concrete curing.ppt

Curing Concrete
Curing can be defined as a procedure for insuring
the hydration of the Portland cement in newly
placed concrete. It generally implies control of
moisture loss and sometimes of temperature.
The hydration of Portland cement is the chemical
reaction between grains of Portland cement and
water to form the hydration product, cement gel:
and cement gel can be laid down only in water-
filled space.
Hydration can proceed until all the cement reaches
its maximum degree of hydration or until all the
space available for the hydration product is filled
by cement gel, whichever limit is reached first.

Curing Concrete
A typical definition of curing is ‘the process of
preventing the loss of moisture from the concrete
whilst maintaining a satisfactory temperature
regime’. This particular definition adds that the
curing regime should prevent the development of
high temperature gradients within the concrete.
Curing requires adequate:
• Moisture
• Heat
• Time
If any of these factors are neglected, the desired
properties will not develop

Curing methods
1.Methods that maintain the presence of
mixing water in the concrete during
the early hardening period. These
include
 Ponding
 Immersion
 Spraying or Fogging
 Saturated wet coverings.
Concrete can be kept moist three curing methods:

Curing methods
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
 plastic sheets
 By applying membrane-forming curing
compounds.

Curing methods
3. Methods that accelerate strength gain
by supplying heat and additional
moisture to the concrete. This is
usually accomplished with
 Live steam,
 heating coils
 Electrically heated forms or pads.
The method or combination of methods
chosen depends on factors such as
availability of curing materials, size,
shape, and age of concrete,
production facilities (in place or in a
plant), and economics.

Curing methods
-On flat surfaces, such as
pavements and floors, concrete
can be cured by Ponding. Earth or
sand dikes around the perimeter
of the concrete surface can retain
a pond of water.
-The most thorough method of
curing with water consists of total
immersion of the finished
concrete element. This method is
commonly used in the laboratory
for curing concrete test
specimens.
Ponding and Immersion

Curing methods
-Fogging and sprinkling with water
are excellent methods of curing
when the ambient temperature is
well above freezing and the
humidity is low. A fine fog mist is
frequently applied through a
system of nozzles or sprayers to
raise the relative humidity of the
air over flatwork, thus slowing
evaporation from the surface.
- Fogging is applied to minimize
plastic shrinkage cracking until
finishing operations are complete.
Fogging and Sprinkling

Curing methods
-Fabric coverings saturated with
water, such as burlap, cotton
mats, rugs, or other moisture-
retaining fabrics, are commonly
used for curing. Treated
burlaps that reflect light and are
resistant to rot and fire are
available.
Wet Coverings

Curing methods
 Impervious paper for curing
concrete consists of two sheets
of Kraft paper cemented
together by a bituminous
adhesive with fiber
reinforcement. Such paper,
conforming to ASTM C 171
(AASHTO M 171), is an
efficient means of curing
horizontal surfaces and
structural concrete of relatively
simple shapes.
Impervious Paper

Curing methods
-Plastic sheet materials, such
as polyethylene film, can be
used to cure concrete.
Polyethylene film is a
lightweight, effective
moisture retarder and is
easily applied to complex as
well as simple shapes
Plastic Sheets

Curing methods
 Liquid membrane-forming
materials can be used to
retard or reduce
evaporation of moisture
from concrete. They are the
most practical and most
widely used method for
curing not only freshly
placed concrete but also for
extending curing of concrete
after removal of forms or
after initial moist curing.
Membrane-Forming Curing Compounds

Curing methods
-Forms provide satisfactory protection
against loss of moisture if the top
exposed concrete surfaces are kept wet.
- The forms should be left on the concrete
as long as practical. Wood forms left in
place should be kept moist by sprinkling,
especially during hot, dry weather. If this
cannot be done, they should be
removed as soon as practical and
another curing method started without
delay.
Forms Left in Place

Curing methods
 Steam curing is
advantageous where early
strength gain in concrete is
important or where additional
heat is required to
accomplish hydration, as in
cold weather. Two methods
of steam curing are used: live
steam at atmospheric
pressure (for enclosed cast-
in-place structures and large
precast concrete units) and
high-pressure steam in
autoclaves (for small
manufactured units).
Steam Curing

Curing methods
 Electrical, hot oil, microwave and infrared
curing methods have been available for
accelerated and normal curing of concrete for
many years. Electrical curing methods include
a variety of techniques:
 (1) Use of the concrete itself as the electrical
conductor,
 (2) Use of reinforcing steel as the heating
element,
 (3) Use of a special wire as the heating
element,
 (4) Electric blankets, and
 (5) The use of electrically heated steel forms
(presently the most popular method).
Electrical, Oil, Microwave, and Infrared Curing

