Dry Lean Concrete (DLC) Methodology, specifications ,joint formation and construction procedure as per MORTH and Indian standard
DLC acts as a sub-base for the cement concrete pavements or pavement quality concrete (PQC) .DLC material has no slump i.e. zero slump.
The design parameters of DLC viz. Width, thickness, grades of concrete, details of joins is given in the drawings.
1 - Sources of Material
The contractor shall indicate to the engineer the source of all material with relevant test data to be used in the dry lean concrete (DLC) work sufficiently in advance and approval of the engineer for same shall be obtained at least 45 days for the scheduled commencement of the work in the trial length. Contractor can take approval of different source of material with relevant test data at least 45 days in advance to use the material by the engineer during the execution of main work.
2 - Cement
Any of the following type of cement may be used with prior approval of the engineer.
If the subgrade soil contains soluble sulphates in a concentration more than 0.5%, sulphate resistant cement conforming to IS:6909 shall be used.
3 - Fly-ash
Fly-ash up-to 20% by weight of cementitious material (cement + fly-ash) may be used along with 43/53 grade cement may be used to replace Ordinary Portland cement (OPC) cement grade 43 up-to 30% by weight of cement.Fly-ash shall conform to IS:3812(part 1).
Aggregate for lean concrete shall be natural material complying with IS:383. Aggregate shall not be alkali reactive.In case engineer consider that the aggregates are not free form dirt, the same may be washed and drained for at least 72 hours before batching, or as directed by the engineer.
Coarse aggregate shall Consist of clean, hard, dense, strong, durable and non-porous pieces of crushed stone and crushed gravel and shall be devoid of pieces of disintegrated stone, soft flaky, elongated, very angular, splintery pieces.The Max. size of aggregate is 26.5 for pavement concrete. No aggregate which has water absorption more than 2% shall be used in concrete mix. The aggregate shall be tested for soundness in accordance with IS:2386(part-5).After 5 cycle of testing , the loss shall not be more than 12% , If sodium sulphate solution is used , 18% If magnesium sulphate solution is used.The combined flakiness index and elongation index of aggregate shall not be more than 35% and the Los Angeles abrasion value shall also not be exceed 35.
The aggregate gradation shall comply with Table 600-1 given below..
The fine aggregate shall consist of clean natural sand and crushed stone sand or a combination of the two and shall conform to IS :383. Fine aggregate shall be free from soft particles, clay ,shale, loam, cemented particles ,mica and organic and other foreign matter. The fine aggregate shall have a sand equivalent value of not less than50 when tested in accordance with the requirement of IS:2720 (part 37).
Water used for mixing and curing shall be clean and free from injurious amounts of oils, acids, alkalis, salts, sugar, organic materials or other substances that may be deleterious to concrete or steel.
Potable water is generally considered satisfactory for mixing concrete. Water found satisfactory for mixing is also suitable for curing of concrete.
The PH value of water shall not be less than 6.
Sea water is not allowed due to presence of harmful salts. Under unavoidable circumstances sea water may be used for mixing or curing in plane concrete not in embedded steel.
Proportioning of the material for the mix
The mix shall he proportioned with a maximum aggregate cementitious material ratio 15:1.
A- Moisture Content
The optimum water content shall be determined and demonstrated by rolling during trail length construction and the optimum moisture content and degree of compaction shall be got approved by the engineer. While laying in main work, the lean concrete shall have a moisture content between the optimum and optimum +2 percent, keeping in view the compaction and compensate evaporation losses.
B- Cement Content
Cement content in DLC shall be such that the strength specified in concrete strength( Next point) is achieved.The minimum cement content shall be 150 kg/cu.m of concrete. In case flyash is blended at site as part replacement of cement, the quantity of flyash shall not be more than 20% by weight of cementitious material and the content of OPC( Ordinary Portland Cement) shall not be less than 120 kg/cu.m.
If this (above described) minimum is not sufficient to produce DLC of the specified strength, it shall be increased by the contractor at his own cost.
C - Concrete Strength
The average compressive strength of each consecutive group of 5 cubes shall not be less than 10 MPa at 7 days. In addition, the minimum compressive strength of each cube shall not be less than 7.5 MPa at 7 days. The design mix shall be got approved from the engineer and demonstrated in the trail length construction.
Construction of DLC Layer ( Construction Procedure)
The DLC shall be laid on the prepared granular drainage layer. The dry lean concrete (DLC) sub-base shall be over laid with concrete pavement (viz. Pavement quality Concrete (PQC)) only after 7 days of sub base construction.
