The reaction of cement with water - hydration
- is a chemical
reaction that produces heat. If this development of heat exceeds
the rate of heat loss, the temperature of the concrete
will rise. Subsequently the concrete will cool and contract. Typical
temperature histories of some concrete sections are shown in Figure 7.
If the contraction
were unrestrained there would be no cracking.
However, in practice there is always some form of restraint inducing tension and hence a risk of cracking. Restraint occurs due to either external
or internal influences.
External restraint
Concrete is externally restrained if, for example,
it is cast onto a previously hardened base, such as a wall kicker, or if it is cast
between two already hardened sections, such as an infill bay in a wall or slab, without the provision
of a contraction joint.
Internal restraint
The surfaces of an element of concrete
will cool faster than the
core, producing a temperature differential, and when this
differential is large, such as in thick sections,
cracks may develop at the surface. In general,
it has been found that by restricting the temperature differential to around 20°C between the core and the
surface, little or no cracking will result.
Factors affecting temperature rise
The main factors that affect the rise in temperature are discussed
below.
Dimensions Thicker sections retain the heat generated,
and will have higher peak temperatures and cool down more slowly. While
peak temperatures increase with increasing thickness, above
thicknesses of about 1.5 m there is little further increase in
temperature.
Cement or combination content The heat generated
is directly related to the cement content.
For Portland cement concretes
in sections of 1 m thickness
and more, the temperature rise in the core is likely to be about 14°C for every 100 kg/m3 of cement. Thinner sections will exhibit lower temperature rises than this.
Cement type Different cement types generate
heat at different rates. The peak temperature and the total amount of heat
produced by hydration
depend upon both the fineness
and the chemistry
of the cement.
As a guide, those cements
whose strength develops most rapidly tend to produce most heat. Sulfate-
resisting cement generally
gives off less heat than CEM I 42,5N and cements that are interground or combined with mineral additions such as pfa or ggbs are often chosen for massive
construction because they have the lowest heat of hydration.
Initial temperature of the concrete
A higher
initial temperature of the concrete results in a greater temperature rise:
for
example, concrete
placed at 10°C in a 500 mm thick section
may
have a temperature rise of 30°C, whereas the same concrete
placed at 20°C may have a temperature rise of 40°C.
Ambient temperature In cooler weather there is likely to be a
greater differential between peak and ambient temperatures, i.e. greater cooling and contraction. During hot weather concrete
will develop a high peak temperature but the differential may be lower.
Type of formwork Steel and CRP formwork will allow the heat
generated to be dissipated more quickly than will timber formwork, which acts as an insulating
layer. Timber formwork and / or additional insulation will reduce the temperature differential between the core and the surfaces.
Admixtures Retarding water-reducers delay the onset of
hydration and heat generation but do not reduce the total heat generated. Accelerating water-reducers increase the rate of heat
evolution and increase
the temperature rise.
The problem of early thermal cracking
is usually confined to slabs
over about 500 mm thick and to walls of all thicknesses. Walls are
particularly susceptible because
they are often lightly reinforced in the horizontal
direction and the timber formwork tends to act as a thermal insulator, thus encouraging a larger temperature rise. The
problem may be reduced by a lower cement content, the use of a
cement with a lower heat of hydration or one containing
ggbs or pfa. There is a practical and economic limit to these measures,
often dictated by the specification requirements for strength and
durability of the concrete itself.
In practice, cracking due to external
restraint is best controlled by
the
provision of crack control reinforcement and the spacing of
contraction joints, which should be determined
by the designer. It should be noted that reinforcement does not prevent crack
formation, although it does control the widths of cracks, and with enough of the right reinforcement, cracks will be
fine
enough so as not to cause leakage or affect durability. With very thick sections,
with
very little external restraint, the temperature differential can usually be reduced by insulating, and thereby
keeping warm, the surfaces of the concrete for a few days.
Plastic cracking
There are two types of plastic
cracks: plastic settlement cracks,
which may develop in deep sections and often follow the pattern of
the reinforcement; and plastic shrinkage cracks, which are more
likely to develop on slabs. Both types form while the concrete
is still in its plastic state, before it has set or hardened
and, depending on
the
weather conditions, form within about one to six hours after the concrete has been placed and compacted.
