### How To Calculate Reinforcement In Circular Slab

We Already Discussed How To Prepare Bar bending Schedule For Rectangular And Square Shape Slab. Now We Are Going To Learn How To Calculate Reinforcement For Circular Slab.

In Circular Slab Reinforcement Length Is Changed Due To Spacing Of The Reinforcement.

For This Cause Using Pythagoras Theorem To Find Out The Length Of Each Reinforcement.

L= Diameter Of Slab – Nominal Cover
For Pythagoras Theorem L1= (R2 – H12)2    H1, H2, H3 =Spacing Of Reinforcement
L2= (R2 – H22)2    R =  Radius Of Slab ( Excluding Nominal Slab )

Same Steps For Other Reinforcements

Example For Circular Slab Reinforcement Calculation

Find The Reinforcement Details Having The Diameter Of 3 m  Providing Main Reinforcement 10 mm Dia And Distributor 8 mm Dia At Spacing 200 mm And Nominal Cover Is 25 mm

✦ L = 3 – 2 ( 0.05 )
L =2.95 m

L1 = (R2 – H12) X 2
L1 = (1.4752 – 0.22)2
L1= 2.92 M

L2= (R2 – H22)2
L2= (1.4752 – 0.42)2
L2= 2.84 M

L3= (R2 – H32) X 2
L3= (1.4752 – 0.62) X 2
L3= 2.70 M

L4 = (R2 – H42) X 2
L4 = (1.4752 – 0.82) X 2
L4 = 2.48 M

L5 = (1.4752 – 12) X 2
L5 = 2.16 M

L6 = (R2 – H62) X 2
L6 = (1.4752 – 1.22) X 2
L6 = 1.72 M

L7 = (R2 – H72) X 2
L7 = (1.4752 – 1.42) X 2
L7 = 0.93 M

Total Length Of Main And Distribution Reinforcements

= L + 2(L1)+ 2(L2)+ 2(L3)+ 2(L4)+ 2(L5)+ 2(L6)+ 2(L7)
= 2.95 + 2(2.92) + 2(2.84)+ 2(2.70)+ 2(2.48)+ 2(2.16)+ 2(1.72)+ 2(0.93)
= 34.45 M

Total Weight Of 10 Mm Dia Main Reinforcement

Weight Of 10 Mm Bar Per Metre Length = 0.617 Kg /M
Weight = 34.45 X 0.617
= 21.26 Kg

Total Weight Of 8 Mm Dia Main Reinforcement

Weight Of 8 Mm Bar Per Metre Length = 0.395 Kg /M
Weight = 34.45 X 0.395
= 13.60 Kg

Total Weight Of Reinforcement For This Slab = 21.26 + 13.60
=34.88 Kg

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### Compaction Of Concrete And Methods Of Compaction Of Concrete

COMPACTION

Compaction is  one of the process of concreting. Compaction is the process of exhaust of air voids from the concrete. Because the presence of air voids make honeycomb in concrete. It will total make the concrete strength less. 1 % of air voids reduce the concrete strength approximately 6 %.

Compaction of concrete can be done either by manually of mechanically. Manual compaction is the hand compaction without use any machines just it is done by using tamping rods.

Mechanical compaction in term machines is used to make compaction they are vibrators.

The compaction process is explained below deeply.

METHODS OF COMPACTION OF CONCRETE

They are many factors consider for choosing compaction method such as reinforcement quantity and spacing, depth of concrete structure, nature of availability of machines, location of concrete structure, concrete paste consistency ,form work difficulty. Methods of compaction are below.

HAND COMPACTION METHOD

Hand compaction is done by three methods Roding, ramming and tamping.

In Roding method a 2 m long and 16 mm dia rod is used. The edges of the rod should be sharp. Thickness of concrete is 150 mm to 120 mm this method is followed

Compaction process is done throughout the concrete area.  Compaction should be proceeding until it is get fully compacted.

Another method is tamping. In tamping method 10 cm x 10 cm size cross beam is used to compact the concrete. Compaction and concrete surface level is maintained uniformly. it is used for road pavement flooring concrete and roof concrete works.

MECHANICAL COMPACTION

In mechanical compaction method vibration technique is mostly used. Vibration causes temporary liquefaction so the stored air between aggregates is removed immediately. Various types of vibrators are used. They are following below.

