Docstoc

concrete mix design

Document Sample
concrete mix design Powered By Docstoc
					                     A

             PROJECT REPORT

                    ON

          CONCRETE MIX DESIGN




                   GUIDE :

             G.A SURYAWANSHI


               SUBMITTED BY :

              SANTOSH LALWANI

TUSHAR BARAPATRE             PALLAVI DHABARDE
ANWAR ASHARF                 PALLAVI DHAWAD
MAYUR RAMTEKE                ASHLESHA GANVIR
NEHA JANBADE                 TRUPTI KSHISAGAR




    APPLIED MECHANICS DEPARTMENT
      GOVERNMENT POLYTECHNIC,
               NAGPUR.
                        CERTIFICATE


This is to certify that Shri………………………………………………
III year Civil Engineering students studying in the institute has
submitted his project work entitled “CONCRETE MIX DESIGN”
In partial fulfillment for the award of Diploma in Civil Engineering
Which is record of his own work carried out by him under
supervision and guidance.


Date :




                              Guide :
                       G.A.SURYAWANSHI
              Lect. in Applied Mechanics Department
                            G.P.Nagpur.




PROF. Y.K.CHANADRANA                   PROF.N.D.NANDANWAR
      H.O.D                                   PRINCIPAL
Applied Mechanics Department               Govt.Polytechnic,
Govt. Polytechnic, Nagpur.                      Nagpur.
                     ACKNOLEDGEMENT


                  We take this golden opportunity to express our
sense of gratitude and sincere thanks to our guide Prof. G.A
SURYAWANSHI.For their valuable guidance. Their ability to
motivate capability to induce ,even willingness to solve our
difficulties, generous suggestions has faced us to make our project
work a unique one and our task easier. The work would not have
seen the light of the day without his existence and encouragement.

              We give sincere thanks to Prof.Y.K CHANDARANA
( Head of Applied Mechanics Dept.) to give us a golden opportunity
to prepare such a project.

                  For their direct & indirect help to our project we
are also thankful of all other our Staff members for giving their
valuable suggestions and their co-operation and help in our project
work.

                   Finally our special thanks to all those who directly
or indirectly helped in during the courses of our project.




                           PROJECTEES

                       SANTOSH LALWANI

TUSHAR BARAPATRE                             PALLAVI DHABARDE
ANWAR ASHARF                                 PALLAVI DHAWAD
MAYUR RAMTEKE                                ASHLESHA GANVIR
NEHA JANBADE                                 TRUPTI KSHISAGAR
               INDEX
Sr.No. Unit                          PageNo.

1.     INTRODUCTION

2.     PROPERTIES & OF INGREDIENTS
       OF CONCRETE
3.     WATER CEMENT RATIO

4.     PRINCIPLES OF CONCRETE MIX
       DESIGN
5.     METHODS OF CONCRETE MIX
       DESIGN
6.     WORKABILITY

7.     CURING OF CONCRETE

8.     TEST ON CONRETE

9.     REFRENCES
 UNIT NO.1

INTRODUCTION
                       1 INTRODUCTION

1.1 CONCRETE:

        Concrete is a composite material form by combination of cement, fine
aggregates, coarse aggregates & water together in a suitable proportion. This mix
form easily workable paste known as plastic wet or green concrete. After mixing
and placing the concrete in position the cement and water undergoes hardening the
cement paste In this way, cement binds the aggregates like hard mass, which IS
called as concrete.

         Now a days concrete is used most commonly as a building material because
of its high strength and durability at low cost concrete is the only building material
which gain strength in presence of water while other decayed_ due to this property
of concrete it widely used in modem engineering construction.

        For economical preparation of the high quality concrete mixes, it is
essential to study the design of such concrete mixes by various methods and
                    -         ~                               ~
adopt one, which IS most economical and durable. For this study of various
Ingredients of concrete, fresh concrete, hardened concrete is very essential.



1.2 ADVANT AGES OF CONCRETE:

       The various advantages of concrete as construction materials are as
below :-

   a) Concrete gains strength in presence of water whereas the materials other
      than concrete decay when they are exposed to water.

   b) Insect does not attack concrete.

   c) Concrete can be molded at any shape at normal temperature and pressure.

   d) Concrete is fire proof and sound proof
   e) Concrete has strength.

   f) Strength of concrete can further increased by reinforcing and prestressing.




    1.3 CHARACTERISTICS OF GOOD CONCRETE:

        A good concrete is obtained by careful selection pf its ingredients, well
grading of aggregates, suitable proportioning, adding sufficient water and adopting
workmanship Through the freshly prepared concrete remains plastic only
temporarily It should fulfill certain characteristics in this state because the quality
and the cost of hardened concrete are much affected by the characteristics of
freshly prepared concrete. The desired properties of good concrete in plastic and
hardened stage are below

Properties of plastic concrete:

a) Workability:-

        Concrete in green stage should have good workability so that it can be
   easily placed and compacted.

b) Freedom from segregation:

      Separations of ingredients of concrete mix so that the concrete mix is no
  longer in a homogeneous and stable condition is called as segregation good
  concrete must be free from segregations.


c) Freedom from bleeding

      Separation of slurry from cement and water from concrete mix is called as
  “Bleeding.”
   Good concrete should be free from bleeding.
Properties of hardened concrete :-

a) Strength

          A good concrete in hardened stage should have good strength.

b) Durability

          A good concrete must be durable enough to resist the effect of weathering agents.

C) Impermeability

                A good concrete should be impervious and airtight




1.4 DISADVENTAGE AND REMEDIES OF
CONCRETE:


    I) Concrete has low tensile strength therefore It cracks under tensile load.
       Bit this can be removed by using reinforcement or prestressing.


   2) Concrete expands and contracts due to temperature variation. To
      overcome this difficult expansion Joints are provided .


   3) Concrete is not totally impervious but it can made Impervious by using water
      proofing agents or rich mixes.


   4) Concrete disintegrates when comes in contact with alkalies or sulphates resisting
      cement.
1.5 CONCRETE MIX DESIGN:

              One of the ultimate aims of studying the various properties of
materials of concrete. plastic concrete and harden concrete, is to unable the concrete
technologist to design a concrete mix for a particular strength.

              The design of concrete mix is not a simple task and the condition that
prevail at the site of work and the condition that are demanded for a particular work
for which the mix is designed . The Structural Engineer stipulates the certain
minimum strength and the concrete technologist design the concrete mix with the
knowledge of the materials, site conditions and standard of supervision available at
site to achieve this minimum strength. Further, the site engineer is required to make
the concrete at Site, closely following the parameters suggested by the" Mix
Designer" to achieve the minimum strength specified by the Structural Engineer.
The Site Engineer makes cubes or cylinders sufficient in numbers and tests them to
confirm the achievement with respect to minimum specified strength .

   "Mix design can be defined as the process of selecting suitable Ingredients of
concrete and determining there relative proportions with the object of producing
concrete of certain minimum strength and durability as economically as possible"
       UNIT NO.2

PROPERTIES OF INGREDIENTS

     OF CONCRETE
                 2 PROPERTIES OF INGRADIENTS
                        OF CONCRETE


2.1 GENERAL:

         The common Ingredients of concrete are coarse aggregates, fine aggregates
and water. By Judicious use of available materials for concrete making and their
proportioning, proportioning, concrete mixes are produced to have the desired
properties in the fresh and hardened states, as the situations demand. In this section,
the physical and chemical properties of materials which Influence the properties and
performance of concrete mixes are discussed Materials are also tested to meet the
requirements of Mix design .


