अप्रत्यक्ष पाण्याचे Water Footprints…!! पाणी वाचवा वाया घालवू नका ..!!

मित्रांनो , सध्या १९७२ च्या दुष्काळा पेक्षाही भयानक असा दुष्काळ उभ्या महाराष्ट्रात पडलेला आहे. उपलब्ध असलेल्या पाण्यासाठी प्रादेशिक पातळीवर राजकारण चालू आहे. दुष्काळ निवारणासाठी packageजाहीर होत आहेत. असही एक मत  आहे कि ७२ चा दुष्काळ हा अन्नधान्याचा दुष्काळ होता आणि सध्या त्या मानाने अन्नधान्य उपलब्ध आहे परंतु पाण्याचे दुर्भिक्ष मात्र अत्यंत जाणवत आहे. काही ठिकाणी अक्षरशः अशी परिस्थिती आहे कि घरी आलेल्या पाहुण्यांना खायला  जे पाहिजे ते देतो मात्र पाणी मागू नका अशी विनंती करावी लागत आहे. महाराष्ट्रातल्या मराठवाड्यात तर महिन्याकाठी रु.३०००/- + फक्त पाण्यासाठी खर्च करावे लागत आहेत. दुसऱ्या बाजूला मात्र आर्थिक दृष्ट्या संपन्न असणारा गट मात्र पाणी आणि पैसा फुटकळ समारंभ आणि मेजावाण्याद्वारे   वारेमाप खर्च करीत आहे. ईतर वेळी कधीच न जाणवणारी ‘पाण्याची किंमत’ आज काळात आहे. एखाद्या वस्तूची टंचाई निर्माण झाल्याशिवाय त्या वस्तूची खरी किंमत काय हा विचार आपण कधीच करत नाही. म्हणूनच आज पाण्याची टंचाई निर्माण झालेली असताना पाण्याच्या खऱ्या किमतीविषयी चर्चा करणे गरजेचे आहे. पाण्याच्या बाबतीत असाच हेळसांड पणा चालू राहिला तर नक्कीच काही काळाने सगळ्या वस्तूंची किंमत हि पाण्याच्या स्वरुपात द्यावी लागेल. अहि कोणतीच वस्तू नाही कि जी प्रत्यक्ष किवा अप्रत्यक्षपणे पाण्याशिवाय निर्माण होऊ शकत नाही.

तांत्रिक दृष्ट्या, कोणत्याही वस्तूच्या निर्मितीमागे आवश्यक असलेल्या पाण्याचा हिशोब जेव्हा अभ्यासाद्वारे आकड्यात मांडली जाते त्यावेळी येणाऱ्या अशा पाण्याच्या स्वरूपातील किमतीला ‘water footprint’ अशी संकल्पना रूढ आहे. यामध्ये एखादी वस्तू निर्माण होत असताना तिच्या निर्मिती प्रक्रियेमधील विविध टप्प्यावर अपेक्षित असणाऱ्या पाणीवापराचा हिशोब करून तो लिटर्स मध्ये मांडला जातो. त्यामध्ये एखाद्या पिकासाठी लागणारे पावसाचे पाणी अथवा सिंचनाचे पाणी लागते त्याला ‘green water footprints’ असे म्हणतात, तर ते पिक पूर्ण हातात येई पर्यंत जे पाणी बश्पिभावानाद्वारे अथवा इतर करणा मुले वाया जाते त्याला ‘blue water footprint’  असे म्हणतात. आणि त्या पिकाचे रुपांतर शेवटी एखाद्या पदार्थ मध्ये अथवा ‘product´’ मध्ये होत असतना जे पाणी प्रदुषित होते त्याला’grey water footprints’ असे म्हणतात. या तिन्हींची बेरीज करून त्या पदार्थाचा अथवा वस्तूचा ‘total water footprint’  या स्वरुपात पाणी वापर काढला जातो.

