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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.
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.
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.
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.
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.
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.
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 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.
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.
2 - 4 kN/m2
more than 4 kN/m2
3 - 6,1m
Reinforced concrete slab
RC concrete beam & slabs
6,1 - 9,1m
RC concrete beam & slabs
Special floor systems
more than 9,1m
Special floor systems
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.
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.
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.
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.
The reaction between the cement and water is exothermic (energy & thus heat generating) and is called hydration.
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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