Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev

Civil Engineering SSC JE (Technical)

Civil Engineering (CE) : Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev

The document Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev is a part of the Civil Engineering (CE) Course Civil Engineering SSC JE (Technical).
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Chapter 1 Introduction (Part 1)

Basic Composition of Concrete:

  • Concrete basically consists of water, cement and aggregate.
  • Aggregate is characterised as coarse aggregate (size > 4.75 mm φ ,) where Φ is square opening of 4.75 mm × 4.75 mm and fine aggregate (size < 4.75 mm φ ).
  • The ratio of fine to coarse aggregate is generally 1 : 2 and it generally depends upon the fineness of sand.
  • Sands are classified in 4 zones. They are zone-1 (coarse), zone-2 (medium), zone-3 (fine), zone-4 (very fine).
  • The ratio of fine to coarse aggregate for finer sands is less than 1 : 2, for medium sands is equal to 1 : 2 and for coarse sands is greater than 1 : 2.

Strength and Durability of Concrete

  • These two important properties depend on water/cement ratio. W/C = 0.4–0.6 (by wt.) for majority of structural concrete. Under moderate conditions of exposure, concrete with W/C = 0.5, durability is very low.
  • In general there are five categories of exposure conditions viz. mild, moderate, severe, very severe and extreme. For these conditions of exposure, minimum cement content (kg/m3), maximum free water-cement ratio, maximum grade of concrete are specified both for plain and reinforced concrete (Refer IS : 456–2000, page 18 and 20).

STRENGTH OF CONCRETE
(a) Compressive Strength of Concrete

  • It is considered as a measure of the quality of concrete, because other desired qualities like permeability, durability etc. depend on its compressive strength.
  • Standard size of the cube = 150 mm.

 Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev

  • Strength obtained is greater for smaller specimen. Maximum size of aggregate must not exceed one fourth of the size of the cube. Maximum size of aggregate generally used is 20 mm.
  • Rate of loading specified is 14 N/mm2 per min. This strength is known as cube strength. fcu = P/A  N/mm2
  • American practice is to test a cylinder instead of a cube. Standard size of cylinder is 150 mm  dia × 300 mm high. Cylinder strength of concrete.  
     fc' = P'/A'  N/mm2.
  • For the same concrete, cylinder strength of concrete is less than cube strength. [fc' ≈ 0.75 to 0.85 fcu].

 Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev

  • End friction restrains the specimen from failure. In case of cube, the end friction acts on the whole length, but in case of cylinder, since the height is taken twice, end friction works only upto 0.85a from each end, so its compressive strength is lower.
  • Cylinder strength is the true compressive strength.
  • Before, testing the cylinder, its top surface needs to be smoothened and the required plainness should be 0.05 mm (which is equal to that of plates).
  • In case of a cylinder we put a cap of 1.5–3 mm and it should be as strong as the cylinder.
  • Strength is reported to the nearest of 0.5 N/mm2.
  • One test consists of 3 specimens. Average of the strengths of the specimens of the strength of the sample.

(b) Variations in Strength.

  • If we test samples of same concrete, they will have different strengths due to the fact that no material is perfectly homogeneous.
  • Now accordingly we draw a histogram of strength vs frequency density where, frequency density.

Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev

  • If the number of samples is very large and the strength interval is reduced then this histogram changes into a curve known as probability distributive curve.

 Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev

  • For most engineering materials this curve is symmetrical about the mean and this type of curve is known as normal probability distribution curve.
  • In the above diagram

 Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev

where n < 30,σ is standard deviation.

  • Spread of the curve depends upon σ. So, σ is an index of the degree of quality control at site (for better control σ should be smaller).

Guidelines for Concrete Mix Design

  • Degree of control for concrete
    Very Good
    Good (Batching by wt.)
    Fair (Batching by volume)
  • At ordinary sites, batching is done by volume. When batching is done by volume, degree of control is fair only.
  • Nominal mix – 1 : 2 : 4 : : cement : fine aggregate : Coarse aggregate
  • Volume of measuring box is equal to the volume of one bag of cement.
  • Volume of one bag of cement = 34.5 litres  Volume = 30 cm × 30 cm × 38 cm.
  • Nominal mix = 1 : 1½ : 3
  • For measuring this half bag we have another box of the dimension (15×15×19) cm3.

