Concrete

CONCRETE

SUBMITTED BY:- GONDALIYA MITALI 01 BHATT HETALI 05 DHAMELIYA URMILA 09 JARIWALA DEWANSHI 17 KHANESHA SAGAR 21

INTRODUCTION COMPONENTS WATER-CEMENT RATIOPROPERTIESMIXING CONCRETE GRADESCONSOLIDATION CURINGTYPES

OVERVIEW

WHAT IS CONCRETE?• Concrete is one of the most commonly used

building materials.• a composite material made from several readily

available constituents (aggregates, sand, cement, water).

• is a versatile material that can easily be mixed to meet a variety of special needs and formed to virtually any shape.

• has many applications and is used to make pavements, pipe, structures, foundations, roads, bridges/overpasses, walls and footings for gates.

ADVANTAGES…

• Ability to be cast• Economical• Durable• Fire resistant• Energy efficient• On-site fabrication

COMPONENTS OF CONCRETE..

concrete

1. cement

2.aggregates

natural

Crushed rocks

Igneous rocks

Sedimentary rocks

Metamorphic rocks

Gravel

sand

artificial

3. steel

4.water

Concrete is basically a mixture of two components: Paste & Aggregates The paste, usually comprised of Portland cement and water, binds the aggregates (sand and gravel or crushed stone) into a rock-like mass as the Paste hardens because of the chemical reaction of the cement and water. Therefore, total 4 components are required, namely:

1. Cement 2. Aggregates 3. Steel (for reinforcement - RCC) 4. Water

1. CEMENT• Before the introduction of ordinary Portland Cement, lime was used as a cementing material.• Most of the cement concrete work is done withPortland cement only.• Used because its ‘rapid hardening cement’ & ‘high alumina cement’ ---- special varieties.• Portland cements are hydraulic cements,i.e., they set & harden by reacting with waterto form a paste. The paste ordinarily constitutes about 25% to 40% of the total volume of concrete.

• is the most energetically expensive. (fuel consumption per ton of clinker produced is high & so have emissions)

Portland cement

Oxides of calcium

Oxides of aluminium

Oxides of silicon

Portlan

d cement

Heating

limeston

e with clay

Grinding

clinker with a source

of sulphate(gypsum)

clinker

2. AGGREGATES (60-80%)• Inert or chemically inactive materials.• Bound together by means of cement.• classified into 2-----Fine and coarse aggregates, makes up the bulk of a concrete mixture.

Fine Aggregates Coarse Aggregates

Natural river sand is used as fine aggregate but where natural sand is not available economically, the finely crushed stone may be used.

Broken stone is generally used.

•The nature of work decides the maximum size of the coarse aggregate. For thin slabs & walls, the max. size of coarse aggregate should be limited to 1/3 the thickness of the concrete section.•Aggregates used should be hard durable & clean, should be completely free from lumps of clay, organic & vegetable matter, fine, dust, etc. The presence of all such debris prevents adhesion of aggregates & hence reduces the strength of concrete.

•May also be classified in the following 2 categories, namely: Natural & Artificial Aggregates

•Natural Aggregates: the term natural suggests that it is used loosely to assign aggregates which need only be removed from their natural deposits as unconsolidated sediments. Thus the aggregates obtained from such deposits are called gravel & sand.•Those produced from boulders stone are known as crushed stones•Thus it can be divided into 3, namely : Crushed rock aggregate, Gravel & Sand

a) Crushed rock: obtained by crushing rock pieces into suitable sizes. Therefore, it is evident that the quality of it will be controlled by nature & type of rock from which it is crushed.

• 3 major divisions of rocks------igneous, sedimentary, metamorphic

Sand Gravel Crushed rocks

Natural Aggregates

Igneous rocks Sedimentary rocks Metamorphic rocks

•originally formed by cooling from a molten rock material known as magma

•formed by the deposition of products of weathering on the pre-existing rocks.

