In this article we will discuss about the process of batching and mixing of concrete.
Batching of Concrete:
The correct measurement of the various materials used in the concrete mix is called batching. It has been observed that batching errors are partly responsible for the variation in the quality of concrete. This operation can be properly and better controlled than other factors responsible for the variation in the quality of concrete.
It has been observed that poor batching is responsible for more variation than any other factor, but good control can reduce this part of variation to an insignificant amount. Thus batching operation is a very important operation.
Type of Batching:
Batching can be done in two ways as follows:
1. Volume batching.
2. Weight batching.
1. Volume Batching:
In this system of batching, the materials are measured by volume. The gauging of cement by volume is most inadvisable as it is difficult to secure accuracy in its measurement as the actual volume of a given weight of cement depends upon how it is filled into a gauge box and whether it is shaken down. The density of cement may vary from 1.12 gram/cm3 if it is lightly poured into the container to 1.6 gram/cm3 if tamped down sufficiently hard.
Therefore it is convenient to use complete bags of cement in a concrete mix so that the batching, of cement on the site is unnecessary except where bulk cement is used. If a fraction of a bag of cement is required, then it should be weighed into a bucket suspended from an ordinary spring balance.
Aggregates can be gauged by volume and for this purpose wooden batch boxes called farmas are used. The size of farmas should be such as to measure the correct quantity of aggregates to be used with a whole bag of cement for the required mix. They should not be made so large as to be unwieldy.
Each cement bag as delivered by the factories is packed to contain a net weight of 50 kg. As one litre of cement weight 1.44 kg. the volume of-50 kg cement will be 50/1.44 = 34.72 lit., but for the sake of convenience the volume of 1 bag of cement containing 50 kg of cement is taken as 35 lit. Thus the farma should normally be made to have a volume of 35 lit., so that it may contain one full bag of cement. On this assumption, for various mixes quantities of materials may be taken.
The quantity of sand as indicated above is based on the assumption that the sand is in a saturated surface dry condition. If surface moisture is present, bulking of sand should be allowed.
Size of Farmas:
The farma should be of convenient size, to measure in multiples of 35 lit. Tall and narrow farmas should be preferred than shallow and wide farmas.
The general dimensions of farmas of different capacities are shown in Table 9.2:
Out of the above dimensions concrete association of India has advocated the dimensions of farma as 40 cms long 35 cms wide and 25 cms deep.
Some authors have suggested the dimensions of gauge boxes as shown in Table 9.3 below. They kept the length and height of boxes same and varied width for various volumes.
The volume of each gauge should be written clearly with paint on the external face of the box.
The farma should be made of 3 cms thick prepared timber, so that it may give a good and strong farma. The joints should be tongued and grooved with the tongue on the inside of the box. This arrangement prevents dirt-getting due to the joint opening either due to shrinkage or rough handling. The faces of the joint should be painted with red lead in linseed oil before the farma is assembled. The inside face should be lined with sheet metal and the top edge of the box should be flush with sheet metal to keep a clean level edge for striking off. The handles should be shaped to provide an easy grip.
Measurement of Water:
Water usually measured by volume, provided the equipment is reliable, properly maintained and subject to calibration checks. The water contained in wet sand and coarse aggregate is difficult to measure accurately. The Value of this water can be assumed as suggested by IS 456-2000 shown in Table 9.4.
Surface Moisture Carried by Aggregate:
Surface moisture carried by aggregates is shown in Table 9.4 below as suggested by IS 456-2000.
2. Batching by Weight:
Nowadays for the production of good concrete all materials are measured by weight. The mixes are incorporated with weight batching devices, such that the weight of any material placed in the loading skip is clearly shown on a dial gauge fitted in it. There are several forms of weigh batching equipment available covering a wide range of capacities to suit the jobs of different sizes.
Mixing of Concrete:
The object of mixing of concrete is to coat the surface of all aggregate particles with cement paste and to blend all ingredients of concrete into a uniform mass. Mixing of concrete can be done either by hand or by machines.
Mixing of concrete by hand is less efficient than mixing by machines, but on small or unimportant works hand mixing is still practiced in India. For mixing concrete by hand, a water tight platform about 3.5 m long and at least 2 m wide should be provided. Concrete mixing by hand should never be done on ground, as earth and dirt dry grass, leaves etc. will mix with it.
