Slabs are critical horizontal structural elements that transfer loads to beams, columns, and walls. This training module covers the complete process of slab construction from types of slabs to reinforcement detailing, formwork, concrete placement, and quality control, ensuring compliance with relevant IS codes.
Training Duration: 1 week
Prerequisite Skills: Basic knowledge of construction drawings, reinforcement details, and concrete works
Slabs are classified based on support conditions, shape, and structural systems. The type of slab influences reinforcement details, formwork requirements, and construction methodology.
- One-way Slab: Supported on two opposite sides with span ratio (long/short) > 2, with main reinforcement running along shorter span.
- Two-way Slab: Supported on all four sides with span ratio < 2, with main reinforcement in both directions.
- Cantilever Slab: Supported on one side only, extended outward with reinforcement at top.
- Flat Slab: Directly supported on columns without beams, may include drop panels or column capitals.
- Ribbed/Waffle Slab: Contains ribs in one or two directions with voids between, reducing self-weight.
- Hollow-core Slab: Precast with continuous voids to reduce weight while maintaining strength.
- Post-tensioned Slab: Contains tensioned tendons to improve load capacity and reduce thickness.
- Composite Slab: Uses profiled steel deck as permanent formwork with concrete topping.
Slab Type |
Typical Thickness |
IS Code Reference |
One-way Slab |
Span/30 to Span/24 |
IS 456:2000, Cl. 23.2.1 |
Two-way Slab |
Span/35 to Span/28 |
IS 456:2000, Cl. 24.1 |
Cantilever Slab |
Span/12 to Span/10 |
IS 456:2000, Cl. 23.2.1 |
Flat Slab |
Span/33 to Span/25 |
IS 456:2000, Cl. 31.3 |
Note: The above thicknesses are indicative and may vary based on loading conditions, concrete grade, and other design factors. Always follow structural drawings and specifications.
- IS 456:2000: Code of Practice for Plain and Reinforced Concrete - Main code governing slab design and construction.
- IS 1786:2008: High Strength Deformed Steel Bars and Wires for Concrete Reinforcement.
- IS 2502:1963 (Reaffirmed 2018): Code of Practice for Bending and Fixing of Bars for Concrete Reinforcement.
- IS 10262:2019: Concrete Mix Proportioning - Guidelines.
- IS 516:1959 (Reaffirmed 2018): Method of Tests for Strength of Concrete.
- IS 13920:2016: Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces.
- SP 34:1987: Handbook on Concrete Reinforcement and Detailing.
Parameter |
Requirement |
IS Code Reference |
Minimum Cover |
20 mm for slabs (mild exposure) |
IS 456:2000, Cl. 26.4 |
Min. Reinforcement |
0.12% of gross area for HYSD bars |
IS 456:2000, Cl. 26.5.2.1 |
Max. Bar Spacing |
3 × slab thickness or 300 mm (whichever is less) |
IS 456:2000, Cl. 26.3.3 |
Development Length |
Ld = (φσs)/(4τbd) where τbd = bond stress |
IS 456:2000, Cl. 26.2 |
Lap Length |
Min. 45d for tension, where d = bar diameter |
IS 456:2000, Cl. 26.2.5.1 |
Important: Always use the latest version of IS codes and check for any amendments. Non-compliance with code requirements can lead to structural deficiencies and safety hazards.
Understanding the difference between main reinforcement and distribution reinforcement is crucial for proper slab construction.
- Main Bars: Carry the primary bending moment and are placed perpendicular to the direction of span (along shorter span in one-way slabs).
- Distribution Bars: Provide load distribution, control cracking, and are placed perpendicular to main bars (along longer span in one-way slabs).
- Reinforcement Ratio: In one-way slabs, distribution bars are typically 0.2% of gross sectional area or ≥ 20% of main reinforcement.
- Two-way Slabs: Both directions have main bars, with the reinforcement proportionate to the moments in respective directions.
- Bottom Reinforcement: Resists positive moment (tension at bottom) at mid-span and extends throughout the slab.