Equation of Normal Distribution Curve 

  • Taking mean as origin, Z =Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev,then equation of the normal probability distribution curve Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev
    y = probability density
  • Area between any two x - ordinates gives the probability of strength falling in the that range, i.e. probability density. 
  • Total area under the curve = 1.

 Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev

  • In the same way probability of strength falling below fm –3σ
    = 1 – (0.5 – 0.341 – 0.136 – 0.022) = 0.001= 0.1%
  • Area under the curve to the left of any ordinate gives the probability of strength below which the test results are likely to fall. 
  • If we want to know the strength below which 5% of the test results are likely to fall then: 0.05 = 1 – (0.5 – x) — x = 0.45
  • x lies between fm –σ and fm – 2σ .

It  comes out to be fm– 1.65σ This is nothing but fck.

Characteristic Strength : It is the strength below which not more than 5% of the test result are expected to fall. fck = fm – 1.65 σ
where

 Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev
and
Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev

  • These values are acceptable for n ≥ 30, other wise error will be too large.

If n < 30 , then

 Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev

  • Concrete is designated by fck where fck is characteristic cube strength of concrete at 28 days.
  • Cement hydrates slowly over a long period so concrete gains its strength over a long time.
  • Hydration of cement takes place only when the capillaries are saturated with water.
  • Strength of concrete after 6 months = 1.2 fck
  • Age7 days28 days6 months
    rel. Strength0.701.01.2

GRADES OF CONCRETE
1. As per IS : 456–2000 

(a) Nominal Mix Concrete
M5, M 10, M 15, M 20

(b) Plain Concrete
M 15, M 20, M 25

(c) Reinforced Concrete
M 20, M 25, M 30, M 35, M 40.

2. Other Categorisation 

(a) Ordinary Concrete
M 10, M 15, M 20.

(b) Standard Concrete
M 25, M 30, M 35, M 40, M 45, M 50, M 55.
(c) High strength concrete
M 60, M 65, M 70, M 75, M 80.
(d) M refer for mix and number indicate the compressive strength of concrete at 28 days.

Compressive Strength of Concrete in a structure:

  • The strength was found to decrease with increase in size of specimen reason being there is a great probability of presence of a weak element in large volume of concrete.

Strength of 50 mm cube = 0.85 times the strength of 150 mm cube.
Compressive strength of concrete in a structure = 0.79 (End friction factor) × 0.85 (size factor) fck = 0.67 fck
where fck is characteristic cube strength of concrete after 28 days.

DO YOU KNOW?
Among the materials and mix variables, water cement ratio is the most important parameter governing compressive strength.
No factor of safety has been included in this strength formulae. It has to be included now.

Flexural Strength or Modulus Rupture

  • Flexural strength is the tensile strength of concrete in flexure (bending)
  • The tensile strength of concrete is closely related to compressive strength of concrete. As compressive strength also increases but at a decreasing rate.
  • Size of the beam — 100 × 100 × 500 mm long.

Distance between the support point and load point is known as shear span. Shear  between two load points is zero and hence the failure occurs in the shear span.

 Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev

If the failure occurs in the shear span and distance > 20 mm the test is rejected. Bending moment at the section of failure.

 Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev

where w = failure load in kN.

 Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev

= 0.4 w N/mm2 
As per IS : 456 – 2000 Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev

  • So, our IS code gives the value of strength on the higher side flexural strength is used to determine the load at which cracks will occur.

(i) Tensile Strength of Concrete or Strength in Direct Tension or Direct Tensile Strength
-Tensile strength is determined by splitting test size- 150 mm dia × 300 mm long.

Generally fct ≈ 0.6 fcr

 Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev
-We generate tensile load by this method to find out tensile strength as it is difficult to apply direct pull on the specimen.

