Distribution bars (bars provided against shrinkage and temperature)
Maximum spacing Between Individual Bars:
1) The maximum diameter of bar used in slab should not exceed 1/8 of the total thickness of slab.
2) For main bars, maximum spacing is restricted to 3 times effective depth or 300 mm whichever is smaller.
3) For distribution bars, the maximum spacing is specified as 5 times the effective depth or 450 mm whichever is smaller.
Hence, diameter of bar, thickness of slab, effective depth and spacing are co-related.
Effective depth = depth of slab – clear cover- half of diameter of bar
Minimum Distance Between Individual Bars:
Minimum Distance Between Individual Bars & main reinforcing bars shall usually be not less than the greatest of the following:
The following shall apply for spacing of bars:
1) The diameter of the bar if the diameters are equal,
2) The diameter of the larger bar if the diameters are unequal and
3) 5 mm more than the nominal maximum size of coarse aggregate.
Clear cover (Nominal Cover) shall be kept in mind while calculating the above parameters.
Source: IS-456 (2000)
Cutting Length Of Bent Up Bars In Slab:
As a site engineer, you need to calculate the cutting length of bars according to the slab dimensions and give instructions to the bar benders.
For small area of construction, you can hand over the reinforcement detailing to the bar benders. They will take care of cutting length. But beware, that must not be accurate. Because they do not give importance to the bends and cranks. They may give some extra inches to the bars for the bends which are totally wrong. So it is always recommended that as a site engineer calculate the cutting length yourself. In this article, we will discuss how to the calculate length for reinforcement bars of slab. Let’s start with an example.
Example:
Where,
Diameter of the bar = 12 mm
Clear Cover = 25 mm
Clear Span (L) = 8000
Slab Thickness = 200 mm
Development Length(Ld) = 40d
Calculation:
Cutting Length = Clear Span of Slab + (2 x Development Length) + (2 x inclined length) – (45° bend x 4) – (90° bend x 2)
As you can see there are four 45°bends at the inner side (1,2,3 & 4) and two 90° bends ( a,b ).
45° = 1d ; 90° = 2d
Cutting Length = Clear Span of Slab + (2 X Ld) +(2 x 0.42D) – (1d x 4) – (2d x 2) [BBS Shape Codes]
Where,
d = Diameter of the bar.
Ld = Development length of bar.
D = Height of the bend bar.
In the above formula, all values are known except ‘D’.
So we need to find out the value of “D”.
D = Slab Thickness – (2 x clear cover) – (diameter of bar)
= 200 – (2 × 25) – 12
= 138 mm
Now, putting all values in the formula
Cutting Length = Clear Span of Slab + (2 x Ld) +(2 x 0.42D) – (1d x 4) – (2d x 2)
= 8000 + (2 x 40 x 12) +(2 x 0.42 x 138) – (1 x 12 x 4) – (2 x 12 x 2)
∴ Cutting Length = 8980 mm or 8.98 m.
So for the above dimension, you need to cut the main bars 8.98 m in length.
Beam Reinforcement Details:
Beams are essentially provided with main reinforcement on the tension side for flexure and transverse reinforcement for shear and torsion.
Tension Reinforcement:
The minimum tension reinforcement is denoted by the given formula
As = (0.85bd/fy)
Where, As = Minimum quantity of tension reinforcement
b = breadth of the beam,
d = effective depth,
fy = strength of reinforcement in N/mm2
D = Overall depth of the member.
The minimum tension reinforcement (As) should not be less than the value of (0.85bd/fy).
And the maximum quantity of tension reinforcement should not be greater than the value of 0.04bd.
Compression Reinforcement:
Stirrups should be provided with the compression reinforcement in beams for lateral restraint.
The maximum quantity of compression reinforcement should not cross 0.04bd.
Side Face Reinforcement:
At the point when the depth of web or rib in a beam crosses 750 mm, the side face reinforcement of cross sectional area at the very least 0.01% of the web zone is to be given and disseminated similarly on two appearances and the dividing of the bars not cross 300 mm thickness whichever is smaller.
Transverse Or Shear Reinforcement:
The minimum quantity of shear reinforcement is computed by the given formula
Asv ≥ (0.4bSv/0.87fy)
Where Asv = Total cross-sectional area of stirrups legs in shear.
Sv = Spacing of stirrups along the length of the member.
b = The breadth of the beam or the web in a flanged member.
fy = Characteristic strength of stirrups reinforcement, which should not cross 415 N/mm2.
For vertical stirrups, the maximum spacing of shear reinforcement should not cross 0.75d.
and for inclined stirrups d is considered as 450.
The maximum limit of spacing is 300 mm.
Slab Reinforcement Details:
The details of slab reinforcement are given below.
Singly Reinforced Beam:
The beam that is longitudinally reinforced only in tension zone, it is known as singly reinforced beam. In Such beams, the ultimate bending moment and the tension due to bending are carried by the reinforcement, while the compression is carried by the concrete.
Practically, it is not possible to provide reinforcement only in the tension zone, because we need to tie the stirrups. Therefore two rebars are utilized in the compression zone to tie the stirrups and the rebars act as false members just for holding the stirrups.
Doubly Reinforced Beam:
The beam that is reinforced with steel both in tension and compression zone, it is known as doubly reinforced beam. This type of beam is mainly provided when the depth of the beam is restricted. If a beam with limited depth is reinforced on the tension side only it might not have sufficient resistance to oppose the bending moment.
The moment of resistance can not be increased by increasing the amount of steel in tension zone. It can be increased by making the beam over reinforced but not more than 25% on the strained side. Thus a doubly reinforced beam is provided to increase the moment of resistance of a beam having limited dimensions.
Besides this, doubly reinforced beams can be utilized under following conditions,
When the outside load is alternating, that means the load is acting on the face of the member.
The load is eccentric and the eccentricity of the load is changing from one side to another side of the axis.
The member is subjected to a shock or impact or accidental lateral thrust.
Singly Reinforced Beam:
Singly Reinforced Beam Design
When the area of steel is provided in tension zone only i.e the reinforcement is given only in tension zone, it will be known as singly reinforced beam.
