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CONCRETE MIX DESIGN

Concrete Technology 2010

Definitions

Selection of concrete proportions involves a balance between economy and requirements for placeability, strength, durability, density, and appearance.

Placeability

Placeability is simply the traits that the wet or plastic concrete has that allow it to be placed and finished.

Placeability includes a mix that doesn't segregate but that can be consolidated.

The ability to be pumped is another consideration.

The classic way to measure placeability is slump.

The purposes of design:

To achieve the stipulated minimum strength and durability

To make the concrete in the most economical manner

Methods of proportioning

Arbitrary proportion Fineness modulus method Maximum density method Surface area method Indian Road Congress, IRC 44 method High strength concrete mix design Mix design based on flexural strength Road note No. 4 (grading curve method) ACI Committee 211 methodACI Committee 211 method DOE methodDOE method Mix design for pumpable concrete Indian standard recommended method IS 10262-82

Example

Design a concrete mix (using DOE method) for a reinforced concrete work. The concrete is to be designed for a mean compressive strength of 45MPa at the age 28 days with 65mm slump. Maximum size of aggregate is 20mm uncrushed aggregate will be used. Sieve analysis shows that 50% passes trough 600µm sieve. Bulk specific gravity of aggregate is found to be 2.65.

Step 1

find all the information from the question Mean compressive strength Type of cement used Slump required Max size of aggregates Type of aggregates Sieve analysis of fine aggregates Bulk specific gravity of aggregates Exposure / special conditions needed Special materials used e.g.: fly ash.

Step 2

calculate the water/cement ratio. This is done in a rather round about method, using table 1 and figure 1

Table 1 gives the approximate compressive strength of concretes made with free w/c ratio of 0.50.

Using this table find out the 28 days strength for the approximate type of cement and types of C.A (course aggregate)

Mark a point on the “Y” axis in fig 1 equal to the compressive strength read from table 1 which is at ratio of 0.50. through this intersection point, draw parallel dotted curve nearest to the intersection point. Using this new curve, we read off the w/c ratio as against target mean strength.

Step 3

Decide water content for the required workability, expressed in terms of slump, taking into consideration the size of aggregate and its type from table 2.

Step 4

find the cement content knowing the w/c ratio and water content.

Water content / (w/c ratio) = cement content

Step 5

Find out the density of fresh concrete from fig 2

The wet density : ________kg/m3Next, find the weight of total aggregateNext, find the percentage of fine

aggregate from fig 3 (b)Lastly, find out the weight of fine and

course aggregates

Table 1: Approximate Compressive Strength Of Concrete Made with a free water/cement ratio of 0.50. According to the 1988 British Method

Type of cement

Type of coarse

aggregate

Compressive strength at the age (cube) of days ( N/mm2)

3 days 7 days 28 days 91 days

Ordinary Portland cement (OPC) or Sulphate Resisting Portland (SRPC)

Uncrushed

Crushed

22

27

30

36

42

49

49

56

Rapid Hardening Portland

Uncrushed

Crushed

29

34

37

43

48

55

54

61

Table 2: Approximate Free Water Contents Required to Give Various Levels of Workability According to 1988 British Method

Slump (mm)V-B (s)

Very low0 – 10> 12

Low 10 – 306 - 12

Medium 30 – 60

3 - 6

High 60 – 180

0 - 3

1. Water content:

Maximum size of aggregate (mm)

Type of aggregate

10 UncrushedCrushed

150180

180205

205230

225250

20 UncrushedCrushed

135170

160190

180210

195225

40 UncrushedCrushed

115155

140175

160190

175205

2. Reduction in water content when fly ash is used:

Percentage of fly ash in cementitious material

Reduction in water content, kg/m3

10 5 5 5 10

20 10 10 10 15

30 15 15 20 20

40 20 20 25 25

50 25 25 30 30

Table 3: Requirements of BS 8110:Part I:1985 to Ensure Durability Under Specified Exposure Conditions of Reinforced and Prestressed Concrete Made With Normal Weight Aggregate.

Condition of exposure

Nominal cover of concrete in mm

MildModerateSevereVery severeExtreme

25----

2035---

20304050-

2025304060

2020253050

Maximum water/cementitious material ratio

0.65 0.60 0.55 0.50 0.45

Minimum content of cementitious material in kg/m3

275 300 325 350 400

Minimum grade MPa

30 35 40 45 50

Figure 1: Relation between compressive strength and free water/cement ratio for use in the British mix design method.

Figure 2: Estimated wet density for fully compacted concrete

Figure 3(a): Recommended proportions of fine aggregate according to percentage passing a 600 m sieve (maximum aggregate size: 10mm)

Figure 3(b): Recommended proportions of fine aggregate according to percentage passing a 600 m sieve (maximum aggregate size: 20mm)

Figure 3(b): Recommended proportions of fine aggregate according to percentage passing a 600 m sieve (maximum aggregate size: 40mm)

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