KTU CE 204 Construction Technology - Module 2

CONSTRUCTION
TECHNOLOGY
Module 2
Fr. Dr. Bennet Kuriakose
Department of Civil Engineering

Syllabus
Department of Civil Engg., SJCET Palai 2
Concrete - aggregates – Mechanical & Physical properties and tests –
Grading requirement
-Water quality for concrete
Admixtures- types and uses – plasticizers – accelerators – retarders – water
reducing agents
Making of concrete – batching – mixing – types of mixers – transportation
– placing – compacting – curing
Properties of concrete – fresh concrete – workability – segregation and
bleeding – factors affecting workability & strength – tests on workability
– tests for strength of concrete in compression, tension & flexure.
Concrete quality control – statistical analysis of results – standard deviation
– acceptance criteria
Mix proportioning (BIS method) – nominal mixes

References

Introduction
• Concrete is a composite material composed of fine and
coarse aggregates bonded together with a cement paste
(cement + water) that hardens over time.
• Often added with other materials – admixtures – for
improving certain properties
• Etymology  Latin word concretus (compact or condensed)

Introduction
• The proportioning of the constituents can be changed to get
the required property for concrete  “mix proportioning” or
“mix design”
• Plastic when it is mixed (known as “fresh/ virgin concrete”)
• strong and durable when hardened. (process is called
“hardening” and concrete “hardened concrete”)
• During hardening „hydration reaction‟ of cement happens 
chemical + physical bonding happens between the
constituents
• It can be molded into any required shape and size.
• It is fire resistant
• Often used as it is  “Plain Cement Concrete” (PCC)
• Mostly used along with rebars  “Reinforced Cement
Concrete” (RCC)

CEMENT (REVIEW)

Raw Materials

Cement Manufacture

Cement Manufacture
Rotary Kiln
Ball mill
Clinker
Silos

Cement Manufacture
Bogue’s Compounts

Types of Cement
• Ordinary Portland Cement – Grades 33, 43 and 53
• Portland Pozzolana Cement (PPC)
• Portland slag cement
• Rapid hardening cement
• Quick setting cement
• Sulphate resisting cement
• Super sulphated cement
• Low heat cement
• Expansive cement
• Coloured cement
• Hydrophobic Cement
• High Alumina Cement

AGGREGATES

Aggregates
• Fine and coarse aggregates make up the bulk of a concrete
mixture (70 to 80 % of concrete volume)
• Provides packing and dimensional stability
• Sand, natural gravel, and crushed stone are used
• Recycled aggregates (from construction, demolition, and
excavation waste) are increasingly used as partial
replacements for natural aggregates
• Manufactured aggregates, including air-cooled blast
furnace slag and bottom ash are also permitted.

Aggregates - Properties
Physical Properties
1. Size
– Size > 4.75 mm  course aggregate
– The largest maximum size of aggregates need to be
limited because of
• Thickness of section
• Spacing of reinforcement
• Clear cover
• Mixing, handling and placing techniques
– Normal applications 20 – 25 mm size
– > 80 mm for Roller Compacted Concrete dams

2. Shape
– Shape affects workability and strength
NO. Type Description Example
1 Rounded Fully water worn with smooth shape River gravels
3 Angular Well defined sharp edges Crushed rock
4 Flaky Angular with one side is small
relative to the other sides
Laminated rocks
5 Elongated One side is large relative to other
sides
Some metamorphic
rocks

– Round aggregates increase workability requiring less
water-cement ratio.
– Flaky and elongated aggregates reduces strength.
– Angular aggregates preferred for concrete.

3. Absorption and porosity
– porous aggregates suck in water
from cement paste  decrease
w/c
– Porous concrete decrease strength
4. Texture
- Relative degree to which particle surfaces are polished
or dull, smooth or rough.
- Surface texture depends on hardness, type of rock, pore
structure.
- As surface smoothness increase, and less bonding with
cement paste – rough texture always preferred.
- Rough texture aggregates produce more friction in roads

5. Bulk Density
– Shows how densely the aggregates is packed.
– Depends on particle size distribution and shape
6. Specific Gravity
– Preferred ranges from 2.6 to 2.8
High density Poor density

Mechanical Properties
1. Impact Value
– Resistance against impact loads
– Aggregates should have high impact value to be used
for machine foundation and roads.
2. Abrasion Resistance
– Resistance to wear and tear
– Crucial when used for roads and floors when heavy
machines are used.
3. Crushing value
– Strength in compression
– Crucial to provide strength of concrete.

