CONCRETE.ppt

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CONCRETE

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INTRODUCTION OF CONCRETE
 Concrete is a mixture of sand, gravel, crushed
rock or other aggregate held together by a
hardened paste of cement and water.
 This mixture, when properly proportioned, is at
first a plastic mass that can be cast or molded
into a predetermined size and shape.
 Upon hydration of the cement by the water,
concrete becomes stone like in strength,
hardness and durability.

Cement
(+ Admixture) → Cement paste
+ Water + → mortar
fine aggregate + → concrete
coarse aggregate

Different between cement and concrete
 Cement is actually an ingredient of concrete.
 Concrete is basically a mixture of aggregates and paste. The
aggregates are sand and gravel or crushed stone; the paste is water
and Portland cement.
 Concrete gets stronger as it gets older. Portland cement is not a
brand name, but the generic term for the type of cement used in
virtually all concrete, just as stainless is a type of steel and sterling a
type of silver.
 Cement comprises from 10 to 15 percent of the concrete mix, by
volume. Through a process called hydration, the cement and water
harden and bind the aggregates into a rocklike mass.
 This hardening process continues for years meaning that concrete
gets stronger as it gets older.
 So, there is no such thing as a cement sidewalk, or a cement mixer;
the proper terms are concrete sidewalk and concrete mixer.

Classifications of concrete
 Based on unit weight
 Ultra light concrete <1,200 kg/m3
 Lightweight concrete 1200- 1,800 kg/m3
 Normal-weight concrete ~ 2,400 kg/m3
 Heavyweight concrete > 3,200 kg/m3

Classifications of concrete (Cont’d)
 Based on strength (of cylindrical sample)
 Low-strength concrete < 20 MPa compressive
strength
 Moderate-strength concrete 20 -50 MPa compressive
strength
 High-strength concrete 50 - 200 MPa
compressive strength
 Ultra high-strength concrete > 200 MPa compressive
strength

Classifications of concrete (Cont’d)
 Based on additives:
 Normal concrete
 Fiber reinforced concrete
 Polymer concrete

Materials used in concrete:
 Cement
 Water
 Aggregates
 Admixture

Cement
 A mixture of compounds made by burning
limestone and clay together at very high
temperature ranging from 1400 to 1500°C. the
production of Portland cement begins with the
quarrying of limestone, CaCO3. Then mixed with
clay (or shale), sand and iron ore and ground
together to form a homogenous powder.

Water
 It is the key ingredient.
 When mixed with cement, forms a paste that binds the
aggregates together
 Water causes the hardening of concrete through process
call hydration.
 The water needs to be pure in order to prevent side
reaction from occurring which may weaken the concrete
or otherwise interfere with hydration process.
 The ratio of cement and water is the most critical factor
in the production of ‘perfect’ concrete.
 Too much water can reduces concrete strength but high
workability
 Too little water will make the concrete unworkable but
high strength

Aggregates
 Chemically inert, solid bodies, held together by the
cement.
 Come in various shapes, sizes and materials ranging
from fine particles of sand to large, coarse rock.
 Soft, porous aggregates can result in weak concrete with
low wear resistance.
 Hard aggregates can make strong concrete with high
resistance to abrasion
 Should be clean, hard and strong. Usually washed to
remove any dust silt, clay, organic matter.

Admixture
 A material, other than aggregates, cement, or
water, added in small quantities to the mix in
order to produce some desired modifications,
either to the physical or chemical properties of
the mix or of the hardened product.
 The most common admixtures affect plasticity,
air entrainment and curing time.

PROPERTIES OF CONCRETE
 Grades of concrete
 Generally graded according to its compressive strength at
28 days
 Concrete hardens and gains strength as it hydrates. The
hydration process continues over a long period of time. It
happens rapidly at first and slows down as time goes by. To
measure the ultimate strength of concrete would require a wait of
several years. This would be impractical, so a time period of 28
days was selected by specification writing authorities as the age
that all concrete should be tested. At this age, a substantial
percentage of the hydration has taken place.
 The various grades of concrete as stipulated in codes of Practice
BS8110 grouped the grade in nine categories which is best
known based on their characteristic strength in N/mm2

Grades of concrete proposed by Code Practice
BS8110
Grade Characteristic
strength
(N/mm2)
Lowest grade suitable for specific purposes
7
10
7.0
10.0
Mass concrete
15 15.0 Reinforced concrete using light weight
aggregates
20
25
20.0
25.0
Reinforced concrete using heavy weight
aggregates
30 30.0 Prestressed post-tensioned concrete
40
50
60
40.0
50.0
60.0
Prestressed pre-tensioned concrete

Workability of concrete
 workability implies the ease with which a concrete mix
can handled from the mixer to its finally compacted
shape.
 Factors affecting workability:
 Water cement ratio
 Aggregates (shape, texture, size)
 Fineness of cement
 Time and temperature
 Admixture
 Measurement of workability
 Slump test
 Compacting factor test
 Flow test
 Kelly ball test
 Vee Bee consistometer test

Slump Test
 This test method covers the determination of
slump of concrete, both in the laboratory and in
the field.
 This test determines slump of plastic hydraulic
cement concretes
 Apparatus :
 Mold - in the form of the lateral surface of the
frustum of a cone with base 200mm in diameter, the
top 100mm in diameter and the height 300mm
inches.
 Tamping rod - round, straight steel rod 16mm inches
in diameter and 600mm in length.

