For most concrete this critical strength is reached within about 48 hours when the temperature of the concrete has been kept above 5°C.
The gain of strength is delayed at low temperatures so it is necessary to protect concrete against cold for some time after placing. Thin sections normally require more protection and for a longer period than thicker ones, corners and edges being particularly vulnerable.
Many of the precautions that can be taken to protect concrete from cold make use of the heat that concrete generates as it
hardens. However, this is effective only if the concrete
temperature is sufficiently high at the time of placing for the heat evolution to start rapidly. To this end the temperature of the concrete when
placed in the for m should never be less than 5°C, preferably not below 10°C. To achieve this, the concrete
temperature in the
mixer or ready-mixed concrete truck needs to be higher to allow for heat losses during transportation and placing. Some ready-
mixed plants can deliver heated concrete.
It should be noted that it is important for prevent water loss from newly laid concrete,
see Curing on page 46.
Raising
the temperature
The easiest way to raise the temperature of the fresh concrete is to
heat the mixing water. Aggregates should be free from ice and
snow because it requires as much heat to melt the ice as to heat the
same quantity of water
from 0°C to 80°C. Aggregates should
be
covered and kept as dry as possible. Heated water should be added to the mixer before the cement so that its temperature will be lowered by contact with the mixer and the aggregates. If this is
not
done there could be a flash set when hot water comes into contact with the cement.
Sometimes, in very cold weather,
aggregates also must be heated
to
achieve the desired concrete temperature. This can be done by injecting live steam or using hot air blowers,
closed steam coils or
electric heating mats. Live steam probably
produces the most uniform heating, but the increased
moisture content needs to be
allowed for in determining batch weights.
If concrete with a sufficiently high initial temperature is prevented from losing heat to its surroundings, the heat evolved during
setting and hardening will protect it from damage by freezing.
Thus, formwork should be insulated (in this respect it should be noted that 19 mm of plywood has fairly good insulating properties
on
its own) and slabs should be covered with insulating mats
immediately after laying (Figure
23). The tops of walls and columns are particularly vulnerable and should be covered with insulating material.
Strength
development
Both the early and subsequent
strength development in cold weather can be accelerated using a water-reducing admixture or
more
conveniently by increasing
the strength class of concrete.
External in-situ paving is particularly vulnerable to the effect of low temperatures because of the large surface area, which loses heat quickly. In addition, slabs are open to drying winds that can add a chilling factor to the effects of low temperature. There is a variety of materials and methods that can be used for protecting and providing insulation to exposed
concrete surfaces, ranging from
plastic sheeting and tarpaulins to proprietary insulating
mats. As an
indication of the relative
merits of different
methods, tarpaulin or plastic sheeting enclosing a 50 mm dead air space has about the same insulating value as 19 mm thick timber; but proprietary
insulation mats (Figure 23) are more effective.
Table : Minimum period before striking formwork (concrete made with CEM I or SRPC).
Mean air temperature
(°C)
|
Sides to beams,
walls and columns
(hours)
|
Soffits to slabs,
props left under (days)
|
Props to slabs
(days)
|
Soffits to beams,
props left under (days)
|
Props to beams (days)
|
3
|
36
|
8
|
14
|
14
|
18
|
10
|
24
|
5
|
8
|
8
|
12
|
16
|
18
|
4
|
6
|
6
|
10
|
NOTES
These times are based on a well-managed concreting operation that includes effective curing.
It may be necessary in cold conditions to instruct the supplier to reduce or eliminate the proportion of any ggbs or pfa
For concretes containing pfa or ggbs recommended striking times
should be increased. Full details are given in CIRIA
Report 1 36.
|
|||||
Minimum striking times
The use of cements with pfa or ggbs in cold weather
presents a particular problem because these concretes are more adversely
affected than ordinary Portland cement concretes
due to their slower gain of
strength. It should be noted that Table 14 applies only to concrete made
with
CEM I or SRPC
Cold weather delays the stiffening of concrete, and ground floor slabs, for example, are likely to take considerably longer than normal before the trowelling
operations can be started.
Plant and equipment
Preparations for winter working should be made well in advance
of the onset of cold weather, and the necessary
plant and equipment made ready
for
use when required. Modifications in site
organisation to help keep work going in winter may not always be
applicable, but they should be considered because their cost is
usually small in relation
to the benefits of a smooth flow of work, a
quicker end to the job and no idle labour. One technique
that may be considered
is the total enclosure of the work area with, for
instance, polythene sheeting
fixed to the scaffolding, and the use of space heaters
within this enclosure. Consideration should also
be
given to the use of cements
of higher strength class such as 42,5R or 52,5 (formerly
known as rapid-hardening Portland
cement) or the use of a higher strength class of concrete,
which will give an increased
rate of strength gain leading to the ability to
strike forms earlier than would be possible with the concrete originally specified.
Weather records
Keeping weather records and planning with an eye to the weather
forecast is necessary for efficient
winter working. Records of
maximum and minimum temperatures, together with a more continuous record during working hours, will help towards an
assessment of maturity and formwork striking times. This assessment should take account of wind and cloud cover because
the
temperature of the concrete
is the factor that matters and this
is
not always the same
as
the air temperature. On a windy, cloudless night concrete can be cooled below the air temperature. The weather forecast is available by telephone or via the internet, and is an invaluable guide to the planning of winter work. Freezing
conditions can usually be predicted
and precautions taken. Specifications frequently call for precautions to be taken at
particular temperatures, depending on whether
the
temperature is
rising or falling.