Methods of curing. Curing in hot and cold weather.

1. PROPERTIES OF CONCRETE
Lecture No.5

2. CURING OF CONCRETE
Lecturer Contents
•Introduction to curing of concrete
•Reasons to cure Concrete
•Duration of Curing of Concrete
•Methods of curing
- Water curing
- Membrane Curing
- Steam Curing
•Curing in Hot and Cold Weather
•Effect of curing on properties of concrete
•Consequences of inadequate curing

3. .
Quality concrete
Cement content
Water-cement ratio
Admixtures
Consolidation Curing

4. CURING
Curing is name given to the procedures used for
promoting the hydration of cement ,and
consists of a control of temperature and of
the moisture movement from and into the
concrete.
It has important role on strength development
and durability of concrete.

5. Concrete derives its strength as a result
of the chemical reaction of the mix water
and the cement, a reaction that starts at
the instant the two materials first come in
contact with each other and can continue
for long time.

6. Curing is done to control the rate and extent of
moisture loss from concrete to ensure an
uninterrupted hydration of Portland cement
after concrete has been placed and finished in
its final position.
Concrete that “dries” out will not reach its
design strength or meet specifications. The
longer the cure, the better the concrete.

7. Why Curing of Concrete is
Important
Cement requires a water/cement ratio
about 0.23 for hydration and a
water/cement ratio of 0.15 for filling the
voids in the gel pores. In other words, a
water/cement ratio of about 0.38 would be
required to hydrate all the particles of
cement and also to occupy the space in the
gel pores.

8. There are several important reasons why one should cure
concrete:
Concrete strength gain
Improves durability of concrete.
Harder, more abrasion-resistant surfaces
Enhanced Serviceability
Improved Microstructure

9. •Concrete strength gain - Concrete strength
increase with age as moisture and a favorable
temperature is present for hydration of
cement.
•An experimental investigation was conducted
by "Cement, Concrete & Aggregates Australia"
(CCAA) on the importance of curing.

10. •Figure-1 illustrates a comparison of the
strength of concrete at 180 days of moist
curing with various periods of moist curing
(0, 3, 7, 14 & 28 days) and then allowing it to
dry out. From the graph below, it can be
observed that concrete allowed to dry out
immediately, achieves only 40% of the
strength of the same concrete water cured
for the full period of 180 days.

12. •Improved durability of concrete – The durability of
concrete is affected by a number of factors
including its permeability, porosity and absorptivity.
•Well cured concrete can minimize thermal, plastic &
drying shrinkage cracks, making concrete more
water tight, thus preventing moisture and water
borne chemicals from entering into the concrete
and thereby increasing its durability.

13. •Enhanced serviceability - Concrete that is
allowed to dry out quickly undergoes
considerable early age shrinkage. Inadequate
curing contributes to weak and dusty
surfaces having a poor abrasion resistance.

14. •Improved microstructure - Material properties are
directly related to their microstructure. Curing assists
the cement hydration reaction to progress steadily
and develops calcium silicate hydrate gel, which binds
the aggregates leading to a rock solid mass, makes the
concrete denser, decreases the porosity and
enhances the physical and mechanical properties of
concrete.

15. Duration of Curing
•The duration of curing of concrete depends
on the grade & type of cement, mix
proportion, desired concrete strength, shape
and size of the concrete member and
environmental & exposure conditions. The
duration may vary from few days to a month.

16. •In case of Ordinary Portland Cement (OPC),
Exposed surfaces of concrete shall be kept
continuously damp or in a wet condition by
ponding or by covering with sacks, hessian
cloth or other similar material and kept
continuously wet for at least 7 days from the
date of placing.

17. •In case of concrete where mineral
admixtures or blended cements are used, it is
recommended that the above minimum
periods may be extended to 10-14 days, for
assisting the secondary reaction.

18. Curing Requires
Adequate…
Moisture
Temperature
Time
If any of these factors are neglected the desired
properties will not develop.

19. Methods to Cure Concrete
Methods of curing concrete broadly fall into the
following categories:
1.Water curing-preventing the moisture loss from
the concrete surface by continuously wetting the
exposed surface of concrete.
2.Membrane curing-minimizing moisture loss from
the concrete surface by covering it with an
impermeable membrane.
3.Steam curing-keeping the surface moist and
raising the temperature of concrete to accelerate
the rate of strength gain.

20. Methods to Cure Concrete
1. Water Curing - is done by spraying or
sprinkling water over the concrete surface
to ensure that the concrete surface
remains continuously moist. This prevents
the moisture from the body of concrete
from evaporating and contributes to the
strength gain of concrete.