The mixing should always be done on a clean, and hard surface, such as concrete floor made by 1:3:6 or 1:4:8 mix or on a wooden floor or in a metal tray. The wooden platform 3.5 m long and 2.0 m wide is shown in Fig. 9.3. This platform is large enough to permit two men using shovels to work upon it at a lime. The platform should be made of 3 to 5 cms thick planks, tongued and grooved so that the platform is rigid and joints water tight. A kerb on three sides should be provided to prevent water and cement draining away or material being showlled off the board.
1. Spread out the measured quantity of sand (Fine aggregate) evenly on the mixing surface.
2. Dump the measured cement on the sand and distribute it evenly. In this case about 10% more cement is used than weight batching.
3. Mix sand and cement with powrah or shovel, turning the mixture over and over again till it is of even colour throughout and free from streaks of brown and grey colour. Such streaks indicate non uniform mixture of sand and cement. The materials should be turned upside down at least three times after mixing the sand and cement spread this mixture over the platform evenly once again.
4. On the top of this mixture, spread the measured quantity of coarse aggregate.
5. Mix again this mixture at least for three times by shoveling from the centre to the side, then back to the centre and again to the side. When it becomes even mixture, make a heap of it with hollow at the centre.
6. In the hollow at the middle pour slowly about half to three quarter water of the total quantity of water required and mix it with the powrah. Add the remaining water slowly, turning the mixture over again until the colour and consistency are uniform throughout the pile.
For hand mixing, a steel mixing trough is a better arrangement than wooden platform, as it is easily portable from one job to another job. It also checks the loss of material while mixing. A steel mixing trough has proved economical than wooden platform due to its longer life.
Mixing by Machines:
Mixing of concrete by hand is expensive in labour in many countries. Thus mixing of concrete is done by machines. Concrete mixing by machines is more efficient and produces concrete of better quality at a faster rate and at less cost. As concrete mixers are manufactured in India itself, so concrete should be mixed by mixers.
Type of Concrete Mixers:
Nowadays concrete mixers of several types are available as follows:
1. Batch mixer.
2. Continuous mixers.
1. Batch Mixer:
In this type of mixers, after mixing one batch of materials, the mixed concrete has to be taken out and then again a second batch is put in.
It has three distinct functions to perform:
(a) Filling material into the loading skip
(b) Discharging skip into mixing drum
(c) Discharging of concrete from the drum.
The completion of all these functions forms one cycle. In the field, usually concrete mixing is done with batch mixers of revolving drum type mixers.
The batch mixers can further be sub divided as follows:
i. Tilting type mixers.
ii. Non tilting mixers.
iii. Open pan mixers.
ii. Tilting Type Mixers:
These classes of mixers have a conical or bowl shaped drum with inside vanes, which revolve on an inclined axis. It is charged by means of loading skip, though the smaller size mixers are normally fed direct into the drum. Concrete is discharged by tilting the drum.
The efficiency of the mixing operation depends on the details of design, but its discharge action is always good as all the concrete can be tipped out rapidly and in an unsegregated mass as soon as the drum is tilted. For this reason, tilting drum mixers are preferable for mixes of low workability and for those containing large size aggregate.
ii. Non Tilting Mixers:
In this class of mixers, the drum is cylindrical and always rotates about a horizontal axis i.e., its axis is always horizontal. In this case the material is charged by means of a cable operated loading skip or a charging hopper. The discharge of concrete is obtained by either inserting a chute into the drum or by reversing the direction of rotation of the drum or by spiking of the drum.
Due to the slow rate of discharge concrete from non-tilting type mixers, sometimes it is liable to segregation. In particular the large size of aggregate may tend to stay in the mixer so that the discharge starts as a mortar and ends as a collection of coated stones.
iii. Pan Mixers:
This type of mixer is not mobile and is thus used either at a central mixing plant or at large concrete project or at a precast factory or in a concrete laboratory. The mixer consists of a circular pan rotating about its vertical axis, with one or two star of paddles rotating about a vertical axis not coincident with the axis of the pan. Sometimes the pan is static and axis of the star travels along a circular path about the axis of the pan.
In either case the relative movement between the paddles and the concrete is the same, and concrete, in every part is thoroughly mixed. Scraper blades prevent mortar sticking to the sides of the pan and the height of paddles can be adjusted to prevent a permanent coating of mortar forming on the bottom of the pan.