In singly reinforced beam, the reinforcement carries the ultimate bending moment and tension due to bending of the beam. On the other hand, the concrete carries the compression of that beam.
The actual NA of singly reinforced beam is calculated by the below given formula.
Generally, these types of beams are balanced, under reinforced or over reinforced type.
In practical work, there is no such way to use reinforcement only in tension area, because we have to bind the stirrups. So, in the compression zone, always two rebars are used to bind the stirrups where, the rebars just withstand those stirrups.
Singly Reinforced Beam Design Procedure:
1. Determine the value of N by the following formula:
[Where N = Critical N.A Constant.]
2. Find the value of J.
Where J = Lever arm constant
3. Determine the moment of resistance coefficient
4. Select appropriate breadth (b) and equate the bending moment and moment of resistance with the effective depth of the section.
The quantity of steel depends on the type of structure, not on concrete volume. The quantity of steel varies from member to member such as beam, column, slab, footings etc. Because the load carrying capacity of different members is different. In this article, I will discuss steel requirements for different RCC elements.
Let’s take an example:
A column is to build with 4% concrete volume. So the steel required for that column is
= (4/100)x1 x 7850 [ Density of 1m³ steel = 7850 kg]
We can follow the following thumb rules for different RCC members.
1. Steel requirement for RCC beam = 1 to 2% or 78.5 kg to 157 kg/m³
2. Steel requirement for RCC column = 0.8 to 6 % or 62.8 kg to 471 kg/m³
3. Steel requirement for RCC Slab = 0.7 to 1% or 55 kg to 78.5 kg/m³
4. Steel requirement for RCC Lintel = 0.7 to 1% or 55 kg to 78.5 kg/m³
5. Steel requirement for Foundation = 0.5 to 0.8% or 34.25 to 62.8 kg/m³
Note:
These are just approximate estimates, If you require detailed estimates, I suggest you follow BBS for different elements.
Doubly Reinforced Beam:
R.C.C beam which comprises of reinforcement both in tension zone, as well as compression zone is called doubly reinforced beam.
Doubly reinforced beam is generally adopted in following conditions:
When the size of the beam is confined.
When the section of the beam is subjected to inversion stress.
When the beam is nonstop more than a few backings.
Critical NA of a doubly reinforced beam is calculated by this given formula:
n = mcd/t+mc
Where, n = Critical NA
m = Modular ratio.
c = Max. compressive stress in the concrete.
d = Effective depth of the beam.
t = Allowable tensile stress in steel.
Actual NA is determined by taking moments of the effective zone about the centroid of the effective segment.
bn2/2 + (1.5m-1)Ac(n-dc) = mAt(d-n)
Where, b = Breadth of the beam.
Ac = Area of compressive steel.
dc= Centre of gravity (c.g) of compressive region of steel from external fibres.
At = Area of tensile steel.
Doubly reinforced beam is inefficient in steel, as steel is utilized as a part of compression zone is never stressed to its full limit. Compressive stress in steel relies on the compressive stress in concrete at that level. Stress in concrete (C1) at level of compressive steel can be determined by using following equation.
C1 = c(n-dc)/n
Moment Of Resistance In Doubly Reinforced Beam:
The total moment of resistance is calculated by adding the moments of following two couples.
1. Couple (M1) having tensile steel A and compressive concrete.
2. Couple (M2) having extra tensile steel At and compressive steel.
The moment of resistance M = M1 + M2 = (bnc/2)(d-n/3) + (1.5-1)AcC1(d-dc)
Where C1 = c(n-dc/n)
Balanced Section:
The section in which the quantity of steel is just sufficiently provided that the concrete in compression zone and steel in tension zone reaches to their permissible stresses simultaneously is called balanced section.
In this section, the critical depth is equal to its actual depth. i.e n = Na = Nc
Under Reinforced Section:
In this section, the quantity of steel is not adequate to make the extreme concrete fibers in the compression area to get compressed to their highest permissible stress.
In this section, the quantity of steel is not adequate to make the concrete to get compressed in compression area to their highest permissible value. That means the steel is provided less than that a balanced section is required. In under reinforced section, the depth of actual Na is less than the critical Na.
i.e; Na<Nc.
Over Reinforced Section:
In this section, the quantity of steel in tension zone is greater than the quantity of steel required to make compressive zone concrete to get compressed to their most extreme admissible value. In other words, when the extreme compressive stress in concrete achieves its allowable limit, the comparing tensile stress in steel will be not as much as its permissible value.
So in overreinforced section, the depth of actual Na is greater than the critical Na.
i.e; Na > Nc
Development Length Of Bars:
The development length can be characterized as the length of the bar required for transferring the stress into the concrete.
A development length is the quantity of the rebar length that is actually required to be enclosed into the concrete to make the desired bond strength between two materials and furthermore to produce required stress in the steel at that area.
The development length Ld of a bar is calculated as following
Where d = diameter of the bar.
σs = stress in the bar at the section considered as design load.
Ï„bd = Design bond stress.
In the below example, we need 10 db development length at the end section so that the concrete-steel bond stays continuous. The bar is bent because there is no space available at the end section. You can see that only 90-degree configuration is used but here we can use more configuration like that.
The ascertained compression or tension reinforcement at every section of an RC member is produced on both sides of that section by hooks embedded length or mechanical gadgets.
If the restraining section of concrete is relatively thin and unable to withheld the position of highly stressed bars the development length is given. In this way, the splitting of bars from concrete is avoided.
The additional embedded length is known as development length. The main aim is to give proper and settled support to the bars.
In compression reinforcement, hooks are not provided but where no or little space is available for extra length, hooks can be used for restraints.
Development Length:
Development length is the length of bar required for transferring the stress into concrete.
In simple words, the quantity of the rebar length that is actually required to be embedded into the concrete to achieve the desired bond strength between concrete and steel by producing required stress for the steel in that area.
The formula for development is given below:
Development length (Ld) =d x σs/τbd
Where
d = Diameter of the bar.
σs = Stress in the bar at the section considered as design load.