समजा,आज आपल्या सकाळच्या एक कप चहाची रुपयातील किंमत आहे रु.८/- परंतु याबरोबरच ‘water footprint’ म्हणजेच पाण्याच्या स्वरूपातील किमतीचा जर हिशोब केला तर तो येतो सुमारे १०० लिटर. म्हणजेच एक कप चहा चा आस्वाद प्रत्यक्ष पणे घेण्यासाठी सुमारे १०० लिटर पेक्षाही जास्त पाणी अप्रत्यक्ष पणे वापरण्यात आलेले असते. मग यामध्ये अगदी चहासाठी लागणारी साखर, त्यासाखरेसाठी उसाला लागणारे पाणीवापर प्रत्यक्षपणे तर चहासाठी लागणारे ईतर निर्मीती प्रक्रियेमध्ये लागणाऱ्या पाण्याचा अप्रत्यक्ष वापर गृहीत धरलेला असतो. त्याचप्रमाणे एक कप कॉफी साठी सुमारे १4० लिटर, तर एक किलो चिकन मागे सुमारे ७,७३६ लिटर्स इतक्या अवाढव्य प्रमाणात  पाणी लागते. आणि एक किलो कापसासाठी सुमारे ११००० लिटर्स आणि त्यापासून निर्माण झालेला एक cotton t-shirt घेतला तर त्याची पाण्याच्या स्वरूपातील किंमत म्हणजेत ‘water footprints’ असतात सुमारे २७०० लिटर्स.

म्हणूनच जागतिक पातळीवर ज्या गोष्टींचे ‘water footprints’  हे जास्त असतात अशा गोष्टी शक्यतो दुसऱ्या देशाकडून आयात करायच्या अशी संकल्पना रूढ होताना दिसत आहे आणि हे अभ्यासाद्वारे स्पष्ट झालेले आहे की भारत युरोपियन युनियन देशामध्ये वर्षाकाठी जो कापूस निर्यात करतो त्या कापसाचे पाण्याच्या स्वरूपातील मूल्य आहे सुमारे ६,६२,३०,००० लाख लिटर्स ‘water  footprints’  म्हणजेच ६६२३००००००००० इतके लिटर्स पाणी आपण एक वर्षात कापूस निर्यात करून युरोपियन युनियन चे वाचवतो. आणि सर्वसाधारण पणे आपण ज्या वस्तू निर्यात करतो त्या बहुतांश वस्तूंचा ‘water  footprints’  हा आपण आयात करत असलेल्या वस्तूंच्या तुलनेत अतिप्रचंड आहे.

आणि सुरुवातीला उल्लेख केल्याप्रमाने आजच्या टंचाई ग्रस्त काळात जेवा लग्न अथवा ईतर समारंभामध्ये मेजावानिद्वारे प्रचंड प्रमाणात अन्नाची नासाडी करन्यात येते तेवा फक्त विचार करा कि आपण किती पाणी वाया घालवतो. त्यामुळेच इथून पुढे ताटात अन्न वाया घालवताना तरी विचार करा कि आपण अन्न तर वाया घालावातोच पण त्यामागे वापरण्यात आलेले लाखो लिटर्स पाणी सुद्धा वाया घालवतो. हाच विचार प्रत्येक वेळी डोक्यात आला तर नक्कीच खात्री आहे कि आपण सगळे पाण्याचा नक्कीच जपून वापर करू. अजून एक महत्वाची गोष्ट अशी कि शाकाहारी भोजनासाठी लागणारा सरासरी पाणीवापर आहे १५०० लिटर्स प्रती दिन आणि मांसाहारी भोजनासाठी ३४०० लिटर्स/प्रती दिन. म्हणजेच नुसते शाकाहाराचे आचरण केले तर आपण निम्यापेक्षा जास्त पाणी वाचवू शकतो, अर्थात निर्णय ज्याचा त्यांनी घ्यायचा....

मतितार्थ एकच, काहीही करा पण पाणी वाचवा....!! वाया घालवू नका ....!!

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Rare construction phase snaps of Bhandrdara dam in Maharashtra

Rare 100 yrs old SnapClip of Bhandardara Dam near Pune,Mumbai, A. Nagar. 

Full courtesy to Shree Bhausaheb Chaskar (sahyagiri.com) .

Gives Good idea of various Dam construction phases.

Informative to civil engineers and irrigation engineers...do watch!!

