mixing of concrete

8/7/2019 MIXING OF CONCRETE

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MIXING OF

CONCRETEThe aim of mixing of concrete is to produce a homogenous, consistent

and uniform coloured concrete. This means that the different constituents of

concrete be uniformly distributed throughout the concrete mass and thecement-water paste should completely coat the aggregates.

The mixing action of concrete thus involves two operations:

I. A general blending of different particle sizes of the ingredients to beuniformly distributed throughout the concrete mass.

II. A vigorous rubbing action of cement paste on to the surface of the

aggregate particles.

There are two methods of mixing concrete:

(1)Hand mixing

(2) Machine mixing.

HAND MIXING

Mixing concrete by hand is hard work. While you can mix the ingredientsin a wheelbarrow, it's usually easier to mix them on a clean, flat surface, suchas an old sheet of plywood, or in a mortar box (also called a concrete barge).When mixing concrete by any method, make sure to protect yourself bywearing long sleeves, long pants, rubber boots, heavy waterproof gloves, andsafety goggles. If concrete accidentally spatters on your skin, wash it offimmediately because it causes burning irritation.For small and unimportantjobs concrete is mixed by hand using shovels. As the mixingcannot be

thorough and enceinte, it is desirable to add 10 per cent more cement to caterforthe inferior concrete produced by this method.


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The following operations are involved in hand mixing.

1. Test the water content of the sand. The sand used to make theconcrete should be wet, but not too wet. As illustrated in figure1, sand

with the correct amount of water will hold a ball shape without wettingyour hand, very wet sand will make a ball but "leak" water, while drysand will crumble and not hold a shape. If the sand is too dry, wet itthoroughly using a garden hose the day before beginning work. Alwayscover your sand pile with a sheet of plastic when you're not working, toprevent the sand from drying out.

2. Measure the ingredients. For mixing purposes, you can measproportions by the bucket, by the shovelful, or with a measuring box, likethe one shown in figure 2. A measuring box is built to hold exactly 1 cubicfoot of dry ingredients, as opposed to a mortar box in which wet and dry

ingredients are combined. Use the same amount of each ingredient foreach batch. Careful measuring ensures correct proportions. Place thematerials in layers on top of each other, beginning with the gravel, then thesand, and finally the cement.

FIGURE-2

3. Mix the dry ingredients. If you're working on a flat surface or mixing theingredients in a wheelbarrow, use a mason's hoe to combine the dryingredients thoroughly before adding the water. When using a mortar box,you can either premix the dry ingredients or mix them together as you addthe water as shown in the figure 3.


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FIGURE-3

4. Add the water. Whether you use a premix bag or a combination ofseparate ingredients, the water used to make the concrete must beclean enough to drink. Start with a gallon of water and keep track of theamount you use so that you can add the same amount to subsequent

batches. Be sure to mix all ingredients thoroughly, scraping anyunmixed cement and aggregates from the sides and bottom of the boxor pile. The concrete mix should be an even colour and have the sameconsistency throughout.

5. If you're mixing on a flat surface, make a shallow depression in the centreof the dry mix using your hoe; then pour in a little water (Fig. 4). Add thewater slowly. Mix thoroughly by pulling dry material from the edges into thewater. Continue to add water until the mix reaches the proper consistencynot crumbly, not sloppy. If using a mortar box, place the dry materials so

that they fill about two-thirds of the box (from one end), leaving the rest ofthe box empty (on the side nearest the forms). Add water to the empty end,and then pull the dry materials into the water, mixing them together as yougo. Continue adding water in small amounts while turning over the mix untilit reaches the proper consistency.

FIGURE-4

6.

Test the mix. You can tell if the concrete has too little or too much water byusing the blade of your hoe or shovel to make ridges in the concrete. If the


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mix is too dry, you won't be able to make distinct ridges; if the mix is toosoupy, the ridges won't hold their shape (you'll also notice water seepingout around the edges of the pile). In a proper mix, the ridges will hold mostof their shape.

FIGURE-5

MACHINE MIXING

Machine mixing is adopted for big projects where large quantities ofconcrete are required are continuously. Machine mixing is not only efficient,but also economical, when the quantity of concrete to be produced is large.