Tests on Aggregates
1. Gradation – Sieve Analysis
– to find out the particle size distribution of
aggregate (gradation)
– IS 2386 (part 1) - 1963
Sieves Sieve shaker

Tests on Aggregates
– Procedure
i. Sample is brought to air-dry condition
ii. Sample is weighted and sieved
iii. The materials retained on each sieve is weighed.
iv. The results are reported as “gradation curve”
gradation curve

Tests on Aggregates
• Then the Fineness Modulus (FM) can be calculated by using
the relation
• Aggregate having fineness modulus > 3.2 is used for
concrete

Tests on Aggregates
2. Flakiness Test
• Flakiness index  percentage by weight of particles in the
course aggregate whose least dimension (thickness) is less
than 3/5 ths of their mean dimension.
• IS 2386 (part 1) - 1963
• Apparatus: metal gauge

Tests on Aggregates
• 200 pieces of each fraction (on sieve) is taken and weighed
• Each fraction is gauged in turn for thickness on the metal
gauge
• The total amount passing in the gauge is weighed.
• Flakiness index = total weight of pieces passing (through
various thickness holes) / total weight of aggregate X 100
• Limit is 15 %
Video 
https://www.youtube.com/watch?v=2ds0DhrnrtI

Tests on Aggregates
3. Elongation Test
• Elongation Index  percentage by weight of particles in the
course aggregate whose greatest dimension (length) is
greater than 1.8 times their mean dimension.
• IS 2386 (part 1) - 1963

Tests on Aggregates
• 200 pieces of each fraction (on sieve) is taken and weighed
• Each fraction is gauged in turn for length on the metal gauge
• Total amount retained by the gauge length is weighed.
• Elongation index = total weight of pieces retained (in various
lengths) / Total weight of aggregate X 100
• Limit is 15 %

Tests on Aggregates
4. Crushing Test
• resistance against gradually applied compressive
load
• IS 2386 (part 4) – 1963

Tests on Agregates
• Aggregates passing through 12. 5 mm and retained on 10
mm is taken.
• The cylinder is filled with 3 layers, each tamped with 25
strokes and weighed (W1)
• The surface is levelled and plunger is inserted.
• The sample is placed under compression testing machine for
40 Tonnes load in 10 minutes.
• The load is released and sieved through 2.36 mm sieve and
the fraction passing through is weighed (W2)
• Crushing value = W2/W1 X100
• Crushing value should not > 30 %

Tests on Aggregates
5. Impact Test
• IS 2386 (part 4) – 1963
• Impact value  resistance to sudden shock or impact

Tests on Aggregates
• Sample of aggregate passing through 12.5 mm and
retained 10 mm sieves are taken.
• Aggregate is filled into cylindrical steel cup with three layers
(W1)
• each layer is tamped with 25 strokes
• The crushed sample is sieved on 2.36 mm sieve and
passing (W2) is weighed.
• Aggregate impact value = W2/ W1 x 100
• The impact value should not be more than 45 %.

Tests on Aggregates
6. Abrasion Test
• IS 2386 (part 4) – 1963
• Los Angeles Machine:
– Hollow steel cylinder closed at both the ends with a dust
tight opening (access cover)
– A removable steel shelf inside the cylinder
– Abrasive charge (steel balls)

Tests on Aggregates
• The sample of aggregates (W1) is placed in the machine
and rotated for 500 revolutions.
• The material is discharged from machine
• Sieved through 1.7 mm sieve (W2)
• Los Angeles Abrasion Value = (W1-W2)/W1 X 100
• Abrasion value should not be > 15 %

Tests on Aggregates
Other tests:
1. Specific gravity of FA and CA
2. Bulk density of CA
3. Bulking of FA (sand)
4. Water absorption tests on FA and CA
5. Moisture content tests on FA and CA
6. Soundness of aggregates
7. Alkali-aggregate reaction test

Grading Requirements
• In order to have workable concrete of good strength,
gradation is important
• The aggregates should pack with minimum voids.
• Well graded mix
– Well graded aggregates give the minimum voids.
– Well graded aggregates will also reduce the cement
requirement for more strength.