2.2 CEMENT:
         Cement is by far the most important constituent of the concrete, in that it forms
the binding medium for the discrete ingredients

          The product manufactured by crushing and burning to an intimate well -
proportioned mixture of calcareous and argillaceous material is called as "Cement. "
Raw materials used to produce cement are as follows -

     Sr.No.      Chemical                Range                    Common
                 Ingredients             %                        proportion
     1           Lime                    60 to70                  63
     2           Silica                  17 to 25                 22
     3           Alumina                 3 to 8                   6
     4           Iron oxide              0.5 to6                  3
     5           Magnesium oxide         0.4 to 4                 2.5
     6           Sulphur Trioxide        1 to 3                   1.75
     7           Alkalies such as        0.2 to 1                 0.25
                 soda & potash
     8           Loss on ignition        1 to 2                   1.5
2.2.1 Ordinary Portland Cement (OPC) (IS:269-1976)
         This type of cement is also normal setting cement since its setting is normal as
compared to other cement when mixed with water
        I t is the general-purpose cement suitable for general concrete construction for use
in general concrete construction, which requires no special

consideration. It should satisfy the requirements as per IS269-1976. The, weight of one bag
of OPC is 50 kg and its volume is 35 lit. or 0.035 M2 .One cubic meter of OPC contains
28.8 bags.

2.2.2 Properties of OPC:

1) Fineness :- The residue of OPC should not exceed when sieved through IS-sieve No.9
(90 micron)

2) Soundness :- Its expansion should not be more than 1Omm for untreated cement and 5
mm for aerated cement.

3) Setting time :- Its initial setting time should not be less than 30 minute & final setting
time should not be more than 600 minute.

4) Compressive Strength:- after 3 days its compressive strength should not be less than 115
kg/cm3 & 175kg/cm3 After 7 days when ordinary sand is used and 160 kg/cm 3 and
220kg/cm3 respectively when standard sand is used.


2.2.3 Uses: -


1) OPC is used in important structures where great strength is required such as heavy
buildings and bridges.

2) It is used in structure subjected to action of water such as foundation, under water
reservoir. Water tight floor, Clock yards etc.

3) It is used for cement mortar, plain concrete, and reinforcement concrete, reinforce
brickwork .

4) It is used for plastering and pointing

It is used for drainage & water supply works.
2.3 AGGREGATES
2.3.0 GENERAL

        The inert material used in preparation of mortar or concrete such as sand ,
gravel ,crushed stone etc. it is termed as aggregate. The aggregate are bound
together by a cement paste to form a hard concrete mass. They are called inert
because , they do not take part in chemical reaction & remains inactive during
setting & hardening of cement.

        Almost 75 % of the volume of concrete is occupied by aggregates , so they
affect various properties of concrete .As aggregate are cheaper than cement, it is
economical to put in concrete keeping in view the durability .Mostly used aggregates
are sand, gravel ,broken bricks, furnace slag are used for plain & fire resisting
concrete .But for R.C.C sand, gravel, crush rock are used as aggregates.

2.3.1 REQUUIRMENTS OF GOOD AGGREGATES :-

       As per IS 383-1975, good aggregate should fulfill the following
requirements:-

       a) It should be chemically inert.

       b) It should be sufficiently strong.

       c) It should be sufficiently hard

       d) It should be sufficiently durable.

       e) It should be preferably cubical or spherical.

       f) It should have rough surface.

       g) It should be free from inherent coating, clay, organic material or any
          other matter, which is likely to reduce setting of cement. and may reduce
          strength of concrete.
2.3.2 PROPERTIES OF AGGREGATES:-

1) SURFACE TEXTURE :-

        Surface texture influences the properties of plastic concrete more than
harden concrete . The strength of the bond between aggregate and cement paste
depends on surface texture or porosity of the aggregates. Aggregates having rough
surface requires more cement and provides better bond. Also smooth aggregates
having surface pores develop good bond. Surface texture may of following type -

           a) Glassy

           b) Smooth

           C) Granular

           d) Rough

           e) Crystalline

           f) Honey combed

     2) STRENGTH OF AGGREGATE:-


        Generally, strength of aggregates refers to crushing strength. It is the
resistance of aggregates crushing that is compressive forces.
         Unless the aggregates are stronger than concrete, it is not likely to influence
strength of concrete. On the other hand, of aggregates is having low crushing
strength its use will adversely affect the strength of concrete


    3) SPECIFIC GRA VITY:-

       As the aggregate contains pores both permeable & impermeable, the
meaning of specific gravity has to be carefully defined and hence specific gravity
may be of several types
a) Apparent specific gravity :-
 J).0,………… specific gravity:-
 K)
 M) Apparent
 L)
 N)
 O)
 P)
 Q)
 R)~1)

       It is defined as ratio of weight of oven dry aggregates to its absolute
excluding all pores. It is also called Absolute specific gravity.

b)Bulk specific gravity:-

       It is defined as the ratio of oven dry aggregates to its absolute volume
including the natural pores in the particles It is termed as simply, " Specific Gravity

                       Sand -- 16 kN/m3

                       Steel -- 78 kN/m3

         Bulk specific gravity is generally used for calculation of yield of concrete ,in
concluding quantities etc.but the actual value of the specific gravity of aggregates is
not a measure of its quality. It is also required in mass concrete structures such as
dams ,retaining walls, where minimum density of concrete is essential for the
stability of the structure.

       The majority of natural aggregates have a specific gravity between 2.6 % to
2.7 %, but values less than 2.6 % does not mean that aggregates are of bad quality.
However no maximum limits of specific gravity has been specified.
       .

4) MOISTURE:-

        Generally termed as surface moisture. It is defined as the difference in
weight of aggregates of in saturated surface dry condition (i.e. an aggregate having
all pores filled with water but having dry surface) and in moisture condition (i.e The
aggregates having all pores filled with water and also having \vet surface) It is a
expressed as percentage.
5) GRADING OF AGGREGA TES:-

        The art of doing grading (Particle sized distribution) of an aggregates as
determined by sieve analysis is know as grading of aggregates Sieve analysis is the
simple operation of dividing the sample of aggregates into fractions, each
consisting of particle of same size

         The principle of grading is that, the smaller particle will fill up the void
between the large particles. This result in most economical use of cement paste
for filling the voids and binding together the aggregates In the preparation of
concrete Thus proper grading of coarse & fine aggregates In a concrete mix
produces a dense concrete with less quantity of cement

         Concrete of desired strength can economically prepared with various
grading of coarse & fine aggregates. The grading of aggregates has a considerable
affect on workability. uniformity and finishing qualities of concrete mix which
thus ultimately affect the economy, strength, durability and ,1ther properties of
concrete .Hence, fine and coarse aggregates to be used in concrete should be well
graded.

       The following IS sieve are generally used for grading of aggregates-

with fine mesh wire cloth -- 3.35 mm , 2,36 mm , 1,18 mm, 600 micron, 300 micron
, 150 micron, 75 micron. With square hole performed plate- 80 mm, 63 mm. 50
mm, 40mm,31.5 mm,25 mm, 20mm, 16 mm, 12.5 mm, 10 mm, 6.3 mm. 4.75mm.
6) FINENESS MODULUS:-

        A single factor calculated from the sieve analysis, which gives an idea
about fineness or coarseness of an aggregate is know as Fineness modulus
(FM.)