The intention of curing is to protect concrete
against:
 • Premature drying out, particularly by solar
radiation and wind (plastic shrinkage)
 • leaching out by rain and flowing water
 • Rapid cooling during the first few days
after placing
 • High internal thermal gradients
 • Low temperature or frost
 • Vibration and impact which may disrupt the
concrete and interfere with bond to
reinforcement.
Why cure concrete

 The depth of the surface
zone directly affected by
curing can be up to 20 mm
in temperate climatic
conditions, and up to 50 mm
in more extreme arid
conditions. Properties of the
concrete beyond this zone
are unlikely to be affected
significantly by normal
curing.
Why cure concrete

 The rate of evaporation of water from
the surface, taking into account the
combined influences of the ambient
temperature and relative humidity, the
concrete temperature, and the wind
velocity can be estimated from Figure
below taken from ACI 308. This
standard requires that curing measures
are taken if the predicted rate of
evaporation exceeds 1.0 kg/m2/h, to
prevent plastic shrinkage cracking, but
also recommends that such measures
may be needed if the rate exceeds 0.5
kg/m2/h.
Why cure concrete

 The question is how early water
can be applied over concrete
surface so that uninterrupted and
continued hydration takes place,
without causing interference with
the W/C ratio.
 The answer is that first of all,
concrete should not be allowed
to dry fast in any situation.
When to start curing

 This condition should be maintained for 24
hours or at least till the final setting time
of cement at which duration the concrete
will have assumed the final volume. Even if
water is poured after this time, it is not
going to interfere with the W/C ratio.
However, the best practice is to keep the
concrete under the wet gunny bag for 24
hours and then commence water curing by
way of ponding or spraying.
When to start curing

This depends upon:
 • The reason for curing (plastic shrinkage,
temperature control, strength, durability,
etc.)
 • The size of the element
 • The type of concrete (especially rate of
hardening)
 • The ambient conditions during curing
 • The exposure conditions to be expected
after curing
 • The requirements of the specification
How long curing should be
applied?

 Regarding how long to cure it is difficult to
set a limit. Since all the desirable properties
of concrete are improved by curing, the
curing period should be as long as practical.
For general guidance, concrete must be
cured till it attains 70% of specified strength.
At lower temperature curing period must be
increased.
 Concrete keeps getting HARDER AND
STRONGER over TIME. For better strength
and durability, cure concrete for 7 DAYS.
The LONGER concrete is cured, the closer
it will be to its best possible strength and
durability.
applied?

applied?

 Cements, or combinations, containing fly
ash (pfa), blast furnace slag (ggbs),
limestone filler (>5 per cent), or condensed
silica fume react more slowly than plain
Portland cement, particularly in cold
weather. Concretes containing blended
cements should therefore be cured
thoroughly and for a longer period than for
PC concrete, particularly if the potential
durability benefits are to be obtained in the
near-surface and cover zone. Concrete
containing condensed silica fume or
metakaolin exhibits only minimal bleeding
and thus requires early protection to prevent
plastic shrinkage cracking.
The effect of cement type

The following circumstances warrant
particular consideration of curing needs:
 • Horizontal surfaces
 • Dry, hot, windy conditions (one or more
of these)
 • Wear-resistant floors
 • High-strength concrete (initial curing is
especially important)
When is curing of particular
importance?

 The hardening of concrete is a chemical
reaction – the rate of this reaction increases
with temperature but so does the rate of
evaporation from an exposed concrete
surface.
 The rate of reaction at 35°C is about twice
that at 20°C which is, in itself, about twice
that at 10°C. The ultimate strength of
concrete cured at low temperature (e.g. in
winter) is generally greater than that of
concrete cured at a higher temperature (e.g.
in summer); but extremes of temperature
generally have a negative effect.
Effect of temperature

 The slow rate of reaction at low
temperatures means the concrete must be
cured for a longer period to achieve the
desired degree of reaction.
 The fast rate of reaction at high
temperatures gives relatively high early
strengths but the long-term strength and
durability are generally reduced.
 The optimum temperature required to
produce the maximum 28-day strength,
based on small laboratory specimens, is
said to be approximately 13°C (Neville and
Brooks, 1987) and ambient temperatures of
15–25°C are generally considered to be
most suitable for concreting operations.

 Hydration will proceed, to some extent, at
temperatures down to as low as –10°C.
Nevertheless, little strength will develop
below 0°C, and below 5°C the early
strength development is greatly retarded
(ACI 308). Even in the temperature range
5–10°C conditions are unfavorable for the
development of early strength. These
effects will be most prevalent in thin
sections, in the near surface of larger
sections, and in concrete made with slow
hydrating cements. The bulk strength of
larger sections will be less affected
because, generally, the internal
temperature will be elevated by the heat of
hydration.