Grading Requirement
• Gap – graded mix also sometimes preferred.
– Workability is less for gap-graded mix.
– Less creep and shrinkage
– But gap graded mix is prone to segregation.
• Main factors governing gradation
1. Surface area of aggregate
2. Relative volume occupied
3. Required workability of mix
4. Tendency of segregation
5. Required strength of the mix

Aggregates Classification
• Based on Origin
– Natural : Sand, gravel, natural stones
– Artificial : blast furnace slag, broken bricks
• Based on Size
– Course Aggregate > 4.75 mm (retained on 4.75 mm
sieve)
– Fine Aggregate: passing through 4.75 mm sieve.
• Based on shape
– Angular
– Rounded
– Flaky
– Elongated

WATER

Water
Functions of Water in Concrete
1. For hydration of cement to form a adhesive gel
2. To prepare a plastic mixture providing workability
3. Wet the surface of aggregates to get adherence with
cement paste
General Points to Ponder
• Add right amount of water
• Excessive water affects the strength of concrete, increase
seggregation, nonhomogenety and bleeding.
• Less water will affect workability (however, admixtures can
be mixed to increase workability)
• „Potable‟ water is preferred

Water
Properties of Water to use in Concrete (Quality)
• Potable water to be used – dark colour and bad smell water
should not be used. Avoid impure water.
• pH value of 9 and above is allowed. Nevertheless, excessive
alkali content will induce alkali-silica reaction (alkali
corrosion).
• Acid content will induce corrosion in reinforcement. Acid also
induce acid-attack in concrete. pH should not be less than 6.
• Chloride cause corrosion, persistent dampness in surface
and efflorescence.
• Sulfate content induce sulfate attack
• Algae or organic content causes air entrainments with loss of
strength.
• Sea water can be used for PCC

Water
Department of aCivil Engg., SJCET Palai 44
Surface of Concrete after Alkali –
Silica Reaction (Alkali Attack)
Efflorescence
Acid Attack
Sulfate Attack

ADMIXTURES

Admixtures
• Defined as a material (other than cement, water and
aggregates) that is used to improve the properties of
concrete.
• Added immediately before or during mixing.
• Contrast to Additive which is added to clinker during
grinding.
• Types:
– Mineral Admixtures/ Pozzolans: Fly ash, GGBFS, Rice
husk ash, silica fume
– Chemical Admixtures

Admixtures
Chemical Admixtures
• Plasticizers
• Superplasticizers
• Retarders and Retarding
Plasticizers
• Accelerators and
Accelerating Plasticizers
• Damp-Proofing and
waterproofing admixtures
• Air–entraining Admixtures
• Alkali-silica Reaction
inhibiting admixtures
• Grouting admixtures
• Corrosion inhibiting
admixtures
• Bonding admixtures
• Fungicidal, Germicidal.
Insecticidal admixtures
• Colouring admixtures.

Plasticizers (Water Reducers)
• Concrete at different situations need different degree of
workability
• High degree of workability required for deep beams, thin
walls, ready-mix concrete, pumpable concrete, hot weather
concreting etc. – plasticizers added
• High strength/high performance concrete  w/c ratio should
be less – in order to have sufficient workability plasticizers to
be added
• Reduction in w/c ratio increase durability – inorder to achieve
this plasticizers are used.
• Superplasticizers and new-gen plasticizers are highly
efficient than plasticizers

Working Principle
Concrete with plasticizers, superplasticizers and new-gen superplasticizers

Retarders
• Retarders slow down hydration so that concrete remains
plastic and workable for a longer time.
• Uses:
– Used to overcome accelerating effect of high temperature
during hot weather concreting.
– Used for casting large number of pours without forming
cold joints.
– Reducing heat of hydration
– Slip form of construction.

Retarders
– Grouting oil wells
– Ready mix concrete
– For ornamental purpose – sometimes retarders are
sprayed to the formwork – later washed off to get
aggregates exposed
• Gypsum, common sugar, casein (skimmed milk powder)

Accelerators
• Increase the rate of hydration for early strength development
• Applications - Precast (prefabricated) construction,
underwater concreting, repair of waterfront structures, earlier
removal of formwork, emergency repair works
• Calcium chloride was popularly used – but induce corrosion
• Now a days use: Fluosilicates and triethenolamines

MAKING OF CONCRETE

The Making…
Steps involved:
1. Batching
2. Mixing
3. Transporting
4. Placing
5. Compacting
6. Finishing
7. Curing