Ï„bd = Design bond stress.
LAP LENGTH:
Lap length is the overlapping length of two bars side by side which gives required design length. In RCC structure if the length of a bar is not sufficiently available to make design length, lapping is done.
Suppose we need to construct a building of 20 m height. But there is no 20 m single bar available in the market. The maximum length of rebar available in the market is usually 12 m, so we need to join two bars of 12 m to get 20 m bar.
The lap length varies from member to member.
Lap length for tension members = 40d
Lap length for compression members = 50d.
Where, d = Diameter of bars.
In the below image you can see some amount of rebar is left for future construction. This extra rebar will be needed for tying bars of column. This extra length of rebar is called lap length.
Lap Length:
Development length and lap length are two important terms in reinforcement. But many of us get confused with the difference between development length and lap length. In our previous article we have already discussed what is development length of bars, today we will discuss what is lap length of bars.
During placing the steel in RC structure if the required length of a bar is not sufficiently available to make a design length then lapping is done. Lapping means overlapping of two bars side by side to achieve required design length.
Suppose, we need to build a 100 feet tall column. But practically 100 ft long bar is not available and it is also not possible to cage. Therefore we need to cut the bars in every second story. Now, we need to transfer the tension forces from one bar to the other at the location of discontinuity of bar. So we have to provide the second bar closed to the first bar that is discontinued and overlapping is to be done. The amount of overlapping between two bars is known as lap length.
In case of RCC structure, if the length of reinforcement bars need to be extended, splicing is used to join two reinforcement bars for transferring the forces to the joined bar.
Lap Length Formula:
Lap length in Tension:
The lap length including anchorage value of hooks shall be
1. For flexural tension – Ld or 30d whichever is greater.
2. For direct tension – 2Ld or 30d whichever is greater.
The straight length of lapping shall not be less than 15d or 20 cm.
Lap length In Compression:
The lap length in compression shall be equivalent to the development length in compression computed but not less than 24d.
For Different Diameter Bars:
In case of bars having different diameter are to be spliced, the lap length is calculated on the basis of smaller diameter bar.
Lap Splices:
Lap splices should not be used for the bars having larger dia than 36 mm. In that case, welding should be done. But if welding is not practicable then lapping may be permitted for the bars larger than 36 mm dia. Additional spirals should be provided around the lapped bars.
Lap length For Concrete Of 1:2:4 Nominal Mix:
Lap length in tension (for plain Grade-1 MS bar) including anchorage value is 58d. So eliminating the anchorage value the lap length = 58 – 2*9d = 40d
where 9d = hook allowance of bars up to 25 mm and k=2
Lap length for compression bar is equal to the value of development length calculated i.e 43.5d.
Lap length For M20 Concrete:
Columns – 45d
Beams – 60d
Slabs -60d.
So if we need to lap 20 mm dia column bars, we have to provide a minimum lap of 45 * 20 = 900 mm.
Basic Rules For Design Of Column:
The basic rules for designing of columns are listed below:
A. Longitudinal Steel:
1. The cross-sectional area of longitudinal steel in a column shall not be less than 0.8 and not more than 6% of the gross-sectional area of the column.
In places where bars from a column below have to be lapped with those in the column to be designed, the maximum percentage of steel should not exceed 4%.
2. The diameter of longitudinal bars should not be less than 12 mm and should not be more than 50 mm.
3. Round columns and columns having helical binders should have at least bars.
4. The minimum cover of concrete to the outside of longitudinal bars shall be 4 cm or the diameter of the bar whichever is greater. In case where the maximum dimension of a column does not exceed 20 cm and the diameter of the longitudinal bars does not exceed 12 mm, the cover of 2.5 cm may be used.
5. Where it is necessary to splice the longitudinal bars, the bars shall overlap for a distance of not less than 24 times the diameter of the smallest bar.
6. The spacing of bars measured along the periphery of the column shall not exceed 300 mm.
B. Transverse Reinforcement:
Transverse steel may be provided either in the form of lateral ties or helical bars (spiral).
1. The minimum diameter of the ties or helical reinforcement shall not less than 1/4th the diameter of the largest longitudinal bars and in no case less than 5 mm.
2. The maximum diameter of the ties or helical steel should preferably be not more than 12 mm and 25 mm respectively.
3. The pitch of the ties should not be more than the least of the following
a) Least lateral dimension of the column.
b) 16 times the diameter of the smallest longitudinal bar nearest to the compression face of the member.
c) 48 times the diameter of the tie.
4. Pitch of the helical reinforcement should not be more than least of the following:
a) 1/6th the diameter of the concrete core.
b) 75mm.
5. The least spacing of the lateral ties may be 150 mm and for spirals, the minimum pitch shall be 25 mm or three times the diameter of the helical bars whichever is greater.
Column Reinforcement Details:
Generally, concrete columns consist of square, rectangular or circular cross sectional area. Columns are essentially required with the primary longitudinal reinforcement and lateral ties to avoid buckling of the primary bars.
The details of minimum and maximum limits of reinforcements, minimum no. of bars, the size of bars, cover requirements, diameter, and spacing are given in the above picture.
In case of RC columns consisting helical ties, 6 basic longitudinal reinforcement must be given to the helical support. The spacing of the longitudinal reinforcement should not be more than 300 mm.
The maximum and minimum values of the pitch of helical reinforcement is restricted to 75 mm and 25 mm. Helically reinforced portions have considerably greater load conveying limit than those have common lateral ties because of higher degree control of concrete in the center.
The safe axial load carrying capacity of different types of columns can be determined as follows:
1. Short Column Having Lateral Ties Or Binders:
Where σcc = Permissible stress in concrete in direct compression.
Ac = (A-Asc) Net cross-sectional area of concrete excluding any finishing material and reinforcing steel.
σsc = Permissible compressive stress for column bars.
Asc = Cross-sectional area of longitudinal steel.
P = Safe load carrying capacity of the column.