A- Batching and Mixing
Plant mix lean concrete shall be transported in tipping trucks at the required point and discharged immediately. Protection from weather is done by covering the tipping trucks with tarpaulin during transit. Each tipping truck shall be washed with water jet before next loading.
Lean concrete shall be placed by a paver with electronic sensor on the drainage layer or as specified in the contract.
DLC material shall be laid in one layer in even manner without segregation with suitable equipments.
The paver machine shall have high amplitude tamping bars to give good initial compaction to the sub-base. One day before placing of the DLC sub base, the surface of the granular sub base /drainage layer shall be given a fine spray of water and rolled with smooth wheeled roller.
Preferably the lean concrete shall be placed and compacted across the full width of the two lane carriage way by constructing it one go.
In roads with carriage way more than 2 lanes a longitudinal joint shall be provided.Transverse butt type joint shall be provided at the end of the construction in a day.
The DLC shall be laid in such way that it is atleast 750 mm wider on each side than the proposed width including paved shoulder of the concrete pavement.
The shall be carried out immediate after the material is laid and levelled.Rolling shall be continued on the full width till there is no further visible movement under the roller and the surface is well closed.
The minimum dry density obtained shall not be less than 98% of that achieved during the trail length construction (of max dry density). The density achieved at the edge I.e. 0.5 meter from the edge shall not be less than 96% of the that achieved during the trail length construction (of max dry density).
The spreading,compacting and finishing of the lean concrete shall be carried out as soon as possible and time between the mixing of first batch of concrete in any transverse section of the layer and the final finishing of the same shall not exceed 90 minutes when the temperature of concrete is between 25°C and 30°C, and 120 minutes if less than 25°C.This period may be reviewed by the Engineer in the light of the results of the trial run but in no case shall it exceed 120 minutes. Work shall not proceed when the temperature of the concrete exceeds 30°C. If necessary, chilled water or addition of ice may be resorted to for bringing down the temperature. It is desirable to stop concreting when the ambient temperature is above 35°C. After compaction has been completed, roller shall not stand on the compacted surface for the duration of the curing period except during commencement of next day's work near the location where work was terminated the previous day.
Double drum smooth-wheeled vibratory rollers of minimum 80 to 100 kN static weight are suitable for rolling dry lean concrete.
The number of passes required to obtain maximum compaction depends on the thickness of the drylean concrete.
Except on super elevated portions where rolling shall proceed from the inner edge to the outer, rolling shall begin from the edges gradually progressing towards the centre. First, the edge/edges shall be compacted with a roller running forward and backward. The roller shall then move inward parallel to the centerline of the road, in successive passes uniformly lapping preceding tracks by at least one half width. A preliminary pass without vibration to bed the Dry Lean Concrete down shall be given followed by the required number of passes to achieve the desired density and, a final pass without vibration to remove roller with vibration marks and to smoothen the surface.
For repairing honeycombed/hungry surface, concrete with aggregates of size 10 mm and below shall be spread and compacted as per Specifications.Any level/thickness deficiency shall be corrected after applying concrete with aggregates of size 10 mm and below after roughening the surface. Surface regularity also shall be checked with 3 m straight edge.
Joints Construction and longitudinal joints shall be provided as per the drawings.
Transverse butt type joint shall be provided at the end of the construction in a day. Longitudinal construction joint shall be provided only when full width paving is not possible. Transverse joints in Dry Lean concrete shall be staggered from the construction butt type joint in Concrete pavement by 800-1000 mm.
Longitudinal joint in Dry Lean Concrete shall be staggered by 300-400 mm from the longitudinal joint of concrete pavement.
As soon as the lean concrete surface is compacted, curing shall commence. One of the following methods shall be adopted:
a) Curing may be done by covering the surface by gunny bags/hessian, which shall be kept wet continuously for 7 days by sprinkling water.
b) The curing shall be done by spraying with approved resin based aluminized reflective curing compound conforming to ASTM-C 309-81. As soon as the curing compound has lost its tackiness, the surface shall be covered with wet hessian for 3 days. The rate of application shall be as recommended by the supplier.
c) Wax-based white pigmented curing compound with water retention index of not less than 90 percent shall be used to cure the dry lean concrete. The curing compound shall conform to BS:7542. The compound shall be applied uniformly with a mechanical sprayer and with a hood to protect the spray from the wind. The curing compound shall be applied over the entire exposed surface of the Dry Lean Concrete, including sides and edges, at the rate of 0.2 litres/sq.m, or as recommended by the supplier.
Measurement for Payment
The unit of measurement for dry lean concrete pavement shall be in cubic metre of concrete placed, based on the net plan area for the accepted thickness shown on the drawings or as directed by the Engineer.
Thanks a lot
Mukesh Kumar (Gyan of engineering)