Stress Strain Curve

  • The peak stresses are cylinder strength of concrete.

 Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev

(a) Conclusions

  • The peak stress is reached at a strain of about 0.2%.
  • The descending part of the high strength concrete is steeper and the high strength concrete crushes at a lower strain, so high strength concrete is brittle.
  • Crushing strain is 0.4 – 0.6 %
  • The ascending part of the curve of all the concrete is similar and is generally second degree parabola.
  • The curve is approximately linear for about 0.6 times peak stress.

(b) Modulus of Elasticity of Concrete (Ec) & (Et)

  • Ec is obtained from the stress strain curve.

  Ec = Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev

  • Et = tangent modulus. It  will depend upon the value of stress.

Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev

  • So we are interested in secant modulus as we want to know the strain corresponding to the given stress to determine the deformation.
  • Permissible compressive stress of concrete in bending = 0.33 fck
  • Ec = Secant modulus corresponding to a stress of 0.33 fck
  • As per IS : 456–2000

Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev
As per ACI
Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev

(c) Concrete Strain at Ultimate Strength:

  • Ultimate strength is the maximum stress attained before failure. So concrete strain at ultimate strength for concrete member axially loaded is 0.2%

 Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev

  • Concrete strain at flexural strength (Concrete strain in extreme compression fibre when it reaches maximum moment of resistance)
    ec = 0.3 – 0.4%
  • In design ec = 0.35%
  • Soon, failure will occur at a strain of 0.4 – 0.6%

(d) Design Stress Strain Curve of Concrete

  • We ignore the curve beyond 0.35% as we are interested only in finding out the ultimate value of strength.

 Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev

  • For design purpose γm in should be used
  • Probability of falling below fck is 5% so we provide factor of safety.
  • γm = Partial safety factor for material strength.
  • Design strength = Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev (for any material)
  • γm = 1, if we are only considering loads
  • γm > 1 for ultimate values
  • For concrete, γm = 1.0, for serviceability limit state.
  • γ= 1.5 for limit state of collapse.

Reinforced Concrete

  • Concrete is weak in tension. Flexural strength = 15% of its compressive strength. So its strength in bending can be fully utilized only when its tension flange is reinforced. Concrete is allowed to crack.  Tension is carried by steel bars and compression is taken by concrete itself.
  • A beam has a ductile failure if the tension steel yields before failure. This will happen only when the beam is under reinforced (& not over reinforced). If we over reinforce a beam then its failure will be a sudden one which is not desirable.

(a) Reinforcement

  • If wire is obtained by cold drawing a bar through sieve of dyes its surface is very smooth. So wires are called cold worked steel bars.
  • Carbon content of mild steel = 0.25%
  • To increase the strength, bars are allowed (Carbon, Nickel etc.). This is done by hot rolling.
  • Bars are also strengthened by cold working.
  • Hot rolled steel as definite yield point.
  • Grades of bars
    1.    Fe 250  250 N/mm2} Plain M.S. bars
    2. Fe 415 415 N / mm2
    3. Fe 500 500 N / mm2 }High Strength
  • fy is characteristic yield strength (5% probability of strength falling below Fe 415) is mostly used.

(b) Cold Worked Steel

  • Yield strength = 0.2% proof stress 
  • 0.2% proof stress is the stress corresponding to 0.2% strain on unloading.

 Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev

(a) Actual Stress-Strain Cuves for Steel

 Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev

(b) Design Stress-Strain Curves for Steel

 Introduction (Part - 1) - Reinforced Cement Concrete Civil Engineering (CE) Notes | EduRev

fy – characteristic yield stress
γm – partial safety factor for steel
γm – 1.0, for serviceability limit state
γm = 1.15, for limit state of collapse.

