Concrete Technology

2. Soon after mixing and while it is still workable,
it is said to be Fresh concrete.
 Fresh concrete – green concrete, wet concrete
or concrete in plastic state
 For good hardened concrete – monitor
workability, bleeding, segregation, desired
compaction.
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3.  Easily mixed and transported
 Uniform throughout a given batch and
between batches
 Fluidity during transportation
 Set in reasonable period of time
 Ability to be fully compacted
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4. Various stages of manufacture of concrete are
 Batching
 Mixing
 Transporting
 Placing
 Compacting
 Curing
 Finishing
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5.  Measurement of materials for making
concrete is known as batching.
i) Volume Batching
ii) Weigh Batching
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6.  Not a good method for proportioning –
difficulty it offers to measure granular
material in terms of volume.
 Volume of moist sand in loose condition
weighs lesser than volume of dry compacted
sand.
 Cement is always measured by weight not in
volume
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7.  Each batch mix – one bag of cement is used
(35 litres)
 Gauge box – measuring FA & CA – gauge box
volume is made equal to volume one bag
cement (35 litres)
 Gauge box are called as farmas – made of
timber or steel plates
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8.  Water measured – either in kg or vol.-
because two units are same – 1 kg =1 litre.
 Quantity of water = W/C ratio X weight of
cement
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9.  Correct method of measuring the materials.
 Important concrete - weighing batching
should be used
 It facilitates accuracy, flexibility and
simplicity.
 Different types of weigh batches are available
based on type of work
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10.  Thorough mixing – production of uniform
concrete
 Mixing ensures – concrete to be
homogeneous, uniformity in colour,
consistency
 Types of Mixing – Hand Mixing, Machine
Mixing
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11.  Hand mixing – small scale unimportant
concrete works
 10% of extra cement is added because mixing
cannot be thorough and efficient
 Hand mixing should be done over an
impervious concrete floor
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12.  Two types of machine mixing- Batch mixers
and continuous mixers
 Batch mixers – produce concrete batch by
batch with time interval
 Continuous mixers – produce concrete
continuously
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13.  Production of concrete at faster rate
 Economic
 Efficient for larger works
 Uniformity in colour and consistency
 Better quality
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17.  Process of carrying the concrete from the
place of mixing to place of deposition
 Concrete should transported to site without
any segregation and drying
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18.  Concrete should be transported to from work
quickly
 Transportation cost – low
 Segregation should be prevented
 Prevention of drying during hot weather
condition and rain conditions
 No water loss during transportation
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19.  Process of depositing the concrete in its
required position
 Placing concrete – good quality of surface
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20.  Natural soil – compact the soil, remove
moisture deficiency
 Rocky base – side cut in vertical side not in
slopes, remove loose particles
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21.  Reinforcement – properly tied, placed and
proper cover
 Correct alignment of form work
 Cement slurry leakage
 Cleaning and oiling
 No addition of water to concrete after placing
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22.  No placing in rainy season
 Old surface – made rough, cleaned – placing
new layer of concrete
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23.  Consolidation of concrete – after placing it in
position – Compaction.
 Removes air – increases density.
 Variation in compaction – concrete porous,
less strength, non – homogeneous.
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24.  Compaction are of two types:
I. Hand Compaction
II. Mechanical Compaction
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25.  Ramming the concrete manually.
 Used – ordinary and unimportant structures.
 Low strength – because of higher W/c ratio.
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27.  Rodding- 16 dia, 2m steel rod-pack the concrete –
continuously done during concreting.
 Layer thickness – 150mm to 200mm.
 Ramming – Compaction on ground-Not in higher
floors.
 Tamping – top surface beaten – wooden cross beam –
levelling and compaction – simultaneously.
 Tamping is done for roof slabs and road pavements.
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29.  Compaction – with machines – vibrator.
 Used – mixes – low W/C ratio.
 Reduces friction – Between particles
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30.  Better Finish and better quality.
 Improves bond – steel and concrete.
 Reduces creep and shrinkage.
 Reduces compaction time.
 Speed of placing concrete increases.
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31.  Internal Vibrator – “Immersion Vibrator” –
common type – direct contact with concrete.
 Operated – petrol or electricity.