Bhandardara is a holiday resort village on the western coast of India. The village is located in the Ahmednagar district of the state of Maharashtra, about 185 kilometers (115 mi) from Mumbai.

Bhandardara sits by the Pravara River, and is a blend of natural beauty, waterfalls, mountains, tranquility, greenery, invigorating air and pristine ambiance. The Bhandardara Lake and the Randha falls are major tourist attractions.

As legend has it, Shri Agasti Rishi meditated here for a year, surviving only on water and air, pleased with this display of devotion, God came down to Earth and blessed Shri Agasti Rishi with a stream of the Ganges river, which is now known as the Pravara River.

From Bhandardara, you can climb up to see the Ratangad and Harishchandragad fort. Or, you can follow the trails that lead to Ajoba and Ghanchakkar peaks. Also, if you are up for a challenge, the highest peak in Maharashtra, Mount Kalsubai (1646 m), stands tall for you to conquer. The starting point of this challenging trek is Bari Village, located 12 km away from Bhandardara. Atop, there is a small temple and standing. Standing at the summit, you can have panoramic view of Sahyadris and Bhandardara.

Facts ~

#¤#
About 65 % of freight and 80%
passenger traffic is carried by the
roads.
#¤#
National Highways constitute only
about 2 % of the road network but
carry about 40 % of the total road
traffic .
#¤#
Number of vehicles has been
growing at an average pace of
10.16 % per annum over the last five
years.

Construction Management

The Art of Construction: Projects and Principles for Beginning Engineers & Architects

10 Things to Remember when doing Concrete Mix Design

Good quality concrete starts with the quality of materials, cost effective designs is actually a by-product of selecting the best quality material and good construction practices. Following are 10 Things to remember during Concrete Mix Design and Concrete Trials.

1. ACI and other standards only serves as a guide, initial designs must be confirmed by laboratory trial and plant trial, adjustments on the design shall be done during trial mixes. Initial design “on paper” is never the final design

 .   

2. Always carry out trial mixes using the materials for actual use.

3. Carry out 2 or 3 design variations for every design target.

4. Consider always the factor of safety, (1.125, 1.2, 1.25, 1.3 X target strength)

5. Before proceeding to plant trials, always confirm the source of materials to be the same as the one used in the laboratory trials.

6. Check calibration of batching plant.

7. Carry out full tests of fresh concrete at the batching plant, specially the air content and yield which is very important in commercial batching plants.

8. Correct quality control procedures at the plant will prevent future concrete problems.

9. Follow admixture recommendations from your supplier

10. Check and verify strength development, most critical stage is the 3 and 7 days strength.

Important note:
Technical knowledge is an advantage for batching plant staff, even if you have good concrete design but uncommon or wrong procedures are practiced it will eventually result to failures.

What is the problem of over vibration of fresh concrete ?

Effect of vibration, jigging and pressure on fresh concrete, (Bulletin 3, Structural materials research laboratory, Lewis Institute, Chicago)

             For proper compaction of concrete by immersion vibrators , the vibrating part of the vibrators should be completely inserted into the concrete . The action of compaction is enhanced by providing a sufficient head of concrete above the vibrating part of the vibrators . This serves to push down and subject the fresh concrete to confinement within the zone of vibrating action. Over vibration should normally be avoided during the compaction of concrete . If the concrete mix is designed with low workability, over vibration simply consumes extra power of the vibration , resulting in the wastage of energy .

             For most of concrete mixes, over- vibration creates the problem of segregation in which the denser aggregates settle to the bottom while the lighter cement paste

tends to move upwards . If the concrete structure is cast by successive lifts of concrete pour, the upper weaker layer caused by segregation forms the potential plane of weakness leading to possible failure of the concrete structure during operation . If concrete is placed in a single lift for road works , the resistance to abrasion is poor for

the laitance surface of the carriageway . This becomes a critical problem to concrete carriageway where its surface is constantly subject to tearing and traction forces exerted by vehicular traffic.

Behavior of Fresh Concrete During Vibration/Aci 309.1R-93Recent researches concerning the vibration of fresh concrete =: Recentes recherches sur la vibration du beton frais (Translation - U.S. Army Engineer Waterways Experiment Station)

What is the function of prime coat in bituminous pavement?

