Methods for Mixing of Concrete and Their Procedure

Mixing of concrete is determined as the “complete blending of the concrete materials or ingredients which are required for the production of a homogeneous concrete”. Mixing can differ from hand to machine mixing of concrete mix, with machine mixing being the most common.

However, no successful mixture can be achieved without the proper batching of all concrete materials.

Points to be Checked before Mixing of Concrete Materials

Manual Mixing

  • A watertight platform of at least size 3m X 3m should be available.
  • The platform should be such that at least two men can work upon the same simultaneously.
  • The platform on which concrete will be mixed should have a peripheral curb of a suitable height to prevent cement slurry from draining away or other materials to overflow.
  • The platform should be clean and free from mortar or concrete laitants and other materials.

Machine Mixing

  • The concrete mixer should be set up on a firm & leveled base.
  • The concrete mixer should be thoroughly checked with respect to the drive system, brake & clutch control arrangement, and wire rope of the skip.
  • Cement, aggregates & water should be checked for quality purposes.
  • Before commencing the concreting, inspect the engine oil, cooling water, and petrol, or other fuel. It should be replenished if necessary. The mixer engine should be permitted to run for a couple of moments to warm up before it is charged with materials.
  • The concrete mixer drum’s baffle plate and/or blades should be clean and dust-free. The gap between the drum and blades should not be bigger than 25 mm so that coarse aggregates are prevented from sliding. If the gap is large, welding a steel rod between the drum and the blade can be easily reduced.
  • The volume of the drum should also be checked for its adequacy/capability. Usually, the drum volume to wet concrete volume ratio should be 2.5:1. The drum should have sufficient empty space within to facilitate proper mixing of concrete ingredients.
  • Mixers without baffle plate don’t perform in a satisfactory manner. Baffle plate helps to mix by dropping the coarse aggregates onto the mortar with every revolution.
  • The concrete mixer operator should be well experienced & should have a good eye judgment of the required mix consistency. The concrete mixer operator must be properly briefed about the danger of adding excess water to the mixer for concrete making.
Machine Mixing

Loading the Skip or Hopper of the Mixer

The following sequence of loading the skip/hopper with dry materials is more advantageous:

  • Coarse Aggregates
  • Fine aggregates
  • Cement

If there is a chance of cement being flown away due to high wind speed, cement is loaded before sand in hopper/skip.

The following sequence of loading the sip or hopper, when sand is moist, is advantageous:

  • One half quantity of coarse aggregates
  • Cement
  • Moist sand
  • Balance half quantity of coarse aggregates

This is to prohibit the choking of the skip mouth due to dampness/moisture in sand.

Smaller size mixers have no skip. Materials are therefore directly added into drum in the following sequence.

  • Cement
  • Fine aggregates
  • Coarse aggregates

The coarse aggregates are added last as they help push in any cement or sand that may adhere/stick to the mouth of the concrete mixer.

Setting the Dial Pointer

The setting of the pointer in the batching plant or on a weigh batcher is more or less identical. The weights of individual ingredients are added in the sequence of their loading into the skip or hopper and cumulative weights are marked on the dial after each ingredient is added.

Addition of Water

Water is added in the mixer drum separately after allowing for adjustment of surface moisture present in the aggregates and absorption that may take place if aggregates are dry.

Water is essential to chemically react with cement and to give the workability to concrete mix so that it can be placed & compacted to the desired shape. Generally, water is essential to be added for cement hydration is very less as compared to water required for workability. Hence, the addition of water into the mix is mainly governed by the workability required for concrete rather than for cement hydration.

However, water addition in concrete must be limited as far as possible. Water addition more than necessary can cause loss of strength & durability problems.

To produce concrete of uniform & consistent quality it is of prime significance that total water content should be the same for each batch throughout the job.

There are different ways to control water addition in each batch. The eye judgment of an experienced concrete mixer operator or the workability test gives a fairly good idea of whether or not the required quantity of water is added. However, on the modern batching plant, the entire water addition and adjustment from batch because of the use of moist aggregate is done automatically.

A water tank with a gauge for measuring the water on a mixer is a rare sight even though BIS 1791-1968 requires all mixers other than 100 T and 140 T to be fitted with water measuring tank.

Normally, the water to be added from batch to batch should not vary considerably. However, due to variations of moisture in sand and variations in the grading of downgraded aggregates and shapes of aggregates, it is quite likely that the water requirement will vary from batch to batch. It is, therefore, always sensible to put in about 2 to 3 liters of water less than the recommended dosage and then gradually add the last few liters depending on the workability test results or eye judgment.

It is always essential to perform the workability test in the first few batches. The slump test is the simplest and most popular in our country. The water content adjustment is then done by increasing or decreasing the dosage of water if the slump is less or more than the design respectively.

Manual Mixing Procedure

The following procedure is required to be followed for manual mixing of mortar and concrete.

  • Spread out the necessary quantity of sand equitably on the mixing platform.
  • Unload the cement on the sand and spread it uniformly over the sand.
  • Mix the sand and cement with shovels. Turn the mixture over and over till it is of even color throughout and free from streaks of brown and grey. The presence of streaks indicates that materials are not yet mixed properly.
  • The mixture of sand and cement is spread and the measured quantity of coarse aggregates is now uniformly spread over the sand cement layer.
  • Mix the materials with a shovel from center to side and then from side to center and back to the side. The materials are blended together by turning them over and over again by a twist of the wrist.
  • The mixture is further spread making a hollow in the center of the pile to receive water.
  • 75% of the total water requirements are poured into the hollow portion and simultaneously the material is turned towards the center with the shovel. The materials are gradually turned and mixed with the remaining quantity of water.
  • The mixing continues till the color of the mix and consistency of the mix are uniform throughout the heap.

Mechanical Mixing

The following points must be practiced during mixing.

  • The most suitable loading sequence must be selected depending on the requirements and the type of mixer.
  • Feed about 10% of the water into the drum. Start the mixer and charge the drum with the batch materials uniformly. The balance quantity of water is simultaneously added keeping in reserve the last few liters to adjust the mix as per consistency/workability requirement.
  • Mixing should be carried out till the mix is observed to be of uniform color, workable, and consistent. It is always preferable if the mixer operator can observe the mixing process in the drum.
  • Care should be taken that the drum is not overloaded. If the drum is overloaded mixing will not be efficient and materials will overflow while mixing.
  • It is always observed that some cement and sand from the first batch of concrete will stick to the sides and blades and therefore, the first batch will be short of sand and cement. This harsh and stony concrete while placing will segregate and result in a honeycombing and poor finish. It is, therefore, very essential to have additional sand and cement batched in the first mix. The first mix will also require more water. However, the water to cement ratio of the mix would remain unchanged.
  • It is recommended that around a 10% increase in sand and cement or reduction of coarse aggregates by about 20% can improve the first batch.
  • The maximum output of mixers is dependent on the mixing time, size of each batch, total batching, and mixing cycle, or in other words the overall efficiency of the system as a whole. Hence, it is important that the total output of the mixer will depend on the total time required to batch, mix, and transport and place the concrete.

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