  • In design stress-strain curve, we ignore the increase in stress in strain hardening range. It is extended only upto 3%.
  • Strain in tension steel at ultimate strengths es = 0.4 to 1%
  • So in this range, increase in stress in quite insignificant, so we ignore it.
  • At ultimate strength, strain in steel is not likely to be more than 3%. In rare cases it is 3% otherwise it is less than 3%. Steel. reaches its ultimate strength at a strain of 9% but reinforced concrete member fails much earlier, so steel never reaches its  ultimate strength in reinforced concrete member as concrete crushes much earlier.
  • This is why we don't designate the reinforced steel bars with their ultimate strength as it is never reached. We designate the bars by their characteristic yield strength.

Non-Destructive Testing Methods:

  • In the non destructive methods of testing the specimen are not loaded to fracture and as such the strength inferred can not be expected to yield absolute values of strength.
  • These methods, therefore attempt to measure some other properties of concrete from which an estimate of its strength, durability and elastic parameters are obtained. Such properties of concrete are hardness, resistance to penetration, rebound.
  • The electrical properties of concrete, its ability to absorb. scatter and transmit X-rays and g -rays, its response to nuclear activation and its acoustic emission allow us to estimate moisture content, density, thickness and its cement content.

Various non destructive methods:
(a) Surface hardness methods.
(b) Rebound test
(c) Penetration and pull out techniques
(d) Dynamic or vibration test
(e) Radio active and Nuclear Methods
(f) Magnetic and electrical methods
(g) Acoustic emission techniques

Chapter 1 Introduction (Part 2)

WET CONCRETE PROPERTIES
(a) Bulking of Aggregate 

  • The free moisture content in fine aggregate results in bulking of volume. Free moisture content forms a film around each particle. This film of moisture exerts what is known as surface tension which keeps the neighbouring particle away from it. 
  • It is interesting to note that bulking increase with increase in moisture content upto a certain limit and beyond that the further increase in the moisture content results in the decrease in the volume. and at a moisture content representing saturation point, the fine aggregates show no bulking. Maximum increase in volume is upto 4% and maximum bulking occurs at a moisture content near to 5%. 
  • Due to bulking, fine aggregate shows unrealistic change in volume. Therefore it is necessary that consideration must be given to the effect of bulking in proportion concrete by volume.

(b) Soundness of Aggregate 

  • Soundness refers to the ability of aggregate to resist excessive changes in volume as a result of changes in physical conditions. These physical conditions that affect the soundness of aggregate are freezing and thawing, temp. variation alternate wetting and drying. 
  • The soundness of cement is determined either by 'Le chatelier Method' or by means of an 'Autoclave Test'. 
  • The test to determine the soundness of aggregate is 'sulphate test'.

(c) Alkali Aggregate Reaction 

  • This reaction takes place between the alkalis in the cement and the active silica or carbonates of aggregates.

Factor promoting alkali aggregate reaction. 

(a) Reactive type of aggregate
(b) High alkali content in cement-alkali in cement 0.6%
(c) Availability of moisture
(d) Optimum temperature conditions (Ideal temperat ure is 10ºC to 38º) 

  • Alkali silica gel formed is the major disruptive product of alkali silica reaction. This silica gel exerts osmotic pressure to cause pattern crakcing particular in thinner sections of concrete like pavements. 
  • The formation of pattern cracks due to the stress induced by the growth of silica gel results in subsequent loss in strength and elasticity. If further accelerates other processes of deterioration of concrete due to formation of cracks. Solutions of dissolved CO2 convert Ca (OH)2 to CaCO3 with subsequent increase in volume.

Control of Alkali Aggregate Reaction:

(a) Selection of non reactive aggregates
(b) By the use of low alkali cement
(c) By the used of corrective admixtures such as pozzolanas.
(d) By controlling the void space in concrete.
(e) By controlling moisture conditions and temperature.

(d) Specific Surface 

  • Specific surface increase with the reduction in size of aggregate particle so the fine aggregate attributes very much more to the surface area then does the coarse aggregate. Greater surface area requires more water for lubricating the mix to give workability but very very fine particles act differently. These particles themselves act like ball bearings to reduce the internal friction b/w coarse particle. 
  • Experience has shown that usually very coarse sand or very find sand is unsatisfactory for concrete making. The coarse sand results in bleeding and segregation, and the fine sand requires a comparatively greater amount of water to produce the necessary fluidity.