2. Short Column With Helical Reinforcement:
These columns are reinforced with closely and uniformly spaced spiral reinforcement in addition to longitudinal steel. These columns are also known as circular columns and are generally spirally reinforced. Sometimes, individual loops may be used instead of spirals. Columns having helical reinforcement shall have minimum 6 longitudinal bars.
For columns with helical reinforcement, the permissible load satisfying the requirements shall be adopted as 1.05 times the permissible load of a similar member of lateral ties.
Note: the ratio of the volume of helical reinforcement to the volume of the core shall not be less than
Where Ag = Gross area of the section.
Ac = Area of the core of the helically reinforced column which is measured to the outside diameter of the helix.
fck = Characteristic compressive strength of concrete.
fy = Characteristic strength of the helical reinforcement but not more than 415 N/mm2
Pitch Of Helical Reinforcement:
Helical reinforcement should be in the regular form having the turns of the helix evenly spaced and the ends should be anchored accurately by giving one and half extra turns of the spiral bar.
The pitch of the helical turns shall not be greater than 75 mm, nor more than 1/6th of the core diameter of the core diameter of the column. nor less than 25 mm, nor less than three times the diameter of the steel bar forming the helix.
Diameter Of The Helical Reinforcement:
The diameter of the helical reinforcement shall be not less than 1/4th the diameter of the largest longitudinal bars and in no case less than 5 mm.
3. Long Columns:
When the ratio of the effective length and the least lateral dimension of a column exceeds 12, the column will be considered as long column. In the design of such columns considering the factor of buckling, lower value of working stresses in steel and concrete is adopted, by multiplying the general working stresses by the reduction coefficient Cr.
So for long column,
Safe stress in concrete = Cr × Corresponding safe stress for short column and
Safe stress in steel = Cr × Corresponding safe stress for short column.
The reduction coefficient can be obtained by the following formula
For more exact calculation
Where Cr = Reduction coefficient.
lef = Effective length of the column.
b Least lateral dimension.
rmin = Least radius of gyration.
Permissible Stresses In RCC Columns:
1. Permissible stresses in concrete (IS: 456-1978) :
M15 – 4 N/mm2
M20 – 5 N/mm2
M25 – 6 N/mm2
2. Permissible Stresses In Steel:
For column bars compression
σsc = 130 N/mm2
For helical reinforcement
σsh = 100 N/mm2.
Design Of Columns As Per ACI:
1. Maximum and Minimum Reinforcement Ratio:
The minimum reinforcement ratio of 1 % is to be used in tied or spirally reinforced columns. This minimum reinforcement is needed to safeguard against any bending, reduce the effect of shrinkage and creep and enhance ductility of columns.
2. Minimum Number of Reinforcing Bars:
Minimum four bars within rectangular or circular sections; or one bar in each corner of the cross section for other shapes and a minimum of six bars in spirally reinforced columns should be used.
3. Clear Distance between Reinforcing Bars:
For tied or spirally reinforced columns, clear distance between bars should not be less than the larger of 150 times bar diameter or 4 cm.
4. Concrete Protection Cover:
The clear concrete cover should not be less than 4 cm for columns not exposed to weather or in contact with ground. It is essential for protecting the reinforcement from corrosion or fire hazards.
5. Minimum Cross-Sectional Dimensions:
For practical considerations, column dimensions can be taken as multiples of 5 cm.
6. Lateral Reinforcement:
Ties are effective in restraining the longitudinal bars from buckling out through the surface of the column, holding the reinforcement cage together during the construction process, confining the concrete core and when columns are subjected to horizontal forces, they serve as shear reinforcement. Spirals, on the other hand, serve in addition to these benefits in compensating for the strength loss due to spalling of the outside concrete shell at ultimate column strength.
7. Ties:
For longitudinal bars, 32 mm or smaller, lateral ties 10 mm in diameter should be used. In our country and in some neighboring countries, ties of 8 mm dia are used for column construction.
Minimum Cover For Reinforcement in Cast-In-Place Concrete:
The clear cover is the distance between the outer surface of concrete to the outer surface of the nearest bar.
Clear cover varies in different conditions. The clear cover for cast-in-place concrete is given in the below table.
Sl. No.
Conditions
Minimum cover (inches)
1
Concrete cast against and permanently exposed to earth
3
2
Concrete exposed to weather or earth
No. 6 to no. 18 bar
2
No. 5 bar, W31 or D31 wire
and smaller
1 ½
3
Concrete unexposed to weather or in contact with the ground.
Slabs, Walls, and Joist
No. 14 and no. 18 bar
1 ½
No. 11 bar and smaller
¾
Beams and columns
Primary reinforcement, ties, stirrups, and spirals
1 ½
Shells and folded plate members.
No. 6 bar and large bars.
¾
No. 5 bar, W31 or D31 wire and smaller.
½
4
Concrete tilt-up panels cast against a rigid horizontal surface, like concrete slab.
No. 8 bar and smaller.
1
No. 9 to no. 18 bar.
2
Stress Distribution Between Steel & Concrete:
As per the basic assumption, plain RCC section before bending remains plain after bending. It is clear that in the flexural member at a specific point the compressive stress and the tensile stress are proportional to their distance from N.A ( Neutral axis). As the bond between concrete and steel becomes excellent, strains induced in concrete as well as in the steel will be equal.
Let strain in concrete and strain in steel be designated by ec and et
According to Hooke’s law
Stress/Strain = Moduli of elasticity
Therefore
Where t = Permissible tensile stress in steel.
Es = Moduli of steel.
and
c’ = Stress in concrete in level with tensile steel.
Ec = Modulus of concrete.
But
Or
From the above expression, it is evident that the stress in the steel is m times the stress in concrete surrounding it.
Water Cement Ratio:
Water cement ratio can be defined as the ratio of the volume of water to the volume of cement used in a concrete mix. Water has a great role on the strength and workability of concrete. After lots of experiments it has been found that for a specific proportion of materials in a concrete mix, there is a certain amount of water that gives maximum strength.
A slight change in the amount of water causes much more differences in the strength of concrete. If less water is used, the resultant concrete will be nearly dry, hard to place in the form and may create difficulties in compaction. Besides this, with less water proper setting will not be guaranteed and thus the strength of concrete get reduced considerably.