•have either igneous or sedimentary origins but as a result of intense heat or pressure or both, they have been altered or changed into rocks possessing different properties

Igneous rocks Sedimentary rocks Metamorphic rocks

• Plutonic(granite & diorite) are brittle due to the presence of large crystals.

• Hypabyssal are medium grained & best road stones

due to the inter grown texture they possess.

• Calcareous---- calcium carbonate predominates & lime stones.

• Dolomites & chalk(unsuitable for construction

purposes) are main varieties.

• Limestones & dolomites have excellent adhesion to cement & bituminous binders.

•the hornfels, formed due to the thermal metamorphism are considered best for road constructions.

•The gneiss & granulites have the same quality of road making as granites of coarse variety.

Igneous rocks Sedimentary rocks Metamorphic rocks

• Volcanic are fine-grained & excellent for building constructions.

Basalt & andesite are the main varieties

• Siliceous-----silicon predominates;

• Sandstones(frequently used in construction work & as road stones where they are available)

• quartzite(quite hard; good in adhesion to cement but poor in bitumen) are the main varieties.

Schist, slate and phyllite are highly foliated; on crushing, they break down into platy fragments which, in mass aggregate, produce large voids

Slate

schist

b) Gravel: the term is used to mean the coarse material resulting from the disintegration of natural rock due to weathering & carried away by water & then deposited on the river banks.

•Therefore, largely it possess properties of basic rock constituents, & usually the hard varieties are found dumped along river banks or along strata which had been under water. •Variety like boulders, do not require any special tests when used as soiling or base course.c) Sand: final residue of the mineral grains resulting from the weathering action

upon the rocks are sand, & has reached after many cycles of deposition & weathering. Most imp mineral is quartz & is hardly affected by the ordinary weathering agents.

• for use in concrete, tested for shape and surface smoothness. If the particles have angular shapes, the sand has a high void content and a high water demand. It takes more fluid (cement paste) to surround an angular. If the fluid present is insufficient to coat the surface area, the concrete mixture is harsh and difficult to place and finish.

• Artificial aggregates: Foamed blast furnace slag(obtained as a by-product in the manufacture of steel) is the only artificially prepared aggregate needed in the construction.

• If slag is manufactured under controlled conditions, it can certainly prove to be an excellent aggregate of uniform quality.Iron

slag(produ

ced with

controlled

amt of water)

Applied to the

molten slag

to trap the

steam

Gives a porous product

s

Crushed to sizes suitable

as an aggregate

3. STEEL• Concrete is strong in compression, as the aggregate efficiently carries the compression

load. However, it is weak in tension as the cement holding the aggregate in place can crack, allowing the structure to fail. Reinforced concrete adds either steel reinforcing bars to carry

tensile loads. • The bars diameter vary from 5mm – 40mm.• Sometimes the square bars or twisted bars or ribbed-tor steel are also used.• For road slabs, the reinforcement may also consist of sheets of rolled steel of suitable thickness. • The hyrib, a steel lath can also be used.For a strong, ductile and durable construction the reinforcement needs to have the following properties at least:•High relative strength•High toleration of tensile strain•Good bond to the concrete•Thermal compatibility, not causing unacceptable stresses in response to changing temp.•Durability

4. WATER: least expensive but most imp ingredient.• Combining water with cement forms a cement paste by the process of hydration.

The cement paste glues the aggregate together, fills voids within it, and makes it flow more freely.

•Impure water used to make concrete can cause problems when setting or in causing premature failure of the structure. •Therefore, only fresh drinking water(free from alkali, acid, oil, etc.) is preferable.

Low water-cement ratio High water-cement ratio

Low polarity=strong & durable concrete

High polarity=weak concrete

Why Use Admixtures?•added to improve qualities or for changing different physical properties in its fresh & hardened stages, known as admixture. •Some admixtures are: alum, aluminium sulphate, barium oxide, bitumen, calcium chloride, coal ash, common salt, iron oxide, lime, mineral oils, organic oils, potassium chloride, silicate of soda, tar products, volcanic ashes, zinc chromate, etc.

advantages of admixtures:i. Adjusting the final setting times of concreteii. Higher early& ultimate strengthiii. Higher slump & self-levelling concreteiv. Increasing durability of concretev. Lesser water-cement ratiosvi. Reducing quantity of cementvii. Reduction in the permeability of concreteviii. Time saving in terms of repair &

maintenance.

