Production of concrete, Batching, Mixing, Compaction and Curing

 CONCRETE

Concrete is a composite material composed mainly of water, aggregate, and cement. Often,
additives and reinforcements are included in the mixture to achieve the desired physical
properties of the finished material. When these ingredients are mixed together, they form a
fluid mass that is easily molded into shape. Over time, the cement forms a hard matrix which
binds the rest of the ingredients together into a durable stone-like material with many uses.
The aim is to mix these materials in measured amounts to make concrete that is easy to:
Transport, place, compact, finish and which will set, and harden, to give a strong and durable
product. The amount of each material (ie cement, water and aggregates) affects the properties
of hardened concrete.

Production of concrete

A good quality concrete is essentially a homogeneous mixture of cement, coarse and

fineaggregates and water which consolidates into a hard mass due to chemical action between

the cement and water. Each of the four constituents has a specific function. The coarser

aggregate acts as a filler. The fine aggregate fills up the voids between the paste and the

coarse aggregate. The cement in conjunction with water acts as a binder. The mobility of the

mixture is aided by the cement paste, fines and nowadays, increasingly by the use of

admixtures.The stages of concrete productionare:Batching or measurement of materials,

Mixing, Transporting, Placing, Compacting, Curing andFinishing.

Batching

It is the process of measuring concrete mix ingredients either by volume or by mass and

introducing them into the mixture. Traditionally batching is done by volume but most

specifications require that batching be done by mass rather than volume.The proportions of

various ingredients are determined by proper mix design.

A concrete mix is designed to produce concrete that can be easily placed at the lowest

cost. The concrete must be workable and cohesive when plastic, then set and harden to give

strong and durable concrete. The mix design must consider the environment that the concrete

will be in; ie exposure to sea water, trucks, cars, forklifts, foot traffic or extremes of hot and

cold. Proportioning concrete is a mixture of cement, water, coarse and fine aggregates and

admixtures. The proportions of each material in the mixture affects the properties of the final

hardened concrete. These proportions are best measured by weight. Measurement by volume

is not as accurate, but is suitable for minor projects.

Cement content as the cement content increases, so does strength and durability.

Therefore to increase the strength, increase the cement content of a mix. WaterContent

adding more water to a mix gives a weaker hardened concrete. Always use as little water as

possible, only enough to make the mix workable.Water to cement ratio as the water to cement

ratio increases, the strength and durability of hardened concrete decreases. To increase the

strength and durability of concrete, decrease the water-cement ratio.Aggregates too much fine

aggregate gives a sticky mix. Too much coarse aggregate gives a harsh or boney mix.Mixing

concrete must be mixed so the cement, water, aggregates and admixtures blend into an even

mix. Concrete is normally mixed by machine. Machine mixing can be done on-site or be a

pre-mixed concrete company. Pre-mixed concrete is batched (proportioned) at the plant to the

job requirements. Truck mixing the materials are normally added to the trucks at batching

plants and mixed for required time and speed at the plant. The trucks drum continues to rotate

to agitate the concrete as it is delivered to the site. Site mixing when site mixing begin by

loading a measured amount of coarse aggregate into the mixer drum. Add the sand before the

cement, both in measured amounts.

Mixing

The mixing operation consists of rotation or stirring, the objective being to coat the surface

the all aggregate particles with cement paste, and to blind all the ingredients of the concrete

into a uniform mass; this uniformity must not be disturbed by the process of discharging from

the mixer. The mixing may done by manually or by mechanical means like, Batch mixer,

Tilting drum mixer, Non tilting drum mixer, Pan type mixer, Dual drum mixer or Continuous

mixers.

There are no general rules on the order of feeding the ingredients into the mixer as

this depend on the properties of the mixer and mix. Usually a small quantity of water is fed

first, followed by all the solids materials. If possible greater part of the water should also be

fed during the same time, the remainder being added after the solids. However, when using

very dry mixes in drum mixers it is necessary to feed the coarse aggregate just after the small

initial water feed in order to ensure that the aggregate surface is sufficiently wetted.

Compaction

The operation of placing and compaction are interdependent and are carried out

simultaneously. They are most important for the purpose of ensuring the requirements of

strength, impermeability and durability of hardened concrete in the actual structure. As for as

placing is concerned, the main objective is to deposit the concrete as close as possible to its

final position so that segregation is avoided and the concrete can be fully compacted. The aim

of good concrete placing can be stated quite simply.