On the other hand, if water is used more, it may develop larger voids and honey-combing in the set concrete, in this way decreasing its density, durability, and strength. Hence, water cement ratio attends an important role in producing concrete of required strength. The lower the ratio, the greater is the strength of concrete.
Required Water-Cement Ratio ( British Standard Specifications):
Formwork is a temporary but rigid structure in which the cast in situ concrete is laid for casting the members to required shape. It is also known as centering or shuttering.
Formwork is placed at its right position before pouring the fresh concrete in it. Poured concrete is then compacted and permitted to solidify to gain strength. The formwork is permitted to stay in position till the concrete achieve enough strength to resist the stresses coming on it without the assistance of the formwork. After this, the formwork is removed.
The formwork is permitted to stay in position till the concrete achieve enough strength to resist the stresses coming on it without the assistance of the formwork. After this, the formwork is removed.
A good formwork should satisfy the following requirements:
1. It should be adequately strong to withstand an extensive variety of dead and live loads. For instance, self-weight, weight of reinforcement, weight of wet concrete, loads of workers, and any other loads during and after casting of concrete.
2. It should be inflexibly built and efficiently propped and supported to hold its shape without undue deflection.
3. The joints in the formwork should be tight enough to prevent leakage of cement grout.
4. The formwork should be created in such a way that it may allow the evacuation of different parts in the desired sequence without shaking or damaging the concrete.
5. The material of the formwork should be inexpensive, easily accessible and can be reused for several times.
6. The surface of the formwork should be plain and smooth, and set properly to the desired line and level.
7. The material of the formwork should not bend or get perverted in presence of sun, rain or water at the time of concreting.
8. It should be lightweight.
9. It should be easy to remove.
Underwater Concreting:
The degradation of concrete under water is a serious and troublesome matter. Underwater concreting is necessarily adopted in marine works and in deep foundations. Due to the continuous loss of cement and segregation of concrete, placing of concrete under water becomes very difficult to unreinforced construction. Today I am going to discuss tremie method that is mostly adopted for concreting under water such as cofferdams, caissons, and such other dewatering method.
Underwater Concreting By Tremie Method:
Tremie method is one of the most common methods that is used for concreting under water. In this method, a long steel pipe (named as tremie) having a diameter of 15 to 30 cm is inserted vertically into the water. The pipe should be long enough that it reaches to the bed of water keeping its one end above the water level.
The tremie is then fitted with a hopper at the upper end for pouring concrete inside the pipe. The lower end of the tremie pipe must be closed with a check valve before inserting it into the water. After that, freshly mixed concrete is poured with the help of hopper. When the concrete is poured, it displaces the air and water present in the pipe and finally reaches to the bed.
During the time of concreting the tremie is continuously lifted keeping the lower end of the tube in the concrete that is already poured. To reduce the extra loss of cement under water, rich concrete mix should be always used. Thus concrete laid underwater should never be compacted or consolidated.
Hot Weather Concreting:
We know that cement possesses faster rate of hydration in hot weather. The rate of hydration of cement increases with the increases in temperature. Generally, 10° C – 27° C is considered as most suitable for hydration.
At higher temperature, the concrete may start to set before placing and compacting in position. Therefore the concrete should be placed as fast as possible after mixing. During concreting in hot climatic condition, the concrete may have to be cooled to protect from ill effects. To achieve this, cooled water and aggregates can be used while preparing the concrete.
Precautions Before, During & After Concreting:
1. The main aim is to decrease the temperature of the concrete itself and furthermore the forms where it will be placed.
2. If possible, mixer machine should be installed below the shade.
3. If concrete is to be transported to a longer distance, concreting materials may be mixed first in dry condition and the water may be included at the job site just before placing the concrete in position.
4. To protect the water content from evaporation, always deep containers should be used for the transportation of concrete.
5. After placing, wet burlap should be used to cover the concrete. It will help to protect coolness and moisture of concrete.
6. Curing should be started immediately as soon as the concrete starts hardening.
7. Retarding admixtures can also be used while adding the water in the mix to slow down the setting action of concrete.
8. Ice cubes or refrigerator can be used to cool the water to be used. But ensure that ice cubes are totally melted and there are no solid ice crystals present in the concrete.
Cold Weather Concreting:
During hydration process temperature plays a great role on the rate of strength development. When the temperature is just above the freezing point the rate of hydration becomes very slow. The hydration that occurs in 1 day at a temperature of 20°C may take up to 7 days at 4°C temperature. Thus the time for removing formwork also extends. So some precautions should be taken while concreting in cold weather.
Precautions For Cold Weather Concreting:
1. Hot water (60°C) and aggregates (15°C) should be mixed before adding cement content in the mix.
2. Concrete should be placed at a temperature of 4° -5° C. After placing and compacting the temperature should be kept at 2°C until it becomes hardened.
3. To prevent flashing, cement should not be added alone with water.
4. Do not use frozen aggregates in the mix.
5. Accelerating admixtures (mainly calcium chloride) can be used to increase the internal temperature of concrete.
6. Damaged concrete due to frost action should be eliminated.
7. Formwork should be cleaned and free of snow or ice.
8. To keep the temperature at constant and to protect the concrete from frost action, the concrete surface should be covered with dry blankets of cement bags, staws, tarpaulins etc.
9. Curing should be extended for a longer period.
10. Formwork should be removed after a longer period.
Mass Concrete:
The concrete placed in different massive structures such as dams, bridge piers, canal locks etc is known as mass concrete.
In mass concrete, larger size aggregates (up to 150 mm maximum) and low slump (very stiff consistency) are used to reduce the amount of cement in the concrete mix (normally 5 bags per m3 of mass concrete).
As the concrete is relatively dry and harsh, it needs immersion type of powder vibrators for full compaction. The concrete is normally placed in open forms. Due to the greater mass of the concrete, the heat of hydration (reaction between cement and water) may increase the temperature considerably.