• decrease water content

• increase workability • retard or accelerate

setting time• reduce segregation• reduce the rate of

slump loss• modify the rate and/or

capacity for bleeding

•Knowing the complete details of any admixture before its recommendation together with the following factors, is very necessary :i. Grading curves of aggregates & their respective propertiesii. Method of constructioniii. Quantity of cement per metre cube of concreteiv. Requirement of slump & retentionv. temp. variationsvi. Type & make of cementvii. Water-cement ratio

• Admixtures are classified into 5 types based on their respective activities in the concrete mix, namely:

i. Acceleratorsii. Air entraining admixturesiii. High range of water reducers or super plasticisersiv. Normal range of water reducers or plasticisersv. Retarders

Water Reducers Accelerators Retarders Shrinkage Reducing

Admixtures

Corrosion Inhibitors

are used for the purpose of reducing the quantity of mixing water required to produce a concrete of given consistency.

are added to concrete for the purpose of shortening set time and accelerating early strength development.

are used to offset acceleration and unwanted effects of high temperature and keep concrete workable during placement and consolidation.

are used to minimize drying shrinkage cracking in concrete .

are used to mitigate corrosion of reinforcing steel in concrete.

SEA WATER FOR MIXING CONCRETE

• It is advisable as stated above, to use clean water fit for drinking purpose for making cement concrete.

• at places where sea water is available in abundance and potable water is costly, the sea water can be used for making cement concrete.

• The problem using sea water for making cement concrete has to be studied from the following two aspects :

1. Strength : below table shows the analysis of average sea water.

• consist about 3.50% of dissolved salt . • The approximate percentage of various salts are 78% of

sodium chloride , 15 %magnesium chloride and magnesium sulphate and the rest 7 % of calcium sulphate, potassium sulphate, etc.

Composition of average sea water

No. Constituent Contents in gm per liter

1 Calcium (Ca) 0.43

2 chloride (Cl2) 19.80

3 Magnesium (Mg) 1.33

4 Potassium (K) 0.44

5 Sodium (Na) 11.00

6 Sulphate (so4) 2.76

Total: 35.72

2. Corrosion of reinforcement: sea water does not lead to the

corrosion, provide the concrete is dense and there is enough cover to the reinforcement.

• The maximum cement content for concreter permanently under sea water should be 3 Kn per m3 & the minimum cover the reinforcement should be 75mm

• however it is not advisable to take the risk of corrosion of reinforcement for prestressed concrete and hence the sea water should not be used for making prestressed concrete.

WATER-CEMENT RATIO

•From the above discussion we came to know that water hardens & set the concrete, also it fills the voids of cement ,i.e., makes the concrete workable.•So it is found that water required is about 0.50-0.60 times the weight of cement. This ratio of the amt of water to the amt of cement is termed as water-cement ratio.

Imp pts to remember : the strength of concrete is inversely proportional to the water-cement ratio. ratio for structures which are exposed to weather should be very carefully decided…

like…structures which are regularly wetting & drying, which are continuously under water.

2 thumb rules which decides the quantity of water in concrete…..

Weight of cement + Weight of total aggregate

28% 4%

30% 5%

Weight of water =

Compressive Strength of various Water-Cement Ratio

PROPERTIES OF CONCRETE

• has relatively high compressive strength• lower tensile strength• The elasticity of concrete is relatively constant at low

stress levels but starts decreasing at higher stress levels as matrix cracking develops.

• has a very low coefficient of thermal expansion, and as it matures concrete shrinks.

• All concrete structures will crack to some extent, due to shrinkage and tension.