        The fineness modulus is defined as the sum of the cumulative percentage
retained on IS sieves, according to the type of aggregates, dividing by 100. The
F.M is roughly proportional to the average size of particles in the aggregates
i.e. coarser the aggregates more are the F.M. It does not represent the particle
size distribution but useful to measure slightly variation in aggregates from same
sources .The IS sieve is used to determine F. M. are:-

       Coarse aggregates -- 80 mm , 40mm, 20mm, 10mm, 4.75mm.

      Fine aggregates -- 475 mm, 2.36 mm, 1.18 mm, 600 micron, 300 micron,
150 micron, etc.
     UNIT NO.3

WATER CEMENT RATIO
                      3. WATER CEMENT RATIO

3.0 GENERAL:-

        Strength of concrete depends on quantity and quality of its grade. A rich mix
provides higher strength not only because of using more cement, but amount of
water to be added plays an important role in this respect.

        Insufficient quantity of water harsh and unworkable mix whereas excess
causes segregation and bleeding. Hence, it is necessary to add property of water to
allow hydration of cement and makes a workable mix enough to be easily placed
inside the form.

Definition'-
        The ratio of volume or weight of water to that of cement in a concrete
mix is called as Water-Cement ratio.

       It is usually expressed in liters of water required per bag (50 Kg of
cement)


3.1 DEVELOPMENT OF STRENGTH OF CEMENT
COMPOUNDS WITH AGE :-

       Lower the ratio, greater is the strength of concrete decrease as the water
cement ratio, increases.

       It can be illustrated graphically by plotting the water cement ratio (By
weight) as abscissa and compressive strength of 28 days as ordinate.

        In case water cement ratio is less than 0.45 the curve is seen bending
downward This indicate that concrete mix having W/C ratio less than 0.45 by weight
is not workable and causes honey combed structures, thus strength of concrete is
decreased. Doted curved shows that with good compaction by mechanical vibration
the strength of concrete is increased with water cement ratio even less than 0.45,
subject to minimum of 0.35. Thus water cement ratio act as a yard stick to obtained
concrete of desired strength.



        In India CRRI (Cement Road Research Institute, New Delhi) charts are
gcneral1y used to calculate water cement ratio of concrete. This chart shows
relation between water cement ratio and compressive strength. The designed
curved for cement concrete mixes in relation to 7 days compressive strength. The
curves are very useful for obtaining water cement ratio for trial mixes of
concrete.
      UNIT NO.4

PRINCIPLES OF CONCRETE

         MIX

       DESIGN
       4.PRINCIPLES OF CONCRETE MIX DESIGN


4.0 BASIC CONSIDERATION :-

       Design of concrete mixes Involves determination of the proportions of the
given constituents, namely cement coarse and fine aggregates, water and
admixtures if any, which would produces concrete possessing specified properties
both in fresh and harden state with the maximum economy.

       Workability is specified as Important property of concrete in fresh state, for
harden state compressive strength and durability are important. The mix design ,
therefore, generally carried out for a particular compressive strength of concrete
with adequate workability so that fresh concrete can be properly placed and
compacted and to achieve required durability .

       The following are the basic assumptions made in design of plastic
concrete mixes or medium strength :-

               a) The compressive strength of the concrete is governed by the its
                  water cement ratio.
               b) For a given aggregates characteristics, the workability of
                  concrete is governed by its water content.
4.1 FACTORS IN THE CHOICE OF MIX DESIGN:-

       Both IS 456--1978 as well as IS 1343--1980 envisage that design of
concrete mix be based on following factors :-

                      a) Grade designations.

                      b) Type of cement.

                      c) Maximum normal size of aggregates.

                      d) Minimum water cement ratio.

                      e) Workability

                      f) Minimum cement content




4.2 OUTLINE OF MIX DESIGN PROPORTIONING:--

        The various factors for detem1ine the concrete proportions and the step by
step procedure for concrete mix design can be represented as in fig. The basic
step involved can be summarized as follows.

                      a) Arrive at the mean target strength from the characteristic
                         strength specified and the level of quality control.

                      b) Choose the water cement ratio for mean target strength
                         and check for requirements of durability

                      c) Arrive at the water cement ratio for workability.
 d) Calculate cement content and check durability.

 e)   Choose the relative proportions of coarse and fine
      aggregates, from the characteristics of coarse and fine
      aggregates .




f) Arrive at the concrete mix proportions for the first trial
   mix.

g)    Conduct trial mixes with suitable adjustment till the final
      mix composition is arrived.
     UNIT NO.5

METHODS OF CONCRETE

       MIX

      DESIGN
5. METHODS OFCONCRETE MIX DESIGN

5.0 GENERAL :-
         The mix design methods are being used in different countries are mostly
based on empirical relationships. Charts &graphs developed from extensive
experiment investigation .Most of them follow the same basic principle include
 in section 4 & only minor variations exist in different mix design methods in the
process of selecting the mix proportions.

METHODS OF MIX DESIGN :-
                       Indian Standard ( IS 1062-1982 )    Method .
                       Road Research Laboratory ( RRL ) Method .
                       American concrete Institute ( ACI ) Method .
                       Road Research Laboratory ( RRL ) Method .
                       Department Of Environment (DOE ) Method .
                       Trial & Error Method .
                       Maximum Density Method .
                       Fineness Modulus Method .

5.1 INDIAN STANDARD METHOD OF MIX DESIN
   GUIDELINES FOR CONCRETE MIX DESIGN ( IS10262-82 ):-
        Indian Standard Institution has brought out the mix design procedure mainly
based on the work done in national laboratory. This method can be applied
to both medium & high strength concrete. In case of fly ash cement concretes water
content arrived at can be reduced by @3% to 5% & proportion of fine aggregate can
be reduced by 2% to 4% .


DATA REQUIRED :-

               DESIGN DATA :
               a.       Grade of concrete (fck ).
           b.         Type of cement .
           c.         Degree of workability .
           d.         Maximum/minimum cement content.
           e.         Water cement ratio .
           f.         Type& Maximum nominal size of aggregates.
           g.         Standard deviation (s ) .
           h.         Type of quality control.


    TEST DATA FOR MATERIALS:-
    a. Specific gravity of cement, coarse & fine aggregate.
    b. Water absorption of coarse & fine aggregate.
    c. Surface moisture of coarse & fine aggregate.

5.2 STEPS INVOLVED IN THE METHOD :-
     1.TARGET MEAN STRENGTH (fck):-

                    ft = fck + K x S
     Where,
                ft = Target mean strength.

            fck = Characteristic strength.

                K = Statistical value.

                S = Standard deviation.



            TABLE NO.5.1
SUGGESTED VALUES OF STANDARD DEVIATION .