INTERNAL VIBRATORS OR NEEDLE VIBRATORS

Internal vibrators or needle vibrators are used for compaction of slabs , beams, columns . Vibrator needle head is having diameter of 2 cm to 18 cm and the shape of the head is cylindrical. Internal vibration is maintained accurately because over vibration time make the concrete segregation. Vibrator needle should be submerged into concrete element.

EXTERNAL VIBRATORS

External vibration method is used where internal vibration can’t be used. Because thin concrete layer or heavy reinforcement structure. External vibrators consume more power compared to internal vibrator, also it give less effect. Form work required extra strong for external vibration method.

TABLE VIBRATORS

This method is used only laboratories. Concrete is placed above table and the vibrator is fixed below table.

SURFACE VIBRATOR

Surface vibrators are fixed a long board, vibrators are activated then the vibration is transferred into concrete surface. This is not effective method. Also this method is normally used where the depth of concrete is 10 to 15 cm like roof slab and floor slab.

RESULT OF IMPROPER VIBRATIONS OF CONCRETE

Concrete structure required adequate compaction otherwise the following problems may be arise.

HONEY COMB

In adequate compaction or without compaction make honey comb in concrete structure. Honey comb is nothing but air voids between it is after settlement of concrete it is formed pit. It is totally affect concrete strength.

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### How To Calculate Volume Of Trapezoidal Footing

Individual Footings Are Mostly Used In Residential Buildings And Small Building. Usually Individual Footing Have The Shape Of Rectangular, Square And Trapezoidal.

Rectangular And Square Shape Footings Are Calculated The Volume By Simple Steps

Volume Of Rectangular Footing = Area ( A x B ) X Height
Volume Of Square Footing = Area ( A2)  X  Height

Also Trapezoidal Footing Volume Calculation is The Easy Process. I Will Explain Broadly About Volume Calculation Of Trapezoidal Footing Below.

V = H / 3 ( A1 + A2 + ( A1 X  A2 )

H = Height Of Trapezoidal Portion
A1 = Area Of Bottom Of Trapezoidal Portion
A2 = Area Of Top Trapezoidal Portion

Example Calculation For Trapezoidal Footing Volume Calculation.

V = H / 3 ( A1 + A2 + ( A1 X  A2 )
Where H= 0.3 M
A1= 1.5 X 1.5 = 2.25 M2
A2= 0.9 X 0.9 = 0.81 M2
V = 0.3/ 3 ( 2.25+ 0.81+ ( 2.25 X  0.81 )
V = 0.1 ( 3.06+ 1.35)
V = 0.441 M3

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### Derivation for d2/162

Hello every civil engineers. We know that d2/162  is the formula to find out the weight of circular steel bar per metre length . but some persons don’t know how  is arrived

So I will explain  how this formula is arrived.
Weight of circular shape steel bar per metre length = area x length x unit weight of steel

Unit weight of steel = 7850 kg/m3
Therefore,  weight = Ï€D2/4 x 1000 x ( (7850/1000 x1000 x 1000 ) )
put all the values in ‘mm’
= 0.7854 x D2 x ((7850/1000 x 1000 ))
= D2 x ( ( 6162.25 / 1000 x 1000 ) )
= D2 X 0.006162
= D2 x 1 / (0.006162 )-1
= D2 /162.28
So we take
Weight of steel bar per metre length = D2 /162

Note : why use this step 1 / (0.006162 )-1  . because 0.006162   is equal to 1 / (0.006162 )-1   but 0.006162  is six digit value It is not remember in our mind therefore this step is used for our remembrance.

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### How To Check The Quality of Sand At Site

Sand is the main part of construction. Sand used for plastering, concreting, brick work and stone masonry.

Sand is one of the main constituent of mortar and concrete, in concrete purpose of mixing of sand is to fill the voids between course aggregates. And in between voids of sand is filled with cement.

Sand is known as fine aggregate. Sand is formed buy the action of weather on the rocks. Many types of sands are used in construction field . They are river sand, Sea sand and pit sand . But cement mortar and concrete works mostly river sand is used.

The size of aggregate live between 75 microns to 4.75 mm is called fine aggregate ( sand) .
If the quality of sand is not good then the quality of work is not satisfied. Because sand is major part of concrete and mortar, so it affects work quality.

How to check quality of sand at site. Below I will provide some simple tests instead of laboratories test. because we don’t have much time and equipment at site. The following main tests are perform quickly at site to find the quality of sand.

1.Clay content test
2.Organic impurities test
3.Silt test
4.Salt test
5.Earth matters.

1. CLAY TEST.

The presence of clay in sand is to affect the actual compressive strength of concrete. The presence of clay in sand is finding out by doing below simple test at site.