Batching
• Batching: The process of measurement of materials for
making concrete
– Volume Batching
– Weigh Batching
Volume Batching
• Easy to do at site
• In fact, not a good method for batching  issues with
measurements of granular materials (aggregates)
– Understand the fine aggregate bulk density – can vary
– “Bulking of sand”

Batching
• One bag of cement  35 litres
• Gauge Box will be of 35 litres
Weigh Batching
• Accurate method
• Difficult to implement in small sites – RMC plant widely used
• Large construction – automated weigh batchers
• Cement bag is treated as – 50 kg

Mixing
• Thorough mixing – the mass become homogenous, uniform
colour and consistency.
– Hand Mixing
– Maching Mixing
Hand Mixing
• 10% more cement is added to accommodate wastage
• Procedure:
– CA and FA are spread on to flat water-tight platform
– Cement poured on top of it
– Thoroughly mixed with shovel
– Spread – water is sprinkled and mixed
– Repeat until required consistency is reached.

Mixing
Machine Mixing
• High efficiency and economy
• Types:
– Batch mixers: produce batch by batch
• Pan type
• Drum type
– Continous mixers: continous concrete delivery

Mixing
Mixing Time and Quality of Concrete
• 25 to 30 revolutions required for well mixing – normal drum
• Reduced time will decrease the quality, thereby create lumps
and balls
• If time is increased
– uneconomical as project is concerned
– Chance of segregation
• Check whether the mixing is uniform

Transporting
• Homogeneity need to be maintained
during transportation
Methods
1. Mortar Pan
– Common method – intensive labour
– More exposure to heat and thereby
water loss
2. Wheel Barrow
– Comparatively labour is less
– Chance of segregation while
transportation  wooden planks are
provided to reduce vibration
– Difficult to be used for higher stories.

Transporting
3. Cranes and Buckets
– High stories
– Cranes can move their booms in
versatile way to place concrete.
– Cranes used for miscellaneous
uses.
– Pouring from height to be avoided.
4. Ropeway
– Massive transport from long
distance
– For massive works like dams

Transporting
5. Belt Conveyors
– Remote places
– Continuous supply of concrete
– Used for massive constructions
– Chance of drying and water loss
– Chance of segregation – remixed
at the destination
6. Chutes
– For transport concrete from higher
level to lower level
– Slight slope

Transporting
7. Trucks and dumpers
– Normally for massive Roller Compacted Concrete (RCC) :
Gravity dams and roads
8. Transit Mixer
– For transporting Ready Mix Concrete (RMC)
– 4 to 7 cu. m. capacity
– Transit mixer permit long haul without drying and
segregation.

Transporting
9. Pumps
– To transport concrete normally from transit mixer
– To high elevation and distance using crane pumps
– Pumps are normally piston type.
– Pumps with booms are popular
– “pumpability” of concrete is crucial
– Used for shotcreting also

Placing
• To be done in ultimate systematic manner to achieve
required quality
• Pool of water, grass, organic matters, roots etc to be
removed.
• Foundation and plinth beams – PCC is provided – (a) to
avoid water loss/ water intrusion (b) level bottom surface
• In case of road or industrial floors– thin polythene sheets
used instead of PCC
• Slabs – Styrofoam sheets are now used
• Formwork to be wetted before concreting
• Old surfaces of concrete to be roughened and grouted
• Not to be poured from more than 1.5 m  segregation

Compaction
• Process of expelling entrapped air
• If not properly compacted, concrete will have
honeycombing, strength and durability reduces.
• To ensure proper compaction workability and
compacting effort need to be ensured.
• Excessive compaction will lead to bleeding
• If compaction is not possible, opt for SCC
• Methods:
– Hand Compaction: Rodding, ramming and
Tamping
• using steel tamping rods (deep members
or columns) or using timber screeds (thin
members like slabs)
• To be done in layers
– Machine (Vibration) Compaction 67

Compaction
Machine (Mechanical) Compaction
• Done through offering vibrations to concrete
to expel air
Types of Vibrators:
1. Needle Vibrator (Internal Vibrator)
– Consist of a power unit and long flexible
tube with needle
– ~7000 vibrations per minute
– Used for normal concreting
2. Form Vibrator (External Vibrator)
– Vibrator clamped to formwork
– Not used for thin and deep structures
– Formwork should be sturdy

Compaction
3. Surface Vibrator
– Also known as screed or pan
vibrator
– Used for screeding slab, ground
slab or roads
– Not efficient for thick concrete
4. Table Vibrator
– Used in laboratories
5. Vibration Rollers
– Used for compacting roller
compacted concrete (RCC)
– Pneumatic vibration is used

