Road Mix Gradations Tips

Some must know calculations/tricks for road Engineers

Material mechanics is very important part of road engineering and hence gradation becomes very important for mechanical stabilisation, it is not possible that every time we recieve exact gradation mix, we have to achive that required gradation by mixing two or more diffrent gradation materials. Sometimes the index properties need to be modified by mixing two materials. there are some simple methods which can be easily applied on field to achive required gradation and Index properties as mentioned below.

1) Achieve required gradation by mixing

Many times material that we recieved on site is not as per gradation which is very important for mechanical stabilisation of the layer, if we have mixes having two different gradations but not as per norms or required gradation then it is possible to achieve required gradation in some proportion by mixing those two gradations, Mixing Proportion is the key here. It can be find out in following way.

Material A			Reccommended Limits			Material B
Num Diff	% Passing	Sieve Size (mm)	Lower Limits	Upper Limits	Arithmetic mean	% Passing	Num Diff
0	100	40	100	100	100	100	0
-8	98	20	80	100	90	73	17
-26.5	94	10	55	80	67.5	55	12.5
-33	83	4.75	40	60	50	42	8
-32	72	2.36	30	50	40	35	5
-32.5	55	0.6	15	30	22.5	21	1.5
-7	17	0.075	5	15	10	9	1
139							45

Material A:B = 45 : 139

Mix Ratio of Materials A:B = 1 : 0.32

Gradation Mixing Calculator

1) Achieve required gradation by mixing

Enter % passing of two materials and recommended limits. Mean will be calculated automatically.

Material A			Recommended Limits			Material B
Num Diff	% Passing	Sieve (mm)	Lower	Upper	Mean	% Passing	Num Diff
0	Total Difference						0

Note (IRC/MoRTH-aligned practice): The method shown is a simplified **difference/sum approach** (trial-and-error variant) for two materials. It calculates "deviation sums" from the mean specification at key sieves and ratios them inversely. This is quick on-site but approximate — always verify combined gradation post-mixing. For precision, use full **trial-and-error** (adjust proportions iteratively in Excel) or **least-squares optimization** (minimize sum of squared deviations from mid-spec). IRC:SP:89 and MoRTH emphasize checking combined gradation against GSB/WMM envelopes (e.g., Table 400-1/400-2) after blending.

2) Mixing of Two Material to get required P.I.

Ratio to Required P.I. A:B= (C-B) : (A-C)

A:B = (6-14) : (2-6)

A:B = 8 : 4 = 2:1

2) Mixing of Two Materials to get required P.I.

Parameter	Value
PI of Material A
PI of Material B
Required PI (C)

Note (IRC:SP:89, MoRTH Section 400): Linear weighted average method is adopted. Always confirm PI in laboratory after field mixing.

Note (IRC:SP:89, MoRTH Section 400): This linear weighted average method is standard for estimating blended PI (assumes linear behavior, valid for similar soil types). For road bases/sub-bases (GSB/WMM), target PI ≤ 6 (often ≤ 4-5 for better performance). High-PI soils (e.g., PI > 10-12) are mixed with low-PI materials (sand/gravel) or stabilized with lime/cement to reduce PI and swell. Always verify lab PI post-mixing — field variability (moisture, clay content) can differ. For expansive soils, PI reduction below 10 is critical to minimize volume changes.

3) Triangular Chart Method

This method is used for aggregate blending, its quite simple method if it can be done on AutoCAD software.

Steps :-

1. Make a triangle with course aggregate (30 to 2 mm), Sand (2 to 0.06 mm) and silt and clay (<0.06 mm) on one side each with 0 to 100%.
2. Mark materials A,B, C and desired gradation D on triangular Chart.
3. Joint Point D with A, B or C (Say C).
4. Extend it to meet the line AB at E.

% of A = EB.DC x 100 / AB.EC

% of B = EA.DC x 100 / AB.EC

% of C = ED/EC x 100

Note (IRC:SP:89, MoRTH Section 400): Linear weighted average method. Target PI ≤ 6 (preferably ≤ 4–5) for GSB/WMM. Always confirm in laboratory after mixing.

3) Triangular Chart Method (for 3 Materials)

Use this when blending three fractions (coarse, sand, fines). Measure lengths on your drawn triangular chart.

Segment	Length
AB
DC
EB
EA
EC
ED

Note (from highway engineering references): Triangular chart (ternary diagram) is ideal for blending **three** aggregate fractions (coarse, sand, fines) to target zones (e.g., maximum density via Fuller curve or Shilstone chart). It visually shows proportions via lever rule (inverse segments). Best used in AutoCAD/Excel for precision. Limitations: Less accurate for >3 materials (use multi-component graphical or software). Alternatives include: - **Rothfuchs balanced-area method** — Balances areas above/below spec curve for better packing. - **Straight-line/rectangular chart** — Plots cumulative % vs log sieve size. - **Power 45 curve** or **0.45 Power chart** — Targets maximum density gradation (common in Superpave/Asphalt Institute). IRC/MoRTH often prefer trial-and-error verified by lab trials for GSB/WMM; graphical methods aid initial proportioning.

Field tips: Always conduct sieve analysis on trial blends to confirm (combined gradation must lie within MoRTH Table 400 envelopes). For mechanical stabilization (e.g., GSB), aim for Cu > 4–5 and fines ≤ 5–12%. Use Excel solver for optimization if proportions are complex.