 Concrete should not be allowed to freeze
before it has gained sufficient strength to
resist damage. According to ACI 308 this
strength is approximately 3.5 MPa. Air
entrained concrete should not be allowed to
undergo any freeze–thaw cycles until it has
reached a strength of approximately 25
MPa. Non air-entrained concrete should, of
course, never be allowed to undergo
freezing and thawing while saturated. The
temperature of concrete during curing
depends on:

 • The dimensions of the element
 • The weather (ambient conditions)
 • cement type cement content
 • Admixtures (accelerators, retarders)
 • The fresh concrete temperature
 • Formwork type/insulation
 • Formwork stripping time
Mostly, temperature control of in-situ concrete
during hardening is only attempted at
temperature extremes where, for example,
there is:
 • A risk of freezing
 • A risk of an excessive peak temperature or
an excessive temperature difference across
the section.

 The peak temperature in a section should
generally be kept below about 65–70°C to
minimize the effect on compressive
strength. Measures to reduce peak
temperatures are beyond the scope of
normal curing techniques and may include
cooling of the fresh concrete by various
means or cooling of the placed concrete by
means of cooling pipes within the section.

 Concrete allowed to freeze before a certain
minimum degree of hardening has been
achieved will be permanently damaged by the
disruption from the expansion of the water
within the concrete as it freezes. This will
result in irretrievable strength loss.
 Excessive evaporation from an exposed
horizontal surface within the first
approximately 24 hours after casting will result
in plastic shrinkage cracking and a weak,
dusty surface. An excessive temperature
difference through the cross-section of an
element will result in early thermal cracking
due to restraint to contraction of the cooling
outer layers from the warmer inner concrete.
What happens if concrete is not
cured properly?

 Inadequate curing will result in the
properties of the surface layer of concrete,
up to 30–50 mm, not meeting the intentions
of the designer in terms of durability,
strength and abrasion resistance.
What happens if concrete is not
cured properly?

-The effect of curing on strength development
is limited to the near-surface of concrete so
its effect on strength will depend on the
element size and type of loading that will be
applied.
-The effect on large elements loaded in
compression will be much less than on
slender elements loaded in flexure. It is
unlikely that the structural capacity of most
elements would be significantly reduced by
poor curing.
The effect of curing on strength

-Required curing duration is sometimes
expressed in terms of the maturity of the
concrete.
-The maturity concept is used to predict the
rate of strength development at different
temperatures; it is mostly used for the
determination of formwork stripping time
and for structural considerations such as
time at loading.
The maturity concept for estimation
of required curing duration

 -Maturity allows the strength of a concrete of
known temperature-time history to be
predicted from laboratory specimens cured
under standard conditions. The maturity is
the sum of the product of temperature
(above a datum level, usually –11°C, the
temperature at which hydration is said to
cease) and the time over which the
temperature prevails:
 M = Σ (T ・ Δt) [°C.h]

Equal maturity should mean equal strength; but
the relationship between maturity and strength
depends on the actual cement type and
strength class used, the water/cement ratio
and whether any significant water loss occurs
during curing. The maturity concept cannot, at
the current state of knowledge, be directly
related to durability aspects rather than
strength. Thus where curing is required for
properties such as abrasion resistance,
permeability, freeze–thaw resistance, etc. it
may be necessary to extend curing beyond
the period predicted by maturity at which a
certain strength develops.

-Practical applications of accelerated curing
include:
 1- Ensuring a daily cycle with, for example,
apartment formwork systems.
 2 -Speeding construction in winter
conditions.
 3 -Ensuring multiple daily use of moulds
used for precast concrete.
 4 -Reducing the time between production
and delivery of precast concrete elements.
Applications of accelerated curing

 1-Steam in pipes (precast technique
mainly).
 2- Gas heating. This system is common for
in-situ construction.
 3- Electric heating
 4- Turbo heaters
Methods of accelerated curing

 Accelerated curing has the following effects
on the properties of concrete:
 1- Reduces the ultimate strength by up to 30
per cent depending on the peak temperature
reached.
 2- A significant increase in coarse porosity
depending on the temperature reached.
 3- If the curing temperature is high, there is a
significant risk of delayed ettringite formation.
 4- For in-situ construction, accelerated curing
will increase the risk of early-age thermal
cracking caused by external restraint.
Effect of accelerated curing on
concrete properties

-The reason for the loss of strength and
increase in coarse porosity is believed to be
due to the hydration products forming close
to the original cement grains and not
spreading uniformly throughout the space
between the cement grains. If the peak
temperature during accelerated curing does
not exceed 65°C, the effect on long-term
properties is not significant.
Effect of accelerated curing on
concrete properties

concrete curing.ppt

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concrete curing.ppt