Case study of EXTREME WEATHER CONCRETING




  
In the present case requirement of concrete temperature is 23° C at the time of mixing and 27° C at the time of placing as furnished by the customer. 

The calculations for quantity of ice to be added in the form of flakes are shown in Tables for both grades of concrete.  However, actual trials shall be done at the site and temperatures be measured.  Necessary adjustments may be done depending upon actual site conditions

CALCULATIONS FOR M30 GRADE OF CONCRETE (TYPICAL)

Material
Mass
(kglcu.m)
Specific
heat
(kJlkg.k)
Joule of vary
temperature
by 1° C
Initial
temperature of
material
C)
Total joule
in material
1
2
3
2x3=4
5
4x5=6
360
0.92
331.2
40
13248
Water
122.8
4.184
513.8
30
15414
Aggregate
1828.2
0.92
1681.9
37
62230.3
Ice
50
4.184
209.2
0
0
Ice (Fusion)
(-)50x33.5
kj/kg)


______________
-16750.0

Total

2736.1

74136.3
Concrete temperature
=
Column 6
Column 4
=
74136.3
2736.1
=
27° C

Percentage of water substituted by ice flakes = 29%                       since{(50/172.8) = 0.2893}
CALCULATIONS FOR M35 GRADE OF CONCRETE (TYPICAL)


Material
Mass
(kg/cu.m)
Specific
heat
(kJ/kg.k)
Joule of vary
temperature
by 1° C
Initial
temperature of
material
C)
Total joule
in material
1
2
3
2x3=4
5
4x5=6
Cement
410
0.92
377.2
40
15088
Water
168.1
4.184
703.3
30 I
21099
Aggregate
1799.9
0.92
1655.9
37
61268.3
Ice
50
4.184
209.2
0
0
Ice
(Fusion)
(-)50x33.5
kj/kg)



-16750.0

Total

2945.6

80705.3

Concrete temperature
=
Column 6
Column 4
=
80705.3
2945.6
=
27° C


Percentage of water substituted by ice flakes = 29%




EXTREME WEATHER CONCRETING






In countries which experience extreme weather conditions special problems are encountered in preparation, placement and curing of concrete. India has regions of extreme hot weather (hot-humid and hot-arid) as well as cold weather. The Indian Standards dealing with extreme weather concreting are: IS: 7861 (Part 1-1975 Reaff. 2007)-Hot weather concreting and IS: 7861 (Part 2-1981 Reaff. 2007) -Cold weather concreting.

 HOT WEATHER CONCRETING:

Special problems are encountered in the preparation, placement and curing of concrete in hot weather. High temperature result in :

Ø  Rapid hydration of cement
Ø  Increased evaporation of mixing water
Ø  Greater mixing water demand
Ø  Large volume changes in concrete resulting in cracks.
Ø  Reduction in strength.

The climatic factors affecting concrete in hot weather are:

Ø  High ambient temperature
Ø  Reduced relative humidity
Ø  Increased wind velocity

Problems associated with hot weather concreting shall be addressed as follows:

Ø  Controlling the temperature of concrete ingredients
Ø  Suitable proportioning of concrete mixes.
Ø  Controlling the temperature of concrete as placed.
Ø  Controlling the processes such as concrete production and delivery
Ø  Carrying out effective protection and curing of placed concrete.

Controlling the temperature of concrete ingredients:

The most direct approach to keep concrete temperature down is by controlling the temperature of its ingredients. The contribution of each ingredient to the temperature of concrete is a function of the temperature, specific heat and quantity used of that ingredient. The aggregates and mixing water exert the most pronounced effect on temperature of concrete. Thus, in hot weather all available means shall be used for maintaining these materials at as low temperatures as practicable.

Aggregates 

Any one of the procedures or a combination of the procedures given below may be used for lowering the temperature or at least for preventing excessive heating of aggregates.

 Shading stockpiles from direct rays of the sun.

Sprinkling the stockpiles of coarse aggregate with water and keeping them moist.
This results in cooling by evaporation, and this procedure is specially effective when relative humidity is low. Such sprinkling should not be done haphazardly because it leads to excessive variation in surface moisture and thereby impairs uniformity of workability. When coarse aggregates are stockpiled during hot weather, successive layers should be sprinkled as the stockpile is-built up. If cold water is available, heavy spraying of coarse aggregate immediately before use may also be done to have a direct cooling action. Coarse aggregates may also be cooled by methods, such as inundating them in cold water or by circulating refrigerated air through pipes or by other suitable methods.