            Grade of         Very Good       Good             Fair
            concrete
               1.                 2.           3.              4.
              M10                2.0          2.3             3.3
              M15                2.5          3.5             4.5
              M20                3.6          4.6             5.6
                M25              4.3          5.3              6.3
                M30              5.0          6.0              7.0
                M35              5.3          6.3              7.3
                M40              5.6          6.6             7.77
                  M45               6.0            7.0            8.0
                  M50               6.4            7.4            8.4
                  M55               6.7            7.7            8.7
                  M60               6.8            7.8            8.8




2. SELECTION OF WATER CEMENT RATIO:-

            Various parameters like type of cement & aggregate, max .size of
aggregate .Surface texture of aggregate etc. shall influence the strength of concrete,
remain a water cement ratio constant, hence it is desirable to establish a relation
between concrete strength & Free water cement ratio with material & conditions to
be used actually at site . In absence of such relationship the free water cement ratio
Corresponding to the target strength may be determine from relationship fig.no.1.

            If 28days strength of cement is known use of figno.2 may be made for
more accurate estimation of water cement ratio .However ,this will need at least
28days for testing of strength of cement, there by delaying whole process by28days.


3. ESTIMATION OF ENTTRAPPED AIR:-

            The air content is estimated from table 5.2 for the normal maximum size
of aggregates used.

                   TABLE NO.5.2
        APPROPRIATE ENTRAPPED AIR CONTENT

            Max.size of aggregate         Entrapped air % of volume
                                                 of concrete
                      10                             3.0
                      20                             2.0
                      40                             1.0
4.SELECTION OF WATER CONTENT & FINE TO TOTAL AGGREGATE
RATIO:-

        The water content and percentage of sand of total aggregate by absolute
volume are determined from table no 5.3 for medium strength concrete, this table IS
based on the following conditions.

         1. Crushed ( angular ) coarser aggregate confirming to IS :383-1970.

         2. Fine aggregate of natural sand confirming to grading zone II of
            table 4 of IS : 383-1970.

         3. Workability corresponds to compacting factor of 0.80.

               Water cement ratio in case of table no. 5.3 is 0.60. for any departure
from above mentioned conditions, corrections have to be applied, based on table5.4 ,
on water content and percentage of sand as percent of total aggregate by absolute
volume, determined from table5.3




                         TABLE 5.4
            APPROXIMATE SAND & WATERPER CUBIC
                    METER OF CONCRETE
                                    W/C = 0.60
                              WORKABILITY = 0.80C.F
  Max.size of aggregate       Water content including   Sand as % of total
                              surface water per cubic aggregate by absolute
                                 meter of concrete           volume
            10                         200                     40
            20                         186                     35
            40                         165                     30
                           TABLE5.4
               ADJUSTMENT OF VALUES IN WATER CONTENT
                   AND SAND % FOR OTHER CONDITION

  Change in condition          Adjustment required in      Adjustment required in %
   stipulated for tables           water content            sand in total aggregate
 For sand confirming to                  0                     +1.5 % for zone I
grading zone I& zone III                                      - 1.5 % for zone III
   or zone IV of table                                        -3.0 % for zone VI
      4.IS:383-1970
 Increase or decrease in               +3.0 %                           0
  value of compacting                  -3.0 %
       factor by 0.1
  Each 0.05increase or                    0                           +1 %
decrease in water cement                                              -1%
           ratio
    For rounded edge                    -15kg                         -7 %




 5. CALCULATION CEMENT CONTENT:-

             The cement content per unit volume of concrete may be calculated from
free water cement ratio and the quantity of water per unit volume of concrete
(cement by mass = water content I water cement ratio)

              The cement content so calculated shall be checked against minimum
cement content for the requirement of durability (Table 5.5 ) and the greater value of
two is to be adopted.
                TABLE 5.5
MINIMUM CEMENT REQUIRED IN CEMENT CONCRETE TO
 ENSURE DURABILITY UNDER SPECIFIED CONDITION OF
                   EXPOSURE .

     Exposure            Plain     Plain concrete     Reinforced      Reinforced
                       concrete      Maximum           concrete.       concrete.
                       Minimum     water content      Minimum         Maximum
                        cement         ( kg )       cement content   water content
                       content (                         ( kg )          ( kg )
                          kg)
        Mild:            220           0.70              250             0.65
For e.g. completely
 protected against
     weather or
     aggressive
condition except for
  a brief period of
exposure to normal
 weather condition
during construction

      Moderate:          250           0.60              290             0.55
 For e.g. sheltered
from heavy & wind
 driven rain against
   freezing whilst
saturate with water,
   concrete soil &
      concrete
continuously under
        water
      Severe:             310            0.50                360                0.45
For e.g. exposed to
sea water alternate
wetting & drying &
freezing whilst wet
 ,subject to heavy
  condensation or
  corrosive fumes




 6. CALCULATION OF AGGREGATE CONTENT :-

      Aggregate content can be determined from the following equation :

                      v = [ W + C/Se + 1/p x Fa/Sfa] 1 /1000 -----------( 1 )

                      Ca = [1- p]/p x Fa x Sca/Sfa ---------------------------( 2 )

Where ,
      V = absolute volume of fresh concrete which is equal to gross volume (m3)
          minus the volume of entrapped air

      W = mass of water (kg) per m3of concrete,

      C = mass of cement (kg) per m3of concrete,

      Sc = specific gravity of cement

      P =    Ratio of fine aggregate to total aggregate by absolute volume,

   Fa, ca = Total mass of fine aggregate and coarse aggregate (kg) per m3 of
             concrete respectively,

  Sfa, Sca= specific gravity of saturated ,surface dry fine aggregate and coarse
            aggregate respectively

7. ACTUAL QUANTITIES REQUIRED FOR THE MIX :-

             It mentioned that above mix proportion has been arrived at on the
assumption that aggregates are saturated surface dry. For any deviation than this
condition i.e. when aggregate are moist or air dry correction has to be applied
uant1ty of mixing water as well as to the aggregate.
        The calculated mix proportion shall be checked by means of trial batches.
Quantities of material for each trial shall be enough for at least three sieve cubes and
concrete required to carry out workability test according to IS :1199-1959.




         Trial mix No. I should be checked for workability and freedom from segregation
and bleeding and its finishing property


       If the measured workability is different from the assumed
in the calculation ,a change in the water cement ratio is to be
made from table 5.4& the whole mix design has to recalculated
keeping the w/c ratio constant .A minor adjustment may be
made to improve the finishing quality or freedom from
segregations & bleeding .This will comprise trial mix no.2 Now
water cement ratio is changed by plus, minus 10 % pre-selected
value & mix proportions are recalculated .These will from trial
mix no.3 & 4 .Testing for trial mix No.2, 3, 4are done
simultaneously .These test normally provide sufficient
information ,including the relationship between compressive
strength & water cement ratio ,from which the mix proportions
for field trials may be arrived.
5.1 1. MIX DESIGN OF M20 GRADE OF CONCRETE
                 BY IS METHOD

  a. Design Data Required :-

     1. Grade of cement (fck )               20Mpa

     2. Maximum size of aggregates           20mm

     3. Degree of workability                Medium

     4. Degree of quality control            Fair

     5. Type of exposure                     Moderate

  b) Test data for materials :-

     1. Compressive strength of cement       43N/mm2

     2. Specific gravity of cement           3.15

     3. i) Sp.Gravity of coarse aggregates   2.85
        ii) Sp.Gravity of fine aggregates    2.54

