Dry Lean Concrete (DLC) - Construction Flow Chart
Methodology for Dry Lean Concrete (DLC)
Scope of Work
The work shall consist of construction of (zero slump) DLC sub-base for cement concrete pavement in accordance with the requirements of MoRTH Specifications Clause 601 and in conformity with the lines, grades and cross sections shown on the drawings.
Reference
- Contract Agreement
- IRC: SP: 84-2014
- Ministry of Road Transport & Highway Specifications (MoRTH - 5th Revision)
- Relevant Drawings
Materials
The Contractor shall indicate to the AE the source of all materials with relevant test data to be used in DLC work sufficiently in advance and the approval of the AE for the same shall be obtained before the scheduled commencement of the work in trial length.
1. Cement
Cement to be used may preferably be obtained in bulk form. Cement shall be transported, handled and stored on the site in such a manner as to avoid deterioration or contamination. Cement shall be stored above ground level in perfectly dry and water-tight sheds and shall be stacked to a height of not more than eight bags.
Wherever bulk storage containers are used, their capacity should be sufficient to cater to the requirement at site. The containers shall be cleaned at least once every 3 months. MTC or laboratory test certificate shall be maintained for each and every batch of cement received.
Following types of cement may be used with prior approval of the AE:
| S. No. | Type of Cement | Conforming To |
|---|---|---|
| 1 | Ordinary Portland Cement | IS:269 |
| 2 | Portland Slag Cement | IS:455 |
| 3 | Portland Pozzolana Cement | IS:1489 Part-I |
If subgrade soil contains soluble sulphates in a concentration more than 0.5%, sulphate resistant cement conforming to IS:12330 or Portland Slag Cement with slag up to 50% shall be used.
2. Fly Ash
Fly ash up to 20 percent by weight of cementitious material (Cement + Fly Ash) may be used as replacement of OPC cement and in such case the cement content shall not be less than 120 kg/cum.
Fly ash shall conform to IS:3812 Part-I.
3. Aggregates
Aggregates for DLC shall be natural material complying with IS:383. The aggregates shall not be alkali reactive. The limits of deleterious materials shall not exceed the requirements set forth in Table 600-2 of MoRTH Specifications.
a. Coarse Aggregates
Coarse aggregate shall comply with Clause 602.2.6.2, except that the maximum size of the coarse aggregate shall be 26.5 mm, and aggregate gradation shall comply with Table 600-1.
b. Fine Aggregates
The fine aggregate shall comply with Clause 602.2.6.3.
4. Curing Compound
The curing compound shall be white pigmented or transparent type with water retention index of 90% when tested in accordance with BS:7542.
5. Water
Water used for mixing and curing shall comply with Clause 602.2.7.
Proportioning of Materials for the Mix
The mix shall be proportioned with a maximum aggregate cementitious material ratio of 15:1 (as per MoRTH). The water content shall be adjusted to the optimum as per Clause 601.3.2 for facilitating compaction by rolling.
The strength and density requirements of concrete shall be determined in accordance with Clause 601.7 and Clause 601.8 by making trial mixes. Care should be taken to prevent one size of aggregate falling into other size of the hopper of the feeding bin while loading the individual size of the aggregates into the bins.
1. Moisture Content
The OMC shall be determined and demonstrated by rolling during trial length construction and the OMC and degree of compaction shall be got approved from AE.
While laying in the main work, the DLC shall have a moisture content between the optimum and optimum +2 percent, keeping in view the effectiveness of the compaction achieved and to compensate for evaporation losses.
2. Cement Content
The cement content in the DLC shall be such that the strength specified in Clause 601.3.4 of MoRTH is achieved.
- Minimum cement content shall be 150 kg/cum of concrete.
- Fly ash replacement shall not exceed 20% by weight of cementitious material.
- Minimum OPC content shall not be less than 120 kg/cu.m.
3. Concrete Strength
Clause 601.3.4 of MoRTH shall be applicable.
- Average compressive strength of each consecutive group of 5 concrete cubes shall not be less than 10 MPa at 7 days.
- Minimum compressive strength of any individual concrete cube shall not be less than 7.5 MPa at 7 days.
