Placing of Concrete – Methods, Requirements and IS 456:2000 Guidelines

Placing is the operation of depositing fresh concrete in its final position in the formwork. It is a critical stage – even perfectly mixed and transported concrete can be ruined by poor placing. Placing of concrete must be done carefully to prevent segregation, ensure proper filling of formwork, and maintain the required cover to reinforcement. This article is a complete guide for RTMNU Concrete Technology students.

1. Objectives of Proper Placing

The placing operation must achieve:

• Concrete deposited as close as possible to its final position – no lateral movement through vibration
• No segregation during deposition
• Complete filling of formwork and all corners
• Proper cover to reinforcement maintained
• No displacement of reinforcement bars or formwork
• No cold joints (poor bond between successive layers or lifts)
• Concrete placed before initial set begins

2. Pre-Placing Requirements

Before placing concrete, the following must be checked and completed:

• Formwork: Clean, adequately braced, joints sealed to prevent leakage of slurry. Formwork must be wetted (not soaked) to prevent absorption of mix water.
• Reinforcement: Correct placement, cover maintained using cover blocks, no loose rust scale, no oily contamination.
• Foundation/sub-base: Existing concrete surface should be roughened, cleaned, and wetted. Remove all standing water.
• Inspection: Site engineer must inspect formwork and steel before giving clearance to place.
• Equipment ready: Vibrators checked, adequate labour available, power supply confirmed.
• Access: Clear route from mixer/truck to placement point without obstruction.

3. General Principles for Placing (IS 456:2000 Clause 13.3)

• Concrete shall be placed as near as practicable to its final position. Do not flow or rake concrete over long distances in the formwork using a vibrator.
• Concrete should not be dropped from a height greater than 1.5 m to prevent segregation.
• Placing must be continuous between planned construction joints to avoid cold joints.
• Concrete shall be thoroughly compacted during placing using mechanical vibrators.
• Concrete should not be placed in running water unless using tremie pipe method.
• In hot weather, placing should be done during cooler periods (early morning/evening).
• No concrete should be placed in very heavy rain (risk of water dilution) unless protected.
• All concrete must be placed within 90 minutes of water addition (IS 456 Clause 13.1).

4. Methods of Placing for Different Structural Members

A. Placing in Foundations (Footings, Raft)

• Remove all loose soil, standing water, and debris from the excavation before placing
• Place bed concrete (lean concrete / PCC) first; allow to set before placing structural concrete
• For large raft foundations: place in alternate panels or bays with planned construction joints
• Avoid placing during rain or in waterlogged conditions
• Ensure vibrators reach all parts of the thick section

B. Placing in Columns

• Place concrete in layers not exceeding 300–450 mm (to allow effective vibration)
• Drop concrete through a funnel/hopper to avoid hitting reinforcement bars (prevents segregation)
• For tall columns: use a tremie/elephant trunk pipe to guide concrete down without free-fall segregation
• Do not pour from the top if column height > 3 m without a drop pipe
• Vibrate each layer thoroughly before placing the next
• Do not disturb previously placed and partially set concrete with vibrator

C. Placing in Beams and Slabs

• Place concrete in beams first (bottom of T-beam or L-beam), then slab
• For flat slabs: place in one continuous operation from one end to the other – no cold joints
• Maximum layer thickness in slabs: 150–200 mm (depending on slab depth and vibrator reach)
• Place concrete on both sides of beam simultaneously to avoid pressure imbalance on formwork
• Ensure concrete flows under beam reinforcement and around stirrups

D. Placing in Walls

• Place in layers of 300–450 mm height
• Work from both ends towards the middle to ensure air and surplus grout can escape
• For tall walls: use access windows (pockets) in formwork for vibrator insertion at lower levels
• Avoid overfilling formwork before consolidating lower layers

E. Placing in Prestressed Concrete Members

• Extra care needed to maintain exact cover to tendons and ducts
• Concrete must not disturb the position of prestressing cables/wires
• High-strength mix (M40+) requires careful vibration to avoid honeycombing around congested reinforcement
• Simultaneous placement on both sides if thin web members

F. Placing in Pavement Slabs

• Concrete spread to required thickness using screed boards, template pavers, or slip-form pavers
• Surface finishing by screeding, floating, and troweling immediately after placing
• No. of passes of screed board: typically 2–3 passes to achieve level surface
• Anti-skid texturing applied while concrete is still green (brushing or burlap drag)

5. Layer Thickness in Placing Concrete

6. Underwater (Subaqueous) Concreting

When concrete must be placed in water (foundations, piles, caissons, sea walls), special methods are used:

A. Tremie Pipe Method (Most Common)

• Vertical steel pipe (200–300 mm diameter) lowered to near the bottom
• Pipe kept continuously immersed at least 600 mm in freshly placed concrete to prevent water entry
• Concrete fed from a funnel at the top continuously; displaces previously placed concrete upward
• Mix: high slump (175–200 mm), rich in cement (minimum 400 kg/m³), small aggregate (<20 mm)
• Once started, must not be stopped or lifted above concrete surface