These can be avoided by placing the concrete in shorter lifts and taking gaps of several days before the next lift. During concreting, cold water should be circulated through the pipes buried in the concrete mass may also be useful. If possible, concreting can be done in the winter season to lower the peak temperature in concrete. Alternatively, the aggregates may be cooled before using in the mix.
The high temperature due to the heat of hydration may result in an extensive and serious shrinkage in the mass concrete. The shrinkage cracks can be prevented by using low heat cement and by rapid curing of the concrete.
The early age strength is very high compared to later strength concrete cured at normal temperatures. During setting and hardening the volume change of mass concrete is very small but it can produce larger creep at a later stage.
Checklist For Concrete Slab:
The Checklist For Concrete Slab are listed below:
1. Make sure that the cover blocks are at right position and stay okay while concreting.
2. If the concrete is prepared at site, carefully handle the process of batching, water content, mixing and testing of fresh concrete.
3. Ensure that the beams are primarily cast in layers and compacted properly by vibrators.
4. Make sure that one carpenter and one helper is always present under the shuttering slab to confirm that it is supported well and will stay constant during and after concreting.
5. Assure the presence of a bar bender to avoid displacement of steel reinforcement.
6. Never permit extremely smooth finishing for the concrete slabs.
7. Try not to permit sprinkling of dry cement during finishing, It will help to prevent micro shrinkage cracks in the concrete.
8. Put the chairs appropriately to avoid disturbance at the top reinforcement bars during concreting.
9. Try to prevent cold joints in the casting slab.
10. In case of concreting in hot weather, protect the concrete surface by covering with wet gunny bags at least for 4 – 5 hours.
11. In case of concreting in rainy, windy, or extremely hot weather, put a cover of tarpaulin over the slab. It will protect the slab from getting damaged.
12. To prevent plastic shrinkage cracks in the concrete slab, avoid drastic drying of green concrete.
13. Ensure full compaction and complete finishing of the concrete.
Workability Of Concrete:
Workability can be defined as the property of fresh concrete which describes the ease and homogeneity of the concrete to be mixed, fully compacted and finished. A workable concrete should possess following two requirements:
1. The concrete should be compacted with minimum efforts.
2. The concrete should not form bleeding and segregation.
Workability of concrete mainly depends on the mix proportion and the properties of concreting materials (water, cement, aggregates). The shape, size, and grades of aggregates also play a great role in the variation of workability. For better workability fine and coarse aggregates should be well graded. It has been found that concrete made of round grain sand is more workable than the concrete of crushed sand. If air entraining admixture is used in the mix, it will also increase the workability and decrease segregation and bleeding.
Factors Affecting Workability Of Concrete:
The factors affecting workability are as following:
1. Amount Of Water In The Mix:
2. Proportion Of Coarse And Fine Aggregates: Workability can be increased by decreasing the amount of coarse aggregates in the mix. Fine aggregates produce more wore workable concrete.
3. Shape Of aggregates: Round shaped aggregates give better workability than angular shaped aggregates.
4. By expanding the cement content in the mix.
5. By including admixtures in the mix.
Apparently, the necessity of workability differs as per the nature of the job and blockage in the full stream of concrete due to the spacing and nature of the reinforcement. The workability of concrete is generally measured by one of the following three tests.
The concrete should be placed and compacted before its setting starts.The method of placing concrete should be such as to prevent segregation. It should not be dropped from a height more than one meter. In case, placing of concrete is likely to take some time it should be kept in an agitated condition.
Before concrete is placed in position, formwork should thoroughly be checked for its stiffness and trueness. The surface of placing concrete should be truly prepared according to requirements and thoroughly soaked with water.
The surface should be cleaned thoroughly to remove any loose matter spread over it. After having checked the formwork and necessary preparation of the surface, concrete placing is started. Following precautions should be taken while placing concrete.
1. Concrete should be laid continuously to avoid irregular and unsightly lines.
2. To avoid sticking of concrete, formwork should be oiled before concreting.
3. While placing concrete, the position of formwork and reinforcement should not get disturbed.
4. To avoid segregation, concrete should not be dropped from a height more than 1 meter.
5. Concrete should not be placed during rain.
6. The thickness of the concrete layer should not be more than 15 – 30 cm in case of RCC and 30 – 40 cm in case of mass concrete.
7. Walking on freshly laid concrete should be avoided.
8. It should be placed as near to its final position as practicable.
Reason For failure Of Concrete Structures:
A reinforced concrete member can fail mostly in the following cases:
1. When the member is subjected to excessive tension, so as to exceed the permissible stress in steel.
2. When the loading is such that the compressive stress in concrete exceeds its safe permissible value.
3. On account of the slipping of the steel bars from concrete.
4. When the concrete is subjected to excessive shear.
5. Due to the bad quality of materials used, shrinkage, creep or thermal effects.
6. When the member is subjected to extremes of temperature, aggressive liquids or gasses.
Different Sources Of Cracks In Concrete:
Cracking is one of the most common problems in concrete and it should be avoided seriously. Different causes of cracks in concrete are described below.
1. Shrinkage:
Shrinkage is one of the major causes of cracking in hardened concrete. In drying shrinkage, the volume of concrete is gradually decreased and if the component is restrained against free movement, tensile stresses are developed which causes cracks.
2. Temperature Changes:
The temperature variation in concrete results in the differential volume change. When the tensile strain capacity of concrete exceeds due to the differential volume change, it will crack.
3. Chemical Reaction:
Due to the alkaline nature of cement, it reacts with the carbon dioxide (CO2) present in the atmosphere resulting in an appreciable increase in the volume of the materials which finally leads to cracking.
4. Poor Construction Practices:
Poor construction practices such as adding excessive water to the mix, lack of curing, poor compaction, using low-grade materials, unreasonable placements of construction joints etc. are also responsible for cracking in concrete.
5. Errors In Design & Detailing:
Errors in design and detailing such as an inadequate amount of reinforcement, improper design of foundation, precast members and slabs, improper selection of materials, lack of sufficient contraction joints etc may result in excessive cracking.
6. Construction Overloads & Early Formwork Removal:
The load induced in the structure during construction can also lead to cracking especially at the younger stage when the formwork is removed earlier.
