British DoE Concrete Mix Design | Mix design of Concrete - lceted

British DoE Concrete Mix Design

 

Objective

To design a concrete mix in accordance with the British DoE mix design method.

 

Theory and Scope

The British DoE method can be applied to produce designed concrete, using cement and aggregates which conform to the relevant British Standards. The method is suitable for the design of normal concrete having  28-day compressive strength as high as 75 MPa for non-air-entrained concretes. The method is also suitable for the design of concretes containing fly ash and GGBFS.

The mixes are specified by the mass of the different materials contained in a cubic metre of fully compacted fresh concrete. The method is based on the following four criteria:

 

1. The volume of freshly mixed concrete equals the sum of the absolute volumes of its constituent materials, i.e., the water, cement, air content and the total aggregate. The method, therefore, requires that the absolute densities of the materials be known in order that their absolute volumes may be calculated.

 

2. The compressive strength class of concrete depends on

a. The free water–cement ratio.

b. The type of coarse aggregate, i.e., whether the aggregate is crushed or uncrushed (gravel).

c. The type of cement, i.e., whether the cement is normal (ordinary) Portland cement or combined cement.

 

3. The consistency (workability) of concrete depends primarily on

a. The free water content.

b. The type of fine aggregate and, to a lesser degree, the type of coarse aggregate.

c. The nominal upper (maximum) size of coarse aggregate.

 

4. The consistency (workability) depends secondarily on

a. The fraction of the fine aggregate is a proportion of the total aggregate content.

b. The grading of the fine aggregate.

C. The free water–cement ratio.

 

Based on the method, the preliminary or trial mixes are made and desired properties of the trial mixes are checked; suitable adjustments are made to produce concrete possessing specified properties both in fresh and hardened states with the maximum overall economy.

 

Apparatus

Sieve sets for finding maximum nominal size, and gradings of coarse and fine aggregates; Weighing balance;  Trowels; Tamping bar; Moulds; Universal compression testing machine; Graduated cylinder; Slump cone apparatus and Buckets.

 

Procedure

Step 1: Perform sieve analyses of both the fine and coarse aggregates available to determine:

a. The nominal upper (maximum) size of coarse aggregate The designations of coarse aggregate are established from the nominal lower and upper sieve sizes for the particular aggregates, the lower size being stated first. For example, an aggregate of a maximum nominal size of 10 mm is designated as 4/10. The maximum aggregate sizes recommended are 10 mm; 20 mm and  40 mm.

b. Gradings of fine and coarse aggregates.

c. Gradings zone of fine aggregate.

If necessary, combine two or more different size coarse aggregate fractions so that the overall grading of coarse aggregate conforms to the desired grading for the particular nominal maximum size of aggregate.

 

 

Step 2: Determine the absolute densities, specific gravities, and absorption capacities of both the coarse and fine aggregates. Also, determine the specific gravity of overall aggregates in the saturated surface dry condition.

 

Step 3: Select the target consistency (workability) of fresh concrete in terms of slump class for the normal working range of zero to 200 mm. Where consistency other than slump is specified it is recommended that a relationship between the two is established.

 

Step 4: Estimate the strength margin factor and the standard deviation for calculation of the target mean corresponding to the 28-day characteristic strength specified. The margin takes into account the degree of safety required for the strength; it is either specified or calculated for a given proportion of defectives. The statistical standard deviation takes into account the conformity rules (quality control) during production. These quantities are different for cylinders or cubes.

 

Note: EN:206 classifies concrete strength in terms of 28-day characteristic strengths on the basis of cylinders and cubes, e.g., C25/30, where the first number is the strength of 150 mm (diameter) × 300 mm  (high) cylinder and the second number is the 150 mm cube strength. However, it should not be presumed that by giving both cube and cylinder strengths, a particular relationship is being assumed for purposes of conversion for concrete design or control.

 

Step 5: Obtain the target mean strength by adding a margin to the stipulated characteristic strength and statistical standard deviation.

If air entrainment is specified, calculate the artificially raised modified target mean strength.