B. Pump Method

• Concrete pumped through a pipe to the bottom of the water body
• Pipe kept immersed in placed concrete similar to tremie method
• Suitable for large underwater pours and deep water placements

C. Pre-packed Aggregate Method (Grouted Concrete)

• Coarse aggregate pre-placed in formwork first
• Cement-sand grout pumped through pipes from the bottom to fill voids between aggregates
• Used for repairs to underwater structures and mass concrete elements

D. Bag Concrete Method

• Concrete placed in permeable hessian (jute) bags and placed manually by divers
• Water penetrates and hydrates the cement inside
• Used for emergency repairs and scour protection only; not for structural concrete

7. Placing in Special Weather Conditions

Hot Weather Concreting (IS 7861 Part 1)

• Concrete temperature at placement: < 30°C (ideally <27°C)
• Use chilled water or ice flakes to replace mixing water
• Pre-cool aggregates by shade storage or water sprinkling
• Place during early morning or evening hours
• Use retarding admixtures to extend workability
• Protect formwork and reinforcement from direct sunlight before placing

Cold Weather Concreting (IS 7861 Part 2)

• Concrete temperature at placement: > 10°C (minimum)
• Use warm mixing water (max 60°C – never add hot water to cement directly)
• Use accelerating admixtures (non-chloride type for RCC)
• Protect placed concrete with insulating blankets or enclosures
• Do not place on frozen ground or against frozen formwork

8. SVG Diagram – Placing Concrete in Columns and Slabs

Placing in Columns Layer 1 (300-450mm) Layer 2 Max drop 1.5 m only Vibrator Use drop pipe for H > 3m Vibrate each layer

Placing in Slabs Place from one end → other No cold joints | Continuous pour Screed to level after placing

IS 456 Key Rules for Placing: Max free drop: 1.5 m | Place near final position | No water addition | Compact every layer | 90 min limit

9. Exam Tips (RTMNU)

• ✅ IS 456:2000 Clause 13.3 governs placing – cite it in every placing-related answer.
• ✅ Maximum free drop height: 1.5 m – most commonly asked placing requirement.
• ✅ Layer thickness for columns/walls: 300–450 mm per lift – standard exam answer.
• ✅ “Why should concrete not be moved laterally by vibrator after placing?” – Causes segregation; coarse aggregate stays, mortar moves.
• ✅ Tremie pipe method: pipe must be immersed minimum 600 mm in placed concrete – frequently asked.
• ✅ Placing in columns: use drop pipe (elephant trunk) for height > 3 m.
• ✅ Cold joint = poor bond between successive concrete pours = structural weakness. Avoid by placing continuously.

10. Key Takeaways

• Concrete must be placed as near as possible to its final position without lateral movement.
• Maximum free-fall height: 1.5 m (IS 456:2000 Clause 13.3).
• Layer thickness: 300–450 mm for columns/walls; full depth for slabs and thin beams.
• Tremie pipe method is used for underwater concreting; pipe must stay immersed ≥ 600 mm in fresh concrete.
• No cold joints: placing must be continuous between planned construction joints.
• Special precautions needed in hot weather (<30°C placing temperature) and cold weather (>10°C).

11. FAQs

Q1. What is the maximum height from which concrete can be dropped during placing?

As per IS 456:2000 Clause 13.3, concrete should not be dropped from a height greater than 1.5 m. Dropping from greater height causes segregation – coarse aggregate separates from mortar, creating honeycombed patches in the hardened concrete.

Q2. What is a cold joint in concrete?

A cold joint is a plane of weakness in hardened concrete formed when a fresh layer is placed against concrete that has already started to set (partially hardened). It results in poor bond between the two layers, creating a structural weakness that can allow water penetration. Cold joints are avoided by placing concrete continuously without allowing any layer to set before the next is placed.

Q3. How is concrete placed in columns to prevent segregation?

For short columns (<3 m), concrete is poured through a hopper. For taller columns, a drop pipe (elephant trunk) – a flexible rubber tube – is inserted into the column and slowly raised as concrete is placed. This prevents free fall and segregation by guiding concrete close to its final position without impacting reinforcement bars.

Q4. What is the tremie pipe method of underwater concreting?

A tremie pipe is a 200–300 mm diameter steel pipe lowered to the base of the water body. A plug is inserted initially to keep water out. When concrete placement starts, the plug is pushed down. The pipe is kept continuously immersed at least 600 mm in the fresh concrete, and concrete is continuously fed from the top, displacing previously placed concrete upward by gravity and pressure.

Q5. What temperature should concrete be at the time of placing?

Per IS 7861, in hot weather, concrete temperature at placing should not exceed 30°C (ideally 27°C). In cold weather, the minimum temperature at placing is 10°C, and concrete must be protected from freezing until it achieves adequate strength (minimum 3.5 MPa).

🔗 Related Reading: Compaction of Concrete – Methods and Vibrator Types

🔗 Related Reading: Transportation of Concrete – Methods and Time Limits

📖 Reference: IS 456:2000 Clause 13.3 and IS 7861 – Extreme Weather Concreting