They are often not
noticed until the
following day. Both types of crack are related to
the extent to which the fresh concrete bleeds.
Bleeding of concrete
Fresh concrete is a suspension of solids in water, and after it has been compacted there is a tendency for the solids (both the
aggregates and the cement) to settle. This sedimentation displaces
the water, which is pushed upwards, and, if the process is
excessive, the water appears as a layer on the surface. This bleed water may not always be seen, as it may evaporate
on hot or windy
days faster than it rises to the surface.
The tendency of a concrete to bleed is affected by the materials and their proportions. Bleeding can generally
be reduced by increasing the cohesiveness of the
concrete by one or more of the following
means:
n Increasing the cement content
n Increasing the sand content
n Using finer sand
n Using less water
n Air-entrainment
n Using a rounded natural sand rather than an angular crushed one.
The rate of bleeding will be influenced by drying conditions,
especially wind, and bleeding will take place for longer on cold days. Similarly, due to the slower
stiffening rate of the concrete,
concrete containing a retarder has a tendency
to bleed for a longer
period of time and their use will, in general, increase the risk of plastic cracking.
Plastic settlement cracks
Plastic settlement cracks are caused by differential settlement and are directly related to the amount of bleeding.
They tend to occur in deep sections, particularly deep beams, but they may also
develop in columns and walls. This is because the deeper the
section the more sedimentation or settlement that can take place. However, cracks will form only where something prevents the concrete 'solids' from settling
freely. The most common cause of
this is the reinforcing steel fixed at the top of deep sections;
the concrete will be seen to 'break its back' over this steel and the
pattern of cracks will directly reflect the layout of the steel below (Figure 8).
Settlement cracks may also occur in trough and waffle slabs
(Figure 9) or at any section where there is a significant
change in the depth of concrete.
If alterations to the concrete, particularly the use of an air- entraining or water-reducing admixture, cannot be made due to
contractual or economic
reasons, the most effective way of
eliminating plastic settlement
cracking is to re-vibrate the concrete
after the cracks have formed. Such re-vibration is acceptable
provided the concrete
is still plastic enough to be capable of being 'fluidized' by a poker, and yet not so stiff that a hole is left when
the poker is withdrawn. The timing will depend on the weather.
Plastic shrinkage cracks
These cracks occur in horizontal
slabs, such as floors and roads.
They usually take the form of one or more diagonal cracks at 0.5 to 2 m centres that do not extend to the slab edges, or they form a very large pattern of map cracking. Plastic shrinkage cracks such
as
those shown in Figure 10 do not usually increase
in length or
width
with the passage of time and seldom have a detrimental effect on the load-bearing capability of suspended slabs or on the
carrying capacity of roads. They may occur in both reinforced and non-reinforced slabs.
Plastic shrinkage cracks are most common in concrete
placed on hot or windy days because they are caused by the rate of
evaporation of moisture from the surface exceeding
the rate of
bleeding.
It has been found that air-entrainment
almost eliminates the risk of plastic shrinkage cracks developing.
Clearly, plastic shrinkage
cracks can be reduced by preventing the
loss of moisture
from the surface of the concrete
in the critical first
few
hours. While sprayed-on
resin-based curing compounds are
very
efficient at curing concrete
that has already hardened,
they cannot be applied to fresh concrete until the free bleed water has evaporated. This is too late to prevent plastic shrinkage
cracking,
Figure 11 : Polythene sheeting supported
clear of a concrete
slab by means of blocks and timber.
Note that all the edges of the
polythene are held down to prevent a wind-tunnel effect.
Remedial measures
The main danger resulting
from plastic cracking is the possible
ingress of moisture
leading to the corrosion of reinforcement. With
both
plastic settlement
and plastic shrinkage
cracks, if the affected
surface will be protected
subsequently either by more concrete or
by
a screed, no treatment
is usually necessary.
Often the best repair is simply to brush dry cement (dampened
down
later) or wet grout into the cracks the day after they form and while they are still clean; this encourages natural or autogenous healing.