Thursday, May 3, 2018

GRADE OF CONCRETE & MIX DESIGN


GRADE OF CONCRETE & MIX DESIGN

INTRODUCTION TO GRADE OF CONCRETE
      Among the many properties of concrete, compressive strength is found to be most important properties
      It is assumed to represent the overall quality of concrete
      Many engineering properties are generally found to be related to compressive strength of concrete
      So, concrete is mostly graded according to its compressive strength.
      Hence, grade of concrete is simply the representation of the compressive strength of concrete

Representation of grade of concrete
      Grade of concrete is represented by letter “M”, which represents mix, followed by number, which is the characteristic compressive strength of 15 cm cube at 28 days, expressed in N/mm2    



Representation of grade of concrete
      For example: Consider grade of concrete is M20
      M refers to mix
      20 refers to the characteristic compressive strength of 15 cm cube at 28 days, expressed in N/mm2    
      Hence, M20 grade of concrete has characteristics compressive strength of 20 N/mm2

Code recommendation for grade of concrete
      As per IS 456:2000, grade of concrete for RCC used shall not be less than M20
      As per IS 456:2000, grade of concrete for PCC used shall not be less than M15
      Grades as per table 5 of IS 456:2000 code is given below:

EXPOSURE CONDITION
PCC
RCC
Mild
-
M20
Moderate
M15
M25
Severe
M20
M30
Very severe
M20
M35
Extreme
M25
M40


Designation of Grade of concrete as per code
Group
Grade designation
Fck
Ordinary concrete
M10
10
M15
15
M20
20
Standard concrete
M25
25
M30
30
M35
35
M40
40
M45
45
M50
50
M55
55
High strength concrete
M60
60
M65
65
M70
70
M75
75
M80
80


Characteristic strength of concrete
      Cube test result for determination of compressive strength of concrete follows normal distribution
      It means that value of strength of material below which not more than 5% of test result results are expected to fall.
      Suppose 100 cubes are tested and specified characteristics strength is 30N/mm2
      Here 5% of cube means 5% of 100 = 5 and 95% of 100 = 95
      So, if strength of more than 95 cubes is more than 30N/mm2 then 30N/mm2 is the characteristic compressive strength of concrete

TARGET MEAN STRENGTH OF CONCRETE
      As per IS code, Target mean strength of concrete is defined as ft = fck + KS
      Where, ft = target mean strength of concrete
      fck = Characteristic strength of concrete
      K is a constant and derived from mathematics of normal distribution
      Here K is 1.65 for not more than 5% of test cubes fall under characteristics strength
      S is standard deviation

Normal distribution curve for test result



Mix design of concrete
      Mix design can be defined as the process of selecting suitable ingredients of concrete and determining their relative proportions with the object of producing concrete of certain minimum strength and durability as economically as possible.
      Following two methods can be used
      Nominal mix
      Design mix

Nominal mix
      In this type of mix, all the ingredients are prescribed and their proportions are specified. 
      Used for relatively unimportant work and simple concrete work
      It is used for concrete of M20 or lower
      Water cement ratio is based on durability criteria, experience & practical trials.
      Water content can be modified according to slump value (field-based test)
      There is no scope for deviation by designer
      Volumetric batching is used
      It is prescriptive type
      No quality controls
      No entrapped air content is considered.

Design mix
      In Design mix, quantity of each ingredients of concrete are calculated based on site condition, environment, etc. as per the code
       It is adopted for important concrete work
      It is used for higher grade of concrete
      Water cement ratio based on concrete grade & 28 days compressive strength of concrete & durability of concrete.
      Water content can be modified according to compaction factor value (laboratory-based test)
      Weight batching is used
      It is performance based
      There is quality control
      Entrapped air content considered according nominal maximum size aggregates.

NOMINAL MIX FOR different GRADE OF CONCRETE
      Nominal mix for some grade of concrete is given below in ratio of
Cement: Sand: Coarse Aggregate
Grade of concrete
Ratio (Nominal mix)
M10
1:3:6
M15
1:2:4
M20
1:1.5:3




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