4. Sub-Grade
The subgrade shall conform to the grades and cross-sections shown on the drawings and shall be uniformly compacted to modified Proctor density not less than 97%.
The lean concrete sub-base shall not be laid on a subgrade softened by rain after its final preparation. Surface trenches and soft spots, if any, shall be properly backfilled and compacted to avoid weak spots.
A day before placing of the sub-base, the subgrade surface shall be given a fine spray of water and rolled with one or two passes of a smooth wheeled roller after a lapse of 2-3 hours in order to stabilise loose surface.
If found necessary, another fine spray of water may be applied just before placing the sub-base.
5. Drainage Layer
A drainage layer conforming to Clause 401 shall be laid above the subgrade before laying the DLC sub-base.
Construction
1. PROJECT UNDERSTANDING & OBJECTIVE
Dry Lean Concrete (DLC) is provided as a structural sub-base layer below PQC (Pavement Quality Concrete).
The main objectives are:
- Provide uniform, strong and stable working platform
- Prevent pumping of fines into pavement concrete
- Distribute wheel loads from PQC to subgrade
- Improve pavement life and structural performance
- Act as non-erodible and non-yielding base layer
The DLC layer is critical because any defect directly impacts PQC performance; therefore, strict control is mandatory.
2. PRE-CONSTRUCTION PLANNING
Before starting DLC work, the following planning is mandatory:
2.1 Work Zone Planning
- Chainage-wise segmentation (typically 100–200 m per block)
- Identification of:
- Plant location
- Haul roads
- Dumping/laying sequence
- Traffic diversion plan (if applicable)
2.2 Resource Mobilization
- Batching plant (fully calibrated)
- DLC paver (compatible or modified WMM paver)
- Vibratory rollers (minimum 8–10 ton, preferably 10–12 ton)
- Survey team (Auto level / Total station)
- QA/QC lab setup at site
2.3 Trial Patch (Mandatory)
- Minimum 50–100 m trial stretch
- Establish:
- Mix behavior
- Compaction pattern
- Loose thickness factor
- Roller passes required
- Workability window
Approval of trial patch is mandatory before mass execution.
3. MATERIAL REQUIREMENTS & CONTROL
3.1 Cement
- OPC 43 or OPC 53 grade
- Fresh, no lumps
- Stored in dry, waterproof godown
- FIFO (First In First Out) strictly followed
3.2 Aggregates
- Crushed aggregate only (no rounded river gravel)
- Well graded as per MORTH grading
- Important properties:
- Los Angeles abrasion ≤ 40%
- Flakiness index controlled
- Free from clay lumps & organic matter
3.3 Water
- Potable quality water
- Chloride/sulfate content within permissible limits
4. MIX DESIGN & PERFORMANCE REQUIREMENT
Typical DLC Mix Parameters (as per practice):
- Cement content: 150–180 kg/m³
- Water content: Controlled for very low slump (0–25 mm)
- Compaction: Heavy vibration required
- 7-day compressive strength: ≥ 7.5 MPa
- Mix type: Zero slump / very stiff mix
DLC is NOT RCC or normal concrete. It behaves like a dry, damp granular cemented mass.
- Overwatering = failure (cracking + pumping)
- Underwatering = poor compaction
5. BATCHING & MIXING OPERATION
The batching plant shall be capable of proportioning the materials by weight, each type of material being weighed separately in accordance with Clause 602.9.2, Clause 602.9.3.1 and Clause 602.9.3.2 of MoRTH Specifications.
5.1 Batching Plant Control
- Fully computerized batching plant preferred
- Daily calibration mandatory
- Moisture correction of aggregates every shift
5.2 Mixing Sequence
- Coarse + fine aggregates fed first
- Cement added uniformly
- Dry mixing (15–20 seconds)
- Controlled water addition
- Wet mixing (minimum 30–45 seconds)
Key Control Point
- Mix must appear uniformly moist but not flowing
- No segregation or cement balling allowed
6. TRANSPORTATION TO SITE
Plant mix DLC shall be discharged immediately from the mixer, transported directly to the point where it is to be laid and protected from weather by covering the tipping trucks with tarpaulin during transit, if necessary.