(e) Gap Grading 

  • Generally it is assumed that the voids present in the higher size of aggregate are filled up by the next lower size of aggregate, and similarly voids created by the lower size are filled by the one size lower than those particle and so on. This was called continous grading. 
  • But later it was realized that the voids created by a particular fraction are too small to accommodate the very next lower size. This would create "particle size interference", which prevents the large aggregate compacting to their maximum density.

Advantage of gap graded concrete:

1. Sand required will be less.
2. Requires less cement and lower w/c ratio.

(f) Workability of Concrete 

  • Hundred percent compaction of concrete is an important parameter for contributing to the maximum strength. Lack of compaction will result in voids whose damaging effect on strength and durability is equally or more predominant than the presence of capillary cavities. 
  • To enable the concrete to be fully compacted with given efforts, normally a higher w/c ratio than that calculated by theoretical consideration must be required that is to say the function of water is also to lubricate the concrete can be compacted with specified effort forth coming at the site of work and the too without segregation. 
  • The quality of concrete satisfying the above requirements is termed as workable concrete Every job requires a particular workability. A concrete which considered workable for concrete foundation is not considered workable for concrete to be used in roof construction. Therefore the word workability assumes full significance of the type of work, thickness of section, extent of reinforcement and mode of compaction. So workability is the property of concrete which determines the amount of useful internal work necessary to produce full compaction.

Factors affecting workability

(a) Water content
(b) Aggregate cement ratio or mix proportion
(c) Size of aggregate
(d) Shape of aggregate
(e) Surface texture
(f) Use of Admixture

Measurement of workability

(a) Slump test
(b) Compacting factor test
(c) Flow test
(d) Kelly ball test
(e) Vee Bee consistometer test

(g) Segregation of Concrete 

  • Segregation is defined as the separation of the constituent materials of concrete. A good concrete is one in which all the ingredients are properly distributed to make a homogeneous mixture. 
  • Segregation may be of three types Firstly the coarse aggregate settling down from the rest of the matrix, secondly the paste or matrix separating away from coarse aggregate and thirdly, water separating out from the rest of the material. 
  • The conditions favourable for segregation are the badly proportioned mix where sufficient matrix is not there to bind and contain in the aggregation. Mixed concrete with excess water content shows a higher tendency for segregation. Dropping of concrete from heights also results in segregation. if too wet mix is excessively vibrated, it is likely that the concrete gets segregated. 
  • The tendency of segregation can be remedied by correctly proportionating the mix, by proper handling, transporting, placing, compacting and finishing.

(h) Bleeding 

  • It is sometimes referred as water gain. It is a particular kind of segregation in which some of the water from the concrete coems out to the surface of concrete Bleeding is predominantly observed in a highly wet mix, badly proportioned and insufficient mixed concrete. In thin numbers like roof slab or road slabs and when concrete is placed in sunny weather. Show excessive bleeding. Sometimes certain quantity of cement also comes out to the surface with water. This formation of cement paste at the surface with water. This formation of cement paste at the surface is known as "Laitance". It results in higher shrinkage cracks and bad wearing quality. This could be avoided by removing the Laitance fully before the next lift is poured. Bleeding can be reduced by proper proportionating and uniform and complete mixing. 
  • Use of finely divided pozzolanic materials reduces bleeding by creating a longer path for the water to traverse use of air entraining agents is very effective in reducing the bleeding. Revibration may also be adopted to overcome the bad effect of bleeding.

Important Fraction of Wet Concrete other than Aggregate

(a) Water
Maximum permissible limits for solids as per IS:456–2000 in water

1. Organic–200 mg/l
2. Sulphates (SO3) – 400 mg/l
3. Chlorides (CI–) – 2000 mg/l for concrete work not containing embedded steel. = 500 mg/l for RCC
4. Suspended – 2000 mg/l
5. Inorganic – 3000 mg/l

  •  The pH value of water shall be not less than 6. 
  • The initial setting time of test block made with the appropriate cement and the water proposed to be used shall not be less than 30 min and shall not differ by ± 30 min for the initial setting time of control test block prepared with the same cement and distilled water. 
  • Average 28 days compressive strength of at least three 150 mm concrete cubes prepared with water proposed to be used shall not be less than 90% of the average of strength of three similar concrete cube prepared with distilled water. 
  • IS 456–2000 prohibits the use of sea water for mixing and curing of reinforced concrete and presetressed concrete work.

