CONCRETE MIX DESIGN

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1) Concrete

  • The word “concrete” originates from the Latin verb “concretus”, which means to grow together.

Advantage of Concrete

  • We have the ability to cast desired shapes Arches, piers, columns, shells
  • Properties can be tailored according to need (strength, durability)
  • Ability to resist high temperatures Will maintain structural integrity far longer than structural steel.
  • Does not require protective coatings
  • Can be an architectural & structural member at the same time.
  • Concrete As A Material used as
  • Infrastructure (bridges, dams, buildings)
  • Roadways/transportation systems

2) The Nature of Concrete

  • It is a composite material
  • Aggregates are 65% – 80% of the volume
  • Fine aggregate:  sand
  • Coarse aggregate: stone
  • Cement: applies to any binder, i) Portland cement, ii) fly ash, iii) ground slag, iv) silica fume, v) Water

3) The Purpose of the aggregates, cement & water

i) Large aggregates

  • provide density (fill space)
  • provide strength

ii) Fine aggregates

  • Fill small voids between large aggregates
  • Increases strength of the cement binder
FINE AGGREGATES

iii) Cement

  • produces a crystalline structure
  • binds aggregates together
CEMENT

iv) Water

  • causes chemical reaction to occur
  • water/cementitious “react”
  • produces workability

4) Properties of Concrete

  • Workability
  • Durability
  • Strength
  • Chloride Penetration Resistance
  • Abrasion Resistance

5) Concrete mix design

  • Concrete mix design may be defined as the art of selecting suitable ingredients of concrete and determining their relative proportions with the object of producing concrete of certain minimum strength & durability as economically as possible.

6) Objectives of mix design

The purpose of concrete mix design is to ensure the most optimum proportions of the constituent materials to fulfill the requirement of the structure being built. Mix design should ensure following objectives: –

  1. To achieve the designed/ desired workability in the plastic stage .
  2. To achieve the desired minimum strength in the hardened stage.
  3. To achieve the desired durability in the given environment conditions.
  4. To produce concrete as economically as possible.

7) Basic Considerations

The following point must be considered while designing concrete mixes: –

  1. Cost
  2. Specification
  3. Workability
  4. Strength and Durability

8) Specifications

The following point may be kept in mind while designing concrete mixes: –

  1. Minimum Compressive Strength required.
  2. Minimum water/ cement ratio.
  3. Maximum cement content to avoid shrinkage cracks.
  4. Maximum aggregate / cement ratio.

9) Workability

The following points related to workability shall be kept in mind while designing concrete mixes: –

WORKABILITY
  • The consistency of concrete should no more than that necessary for placing, compacting and finishing.
  • For concrete mixes required high consistency at the time of placing, the use of water-reducing and set-retarding admixtures should be used rather than the addition of more water,
  • Wherever possible, the cohesiveness and finish of concrete should be improved by increasing sand/ aggregate ratio than by increasing the proportion of the fine particles in the sand.

10) Strength and Durability

STRENTH AND DURABILITY
  • Strength and durability require lower w/c ratio. It is usually achieved not by increasing the cement content, but by lowering the water at given cement content.
  • Water demand can by lowered by throughout control of the aggregate grading and by using water reducing admixtures.

11) Grade of Concrete

  • The grade of concrete gives characteristic compressive strength of concrete. It is one of the important factors influencing the mix design.
  • The grade M 20 denotes characteristic compressive strength of 20 N/mm2. Depending upon the degree of control available at site, the concrete mix is to be designed for a target mean compressive strength applying suitable standard deviation.

The concrete shall be in grades designated: –

GRADE OF CONCRETE

12) What is M 20?