The principal function of prime coat in
bituminous pavement is to protect the
subgrade from moisture and
weathering. Since the presence of
moisture affects the strength of
subgrade, the prevention of water
entry
during construction is essential to
avoid the failure of the pavement. In
cold countries, by getting rid of
moisture from subgrade, the danger
of frost heave can be minimized .
Prime coat is an asphalt which, when
applied evenly to the surface of sub-
base or subgrade, serves to seal the
surface to hinder the penetration of
moisture into subgrade. Vehicular
traffic should be avoided on the
surface sprayed with prime coat
because the traction and tearing
action of vehicles would damage this
asphalt layer .

Facts About Construction In India

# Sardar Sarovar Dam being executed by the group is the third largest in the world for volume of chilled concrete to be placed - nearly 7 million cum

# Indira Sagar a 1000 MW Power
house is the second largest surface
power house in the country.

# Nathpa Jhakri a 1500 MW Power
House is the largest underground
power house in India .

# Tehri Dam is the third tallest rockfill dam in the world, and the largest in Asia invloving placement of over 25 million cum of all types of fill material.

# Baglihar Hydroelectric project
involved construction of 30km of
project road along with three bridges.

# Brahmaputra Guide Bund
completed in a record time of 7
months.

# Baspa-II and Chamera -II projects
involved continuous concrete
shuttering for tunnel lining which is
used for the first time in the country.

# Teesta V project has been provided with Jet Grouting curtain is being provided below the coffer dams for the first time in India.

# Alimineti Madhva Reddy Irrigation project is the longest underground face to face tunnel in the world.

What is the difference between epoxy grout, cement grout and cement mortar?

   Epoxy grout consists of epoxy resin, epoxy hardener and sand/aggregates. In fact, there are

various types of resin used in construction industry like epoxy, polyester, polyurethane etc.

Though epoxy grout appears to imply the presence of cement material by its name, it does

not contain any cement at all. On the other hand, epoxy hardener serves to initiate the

hardening process of epoxy grout. It is commonly used for repairing hairline cracks and

cavities in concrete structures and can be adopted as primer or bonding agent.

Cement grout is formed by mixing cement powder with water in which the ratio of cement

of water is more or less similar to that of concrete. Setting and hardening are the important

processes which affect the performance of cement grout. Moreover, the presence of

excessive voids would also affect the strength, stiffness and permeability of grout. It is

versatile in application of filling voids and gaps in structures.

Cement mortar is normally a mixture of cement, water and sand. They are used as bedding

for concrete kerbs in roadwork.

What is the advantage of sliding bearings over roller bearings?

In roller bearing for a given movement the roller bearing exhibit a change in pressure
centre from its original position by one-half of its movement based on David J. Lee.
However, with sliding bearing a sliding plate is attached to the upper superstructure and the
moving part of bearing element is built in the substructure. It follows that there is no
change in pressure center after the movement.

Low friction sliding surfaces for bridge bearings: PTFE weave (RRL report)

What is sustainability?

There are many definitions, as with any new buzz term, people queue

up to add their defi nition in order to gain their fi ve minutes of fame!

In reality, it would appear to mean different things to different people

in different parts of the world, depending on their circumstances.

Consequently, there may never be a consensus view on its exact

meaning. However, one way of looking at sustainability is ‘ The ways

in which built assets are procured and erected, used and operated,

maintained and repaired, modernised and rehabilitated and reused

or demolished and recycled constitutes the complete life cycle of sustainable

construction activities.’

Why is construction signifi cant in the sustainability big picture?

• Over 90 million tonnes of construction and demolition waste arises

annually in the UK alone

• The construction industry spends over £200 million on landfi ll tax

each year

• 13 million tonnes of construction and demolition waste is material

that is delivered to sites but never used!

• Over 5 million tonnes of hazardous waste is produced in England

and Wales, 21% of which is produced by construction and

demolition

• Construction and demolition waste form nearly 30% of all Environment

Agency recorded fly tipping incidents

• In addition, around 40% of total energy consumption and greenhouse

gas emissions are directly attributable to constructing and

operating buildings.