• can be damaged by fire, aggregate expansion, sea water effects, bacterial corrosion, leaching, physical damage and chemical damage (from carbonation, chlorides, sulfates and distillate water).

Desired Properties of Fresh Concrete

CONSISTENCY WORKABILTY SEGREGATION•It is desirable that freshly mixed concrete be relatively easy to transport, place, compact and finish without harmful segregation. • A concrete mix satisfying these conditions is said to be workable.

•Segregation refers to a separation of the components of fresh concrete, resulting in a non-uniform mix•The primary cause for this is differences in size of constituents of concrete. Moreover, improper mixing, placing and consolidation also lead to segregation.

• Consistency is the fluidity or degree of wetness of concrete.• it is a major factor indicating the workability of freshly mixed concrete.

•Test methods for measuring consistency is SLUMP TEST (most widely used test)- Given in the Next slide

Slump Test• The goal of the test is to measure the consistency of

concrete through out the mix.

• "Slump" is simply a term coined to describe how consistent a concrete sample is.

• The test also further determines the workability of concrete, how easy is it to handle, compact, and cure concrete.

• By adjusting the cement-water ratio or adding plasticizers to increase the slump of the concrete will give a desired mix.

10 cm

20 cm

30 cm

The slump cone is filled in 3 layers. Every layer is evenly rodded 25 times.

Measure the slump by determining the vertical difference between the top of the mold and the displaced original center of the top surface of the specimen.

UNIFORMILTY DURABILITY

•Concrete uniformity is checked by conducting tests on fresh and hardened concretes.o Slump, unit weight, air content

testso Strength tests

Due to heteregeneous nature of concrete, there will always be some variations--- inadequate mixing leads to non-homogeneous nature.

Shrinkage: decrease in volume of concrete due to loss of water from structure is the major cause of cracking in concrete, high water content increases shrinkage, high aggregate content decreases shrinkage, moist curing decreases shrinkage.

MIXING THE MATERIALS OF CONCRETE

• The aim of mixing is to blend all of the ingredients of the concrete to form a uniform mass and to coat the surface of aggregates with cement paste.

• The process of rolling, folding and spreading of particles is known as the mixing of concrete. The thorough mixing also ensures that cement water paste completely covers the surfaces of aggregates.

• The materials of mixing of concrete can be done either with hand or with the help of a machine.

Mixing time should be sufficient to produce a uniform concrete. The time of mixing depends on the type of mixer and also to some properties of fresh concrete.

Undermixing → non-homogeneity Overmixing → danger of water loss, breakage

of aggregate particles

Undermixing → non-homogeneity Overmixing → danger of water loss, breakage of aggregate particles Ready-Mix concrete: In this type ingredients are introduced into a mixer

truck and mixed during transportation to the site. • Wet – Water added before transportation• Dry – Water added at site

Mixing at the site• Hand mixed• Mixer mixed

1. HAND MIXING

• The mixing by hand is allowed in case of small works or unimportant works where small quantity of concrete is required.

• For important works, if hand mixing is so to be adopted, it is advisable to use 10 per cent more cement than specified.

Materials (wood

brick or steel)

stacked on a

water tight

platforms

thoroughly

mixed, at least 3 times, before

water is added

This mix should

be consume

d in minutes

after adding water.

MACHINE MIXING

all the materials including water, are

collected in a revolving

drum.

the drum is rotated for a

certain period.

The resulting mix is then taken out

The features of machine mixing are:• is more efficient and it produces concrete of better quality in a short time.• The mixers of various types and capacities are available in the market.

They may either be of tilting type or non-tilting type. • For small works, a mixer capable of producing concrete of one bag of

cement, is used. For works such as roads , aerodromes, drams, etc., special types of mixers are used.

•should be thoroughly washed and cleaned after use. If this precaution is not taken, the cakes hardening concrete will be formed & are difficult to remove at a later stage, also affects the efficiency of the mixer.•inside portion of the mixer should be inspected carefully at regular intervals. The damaged or broken blades should be replaced.•The time of mixing the materials and the speed of the mixer are very important factors, decides the strength of concrete formed. The mixing time should be at least one minute and preferably two minutes. The mixer should be rotated at a speed as recommended by the manufactures of the mixer.• the concrete discharged by the mixer should be consumed within 30 minutes.

