INTRODUCTION –
STRUCTURAL CONCRETE
Concrete
is an artificial engineering material made from a mixture of cement, water,
fine and coarse aggregates, and a small amount of air. It is the most widely
used construction material in the world.
Concrete
is the only major building material that can be delivered to the job site in
a plastic state. This unique quality makes concrete desirable as a building
material because it can be molded to virtually any form or shape.
Other
desirable qualities of concrete as a building material are its strength,
economy, and durability. Concrete has a very high compressive strength,
unfortunately the tensile strength of concrete is much lower, but by using
properly designed steel reinforcing, structural members can be made that are
as strong in tension as they are in compression.
CONCRETE
CONSTRUCTION IN SOUTH AFRICA
South
Africa, especially the inland regions, is one of the areas in the world with
the highest count of sunny days per year in the world therefore I will have
an in-depth look at special precautions, and recommendations for the use of
concrete in hot conditions.
In
addition to ambient temperature, other climatic factors occur in varying
combinations which also need to be taken into account in order to assess the
severity of the effects of weather conditions on concrete. These factors are
relative humidity, wind velocity, precipitation and solar radiation. They
affect the properties of fresh and hardened concrete, and thus determine the
nature and extent of the precautions to be taken for satisfactory execution
of the job.
In South
Africa the severity of hot weather conditions becomes greater when high
temperatures occur in combination with low relative humidity and, worse,
with high winds if they prevail during the construction and curing stages as
in many coastal areas of the country.
CLIMATIC FACTORS
Besides
air temperature, the other important factors which influence the effects of
hot weather on concrete are relative humidity, wind and solar radiation.
Where poor combinations of these factors prevail, the temperature of the
concrete and the rate of evaporation of the water increase rapidly, the
thermal movements and restraints become large, and the
chemical
reactions of hydration of cement and chemical attack are accelerated.
Substantial
changes in air temperature are accompanied by major changes in relative
humidity which, in turn, are influenced by location (proximity to the sea)
and by the direction of wind.
Low relative
humidity leads to rapid evaporation of the mixing water in concrete which,
in hot weather, causes accelerated stiffening of concrete. In coastal
locations salts, if present in the atmosphere, are deposited on the surface
in increasing concentrations and make their way through cracks and pores
into the body of the concrete, causing deterioration of the concrete and
corrosion of the embedded steel reinforcement.
The rate of
evaporation of mixing water from fresh concrete increases substantially in
windy conditions, and can reach serious proportions if wind velocities
exceed 15 km/h which is often the case in a lot of coastal South African
areas. With the three adverse factors present simultaneously, it often
becomes difficult to prevent the surfaces of freshly placed concrete from
drying out rapidly. This can seriously impair its strength and the
durability of structures made with it.
Solar
radiation affects the properties of fresh concrete in several ways; for
instance, the temperature of stored raw materials, such as cement,
aggregates and water, is increased. Heating of reinforcement fixed in
position, as well as of; metal formwork, further aggravates the situation.
As a consequence, there is a reduction in workability and an acceleration in
the stiffening of the concrete.
EFFECT OF
HOT WEATHER ON FRESH CONCRETE
Hot weather
unfavourably affects workability and thus the stiffening time of concrete.
Since the site engineer cannot work with these constraints, recourse is
often had to increasing the water content in order to improve the
workability, and to increasing the handling time of the concrete.
Any arbitrary
increase in the unit water content beyond that based on the water/cement
ratio would decrease the strength, durability, water-tightness and other
related properties.
The
temperature at which concrete is placed is an important factor influencing
the stiffening behaviour of concrete. As a general rule, the rate of
hydration in fresh concrete is approximately doubled for every 10°C increase
in its temperature. At higher temperatures, the concrete is stiffer at a
given time after adding water than it would be at a lower temperature. Even
if the batch is produced at the desired workability, this workability will
be lost more rapidly at a higher temperature. Set-retarding admixtures may
be used advantageously to delay the stiffening of concrete.
Bleeding is a
form of segregation in which the mixing water rises to the upper surface and
the solids, i.e. the aggregates and cement particles, settle down. In
unfavourable atmospheric conditions of high temperature and high wind
velocity, the rate of evaporation of bleed water from the upper surface can
be excessive, resulting in settlement and plastic shrinkage cracking.
MATERIALS
Cements of
greater fineness exhibit accelerated setting and increased heat generation.
Such cements should be used with caution. The use of rapid-hardening
cements should be avoided as far as possible.
The quality of
aggregates available in hot arid regions can often be porous, dusty and
poorly graded.
The quality of
water used for the mixing of concrete should conform to the relevant
national codes with respect to the permissible limits for chlorides,
sulphates and organic matter.
Curing water
should also be of an acceptable standard, particularly when curing
reinforced or pre-stressed concrete. The ingress of salts into the concrete
leads to the corrosion of reinforcement and to cracking of the concrete.