PROCEDURE :
 Dampen the mold and place it on
a flat, moist, non absorbent
surface.
 Rod each layer with 25 strokes of
the tamping rod. Rod the top,
second and bottom layer
throughout its depth.
 In filling and rodding the top
layer, heap the concentrate
above the mold before rodding is
started. Remove the mold
immediately from the concrete by
raising it carefully in a vertical
direction.
 Immediately 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.

(a) True slump
(b) Shear slump
(c) Collapse slump
Conventionally, when
shear or collapse slump
occur, the test is
considered invalid.

 The slumped concrete takes various shapes, and according to the
profile of slumped concrete, the slump is termed as true slump,
shear slump or collapse slump. If a shear or collapse slump is
achieved, a fresh sample should be taken and the test repeated. A
collapse slump is an indication of too wet a mix. Only a true slump is
of any use in the test. A collapse slump will generally mean that the
mix is too wet or that it is a high workability mix, for which slump test
is not appropriate
 Very dry mixes; having slump 0 - 25 mm are used in road making,
low workability mixes; having slump 10 - 40 mm are used for
foundations with light reinforcement, medium workability mixes; 50 -
90 for normal reinforced concrete placed with vibration, high
workability concrete; > 100 mm

Compacting Factor Test
 The compacting factor test gives the behavior of
fresh concrete under the action of external
forces, i.e to measure the degree of compaction
obtained by doing a standard amount of work on
the concrete. The method of determining the
compacting factor test is described in BS 1881:
Part 103 : 1983.

Compacting Factor Test Equipment
 The compacting factor
equipment consists of two
conical hoppers mounted
vertically above a cylinder.
 Each of the conical hoppers
comprise of a hinged flange
and a quick release
mechanism to allow the
concrete sample to flow freely
into the cylinder.
 The hoppers and cylinder is
mounted on a steel rigid frame
and are easily removed for
cleaning. The apparatus is
protested against corrosion.

 In making the test, the top hopper is filled with a representative
sample of the concrete.
 When completely filled, a hinged door at the bottom is released and
the concrete allowed to fall into the second hopper.
 The filling of the second hopper is thus affected by a standard
method. The concrete is similarly released from the second hopper
and falls into the cylindrical container.
 Surplus concrete is struck off by simultaneously working two steel
floats from the outside to the center. The contents of the cylinder are
then weighed to the nearest 10 grams giving the weight of partially
compacted concrete.
 The cylinder is then refilled from the same sample in layer
approximately 50mm deep, the layers being rammed to obtain full
compaction.
 The top surface is gain struck off level with the top of the cylinder
and the weight the concrete container again determined which is
known as the weight of fully compacted concrete.

 The compacting factor is the ratio of the weight
of partially compacted concrete to the weight of
fully compacted concrete. The difference in the
two weights is due to air voids, and the closer
the values, the less the air voids and the higher
the compacting factor. The workability is
therefore increase as the compacting factor
approaches unity.
 Compacting factor = weight of partially compacted
Fully compacted

Mixing concrete
 Concrete can be mixed on site or brought to site as
ready mix from works where it is mixed in large
quantities and distributed to sites.
 Mixing directly on site will only happen for small jobs or
those which are so large, as in the case of civil
engineering contracts for bridges, reservoirs or
motorways, that large-scale mixing is the only solution.
 Mixing directly on site can be manual and use the
machine (drum concrete mixer)
 All machines used for mixing concrete have to be
cleaned everyday, usually with water and loose
aggregates
 The ingredients (cement, aggregates, water) can be
count by weight or volume.

DRUM MIXER & READYMIX CONCRETR TRUCK

Transporting concrete
 The various methods used to move the concrete
from the mixer or truck to the forms depend
largely upon the job conditions.
 On small jobs, wheelbarrows are the usual
means of transportation.
 However, concrete can be handled and
transported by many methods, including the use
of chutes, buggies operated over runways,
buckets handled by cranes or cable ways, small
rail cars, trucks, pumps to force the concrete
through pipelines, and equipment to force the
concrete through hoses pneumatically.

Transporting concrete

Placing Concrete
 All concrete forms must be clean, tight, adequately
braced, and constructed of materials that will impart the
desired texture to the finished concrete.
 Sawdust, nails, and other debris should be removed
from the forms before the concrete is placed.
 Wood forms should be moistened before the concrete is
placed, otherwise they will absorb water from the
concrete and swell.
 In addition, the forms should be oiled or lacquered to
make form removal easier.
 Reinforcing steel should be clean and free of loose rust
or mill scale at the time the concrete is placed. Any
coatings of hardened mortar should be removed from the
steel.