Although high on the face of it, the true cost of waste is generally

around 20 times that of the costs due to the following:

• Purchase cost of materials

• Cost of storage, transport and disposal of waste

• Loss of income from selling salvaged materials.

The so-called waste hierarchy has been described as follows:

• Eliminate – avoid producing waste in the fi rst place

• Reduce – minimise the amount of waste you produce

• Re-use – use items as many times as possible

• Recover (recycling, composting, energy) – recycle what you can

only after you have re-used it

• Dispose – dispose of what is left in a responsible way.

Concrete Mix Design Secrets

                                   In order to make a concrete mix design that works, you should master all the concrete theories in combination with experiences of concreting at work. Here is some important things you need to know when design concrete mixes.
A. What do you need to know before designing concrete?

1. What are the strength requirements?

- Compressive (on cube or cylinder specimen) strength

- Flexural strength

- Tensile strength
2. What is the placing method? By pump or direct pouring?

3. How far is the job site from the batching plant?

4. What is the structure for casting? Pavement, foundation, elevated slab, etc.

5. What are the projects specification?

- Maximum or minimum cement contents

- Maximum water/cement ratio

- Slump or consistency limit

- Minimum Strength requirement @28 days

- Material specifications (what is the maximum size of aggregate?)

6. Latest testing results of materials is needed in the preliminary selection of materials and design calculation
B. What are Design Precautions and Things to Remember when design concrete mixes?

1. Increasing the sand/total aggregate ratio, increases the water requirement at the same consistency.

2. Increasing the water/cement ratio decreases the strength of concrete at the same cement content.

3. Remember that adding 5 liters of water per cubic meter increases the slump by 2.5cm.

4. Remember that adding 5 liters of water per cubic meter decreases strength by approximately 4%.

5. Always follow recommended admixture dosage.

6. Always have “control” when performing trial mixes, always perform trial mixes with another mix using the same materials. This data can be useful in diagnostics if a problem occurs.

7. Always adjust batching quantities to the actual moisture condition of the aggregates.

8. Volume tolerance for 1m3 concrete is 1 ± 0.2m3.

9. Range of normal weight concrete is from 2,200 kg/m3 to 2,400 kg/m3






WITH THANKS FROM ENGINEERING CIVIL DOT COM

Low Cost Housing

Low Cost Housing is a new concept which deals with effective budgeting and following of techniques which help in reducing the cost construction through the use of locally available materials along with improved skills and technology without sacrificing the strength, performance and life of the structure.There is huge misconception that low cost housing is suitable for only sub standard works and they are constructed by utilizing cheap building materials of low quality.The fact is that Low cost housing is done by proper management of resources.Economy is also achieved by postponing finishing works or implementing them in phases.
Building Cost
The building construction cost can be divided into two parts namely:
Building material cost : 65 to 70 %
Labour cost : 65 to 70 %
Now in low cost housing, building material cost is less because we make use of the locally available materials and also the labour cost can be reduced by properly making the time schedule of our work. Cost of reduction is achieved by selection of more efficient material or by an improved design.


Areas from where cost can be reduced are:-


1) Reduce plinth area by using thinner wall concept.Ex.15 cms thick solid concrete block wall.


2) Use locally available material in an innovative form like soil cement blocks in place of burnt brick.


3) Use energy efficiency materials which consumes less energy like concrete block in place of burnt brick.


4) Use environmentally friendly materials which are substitute for conventional building components like use R.C.C. Door and window frames in place of wooden frames.


5) Preplan every component of a house and rationalize the design procedure for reducing the size of the component in the building.


6) By planning each and every component of a house the wastage of materials due to demolition of the unplanned component of the house can be avoided.


7) Each component of the house shall be checked whether if it’s necessary, if it is not necessary, then that component should not be used.


