Curing of concrete

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