Codes referenced
Location, treatment, and acceptance of construction joints.
Formwork cycle timing, shore removal, re-shoring.
Bar continuation across construction joints, lap placement.
Plumb, level, alignment tolerance for sequential pours.
Why sequencing matters
Three failure modes from wrong sequence:
- Cold joints — new concrete cannot bond to already-set concrete; weakness plane at interface
- Premature loading — slab loaded before column strength develops; deflection and cracking
- Formwork failure — shore removal too early, or unsupported span too long
Standard column-beam-slab order
For RCC frame buildings:
- Columns — cast first; pour column up to beam bottom level (soffit). Allow ≥ 24 h before beam pour (or use retarder to extend).
- Beams — cast after columns have set (≥ 24 h, or until column concrete has initial set). Beam bottom integrated with slab formwork.
- Slabs — cast after beams have set (≥ 24 h). Slab pours may be sub-divided by bays.
For raft foundations:
- Raft — single continuous pour if possible (limit 200-500 m³ depending on plant capacity)
- Walls — pour walls separately after raft reaches 70% strength (≥ 7 days)
- Columns above walls — separate pour after walls
Bay-by-bay planning
Bays are sub-divisions of the structure matching the formwork cycle and daily concrete supply rate. Standard bay sizing:
Typical 80-150 m² per bay; volume depends on slab thickness. M30 slab 150 mm thick = 1.5 m³/m² × 100 m² = 150 m³ per bay.
RMC supply typically 30-60 m³/h. At 45 m³/h, 150 m³ bay = 3.3 hours pour. Plan 1-3 bays per day.
Cold joint prevention
A cold joint forms when fresh concrete cannot bond to already-set concrete. Prevention:
- Pour rate — keep pour rate ≥ 30 m³/h to ensure adjacent lifts placed within initial-set window (typically 2-4 h at 25°C)
- Retarder — in hot weather (> 30°C), use retarder to extend initial set to 4-6 h
- Vibration — re-vibrate the joint zone (penetrate 100-150 mm into previous layer) to weld the layers
- Surface prep — at construction joints, roughen + wash the old surface to expose aggregate; apply bonding agent if specified
Special cases
> 500 m³ raft: 24/7 operation; multiple pumps; 3+ RMC plants on call. Plan concrete supply with 20% spare capacity. Need ≥ 50 m³/h sustained.
Pour in 2 lifts with ≥ 30 min between lifts (avoids segregation from free-fall). Tremie pipe for > 5 m height. Avoid horizontal construction joints in columns at mid-height (in high moment zone).
Worked example — G+5 slab pour sequence
| Parameter | Value |
|---|---|
| Floor area | 900 m² (30 × 30 m) |
| Slab thickness | 150 mm |
| M30 volume per floor | 135 m³ |
| Bay size | 150 m² each (3 × 6 = 18 bays per floor, 6 bays per direction) |
| RMC supply rate | 1 pump, 45 m³/h |
| Pour rate per bay | 150 m² × 0.15 m = 22.5 m³ in ~30 min |
| Daily plan | 6 bays per day = 135 m³ (matches one floor) |
| Sequence | Bays 1-3 on day 1, bays 4-6 on day 2 (after forms stripped + reshore) |
| Cold joint window | 30 min pour + 2 h initial set window = 2.5 h |
| Construction joint location | Bay boundaries at 1/3 span (low moment zone per IS 456 Cl 13.3) |
Common deviations seen on Indian sites
Slab loading premature relative to column strength causes deflection, cracking, and potential progressive collapse. Always wait ≥ 7 days for OPC columns (or per mix design specification) before loading slab.
Pouring until "concrete is finished" leaves joint locations uncontrolled. Plan all joints upfront on the pour card; align with design construction joints per IS 456 Cl 13.3.
Joints at mid-span of beams create weakness in the highest-stress zone. Always at 1/3 to 1/2 span, never mid-span.