 The concrete should be placed between the forms or
screeds as close as possible to its final position.
 To consolidate the concrete, it should be mechanically
vibrated or spaded as it goes into the form.
 Then the concrete is thoroughly spaded next to the
forms to eliminate voids or honeycombing at the sides.
 In inaccessible areas, the forms can be tapped lightly
with a hammer to achieve consolidation.
 This operation makes a dense concrete surface by
forcing the coarse aggregate away from the form or face.
 The concrete should not be overworked while it is still
plastic. Overworking will cause too much water and fine
material to be brought to the surface. This may later lead
to scaling or dusting.

Segregation (separation)
 Segregation is when the aggregates separate
from the rest of the concrete. This causes
weakening and excessive curling and
shrinkage. Some of the ways to avoid
segregation include:
 Placing the concrete as close as possible to its final
position.
 Do not drop from higher that 2-3 feet.
 Avoid high slumps.
 Do not move the concrete with a vibrator.

Bleeding (water concentration)
 Bleeding means the concentration of water at
certain portions of the concrete.
 The locations with increased water
concentration are concrete surface, bottom of
large aggregate and bottom of reinforcing steel.
 Bleed water trapped under aggregates or steel
lead to the formation of weak and porous zones,
within which micro cracks can easily form and
propagate.

Compacting concrete
 After placing the concrete it has to be compacted by
removing voids.
 This can be achieved by overfilling and physically
tamping the concrete into place, or by using mechanical
vibration.
 Poker vibrators are used which allow air bubbles to rise
to the surface with a cement-rich thin film.
 When this activity stops the poker can be moved along
usually at intervals of between 300 and 500mm.
 When pre-cast elements are made, the concrete is
poured into forms which are vibrated as a whole on
tables.
 Surface vibrators are only used for concrete which has a
maximum depth of 150mm for floors or roads.
 There is an approximate loss of strength of 5% for every
1% of air in the mix. For a concrete mix to be durable it
must be dense.

Curing Concrete
 Concrete hardens because of hydration, the
chemical reaction between Portland cement and
water.
 As long as the temperatures are favorable and
moisture is present to hydrate the cement, the
following properties of concrete improve with
age: durability (resistance to freezing and
thawing), strength, watertightness, wear
resistance, and volume stability.

Effect of Curing
 All of the desirable properties of concrete are
improved by the proper curing process.
 Soon after the concrete is placed, the increase
in strength is very rapid (for a period of 3 to 7
days). The strengthening then continues slowly
for an indefinite period.
 Concrete which is moist cured for 7 days is
about 50 percent stronger than that which is
exposed to dry air for the same period.
 If the concrete is kept damp for one month, the
strength is about double that of concrete cured
in dry air.

FACTORS AFFECTING THE PERFORMANCE OF
CONCRETE
FRESH CONCRETE
 CEMENT
 Composition
 Quantity
 AGGREGATES
 Size
 Shape
 Grading
 Quantity
 Moisture
 WATER
 Quantity
 MIXING
 TRANSPORTING
 PLACING
 COMPACTING
HARDENED CONRETE
 CURING

Assignment 1
 Why is Concrete Important?
 need to create a list of the importance of concrete
and explain how it affects your lives.
 Applications of Concrete
 Need to create a list of the past, present, and future
applications of concrete.
 Describe the process of curing.

Factors affecting concrete strength
 Concrete porosity: voids in concrete can be
filled with air or with water. Air voids are an
obvious and easily-visible example of pores in
concrete. Broadly speaking, the more porous the
concrete, the weaker it will be. Probably the
most important source of porosity in concrete is
the ratio of water to cement in the mix, known as
the 'water to cement' ratio. This parameter is so
important it will be discussed separately below.

 Water/cement ratio: this is defined as the mass of water
divided by the mass of cement in a mix. For example, a
concrete mix containing 400 kg cement and 240 litres
(=240 kg) of water will have a water/cement ratio of
240/400=0.6. The water/cement ratio may be
abbreviated to 'w/c ratio' or just 'w/c'. In mixes where the
w/c is greater than approximately 0.4, all the cement
can, in theory, react with water to form cement hydration
products. At higher w/c ratios it follows that the space
occupied by the additional water above w/c=0.4 will
remain as pore space filled with water, or with air if the
concrete dries out.
 Consequently, as the w/c ratio increases, the porosity of
the cement paste in the concrete also increases. As the
porosity increases, the compressive strength of the
concrete will decrease.

 Soundness of aggregate: it will be obvious that
if the aggregate in concrete is weak, the
concrete will also be weak. Rocks with low
intrinsic strength, such as chalk, are clearly
unsuitable for use as aggregate.
 Aggregate-paste bond: the integrity of the
bond between the paste and the aggregate is
critical. If there is no bond, the aggregate
effectively represents a void; as discussed
above, voids are a source of weakness in
concrete.

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