(b) Admixtures 

  • Admixture is defined as a material, other than cement, water and aggregates, that is used as an ingredient of concrete and is added to the batch immediately before or during mixing. Additive is a material which is added at the time of grinding cement clinker at the cement factory. 
  • Following Admixtures are used in Concrete: Plasticizers, super plasticizers, retarders and retarding platicizers, accelerators and accelerating plasticizers, air entraining admixtures, pozzolanic or mineral admixture, workability admixture.

Other Minor Admixture: Damp proofing and water proofing, gas forming admixtures, air entraining, grouting admixtures, bonding admixtures, fungicidal, germicidal and colouring admixtures.

(c) Plasticizers (Water Reducers) 

  • A high degree of workability is required in situation like deep beams, thin walls of water retaining structures with high percentage of steel reinforcement, tremie concreting, pumping concreting. 
  • The use of plasticizers can help in difficult conditions for obtaining higher workability without using excess of water. 
  • "The organic substances or combination of organic and inorganic substances, which allow a reduction in water content for the given workability, or give a higher workability at the same water cotent, are termed as plasticizing admixtures. Calcium, Sodium and Ammonium Ligno sulphonates are mostly used. They are used in the amount of 0.1% to 0.4% by wt. of the order of 5 to 15%. This naturally increases the strength. 
  • The increase in workability that can be expected at the same w/c, may be anything from 30 mm to 150 mm slump, depending on the dosage, initial slump of concrete, cement content and type.

(d) Super Plasticizers 

  • They are high range water reducers. Their use permit the reduction of water to the extent of 30% without reducing workability. 
  • Thus the super plasticizers produce a homogeneous, cohesive concrete and bleeding. Some super platicizers are:
    1. Sulphonated melamine formaldehyde condensate (SMF)
    2. Sulphonated napthalene formaldehyde condensates (SNF)
    3. Modified Ligno Sulphonates.

(e) Retarders 

  • It slows down the chemical processes of hydration so that concrete remains in plastic and workable for a longer time than concrete without the retarder. Generally good for hot weather concretig. Retarders are used in grouting oil wells. 
  • The most common retarder is Calcium Sulphate. (Gypsum). Other materials used for this purpose are starches, cellulose products. sugars, acids or salt of acids, Ligno Sulphonic Acids and their salts, hydroxylated carboxylic acids and their salts. The last two also reduce the water content. 
  • All the plastizers and superplastizers by themselves show certain extent of retardation.

(f) Accelerators 

  • Accelerating admixture are added to concrete to increased the rate of early strength development in concrete to:
    1. Permit earlier removal of formwork.
    2. Reduce the required period of curing.
    3. Advance the time that the structure can be placed in service.
    4. In the emergency repair work.
    5. In cold weather to compensate for retardation. 
  • Some accelertors ar e soluble carbonates, silicates, some of the organic compounds such as trietheriolamine are used.

(g) Air Entraining Admixture 

  • Air entrained concrete is made by mixing a small quantity of air entraining agent or by using air entraining cement. These air entraining agents incorporate millions of non-collapsing air bubbles, which will act as flexible ball bearings and will modify the properties of plastic concrete regarding workability, segregation, bleeding and finishing quality of concrete. 
  • It also modifies the properties of hardened concrete regarding its resistance to frost action and permeability. 
  • Air entraining agents and Natural wood resins, animal and vegetable fats and oils and their fatty acids and such as stearic and oleic Acids, wetting agents such as alkali salts or sulphated or sulphated or sulphonated organic compounds.