  • M refers to Mix.
  • 20 refers to characteristic compressive strength of 150 mm cube at 28 days in N/mm2.
  • The minimum Grade of Plain Concrete (PCC) shall be 15 N/mm2.
  • The minimum grade of reinforced Concrete (RCC) shall be 20 N/mm2.
M20

13) Nominal Concrete Mixes And Design Mix Concrete

i) Nominal Concrete Mixes

  • The wide use of concrete as construction materials has led to the use of mixes of fixed proportion, which ensures adequate strength. These mixes are called nominal mixes.
  • They offer simplicity and Under normal circumstances, has margin of strength above that specified.
  • Nominal mix concrete may be used for concrete of grades M5, M 7.5, M10, M15 and M20.
NOMINAL CONCRETE MIXES
Proportions of Ingredients in Nominal Mixes
  • The proportions of materials for nominal mix shall be in accordance.
PROPORTIONS OF INGREDIENTS IN NOMINAL MIXES

ii) Design Mix Concrete

  • The concrete mix produced under quality control keeping in view the strength, durability, and workability is called the Design Mix.
  • Others factors like compaction equipment’s available, curing method adopted, type of cement, quality of fine and coarse aggregate etc. have to be kept in mind before arriving at the mix proportion.
  • The design mix or controlled mix is being used more and more in variety of important structures, because of better strength, reduced variability, leaner mixed with consequent economy, as well as greater assurance of the resultant quality.

14) Factors Influencing Choice of Mix Design

  • Grade of Concrete
  • Type of Cement
  • Maximum nominal Size of Aggregate
  • Grading of Combined aggregate
  • Maximum Water/ Cement Ratio
  • Workability
  • Durability
  • Quality Control.

i) Grades of Concrete

GRADES OF CONCRETE

ii) Type of Cement

TYPE OF CEMENT
  • The rate of development of strength of concrete is influenced by the type of cement.
  • The higher the strength of cement used in concrete, lesser will be the cement content. The use of 43 grade and 53 grade of cement, gives saving in cement consumption as much as 15 % and 25 % respectively, as compared to 33 grade of cement.
  • For concrete of grade M25 it is advisable to use 43 and 53 grade of cement.

iii) Maximum Nominal Size Of Aggregates

  • The maximum size of C.A is determined by sieve analysis. It is designated by the sieve size higher than larger size on which 15 % or more of the aggregate is retained. The maximum nominal size of C.A. should not be more than one-forth of minimum thickness of the member.
  • The workability of concrete increases with an increase in the maximum size of aggregate. But the smaller size of aggregates provide larger surface area for bonding with the mortar matrix which gives higher strength.

iv) Grading of Combined Aggregates

  • The relative proportions of the fine and coarse aggregate in a concrete mix is one of the important factors affecting the strength of concrete.
  • For dense concrete, it is essential that the fine and coarse aggregate be well graded.
  • In the case when the aggregate available from natural sources do not confirm to the specified grading, the proportioning of two or more aggregate become essential.

v) Maximum Water/ Cement Ratio

  • Abram’s water/Cement ratio states that for any given condition of test, the strength of a workability concrete mix is dependent only on water/cement ratio. The lower the water/Cement ratio, the greater is the compressive strength.

vi) Workability

  • Workability of fresh concrete determines the case with which a concrete mixture can be mixed, transported, placed, compacted and finished without harmful segregation.

vii) Durability

  • Durability require low water/Cement ratio. It is usually achieved not by increasing the cement content, but by lowering the water demand at a given cement content.
  • Water demand can be lowered by through control of the aggregate grading and by using water reducing admixtures.

14) The Slump test

  • The slump test is a means of assessing the consistency of fresh concrete. It is used, indirectly, as a means of checking that the correct amount of water has been added to the mix.
  • The steel slump cone is placed on a solid, impermeable, level base and filled with the fresh concrete in three equal layers.
  • The cone is carefully lifted up, leaving a heap of concrete that settles or ‘slumps’ slightly. The upturned slump cone is placed on the base to act as a reference, and the difference in level between its top and the top of the concrete is measured and recorded to give the slump of the concrete.
  • When the cone is removed, the slump may take one of three forms.
  • In a true slump the concrete simply subsides, keeping more or less to shape.
  • In a shear slump the top portion of the concrete shears off and slips sideways.
  • In a collapse slump the concrete collapses completely.
THE SLUMP TEST
  • Only a true slump is of any use in the test.
  • If a shear or collapse slump is achieved, a fresh sample should be taken and the test repeated.
  • A collapse slump will generally mean that the mix is too wet or that it is a high workability mix.

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