Grade Mixes of Concrete…

General expression cement : sand : aggregate = 1 : n : 2n

As per IS:456-2000, concrete is designated in 7 grades---- M10, M15, M20, M25, M30, M35 & M40 (M = mixnumber = specified compressive strength of that mix at 28 days)

•M5 & M7.5…simple foundations for masonry walls< M15 ….not to be used in RCC work

M5 – 1: 5: 10

M7.5-1: 4: 8

M10-1: 3: 6

M15-1: 2: 4

M20-1: 1.5: 3

M25-1: 1: 2

CONSOLIDATION OF CONCRETE • The main consideration of consolidation of concrete

is to eliminate air bubbles and thus to give maximum density to the concrete

• The importance of consolidation of concrete can be seen from the fact that a presence of 5% of voids reduces 30% strength of concrete

• The difference between voids and pores is

o voids are the gaps between two individual particles.

o pores represent the openings within the individual particles

THE PROCESS OF CONSOLIDATION IS DIVED IN TO TWO PARTS

HAND CONSOLIDATION VIBRATORS CONSOLIDATION

HAND CONSOLIDATION

• The consolidation of concrete is carried out by hand methods which includes ramming , tamping , spading and slicing with suitable tools and its require use fairly wet concrete .

• It should however be remembered that wherever feasible , the hand compaction should be preferred because the use of vibrator may lead to segregation of the aggregates .

• The concrete mixes which can be hand compacted should not be compacted by the use of vibrators

VIBRATORS • THESE ARE THE MECHANICAL DEVICES USED TO COMPACT

CONCRETE IN THE FORMWORK.

• Vibrators to make harsh and stiff concrete mix, with a slump of about 40 mm or less is workable• The quality of concrete can be improved by use of vibrators as less water will be required or in other way,

economy can be achieved by adopting a leaner mix when vibrators are used.

• The use of vibrators result in the reduction of consolidation time they are used where the rapid progress of work is great importance.

TYPES OF VIBRATORS

INTERNAL VIBRATORS

SURFACE VIBRATORS

FORM VIBRATORS

VIBRATING TABLE

INTERNAL VIBRATORS

• These vibrators consist of steel tube which inserted in fresh concrete .

INTERNAL VIBRATORS SURFACE VIBRATORS

These vibrators consist of steel tube which called the POKER its inserted in fresh concrete .Its connect with electric motor.

These vibrators are mounted on platforms or screeds. They are used to fine concrete surface on bridge floors, roads slabs, station platforms.

They are available in 40 mm to 100 mm dia. The size is depend on us the spacing between reinforcing bars in concrete . The frequency is about 3000 to 6000 r.p.m

These vibrators are found to be more effective for compacting very dry concrete because vibrators acting same direction of gravity to the concrete

When poker inserted then the internal vibrators should be inserted and withdrawn and its working continues when its withdrawn .

The vibrators used to the movement of fine material to the top and its finishing operations .

We can used vibrators vertically or slight inclination The vibrators are efficient then other types and its commonly used .

FORM OR SHUTTER VIBRATORS VIBRATING TABLE

These vibrators are attached to the formwork and external centering of wall, columns

These are in the form of rigidly built steel platforms mounted on flexible springs and they are operated by electromagnetic.

These vibrators require more power because of loss of some in vibrating the rigid shutters.

They are found to be very effective in compacting stiff and harsh concrete mixes and they are invariably used in the preparation of pre-cast structural products in factories and test specimens in laboratories.

They are also have and they cannot be clamped at as many points as possible for uniform compaction of concrete.

The table are vibrated either mechanically or by placing the springs under the supports of tables. Its 3000 to 7200 vibrations per minute.

It means the consolidation of concrete will be achieved in less time and vice versa.