Take a glass bottle fill some amount of sand then add some quantity of water. Then shake it 10 times and allow some times to settle. If the clay is present in sand. It will form layer at the water surface. So the quality of sand is not good.

In second method take some amount of wet sand in your hand then drop it. If the presence of clay in sands it make cohesion in your hand.

2. PRESENCE OF ORGANIC IMPURITIES IN SAND.

To find the presence of organic impurities in sand . Take some amount sand and add it to sodiyem hudroxide.then shake the solution for 1 minute. If the colour is change to brown. It indicate the presence of organic impurities in sand.

3. SILT CONTENT TEST

Take a glass bottle add some amount of sand and add some amount of water as well as salt into the bottle. Then shake it sometimes after shaking then allow it to settle at least 10 hours. If the silt is present in sand it will form a layers above the sand layer.

4.SALT TEST

This is simple and easy method to check the presence of salt in sand.Take small amount of sand in your hand and taste it . If the presence of salt in sand it will make salty taste.

5.EARTHY MATTERS.

Almost earthy matters are present in sand. It is the common issue. To check the presence of earthy matters in sand at site we mainly prepare following physical test.

Take partially sand in your one hand and rub it with in your other hand. Then leave the sand from your hand. If the light marks are appear in your hand it indicate the presence of earthy matters ( sand ) .

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### HOW TO CHECK QUALITY OF BRICK AT SITE

Bricks will take place major parts of the building . If the quality of brick is worst , then it can cause serious damage to whole parts of the building . Therefore it is necessary to check the minimum quality of brick before using of construction. there some simple field tests that we can conduct in the field in order to check the quality of brick

THE FOLLOWING TESTS ARE

1. Water absorption test
2. Size colour and shape
3. Dimension
4. Hardness
5. Soundness
6. Structure
7. Efflorescence test

WATER ABSORPTION TEST

For the proposer of find out the water absorption test. Taken for five procurement bricks . Then weight it in dry condition and the average dry weight is calculated.

Then immerse this five bricks into water for 24 hours. After 24 hours taken it out and weight it again in wet condition and calculate the average weight of five bricks.

The difference between average weights of the of wet bricks to the dry brick Indicate the water absorption of brick . The water absorption does not exceed 20 % of weight of dry brick .

If the brick tend to absorb more water than its limits. it will cause decrease the strength of mortar because the brick absorb water from mortar.

SIZE COLOUR AND SHAPE.

The colour of well burnt brick is red or copper . other than this two colors that bricks are over burnt or under burnt . So this bricks are not used for any construction works.

Size of brick is uniform does not bulk in any sides. More bulk need more mortar so the cost of construction is exceed actual cost.

DIMENSION

Edges of bricks should be sharp. Variation in dimensions are allowed only with this limit +- 3 % tolerance for first class and +,- 8 % tolerance of second and third class bricks.

HARDNESS AND SOUNDNESS

Good quality brick so hard. In this test just scratch a brick with your finger nail .if the brick is good then there is no impression on the brick.

Struck one brick to another brick. if a good brick make clear ring sound. Otherwise the quality of brick is low.

Take  a brick and drop it above the hard floor from 1.2 m to 1.5 m height . if the brick is  good then the brick not break.

EFFLORESCENCE

If sodium and potassium salts are presence in the brick this will cause white patches on the brick surface that is called efflorescence.

Brick production soil should be free from sulphate, potassium and sodium . When bricks are contact to water this sale dissolved and this will cause major defects on plastering surface. So this bricks are not allowed for construction.

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### METHOD OF DEWATERING AT CONSTRUCTION SITE

WHAT IS DEWATERING

For construction of building, dams, other structures it is required excavation for foundation most of the times foundation  depth may be below the water table at that time seepage of water disturbed the work therefore seepage of water is removed from the excavation that process is dewatering.
Dewatering process of each site varies depends upon depth of excavation geological conditions and characteristics of soil.

METHODS   OF DEWATERING

THE FOLLOWING METHODS OF DEWATERING SYSTEMS ARE APPLIED AT CONSTRUCTION SITE.

1.pumping
2, providing sumps and side chairs
3.well point systems
a. single stage system
b. multible stage system
c, vacuum system
4, chemical grouting
5. freeing process
6.electro osmosis process

PUMPING

For these methods pumps are installed in excavated foundation trenches at suitable place which point is water stagnation. The stagnation water is removed from foundation trenches through the pumping system.