     4. Water absorption
        i) Coarse aggregate                  0.3%
        ii) Fine aggregate                   0.2%

     5. Free moisture content
         i) Coarse aggregate                 Nil
        ii) Fine aggregate                   1.5%
 c) Target mean strength of concrete :-

               ft = fck + K.S

               fck = 20MPa

               K = 1.65

               S   = 5.6


Target mean strength   = 20 + 1.65 x 5.6

                       = 29.24N/mm2



 d) Selection of water cement ratio :-


           From fig. 5.1. the water cement ratio required for the target mean
     strength of 29.24 N/mm2 is 0.5 this is lower the maximum value of
     Prescribed for “Moderate “ exposure.( see table no.5.5 )


 e) Selection of water content & fine to total aggregate ratio :-


         As grade of concrete is M20 which is less than M35


 Therefore ,

     Water content per M3 of concrete      = 186lit

     Sand as % of total Agg. by absolute volume = 35%




                                                          Contd…….
Change in Condition                     Water Content        Adjustment Required
                                             %              % sand in total aggregate

For decrease in water cement ratio by
0.05 ( 0.50 -0.60 )                                 Nil         [-0.1x1]/0.005
                                                                   -2%

For increase in compaction factor
By 0.1 ( 0.92 – 0.80 )                              +3.6%          Nil


For sand confirming to zone I                       Nil            +1.5%

                                TOTAL               +3.6%           -0.5%

Therefore ,
      Sand content as percentage of total aggregate by absolute volume
                    = 35 – 0.5
                    = 34.5%

       Required water content = 186+3.6 x [186/100]

                                    = 192069 lit.


f) Determination of cement content :-

       Water cement ratio       = 0.50

       Water                    = 192.69

       Cement                   = 192.69/0.50

                                =     385.38 kg

                                =      345 kg (Assumed)


   This cement content is adequate for “Moderate “ exposure condition.
g) Determination of coarse & fine aggregates content :-

        From table 5.2, For the specified maximum size of aggregate of 20mm
the amount of entrapped air in the wet concrete is 2% .Taking this into account
& applying equation,


           V = [ W + C/Sc + 1/P x Fa/Sfa ] x 1/1000,&

           V = [ W + C/Sc + 1/( 1-P )x Ca/Sca ] x 1/1000

Fine aggregates :-

       980 = [192.69 +345/3.15 + 1/0.345 x Fa/2.54] x 1/1000


       Fa = 594.55 kg



Coarse aggregates :-

       980 = [192.69 +345/3.15 + 1/[1-0.345] x Ca/2.85] x 1/1000


       Ca = 1264.53 kg


The mix proportion thus become,

    Cement           :    Water    :     Sand    :    Coarse aggregate

      345            :    192.69    :    594.55 :       1264.53

                                   OR

       1             :     0.50     :     1.72    :     3.66
h) Determination actual quantity required for mix :-

           Adjustment required for water absorption.

   1. Water absorbed by C.A      = [Qty x % of saturation] / 100

                                 = [ 1264.53 x 0.3 ] /100

       Water absorbed by C.A         = 3.79 lit

   2. Free water in fine aggregate = [ Qty x %of surface moisture]/100

                                     = [ 594.55 x 1.5 ] /100

   Free water in fine aggregate = 8.918 lit

   Therefore,

   a) Actual Qty of Cement required         = 345kg

   b) Actual Qty of Water required          = 192.69 + 3.79 – 8.918

                                            = 187.56 lit

   c) Actual Qty of C.A required            = 1264.53 -3.79

                                            = 1260.74 kg

   d) Actual Qty of Sand required           =     594.55 + 8.918

                                            =     603.46 kg

   Therefore,
   The actual quantity of material required are :-

   Cement       : water          :     sand         : Coarse aggregate

   345          :     187.69 : 603.46               :    1260.74
                         OR

   1            :     0.5       :       1.75        :      3.65
            The mix should be prepared according to the number of cubes to be
   prepared from the above data & changing the water cement ratio , other trial
   mixes should be produced & tested for workability & strength.
5.1 1. MIX DESIGN OF M25 GRADE OF CONCRETE
                 BY IS METHOD

  b. Design Data Required :-

     1. Grade of cement (fck )               25 Mpa

     2. Maximum size of aggregates           20 mm

     3. Degree of workability                Medium

     4. Degree of quality control            Fair

     5. Type of exposure                     Moderate

  f) Test data for materials :-

     1. Compressive strength of cement       53 N/mm2

     2. Specific gravity of cement           3.15

     3. i) Sp.Gravity of coarse aggregates   2.8
        ii) Sp.Gravity of fine aggregates    2.5

     4. Water absorption
        i) Coarse aggregate                  0.3%
        ii) Fine aggregate                   0.2%

     5. Free moisture content
         i) Coarse aggregate                 Nil
        ii) Fine aggregate                   1.5%
 g) Target mean strength of concrete :-

               ft = fck + K.S

               fck = 25 MPa

               K = 1.65

               S   = 6.3


Target mean strength   = 20 + 1.65 x 6.3

                       = 35.395 N/mm2



 h) Selection of water cement ratio :-


           From fig. 5.1. the water cement ratio required for the target mean
     strength of 35.395 N/mm2 is 0.47 this is lower the maximum value of
     Prescribed for “Moderate “ exposure.( see table no.5.5 )


 i) Selection of water content & fine to total aggregate ratio :-


         As grade of concrete is M25 which is less than M35


 Therefore ,

     Water content per M3 of concrete      = 186lit

     Sand as % of total Agg. by absolute volume = 35%




                                                         Contd…….
Change in Condition                     Water Content       Adjustment Required
                                             %             % sand in total aggregate

For decrease in water cement ratio by
0.05 ( 0.47 -0.60 )                                  Nil       [-0.13 x 1]/0.005
                                                                  -2.6 %

For increase in compaction factor
By 0.1 ( 0.92 – 0.80 )                            +3.6%           Nil


For sand confirming to zone I                      Nil            +1.5%

                                TOTAL             +3.6%            -1.1 %

Therefore ,
      Sand content as percentage of total aggregate by absolute volume
                    = 35 – 1.1
                    = 33.9 %

       Required water content = 186+3.6 x [186/100]

                                    = 192.69 lit./m3


f) Determination of cement content :-

       Water cement ratio       = 0.47

       Water                    = 192.69

       Cement                   = 192.69/0.47

                                =     409.99 kg/m3

                                =      410 kg/m3 (Assumed)


   This cement content is adequate for “Moderate “ exposure condition.
i) Determination of coarse & fine aggregates content :-

        From table 5.2, For the specified maximum size of aggregate of 20mm
the amount of entrapped air in the wet concrete is 2% .Taking this into account
& applying equation,


           V = [ W + C/Sc + 1/P x Fa/Sfa ] x 1/1000,&

           V = [ W + C/Sc + 1/( 1-P )x Ca/Sca ] x 1/1000

Fine aggregates :-

       980 = [192.69 + 410 / 3.15 + 1 / 0.339 x Fa/2.5] x 1/1000


       Fa = 563.6 kg/m3



Coarse aggregates :-

       981 = [192.69 +410 / 3.15 + 1 / [1-0.339] x Ca/2.85] x 1/1000


       Ca = 1216.96 kg/m3


The mix proportion thus become,

    Cement           :    Water    :    Sand     :    Coarse aggregate

      410            :    192.69    :   563.6    :      1264

                                   OR

       1             :     0.50     :     1.72    :     3.66
j) Determination actual quantity required for mix :-

           Adjustment required for water absorption.

