ROAD NOTE METHOD NO. 4

MIX DESIGNThe process of selecting suitable ingredients of concrete and determining their relative quantities with the purpose of producing an economical concrete which has certain minimum properties, workability, strength and durability.

BASIS OF THE METHOD

According to the Road Note No.4 is mainly based on the experimental investigations by Glanville et al

Deals with the effect of aggregate grading on the strength and workability of concrete

Its mandatory to note that designed concrete must be satisfactory both in the Plastic as well Hardened state

Necessary the choice of Mix proportions is governed by both these conditions

The required W/C ratio to produce the design characteristic strength depends upon the characteristics of the cement

By using same W/C ratio, different strengths are produced by Ordinary Portland, Rapid hardening Portland and High alumina cements.

The Road note No.4 presents a set of curves relating W/C ratio and Compressive strength at different ages and for different types of cement.

A rough indication of the degree of workability is obtained by the corresponding slump and compacting factor of the concrete at the work site.

Extensive Laboratory investigations were conducted at the ROAD RESEARCH LABORATORY to study the relation among the various parameters such as1. Aggregate/Cement Ratio2. Water/Cement Ratio3. Degree of workability4. Grading 5. Shape and size of aggregates

Based on the results, design tables are presented in Road Note No.4 to select suitable aggregate/Cement ratios for a concrete mix and filling up other parameters.

MIX DESIGN PROCEDURE

The following concrete mix design operations of low and medium strength has to be carried out

STEP 1

The average compressive strength of the concrete mix is to be obtained by applying the control factors to the minimum compressive strength.

The relation between the minimum and average compressive strength depend on the degree of quality control and shown in the below table 1

CONDITIONSMINIMUM STRENGTH AS % OF AVERAGE STRENGTH

Very good control with weigh batching.Constant Supervision75

Fair control with weigh batchingOccasional supervision60

Poor control, inaccurate volume batching of all in aggregatesNo supervision40

Table.1 Estimated relation between minimum and mean compressive strengths of site cube (ROAD NOTE NO.4)

STEP 2

The W/C ratio is required to give the necessary average compressive strength, at the desired age and for the type of cement used. Read Table 2 and get the value of W/C ratio.

Table.2 Relation between compressive strength and water cement ratio for 10 cubes of fully compacted concrete

STEP 3 The proportion of combine aggregate to cement is determined from Table 3 & 4 for aggregates of maximum size 40 and 20mm respectively.

The tables provide for four different degrees of workability detailed in Table 5.

STEP 4 If the aggregate available at the work site differ from the standard grading, the coarse and fine aggregates must be combined so as to produce one of the standard grading.

STEP 5 The proportion of cement, water, fine and coarse aggregates is determined having knowledge of the water and aggregate/cement ratios of the mix.

STEP 6 The quantities of ingredients required to produce one cubic metre of concrete is calculated by the absolute method, using the specific gravities of cement and aggregates.

METHOD OF COMBINIG AGGREGATES

Generally the aggregates quarried and stockpiled at work site do not conform to the standard grading.

In such cases it is desirable to proportion the available materials in such a way that the grading of combined aggregate corresponds to one of the four standard grading curves

This can be done either analytical or graphical calculations by using Road Note NO.4.

ANALYTICAL METHOD

The grading of fine and coarse aggregate available at work site are detailed in table 6.5 The fine aggregate and coarse aggregate of 20mm maximum size have to be combined so as to approximate to the coarsest grading of the standard grading curve No.1(Fig.2.2) The most economical mix having the highest permissible aggregate/cement ratio. On this curve 30% of the total aggregates passes the 4.75mm, IS sieve. If x y are the proportions of fine and coarse aggregates in the combined state, then to satisfy the condition that 30 % of the combined aggregate passes the 4.75mm I.S. Sieve, we have

1.0x + 0.07y = 0.3 ( x + y )

x : y = 1:3 Hence the fine and aggregate s have to be combined in the proportions of 1:3. The grading of the resulting combined aggregate is determined by columns (a) & (b) of Table 6.5 by 1.0 and 3.0 respectively and dividing the sum of these products by 4.0 The resulting combined grading is shown in column (f), the values being rounded off to nearest percent. In comparison with the standard grading curve shown in column (g), the combined grading is found to be in good agreement, except for minor deviations.

