2. • Concrete a composite man-made material
• It consists of binding material such as lime or cement, well graded fine and
coarse aggregates, water and admixtures (to produce concrete with special
properties).
• In a concrete mix, cement and water form a paste or matrix fill the voids of
the fine aggregate, coats the surface of fine and coarse aggregates and binds
them together.
• The matrix is usually 22-34% of the total volume.
• Freshly mixed concrete before set is known as wet or green concrete whereas
after setting and hardening it is known as set or hardened concrete.
• The moulded concrete mix after sufficient curing becomes hard like stone
due to chemical action between the water and binding material.
• reason its most used is its properties can be controlled within a wide range
by using appropriate ingredients and by special mechanical, physical and
chemical processing techniques.
3. Classification of cement concrete
• M refers to the mix. This type of concrete mix
is also known as nominal mix., IS: 456 restricts
its use only up to M-20 grade.
4. Design mix concrete
• When the concrete properties such as
strength, water-cement ratio, compaction
factor, slump, etc., are specified the concrete
may be classified as designed-mix concrete.
For a design mix concrete the mix is designed
to produce the grade of concrete having the
required workability and a characteristic
strength not less than the appropriate values
as specified in Table below.
6. Based on bulk density
• On the basis of density, concrete is classified
as super heavy (over 2500 kg/m3 ),
• dense (1800-2500 kg/m3 ),
• light weight (500–1800 kg/m3 ) and
• extra light weight concrete (below 500 kg/m3)
7. Based on place of casting
• When concrete is made and placed in position
at the site it is known as in-situ concrete
• and when used as a material for making
prefabricated units in a factory is known as
precast concrete.
8. Constituents of concrete
• Cement which is a binding material should have a
minimum compressive strength .
• Fine Aggregate materials help to make concrete
mixes more compact.They also decrease the
consumption of cement and water and contribute
to the mechanical strength of the concrete,
making them an indispensable ingredient in the
construction and maintenance of rigid structure.
• Coarse aggregate is usually greater than 4.75
mm.The compressive aggregate strength is an
important factor in the selection of aggregate.
9. Process of production of concrete
• 1. Batching or measurement of materials
• 2. Mixing
• 3. Transporting
• 4. Placing
• 5. Compacting
• 6. Curing
• 7. Finishing
10. Batching or measurement of
materials
• The aggregates, cement and water should be
measured with an accuracy of ± 3 per cent of
batch quantity and the admixtures by 5 per cent
of the batch quantity.
• There are two methods of batching materials, the
volume batching and the weigh batching.
• For most important works weigh batching is
recommended.
• Cement is always measured by weight,
irrespective of the method of batching.
11.
12. Transporting
• A maximum of 2 hours from the time of
mixing is permitted if trucks with agitator and
1 hour if trucks without agitators are used for
transporting concrete
13. placing
• For dry mixes in hot weather delay of half to
one hour is allowed whereas for wet mixes in
cold weather it may be several hours.
• Concrete foundations for walls and columns
are provided below the ground surface.
• Before placing the concrete, the forms must
be examined for correct alignment.
14. compaction
• The vibrations imparted to the fresh concrete
reduce the internal friction between the
particles of concrete by setting the particles in
motion and thus produce a dense and
compact mass.
15. curing
• Cement gains strength and hardness because
of the chemical action between cement and
water. This chemical reaction requires
moisture, favourable temperature and time
referred to as the curing period
• Curing of freshly placed concrete is very
important for optimum strength and
durability.
18. Test in concrete
• The flexural tensile strength test is performed
to estimate the tensile load at which concrete
may crack.
• compression testing machine
• briquette test (direct method) and split tensile
strength test
• Workability test
19.
20. Methods of curing concrete
• Concrete can be kept moist (and in some cases at a
favorable temperature) by three curing methods: 1.
Methods that maintain the presence of mixing water in
the concrete during the early hardening period. These
include ponding or immersion, spraying or fogging, and
saturated wet coverings. These methods afford some
cooling through evaporation, which is beneficial in hot
weather. 2. Methods that reduce the loss of mixing
water from the surface of the concrete. This can be
done by covering the concrete with impervious paper
or plastic sheets, or by applying membrane-forming
curing compounds
21. • 3. Methods that accelerate strength gain by
supplying heat and additional moisture to the
concrete. This is usually accomplished with
live steam, heating coils, or electrically heated
forms or pads.
