Mixing of Concrete – Methods, Types and Quality Control

Mixing is one of the most critical operations in concrete production. No matter how carefully you select your materials, a poor mixing operation will ruin the entire batch. Mixing of concrete ensures that cement, aggregates, water, and admixtures are uniformly combined to produce a homogeneous, consistent mix. This article covers everything about concrete mixing for RTMNU Concrete Technology students.

1. Objective of Mixing Concrete

The primary objective of mixing is to produce a uniform, homogeneous concrete mix in which every portion of the mix has the same composition – the same proportions of cement, aggregate, water and admixtures. Proper mixing ensures:

• Each aggregate particle is coated with cement paste
• Cement is uniformly hydrated and dispersed
• No pockets of unmixed dry cement or aggregate
• Consistent workability and strength throughout the batch
• Admixtures are evenly distributed

IS Code: IS 456:2000 (Clause 9) governs mixing of concrete. IS 1791 covers concrete mixers.

2. Hand Mixing

Hand mixing involves manually combining concrete ingredients using shovels, spades, and hoes on a clean, hard, impervious platform (steel sheet or concrete floor).

Procedure for Hand Mixing

1. Measure out the required quantities of fine aggregate, cement, and coarse aggregate by weight (or volume with bulking correction for FA).
2. Spread fine aggregate in a thin layer on the platform.
3. Spread the measured cement over the fine aggregate layer.
4. Turn the dry mixture (FA + cement) at least 3 times until uniform grey colour with no streaks.
5. Add coarse aggregate and turn the entire mix 3 more times until uniform.
6. Form a hollow in the centre (crater) of the dry mix pile.
7. Add 75% of the gauged water and mix thoroughly, turning at least 3 times.
8. Add remaining 25% water gradually and mix until workable and uniform.

Limitations of Hand Mixing

• Not uniform – depends on skill and effort of workers
• Suitable only for very small quantities (typically < 0.5 m³)
• Not permitted for RCC work requiring M20 and above per IS 456:2000
• Slower and more labour-intensive
• Not suitable if admixtures need precise dispersion

IS 456 restriction: Hand mixing is generally not recommended for structural concrete. Where permitted for minor works, 10% extra cement is added to compensate for non-uniformity.

3. Machine Mixing

Machine mixing uses concrete mixers that mechanically combine all ingredients to produce a homogeneous, consistent mix. It is mandatory for all structural concrete work (IS 456:2000).

Advantages of Machine Mixing

• Uniform and consistent mix quality
• Faster production – higher output per hour
• Suitable for all concrete grades including high-strength mixes
• Accurate water-cement ratio control
• Better dispersion of admixtures
• Less dependent on worker skill

4. Types of Concrete Mixers

A. Tilting Drum Mixer

• Drum tilts to discharge concrete (tilting axis)
• Capacity: 0.05 to 0.5 m³ (50 to 500 litres)
• Blades fixed inside rotating drum mix the concrete
• Suitable for mixes containing large aggregate (up to 40 mm)
• Simple design, easy to maintain
• Most common on Indian construction sites

B. Non-Tilting Drum Mixer

• Drum axis is horizontal and fixed; drum rotates around horizontal axis
• Concrete discharged through a chute at one end
• Less efficient for stiff mixes; prone to segregation during discharge
• Suitable for wet, workable mixes only

C. Reversing Drum Mixer

• Drum rotates in one direction for mixing, reverse direction for discharging
• More efficient discharge than non-tilting type
• Common for medium-to-large site operations

D. Pan (Forced Action) Mixer

• Stationary pan with rotating paddles/blades inside (forced action mixing)
• Produces highly uniform mix – especially for stiff, high-strength, and SCC mixes
• Capacity: 0.1 to 3.0 m³
• Used in precast factories, ready-mix plants, and research labs
• Excellent for mixes containing silica fume, fibers, or other special materials

E. Transit Mixer (Truck Mixer) – For Ready Mix Concrete

• Drum mounted on a truck; rotates during transit to prevent segregation
• Capacity: 4–10 m³ per truck
• Drum rotates at mixing speed (8–12 rpm) during mixing; agitation speed (2–4 rpm) during transit
• Maximum transit distance: 100–150 km or 90 minutes from batching (IS 4926)
• Used for Ready Mix Concrete (RMC) delivery

5. Mixing Time

Adequate mixing time is critical for uniformity. As per IS 456:2000 and IS 1791:

• Minimum mixing time after all ingredients are added: 2 minutes for conventional drum mixers
• For large mixers (>1 m³): may need 3–5 minutes
• For pan mixers: 1.5–2 minutes (more efficient action)
• Under-mixing: non-uniform, weak concrete with pockets of dry cement
• Over-mixing: grinding of aggregate, loss of workability, air entrainment changes

Rule of thumb: Minimum mixing time = 2 minutes OR 30 revolutions of drum, whichever gives more mixing – per IS 1791.