Finishing
• Produce durable surface
• Fine hair-cracks can develop (craziness) – to be avoided
• Satisfactorily withstand wear and tear
• Should be impervious to water and chemicals
• Texture for suitable appearance
• If tiles need to be sticked, rough texture is provided
• Sometime “screed” is provided for better finishing of concrete
• Bull floating and trowelling (power float) operations are used
for finishing

Curing
• Process of keeping the concrete moist and warm enough so
that the hydration can continue for proper setting and
hardening
• Start immediately after placement and finishing
• Ensure curing at all the parts, depths and faces of concrete
• Importance (functions):
– Plays an important role in strength development and
durability by maintaining proper water content
– Reduces drying shrinkage by preventing loss of water
– Maintain uniform heat for hydration and strength
development
• If curing is not performed properly (i.e., air-cured concrete),
self-desiccation happens

Curing
If curing is not done properly….
Crazing (hair-line cracks) Shrinkage cracks Low strength

Curing
Methods
1. Water curing
a. Ponding and Immersion
• Flat surfaces of smaller works
b. Spraying and Fogging
• When humidity is very low
• Need to be done frequently
c. Saturated wet covering
• Jute bags, straw, cloths, saw
dust, earth or sand
• Moisture to be maintained
• Used for inclined/vertical
surface – small works

Curing
2. Membrane curing
– Prevent evaporation of water within
concrete
– Done when water is scarce or
cannot be properly supervised
3. Curing/Sealing Compounds
– Reduce water evaporation from
surface
– Hot regions – white pigmented
compounds are preferred
– Bituminous compounds,
polyethelene films, waterproof
paper, rubber compounds etc.

Curing
4. Accelerated Curing
– Provide heat to accelerate the
hydration
– Almost 1 day is enough
a. Steam Curing
b. Heating Coils
c. Electrical pads/ Power
Blankets
5. Chemical Curing
– Chemicals like calcium chloride
– Retains water content at surface

Curing
6. Hot Mix Method (preheating)
– The temperature of fresh concrete is raised upto ~30 °C
– Achieved by water
• Heating aggregates
• Adding hot water for mixing
• Injecting steam while mixing
– Formwork need to be insulated
7. Infrared Radiation Method
– Infrared curing lamps

Formwork removal (Stripping Time)
• Depends on type of structure, nature of concrete, type of
curing

PROPERTIES OF
CONCRETE

Properties - Introduction
Fresh Concrete Properties
• Workability
• Flowability
• Pumpability
• Seggregation
• Bleeding
Hardened Concrete Properties
• Strength – compressive, tensile, flexural
• Durability
• Creep
• Shrinkage
Fresh
Concrete
Green
Concrete
Hardened
Concrete

Workability
• “Workability is the property of fresh concrete which
determines the ease and homogeneity with which the
concrete can be mixed, transported, placed, compacted
and finished.” (ACI 116R-00)
• “Property determining the effort required to manipulate a
freshly mixed quantity of concrete with minimum loss of
homogeneity” (ASTM)
• “the property of concrete which determines the
amount of useful internal work necessary to
produce complete compaction.” (IS 1199:1959)
• Workability requirements depends on the type of
structure, mixing method, transportation, placement
method, compaction etc.

Workability
• Classification
– Unworkable/Harsh concrete
– Medium workable concrete
– Highly workable concrete
Factors Affecting Workability
1. Cement Content
2. Water Cement Ratio
3. Size, shape, texture and gradation of aggregates
4. Use of admixtures
5. Supplementary cementitious materials ( mineral
admixtures) – increase at lower quantity, reduces with
higher quantities
6. Occurrence of fibres

Tests for Workability
1. Slump Test
2. Compacting factor test
3. Vee Bee Consistometer test
4. Flow test – V funnel
5. Kelly ball test

Slump Test
• Instrument: Abram‟s Cone
• Procedure (IS 1199:1959) :
– Slump cone, initially applied with grease/oil is placed on
a smooth – flat and non-absorbent surface
– Place the concrete to ¼ of cone‟s height and compact
using tamping road for 25 times (1)

– Place the concrete about half height of the cone and
compact again (25 times tamping). (2)
– Place the concrete about ¾ th height and tamp 25 times (3)
– A final layer is placeed and tamped. Strike off the top
surface with trowel (4)
– Lift the cone in the vertical direction (5)
– Measure the subsidence in millimeters and reported as
“slump” of concrete (6)