CURING OF CONCRETE

Concrete surfaces are kept wet for a certain period after placing of concrete so as to promote the hardening of cement.

Consists of a control of temp. & of the moisture movement from & into the concrete.

Such processes are termed as CURING

Following are the purposes of curing:1. protects concrete surface from sun and wind 2. the presence of water is essential to cause the chemical action which

accompanies the setting of concrete . 3. Strength of concrete gradually increases with age, if curing is efficient.4. By proper curing, durability is increased & shrinkage is reduced.5. Resistance of concrete to abrasion is increased.

Effects of improper curing:• Compressive strength is reduced• Cracks will be formed due to shrinkage• Durability decreases• Frost & weathering resistances are decreased• Rate of carbonation increases• Surfaces are coated with sand & dust which leads to reduction of abrasion

resistance

Above mentioned disadvantages are more prominent for those structures which are either directly exposed or those which have large surfaces compared to depth such as roads, canals, bridges, etc, to avoid the cracks (necessary to protect even before setting).

Factors affecting evaporating of water from concrete:a) Air temp.b) Fresh concrete temp.c) Humidityd) Wind velocity• The evaporation of water in the first few hours can leave very low amt of

water in the concrete for hydration, leads to several cracks (average loss of water varies from 2.5-10N/m2 per hour).

• Major loss occurs in the top 50mm layer & over a period of 3 hours, the loss could be about 5% of the total volume of that layer.

Curing Tips

Period of curing

Curing Methods –Factors to be considered while selecting any mode of curing :a. temp. should be kept min. for dissipation of heat & hydrationb. Prevention of water evaporationTherefore, the methods should be employed in proper way & proper

supervision.

Specialized curing techniques are :1) Ponding with water2) Covering concrete with wet jute bags3) Covering concrete with wet sand, saw dust, etc.4) Covering concrete with water-proof paper & polyethylene sheets5) Sprinkling or spraying with water6) Applying curing compounds

•Methods 1 to 5 are conventional ones & all suffer from the common defect of late beginning by which time some harm has been affected to the concrete.

•Ponding is the best method….consists of little earthen dams which are built over the entire surface to be cured.•The squares thus formed are then flooded with water to a depth of about 50mm or so.•Effective for horizontal surfaces & not vertical.

•The method of covering concrete surfaces with wet jute bags, wet sand, saw dust, polyethylene, etc. is difficult in maintaining, especially at places having winds blowing at high speeds.

•The last method of applying the curing compounds is a simple one & can be brought about by spraying while the concrete is wet.•The application is carried out by tree sprayers which are use for spraying insecticides.•Nozzle should be regularly cleaned by water or white cement, should be held at a distance of 0.7m-1m from the surface & should be confirmed that full area is covered .•Pressure should be maintained in the pump to effect fine spray.

Spraying of water is the most common method but is not properly employed because there should be continuous supply of water to the surface.

TYPES OF CEMENT CONCRETE

Water proofing cement concreteColoured concreteLight weight concreteNo fines concretePre cast concreteReady mix concreteTransit mix concrete

WATER PROOFING CEMENT CONCRETEInfo… How to obtain dense concrete…

It is used for certain types of works like water storage tanks, reservoirs,basement walls,roofs,swimming pools, sewage units,etc. So, the concrete should be dense and free from cracks.

•Using high class Portland cement of guaranteed quality.•Taking extreme care to adopt correct grading and proportioning of the sand, aggregate and cement.•Using clean and non-porous aggregates.•Mixing thoroughly to the right consistency using the right amount of water.•Placing, tamping and curing carefully.•Making use of suitable water-proofing compound.

COLOURED CONCRETEMaking… Use…

•By addition of suitable colouring pigments to the extent of about 8 to 10% of the weight of cement.•By using coloured cement for the preparation of cement concrete.•By selecting agregates possessing the required colours.