THE PUMPING SYSTEM MUST HAVE THE FOLLOWING FEATURES

The pump should be small and portable because easily moved where we want to dewatering

The pump is must have the capable of removing of water mixed with sand clay etc.

The pump should be made of quality materials and strong
The efficiency of the pump require good
Providing sumps and side drains

In this method drains are formed along the longitudinal direction of the foundation trench. For every 40m to 60m distance sumps are constructed. small gradient is provided in drain for easy drain of water to the sumps. Sump is collected water from drain, then this water is pumped through centrifugal pump. The sump is formed in semi circular shape with diameter 0.2m and above.

WELL POINT SYSTEM

Well point dewatering method series of wells are formed into required depth in excavated area from where the water has to be released  out
In this wells are formed in series line in 2m spacing.

WELL POINT

Well point is a pipe it is one metre long and 0.4m to 0.5m in diameter. In bottom end of the well point pipe a valve is fitted. the valve is opened when water is pumped into pipe and it is closed when suction is applied pipe and it is closed when suction is applied into well point pipe. Inflow pipe is provided into well point pipe. Inflow pipe is provided holes and these holes are covered with mesh screen.

RISER

Riser is the vertical pipe driven into the ground. riser is connecting well point pipe to the flexible swipe pipe. Responsible of the riser is discharging the water away from the excavated depth. Top end of the riser a flexible swing pipe is connected. The purpose of swinger pipe is just for understands what being pumped. Dia of riser is 40 mm to 50 mm.

Header is the horizontal pipe. all the swinger pipes are connected with this header pipe. Header pipe is connected to water pump.

PROCESS OF WELL POINT SYSTEM.

The area to be dewatered is surrounded by number of well points. The spacing of dewatering system is depends upon nature of soil condition and ground water table. minimum spacing is 1m.

Water is forced through the well point at the rate of 20 to 25 litters per second. Well point is automatically inserted into the ground with the help of water pressure and without process of driving.

When the well point is reached the required depth water forces is stopped . for this high pressure of water make annular spacing around the well point.

Then around the spacing of well point system. Filter media of sand and gravel is filled for the purpose of preventing the entry of soil into the system. then forcing of water down is stopped and suction is applied through the well point with the help of water pumping system. Water is removed from the ground and thus the original water table is lowered.

The work of excavation is started. when the work of excavation is completed this suction process is stopped . then water is pumped force  into the pipe this make around the well point loosen the ground and the system is recovered.

SINGLE STAGE SYSTEM

The water lifting depth is about 5m or so . The depth of excavation is not to exceed 5m in this system the whole setup is not sifted until the excavation process is done.

MULTIPLE STAGE SYSTEM.

When the depth of excavation is exceed 5 metres below the water table second stage well point is provided. For first stage well point is provided up to 5m depth . In the second stage well point is provided next 5m depth from first stage excavation surface.

CHEMICAL GROUTING

This method is similar to well point system first one chemical is forced through well point system and it will be stopped when the desired depth is reached. Then another chemical is forced through pipe and the pipe is withdrawn. Two chemical are react and make the soil solidify.

TYPES OF CHEMICAL GROUTING

INORGANIC CHEMICAL – include sodium silicate lingo chemical and lingo sulphate etc.

ORGANIC CHEMICALS- include epoxy resin polyester and other resins.

FREEZING PROCESS

In this method. the surrounding area of excavation is frozen and thus a solid wall of frozen earth formed. The process of freezing process is cold water is applied into earth the cold water freezing the ground water and thus solid soil wall is formed with the help of freezing water.

PROCEDURE

A refrigeration plant is installed near to the site.

A 10 cm to 15 cm dia pipe is inserted into ground desired depth . the spacing between the pipe is 1m to 1.5m . the pipes are closed at bottom.

Another pipe of 2.5 cm to 5 cm pipe is inserted into larger dia pipe. The bottom of the pipe is not closed.

All the small pipes are connected to the single pipe and closed circuit is formed. Larger pipes are connected similarly.

The cold liquid temperature about -23 0 c to 300  c is passed through larger pipe and come out through smaller pipe.

This cause the ground to freeze around the pipe. A solid wall of frozen earth of considerable thickness is formed.

ELECTRO OSMOSIS PROCESS

In this process two electrode one positive and other one is negative are driven in the saturated soil. Then electricity is applied tow electrodes.

The water contained in soil is repeated by positive electrode and is attracted negative electrode the water can be collected in it and then it can be pumped out.

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