Setting time of concrete 

  • Setting time of concrete depends upon the w/c ratio, temperature conditions, type of cement, use of minerals admixture, use of plasticizers. 
  • The setting time of concrete is found by penetrometer test. Plot a graph of penetration resistance as ordinate and elapsed time as abscissa. 
  • From penetration resistance equal to 3.5 MPa draw a horizontal line. The point of intersection of this with a smooth curve is read on the X-axis which gives the initial setting time. Similarly a horizontal line is drawn from the penetration resistance of 27.6 MPa and the point it cuts the smooth curve is read on the X-axis which gives the final set time.

Various Stages of Manufacture of Concrete (a) Batching (b) Mixing (c) Transporting (d) Placing (e) Compacting (f) Curing (g) Finishing

(a) Batching 

  • It is of two types, volume batching and weight batching. Volume batching is not considered good. 
  • when volume batching is done the bulking of sand is consider

(b) Mixing 

  • Hand mixing and machine mixing are the commonly adopted mixing methods. Hand mixing is adopted for small scale and unimportant work while machine. mixing is carried out for reinforced concrete work for medium or large scale concrete work. Machine mixing is not only efficient but also economical (when the quantity of concrete to be provided is large) 
  • Concrete mixers are generally designed to slum at a speed of 15 to 20 rpm. For proper mixing it is seen that about 25 to 30 revolutions are required in a well designed mixer.

(c) Transporting of Concrete 

The methods adopted for transportation are:
(a) Mortar pan
(b) Wheel barrow
(c) Crane, bucket and rope way
(d) Truck Mixers and dumpers
(e) Belt conveyor
(f) Chute
(g) Skip and Hoist
(h) Transit Mixer
(i) Pump and pipe line

(d) Placing of Concrete 

  • Concrete must be placed in systematic manner to yield optimum results.
  • Concrete is often required to be placed under water or in a trench filled with bentonite slurry. In such cases use of bottom dump bucket or tremie is made use of.

(e) Compaction of Concrete 

  • It is process adopted for expelling the entrapped air from the concrete. The lower the workability, higher is the amount of air entrapped. In other words, stiff concrte mix has high percentage of entrapped air and therefore would need higher compacting efforts than high workable mixes.  It must be borne in mind that 100% compaction is important not only from the point of view of strength but also from point of view of durability.

Methods of Compaction

(a) Hand Compaction 
1. Rodding 2. Ramming 3. Jumping

(b) Compaction by Vibration

1. Internal Vibrator (Needle Vibrator)
2. Form Work Vibrator (external Vibrator)
3. Table Vibrator
4. Platform Vibrator
5. Surface Vibrator 6. Vibratory Roller

(c) Compaction by Pressure and Jolting 

(d) Compaction by Spinning 

(f) Curing of Concrete

  •  Curing can be considered as creation of a favourable environment during the early period of uninterupted hydration. 
  • More elaborately it can be described as the process of maintaining a satisfactory moisture content and a favorable temperature in concrete during the period immediately following placement so that hydration of cement may continue until the derived  properties are developedto a sufficient degree to meet the requirement of service.

Curing Methods may be broadly divided into 4 categories.

(a) Water Curing (b) Membrane Curing (c) Application of heat (d) Miscellaneous 

  • Water curing is considered the best method of curing. Even if membrane method is adopted, it is desirable that a certain extent of water curing is done before the concrete is covered with membranes. Immersion, ponding, spraying or fogging, wet cover are commonly adopted for water curing. Membrane curing is adopted where there is acute shortage of water. Membrane curing is the application of membrane or scaling compound. The materials used for this purpose are bituminous compounds, polyester film, water proof paper or rubber compound etc. Bituminous compound, polyster film, water proof paper or rubber compound etc. Bituminous compound are generally not used now as they have a great heat absorbing property. 
  • In the application of heat, generally steam curing is adopted as it fulfills both teh purposes of maintaining temperature and that too without loss of m/c. Steam curing is done normally at ordinary pressure but in certain circumstances under high pressure curing is done by infrared radiation Electrical curing are other curing methods. 
  • Curing is done at the temperature of 27 ±  3ºC and humidity 90%.
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