•Manufacture of items for public welfare•Ornamental finishes in buildings•Preparing park lanes•Separating lines of traffic of road surfaces•Underground pedestrian crossings, etc

LIGHT-WEIGHT CONCRETEInfo… Advantages…

The bulk density of ordinary concrete is about 23 kN/m^3. The concrete having bulk density between 5 to 18 kN/m^3 is known as the light-weight concrete .

•The local industrial waste, if found suitable for light-weight concrete, can be economically utilized.•The reduction in weight of concrete helps easy removal, transport and erection of pre-cast products.•The use of light-weight concrete results in the reduction of cost to the extent of about30 to 40% or so.•The light-weight concrete has comparatively less tendency to spall. Hence, its fire resistance is greater as compared to the ordinary concrete.•The light-weight concrete has generally a lower thermal expansion than ordinary concrete.

NO-FINES CONCRETEInfo… Advantage… Limitations…

The no-fines concrete consists of cement, coarse aggregate and water.

•As compared to the conventional concrete, the drying shrinkage of no-fines concrete is relatively low.•As there is absence of capillary passages, there is no transmission of water by capillary action.•It is a type of light-weight concrete and hence it grants the advantages associated with the light-weight concrete construction.•It possesses better insulating characteristics than conventional concrete because of the presence of large voids.

•As no-fines concrete has little or no cohesion in the fresh state, it requires long time for the removal of forms.•It is highly permeable as compared to the conventional concrete and hence, the rendering of walls becomes essential. •The compressive, bond and flexural strengths of no-fines concrete are considerably lower than those of conventional concrete.

Info… Advantage… Limitations…

•The unit weight of no-fines concrete is about two-thirds of the unit weight of conventional concrete. Hence, the pressure on formwork is greatly reduced. •As no-fines concrete does not segregate, it can be dropped from a considerable height and placed in very high lifts.

PRE-CAST CONCRETEInfo… Advantage… Limitations…

The main difference between pre-cast concrete and cast-in-situ concrete is that the former is a factory made product while the latter is prepared at site of work.

•Very rapid speed of erection•Good quality control•Entire building can be precast-walls , floors ,beams ,etc.•Rapid construction on site•High quality because of the controlled conditions in the factory•Prestressing is easily done which can reduce the size and number of the structural members.

•Very heavy members•Connections may be difficult•Somewhat limited building design flexibility•Need for repetition of forms will affect building design.•Joints between panels are often expensive and complicated.•Skilled workmanship is required in the application of the panel on site.•Cranes are required to lift panels.

The procedure for preparing pre-cast products is as follows…..•The moulds, which may be of timber, steel or sand, are prepared to the shape of the product.•The reinforcement, if any, is put up in the moulds.•The concrete is mixed in the desired proportion and placed in the mould.•The finishing of the product is then carried out. The ordinary products such as fence-posts, sleepers, etc. are left as they are, while products such as spun pipes are finished during the process of manufacture.•The products are then sufficiently cured in specially constructed tanks.•The products are then dispatched for use at site of work.

READY MIX CONCRETEInfo… Advantages of ready-mix

concrete over site-mix concrete…

Disadvantages of ready-mix concrete over site-mix concrete…

Ready mix concrete is a type of concrete that is manufactured in a factory or batching plant, according to a set recipe, and then delivered to a worksite, by truck mounted transit mixers.

•A centralized concrete batching plant can serve a wide area.•Better quality concrete is produced.•Storage space for basic materials at site is not required.•It eliminates procurement/hiring of plant and machinery.•Wastage of basic materials is avoided.•It saves labour associated with production of concrete.•It reduces time required to prepare concrete.•It also reduces noise and dust pollution at site.

•The materials are batched at a central plant, and the mixing begins at that plant, so the travelling time from the plant to the site is critical over longer distances.•Some sites are just too far away, though this is usually a commercial rather than technical issue.•Access roads and site access have to be able to carry the weight of the truck and load.•Ready mix concrete is to be placed within a limited time span after batching at the plan.

TRANSIT-MIX CONCRETE

The concrete that is mixed, either wet or dry, en route to a job site is called TRANSIT-MIX CONCRETE.