Limitations of Slump tests
1. Not suitable for concrete containing aggregate larger than
40 mm
2. Not suitable for dry mix
3. No suitable for very wet concrete (SCC)
4. Not always reliable (repeatability is less)

Compaction Factor Test
• compaction factor test apparatus
• workability (measured in terms of “compaction factor”) is
defined as the amount of work required to compact
concrete to its maximum density

• Procedure (IS 1199:1959) :
– The apparatus is cleaned, dried and oiled
– Concrete is filled in upper hopper
– Trap of hopper is opened to facilitate the concrete to fall
into bottom hopper. The concrete sticking on the sides of
upper hopper is pushed with steel rod.
– Trap of bottom hopper is opened to allow the concrete to
fall into cylinder
– Surplus concrete is removed from top of cylinder with the
help of trowel. Outside surface of cylinder is wiped off and
weighed (weight of partially compacted concrete = W1)

– Concrete in cylinder is removed and filled with fresh concrete
with layers each of 5 cm thick, heavily tamped to achieve full
compaction and weighed. (weight of fully compacted concrete
= W2)
– Compaction factor = W1/ W2

Degree of
workability
Slump
(mm)
Compa
cting
factor
Use for which concrete is suitable
Very low – 0.80 Roads vibrated by power-operated machines. At the more
workable end of this group, concrete may be compacted in
certain cases with hand-operated machines.
Low 25 – 75 0.85 Roads vibrated by hand-operated machines. At the more
workable end of this group, concrete may be manually
compacted in roads using aggregate of rounded or
irregular shape. Mass concrete foundations without
vibration or lightly reinforced sections with vibration.
Medium 50-100 0.90 At the less workable end of this group, manually
compacted flat slabs using crushed aggregates. Normal
reinforced concrete manually compacted and heavily
reinforced sections with vibration.
High 100-
150
0.95 For sections with congested reinforcement. Not normally
suitable for vibration, for pumping and tremle placing.
Very high – – Flow table test is more suitable.

Vee Bee Consistometer Test
• For concrete having very low workability

Procedure:
– Place the slump cone in the cylindrical container
– Fill the slump cone with four layers, each tamped with 25
strokes. The excess concrete is trowelled off.
– The glass disc (Perspex) is placed just on top of the
Abram‟s cone and reading is noted (R1)
– The slump cone is lifted off.
– The glass disc (Perspex) is placed just on top of the cone
of concrete and the initial reading is noted (R2).
– Slump = R2-R1
– The vibrator is started and proceeded until the concrete
surface become horizontal inside the cylinder. Time
required is reported as Vee-bee time in seconds

Segregation
• Separation of the constituent of concrete
• Segregated concrete will be weak, less durable
• Types:
– Type 1: Coarse aggregates seperating out or settling
down from matrix (result in Fat Concrete)
– Type 2: Matrix separating away from coarse aggregate
(result in Lean Concrete)
– Type 3: Water separating out of matrix and aggregate
(called as bleeding)

Segregation
Causes (Factors affecting)
1. Badly proportioned mix without sufficient matrix (Improper
mix design)
2. Insufficient mixing with excessive w/c ratio
3. Mixing of concrete with mixer of damaged blades
4. Conveyance of concrete through conveyor belts
5. Dropping of concrete from heights
6. Discharging concretes against obstacles (rebars,
formworks etc.)
7. Over-compaction

Bleeding
• Form of segregation where some of the water of concrete
rise to the surface
• Arises due to the inability of solid components to hold all the
water when they settle downward
• Cause shrinkage cracks to develop
• Affect durability
• Causes:
1. Improper mixing
2. Improper mix design
3. Highly w/c ratio
4. Over-compaction

Strength
• In construction, concrete is used only for compression, since
its contribution to tensile resistance is negligible
• Compressive strength, tensile strength and flexural strength
• Factors affecting strength:
1. Water – cement ratio
Duff Abrams

Strength
2. coarse aggregate properties: angular well graded
aggregates with less elongation and flakiness
3. Coarse aggregate strength: strong aggregates is
essential for high strength concrete
4. Type of fine aggregate
5. Air entrainment
6. Degree of compaction
7. Existence of segregation and honeycombing
8. Aggregate-cement ratio – less influence
9. Age of concrete
10. temperature of concrete
11.Curing

Strength
Compression Test
• Done on either 150 х 150 х 150 mm cube specimen or 150
mm dia х 300 mm long cylindrical specimen
• Test done as per IS 516: 1959
– The moulds are cleaned and oiled.
– Concrete is poured and compacted by tamping or vibration
– After 24 hours, the specimen is demould and stored in
clean water (curing)
– After the required time (3 days, 7 days, 14 days, 28 days
etc.), the specimen is taken out and wiped.