   1. Water absorbed by C.A      = [Qty x % of saturation] / 100

                                 = [ 1264.53 x 0.3 ] /100

       Water absorbed by C.A         = 3.79 lit

   2. Free water in fine aggregate = [ Qty x %of surface moisture]/100

                                     = [ 594.55 x 1.5 ] /100

   Free water in fine aggregate = 8.918 lit

   Therefore,

   a) Actual Qty of Cement required         = 345kg

   b) Actual Qty of Water required          = 192.69 + 3.79 – 8.918

                                            = 187.56 lit

   c) Actual Qty of C.A required            = 1264.53 -3.79

                                            = 1260.74 kg

   d) Actual Qty of Sand required           =     594.55 + 8.918

                                            =     603.46 kg

   Therefore,
   The actual quantity of material required are :-

   Cement       : water          :     sand         : Coarse aggregate

   345          :     187.69 : 603.46               :    1260.74
                         OR

   1            :     0.5       :       1.75        :      3.65
            The mix should be prepared according to the number of cubes to be
   prepared from the above data & changing the water cement ratio , other trial
   mixes should be produced & tested for workability & strength.
5.1 1. MIX DESIGN OF M40 GRADE OF CONCRETE
                 BY IS METHOD

  c. Design Data Required :-

     1. Grade of cement (fck )               40 Mpa

     2. Maximum size of aggregates           20 mm

     3. Degree of workability                High

     4. Degree of quality control            Very good

     5. Type of exposure                     Severe

  j) Test data for materials :-

     1. Compressive strength of cement       43N/mm2

     2. Specific gravity of cement           3.15

     3. i) Sp.Gravity of coarse aggregates   2.97
        ii) Sp.Gravity of fine aggregates    2.57

     4. Water absorption
        i) Coarse aggregate                  0.3%
        ii) Fine aggregate                   0.2%

     5. Free moisture content
         i) Coarse aggregate                 Nil
        ii) Fine aggregate                   1.5%
 k) Target mean strength of concrete :-

               ft = fck + K.S

               fck = 40MPa

               K = 1.65

               S   = 5.6


Target mean strength   = 40 + 1.65 x 5.6

                       = 49.24 N/mm2



 l) Selection of water cement ratio :-


           From fig. 5.1. the water cement ratio required for the target mean
     strength of 49.24 N/mm2 is 0.43 this is lower the maximum value of
     Prescribed for “Severe “ exposure.( see table no.5.5 )


 m) Selection of water content & fine to total aggregate ratio :-


         As grade of concrete is M40 which is higher than M35


 Therefore ,

     Water content per M3 of concrete      = 180lit

     Sand as % of total Agg. by absolute volume = 25%




                                                          Contd…….
Change in Condition                     Water Content        Adjustment Required
                                             %              % sand in total aggregate

For decrease in water cement ratio by
0.05 ( 0.43 -0.35 )                                   Nil       [-0.35 x 1]/0.05
                                                                   +1.6 %

For increase in compaction factor
By 0.1 ( 0.95 – 0.80 )                           + 4.5 %            Nil


For sand confirming to zone I                         Nil          +1.5%

                                TOTAL             +4.5 %            +3.1 %

Therefore ,
      Sand content as percentage of total aggregate by absolute volume
                    = 25 + 3.1
                    = 28.100 %

       Required water content = 180+ 4.5 x [180/100]

                                    = 188.1 lit./m3


f) Determination of cement content :-



       Water cement ratio       = 0.43

       Water                    = 188.1

       Cement                   = 188.1/0.43

                                =     437.44 kg/m3



   This cement content is adequate for “Severe “ exposure condition.
k) Determination of coarse & fine aggregates content :-

        From table 5.2, For the specified maximum size of aggregate of 20mm
the amount of entrapped air in the wet concrete is 2% .Taking this into account
& applying equation,


           V = [ W + C/Sc + 1/P x Fa/Sfa ] x 1/1000,&

           V = [ W + C/Sc + 1/( 1-P )x Ca/Sca ] x 1/1000

Fine aggregates :-

       980 = [188.1 + 438/3.15 + 1 / 0.281 x Fa/2.57 ] x 1/1000


       Fa = 472 kg/m3



Coarse aggregates :-

       980 = [188.1 + 438 / 3.15 + 1 / [1-0.281 ] x Ca/2.97] x 1/1000


       Ca = 1389 kg/m3


The mix proportion thus become,

    Cement           :    Water    :     Sand    :    Coarse aggregate

      438            :    188.1   :      472     :      1389

                                  OR

       1             :     0.43    :      1.1     :     3.1
l) Determination actual quantity required for mix :-

           Adjustment required for water absorption.

   1. Water absorbed by C.A      = [Qty x % of saturation] / 100

                                 = [ 1389 x 0.3 ] /100

      Water absorbed by C.A          = 4.2 lit

   2. Free water in fine aggregate = [ Qty x %of surface moisture]/100

                                     = [ 472 x 1.5 ] /100

   Free water in fine aggregate = 7.1 lit

   Therefore,

   a) Actual Qty of Cement required         = 438 kg/m3

   b) Actual Qty of Water required          = 188.1 + 4.2 – 7.1

                                            = 186.18 lit/m3

   c) Actual Qty of C.A required            = 1389 - 4.2

                                            = 1384.8 kg/m3

   d) Actual Qty of Sand required           =    594.55 + 8.918

                                            =    603.46 kg

   Therefore,
   The actual quantity of material required are :-

   Cement       : water          :      sand       : Coarse aggregate

   438          :     186.18 : 479                 :    1384.8
                              OR
     1          :     0.43   : 1.1                :       3.16
                  The mix should be prepared according to the number of cubes
   to be prepared from the above data & changing the water cement ratio , other
   trial mixes should be produced & tested for workability & strength.
5.1 1. MIX DESIGN OF M45 GRADE OF CONCRETE
                 BY IS METHOD

  d. Design Data Required :-

     1. Grade of cement (fck )               45Mpa

     2. Maximum size of aggregates           20mm

     3. Degree of workability                High

     4. Degree of quality control            Very good

     5. Type of exposure                     Severe

  n) Test data for materials :-

     1. Compressive strength of cement       43N/mm2

     2. Specific gravity of cement           3.15

     3. i) Sp.Gravity of coarse aggregates   2.97
        ii) Sp.Gravity of fine aggregates    2.61

     4. Water absorption
        i) Coarse aggregate                  0.3%
        ii) Fine aggregate                   0.2%

     5. Free moisture content
         i) Coarse aggregate                 Nil
        ii) Fine aggregate                   1.5%
 o) Target mean strength of concrete :-

               ft = fck + K.S

               fck = 45MPa

               K = 1.65

               S   = 6.0


Target mean strength   = 20 + 1.65 x 6.0

                       = 54.9 N/mm2



 p) Selection of water cement ratio :-


           From fig. 5.1. the water cement ratio required for the target mean
     strength of 54.9 N/mm2 is 0.45 this is lower the maximum value of
     Prescribed for “Severe “ exposure.( see table no.5.5 )


 q) Selection of water content & fine to total aggregate ratio :-


         As grade of concrete is M45 which is Higher than M35


 Therefore ,

     Water content per M3 of concrete      = 180lit

     Sand as % of total Agg. by absolute volume = 25%




                                                          Contd…….
Change in Condition                     Water Content        Adjustment Required
                                             %              % sand in total aggregate

