Mixing of Concrete
Concrete mixing is a process of mixing the ingredient of concrete such as cement, sand, aggregate, water, and admixture together to make concrete of suitable grade.
Mixing Concrete is a very complex process. For making good quality concrete, we just have to follow some standard process of mixing its ingredients. It just does not up to making concrete, but making good quality concrete is important.
Production of good quality and bad quality concrete includes the same material, but the proportion and mixing method can be a differentiating factor. It requires proper care and knowledge for making good quality concrete.
Mixing of Concrete Methods
Proper and sufficient mixing of the materials is essential for the production of uniform concrete. The mixing process ensures that concrete becomes homogeneous, uniform in color, and consistent. There are two methods used for mixing concrete.
The following methods are used for concrete mixing,
- Hand Mixing of Concrete
- Machine Mixing of Concrete
1. Hand Mixing of Concrete
Hand Mixing of Concrete is practiced for small-scale unimportant concrete works. In this method, as material mixing is not sufficient compared to machine mix it is desirable to add 10 percent more cement to cater to the inferior concrete produced by this method
If hand mixing is adopted for making concrete,
It should be done over an impervious concrete or brick floor of sufficiently large size to take one bag of cement.
First, spread out the coarse aggregate and fine aggregate in alternate layers insufficient or measured quantity. Drop cement on top of this, and mixes them dry by shovel, turning the mixture over and over again until uniformity of color is achieved. With the help of a suitable tool, properly mix these materials.
Water is added after materials are well mixed. After adding water, the mixing operation is continued until such time a good uniform, homogeneous mix is obtained.
Do not add extra water in concretes and try to use maximum water that is added in the mix. After the concrete is well mixed ready to be used.
2. Machine Mixing of Concrete
For large-size construction, the concrete mixing is done by machine only, for reinforced concrete work, and for medium or large-scale mass concrete work. Mixing Concrete in a mixer is not only efficient but also economical when the quantity of concrete to be produced is large.
These types of mixers produce concrete batch by batch, whereas continuous mixers produce concrete continuously without stoppage till such time the plant is working.
The process is continuous material for the next batch and is filled by screw feeders and the materials are continuously mixed and continuously discharged.
The batch mixer may have two types of pan or drum. The drum type of mixer may further be classified as tilting, non-tilting, reversing, or forced action type.
There is not any analysis of the mixing efficiency of the various types of mixers, but some evidence is there to suggest that pan mixers with a revolving star of blades are more efficient.
This type of mixer is mainly suitable for stiff and lean mixes, which present difficulties with most other types of mixers, mainly due to the sticking of mortar in the drum.
The efficiency of the mixer is mainly affected by the shape of the drum, the angle, and the size of the blades, the angle at which the drum is held.
It is found that a tilting drum to some extent is more efficient than a non-tilting drum and the discharging is not a lilting drum; a chute is introduced into the drum by operating a lever. In these concrete mixed in the drum, falls into the inclined chute and gets discharged out.
Generally, it is recommended to make a batch of concrete having 50 kg of cement. If you are going the purchase of mixer you should ask for a capacity mixer that should hold all the materials for one bag of cement
For example, for 1: 2: 4 mixes, the ideal mixer is of 200 liters capacity,
In the case of a mix ratio of 1: 3: 6, the requirement will be 280 liters capacity to facilitate one bag mix. The mixer of 200-liter capacity may be inefficient for 1 : 3: 6 mix and also mixer of 280 liters is too big, hence uneconomical for 1: 2: 4 concrete.
For getting a good quality concrete mix follow the below steps
- Firstly, place a half quantity in the drum and over which about half the quantity of fine aggregate is poured.
- In this place full quantity of cement, one bag is poured over which the remaining portion of coarse aggregate and fine aggregate are deposited in sequence.
- This sequence of adding material can save the spilling of cement while discharging into the skip and also this prevents the blowing away of cement in windy weather.
- Add 25 percent of the total quantity of water required for mixing before loaded skip discharge material into the drum to wet the drum and to prevent any cement from sticking to the blades or at the bottom of the drum.
- Immediately after discharging the dry material into the drum, the remaining 75 percent of water is added to the drum.
Concrete Mixing Time
The mixer is generally designed to run at a speed of 15 to 20 revolutions per minute. For well mix concrete, it is seen that about 25 to 30 revolutions are required in a well-designed mixer.
On the construction site, the normal tendency is to speed up the outturn of concrete by reducing the mixing time.
This results in the poor quality of concrete. If the mixer runs for a long time, it is uneconomical from the point of view of the rate of production of concrete and fuel consumption.
Therefore mixing time of concrete in the mixer has an important factor, which will accrue optimum benefit.
It is research data result that the quality of concrete in terms of compressive strength will increase with the increase in the time of mixing.
In case mixing time is more than 2 minutes the improvement of compressive strength is not significant. Fig. shows the Comparison of the mixing time of concrete and its compressive strength.
Generally mixing time of the mixer is related to the capacity of a mixer. The time of mixing in the drum varies between1½ 2½ minutes. The larger the size or capacity drum the more is the mixing time.
However, for producing ready-mix concrete, the mixer can well mix in about 15 to 30 secs. Concrete mixer having one cubic meter capacity high-speed Pan Mixer takes only about 2 minutes for batching and mixing. The RMC batching plant takes about 12 minutes to load a transit mixer of 6 m3 capacity.