Approximate compressive strength of concrete with a water-cement ratio as 0.5

 

Step 6:  Select the maximum free water–cement ratio which will provide the target mean strength for concrete made from the given types of coarse aggregate and cement as follows: For the given type of cement and aggregate, the compressive strength at the specified age corresponding to the reference water-cement ratio of 0.50 is obtained from Table above For example, when normal Portland cement and uncrushed aggregate are used, the compressive strength is 43 MPa at  28 days. With this pair of data (43 MPa and water-cement ratio = 0.50) as a controlling or reference point, a strength versus water-cement ratio curve is located in Fig. below In this particular case, it is the fourth (dotted) curve from the top of Fig. below passing the controlling point. Using this curve, the water-cement ratio is determined corresponding to the computed target mean strength. In case an  existing curve is not available that passes through the controlling point, the curve is interpolated  between two existing curves in Fig. below

Variation of compressive strength with water–cement ratio (DoE)

 

Compare this water-cement ratio with the maximum water-cement ratio specified for the durability from the  Table below and adopt the lower of the two values. The maximum water-cement ratio based on durability considerations includes a set of exposure classes related to different mechanisms of deterioration. Exposure class XO  exists on its own and there are no requirements for the water-cement ratio or the minimum cement content.

 

Minimum cement content and maximum water–cement ratio for different exposures

 

 

Abbreviations: w = with; wo = without; s = de-icing salt

 BS EN:2306-1 does not contain abrasion classes

 

Note: For a concrete designed using EN: 206 specifications for durability, the EN:206 specifications allow to count of the proportion (k) of addition in the combination with cement towards satisfying specified limits for minimum cement content and maximum water–cement ratio, rather than just the cement content. Since generally the presence of Type 2 (pozzolanic or latent hydraulic) addition reduces the heat of hydration and improves the durability of a mix. Here, the factor k called the efficiency or strength factor of the addition refers to relative strength of addition with respect to the cement.

 

Approximate water content required for target consistency (Workability)

 

Step 7:  Select the approximate free water content from the Table above, which will provide the target consistency  (specified in terms of a slump or flow diameter or Vee-Bee time) for the concrete made with the given fine and coarse aggregate types and nominal upper size of coarse aggregate.

When the coarse and fine aggregates used are of different types, the water content is estimated by the expression given by Eq. (below).

W = (2W_F/3)+(W_C+3)

 

Where,

Wf = water content appropriate to type of fine aggregate.

Wc = water content appropriate to type of coarse aggregate.

If the free water content has been determined for target consistency, adjust it for the specified air entrainment, and further adjust if the water reducing admixture is specified.

 

Step 8: Determine minimum cement content by dividing the free water content obtained in Step 7 by the free water–cement ratio obtained in Step 6.

Cement content (kg/m³) = (Water content/water-cement ratio)

a. Compare the computed cement content with the maximum cement content which is permitted. If the calculated cement content is higher than the specified maximum, then the target strength and target consistency (workability) cannot be achieved simultaneously with selected materials. In such a situation, the process is repeated by changing the type of cement, the type and the upper size of the aggregate.

 

b. Compare the computed cement content required for target strength with the minimum cement content which is specified for durability; adopt the greater of the two in the concrete.

 

Thus the cement content is the minimum given by a free water-cement ratio that is low enough to provide the target strength and durability.

 

Step 9: Determine the free water content which is available to react with the cement; it is the sum of (a)  the added water; (b) the surface water of the aggregates and (c) the water content of admixtures less (d) the water absorbed by the aggregate during the period between the mixing and the setting of the concrete.

 

Step 10: Divide the free water content by the cement content used in the concrete to obtain a modified free water–cement ratio.

Estimated wet density of fully compacted concrete (DoE)

 

Step 11: Compute the total absolute volume of aggregates as follows:

The total aggregate content (kg/m3) can be computed from the wet density of concrete obtained from Fig, above The wet density of concrete depends on the specific gravity of overall aggregates in the saturated surface dry condition.

Alternatively, the absolute volume fraction of the aggregate is calculated by subtracting the proportional volumes of the free water and cement from a unit volume of concrete using Eq. (below).