6. Sequence of Charging the Mixer

The order in which ingredients are fed into the mixer affects mixing efficiency and uniformity:

1. Add approximately 10% of the total water to the drum first (wets the drum)
2. Add coarse aggregate (helps scour the drum)
3. Add cement
4. Add fine aggregate
5. Add remaining 90% water gradually during mixing
6. Add admixtures dissolved in a portion of the mixing water (last, unless specified otherwise)

Note: Cement should never be added to a dry drum or directly onto dry coarse aggregate – it will form balls and not mix properly.

7. Quality Control in Mixing

• Check slump of first batch and compare with target slump – adjust water if needed
• Clean drum at end of every shift and after each grade change
• Do not add water to restore workability of a concrete batch that has started to stiffen
• Monitor mixing time with a timer – do not estimate by eye
• First batch of the day: prime the drum with mortar (cement + sand + water) before loading full mix
• Never mix more concrete than can be placed and compacted within 90 minutes
• Weigh batching is mandatory for M20 and above – volume batching only for minor works

8. SVG Diagram – Types of Concrete Mixers

9. Exam Tips (RTMNU)

• ✅ IS 456:2000 Clause 9 and IS 1791 govern mixing of concrete – cite both.
• ✅ Minimum mixing time = 2 minutes after all materials added (IS 1791) – most frequently asked.
• ✅ Hand mixing: add 10% extra cement (IS 456); not suitable for M20 and above.
• ✅ Pan mixer = forced action mixer – best for SCC, HPC, precast – explain why (paddles give uniform forced mixing).
• ✅ Transit mixer: maximum 90 minutes or 100–150 km from batching plant (IS 4926).
• ✅ Sequence of charging: water first → CA → cement → FA → remaining water → admixtures.
• ✅ “What happens if you over-mix concrete?” – aggregate grinding, workability loss, air changes.

10. Key Takeaways

• Mixing ensures uniform distribution of all concrete ingredients – critical for consistent strength.
• Hand mixing: 3 dry turns + 3 wet turns; add 10% extra cement; only for minor works.
• Machine mixing is mandatory for all structural concrete (IS 456:2000).
• Minimum mixing time: 2 minutes OR 30 drum revolutions (IS 1791).
• Pan mixer gives best uniformity for stiff, high-strength, SCC, and fiber-reinforced mixes.
• Transit mixer carries RMC; maximum transit time: 90 minutes (IS 4926).
• Correct charging sequence: 10% water → CA → cement → FA → 90% water → admixtures.

11. FAQs

Q1. What is the minimum mixing time for concrete?

As per IS 1791, the minimum mixing time after all ingredients are in the drum is 2 minutes for conventional drum mixers, or a minimum of 30 revolutions – whichever gives more mixing. For pan mixers, 1.5–2 minutes is typically adequate due to forced action.

Q2. Why is hand mixing not recommended for structural concrete?

Hand mixing cannot guarantee uniform distribution of cement throughout the mix, especially for larger quantities. The resulting concrete is non-uniform with variable strength. IS 456:2000 generally prohibits hand mixing for structural work. When exceptionally permitted, 10% extra cement must be added to compensate.

Q3. What is the difference between a tilting drum mixer and a pan mixer?

A tilting drum mixer uses gravity and drum rotation to mix concrete – suitable for general site work. A pan (forced action) mixer has stationary pan with rotating paddles that mechanically force the ingredients together. Pan mixers produce more uniform mixes, especially for stiff, low-slump, high-performance, and SCC concrete.

Q4. Why is the charging sequence important?

Correct charging sequence prevents cement balling and ensures uniform mixing. Adding 10% water first wets the drum and prevents dry cement sticking. Coarse aggregate added first scours the drum. Cement and fine aggregate follow, and remaining water is added gradually to control workability uniformly throughout the mix.

Q5. What is a transit mixer and how long can concrete be kept in it?

A transit mixer is a concrete truck with a rotating drum that keeps freshly mixed concrete agitated during transport to the construction site. As per IS 4926 (Ready-Mixed Concrete), concrete must be discharged within 90 minutes of water addition at the batching plant, or before the drum has completed 300 revolutions, whichever comes first.

🔗 Related Reading: Transportation of Concrete – Methods and Requirements

🔗 Related Reading: Batching of Concrete – Volume vs Weight Batching

📖 Reference: IS 456:2000 Clause 9 and IS 1791 – Concrete Mixers