The following points highlight the DOE methods of concrete mix design. The methods are: 1. The Department of Environment’s Design Method (DOE Method) 2. Concrete Mix Design by DOE Method using Fly Ash.
1. The Department of Environment’s Design Method (DOE Method):
The DOE method of mix design is an improvement over Road Note No. 4 method. This method of concrete mix design or proportioning mainly is based on the extensive field and laboratory experiments carried out by Road Research laboratory U.K. The Road Note 4 method was published for the first time in 1950. This method of mix design was most popular and widely used upto 1970 all over the world. Most of the Indian Concrete roads and air fields were designed by this method, but it is obsolete now.
The DOE method was first published in 1975 and revised in 1988. The DOE method of mix design is applicable to all types of concrete mixes including roads, while Road Note No. 4 was applicable to roads and air fields only. This method can also be used for concrete containing fly ash. DOE method is a standard method of mix design in U.K. now. This method used the relationship between water/cement ratio and compressive strength depending on the type of cement and aggregate used.
The water contents required to give various levels of workability, as very low, low, medium and high expressed in term of slumps or Vee Bee time or compacting factor, are determined for the two types of aggregates as crushed aggregate and gravel. This method is suitable for mix design of normal concrete mixes having 28 days cube compressive strength upto 75 MPa for non-air entrained concrete.
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The step by step procedure of mix design is given below:
Procedure:
Step 1:
Determine the target mean strength from the specified characteristic strength. Target mean strength = specified characteristic strength + standard deviation x Risk factor.
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(Risk factor is taken on the assumption that 5% of results are allowed to fall below the specified characteristic strength).
Step 2:
Calculate the water/cement ratio from Fig. 20.10. From table 20.46 the approximate compressive strength of concrete made with free water/cement ratio 0.5 is obtained.
From table 20.46, find out the 28 days strength for the type of cement and type of C.A. Now mark a point on the y-axis of the Fig. 20.10 equal to the compressive strength read from table 20.46, which is at w/c ratio 0.5.
From this point draw a parallel dotted curve nearest to the point of intersection using this curve, read off the w/c ratio as against target mean strength. The method will be more clear from the example below.
Step 3:
Determine the water content for the required workability, taking into account the size of aggregate and its type from table 20.47 and 20.48.
Step 4:
Determination of cement content ― The amount of cement can be determined from the total water content and w/c ratio.
i.e., Cement content = [(Weight of water)/(w/c ratio)]
The quantity of cement obtained by the above relation should be compared with the minimum cement content specified from 2800 durability. The higher of the two quantities should be adopted. If maxi 2600 mum cement content is specified, then the calculated cement content should be less than the specified maximum cement content.
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Step 5:
Determination of total aggregate content ― For determining the total aggregate content, the estimation of wet density of fully compacted concrete is required. This can be obtained from Fig. 20.11 for approximate water content and known specific gravity of aggregate. If the specific gravity of aggregate is not known its value for un crushed aggregate may be taken as 2.6 and 2.7 for crushed aggregate may be assumed as shown on curves A and B of the Fig. 20.11. The aggregate content is determined by subtracting the weights of cement and water content from the weight of fresh concrete read from the Fig. 20.11.
Step 6:
Determination of fine aggregate ― The proportion of fine aggregate in the total aggregate is determined from Fig. 20.12. Fig. 20.12 (a) is for 10 mm size, 20.12 (b) for 20 mm size, Fig. 20.12 (c) is for 40 mm size coarse aggregate. The parameters involved in Fig. 20.12 are max. size of aggregate, the water content, degree of workability and the percentage of fine aggregate passing through 600 micron sieve.
Knowing the proportion of fine aggregate from Fig. 20.12, the weight of fine aggregate may be obtained by multiplying the total weight of aggregate by this proportion. Knowing the fine aggregate, the weight of coarse aggregate can be found. The coarse aggregate can further be divided into different fractions depending upon the shape of aggregate. Generally figures given in Table 20.49 may be adopted.
Note:
1. From the proportions so obtained trial mix may be prepared and samples should be prepared and tested to confirm its suitability for the proposed concrete structure.
2. If fly ash is used along with cement, then water content may be reduced as shown in Table 20.48.
Example 2:
Design a concrete mix for targeted 28 days cube strength of 45 MPa from the following data:
1. Max. size of uncrushed aggregate to be used = 20 mm
2. Low workability, slump = 10-30 mm
3. Specific gravity of aggregate = 2.65
4. The aggregate percentage passing 600 micron = 50%
5. Exposure to concrete is moderate
6. The cover to the reinforcement to be provided is 25 mm.
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.