ROAD NOTE METHOD NO. 4
MIX DESIGN
The 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
It’s 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 as
1. Aggregate/Cement Ratio 2. Water/Cement Ratio 3. Degree of workability 4. Grading
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
CONDITIONS MINIMUM STRENGTH AS % OF
AVERAGE STRENGTH Very good control with weigh batching.
Constant Supervision 75
Fair control with weigh batching Occasional supervision
60 Poor control, inaccurate volume batching of all in aggregates
No supervision
40
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.
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.
1 Nf Nc w
Where,
C – Weight of cement required per cubic metre of concrete ρc – Specific gravity of cement
ρfa – Specific gravity of fine aggregate
ρca – Specific gravity of coarse aggregate
ν – Percentage of entrained air in concrete ρw – Density of water
Thus the weight of cement ‘C’ is evaluated by the relation
C + Nf.. C + Nc C + W.C + 0.01 ν = 1 m3 ρc ρw ρfa ρw ρca ρw ρw
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:
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 Weight
B) 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
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 m3 Water=220/1000=0.220 F.A=680/1860=0.405 C.A=1020/1520=0.674