7. Elastic Deformation And Creep:
The different components of the building such as wall, column, beam. slab etc undergo elastic deformation when loaded. The deformation of concrete depends on the type of building materials used in the construction such as bricks, cement concrete blocks etc. This unusual deformation of concrete results in cracking.
8. Corrosion Of Concrete:
The corrosion of steel develops a huge amount of iron oxides and hydroxide that have a much greater volume than the volume of metallic iron. Hence the volume is increased and cracks.
Concrete Shrinkage Or Shrinkage Of Concrete:
The volumetric changes of concrete structures due to the loss of moisture by evaporation is known as concrete shrinkage or shrinkage of concrete. It is a time-dependent deformation which reduces the volume of concrete without the impact of external forces.
Types Of Shrinkage:
The types of concrete shrinkage are listed below:
1. Plastic Shrinkage:
Plastic shrinkage crack
Plastic shrinkage occurs very soon after pouring the concrete in the forms. The hydration of cement results in a reduction in the volume of concrete due to evaporation from the surface of concrete, which leads to cracking.
2. Drying Shrinkage:
The shrinkage that appears after the setting and hardening of the concrete mixture due to loss of capillary water is known as drying shrinkage. Drying shrinkage generally occurs in the first few months and decreases with time.
3. Carbonation Shrinkage:
Carbonation shrinkage occurs due to the reaction of carbon dioxide (Co2) with the hydrated cement minerals, carbonating Ca(Oh)2 to CaCo3. The carbonation slowly penetrates the outer surface of the concrete. This type of shrinkage mainly occurs at medium humidities and results increased strength and reduced permeability.
4. Autogenous shrinkage:
Autogenous shrinkage occurs due to no moisture movement from concrete paste under constant temperature. It is a minor problem of concrete and can be ignored.
Factors Affecting Shrinkage:
The shrinkage of concrete depends on several factors which are listed below.
1. Water-Cement Ratio:
shrinkage is mostly influenced by the water cement ratio of concrete. It increases with the increases in the water-cement ratio.
2. Environmental Condition:
It is one of the major factors that affect the total volume of shrinkage. Shrinkage is mostly occurred due to the drying condition of the atmosphere. It increases with the decrease in the humidity.
3. Time:
The rate of shrinkage rapidly decreases with time. It is found that 14-34% of the 20 years shrinkage occurs in two weeks, 40-80% shrinkage occurs in three months and the rest 66-85% shrinkage occurs in one year.
4. Type of Aggregate:
Aggregates with moisture movement and low elastic modulus cause large shrinkage. The rate of shrinkage generally decreases with the increase of the size of aggregates. It is found that concrete made from sandstone shrinks twice than the concrete of limestone.
5. admixtures:
The shrinkage increases with the addition of accelerating admixtures due to the presence of calcium chloride (CaCl2) in it And it can be reduced by lime replacement.
Other Factors:
The type and quantity of cement.
Granular and microbiological composition of aggregates.
The strength of concrete.
The method of curing.
The dimension of elements etc.
Factors Affecting Properties Of Concrete:
The factors which affect the properties of concrete (workability, bond strength, tensile strength, creep, shrinkage, bleeding, segregation, etc) are described below.
1. Water-cement Ratio:
Strength elasticity, durability, and impermeability of concrete are increased with the decrease in water-cement ratio, provided the concrete is workable. Shrinkage is increased with greater w/c ratio.
2. Cement Content:
With increases in cement content, w/c ratio decreased and consequently, strength, elasticity, durability, and permeability is increased. More cement improves workability but it also increases shrinkage which is undesirable.
3. Temperature:
The rate of setting and hardening of concrete is high at higher temperature. If the temperature of concrete falls below 0°C, free water in concrete turns into ice crystals and since ice has greater volume than the same quantity of water, the concrete is completely disrupted.
Such concrete on thawing will have no strength. If the temperature is more than the freezing temperature, cool concreting gives better ultimate strength, durability and less shrinkage.
4. Age Of Concrete:
The strength of concrete goes on increasing with age, though the rate of increase becomes very slow with the passage of time. The following table gives some ides of strength development with age:
Age
Strength in percentage
Ordinary cement
Rapid hardening cement
7 days
35%
65%
28 days
60%
90%
3 months
85%
95%
1 year
100%
100%
5. Aggregate:
Size, shape, and grading of aggregates, control concrete properties to a large extent. Rounded aggregates give better workability than flaky and angular aggregates. Larger the size of the aggregate, greater will be the strength, provided concrete mix is workable. Property graded aggregates give better workability and strength.
6. Curing:
Curing is the process of keeping the setting concrete damp so that complete hydration of cement is brought about. Besides strength the curing affects following qualities:
a) It improves wear-resisting and weather resisting qualities.
b) It increases impermeability and durability.
c) It reduces shrinkage.
7. Frost:
The frost causes disintegration of concrete and as such strength, durability and impermeability are reduced. Resistance to frost action depends upon the structure of the pores in the concrete.
8. Entrained Air:
The entrained air in concrete is due to incomplete compaction. It has the effect of reducing the strength of concrete. With 1% of entrained air, the strength of concrete is reduced by 5%. It also increases permeability of concrete.
Common Concrete Problems And Their Prevention:
There are many problems we might be facing during and after concreting. To produce high quality concrete we must take some precautions to avoid those common problems during concreting. In this article, we will discuss common concrete problems and how to prevent them.
1. Bleeding:
Bleeding refers to as a tendency of water to appear on the top surface of concrete after finishing. Due to bleeding some measure of water (with sand particles and other cementing materials) appears at the surface of the concrete.
Following precautions should be taken to reduce bleeding in concrete.
1. Design the mix appropriately.
2. Include least water content in the mix.
3. Use greater amount of cement content.
4. Use greater amount of fine particles.
5. Utilize a little measure of air entraining admixture.
2. Segregation:
Segregation means separation of coarse aggregates from the concrete surface due to poor compaction. It is generally seen in the plastic stage of concrete. As a result honeycomb, laitance, scaling, porous layer, bond failure etc. can be formed in concrete. Following precautions should be adopted to prevent segregation in concrete.