- Top Reinforcement: Resists negative moment (tension at top) at supports and corners.
- Continuous Slabs: Require top reinforcement at intermediate supports (typically extending 0.3L on either side of support, where L = effective span).
- Corner Reinforcement: Special top reinforcement at corners of two-way slabs to resist corner uplift.
Proper lapping of reinforcement is critical to ensure structural continuity and load transfer. IS 456:2000 provides detailed requirements for lapping.
- Lap Length Calculation: Min. lap length = Ld or 30d (whichever is greater) for tension bars, where Ld = development length, d = bar diameter.
- Bottom Bars Lapping: Preferably done at mid-span (away from maximum moment zone) or at supports for alternate bars.
- Top Bars Lapping: Preferably done away from supports (zone of maximum negative moment).
- Staggered Laps: Laps should be staggered, with not more than 50% of bars lapped at one section.
- Minimum Offset: Minimum clear distance between lapped bars = 2 × bar diameter.
- Binding: All laps must be securely tied with binding wire with minimum 3 ties per lap.
Critical: Per IS 456:2000, never lap more than 50% of bars at any section. Avoid lapping in high-stress zones. For larger diameter bars (> 36mm), lapping is not recommended; use mechanical couplers or welding.
Proper positioning and maintaining cover for reinforcement is achieved using spacers and chairs.
- Cover Blocks: Precast cement concrete blocks (same grade as structural concrete) to maintain bottom cover.
- Chairs/Bar Supports: Metal or plastic supports to hold top reinforcement at correct height.
- Spacing of Spacers: Place at maximum 1.0m center-to-center in both directions.
- Chair Spacing: Maximum 0.75m center-to-center to prevent sagging of top reinforcement.
- Types of Chairs: Individual high chairs, continuous high chairs, or individual bar supports depending on slab thickness.
- Size Calculation: Chair height = slab thickness - (bottom cover + top cover + bottom bar diameter + top bar diameter).
Best Practice: Use only factory-made plastic or precast concrete spacers for maintaining cover. Never use brick pieces, stone chips, or wooden blocks as they can lead to corrosion and compromise durability.
- Measuring tape (30m & 5m).
- Chalk line/chalk powder.
- Spirit level.
- Marker pen/paint.
- Plumb bob.
- String/thread.
- Slab reinforcement drawing.
- Verify beam top levels are as per drawing.
- Mark slab thickness above beam top level.
- Mark locations for any drops, openings, or cutouts.
- Indicate position of services (electrical conduits, plumbing sleeves).
- Mark locations for any inserts or embeds.
- Indicate construction joint locations if applicable.
- Verify all dimensions against the drawing.
- Mark main bar spacing on formwork or shuttering edges.
- Mark distribution bar spacing perpendicular to main bars.
- Indicate extra reinforcement zones (near supports, openings).
- Mark locations for top reinforcement chairs.
- Double-check spacing against Bar Bending Schedule.
Important: Ensure proper level checking before marking. Incorrect levels can lead to varying slab thickness and structural issues.
- Timber Formwork: Traditional system using wooden planks and runners.
- Plywood Formwork: Water-resistant plywood sheets with timber or metal framing.
- Steel Formwork: Steel sheets and frames, suitable for repetitive use.
- Aluminum Formwork: Lightweight, high precision system for rapid construction.
- Table/Flying Formwork: Large pre-assembled units for rapid cycling.
- Proprietary Systems: DOKA, PERI, MIVAN, etc.
- Set up primary supports/props at required spacing (typically 0.9-1.2m).
- Install secondary beams/runners perpendicular to primary beams.
- Place formwork sheets/boards on secondary beams.
- Apply release agent evenly on formwork surface.
- Install edge formwork with proper bracing.
- Provide camber if specified in drawings (typically L/500).
- Seal all joints to prevent grout leakage.
- Install sleeves for services and blockouts for openings.