21. Ponding: This is the most common and
inexpensive method of curing flat surfaces
such as floor slabs, flat roofs, pavements and
other horizontal surfaces.
A dike around the edge of the slab, which may
be sub-divided into smaller dikes, is erected
and water is filled to create a shallow pond.
Care must be taken to ensure that the water
in the pond does not dry up, as it may lead to
an alternate drying and wetting condition.
Methods to Cure Concrete
1. Water Curing

22. Methods to Cure Concrete
1. Water Curing

23. •Sprinkling, fogging & mist curing: Using a
fine spray or fog or mist of water can be an
efficient method of supplying water to the
concrete surface especially during hot
weather, which helps to reduce the
temperature of concrete, eventually
conserving moisture inside the body of
concrete.
Methods to Cure Concrete
1. Water Curing

24. Methods to Cure Concrete
1. Water Curing

25. •Wet coverings: Water absorbent fabrics such
as hessian, burlaps, cotton mats, rugs etc.
may be used to maintain water on the
concrete surface by completely covering the
surface immediately after the concrete has
set. They must be continuously kept moist to
prevent the fabric from absorbing water
from the body of concrete, due to capillary
action.
Methods to Cure Concrete
1. Water Curing

26. Methods to Cure Concrete
1. Water Curing

27. 2. Impermeable Membrane Curing - Sometimes,
concrete works are carried out in places
where there is acute shortage of water .
•The quantity of water, normally mixed for
making concrete is more than sufficient to
hydrate the cement, provided this water is not
allowed to go out from the body of concrete.
•Concrete could be covered with membrane
which will effectively seal off the evaporation of
water from concrete.
Methods to Cure Concrete

28. •Formwork Leaving the formwork in place
during the early age of concrete is one of the
most efficient methods of curing, especially
for columns. However, the turn around time
of the formwork reduces considerably.

29. •Plastic sheeting Plastic sheets form an
effective barrier to control the moisture losses
from the surface of the concrete, provided
they are secured in place and are protected
from damage. They must be placed
immediately after the final set of concrete
without causing any damage to the surface.
•On flat surfaces like slabs, pavements, etc they
must be properly secured to the surface and
must extend beyond the edges of the slab, so
that they are not blown away by gusty winds.

32. •Membrane curing compounds - Curing
compounds are wax, acrylic and water based
liquids which are sprayed over the freshly
finished concrete to form an impermeable
membrane that minimizes the loss of
moisture from the concrete. These are cost
effective methods of curing where standard
curing procedures are difficult to adopt.

33. •Membrane curing compounds: When used to
cure concrete the timing of the application is
critical for maximum effectiveness. They must be
applied when the free water on the surface has
evaporated and there is no water sheen on the
surface visible. Too early application dilutes the
membrane, where as too late application results
in being absorbed into the concrete. Care must
be taken to avoid foot, machinery and vehicular
traffic over the concrete surface to prevent
damage of the coating.

34. 3. Steam Curing- Steam curing is a process
for accelerating the early hardening of
concrete and mortars by exposing it to steam
and humidity.
This type of system is most commonly used
for precast concrete products where standard
products are manufactured in the factory and
the turnaround time of the formwork is very
quick.

35. In the curing chamber, the control of
temperature and humidity is of prime importance
or else the concrete products are likely of
fracture, crumble and develop other problems
later in their service lives.
This type of curing systems are generally adopted
for railway sleepers, concrete blocks, pipes,
manhole covers, poles, pipe culverts, prestressed
precast concrete products, and so forth.

36. Curing in Hot weather
 During hot weather, concrete must be
protected from excessive drying and from
direct sun and wind.
 Curing materials which reflect sunlight to
reduce concrete temperature must be used.
 Water curing is recommended and care
should be taken to prevent excessive stress
caused by alternative wetting and drying or
by cold water on warm concrete. Framed
enclosures of canvas tarpaulins or sun
shades may be used to protect the concrete
from direct sunlight.

37. Curing in Cold Weather
Some problems associated with temperature
below 4o C are:
•Freezing of concrete before adequate
strength is developed
• Slow development of concrete strength
• Thermal stresses induced by the cooling of
warm concrete to cooler ambient
temperatures.

38. •In cold weather, some procedures like
heated enclosures, insulating blankets &
curing compounds may be used.
•The temperature of fresh concrete must
be kept above 100C by using heated
raw materials and the curing shall be
continued for a longer period of time till
concrete gains the desired strength.

39. Conclusion
•The chemical reactions between cement
& water produces C-S-H gel which
bonds the ingredients of concrete, viz.
coarse & fine aggregates, mineral
admixtures, etc, and converts these
fragments into a rock solid mass.
•This is possible only if continuous curing
is done for at least 14 days; irrespective
of the type of cement used.

40. •To have a dense microstructure and
impermeability, prolonged curing is a
must which leads to enhanced
durability.
•Well designed concrete may give poor
durability if not properly cured and on
the other hand a moderately designed
concrete if well cured can give a better
durability. Hence importance of curing
should never be ignored.
Conclusion

41. •It has been observed that at several
sites, curing of concrete is left to the
decision and comfort of the unskilled
laborer.
•Site engineers & supervisors should put
an extra effort to ensure that curing is
not ignored at site & they should
provide the necessary resources to
maintain satisfactory levels of curing, by
using the best technique available at
site.
Conclusion

42. •Just as a new born baby, when it comes into
this world needs the utmost care for its
development and protection from this new
environment, in the similar manner, a freshly
placed concrete requires proper protection
and care from the encapsulating & aggressive
environment.
•Strictly adopting good curing practices at site
will help concrete to achieve the properties of
designed strength, enhanced durability,
improved microstructure and a long lasting
serviceability.
Conclusion