GRAPHICAL METHOD

The graphical method of combining the coarse and fine aggregates to conform to a standard grading is illustrated in Fig 6.2 for the same aggregates.

DETERMINATION OF THE WEIGHTS OF INGREDIENTS BY ABSOLUTE VOLUME METHODS

When mix proportions have been determined based on the knowledge of W/C, A/C ratios and relative proportions of the aggregates of various sizes, the weight of cement required to produce one cubic metre of concrete can be calculated using Absolute volume Method.

CEMENT: FINE AGGREGATE:COARSE AGGREGATE: WATER 1NfNc w

Where,

C Weight of cement required per cubic metre of concrete

c Specific gravity of cement fa Specific gravity of fine aggregate

ca Specific gravity of coarse aggregate

Percentage of entrained air in concrete

w Density of water

Thus the weight of cement C is evaluated by the relation

C + Nf.. C + Nc C + W.C + 0.01 = 1 m3 c w fa w ca w w

LIMITATIONS OF ROAD NOTE NO.4 METHOD

Design tables of aggregate/ Cement ratio recommended in Road Note No.4 cover only three shapes of aggregates and four type gradings But in practice aggregates having different shapes, sizes and properties have to use for the mixes. This method cannot be used directly for the design of air entrained concrete The nominal mix designed by using the data can be suitably adjusted for the desired air content in the mix.

PROBLEM ON ROAD NOTE METHOD NO.4

1. Design a concrete Mix to suit the following data using the Method of Road Note No.4

Specified works cube strength -200 kg/cm2 at 28 days

Degree of workability- High since concrete is required weigh batching and constant supervision

Type of cement Ordinary cement

Type of fine aggregate-Natural sand

Type of coarse aggregate-Crushed granite

Angular aggregate of 20mm max size. The aggregate available at works site have the following grading:

The specific gravity and bulk density of the various ingredients of the mix are as follows

Design the concrete mix and set out field mix proportions for weigh batching and volume batching. Also calculate the quantities of materials required 1 cubic metre of concrete.A) By WeightB) By Volume

DESIGN OF CONCRETE MIX

For the degree of control used refer table 1. Minimum strength is 75% of the average strength.

Mean design strength =200/0.75= 270 kg/cm2

Water cement ratio (from table 2) = 0.62

For High workability, the required

Aggregate/cement ratio (from table 3) = 4.8

The given aggregate do not belong to any particular standard grading curve.

They should be combined in suitable proportions to corresponding to the nearest practical grading, which is in this case is grading 3.

The fine aggregate and coarse aggregate are combined graphically as shown in figure Fig 6.3

The proportion of fine to total aggregate = 40%

Hence the proportions by weight of ingredients are given by

CEMENT: FINE AGGREGATE:COARSE AGGREGATE

1 :4.8x60: 4.8x100 100 100

2 : 1.92: 2.88

PROPORTION BY VOLUME

1 :1.92x1472: 2.88x1472 1680 1520

Or

Quantity of materials required for 1m3 of concrete

If C= weight of cement required per m3 of concrete

Then by absolute volume method, we have

C + 1.92C + 2.88C + 0.62 C = 1 m3 3.15x103 2.6x103 2.6x103 103

C=355 kg

Water= 0.62x355=220 kg

F.A=1.92x355=680kg

C.A=2.88x355=1020kg

Density of fresh concrete=2275kg/m3

Material quantities by volume

Cement=355/1472=0.231 m3Water=220/1000=0.220

F.A=680/1860=0.405

C.A=1020/1520=0.674