- Transport via dumper or transit mixer (modified discharge)
- Covered with tarpaulin to prevent moisture loss
- Maximum travel time: 30–45 minutes recommended
- No retempering allowed at site
7. PREPARATION OF SUBGRADE
Before DLC placement:
- Subgrade must be properly compacted (≥ 95–97% MDD)
- Trimmed to final formation level
- Free from loose soil, debris, standing water
Pre-Laying Treatment
- Light water sprinkling (SSD condition)
- No muddy condition allowed
- Proof rolling done to identify weak spots
8. LAYING & SPREADING OF DLC
DLC shall be placed by a paver with electronic sensor on the drainage layer.
- In roads with carriageway more than 2 lanes, a longitudinal joint shall be provided.
- Transverse butt type joints shall be provided at the end of construction in a day.
- The DLC shall extend beyond the PQC (including shoulder portion, if any) by 0.75 m on either side.
8.1 Mechanical Paver Laying (Preferred Method)
- DLC paver or modified WMM paver used
- Automatic level sensors (string wire or sensor beam)
- Continuous feed from dumpers
8.2 Manual Laying (Only for small patches)
- Spread using shovels
- Avoid dumping in heaps
- Maintain uniform loose thickness
8.3 Thickness Control Strategy
- Loose thickness = Design thickness + compaction allowance
- Typically 10–20% extra provided initially
- Level pegs at 10 m interval for control
9. COMPACTION PROCEDURE (CRITICAL STAGE)
The compaction shall be carried out immediately after the material is laid and levelled. Rolling shall be continued on the full width till there is no visible movement under the roller and the surface becomes well closed.
9.1 Rolling Sequence
- Initial static pass (smooth drum roller)
- Vibratory rolling (4–8 passes, overlap 1/3 drum width)
- Final finishing pass (static mode)
9.2 Compaction Requirements
- Minimum dry density shall not be less than 98% of that achieved during the trial length construction
- Density at edges (0.5 m from edge) shall not be less than 96% of trial density
- Double drum smooth wheeled vibratory rollers of 80–100 kN static weight shall be used
9.3 Time & Temperature Control
The spreading, compacting and finishing of lean concrete shall be completed within the permissible time limits based on concrete temperature:
| Concrete Temperature | Maximum Time Limit |
|---|---|
| 25°C to 30°C | 90 Minutes |
| Less than 25°C | 120 Minutes |
This period may be reviewed by the Engineer based on trial run results, but in no case shall it exceed 120 minutes.
9.4 Operational Controls
- Work shall not proceed when concrete temperature exceeds 30°C
- Chilled water or ice may be used to control mix temperature if required
- Concreting should preferably be stopped when ambient temperature exceeds 35°C
- Rollers shall not stand on compacted DLC during curing period except during commencement of next day’s work
9.5 Control Warning Signs
- Cracking during rolling → mix too dry or delayed compaction
- Pumping → over-wet mix or weak subgrade
- Surface tearing → improper roller timing
10. FINISHING & SURFACE CONTROL
- Surface checked using 3 m straight edge
- Permissible variation: ±10 mm
- Cross fall and camber strictly maintained
Defect Rectification
- Low areas → fresh DLC fill + recompaction
- High spots → trimming + rerolling
11. CONSTRUCTION JOINTS
Construction and longitudinal joints in Dry Lean Concrete (DLC) shall be executed strictly as per approved drawings, ensuring structural continuity with PQC and maintaining uniform load transfer characteristics.
11.1 Types of Joints & Location Control
- Transverse Construction Joints: Provided at the end of each day’s work or unavoidable stoppage of paving operations.
- Longitudinal Construction Joints: Provided only where full-width paving is not feasible due to operational constraints.
- Joint Staggering Requirement:
- Transverse DLC joints shall be staggered from PQC joints by 800–1000 mm.
- Longitudinal DLC joints shall be staggered from PQC joints by 300–400 mm.