Cost reduction through adhoc methods


Foundation
Normally the foundation cost comes to about 10 to 15% of the total building and usually foundation depth of 3 to 4 ft. is adopted for single or double store building and also the concrete bed of 6″(15 Cms.) is used for the foundation which could be avoided.
It is recommended to adopt a foundation depth of 2 ft.(0.6m) for normal soil like gravely soil, red soils etc., and use the uncoursed rubble masonry with the bond stones and good packing. Similarly the foundation width is rationalized to 2 ft.(0.6m).To avoid cracks formation in foundation the masonry shall be thoroughly packed with cement mortar of 1:8 boulders and bond stones at regular intervals.
It is further suggested adopt arch foundation in ordinary soil for effecting reduction in construction cost up to 40%.This kind of foundation will help in bridging the loose pockets of soil which occurs along the foundation.
In the case black cotton and other soft soils it is recommend to use under ream pile foundation which saves about 20 to 25% in cost over the conventional method of construction.


Plinth
It is suggested to adopt 1 ft. height above ground level for the plinth and may be constructed with a cement mortar of 1:6. The plinth slab of 4 to 6″ which is normally adopted can be avoided and in its place brick on edge can be used for reducing the cost. By adopting this procedure the cost of plinth foundation can be reduced by about 35 to 50%.It is necessary to take precaution of providing impervious blanket like concrete slabs or stone slabs all round the building for enabling to reduce erosion of soil and thereby avoiding exposure of foundation surface and crack formation.


Walling
Wall thickness of 6 to 9″ is recommended for adoption in the construction of walls all-round the building and 41/2 ” for inside walls. It is suggested to use burnt bricks which are immersed in water for 24 hours and then shall be used for the walls


Rat – trap bond wall
It is a cavity wall construction with added advantage of thermal comfort and reduction in the quantity of bricks required for masonry work. By adopting this method of bonding of brick masonry compared to traditional English or Flemish bond masonry, it is possible to reduce in the material cost of bricks by 25% and about 10to 15% in the masonry cost. By adopting rat-trap bond method one can create aesthetically pleasing wall surface and plastering can be avoided.


Concrete block walling
In view of high energy consumption by burnt brick it is suggested to use concrete block (block hollow and solid) which consumes about only 1/3 of the energy of the burnt bricks in its production. By using concrete block masonry the wall thickness can be reduced from 20 cms to 15 Cms. Concrete block masonry saves mortar consumption, speedy construction of wall resulting in higher output of labour, plastering can be avoided thereby an overall saving of 10 to 25% can be achieved.


Soil cement block technology
It is an alternative method of construction of walls using soil cement blocks in place of burnt bricks masonry. It is an energy efficient method of construction where soil mixed with 5% and above cement and pressed in hand operated machine and cured well and then used in the masonry. This masonry doesn’t require plastering on both sides of the wall. The overall economy that could be achieved with the soil cement technology is about 15 to 20% compared to conventional method of construction.


Doors and windows
It is suggested not to use wood for doors and windows and in its place concrete or steel section frames shall be used for achieving saving in cost up to 30 to 40%.Similiarly for shutters commercially available block boards, fibre or wooden practical boards etc., shall be used for reducing the cost by about 25%.By adopting brick jelly work and precast components effective ventilation could be provided to the building and also the construction cost could be saved up to 50% over the window components.


Lintals and Chajjas
The traditional R.C.C. lintels which are costly can be replaced by brick arches for small spans and save construction cost up to 30 to 40% over the traditional method of construction. By adopting arches of different shapes a good architectural pleasing appearance can be given to the external wall surfaces of the brick masonry.


Roofing
Normally 5″(12.5 cms) thick R.C.C. slabs is used for roofing of residential buildings. By adopting rationally designed insitu construction practices like filler slab and precast elements the construction cost of roofing can be reduced by about 20 to 25%.


Filler slabs
They are normal RCC slabs where bottom half (tension) concrete portions are replaced by filler materials such as bricks, tiles, cellular concrete blocks, etc.These filler materials are so placed as not to compromise structural strength, result in replacing unwanted and nonfunctional tension concrete, thus resulting in economy. These are safe, sound and provide aesthetically pleasing pattern ceilings and also need no plaster.


For more on filler materials check Filler Materials Used in Concrete


Jack arch roof/floor
They are easy to construct, save on cement and steel, are more appropriate in hot climates. These can be constructed using compressed earth blocks also as alternative to bricks for further economy.


Ferrocement channel/shell unit
Provide an economic solution to RCC slab by providing 30 to 40% cost reduction on floor/roof unit over RCC slabs without compromising the strength. These being precast, construction is speedy, economical due to avoidance of shuttering and facilitate quality control.