Water

The mixing water has the greatest effect on temperature of concrete, since it has a specific heat of about 4.5 to 5 times that of cement or aggregate. The temperature of water is easier to control than that of other ingredients and, even though water is used in smaller quantities than the other ingredients, the use of cold mixing water will effect a moderate reduction in concrete placing temperatures. For a nominal concrete mixture containing 336 kg of cement, 170 kg water, 1850 kg of aggregate per ma, a change in 2°C water temperature will effect a 0.5 º C change in the concrete temperature.

Efforts shall be made to obtain cold water, and to keep it cold by protecting pipes, water storage tanks, etc. Tanks or trucks used for transporting water shall be insulated and/or coloured and maintained white or yellow. Under certain circumstances, reduction in water temperature may be most economically accomplished by mechanical’ refrigerator or mixing with crushed ice. Use of ice as a part of the mixing water is highly effective in reducing concrete temperature since, on melting alone, it takes up heat at the rate of 80 kcal/kg. To take advantage of heat of fusion, the ice shall be incorporated directly into the concrete as part of the mixing water. Conditions shall be such that the ice is completely melted by the time mixing is completed.

NOTE :- If the ice is not melted completely by the time mixing is completed, there can be a possibility of Ice melting after consolidation of concrete and thus leaving hollow pockets in concrete, with detrimental effects.

Recommended procedure for concreting during hot weather conditions is given below:

Ambient temperature shall be below 40° C at the time of placement of concrete.  Concreting may be planned during morning and evening hours.
The period between mixing and delivery (placing) shall be kept an absolute minimum.

Keep aggregates under shade and cool aggregates by sprinkling water.
Formwork, reinforcement shall be sprinkled with cool water just prior to placement of concrete.



COLD WEATHER CONCRETING:

The production of concrete in cold weather introduces special and peculiar problems which do not arise while concreting at normal temperatures. Quite apart from the problems associated with setting and hardening of cement concrete, severe damage may occur if concrete which is still in the plastic state is exposed to low temperature, thus causing ice lenses to form and expansion to occur within the pore structure. Hence it is essential to keep the temperature of the concrete above a minimum value before it is placed in the formwork. After placing, concrete may be kept above a certain temperature with the help of proper insulating methods before the protection is removed. During periods of low ambient temperature, special techniques are to be adopted to cure the concrete while it is in the formwork or after its removal.


The Precautions to be taken and methods adopted for concreting in sub-zero temperature is listed below.

  1. Utilization of the heat developed by the hydration of cement and practical methods of insulation.

  1. Selection of suitable type of cement

  1. Economical heating of materials of concrete
(Heating of water is the easiest to be adopted)

  1. Admixtures of anti-freezing materials

  1. Electrical heating of concrete mass

  1. Use of air-entraining agents.

STRUCTURAL MEMBERS REQUIRED FOR TOP-DOWN CONSTRUCTION (Top Down Construction- Part 2)




Top Down Construction -Part 1


STRUCTURAL MEMBERS REQUIRED FOR TOP-DOWN CONSTRUCTION
Design and construction principles for top-down method primarily call for two major structural elements.
·        Columns with sufficient capacity must be pre-founded in bored piles or barrettes to sustain the construction load and to utilize as part of bracing system.
·         Excavation for basement must be carried out with the support of permanent
           retaining wall so that basement floor slabs can be utilized as lateral bracing.
Diaphragm wall of 0.8m to 1.2m in thickness with sufficient embedment in firm soil layers is commonly used as a retaining wall whereas prefabricated steel columns known as Stanchions embedded in either large diameter deep-seated bored piles or barrettes are utilized as structural columns. Figure 1 illustrates the top-down construction method with utilization of stanchions and diaphragm wall.


Installation of Diaphragm wall
Construction of guide wall
A guide wall is constructed to set out the position of the diaphragm wall.
Excavation of panel
The grab/trench cutter cuts and removes the soil to form the panel. The excavation is stabilised by filling it with bentonite slurry to support the wall of the excavation.

Installation of Rebar cage and concreting

The crane lifts up the reinforcement-bar cage and places it within the panel. Concrete is poured into the panel to form the panel wall.
Repetition of process
Process 2 - 4 repeats for the remaining soil in between the panels till the entire length of the diaphragm wall construction is completed.



