British Standard Mix Design Provisions(DOE METHOD)

Introduction

• The British method of concrete mix design, popularly referred to as the "DOE method is used in the United Kingdom and other parts of the world and has a long established record. The method originates from the "Road Note No 4 which was published in Great Britain in 1950

• The DOE method utilizes British test data obtained at the Building Research Establishment, the Transport and Road Research Establishment, and the British Cement Association. The aggregates used in the tests conformed to BS 882 and the cements to BS 12 or BS 4027

The DOE method is based on various assumptions and requirements

• Mixes are specified by the weights of the different materials contained in a given volume of fully compacted concrete.

• It is assumed that the volume of freshly mixed concrete equals the sum of the air content and of the absolute volumes of its constituent materials. The method therefore requires that the absolute densities of the materials be known in order that their absolute volumes may be calculated.

It is assumed that the strength of a concrete mix depends on: The Free water/Cement Ratio

• The Free water/Cement Ratio;• The Coarse Aggregate Type• The Cement Properties.

• On the basis of tests the DOE Method provides a Table and a Figure from which one can estimate the free water/cement ratio, which will provide a given strength for concrete made from given coarse aggregate and cement types.

It is assumed that the workability of a concrete mix depends primarily on:

• The Free Water Content;• The Fine Aggregate Type and, to a lesser degree, the Coarse

Aggregate Type;• The Maximum Size of Coarse Aggregate.

On the basis of tests the DOE Method provides Figures from which on can estimate the percentage of fine aggregate, which for given a free water/cement ratio will provide a given workability for concrete made from fine aggregate of a given grading.

The DOE Method also provides guidance on the effects of air entrainment in a concrete mix.

The mix design is carried out according to the DOE Method in the following Five stages.

Stage (I):Determine Free Water/Cement Ratio Required for Strength

Stage (II). Determine Free Water Content Required for Workability

Stage (III). Determine Required Cement Content

Stage (IV). Determine Total Aggregate Content

Stage (V). Determine Fine Aggregate Content

Stage (I). Determine Free Water/Cement Ratio Required for Strength

• Either use a specified margin or calculate a margin for a given proportion of defectives and statistical standard deviation.

• Obtain the target mean strength by adding the margin to the required characteristic strength.

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

• Either accept a specified free water/cement ratio or obtain the maximum free water/cement ratio which will provide the target mean strength for concrete made from the given coarse aggregate type and from cement with the given properties.

Stage (II). Determine Free Water Content Required for Workability

• Either use a specified free water content or obtain the minimum free water content, which will provide the desired workability for concrete made with the given fine aggregate type, coarse aggregate type and maximum size of coarse aggregate.

• If the free water content has been determined for workability, adjust the required free water content if air entrainment is specified, and adjust further if a water-reducing admixture is specified.

Stage (III). Determine Required Cement Content

• Obtain the minimum cement content, which is required for strength, by dividing the free water content obtained in Stage (II) by the free water/cement ratio obtained in Stage (I).

• Check the minimum cement content, which is required for strength, against the maximum cement content, which is permitted, and give a warning if the former exceeds the latter.

• Check the minimum cement content, which is required for strength, against the minimum cement content, which is allowable for durability, and adopt whichever is greater to be the cement content in the mix.

• Divide the free water content by the cement content used in the mix to obtain a modified free water/cement ratio.

Stage (IV). Determine Total Aggregate Content

• Obtain a value for the overall aggregate density.• Obtain the fractional volume of the aggregate by

subtracting the proportional volumes of the free water and the cement from a unit volume.

• Calculate the total aggregate content by dividing the volume of the aggregate by the aggregate density.

Stage (V). Determine Fine Aggregate Content

• Either use a specified value of the percentage of fine aggregate, or obtain the percentage of fine aggregate, which will provide the desired workability for concrete made with the given grading of fine aggregate, maximum size of coarse aggregate and the free water/cement ratio obtained in Stage (III).

• Calculate the fine and coarse aggregate contents from the total aggregate content obtained in Stage (IV) and the percentage of fine aggregate.

Step 1Find the target mean strength from the specified characteristic

strength

Target mean strength = specified characteristic strength + Standard Deviation X Risk Factor

Risk factor is on the assumption that 5% of results are allowed to fall less than the specified d characteristic strength

STEPS INVOLVED IN DOE METHOD

Step 2.Calculate the water /cement ratio

• This is done in a rather round about method using the table and Figure given below

TABLE

• Table gives the approximate compressive strength of concretes made with a free w/c ratio of .50

• Using the table find out the 28 days strength for the approximate type of cement and types of Coarse aggregate

• Mark a point on the Y axis in Fig equal to the compressive strength read form table which is at a W/C ratio of .50.through this intersection point ,draw a parallel dotted curve nearest to the intersection point Using this new curve ,we read off W/C ratio as against target mean strength.

STEP 3

• Next decide water content for the required workability expressed in terms of slump taking into consideration the size of aggregate and its type from Table

Step 4

• Find the cement content knowing the water/ cement ratio and water content. Cement content is calculate simply dividing water content by W/C ratio .

• The cement content so calculated should be compared with minimum cement content specified from the durability consideration as given in table 9.20 and higher of the two should be adopted .

• Sometime maximum Cement content is also specified .The calculated cement content must be less than the specified maximum cement content

Step 5

• Find out the total aggregate content .This requires an estimate of the wet density of the fully compacted concrete .This can found out from Figure for appropriate water content and specific gravity of aggregate .If the specific gravity is unknown, the value of 2.6 for uncrushed aggregate and 2.7 for crushed aggregate can be assumed. The aggregate content is obtained by subtracting the weight of cement and water content from weight of fresh Concrete.

Step 6

• Then, proportion of fine aggregate is determined in the total aggregate using Figure 11.5.The parameters involved in Fig are maximum size of coarse aggregate ,level of workability ,the water/cement ratio ,and the percentage of fines passing 600 micron seive.Once the proportion of F.A is obtained ,multiplying by weight of total aggregate give the weight of fine aggregate. then the weight of C.A can be found out. Course aggregate .As a general Guidance the figure given below can be used

• The proportion so worked out should be tried in a trail mix and confirmed about its suitability for the given concrete structure

Thank You