FORMWORK
Steel and
timber are used extensively for the construction of formwork; each has its
own advantages. Apart from economics, the choice of material in hot weather
conditions depends upon whether the formwork is exposed to solar radiation
and the duration for which it is retained. Steel has the advantage of
dissipating heat while timber, because of its low conductivity, retains
heat. Where formwork has to be retained for a long time, steel forms would
be preferable because the concrete would reach ambient temperature earlier
than with timber forms.
PRODUCTION
The
temperature of fresh concrete depends upon the temperature of its
ingredients. The two immediate effects of a rise in temperature of fresh
concrete are evaporation of water and rapid loss of workability. It becomes
necessary, therefore, to adopt measures to minimize the temperature of mixed
concrete by using any of the available methods of cooling.
It is well known that
poor workmanship leads to bad concrete, even with the best of materials. The
unfavourable effects of poor workmanship are aggravated in hot weather
conditions.
Joints are a common
source of weakness and, therefore, it is desirable to avoid them. If this is
not possible, their number should be minimized if possible. The location of
all joints must be predetermined.
Concreting
operations should be planned so that mixing and placement of concrete are
preferably done during early morning and night hours.
Concrete tends
to stiffen rapidly in hot weather. It should therefore be vibrated as
quickly as possible.
CURING OF CONCRETE
Curing assumes
special importance under hot weather conditions, as there is the risk of
elimination of mixing water from concrete in the absence of a moist
environment. The essence of good curing practice lies in preventing the
concrete from drying out at any time during curing.
FLOOR
SLAB DESIGN OBJECTIVES
When
designing floor slabs for buildings the following parameters will influence
your choice of system:
Loading & span
No
specific type of slab can be seen as the best for a specific application,
one will have to consider all the factors below and then decide. If more
than one floor system can be used for the same span, the next factors that
might strongly guide you in a direction is the cost of erection and speed of
erection.
Cost
(Economic considerations of spanning)
In general
spans of less than 6m is most economically done with reinforced concrete.
When the span is between 6m – 9m, various other concrete slab systems can be
used and be just as economical or even more economical that reinforced
concrete. When the span is more that 9m, normal reinforced concrete is not
economical anymore and one will have to look at pre-stressed or other
special beam systems. (see figure)
Pre-cast
members can also be considered for economical reasons in large building
construction for there is a lot of repetition of standard components and can
reduce costs.
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SPAN RANGE
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LOADING
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2 - 4 kN/m2
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more than 4 kN/m2
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3 - 6,1m
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Reinforced concrete slab
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RC concrete beam & slabs
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6,1 - 9,1m
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RC concrete beam & slabs
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Special floor systems
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more than 9,1m
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Special floor systems
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Speed of erection
Nowadays
speed of building construction plays an all the more important role,
especially for the money saving factor and this has to be looked at
intensively when a project is started.
Depending
on the application, in-situ cast reinforced concrete can be quite
economical, but can take longer because of the temporary support required
until the concrete has set and is not suitable for large multi-storey
buildings. This is best for large residential buildings & small office
blocks of up to 3-4 storeys. With precast members the shuttering is reduced
or eliminated totally and thus results in shorter construction times and can
be very effective for specific applications but one has to look at the other
factors before a decision is made.
Logistics
Post-tensioning in stead of pre-tensioned members is recommended where the
elements require excessive transportation because pre-stressed members, is
not cast on site and the sizes of the members might have an increase in
transportation cost resulting because of the larger transport vehicles
necessary to move them. The accessibility of the specific site also
influences this factor, for some larger vehicles may not be capable of
accessing the site.
Services
This is
especially important when one makes use of tensioned eg: (pre-stresses or
post stressed concrete members) where openings in slab is to be provided one
must make sure not to damage the tensioned cables as this may greatly reduce
their strength and structural failure might occur. Any services protruding
the slab at any point is to be designed beforehand to prevent such failures.
DEFINITIONS &
TERMINOLOGY
Building
Loads
The loads
imposed on a building are classified as either “dead” or “live.” Dead loads
include the weight of the building itself and all major items of fixed
equipment. Dead loads always act directly downward, and are additive from
the top of the building down.
Live loads
include wind pressure, seismic forces, vibrations caused by machinery,
movable furniture, stored goods and equipment, occupants, and forces caused
by temperature changes. Live loads are temporary.
In
general, the design of a building must accommodate all possible dead and
live loads to prevent the building from settling or collapsing and to
prevent any permanent damage to the structure.
Portland cement
The name
‘Portland’ in Portland cement comes from a building stone from Portland,
England. The main ingredients are limestone & shale which is fired at a very
high temperature to form clinker. After cooling, a small amount of gypsum is
added to control setting time, then it is ground to a fine powder which is
known as PC (Portland cement) and is today the most widely used cement in
the world.
Hydration
The
reaction between the cement and water is exothermic (energy & thus heat
generating) and is called hydration.
Creep
If
concrete is loaded continuously for a long time, and the load is then
removed, not all of the distortion is then recovered. This behaviour of
concrete is called creep.
Plastic
shrinkage
If water
is removed from compacted concrete before it sets, the volume of the
concrete is reduced by the amount of water removed. This volume reduction is
known as plastic shrinkage.
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