11.2 Construction Procedure at Joint Face
- Joint shall be formed as a true vertical face without irregularities
- Before restarting work, the joint face shall be properly trimmed and cut back to sound material
- All loose, weak, or segregated material shall be removed completely
- Surface shall be lightly moistened to ensure proper bonding with fresh mix
11.3 Quality & Execution Controls
- No cold joints shall be permitted without prior approval and proper treatment
- Continuity of DLC layer shall be maintained without segregation or honeycombing at joints
- Joint edges shall be properly compacted using hand/plate compaction where required
- Any visual defect at joint locations shall be rectified immediately before proceeding further
11.4 Engineering Significance
Proper joint construction ensures uniform load transfer between DLC panels and prevents weak planes that may lead to:
- Reflection cracking in PQC layer
- Localized settlement at joint locations
- Loss of structural continuity in pavement system
12. CURING PROCESS (VERY CRITICAL)
Curing shall be carried out immediately after finishing of Dry Lean Concrete (DLC) to ensure adequate hydration and strength development. Continuous moisture retention is essential to prevent shrinkage cracks and loss of durability.
12.1 Permitted Curing Methods
a. Wet Hessian / Gunny Bag Curing
The surface shall be covered with wet gunny bags or hessian cloth and kept continuously moist for a minimum period of 7 days through regular sprinkling of water.
b. Resin-Based Curing Compound
Approved resin-based aluminized reflective curing compound shall be sprayed in accordance with Clause 602.9.12. After the curing compound loses tackiness, the surface shall be covered with wet hessian for at least 3 days to ensure sustained moisture retention.
c. Wax-Based Curing Compound
Wax-based white pigmented curing compound having a water retention index not less than 90% shall be applied uniformly using a mechanical sprayer fitted with a wind protection hood to ensure even coverage.
12.2 Duration of Curing
- Minimum curing period: 7 days (continuous)
- No interruption in moisture application shall be permitted during curing period
12.3 Execution Controls
- Curing shall begin immediately after final finishing and surface acceptance
- Surface shall remain continuously moist throughout curing period
- No vehicular or construction activity shall be permitted during curing
13. QUALITY CONTROL PLAN (QCP)
Field Tests
- Field density (every 250–500 m²)
- Thickness check (random grid)
- Moisture content check
Lab Tests
- 7-day compressive strength (mandatory)
- Aggregate gradation
- Cement content verification (if required)
14. ACCEPTANCE CRITERIA
- Strength ≥ 7.5 MPa at 7 days
- Density ≥ 98% MDD
- Thickness tolerance: ±10 mm
- Surface irregularity ≤ 10 mm (3 m straight edge)
15. SAFETY & ENVIRONMENT
Safety Measures
- PPE mandatory (helmet, vest, shoes)
- Roller movement controlled with flagmen
- No unauthorized entry in work zone
- Night lighting for late work
Environmental Controls
- Dust suppression on haul roads
- No slurry discharge
- Proper cement handling to avoid spillage
16. KEY CONTROL points:
- Continuous material flow (no stoppage)
- Synchronization of plant–transport–paver–roller
- Strict timing discipline (critical factor)
- Daily calibration of batching plant
- Skilled roller operators
- QA/QC engineer presence full-time
17. COMMON FAILURE CAUSES :
- Delay between mixing and compaction
- Excess water in mix
- Poor subgrade preparation
- Insufficient roller passes
- Segregation during transport
- Inadequate curing in initial 24 hours
18. Quality Control
Control of quality of materials and workmanship shall be exercised by the Engineer in accordance with Section 900 of MoRTH Specifications.