Pan mixers are efficient with stiff and cohesive mixes and thus are useful for precast factories. They are also suitable for mixing small quantities of concrete due to scrapping arrangements smaller pan mixers of 40 lit capacities can be used in the laboratory. For discharge, such mixers have to be lifted physically, where as in big mixers there is one gate in the bottom covered with a gate, through which concrete is taken out.
2. Continuous Mixers:
These mixers do not require separate batching plant. They are designed to feed materials into the drum at rates related to the mix proportions required. The drum, which is mounted horizontally, consists of a long cylinder open at both ends and with internal blades of spiral shape so that they move the materials forward as the drum rotates. The materials are fed in by means of spiral conveyers and bucket elevators, the proportions are determined by the speed of the separate spirals.
In case of continuous mixers, the proportion of materials is done on the basis of volume batching. Thus concrete obtained by the mixer has less strength than obtained from batch mixers. Further control on mixing of ingredients in continuous mixer is also difficult. These mixers are used on big concrete projects such as construction of dam etc.
Size of the Concrete Mixer:
The size of the mixer is described by the volume of concrete after compaction. Previously the volume of the unmixed ingredients in a loose state was also included in its description. The latter may be upto 50% greater than compacted volume. Mixers are made in a variety of sizes from 40 liters for laboratory use upto 13 m3. Capacity upto 10% over load of the mixer is considered harmless. The actual size depends upon the nature and size, of work. The capacity of mixers usually is given in cubic metres or litres in M.K.S units and in cubic feet in F.P.S. units.
As the mixed concrete is more dense than its ingredients, for a particular quantity of concrete, the quantity of ingredients to be used is more by about 50%. Thus the capacity of mixers in M.K.S. units is described as 140/100, 200/140, 280/200, 400/280, and 600/400, which shows that to obtain 100 litre mixed concrete 140 litres ingredients should be Used. Numerator shows the quantity of unmixed ingredients while denominator shows the quantity, of mixed concrete.
Following Table 9.5 gives the capacity of mixers and their hourly output:
According to I.S. 1791-1961, the batch mixers are designated as follows:
Tilting drum type — 100 T, 140 T, 200 T
Non-tilting type — 140 NT, 200 NT. 280 NT, 400 NT and 800 NT etc.
The equivalent capacity in cubic feet is as follow:
F.P.S. units — 3½ 5, 7, 10, 14, 28 ft.3
100, 140, 200, 280, 400, 800 litres
The above figures are nominal mixed batched capacities. However Mixers can be used to mix 10% higher quantities. 200 litres capacity mixer is the most popular, size of the mixers. For any given size of mixer, it is necessary to know the number of 50 kg bags of cement which will produce a batch of concrete of given proportions.
This information is given in Table 9.6 below:
Mixing Time of Concrete:
On a site it is often a tendency to mix concrete as rapidly as possible, therefore it is important to know the minimum mixing time necessary to produce a concrete of uniform consistency and composition to give satisfactory strength. Actually the time of mixing of concrete varies with the size and type of mixer. Strictly speaking, it is not the mixing time but the number of revolutions of the mixer that is the criterion of adequate mixing.
Generally about 20 revolutions are sufficient for adequate mixing. Since an optimum speed of rotation is recommended by the manufacturers of the mixer, hence the number of revolutions and time of mixing are inter dependent. The variation in uniformity with mixing time based on Shalon’s work is shown in Fig. 9.7. From the study of this figure, it will be seen that mixing for less than 1 minute to 1¼ minute produces an appreciable more variable concrete, but mixing beyond these values results in no significant improvement in uniformity. Thus mixing time of 2 minutes is quite reasonable.
The average strength of concrete also increases with an increase in mixing time as shown in Fig. 9.8 based on Abram’s work. From the study of Fig. 9.8 it will be seen that the rate of increase of strength falls rapidly beyond about one minute mixing and the increase in strength is not large. Beyond 2 minutes mixing there is insignificant gain in strength, sometimes even a slight decrease in strength has been observed. Within the first minute, the influence of mixing time on strength is of considerable importance.
Increasing the time of mixing from 30 seconds to 1 minute Shalon found significant increase in strength and for a given required strength a saving of 30 kg cement per cubic metre of concrete was possible. However mixing for prolonged period causes segregation. As mentioned above, the exact value of the minimum mixing time depends upon the type and size of the mixer.