Finishing Work
The cost of finishing items like sanitary, electricity, painting etc., varies depending upon the type and quality of products used in the building and its cost reduction is left to the individual choice and liking.


Conclusion
The above list of suggestion for reducing construction cost is of general nature and it varies depending upon the nature of the building to be constructed, budget of the owner, geographical location where the house is to be constructed, availability of the building material, good construction management practices etc. However it is necessary that good planning and design methods shall be adopted by utilizing the services of an experienced engineer or an architect for supervising the work, thereby achieving overall cost effectiveness to the extent of 25% in actual practice
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WITH THANKS FROM ENGINEERING CIVIL DOT COM

Q:In designing concrete structures, normally maximum aggregate sizes are adopted with ranges from 10mm to 20mm. Does an increase of maximum aggregate size benefit the structures?

To answer this question, let’s consider an example of a cube. The surface area to volume

ratio of a cube is 6/b where b is the length of the cube. This implies that the surface area to

volume ratio decreases with an increase in volume. Therefore, when the 

size of maximum  aggregate is increased, the surface area to be wetted by water per unit volume is reduced.

Consequently, the water requirement of the concrete mixes is reduced accordingly so that

the water/cement ratio can be lowered, resulting in a rise in concrete strength.

However, an increase of aggregate size is also accompanied by the effect of reduced

contact areas and discontinuities created by these larger sized particles. In general, for

maximum aggregate sizes below 40mm, the effect of lower water requirement can offset

the disadvantages brought about by discontinuities as suggested by Longman Scientific and

Technical (1987).

Q:What are the major problems in using pumping for concreting works?

In pumping operation, the force exerted by pumps must overcome the friction between
concrete and the pumping pipes, the weight of concrete and the pressure head when placing
concrete above the pumps. In fact, as only water is pumpable, it is the water in the concrete
that transfers the pressure.
The main problems associated with pumping are the effect of segregation and bleeding. To
rectify these adverse effects, the proportion of cement is increased to enhance the cohesion
in order to reduce segregation and bleeding. On the other hand, a proper selection of
aggregate grading helps to improve the pumpability of concrete.

Q:In concrete compression test, normally 150mmx150mmx150mm concrete cube samples is used for testing. Why isn’t 100mmx100mmx100mm concrete cube samples used in the test instead of 150mmx150mmx150mm concrete cube samples?

A:  

   Basically, the force supplied by a concrete compression machine is a definite value. For

normal concrete strength application, say below 50MPa, the stress produced by a

150mmx150mmx150mm cube is sufficient for the machine to crush the concrete sample.

However, if the designed concrete strength is 100MPa, under the same force (about

2,000kN) supplied by the machine, the stress under a 150mmx150mmx150mm cube is not

sufficient to crush the concrete cube. Therefore, 100mmx100mmx100mm concrete cubes

are used instead to increase the applied stress to crush the concrete cubes.

For normal concrete strength, the cube size of 150mmx150mmx150mm is already

sufficient for the crushing strength of the machine.

Measures to avoid cracking in fresh concrete

               Generally, the contractor shall allow for all necessary measures to monitor and avoid cracking in fresh hydrating concrete, regardless the size or volume of the pour. Such measures shall be to the satisfaction of the Engineer and shall be such that maximum surface crack width on hardened concrete measure immediately after the pour does not exceed 0.004 times the nominal cover of the main reinforcement.

The contractor shall allow for and provide approved instrumentation for the measurement of internal temperature changes in large pours. The maximum concrete temperature at the point of delivery shall not in general exceed the lower of either 37 degree C, or 6 degree C above the prevailing shade temperature in accordance with the recommendations of ACI. The limiting internal temperature differential measured across the extreme faces of concrete mass shall not exceed 25 degrees C at any time.

            Curing of hardened concrete shall be executed in accordance with the curing specification. Generally, the element surface shall not be cooled to dissipate heat from the concrete. Curing methods, such as the wetting of heated concrete elements exposed to prolonged and direct radiation, which induce temperature gradients within the concrete mass are strictly prohibited.