Concrete mixers are classified as under:

1. According to the operating conditions:i. Batch mixersii. Continuous mixers.

2. According to the principle of mixing:

i. Gravity typeii. Type with forced mixing.

3. According to the condition of use:

i. Stationary concrete mixersii. Portable concrete mixers.

Batch Mixers:

In this case, the mixers are charged with materials in batches. Each newbatch will be charged in to the drum of the mixer only when the preceding

batch has been discharged. Batch mixers are used for normal concrete works.They produce concrete batch by batch with time interval.


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Batch mixers can be classified in to three main types. They are,

a) Tilting type

b) Non-tilting typec) Reversing type

As per IS: 1971-1985, concrete mixers are designated by a numberrepresenting its nominal mixed batch capacity in litres. The following are thestandardized sizes of three types:

i. Tilting type : 85 T, 100 T, 140 T, 200 T

ii. Non-tilting type: 200 NT, 280 NT, 375 NT, 500 NT, 1000 NT

iii. Reversing type: 200 R, 280 R, 375 R, 500 R, 1000 R

The letters T denote tilting type, NT denote non-tilting type and R denotereversing type.

TILTING TYPE CONCRETE MIXTURE

NON TILTING TYPE CONCRETE MIXTURE:-


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The non-tilting type mixture essentially consists of a cylindrical drum with twocircular openings at the ends and blades fixed inside the drum. The drumrotates about a horizontal cannot be tilted. The mixer is loaded through acentral opening at one end of drum and, after mixing, the concrete is

discharged through the opening at the other end by a chute. Owing to a ratherslow rate of discharge, the concrete is sometimes susceptible to segregation.In particular, the Iarger size aggregate may tend to stay in the mixer. Hencethe discharge may initially consist of mortar and then as a collection of a largesize coated aggregate. However it is worthwhile to check the performance ofthe mixer for a particular type of mix before it is actually used.

REVERSING TYPE CONCRETE MIXTURE:-

Reversible concrete mixer, Reversible Drum Mixer are strong well-engineered pieceof construction equipment which is meant for carrying mixing mortar from batchplant directly to the place where it is to be poured. It has a wide range ofapplications specially used in construction of canals, bridges, highways, dams, multistoried & industrial buildings, factories and for ready mixed concrete works.Reversible Mixer drum is rotating on well align heavy duty gear ring guided by threeroller for smooth rotation. Its compact design is easy for transportation. Rotation of

the concrete drum, reversed is auctioned by push on the elecpanel.


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CONTINUOUS TYPE CONCRETE MIXTURE:-

The second category of mixers is continuous mixers. As the name indicates,the materials are continuously fed into the mixer at the same rate as theconcrete is discharged. They are usually non-tilting drums with screw-typeblades rotating in the middle of the drum. The drum is tilted downward towardthe discharge opening. The mixing time is determined by the slope of the drum

(usually about 150). These mixers are used for applications that require a shortworking time, long unloading time, remote sites (not suitable for ready-mix)and/or small deliveries. A major use of these types of mixers is for low slump.

STATIONARY CONCRETE MIXTURES:-

A concrete mixing plant is a huge set up of machines meant to combinecement, aggregates such as sand or gravel, and water to form concrete.Concrete mixing plants apart from various other equipments also uses a

revolving drum to mix the components. Concrete mixers may be mobile orstationary. Its size range from portable mixer to heavy industrial mixing plant.

Stationary plant-mounted mixer mixes the concrete before it is discharged intoa truck mixer. Revolving drums, conveyor and diesel or electric poweredengines are used in this plant. The truck mixer is used primarily as an agitatinghaul unit at a central mix operation. Dump trucks or other non-agitating unitsare sometimes used for low slump and mass concrete pours supplied bycentral mix plants.


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PORTABLE TYPE CONCRETE MIXTURE:-

The name says it all. Portable cement mixer can be transported from one siteto another with ease. Making it the most commercialized concrete mixer outthere. That also means that they are the most cement mixers been abusedand over worked. This is something you may want to consider when buyingthese types of cement mixers. One of the main advantages of this type ofconcrete mixer is that you dont need to add extra water for the mixingprocess. This is the type of cement mixers that you would use for a typical

house project. The rotation of the concrete mixture is powered by means ofelectricity. The lever allows the concrete/mortar to be tipped ia wheelbarrow.

mixing of concrete

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