizes. It shall consist of clean, hard, strong, dense, non-porous and durable pieces of crushed stone or a suitable combination thereof as specified & shall conform to the quality requirements as per specifications.
Fine aggregate: Fine aggregate shall be clean, hard, strong and durable pieces of natural sand, crushed stone or suitable combination of the two & shall confirm to the road project specifications. Fine aggregate shall not contain dust, lumps, soft or flaky, materials, mica or other deleterious materials in such a quantity to reduce the durability and strength of finished concrete.
Cement: Ordinary Portland cement 43-grade shall be used in the DLC construction.
Water: Quality of the water shall conform to the specifications as applicable.
Proportioning of Material
The mix design/ job-mix formula used for DLC construction shall be as per mix design approved by the client.
Mix shall be proportioned with maximum aggregates cement ratio of 15:1.
The water content shall be adjusted to the optimum to facilitate the compaction by rolling.
To determine the optimum water content, selected proportion of the mix shall be compacted in the cube mould using steel tamper or by vibrating hammer at various moisture contents.
Cubes shall be weighed and weight at different moisture contents shall be recorded.
Also density shall be calculated at various moisture contents and a graph plotted between density and moisture content.
The peak point of the parabola shall give the maximum value of the density and perpendicular drawn from the peak point to the base shall give the optimum moisture content.
Generally, the moisture in the DLC mix shall be OMC +2%.
Actual Moisture content required shall be determined in the field during trial laying of DLC, taking in account evaporation losses.
Cement Content: Ordinary portland cement OPC 43 grade cement shall be used in the mix. Minimum quantity of cement shall be 150Kg/M3 of the lean concrete.
Concrete strength: The average compressive strength of the group of five cubes shall not be less than 10 N/mm^2 at 7 days’ age and minimum strength of any individual cube shall not be less than 7.5 N/mm^2.
Trial Mix
Trial Mixes shall be made on various moisture content using Specified cement content, aggregate grading and aggregate cement ratio.
Optimum moisture and density shall be established by preparing cubes with varying moisture contents.
After establishing the optimum moisture, a set of six cubes shall be casted at optimum moisture & tested for compressive strength on the third & the seven day.
Trial mixes shall be repeated by increasing cement content if the strength is not satisfactory.
Trial Bay Construction
After completion of lab mix design, a trial length of 100 m shall be laid to establish the Construction methodology & final mix.
Trial length shall contain one transverse joint full involving DLC and filter layer and ability to do the job satisfactory shall be demonstrated.
After the construction of trial length, in situ density of freshly laid material shall be determined using sand replacement method with 200 mm Dia density cone.
Three density holes shall be made along the diagonal leaving 500 mm from the edge, which bisects the trial length at equal distance and average of these densities determined.
This average density shall be the reference density and considered 100%.
The field density of regular work shall be compared with reference to this density.
Size of trial bay for DLC shall be min. 9m x 65m.
Sequence of Work for Dry Lean Concrete Construction
The time gap between the layer of PQC over DLC shall be maintained minimum 7 days to ensure complete curing of DLC.
Batching and mixing shall be carried out in a force action central batching and mixing plant having necessary automatic control to ensure accurate proportioning and mixing. The accuracy of weighing scales of the batching plant shall be within + 2 % in the case aggregates and +1 % for cement and water.
Mixed lean concrete shall be transported immediately to the point where it’s to be laid and protected from the weather by covering tipper/dumpers with tarpaulin during transit.
Sufficient tippers shall be employed to ensure the uninterrupted supply to the paver to laying DLC at a uniform speed.
It shall be paved and compacted within 90 minutes from the mixing when temperature of concrete between 25 °C to 30 °C and 120 minutes if less than 25°C.
Placing of Dry Lean Concrete
Lean concrete shall be laid using paver fitted with electronic sensor.
It shall be laid in single layer of 150 mm compacted thicknesses.
High amplitude paver shall be used in laying to achieve good initial compaction.
The laying shall be done lane by lane.
DLC shall be laid in such a way that it is at least 500mm wider on each side than the proposed width including paved shoulder of PQC.
Transverse and longitudinal joints shall be staggered by 800-1000 mm and 300-400 mm respectively from the corresponding joints in the overlaying of concrete slab.
Compaction
Compaction shall be carried out with tandem vibratory roller immediately after the material is laid.
In order to ensure thorough compaction, rolling shall be continued till there is no further movement under the roller.
Layer density shall be tested by sand replacement method while the mix is wet to check the conformance to requirement of minimum 98% density of that achieved during Trial Length.
The density achieved at the edge shall not be less than 96%.
Spreading, compacting and finishing of the lean concrete shall be carried out as per project specifications.
Ensuring the time gap between first batch mixing in any transverse section of the layer and the final finishing of the same within 90 minutes between 25-30-degree C and 120 minutes if mix temperature below 25 degree C.
Joints: Construction and Longitudinal joints shall be provided as per the drawing. Transverse and Longitudinal construction joints shall be staggered by 800-1000mm and 300 – 400mm respectively from the corresponding joints in the overlay concrete slab.
Protection and Curing: Surface shall be kept damp by covering with wet hessian/sprinkling of water of the approved quality. The application procedure of curing compound shall be as per the manufacturer’s instructions manual.
Quality Requirements for DLC
The average compressive strength determined from any group of five cubes shall not be less than 10 N/mm2 at 7 days.
Strength of any individual cube shall not be less than 7.5 N/mm2 at 7 days.
The minimum dry density shall be 98% of that achieved during the trial length construction.
Level tolerance shall be +6 mm or as per the specs.
Edge of the carriage way shall be within tolerance of + 25 mm.
Test frequency and surface finishing shall conform to project specification requirements.
All debris shall be disposed-off at designated locations.
Site shall be kept clean of all oil spills, grease etc. Disposing these off as per the safety procedures given in project safety manual.
Provisions given in the Contract Agreement, specifications, drawings, approved mix designs & ITP shall get precedence over this method statement as applicable.