Dry Lean Concrete (DLC)
Cement - Inspection at Source
| Test | Standard | Frequency | Requirement |
|---|---|---|---|
| Normal Consistency | IS 4031 | Each batch | IS 12269 / IS 8112 / IS 1489 |
| Initial & Final Setting Time | IS 4031 | Each batch | Initial ≥ 30 min, Final ≤ 600 min |
| Soundness | IS 4031 | Each batch | Max 10 mm |
| Compressive Strength (Mortar) | IS 4031 | Each batch | 28-day strength as per grade |
| Chemical Test | IS 4031 | Every 6 months | As per IS 269 / IS 1489 |
Water - Inspection at Source
| Test | Standard | Frequency | Requirement |
|---|---|---|---|
| Phenolphthalein Test | IS 3025 | Every 3 months | Max 5 ml of 0.02N NaOH |
| Methyl Orange Test | IS 3025 | Every 3 months | Max 25 ml of 0.02N H₂SO₄ |
| pH | IS 3025 | Every 3 months | ≥ 6 |
| Organic | IS 3025 | Every 3 months | Max 200 mg/l |
| Inorganic | IS 3025 | Every 3 months | Max 3000 mg/l |
| Sulphate | IS 3025 | Every 3 months | Max 400 mg/l |
| Chlorides | IS 3025 | Every 3 months | Max 500 mg/l |
| Suspended Matter | IS 3025 | Every 3 months | Max 2000 mg/l |
Coarse & Fine Aggregates
| Test | Standard | Frequency | Requirement |
|---|---|---|---|
| Sieve Analysis | IS 2386 Part-1 | 1 Test/day/plant |
26.5mm: 100% 19mm: 75–95% 9.5mm: 50–70% 4.75mm: 30–55% 2.36mm: 17–42% 0.600mm: 8–22% 0.300mm: 7–17% 0.150mm: 2–12% 0.075mm: 0–10% |
| Aggregate Impact Value | IS 2386 | 1/day | Max 45% (30% for wearing surface) |
| Los Angeles Abrasion | IS 2386 | As required | Max 35% |
| Flakiness & Elongation | IS 2386 | Weekly | Max 35% |
| Soundness | IS 2386 | Once/source | 12% Na₂SO₄ / 18% MgSO₄ |
| Water Absorption | IS 2386 | Once/source | Max 2% |
Concrete - In Process Inspection
| Test | Standard | Frequency | Requirement |
|---|---|---|---|
| Alignment, Cross Section & Level | Standard Survey Practice | Regular | As per specification |
| Field Compaction | IS 2720 | 3 density tests / 2000 m² | ≥ 98% of trial density (≥96% edges) |
Acceptance Criteria
| Test | Standard | Frequency | Requirement |
|---|---|---|---|
| Strength of Concrete | IS 516 | Min 3 samples / 1000 m² | Min 10 MPa at 7 days |
| Surface Level Tolerance | Standard Survey Practice | Every 10 m | ±6 mm |
Common Site Problems and Solutions
| Problem | Cause | Solution |
|---|---|---|
| Low Density | Inadequate roller passes | Increase vibration and adjust moisture |
| Segregation | Poor plant control | Ensure proper aggregate blending |
| Cracking | Early moisture loss | Immediate curing |
| Thickness Variation | Improper paver calibration | Sensor check and trial adjustment |
Safety Measures
- PPE shall be mandatory
- Equipment operators shall be trained
- Barricading and traffic diversion shall be provided
- Dust control measures shall be implemented
- Emergency first aid facility shall be available
19. DOCUMENTATION REQUIREMENTS
The following records shall be systematically maintained at site to ensure quality assurance, traceability, and audit compliance:
- Trial mix approval records
- Concrete cube test reports (7-day and 28-day)
- Field density test reports
- Daily progress reports (DPR)
- Equipment calibration certificates (batching plant, rollers, etc.)
Proper documentation ensures full compliance with contractual requirements and supports third-party and client audits.
20. ENVIRONMENTAL CONSIDERATIONS
- Effective control of cement dust during handling and mixing operations
- Prevention of material spillage during transportation and laying
- Proper disposal and management of waste concrete and slurry
- Noise control measures during night-time construction activities
21. CHECKLIST BEFORE CONCRETING
- ✔ Approved trial mix design and laboratory validation
- ✔ Approved GSB layer level and surface readiness
- ✔ Calibrated batching plant and equipment
- ✔ Verified OMC (Optimum Moisture Content) of subgrade
- ✔ Approved rolling pattern established from trial stretch
- ✔ QA/QC team fully deployed at site
22. CONCLUSION
Dry Lean Concrete (DLC) is a critical structural layer in rigid pavement construction. Its performance directly influences the durability and service life of the pavement system.
Strict control over mix design, moisture content, compaction, curing, and quality testing is essential to achieve the required performance standards.
Compliance with MoRTH Section 600, supported by systematic field verification, documentation, and engineering supervision, ensures long-term success of highway infrastructure projects.
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