 Needle diameter – 20mm to 75mm.
 Length – 250mm to 900mm.
 Frequency – 6000 to 12000 cycles/ min.
 Period of vibration – 30 sec to 2 min.
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33.  Form Vibrator – External Vibrators – less
effective – consume more power.
 Uses – thin sections, heavy reinforcement.
 Compaction of stiff concrete.
 Frequency – 3000 to 9000 cycles/ min.
 Period of vibration – 1 to 2 min.
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34.  Surface Vibrator –Surface vibration.
 Uses – roof slabs, road pavements.
 Frequency – 4000 cycles/ min.
 Not effective beyond 150 mm depth.
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38.  Form work –strong.
 Concrete laying – 150 to 600 mm – to avoid
air trapping.
 Vertical insertion of vibrators.
 Remove vibrator – appearance of mortar on
the surface.
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39.  Do not touch the form work with vibrators.
 Don’t use vibrators for long time.
 Vibrators used only for compaction – not for
pushing – leads to segregation.
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40.  Creating an environment – setting and
hardening.
 Desirable conditions – suitable temperature,
controls rate of hydration and water.
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41.  Prevent loss of water by evaporation.
 To supplement water consumed in heat of
hydration.
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42.  Maintains hydration – prevents loss of water
by evaporation.
 Reduces shrinkage.
 Preserve the properties of concrete.
 Increases Impermeability and durability.
 Maintains uniform temperature.
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43.  Water Curing
 Membrane Curing
 Steam Curing
 Accelerated Curing
 Self Curing
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44.  Best method – satisfies all requirements.
 Water directly applied.
 Types – Direct application and Wet covering.
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45.  Immersion
 Ponding
 Sprinkling
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46.  Immersion - Prefabricated elements –
immersed in water tank.
 Precast elements – concrete poles, man hole
covers, pipes etc – cured in tanks.
 Ponding – surface members – pavements,
floors, roofs, slabs etc.
 Small water retaining material – perimeter of
surface.
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47.  The enclosed area – divided into smaller
rectangles – flooded with water – 2 to 3
times a day.
 Sprinkling – spraying of water – very
efficient – less water demand.
 Uses – floor slab, plastered surface,
columns and piers.
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50.  In case wet curing – cannot ensure complete
curing.
 Vertical and inclined members – covered –
wet gunny bags, jute matting – keeping moist
for longer time.
 Uses – vertical and inclined members.
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52.  Regions – lack of water – membrane curing done.
 Apply – membrane forming compounds.
 Membrane – not only liquid compounds – solid
sheets – covering concrete.
 Forming Impervious film – prevents water loss by
evaporation.
 Apply membrane – immediately finishing concrete.
 Keep concrete surface wet till membrane is applied.
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54.  Steam curing – two favourable conditions –
concrete subjected to high temperature and
maintains the wetness in concrete.
 Strength development – very rapid.
 Steam curing – at atmospheric pressure or
under pressure.
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55.  1 day steam curing concrete = 28 day normal
curing concrete.
 Steam Curing – employed to achieve 50 -70%
of strength.
 Followed by 7 days curing atleast.
 Uses – Precast prestressed concrete sleepers
and bridge girders.
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57.  Low Pressure Steam Curing
I. Steam curing – atmospheric pressure.
II. Can be continuous or intermittent.
 High Pressure Steam Curing
I. Cylindrical chambers used – Autoclave.
I. Exhibits higher strength and durabillity.
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58.  Calcium chloride – coating material and
admixtures.
 Obsorbs moisture – retains on concrete
surface.
 reduces evaporation – increases rate of
hydration.
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59.  Curing method – providing moisture from within
the concrete.
 LWA – ability to retain water – additional moisture
to concrete.
 For lower W/C ratio (0.3) – around 60 – 180
kg/m3 of LWA – provide additional moisture-
increase in strength and durability.
 Internal moist curing accompanied by external
curing method
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60. Do’s
 Use fresh cement.
 Well graded aggregates.
 Concreting – skilled supervision.
 Concreting – man power, equipment and
tools.
 Favourable weather conditions.
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61.  Suitable mix.
 Use water free from impurities.
 Do slump testing before concreting.
 Verify – site conditions, reinforcement covers
etc..