Faster than 30 m³/h without enough vibrators leaves unconsolidated zones. Plan vibrator count: 1 per 10 m³/h + 1 standby.
Field-checklist blocks
A short questionnaire, an inspector's checklist, and three worked examples that show how the pour rate and the joint location decide whether the floor goes up clean or cold-jointed.
Questionnaire — ask before the pour card is written
- Is the sequence column, then beam, then slab, with at least 24 h between adjacent stages?
- Are the construction joint locations marked on the drawing and on the pour card, at 1/3 to 1/2 span and never mid-span?
- Will adjacent lifts be placed inside the initial-set window — 2 to 4 h at 25 °C, extended to 4 to 6 h with retarder in hot weather above 30 °C?
- Does the planned pour rate hold at or above 30 m³/h, so the pour neither outruns the initial-set window nor the vibration coverage?
- Are the vibrators planned at one per 10 m³/h plus one standby?
- Is the bay sized to the daily supply rate — typically 80 to 150 m² per bay, and 1 to 3 bays a day?
- Is the formwork cycle lined up with the pour cycle, including stripping and re-shoring before the next bays go in?
Inspector's checklist — what to verify on the pour card and on the deck
- Bay-by-bay plan issued, with the bay sequence, the volume per bay and the pour rate per bay written on it.
- Joint locations agreed with the designer and marked physically on the structure before the pour starts.
- Old surface prepared at every construction joint — roughened and washed to expose aggregate, with bonding agent applied where specified.
- Joint zone re-vibrated, penetrating 100 to 150 mm into the previous layer.
- Supply confirmed against the plan, with 20 % spare capacity for a large raft and a sustained rate of 50 m³/h or better.
- Shore removal and re-shoring checked against the formwork cycle, so the next bay is not standing on stripped formwork.
- Column strength confirmed before the slab is loaded — at least 70 % of design strength, typically at least 7 days for OPC.
What happens if… three worked examples
What happens. 135 m³ per floor in 18 bays of 150 m², six bays in each direction. One pump at 45 m³/h puts 22.5 m³ into a bay in about 30 minutes, and six bays make one floor.
Likely outcome. Six bays a day at 135 m³ matches one floor a day, with a cold joint window of the 30-minute pour inside the 2-hour initial-set window, so 2.5 h in total. Bays 1 to 3 go in on day 1, bays 4 to 6 on day 2 after the forms are stripped and re-shored.
Correction. Keep the pump rate and the bay size matched. If the pump slows, the bay has to get bigger or the day has to lose a bay — never the joint window.
What happens. The slab is loaded while the column concrete is still green, because the cycle wants the floor up.
Likely outcome. Columns must reach at least 70 % of design strength before slab loading, which typically takes at least 7 days for OPC. Premature loading and premature stripping cause deflection and cracking, with the potential for progressive collapse.
Correction. Hold the slab pour until the column strength is confirmed, and keep the shores on until the formwork cycle releases them. The formwork cycle follows the concrete, not the other way round.
What happens. The pump runs at 45 m³/h, but the vibrating plan was set at the minimum rate.
Likely outcome. The rule is one vibrator per 10 m³/h plus one standby, and that count is sized for 30 m³/h. Above that without more vibrators, the pour outruns the coverage and you get unconsolidated zones behind the reinforcement.
Correction. Size the vibrator crew to the planned rate, not to the minimum, and hold the pour rate at or above 30 m³/h so the initial-set window is not exceeded either.
Frequently Asked Questions
What's the minimum time between adjacent pours?
Can slabs be poured before columns reach strength?
Where should pour boundaries be placed?
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Amit Haridas
Founder & Proprietor, ConcreteInfo. 25+ years experience in construction QA/QC, concrete technology, RMC plant operations, consulting and technical training across India. For pile-integrity witnessing or waterproofing consultancy on your project, contact amit@concreteinfo.in.
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