22. • Concrete may be kept moist by a number of
ways. The methods consist in either supplying
additional moisture to concrete during early
hardening period by ponding, spraying,
sprinkling, etc. or by preventing loss of
moisture from concrete by sealing the surface
of concrete by membrane formed by curing
compounds. Following are some of the
methods of curing.
23. Water curing
• done by covering the concrete surface with gunny
bags and then sprinkling water over them
regularly .
• Ponding, may, affect the strength if the concrete
is flooded too soon.
• When sprinkling of water is done at intervals, care
must be taken that the concrete does not dry out
between applications to prevent the possibility of
crazing—the fine cracks that may occur in the
surface of new concrete as it harden
26. Membrane curing
• In membrane curing the surface is coated with
a bitumen layer to prevent loss of moisture by
evaporation. Sealing compounds such as
rubber latex emulsion, resins, varnish and wax
may also be used as an alternative to bitumen
layer. However, the concrete here may not
achieve full hydration as in moist curing.
28. Steam curing
steam Curing can be also accomplished by artificial heat while
the concrete is maintained in moist condition.
Both of these conditions can be fulfilled by the use of steam
curing. This method of curing is also known as accelerated
curing since an increased rate of strength development can be
achieved.
The accelerated process of curing has many advantages in the
manufacture of precast concrete products.
29. Curing by Infra Red Radiation:
• A much more rapid gain of strength can be
obtained with the help of infra red radiation
than even with steam curing. The rapid initial
rise of temperature does not affect the ultimate
strength. It is particularly suitable for the
manufacture of hollow concrete products in
which case the heaters are placed in the hollow
spaces of the product. The normal operative
temperature is 90°C.
30. Electrical Curing:
• Concrete products can be cured by passing
alternating current of low voltage and high
amperage through electrodes in the form of plates
covering the entire area of two opposite faces of
concrete. Potential difference between 30 and 60
V is generally adopted. Evaporation is prevented
by using an impermeable rubber membrane on the
top surface of the concrete. By electrical curing,
concrete can attain the normal 28-day strength in
a period of 3 days. The technique is expensive.
31. • Chemical Curing: Chemical membranes can be sprayed on to cure concrete. Liquid
membrane forming curing compounds such as sodium silicate (water glass)
solution retard or prevent evaporation of moisture from concrete by forming a
film, filling the pores, sealing the surface voids and preventing evaporation. The
application should be made immediately after the concreting has been finished. If
there is any delay, the concrete should be kept moist until the membrane is
applied. Membrane curing compound should not be applied when there is free
water on the surface, because this water will be absorbed by the concrete and the
membranes broken.
• Nor should the compound be applied after the concrete has dried out The correct
time to apply the membrane is when the water sheet disappears from the surface
of the finished concrete. Adequate and uniform coverage of curing compounds is
essential. In most cases two applications are required. Chemical membranes are
suitable not only for curing fresh concrete but also for further curing of concrete
after removal of forms or after initial moist curing.
32. Water –cement ratio
• The water-cement ratio (w/c) is one of the major
factors but not the only one influencing the
strength of concrete. It is responsible mainly for
the porosity of the hardened cement paste.
• For proper workability the w/c ratio varies from
0.4–0.6
• maximum strength is derived at w/c = 0.4 at
which minimum capillary cavities are expected to
form.
33. • Factors depending on testing methods are
• size of test specimen,
• size of specimen relative to maximum size of aggregate,
• moisture condition of specimen,
• rate of loading adopted,
• and type of testing machine used;
• and those independent of testing method are
• type of cement and
• age of cement,
• type and size of aggregates,
• degree of compaction,
• water-cement ratio,
• aggregate-cement ratio,
• airvoids,
• curing method and curing temperature,
• and type of stress situation that may exist (uniaxial, biaxial and triaxial).
34. • The maturity of concrete is defined as the
summation of product of time and
temperature. Maturity = (time × temperature)
Its units are °C hr or °C days. A sample of
concrete cured at 18°C for 28 days is taken to
be fully matured which is equal to M28 days =
28 × 24[18 – (–11)] = 19488°C hr