        For large pours, the contractor shall allow for and take extra precautions to reduce concrete temperature gradient and to prevent the loss of surface moisture. Such measures include but are not limited to:

• Keeping all mix constituents shaded where possible to reduce their temperatures in the stockpile

• Cooling of mixing water and/or replacing part or whole of the added water with ice.

• Reducing the cement content by the use of admixtures (but not below that required for the durability)

• Using a cement with a lower heat of hydration

• Injecting liquid nitrogen after mixing of concrete

• Restring the time between mixing and placing of the concrete to not more than 2 hours

• Providing approved surface insulation continuously over all exposed surfaces to prevent draughts and to maintain uniform temperature through the concrete mass

• Initiating curing immediately after final tamping and continue until the approved surface insulation system is fully in place

• Providing shade to the concrete surface to prevent heat gain from direct radiation.

If the surface exhibits crack after compaction, it shall be retamped to close the cracks while the concrete is still in plastic stage.

Safety in Concrete Construction

While there are many things important to concrete construction, such as quality work and making a profit, safety must always be the No. 1 priority. For that reason, we are starting with safety to emphasize its importance as critical to a successful project.

Most accidents are preventable. Accidents are often due to carelessness and not thinking through what you are doing. You MUST plan for safety. The following list of things to watch out for on a concrete construction jobsite is not intended to be comprehensive. It does, however, serve to alert you to some of the more common safety in concrete construction:

* Fresh concrete can cause eye injuries and skin burns. When working with fresh concrete, wear protective clothing (a long-sleeved shirt, rubber boots, and rubber gloves) and eye protection to avoid getting fresh concrete on your skin or in your eyes. If you do get fresh concrete on your skin, wash it off with clean water. And remember that the tool clean-off bucket is not clean water.

* Finishers should wear long pants, work boots, knee pads (and use knee boards), and gloves. Immediately remove clothing that has become saturated with wet concrete.

* The simple use of personal protection equipment (PPEs–hard hats, gloves, boots, eye protection, fall protection, respirators, and so forth.) can save workers from the short-term and long-term effects of construction site conditions.

* Ear plugs must be used when the noise level gets to the point where you have to raise your voice to speak to the person working next to you. It doesn’t take much exposure to noise to permanently damage your hearing.

* Dust masks or respirators must be worn whenever there’s a chance of inhaling dirt, dust, chips, or mist; when you are cutting, grinding, or chipping hardened concrete; or when you are mixing epoxy or grout. Be sure to ask for training in the selection and use of a proper respirator. Another solution to this problem is to use wet methods or “dustless” vacuum tools.

* Ladders and stairways are major sources of injuries and fatalities among construction workers. Employers should ensure that employees are trained by a competent person in the nature of fall hazards; the correct procedure for erecting, maintaining, and disassembling fall protection systems; proper construction, use, placement, and care in handling stairways and ladders; and the maximum intended load-carrying capacity of ladders.

* Scaffolding should be solidly constructed, even if it is to be used only for a short time. Be sure uprights are uniformly spaced, plumb, and set on a good solid foundation. Use horizontal or diagonal bracing for stability. Planking should overlap the support by a minimum of 12 inches. Scaffolding should be tied to walls, buildings, or other structures. A competent person should inspect the scaffolding daily.

* The most hazardous moment when working at heights is when you are moving from place to place. That’s why you always need to be tied off to somethine substantial–something that can support a dead weight of 5000 pounds. Any time you go over a guardrail to perform work, you must be tied off. Fall protection should also be worn when working at ground level around open excavations 6 feet or more in depth. Be sure to place guardrails around all openings in decks.

* Use ground-fault circuit interruption devices at all times when using vibrators and other electrical tools. Wet concrete and water are excellent conductors. GFCI devices will prevent electrocution.

* Make sure that all wire, rope, slings, shackles, and other lifting devices are sized correctly and inspected thoroughly before using. If something breaks under a lifting load, a lot of energy can be released. A flying cable can remove an arm or leg in an instant.

Remember, accidents don’t just happen. They are more often than not the results of poor planning, improper training, or not thinking through each of your work activities. Safety in Concrete Construction always the first priority, keep it in mind.