The values of mixing time as recommended by different Institutions and Bureaus are shown in Table 9.7. below:
The mixing time is reckoned from the time when all the solid materials have been put in the mixer and all the water has been added not later than one quarter of the mixing time has elapsed.
Output of Concrete Mixer:
The output of a concrete mixer depends upon the capacity of the mixer and the concrete mix adopted. However the output per hour is also affected by the length of mixing period and the time taken to charge and discharge the drum i.e. time of mixing cycle. The delays that arise due to one cause or the other even under normal circumstances invariably give a considerably less output than given by the manufacturers. Let us take the case of 200 litre capacity mixer.
The time taken in mixing one batch will be as follows:
Raising skip and loading ingredients into the drum = 0.5 mint.
Adding water and mixing in drum = 1.5 mint.
(During this time the skip is being lowered and recharged)
Discharging drum and completing loading of skip = 1.0 mint.
Total time = 3.0 mint.
No of batches in one working hour = 60/3 = 20 batches.
In a shift of 8 hours working, there will be only 7 hours of effective mixing time available.
So the batches produced in one day = 20 x 7 = 140 batches.
If each batch represents 140 litre of wet concrete, then output per day
= 140 x 140 = 196000 litres or 19.6 m3 per day
For mixers having capacity more than 800 litres Orchard has suggested the output as
= K x nominal capacity of mixer
K = 0.65 for tilting drum mixers, and
K = 0.72 for non-tilting mixers.
Feeding the Ingredients into Concrete Mixer:
No general rules on the feeding of ingredients into the mixer can be given as they depend upon the properties of the mix and the mixer. Generally a small amount of water should be fed first, followed by all the solid materials, preferably fed uniformly and simultaneously into the mixer. If possible greater part of water should also be fed during the same time, remainder of the water may be added after the solids.
I.S. 516-1959 has suggested the following sequence of feeding the ingredients:
1. Power Operated Mixer:
(b) Half coarse aggregate
(c) Fine aggregate
(d) Cement, and
(e) Remaining aggregate.
2. Hand Operated Mixer:
(a) Coarse aggregate
(b) Fine aggregate
(c) Cement, and
Checking of Concrete Mixer:
Before commencing operating mixer, following points should be attended:
1. The mixer should be placed on the firm and level ground.
2. The speed of the mixer should be kept 20 revolutions per minutes as given by manufacturers. The speed should be checked.
3. If the water is fed from tanks incorporated in the mixer, the water controlling device should be checked, so that correct amount of water may be added to the mix.
4. Inspect the engine, check oil, cooling water, petrol etc.
5. The engine should be allowed to run for few minutes to warm up.
6. Examine the mixer drum and blades and sec that they are clean and free from adhering lumps or concrete.
7. Also examine that all transporting machines are in working order.
8. While loading the skip, the ingredients should be put in the following order- (a) Coarse aggregate (b) Fine aggregate (c) Cement.
While these materials are put into the drum, the reverse order will follow and the coarse aggregate will fall in the last. It will help to push any sand or cement which might have adhered to the mouth of the skip.
Maintenance of Concrete Mixer:
1. Before closing down a shift for the day or in lunch break, the mixer should not be left loaded with concrete mix and the interior of the drum and blades should be flushed clean of all adhering mortar or concrete. If it is not cleaned, the age and efficiency of the mixer will be affected to a great extent.
The quality of concrete also will be affected as the adhered part of mortar or concrete may mix with the green concrete which will affect its setting and strength. The cheapest and simplest method of cleaning the mixer is, that after emptying it, water, coarser pieces of coarse aggregate or brick bats are put in the drum and it is run for 10 revolutions and the water thrown out by emptying it. If need be, this process may be repeated.
2. The interior face of the drum of mixer, blades surface etc. should be greased or oiled.
3. The outer surface of the mixer should also be kept clean and free from adhering mortar or concrete.
4. If the mixer is run by a petrol engine, then the rope and belt should be checked, lest it may break and cause accident.
5. Wash the machine every day using a hose pipe and grease or oil all parts of the mixer.
6. When the mixer is working at one place for a long time, its wheel and axles should not be allowed to be buried into the accumulated materials.
7. To speed the discharge of adhering sand and cement, the loading skip should not be allowed hitting or hammering. It will shorten its life and impair its action.
8. Engine should be attended every day and the fuel and cooling water is replenished.