Construction Methodology of Dry Lean Concrete
Dry Lean Concrete (hereinafter referred to as “DLC”) is first layer of rigid pavement, mechanically rolled and compacted over sub-base (GSB). For all major State Highway and National Highway projects, a thickness of at least 150 mm is advised. When DLC is used as a sub base for roads other than the ones mentioned above, a thickness of 100 mm is advised.
Machinery
- Excavator or JCB
- Loader
- Tippers/Dumpers
- Concrete Batching Plant
- Paver with Sensor or Motor Grader
- vibratory Rollers
- Water tanker
- Survey kit etc.,
Material
All the material required in the construction will be from the approved sources by the Engineer and according to the relevant clauses of MoRT&H specifications mentioned in the Contract.
Cement – 601.2.2
Coarse Aggregates – 602.2, 4.2
Fine Aggregates – 602.2, 4.3
Water – 601.2, 4
Aggregate gradation given in table 600-1 of MoRT&H specifications shall be adopted after blending the coarse & fine aggregates.
Aggregates are fed into the bins from the approved stacks. Cement is obtained in bags for feeding the hopper of the concrete batching plant.
Mix for dry lean concrete will be appropriately proportioned to achieve the strength requirement mentioned in the specifications and prior approval will be taken from the Engineer before adopting for the site trials.
Transportation
The concrete is transported in tippers from the batching plant and to the place of laying. The tippers are covered with tarpaulins in order to protect them from the weather during transit. It will ensure that there will be a sufficient number of trippers for an uninterrupted supply of material to the paving equipment. The lead time will be in accordance with clause 601.5.5.2.
Paving
The grader is set in position and the concrete is dumped in the hopper in the case of a normal sensor grader and grading is done with a loose thickness of about 200mm for achieving the compacted thickness of 150mm. the loose thickness is ascertained by the trail laying of DLC. The grading will be done in two lanes. After compaction of an average length of 300mm, the second lane is done on the same day matching with the first lane.
A combination of two graders for a 7m width may also be tried for grading full width at a time with a little time gap between the two grading lanes.
It will ensure that transverse and longitudinal construction joints are staggered by 500 – 1000mm and 200 – 400mm respectively from the corresponding joints in the overlaying pavement quality concrete.
Compaction
The compaction is carried out immediately after the material is laid and leveled, rolling will be continued at full width till there is no further visible movement under the roller and the surface is closed.
The minimum dry density obtained will be 98% of that achieved during the trail length construction vide clause 601.7. The densities achieved at the edges i.e. 0.50m from the edge will not be less than 96% of that achieved during the trail construction vide clause 601.7.
The required compactive effort in terms of a number of passes is ascertained by employing one single drum smooth wheeled vibratory roller and one double drum smooth wheeled vibratory roller on the trail stretch of DLC. Initial two passes will be plain in order to roll down the loose DLC, and then passes with vibration will be given followed by plain passes for providing a finished and closed surface.
Curing
Curing of the DLC will be done by covering the surface with gunny bags or Hessian cloth, which will be kept continuously moist for 7 days by sprinkling water using water tankers.
Quality Control
After the construction of the trail length, the in-situ density of freshly material will be determined by the sand replacement method with a 20mm dia density cylinder. The density will be taken at three locations along the diagonal which bisects the trail length.
The locations for testing will be at least 50cm from the edges. The average bulk density of these holes will be considered as 100 percent and the field bulk density of regular work will be compared with this reference density in accordance with clauses 601.5.5.1 and 903.5.1.2 if MoRT&H specifications.
A DLC mixture’s average compressive strength shouldn’t be less than 10 MPa over 7 days. The primary acceptance criteria for dry lean concrete mixtures is the cube’s compressive strength after 7 days.
The routine quality control tests for levels alignment and material will be exercised as per the frequency norms given in table 900-6. other tests will be done at the frequency mentioned herein. If there can be any reduction in the total number of tests depending on the consistency of the results, the issue will be taken up with the Engineer.
Repairs
Rectification, if any, will be taken up before overlaying. the low spots, loose material, potholes, etc., will be made good by using fresh lean concrete material duly compacting the same as per specifications. For repairing honeycombed surface, and any other spots which can not be repaired with DLC material, concrete with aggregates of size 10mm as below will be used after roughening the surface for proper laying in.
Uses
- It is used as a Sub-Base for Rigid Pavement
Advantages
- It has high resistance to deformation
- High efficiency to transfer loads in joints
Above all, this is the general procedure for the laying of Dry Lean Concrete (DLC).
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