DOE Method of concrete Mix Design
Design:
Step 1:
Determine the target strength from the relation.
Target strength = specified characteristic strength + standard deviation x Risk factor In this case the target strength is directly given as 45 MPa at 28 days.
Step 2:
Find out the water/cement ratio for targeted strength 45 MPa.
(a) For finding out the w/c ratio, refer to table 20.46. From this table for ordinary port-land cement and uncrushed 20 mm aggregate the 28 days strength is 42 MPa.
(b) From Fig. 20.12 find the point of inter section of 42 MPa at 0.5 water/cement ratio. Through this point draw a dotted line curve parallel to the neighboring curve. Now draw a strength line from the desired strength on y-axis, where this line cuts the curve, draw a perpendicular on x-axis which will give w/c ratio for the targeted strength i.e., 45 MPa.
From this curve for 45 MPa strength the water/cement ratio comes out 0.54.
For uncrushed aggregate of 20 mm max. size aggregate approximate water content from table 20.47, is 160 kg/m3.
For moderate exposure and 25 mm cover maximum w/c ratio from durability consideration is permitted as 0.5. Actually the lower value of the two should be adopted. Hence adopt w/c ratio as 0.5 water content for 10 to 30 mm slump from table 20.47 is 160 kg/m3.
Cement Content:
From w/c ratio 0.5 and water content 160 kg, 160
Cement content = 160/0.5 = 320.4 kg.
From Durability consideration, the quantity of cement should be used as 350 kg/m3. Thus adopt greater cement content as 350 kg/m3.
Determination of Density of Fresh Concrete:
From Fig. 20.11, for water content of 160 kg/m3, 20 mm uncrushed, the wet density of concrete = 2490 kg/m3
Determination of total weight of aggregate.
Total weight of aggregate = Total weight of concrete-weight of water-weight of cement.
= 2490 – 160 – 350
= 2490 – 510 = 2180 kg/m3
Fine aggregate from Fig. 20.12 (b) for slump 10-30 and aggregate passing through 600 micron as 50% for w/c ratio 0.5
F.A. = 27%
∴ weight of fine aggregate = (2180 x 27)/100 = 588.6 = 589 kg/m3
weight of coarse aggregate = 2180 – 5886 = 1591.4 kg/m3
Estimated quantities in kg/m3 are:
(a) Cement = 350 kg/m3
(b) F.A. = 588.6 kg/m3
(c) C.A. = 1591.4 kg/m3
(d) Water = 160 kg/m3
(e) Wet density = 2490 kg/m3
From these quantities the trial mix is prepared, samples cast and tested at 28 days to judge the suitability of concrete for the desired work. If need arises, adjustment in quantities be made.
2. Concrete Mix Design by DOE Method using Fly Ash:
The use of fly ash as a part substitute of cement is fast gaining popularity throughout the world. Thus one should be acquainted with the procedure of concrete mix design containing fly ash. The method of design of mix is illustrated by the following example.
Example 3:
Design a concrete mix using fly ash from the following data:
(a) Characteristic 28 days mean strength = 25 MPa
(b) Risk factor may be taken =1.65
(c) Standard deviation = 5.0
(d) The max. size of coarse, aggregate C.A. = 20 mm
(e) 40% F.A. passes through 600 micron sieve.
(f) Slump range = 30-60 mm
(g) Cover to reinforcement = 25 mm
(h) Specific gravity of F.A. and C.A. = 2.6 and 2.7 respectively
(i) Percentage of fly ash to be used = 30%
Design Procedure:
By DOE method of mix design using fly ash.
Cement content is given by the relation:
where, F is the quantity of fly ash and it is given as:
where, P is the percentage of fly ash in the total cementitious material:
W = Free water content W
... W/(C + 0.3F) is the free water Cementitious ratio for the design strength Fig. 20.12
Specified targeted strength = 25 + 5 x 1.65 = 25 + 8.25 = 33.25 MPa
Taking 33 MPa as the target strength we proceed as follows:
From table 20.46 for 0.50 w/c ratio {Here W/(C + 0.3F)}, the 28 days compressive strength of concrete with ordinary cement is 42 MPa. For target mean strength of 33MPa, from Fig. 20.10, the value of free water W/(C + 0.3F) is 0.65.
Note:
This value is not to be compared with maximum value from durability consideration of table 17.3 it is to be used for strength purpose only. From table 20.47, for crushed 20 mm max. size aggregate and 60 mm slump, the quantity of water is 210 kg/m3. In the present case, the fly ash used is 30%. Hence reduce water content as partable 20.48. From this table reduction for 30% fly ash is 20 kg/m3.
Hence water content to be used = 210 – 20 = 190 kg/m3
From table 17.3 Cement content satisfy the durability requirements, but water/cementitious material ratio does not satisfy this durability condition. Hence adopt w/cementitious material ratio as 0.50 instead of 0.51.
Then water content = 370 x 0.5 = 185 kg/m3
For water content of 185 kg/m3, and average specific gravity of 2.65, the density of fresh concrete from Fig. 20.11 = 2420 kg/m3
Total weight of aggregate = 2420 – (259 + 111 + 185)
= 2420 – 555 = 1865 kg/m3
From Fig. 20.12 (b) for free water/cementitious material as 0.5 and for 40% F.A. passing through 600 micron sieve and for slump 30-60 mm is 38%.
∴ weight of F.A. = 38/100 x 708.7 kg. say 709 kg.
weight of C.A. = 1865 – 709 = 1156 kg.
Hence estimated quantities of materials on the basis of saturated surface dry condition are as follows:
The weight of aggregates may be adjusted according to free moisture content and absorption characteristics of the aggregates.
The trial mix may be prepared to determine its characteristic strength which should satisfy the desired requirements.
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