Strength
– The specimen is placed on a compression testing
machine or UTM. (cube – both sides should not be
casting side)
– Load is applied without any shock and max load is
recorded.
– Compressive strength = max load / area of cross section
• Normally 3 specimens are tested and the average value is
reported as the compressive strength

Strength
• Cube compression value is higher than
cylindrical value for same concrete –
Platen effect
– Cyl. Strengh = 80 % cube strength
• Concrete mix is classified based on
‘characteristic’ compressive strength
– The letter M refers to the mix and the
number to the specified 28 day cube
strength of mix in N/mm2.
– The concrete can be designated as
M10, M15, M20, M25, M30, M35 and
so on

Strength
Tension Test
• tensile strength of concrete is not important for designing
structures.
• However, tensile strength is significant for predicting
cracking of concrete in various situations.
• Generally, the tensile strength is only ~1/10 of the
compressive strength.
• Three methods are commonly used to determine the tensile
properties: Direct tension, Split tension, and Flexure.
• The direct tension method is the most accurate for
measuring the tensile response of concrete. However, this
experiment is very difficult to perform, because of the
difficulty in gripping the ends of the specimen for the test.

Strength
Split Tensile Test (Brazilian Test)
• split tension test is conducted by loading a cylindrical
concrete specimen along its length.
• This results in the development of tensile stresses along the
central diameter in the lateral direction (except for
compression very close to the loading points).
• When these stresses exceed the tensile capacity of the
concrete, the specimens simply splits into two halves.

Strength
• Procedure IS 5816: 1999
– Standard test cylinder 150 mm dia (D) X 300 mm long (L)
is placed horizontally between loading surfaces of CTM
(with strip of plywood)
– Compression load is applied diametrically without shock
and uniformly along the length of cylinder until it split and
load at failure is noted (P)
– Split tensile strength =
– Split tensile strength ~10 % greater than real (direct)
tensile strength.
2P
DL

Strength
Flexural Test
• One of the method to estimate tensile strength of concrete
• Based on the principle that on bending, one part of cross
section of the beam will be under tension – as concrete is
weak in tension – failure will be a „bending tension‟ one
• Procedure IS 516: 1959
– Specimen 750 mm long beam of 150 mm X 150 mm
cross section without reinforcement
– The specimen is taken out of water and wiped
– The specimen is placed in test setup with load (Third-
point loading) acting on the top cast surface

Strength
– Load is gradually increased without shock
– Load at failure is noted.
– Modulus of rupture (max tensile strength in flexure)
• Approx value of Modulus of rupture = (IS 456:2000)
2
PL
bd

0.7 ckf

CONCRETE QUALITY
CONTROL

Concrete Quality Control
• Concrete have variability based on the mixing process,
transportation, placement, compaction etc.
• Therefore, final properties are difficult to assess.
• Quality control measures are adopted to minimise such
variability.
• Quality accomplished by following stages:
– Quality control before concreting
– Quality control during concreting
– Quality control after concreting

Quality Control before Concreting
• Excavations shall be done properly and checked for
presence of water or organic contents like leaves.
• Formwork shall be thoroughly checked for level and leak
• Reinforcements shall be corrosion free – otherwise
sandblasting to remove corrosion.
• All the ingredients shall be checked for quality – water,
aggregates and cement.
• Cement shall be checked for clumps – stored in moisture-
free environment

Quality Control during Concreting
• Concrete should be mix designed
• Weigh batching is preferred. Volume batching may be
permitted under strict supervision.
• No segregation of concrete during any stage
• Concrete should not be dropped from height more than 1 m
• Vibrators not to be used on reinforcement.
• Curing should be done for the specified period.
• Deshuttering may be done only after specified period.