For decrease in water cement ratio by
0.05 ( 0.45 -0.35 )                                  Nil        [-0.1x1]/0.005
                                                                   +2%

For increase in compaction factor
By 0.1 ( 0.95 – 0.80 )                             +4.5 %           Nil


For sand confirming to zone I                      Nil             +1.5%

                                TOTAL              +4.5 %           +3.5 %

Therefore ,
      Sand content as percentage of total aggregate by absolute volume
                    = 25 + 0.45
                    = 28.5 %

       Required water content = 180+ 4.5 x [180/100]

                                    = 188.1 lit.


f) Determination of cement content :-


       Water cement ratio       = 0.42

       Water                    = 188.1

       Cement                   = 188.1 / 0.42

                                =     447.85 kg/m3



   This cement content is adequate for “Severe “ exposure condition.
m) Determination of coarse & fine aggregates content :-

        From table 5.2, For the specified maximum size of aggregate of 20mm
the amount of entrapped air in the wet concrete is 2% .Taking this into account
& applying equation,


           V = [ W + C/Sc + 1/P x Fa/Sfa ] x 1/1000,&

           V = [ W + C/Sc + 1/( 1-P )x Ca/Sca ] x 1/1000

Fine aggregates :-

       980 = [188.1 +447.85/3.15 + 1 /0.285 x Fa/2.6] x 1/1000


       Fa = 488.30 kg/m3



Coarse aggregates :-

       982 = [188.1 +447.85/3.15 + 1/[1-0.285] x Ca/2.97] x 1/1000


       Ca = 1402.57 kg/m3


The mix proportion thus become,

    Cement           :    Water    :    Sand     :    Coarse aggregate

      447.85         :    188.1    :    488.30 :        1402.57

                                  OR

       1             :     0.42    :      1.10    :     3.13
n) Determination actual quantity required for mix :-

           Adjustment required for water absorption.

   1. Water absorbed by C.A      = [Qty x % of saturation] / 100

                                 = [ 1402.57 x 0.3 ] /100

      Water absorbed by C.A          = 4.2 lit

   2. Free water in fine aggregate = [ Qty x %of surface moisture]/100

                                     = [ 488.30 x 1.5 ] /100

   Free water in fine aggregate = 7.32 lit

   Therefore,

   a) Actual Qty of Cement required         = 447.85kg/m3

   b) Actual Qty of Water required          = 188.1 + 4.2 – 7.32

                                            = 184.98 lit/m3

   c) Actual Qty of C.A required            = 1402.57 – 4.2

                                            = 1389.37 kg/m3

   d) Actual Qty of Sand required           =    488.30 + 7.32

                                            =    495.62 kg/m3

   Therefore,
   The actual quantity of material required are :-

   Cement       : water          :      sand       : Coarse aggregate

   447.8        :    188.1       : 495.62          :    1398.37
                                  OR
     1          :     0.42      : 1.10            :     3.12
            The mix should be prepared according to the number of cubes to be
   prepared from the above data & changing the water cement ratio , other trial
   mixes should be produced & tested for workability & strength.
 UNIT NO.6

WORKABILITY
                            6. WORKABILITY

6.0 GENERAL:-

      The ease with which the concrete is placed in the formwork and
compacted is termed as Workability of concrete.

      Higher workability is needed for thin and heavily reinforced concrete.
The workability of concrete depends on the various properties of its ingredients.

6.1 SEGREGATION:

        The separation of coarse aggregate from rest of the concrete mix due to
difference in the their particle size and specific gravity is called as
"SEGREGA TION"

        It should be prevented under any circumstances, as it is harmful to the
concrete properties. Ready mix concrete in which segregation is more should not be
used in any case. Using segregated mix results into honey combing, weak and pores
layer, surface scaling in the hardened concrete.

        With cement content less, the segregation is more. The mixes which are not
designed properly or which have excess quantity of water are liable to segregation.
Segregation is more when the concrete is to be placed under water or pumped. It
usually results from the shaking during loading or discharging from considerable
height. Segregation indicates poor grading of aggregates. Care should be taken to
avoid segregation, however following points should be kept in mind in order to
eliminate or minimize segregation.

a) The mixes must be correctly designed.

b) The water cement ratio must be kept constant.

c) The height of fall of concrete must not exceed 3m in any case.
d) The concrete operations must me strictly supervised.

e) The concrete should be placed from the final position as near as possible.

f) The concrete should be mixed until it attains uniform co lour.

g) The air-entering agents must be used for reducing segregation.

h) The water or sand content or both should be charged for preventing
segregation.



6.2 BLEEDING:

        The appearance of water on the surface of the concrete after compaction is
called "BLEEDING" .

         Bleeding indicates the presence of excess of water in the concrete or
deficiency of fine aggregate or too much finishing. This layer is called as
"Loitance". It should be removed if new layer of the concrete is to be placed on old
one. If the bleeding goes on increasing, the concrete becomes permeable.

        The aggregate are generally formed in dry state. These have to be washed for
removing imparities which increase their moisture content. It varies from time to
time depending upon the climatic condition e.g. the aggregates if dry will absorb
moisture from the mix and if wet, will have more moisture. Therefore, while
calculating the amount of water to be added, consideration must be given to the
surface condition of the aggregates. Concrete with fine particles require more water
than concrete with coarse particles from the same workability.

       Following points must be remembered in order to prevent bleeding of
concrete:-

    a) The mix must be carefully and properly designed.

    b) The water content ratio must be kept constant.

    c) The richer concrete should be used.
   d) The air entraining agents should be used.

   e) The sand used should not be too fine

6.3 HARSHNESS:-

       The concrete mix which causes difficult in obtaining smooth finish or
good contact with formwork is known as "Harsh mix'.

        It occurs due to excess of middle size particles. It can also be due to
deficiency of fine aggregate to fill the voids in the coarse aggregate It can be
eliminated with sufficient proportion of mortar till the voids in the coarse
aggregates

6.4 FACTORS AFFECTING WORKABILITY:-

       The various factors affecting workability of concrete are listed as below

1) Water Content :-

       The workability of any concrete mix mainly depends upon the quantity of
water added into the mix. The determination of the correct quantity of water,
depends on the shape of the aggregates and size of aggregates, proportioning of
ingredients, methods of compaction and climatic conditions.

       Water plays an important role of lubricants when added into the mix. But on
the other hands, if its amount is increased, the strength of the concrete is reduced.
When more water is added, it occupies space in concrete. So evaporation takes
place from the concrete surface, it leaves behind voids and concrete cracks. It is
very essential to limit the water content and should be sufficient enough to produce
a workable mix.

       The quantity of water added influence not only the strength but only the
consistency of the concrete. If concrete is to be placed in congested section, then
water content is increased to obtain higher degree of workability. An increase in
water content must be accompanied by an increase in cement content in order to
maintain the strength of concrete.
2) Shape Of The Aggregate:

       An angular, rough, flaky and elongated aggregate reduces workability.
The rounded and smooth aggregate requires comparatively less quantity of
water for lubrications and hence they improve the workability of the concrete
with same quantity of water.