Check |
Method |
Tolerance |
Level |
Auto level/water level |
±3mm in 3m length |
Dimensions |
Measuring tape |
±10mm |
Verticality of edges |
Plumb bob/spirit level |
±3mm in height |
Support spacing |
Measuring tape |
As per design, typically 0.9-1.2m |
Surface finish |
Visual inspection |
No dents, damages or warping |
Safety Note: Formwork failure is a major cause of construction accidents. Ensure adequate bracing, solid base for props, and use safety shores during concrete pouring. Follow IS 14687:1999 for formwork design and safety guidelines.
- Use rigid PVC conduits (min. 2mm thick) for electrical wiring.
- Place conduits between top and bottom reinforcement layers.
- Avoid placing conduits in highly stressed areas (column strips).
- Secure conduits to reinforcement with binding wire.
- Minimum 20mm clearance between conduits and reinforcement.
- Maximum diameter of conduits limited to 1/3 of slab thickness.
- Install proper sleeves for all penetrations.
- Provide 15-25mm clearance around pipes for movement.
- Seal sleeves to prevent concrete leakage.
- For large openings (>150mm), provide additional reinforcement.
- Avoid clustering of sleeves in one area.
Important: Embedding services must be carefully planned to avoid weakening the slab. For large ducts or multiple services, increase slab thickness locally or use drop panels. Always get approval from the structural engineer before installing unplanned services.
Typical concrete for slabs is M20 or M25 grade, designed for good workability and finish.
Grade |
Cement (kg/m³) |
W/C Ratio |
Slump (mm) |
28-day Strength (MPa) |
M20 |
320-340 |
0.50-0.55 |
75-100 |
20 |
M25 |
350-370 |
0.45-0.50 |
75-100 |
25 |
M30 |
380-400 |
0.40-0.45 |
75-100 |
30 |
- Obtain approval of reinforcement from structural engineer.
- Verify formwork stability, alignment, and levels.
- Check concrete mix design approval.
- Verify cover blocks and chairs are properly placed.
- Ensure electrical conduits and sleeves are secured.
- Arrange for concrete cube samples for testing.
- Check availability of vibrators, screeds, and finishing tools.
- Plan pouring sequence and ensure adequate labor.
- Arrange for proper lighting if concreting extends to evening.
- Prepare curing arrangements.
- Perform slump test before placement (75-100mm recommended).
- Cast cube samples for 7 and 28-day strength tests.
- Start pouring from farthest point and progress systematically.
- Pour in strips or panels based on planned sequence.
- Avoid concrete fall from height exceeding 1.5m.
- Use needle vibrators at 400-600mm intervals to compact concrete.
- Avoid over-vibration to prevent segregation.
- Check level continuously during pouring.
- Use screed boards to achieve uniform level.
- For large slabs, use defined construction joints with proper detailing.
- Wait until bleeding water disappears before finishing.
- Use wooden float for initial leveling.
- For smooth finish, use steel trowel.
- For non-slip finish, use broom or textured roller.
- Check levels with straight edge during finishing.
- Ensure appropriate slopes for drainage where required.
- Apply curing compound or cover with wet burlap once concrete has set.
- Ponding: Create earth bunds and pond water (most effective method).
- Wet Covering: Use wet burlap/gunny bags kept continuously moist.
- Curing Compounds: Apply membrane-forming compounds after final set.
- Duration: Minimum 7 days for OPC concrete, 10 days for PPC/PSC concrete.
- Water Quality: Use clean, potable water free from impurities.
IS 456:2000 (Clause 11.3) specifies minimum periods before formwork removal:
Part of Structure |
OPC Days (T>20°C) |
OPC Days (T<20°C) |
PPC/PSC Days |
Vertical sides of beams, walls, columns |
16-24 hours |
24-48 hours |
24-72 hours |
Slabs (props left under) |
3 days |
4 days |
5 days |
Beam soffits (props left under) |
7 days |
10 days |
14 days |
Removal of props to slabs |
14 days |
18 days |
21 days |
Removal of props to beams |
21 days |
25 days |
28 days |
- Check concrete strength before removal (by cube tests or NDT).