 Concrete – place within 15 – 20 min after adding
water.
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62.  Proper compaction.
 Check the stability of form work frequently.
 Proper finishing at joints.
 Attention towards bleeding.
 Proper curing.
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63. Don’ts
 No damage form work.
 No unwashed aggregates.
 No volumetric batching.
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64.  No addition of water while concreting for easy
placements.
 No semi dry concrete – honey comb.
 Don’t drop concrete from greater heights –
segregation.
 No early removal of form work.
 No concreting during rain.
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65.  Concrete placed in massive structures like dams,
bridge piers etc. – mass concrete.
 Low slump – stiffer concrete.
 Main problem in mass concrete – heat of hydration.
 Excessive heat leads to shrinkage cracks.
 Low heat cement used – prevents shrinkage cracks.
 Continuous curing.
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66.  Placing the concrete in small lifts.
 Concreting is done in winter.
 Cast concrete should be cured in cold water.
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67.  Workability
 Segregation
 Bleeding
 Consistence
 Mobility
 Compactability
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68.  Property determining the effort required
manipulate a freshly mixed quantity of
concrete with minimum loss of homogeneity.
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69.  Water Content – higher water content higher
fluidity.
 Mix Proportion
 Size of aggregates –Bigger the size – lesser
surface area – requires less water to wet the
surface – higher workability.
 Shape of aggregates – angular or flaky
aggregates – harsh conncrete
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70.  Surface texture of aggregates - rough surface
– poor workability – smooth surface – better
workability.
 Grading of aggregates – well graded – lesser
voids- higher workability.
 Use of Admixtures – improves the workability.
 Temperature – increases – workability
decreases.
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71.  Slump test
 Compaction Factor test
 Vee – Bee consistometer test
 Flow test
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72.  Most widely used method to determine the
workability.
 Suitable – for medium workability concrete
mix – not for very wet or very dry mix.
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74.  Mould - Internal surface is cleaned.
 Mould is placed over a smooth, horizontal and
non – absorbent surface.
 Concrete is filled in 4 layers – 25 tamps each.
 Remove mould by lifting slowly – subside of
concrete is called slump.
 Height of mould – height of subside concrete =
SLUMP taken in “mm”
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75.  More efficient than slump test
 Suitable for very low workability concrete.
 C.F = W/W’
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77.  Fill the hopper A by concrete at top.
 Opening the trap door of hopper A concrete
allowed to fall to hopper B.
 Opening the trap door of hopper B concrete
allowed to fall to cylinder C.
 Take weight of the concrete “W”.
 Now compact the concrete in cylinder in
layers to achieve full compaction. Let the
weight be “W’”.
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80.  Separation of coarse aggregate from mortar
resulting in their non-uniform distribution
 difference in size of particles and in specific
gravity of constituents – prime cause
 Separation of CA from mortar leaves voids in
CA unfilled and it is called as
“Honeycombing”
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83.  Improper mix proportioning
 Dropping concrete from greater heights
 Over compaction
 Leakage of mortar through form works
 Too vibratory or too long transportation
methods
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84.  Proper grading of aggregates
 Strict supervision – all concrete activities
 Restricting height of pour
 Optimum usage of water in mixing
 Reducing continuous vibration for long time
 Adding air entraing agents – reduces water usage
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85.  Also known as ‘Water Gain’
 Rise of water with cement particles to the
surface of freshly laid concrete –Bleeding
 More water than required – solid constituents
are able to hold all mixing water
 Result – porous, weak, non-durable concrete
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87.  Excess of water
 Deficient of FA in mix
 Over compacting
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88.  Controlling the water content
 Using finely ground cement
 Controlled compaction
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89.  Consistence – describes how stiff or runny.
 Soft enough to work with it- not too wet
 Indicator of workability
 Every batch of concrete – consistent quality –
achieved by using same materials for all
batches
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90.  Ease with which concrete can flow into the
formwork around steel, ability to be moulded.
 Depends on type of formwork, arrangement
of steel, time lag b/w mixing and pouring
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91.  Ease with which concrete can be compacted
 Compactability depends – amount, che.
Composition of cement, amount of water,
grade & shape of aggregates etc.
 Addition of admixture – increases
compactability without increase in slump
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