Quality Control after Concreting
• Hardened concrete shall be checked for trueness of
dimensions. Surfaces shall also be checked.
• Once concrete is laid and finished, concrete cubes are
normally made to check the strength.
• Sampling core can be cut out of real structure to test the real
quality in-situ
• NDT tests (Schmidt rebound hammer, rebar locator, x-ray
refractometer etc.) can be used to test the quality of concrete
• Chemical analysis of hardnened concrete can be used to
estimate the content of concrete (Forensic concrete
technology)

Statistical Analysis
• Some of the test results fail, and need not be rejected. Can
be accepted based on statistical theory  Statistical Quality
Control
• Scientific method for understanding the variability of material
properties
• If the test results of cubes are plotted against frequency, it
follows a bell shaped curve, idealised as „normal distribution
curve‟/ „lognormal distribution curve‟.

• Important Statistical parameters
– Mean strength = sum of strengths of all cubes/ no. of
cubes
– Variance  measure of variability of single cube strength
with respect to mean strength.
– Standard Deviation  root mean square deviation of all
results
– Coefficient of Variation  Non-dimensional measure of
deviations
 c i
cm
f
f
n


 
2
c cmi
f f  
 
2
1
c cmi
f f
s
n
  


CoV 100
cmf

 

Characteristic Compressive Strength
“The characteristic strength is defined as the strength of
the concrete below which not more than 5% of the test
results are expected to fall.”

MIX PROPORTIONING

Concepts
• Consideration of the right mix of ingredients in concrete is
important to get required strength.
• Concrete can be classified as:
– Nominal Mix Concrete –
• Approximate proportion of ingredients are provided
based on past experience.
• Small works
• Less quality control – strength and durability cannot be
assured.
– Design Mix Concrete
• Right mix of ingredients are found out from properties
of all the ingredients.
• High quality control is obtained

Nominal Mix
• When testing facility are not available, or the volume of
concrete is small to design a mix, nominal mixes are used.
• For temporary works, nominal mix are used.
• Highly chemically exposed conditions, design mix to be used

Mix Design
• All the qualities of concrete is governed by the proportion of
ingredients.
• In order to have strict control over quality, the properties of
materials are found out and right proportion is calculated.
• Variables involved in design:
1. Water-cement ratio
2. Cement content (cement-aggregate ratio)
3. Gradation of aggregates
4. Consistency of cement paste
• Design is performed by optimising the above quantities,
i.e., reducing w/c ratio, reducing cement content, preferring
right gradation and so on…

Mix Design
Methods of Mix Design
1. Trial and error method
2. Fineness modulus method
3. Maximum density method
4. Surface area method
5. Road note NO. 4 method (UK)
6. DOE method (British)
7. ACI 211 method (US)
8. KDOT method (US)
9. IRC 44 method (Indian)
10. IS 10262: 2009 method (Indian)

BIS Method
IS 10262: 2009, IS 456: 2000
• Required parameters (general):
– Required fck
– Degree of workability
– Specific gravity of cement
– Bulk density of coarse aggregates
– Gradation of aggregate
– Moisture content of fine aggregate
– Exposure condition

BIS Method
Step 1: Target Mean Strength
Assume standard deviation

BIS Method
Step 2: Water-Cement Ratio (IS 456)

BIS Method
Step 3: Selection of Water Content
Step 4: Calculation of cement content
Cement by mass = water content / water-cement ratio

Step 5: Volume of Entrapped Air
Step 6: Volume of Fine aggregate per volume of total
aggregate (P)

BIS Method
Step 6: Aggregate Contents

BIS Method
Example Problem: Design of M20 Concrete
Given Values
Fine aggregate gradation – zone III

BIS Method
Step 1: Target Mean Strength
Step 2: Selection of Water-Cement Ratio
- Maximum Water-cement ratio = 0.55
Select water-cement ratio of 0.5
Step 3: Selection of Water Content
Water content per cubic meter of concrete = 186 kg
= 186 Litre
Step 4: Cement Content
Cement content = 186 / 0.5 = 372 kg per meter cube of
concrete

BIS Method
Step 5: Volume of Entrapped air
Volume of entrapped air = 2 %
Step 6: Volume of FA per volume of total aggregate
P = 1- 0.64 = 0.36
Step 7: Volume of FA and CA
Proportion:
372 (cement): 632 (FA): 1123 (CA)
or 1: 1.7 : 3
372 1 1
0.98 186
3.15 0.36 2.6 1000
af 
     
632 kgaf 
1 0.36 2.6
632 1123kg
0.36 2.6
aC

   

KTU CE 204 Construction Technology - Module 2

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