It is to get the concrete into position at a speed, and in a condition, that allow it to be

compacted properly. To achieve proper placing following rules should be kept in mind:The

concrete should be placed in uniform layers, not in large heaps or sloping layers.The

thickness of the layer should be compatible with the method of vibration so that entrapped air

can be removed from the bottom of each layer.The rate of placing and of compaction should

be equal. If you proceed too slowly, the mix could stiffen so that it is no longer sufficiently

workable. On no account should water ever be added to concrete that is setting. On the other

hand, if you go too quickly, you might race ahead of the compacting gang, making it

impossible for them to do their job properly. Each layer should be fully compacted before

placing the next one, and each subsequent layer should be placed whilst the underlying layer

is still plastic so that monolithic construction is achieved. Collision between concrete and

formwork or reinforcement should be avoided.For deep sections, a long down pipe ensures

accuracy of location of concrete and minimum segregation.You must be able to see that the

placing is proceeding correctly, so lighting should be available for large, deep sections, and

thin walls and columns.Once the concrete has been placed, it is ready to be compacted. The

purpose of compaction is to get rid of the air voids that are trapped in loose concrete.

It is important to compact the concrete fully because:Air voids reduce the strength of

the concrete. For every 1% of entrapped air, the strength falls by somewhere between 5 and

7%. This means that concrete containing a mere 5% air voids due to incomplete compaction

can lose as much as one third of its strength.Air voids increase concrete's permeability. That

in turn reduces its durability. If the concrete is not dense and impermeable, it will not be

watertight. It will be less able to withstand aggressive iquids and its exposed surfaces will

weather badly.Moisture and air are more likely to penetrate to the reinforcement causing it to

rust. Air voids impair contact between the mix and reinforcement (and, indeed, any other

embedded metals). The required bond will not be achieved and the reinforced member will

not be as strong as it should be.Air voids produce blemishes on struck surfaces. For instance,

blowholes and honeycombing might occur. There are two methods for compaction which

includes: vibration by vibrators or by tamping using tamping rods.

Curing

Curing is the process of making the concrete surfaces wet for a certain time period after

placing the concrete so as to promote the hardening of cement. This process consists of

controlling the temperature and the movement of moisture from and into the concrete.

Curing of concrete is done for the following purposes. Curing is the process of controlling the

rate of moisture loss from concrete to ensure an uninterrupted hydration of Portland cement

after concrete has been placed and finished in its final position.Curing also helps maintain an

adequate temperature of concrete in its early stages, as this directly affects the rate of

hydration of cement and eventually the strength gain of concrete or mortars.

Curing of concrete must be done as soon as possible after placement and finishing and must

continue for a reasonable period of time, for the concrete to achieve its desired strength and

durability.Uniform temperature should be maintained throughout the concrete depth to avoid

thermal shrinkage cracks.

Material properties are directly related to micro-structure. Curing assists the cement

hydration reaction to progress steadily and develops calcium silicate hydrate gel, which binds

aggregates leading to a rock solid mass,makes concrete denser, decreases the porosity and

enhances the physical and mechanical properties of concrete.

Some other purposes of curing can be summed up as: curing protects the concrete surfaces

from sun and wind, the process of curing increase the strength of the structure, the presence

of water is essential to cause the chemical action which accompanies the setting of concrete.

Generally there is adequate quantity of water at the time of mixing to cause the hardening of

concrete,but it is necessary to retain water until the concrete is fully hardened.

If curing is efficient, the strength of concrete gradually increases with age. This

increase in strength is sudden and rapid in early stages and it continues slowly for an

indefinite period.By proper curing, the durability and impermeability of concrete are

increased and shrinkage is reduced.The resistance of concrete to abrasion is considerably

increased by proper curing.

What is Cement? What are the types of cement and their uses?

Composition of Cement clinker and Hydration of Cement 

Types of Tests on Cement to Check the Quality

PPC or OPC And Their Difference

All Types of Cement in Construction

Must-Know Points to Check on a Cement Bag Before Buying It

CONCRETE Curing period,Disadvantages of improper curing,Factors affecting evaporation of water from concrete and Methods of curing

What is a Cavity Wall? Construction and Advantages of Cavity walls and their perpose