- Remove formwork gradually without shock or vibration.
- For large spans, start removal from center and move to supports.
- Leave re-shoring/back-propping in place for required duration.
- Inspect concrete for any defects immediately after formwork removal.
- Continue curing even after formwork removal.
Safety Warning: Premature formwork removal is a major cause of structural failures in construction. Always follow the specified timeline and verify concrete strength. Never remove supports suddenly or simultaneously.
- Slump test: For workability check (75-100mm for slabs).
- Cube test: 7 and 28-day compressive strength (IS 516).
- Steel reinforcement testing: Tensile strength, bend test.
- Level survey: To check final slab thickness and levels.
- Cover measurements: Using cover meter.
- Surface regularity: Using 3m straight edge.
- Non-Destructive Testing (NDT): Rebound hammer test (IS 13311 Part 2) or Ultrasonic Pulse Velocity (UPV) test (IS 13311 Part 1) for in-situ concrete quality assessment.
- Approved architectural and structural drawings (latest revisions).
- Reinforcement inspection records and Bar Bending Schedule (BBS).
- Formwork inspection records and approval.
- Concrete mix design approval and concrete pour cards.
- Concrete cube test reports (7-day and 28-day strength).
- Material test certificates for cement, aggregates, and steel.
- Daily progress reports and site diaries.
- Level survey reports for formwork and finished slab.
- Curing records (method, duration, start/end times).
- Photographic evidence at critical stages (formwork, rebar, concreting).
- Records of any site instructions, non-conformities, and remedial actions.
Effective on-job training quality relies heavily on continuous supervision and immediate correction of deviations. The site engineer and quality control team play a pivotal role.
- Pre-Pour Checklist: Implement a mandatory checklist before every concrete pour covering formwork, reinforcement, and service installations.
- Visual Inspection: Regular visual inspection of rebar cage for correct spacing, cover, and binding before and during concrete placement.
- Slump Test Verification: Site engineer must personally witness and approve slump tests for every concrete delivery.
- Vibration Monitoring: Ensure proper use of vibrators to achieve full compaction without segregation or honeycombing. Train labor on correct vibration techniques.
- Level Monitoring: Continuously check slab levels during concrete pouring to avoid undulations and ensure correct thickness.
- Curing Supervision: Strict supervision of curing methods and duration. Emphasize the importance of continuous moisture.
- Immediate Rectification: Any non-conformity (e.g., misplaced rebar, formwork deflection) must be identified and rectified immediately before proceeding.
- Toolbox Talks: Conduct daily toolbox talks with labor and supervisors on critical quality aspects for the day's work.
Role of Site Engineer: The site engineer is the primary custodian of on-site quality. Their constant presence, vigilance, and proactive intervention are crucial for ensuring that construction practices align with design specifications and quality standards, turning theoretical knowledge into practical, high-quality execution.
Defect |
Possible Cause |
Remedy |
Honeycombing |
Inadequate vibration/high slump |
Repair with PMM after chipping loose material |
Crazing/map cracks |
Rapid drying of surface/over-troweling |
Proper curing and finishing techniques |
Low spots/undulations |
Formwork settlement/improper screeding |
Self-leveling compounds for minor issues |
Bleeding |
Excess water in mix/overworking |
Revise mix design, delay finishing |
Proper slab construction is crucial for structural integrity, serviceability, and durability of buildings. Following the correct procedures for slab type selection, reinforcement detailing, formwork, concrete placement, and curing ensures a high-quality finished product.
Key to successful slab construction is understanding and implementing the proper reinforcement details, particularly the correct identification of main and distribution bars, accurate lap locations, and proper positioning using spacers and chairs. Strict adherence to IS code provisions, especially IS 456:2000, ensures structural safety and compliance.
Remember that quality control at every stage, proper documentation, and timely inspection are essential to prevent defects and ensure successful slab construction.
Key Takeaway: A well-constructed slab is vital for the safety and longevity of the entire structure. Prioritize quality at every step.