 

Absolute volume of aggregates = 1 – (c/1000Sc) – (W/1000)

 

where C and W are the cement and water contents, respectively, and Sc is the specific gravity of cement particles. Therefore,

 

Total aggregate content (kg/m³) = (1000S_a) x absolute volume of aggregates      (2)

 

where Sa is the specific gravity of aggregate particles. If no information is available Sa may be taken at 2.6 for uncrushed aggregate and 2.7 for crushed aggregate i.e. curves A and B can be used.

 

Step 12: Determine the fine and coarse aggregate contents as follows:

(a) Obtain the percentage of fine aggregate from Fig. below expressed as a percentage of total aggregate that will provide the target consistency of the fresh concrete to be made with the given grading of fine aggregate, the nominal upper size of coarse aggregate and the free water-cement ratio obtained in Step 10.

 

(b) Calculate the content of coarse aggregate from the total aggregate content obtained in Step 8 as follows:

 

Coarse aggregate content (per cent) = 100 – content of aggregate (percent)

 

(C) Divide the coarse aggregate further into different size fractions. Coarse aggregate fractions listed in the Table above can be used as a general guideline.

Recommended proportions of fine aggregate for different grading zones (DoE)

 

Proportions of different sizes of coarse aggregates

Aggregate size range (mm)	(2.36 /4) - (4 /10)	(4 /10) - (10 /20)	(10 /20) -(20 /40)
Type-I	33	67	–
Type-II	18	27	55

 

Step 13: Determine the concrete mix proportions for the first trial mix or trial mix no. 1. Measure the workability of the trial mix in terms of a slump; carefully observe the mix for freedom from segregation and bleeding and its finishing properties. If the slump of the first Trial mix is different from the stipulated value, adjust the water and/or admixture content suitably to obtain the correct slump.

 

Step 14: Make adjustments for aggregate moisture and determine final proportions. Since aggregates are batched on an actual weight basis, adjust the amount of mixing water to be added to take into account the aggregate moisture.

 

Step 15: Recalculate the mix proportions keeping the free water-cement ratio at the pre-selected value;  this will comprise Trial mix no. 2. In addition formulate two more trial mixes no. 3 and 4 with the water content same as Trial mix no. 2 and varying the free water-cement ratio by ±10 per cent of the preselected value.

 

Step 16: Test the fresh concrete for unit weight, yield and air content. Prepare trial mix and cast three 150  mm cubes and test them after 28 days of moist curing.

 

Step 17: Analyse mix nos. 2 to 4 for relevant information, including the relationship between compressive strength and water–cement ratio. Compute the water-cement ratio required for the mean target strength using the relationship. Recalculate the mix proportions for the changed water–cement ratio keeping water content at the same level as that determined in trial no. 2.

For field trials, produce the concrete by the actual concrete production method used in the field.

 

Observations and Calculations

 

The compressive strength of concrete mix is……….. 

The designed mix is suitable/it needs further revision The mix proportions are…………………

 

Precautions      

The slump test, cube casting, curing and testing should be done according to the specifications.

The fresh concrete should be carefully observed for freedom from segregation and bleeding, and finishing properties.

 

Discussion

EN:206 exerts relatively little influence directly on the process of design of concrete mixtures which is a  key part of concrete production. However, it does of course have a considerable indirect effect through the requirements for specification and conformity.

An exposure class which requires the greatest resistance in the form of the lowest water-cement ratio along with the highest minimum cement content and the highest concrete strength class is selected. However, the minimum cement contents are independent of the type of cement used. EN:206 specifies design margins in the minimum cement content of minus 10 kg and in maximum water-cement ratio plus 0.02 in trial batch tests.

The free water content which is available to react with the cement is the sum of (a) the added water; (b)  the surface water of the aggregates and (c) the water content of admixtures less than (d) the water absorbed by the aggregate during the period between the mixing and the setting of the concrete.

Target air content of fresh concrete For non-air entrained concrete, air content is not specified but entrapped air is as usual considered in the design for EN:206 concrete. For air entrained concrete, EN: 206 specifies minimum total air content with a maximum total air content being 4 per cent higher than the specified minimum.