TYPES OF STANCHION AND THEIR APPLICATION



TYPES OF STANCHION
General information

Material & Example Size


Application

limitation




Light stanchion




Steel H-beams
350x350x137kg/m



For semi top-down
construction
• For temporary decking



Limited capacity of light stanchion does not allow for construction of super
structure until completion of
basement construction



Medium sized stanchion


Steel H-beams
350x350x390 kg/m

For semi and full top-down
construction of shallow to
medium deep excavation



Limited number of super
structure floors construction





Heavy stanchion

Steel H-beams
508x457x738kg/m
• Composite steel columns
built up by 2 or more small
to medium size H-beams
• Large section pre-cast RC
column (seldom use)



Full top-down construction in
deep excavation

   

       Depending on the loading
condition, numbers of
superstructure floors can be
constructed before
completing basement
excavation



STANCHION INSTALLATION METHODS
Stanchion installation method is usually selected by the piling contractor who takes into consideration three main factors such as installation depth, size of stanchion and size of bored or barrette piles. Though installation details may be different from one contractor to another, stanchion installation can be categorized under two main methods, post-concreting or plunging installation and pre-concreting installation or placing stanchion prior to concreting.
Post-concreting installation or plunging method
In this method, stanchion is installed immediately after completion of bored pile concreting process. General construction sequence involved in this method is demonstrated in Figure 2.
Guide frame is used to install thestanchion at the correct position. 

Pre concreting method
In this method, stanchion is installed immediately after completion of drilling and reinforcement lowering prior to concreting process. In some projects stanchion is attached to the last section of reinforcement and installed together with the reinforcement. General construction steps involved in this method are demonstrated in Figure 3.

Advantages and disadvantages
Advantages:
1. The shortened construction period due to simultaneous construction of the basement and the superstructure.
2. More operational space gained from the advanced construction of floor slabs.
3. The higher stiffness of floor slab compare to steel struts improves the safety of excavation.
4. It is highly suitable for construction for tall buildings with deep basements to be constructed in urban areas.
Disadvantages:
1. Higher cost (due to the construction of pile foundation)
2. Since the construction period of the basement is lengthened, the lateral displacement of retaining wall or ground settlement may possible increase due to the influence of creep if the soil layers are encountered.
3. The construction quality may influence because of worsened ventilation and illumination under floor slab.
4. It requires highly skilled supervision and labour force.

Case Study
a)  The Top-Down Construction in Indonesia:
The top down construction in Indonesia was introduced for the first time in 1994. The project was the Bank Indonesia C-Building Projects (12 stories, and 3 basement), and PT. (Persero) Wijaya Karya was the main contractor.
In, 1995 another project was Menara Merdeka Building (30 stories, 3 basement) also built by top-down construction. The main contractor was joint operation of PT. (Persero) Pembangunan Perumahan and PT. (Persero) Wijaya Karya.
The largest project in Indonesia that use the top-down construction was Pasar Baru Bandung Project , 2003. The huge building was consisted 120,000 sq.m, 10 stories plus 3 basements, and the time line for accomplishing the project was only 8 months. The main contractor was PT. (Persero) Pembangunan Perumahan.
b)    The one seen here depicts the basement entrance for the Cheung Kong Center projectin hong kong, which best illustrates the method. The top-down construction method is virtually the only method suitable for the construction of deep basements on a large scale. One irresistible advantage of this method is that substructure and superstructure work can be carried out at the same time.
C)   Top-down basement construction: the permanent RC structure at the top has been completed but remains supported on temporary steel stanchions, while the excavation and construction work to the lower basement is yet to be continued. (Lee Gardens)
d)    Another feature of top-down construction is that the ground floor slab (indeed, any slab close to ground level) is cast first as a starting level to provide the necessary rigidity to the side supports. This example, from the Festival Walk project, best illustrates the arrangement



Conclusion
From the above report we can conclude that top down constuction has its suitability for certain kind of mega structures. It is suitable for structures with deep basements like undergrond rails, car parks etc. It is also very efficient way of doing two way construction to save time. Skyscrapers with deep basements in urban areas should be constucted using top down construction. But top down construction needs very efficient planning and designing and skilled supervision and labour force. Top down construction is the need of the hour as it is highly time efficient and is becoming popular and is coming more and more in practice with every passing day.




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