Batching of Concrete
Measuring the proper and sufficient quantity of material for making concrete is known as batching concrete.
Concrete batching is done by two methods,
- Weigh batching
- Volume batching
1. Volume Batching
Volume batching of concrete is a too accurate method for making concrete because of the difficulty it offers to measure granular material in terms of volume.
It is a fact that moist sand volume in a loose condition weighs much less than the same volume of dry compacted sand.
The solid material quantity in a cubic meter is an indefinite quantity. For more accurate and good quality concrete weigh batching is more preparable.
However, it is not possible for in-site mixed concrete to weigh this large amount of volume of material. For such a situation volume batching can be used. The cement used is always measured by weight not by volume.
Normally, for each batch of concrete, one bag of cement is used. The volume of a single bag of cement is thirty-five (35) liters.
For taking a quantity of fine and coarse aggregates gauge box or volume box is used which has a fixed size. The standard size gauge box is shown in Figure 6.12.
The box is made of volume which is equal to the weight of one bag of cement and which has the volume of 35 liters or multiple thereof.
The gauge boxes are made with the top narrow surface rather than shallow with a wider surface to facilitate easy estimation of top-level.
Gauge boxes or volume boxes are generally called farms. They can be made of timber or steel plates. In Indian conditions, volume batching is also used for large concreting operations.
For important major sites, the following gauge boxes the site cater to the change in Mix Design or bulking of sand. The volume of each gauge box is clearly marked with paint on the external surface.
Water can be measured in kg or liters. It is due to the fact that the density of water of 1 kg per liter. The amount of water required is a product of the water/cement ratio and the weight of cement.
For example, if the water/cement ratio of 0.5 is specified, the quantity of mixing water required per bag of cement is 0.5 x 50.00 = 25 kg. or 25 liters. The quantity is, of course, inclusive of any surface moisture present in the aggregate.
Approximate moisture content carried by aggregate is given in the below table:
How to Make Concrete – Mixing and Placing of Concrete
Table 6.5. Approximate Surface moisture in aggregate–I.S. 456-2000
2. Weigh Batching
Weigh batching is an accurate method of making concrete. It is recommended in many countries‘ IS codes to use the weight-batching process for making concrete for important government work.
Weight batching of concrete facilitates accuracy, flexibility, and simplicity. Different types of weighing batches are available,
The particular type to be used depends upon the nature of the job. Large weigh batching plants have automatic weighing equipment.
The use of this automatic equipment for batching is one of sophistication and requires qualified and experienced engineers.
On large worksites, weigh bucket-type equipment is used. This supplies the material from a large storage hopper and it discharges by gravity, straight into the mixer. The material weighting is done through a lever–arm system and two interlinked beams and jockey weights.
The required quantity of say, coarse aggregate is weighed, having only the lower beam in operation. After balancing, by turning the smaller lever, to the left of the beam, the two beams are interlinked and the fine aggregate is added until they both balance.
The final balance is indicated by the pointer on the scale to the right of the beams. It is discharged by a swivel gate at the bottom. These types of automatic plants are available in small and large capacities.
In this, the operator has only to press one or two buttons to put into motion the weighing of all the different materials, the flow of each being cut off when the correct weight is reached. In their most advanced forms, automatic plants are electrically operated on a punched card system.
These types of plants are majorly suitable for making ready mix concrete in which very frequent changes in mix proportions have to be made to meet the varying requirements of different customers.
Measurement of Water For Concrete Mixing
While we choose to weigh batching method, the measurement of water must be done accurately. The addition of water buckets having graduation in liters will not be accurate enough for the reason of spillage of water etc.
Generally, it is measured in a horizontal tank or vertical tank fitted to the mixer.
These tanks are empty during the batch and again filled for the next batch. The tank filling has such an accuracy to admit any desired quantity of water.
For this work sometimes water meters are fixed in the main water supply to the mixer from which the exact quantity of water can be let into the mixer.
Transporting of Concrete
Transportation of Concrete is done by various methods and equipment on site. The only care that should be taken is that the homogeneity of concrete obtained at the time of mixing should be maintained until it reached its final destination. Following are the methods adopted for transportation are:
The following are methods of transportation of concrete,
- Mortar Pan
- Wheel Barrow
- Hand Cart
- Ropeway, Crane, Bucket
- Truck Mixer and Dumpers
- Belt Conveyors
- Skip and Hoist
- Transit Mixer
- Pump and Pipe Line
Mixing of Concrete
Concrete mixing is a process of mixing the ingredient of concrete such as cement, sand, aggregate, water, and admixture together to make concrete of suitable grade. To make the different grades of concrete, the mixing of concrete materials should be done properly as per the mix design of concrete to achieve the design strength of concrete.
What is the ratio for mixing concrete?
The concrete mixing ratio depends on what strength you are trying to achieve. In the case of a mix ratio of 1: 3: 6, it means 1 part cement, 3 part sand, and 6 part aggregate to be added for concrete making.
What are the methods of mixing concrete?
There are two most popular concrete mixing methods used on construction sites,
1. Hand Mixing of concrete
2. Machine Mixing of concrete
What is a 1 2 3 mix for concrete?
The general concrete mix for concrete is 1:2:3, which means 1 part cement, 2 part sand, and 3 part is aggregate.
What is the strongest concrete mix ratio?
The strongest mix ratio for concrete making is 1:3:5 (Cement, Sand, Coarse Gravel). In this case, both the sand and gravel are the aggregate.
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