3) Size Of Aggregates:-

        Smaller size aggregate offers more surface area than large size aggregates
and thus they require more quantity of water for lubrication, hence for same degree
of workability less water is required for large size aggregates and they contribute to
the improvement of workability. Moreover less quantity of water is used in large
size aggregates also reduces quantity of the cement for the given water cement ratio

4) Grading of Aggregates:

        It plays its vital role when lean concrete mix of high workability is
required, for lean concrete mixes the grading should be continuous whereas for
rich concrete mixes grading should be coarse.

5) Surface Texture of Aggregate :

      The aggregate with rough surface requires more cement to produce
workable concrete mix and provide a better bond than the aggregate with
smooth surface. Thus aggregate with smooth surface will contribute to
improvement of workability.

6) Porosity and Absorption of Aggregates:

        A porous and non saturated aggregate requires more water than the non
porous and unsaturated aggregate. For the same degree of workability non porous
and saturated aggregate requires less quantity of water. Thus non porous and
saturated aggregate contributes to the improvement of workability.
UNIT NO.7


 CURING
   OF
CONCRETE
                      7. CURING OF CONCRETE

7.0 GENERAL:

       Curing is defined as the operation of maintaining the humidity and
temperature of freshly placed concrete during some definite period following
placing, finishing to assure satisfactory hydration of cement and hardening of
cement


7.1 NECESSITY

         The concrete hardens because of hydration i.e. the chemical reaction
between water and cement. The chemical action, which accompanies the setting of
concrete, is dependent on the presence of water. Although there is a sufficient water
at the time of mixing. It is necessary to ensure that the water is retained to enable the
chemical reaction to continue until the concrete is fully hardened. Properties of
concrete such as strength, water tightness, durability, wear resistance and volume
stability improves with the passage of time. Three gallons of water is approx.
required to hydrate one bag of cement

       If the loss due to evaporation is more from newly placed concrete, the
hydration process will stop and concrete will shrinks thus creating tensile stresses at
the drying surface. The development of the tensile stresses will result in the
formation of plastic shrinkage cracks. Hence curing is necessary and must be done
immediately after concrete is placed and compacted.
7.2 METHODS OF CURING:

       There are various methods of curing. The method to be adopted depends on
the nature of work and climatic conditions. Following are the methods of curing
the concrete:

a) Shading concrete works:-

       The object of shading concrete work is to prevent evaporation of water
from the surface even before setting this is adopted mainly in case of large
concrete work such as road slabs. This is essential in dry weather to protect the
concrete from the heat, direct sunrays and wind.



b) Membrane curing:-

        The method discussed above comes under moist curing. Another method of
curing is to cover the wet concrete surface by a layer of waterproof material, which
is kept into contact with the concrete surface for 7 days. This method of curing is
done or termed as membrane curing. A membrane will prevent the evaporation of
water form the concrete .The membrane can be either solid or liquid. They are
known as sealing compounds.

        This method does not require constant supervision .This method is
usually adopted where there is a shortage of water for moist curing. This method is
efficient as compared to other moist curing method as rate of hydration is
less. Moreover, the strength of concrete cured by membrane curing is less then that
of concrete, which is moist cured. When membrane is damaged, the curing is
badly affected.

c) Steam curing:-

       By using this method, the strength of the concrete can be increased rapidly
This can be used in precast concrete works. In steam curing the temperature of the
steam should be restricted to 75 degree C as in the absence of proper humidity the
concrete may dry to soon. In case of hot water curing the temperature may be raised
to any limit, say 100 degree ,at this temperature the gain of strength is about 70
%.of the 28 days strength after 4 to 5 hours.
d) Sprinkling of water:-

        Sprinkling of water continuously over the concrete surface provides an
efficient curing. It is mostly used for curing floor slabs. The concrete should be
allowed to set sufficiently before sprinkling is started. On small jobs sprinkling can
be done by hand. Vertical and sloping surfaces can be kept wet by sprinkling water
on the top surface and allow it to run down between forms and concrete. For this
method water requirement is more.


e) Pondind method:-

 This is the best method of curing. It is suitable for curing horizontal surfaces such
 as floors, roof slabs, roads and airfield pavements. The horizontal top surface of the
 beam can also be ponded. After placing the concrete, its exposed surface is first
 covered and removed and small bands of clay and sand, divided into numbers of
 square and rectangles. The water is filled in bands. The filling of water in this
 ponds is done twice of or three a day depending upon the atmospheric conditions.
   CHAPTER NO. 8

TESTS ON CONCRETE
COMPRESSIVE STRENGTH :-

         The compressive strength of concrete is one of the most important and useful
property. In most structural applications concrete is employed primarily to resist
compressive stress. In those cases where strength in tension or shear is primary
importance, the compressive strength is frequently used as a measure of these
properties. Therefore the concrete making properties of various ingredients of mix
are usually measured in terms of compressive strength. Compressive strength is also
used as a qualitative measure for other properties of hardens concrete. No exact
qualitative relationship between compressive strength and flexural strength, tensile
strength, modulus of elasticity, wear resistance, fire relationship or permeability
have been established or nor are they likely to be. However, approximate or
statistical relationship in same case has been established and this gives much useful
information to the engineers. It should be emphasized that compressive strength
gives only an approximate value of these properties and that other tests specially
designed to determine these properties should be useful more precise result are
required. For instance, the indicated compressive strength increases as the specimen
size decrease. The modulus of elasticity in this case does not follow the
compressive strength. Concrete containing about 6% of entrapped air is relatively
weaker in strength is found to be more durable than the dense and strong concrete

        Among the materials and mix variables, water cement ratio is the most
important parameter governing compressive strength. Beside water cement ratio,
the following factors also effect the compressive strength of concrete:-

       i. The characteristics of cement.

       ii. The characteristics and properties of aggregates

       iii. The degree of compaction

       iv. The efficiency of curing.

       v. The temperature during the period of curing.

       vi. The age at the time of compaction.

       vii. The condition of test.

       Compressive test is carried out on specimen cubical or cylindrical in shape.
Procedure:

        First of all ingredients are measured as per the design. Then the cement is
mixed dry with fine aggregates in concrete mixer until it attains uniform colour.
After than the coarse aggregate are mixed. Then the water is added as per the water
cement ratio which is derived from the mix design. After this the all ingredients are
mixed for five minutes or until the homogeneous is mixture is formed.

       Simultaneously, the inter surface of the mould is oiled. After this the
concrete is placed in moulds in approximately three layers, each layer is compacted
twenty five times with 16mm diameter and 60cm long tamping rod. Compaction
should be uniform.

        After the mould is filled complete the moulds are placed on the vibrator.
After vibrating the top layer of concrete is struck off with trowel, then the moulds in
air for about twenty four hours at a minimum 90% relative humidity and at a
temperature of 27oC. After the given period the moulds are opened and the prepared
concrete blocks are taken out and place for curing in curing tank.

        Then the compressive strength test is performed after seven and twenty eight
day of curing on compression testing machine. For this the cubes are taken out of
curing tank for at least one hour before testing. The loaded indicated by the pointer
on the compression machine are recorded.
Result:-

       The results of compressive strength of cubes of M20 grade of concrete for
given period of curing are tabulated below:



 W/C Ratio        Curing          Load at       Compressive        Average
                  Period          Failure         strength       compressive
                                   (kgf)         (N/mm2)           strength
                                                                   (N/mm2)




5)

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:599
posted:7/16/2012
language:
pages:68