1. Design the mix appropriately.
2. Never use excessive water content.
3. Take care of handling, placing, and proper compaction of concrete.
4. Do not allow the concrete to be dropped from more heights.
5. Use air entraining admixture.
6. Keep the formwork to be watertight.
3. Laitance:
The appearance of cement-sand particles on the surface of freshly placed concrete is known as laitance. It is mainly occurred due to the bad effect of bleeding and segregation of concrete. The bond between subsequent layers of concrete becomes weaker and as a result, laitance is developed.
Following precautions can be taken to stop the occurrence of laitance in concrete.
1. Clay, dust, silt content etc should be removed before mixing the concrete.
2. Water-cement ratio should be maintained properly.
3. Water should not be sprayed on the concrete surface during finishing work.
4. Use well graded fine aggregates in the mix.
5. Add little amount of water reducing admixture in the concrete mix.
4. Scaling:
Scaling is the physical deterioration of concrete in which the surface layer of concrete broke down, pitted or flaked away. Due to this effect concrete surface becomes worse. Scaling can be prevented by taking same precautions adopted for laitance.
5. Plastic Shrinkage Cracks:
When the evaporation rate of water mixed in the concrete is greater than the bleed water of concrete, plastic shrinkage cracks are developed on the surface of the concrete. Basically, this type of cracks occurs in very hot climate.
6. Dusting:
Dusting can be prevented by taking following precautions.
1. Maintain a suitable water/cement ratio in the concrete.
2. Utilize dust free aggregates in the mix.
3. Guarantee appropriate hydration of concrete.
4. Avoid early surface finishing of concrete.
What Is Segregation?
The tendency of separating coarse aggregate particles from the concrete mix is known as segregation. Generally, it is observed in the plastic stage of concrete. Segregation mostly occurs in very lean and wet concrete. Honeycomb, sand streaks, porous layers, rock pockets etc are the results of segregation in hardened concrete.
Causes of Segregation In Concrete:
Different causes of segregation in concrete are as following:
1. Excessive water content in the mix.
2. Use of poor graded aggregates.
3. Improper design of the mix.
4. Poor compaction of concrete.
5. Over vibration of concrete.
How To Reduce Segregation In concrete:
Segregation can be avoided by taking following precautions.
The design of the concrete mix should be done properly.
Water content should not be added more than the desired amount.
Handling, placing, and compaction of freshly mixed concrete should be done carefully. A proper vibration also reduces the chances of segregation.
Formwork should be always watertight to prevent leakages.
Honeycomb In Concrete:
Honeycomb is the rough pitted surface or voids in concrete formed due to improper compaction or incomplete filling of the concrete.
In this formation, concrete not filled properly and create gaps/voids in between concrete and aggregates ( As shown in the above image). Honeycomb is mostly seen in columns and beams and can easily be detected just after removing the formwork. Actually, it looks like a honey bee nest.
Honeycomb is a serious problem of concrete which should be treated carefully. Otherwise, the structure or member may lose its strength.
The various reasons for honeycomb in concrete are as following:
1. Inappropriate workability of concrete.
2. Use of stiff concrete mix or the concrete is already set before placing.
3. Improper vibration of concrete in formwork.
4. Over reinforcement.
5. Use of larger size aggregates in excessive amount.
6. Formwork is not rigid and watertight.
7. Concrete is poured from more than allowable height.
8. Congestion of steel is preventing the concrete to flow over all corners.
Prevention:
So if we overcome the above reasons, we can easily prevent honeycomb in concrete. Here I will discuss further how to repair honeycomb if it is already formed in the concrete.
Repairing Of Honeycombs In Concrete:
1. First, remove the loosened aggregates and concrete particles from the affected surface by using a wire brush and a chipping hammer.
2. Clean the surface thoroughly with a brush to remove finer particles and then wash the surface with water.
3. Let the surface to dry well and apply Chemi-fix glue on the area.
4. Now mix the concrete grout with white cement and add required amount of water ( as per the specification recommended by the manufacturer).
5. Then pour/paste the mixture in the affected area to fill it completely. In case of large honeycomb, the concrete mixture should be poured after creating a pocket.
6. Remove the formwork after 12 hours and then cure it well.
Slab Reinforcement Details:
Where d = diameter of the bar.
σs = stress in the bar at the section considered as design load.
Ï„bd = Design bond stress.
In the below example, we need 10 db development length at the end section so that the concrete-steel bond stays continuous. The bar is bent because there is no space available at the end section. You can see that only 90-degree configuration is used but here we can use more configuration like that.
The ascertained compression or tension reinforcement at every section of an RC member is produced on both sides of that section by hooks embedded length or mechanical gadgets.
If the restraining section of concrete is relatively thin and unable to withheld the position of highly stressed bars the development length is given. In this way, the splitting of bars from concrete is avoided.
The additional embedded length is known as development length. The main aim is to give proper and settled support to the bars.
In compression reinforcement, hooks are not provided but where no or little space is available for extra length, hooks can be used for restraints.
Where t = Permissible tensile stress in steel.
Es = Moduli of steel.
and
c’ = Stress in concrete in level with tensile steel.
Ec = Modulus of concrete.
But
Or 
From the above expression, it is evident that the stress in the steel is m times the stress in concrete surrounding it.
A slight change in the amount of water causes much more differences in the strength of concrete. If less water is used, the resultant concrete will be nearly dry, hard to place in the form and may create difficulties in compaction. Besides this, with less water proper setting will not be guaranteed and thus the strength of concrete get reduced considerably.
On the other hand, if water is used more, it may develop larger voids and honey-combing in the set concrete, in this way decreasing its density, durability, and strength. Hence, water cement ratio attends an important role in producing concrete of required strength. The lower the ratio, the greater is the strength of concrete.
During the time of concreting the tremie is continuously lifted keeping the lower end of the tube in the concrete that is already poured. To reduce the extra loss of cement under water, rich concrete mix should be always used. Thus concrete laid underwater should never be compacted or consolidated.
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