Civil Engineering · Aggregate Testing · IS 2386 (Part 1)
Flakiness Test of Aggregates: Procedure, Formula & Permissible Limits
The flakiness test is the standard laboratory method civil engineers use to measure how many flat, plate-shaped particles are hiding inside a sample of coarse aggregate — and why that shape quietly weakens roads and concrete. This guide covers the full flakiness index definition, apparatus, step-by-step how to procedure, formula, IS code limits, advantages, disadvantages, safety, uses and a complete FAQ section.
The test is defined in IS 2386 (Part 1) – 1963 in India, and closely mirrored by BS 812-105.1 in the UK and ASTM D4791 in the US (which uses a slightly different flat-and-elongated ratio). It applies only to coarse aggregate between 6.3 mm and 63 mm, because standard gauge slots and length bars are only calibrated for that size range.
A closely related test, the elongation test, measures the opposite problem — particles that are too long rather than too flat. The two are frequently reported together as the combined flakiness and elongation index, since both shapes weaken a compacted layer in similar ways.
Why It Matters
Why is the Flakiness Test Important?
Particle shape controls how aggregate particles pack, interlock and carry load. Flaky aggregate behaves very differently from cubical aggregate under compaction and traffic, which is why the flakiness index is checked on almost every road and concrete project.
Weaker Interlock
Flat particles lie flush under load instead of interlocking, creating planes of weakness in the compacted layer.
More Voids
Flaky particles pack poorly, leaving extra voids that must be filled with more binder, cement paste or fines.
Higher Binder Demand
Their large flat surface area soaks up more bitumen or cement paste than an equal weight of cubical aggregate.
Breaks Under Rollers
Thin, flat particles fracture more easily under compaction rollers and repeated traffic loading.
Lower Strength
Concrete and pavement layers made with flaky aggregate typically show lower crushing and impact strength.
Poor Workability
Mixes become harder to place and compact uniformly, affecting surface finish and long-term durability.
Classification
Types of Non-Cubical (Undesirable) Particles
Aggregate particle shape is generally classified into four broad types: rounded, angular, flaky and elongated. The flakiness test and elongation test specifically target the last two, since they are the shapes most likely to reduce the quality of a finished layer.
Flaky Particle
Thickness is small compared to its length and width — it looks like a thin slab or plate. Identified with the thickness gauge.
Thickness < 0.6 × Mean DimensionElongated Particle
Length is large compared to its width and thickness — it looks like a rod or needle. Identified with the length gauge.
Length > 1.8 × Mean DimensionWell-shaped, angular to sub-rounded cubical aggregate is the target for all major applications, since it interlocks efficiently, resists crushing, and needs the least binder or paste to achieve full workability.
Equipment
Apparatus Required for the Flakiness Test
| Apparatus | Purpose |
|---|---|
| Standard thickness (slot) gauge | Metal gauge with slots of varying width matched to each aggregate size fraction; particles are hand-gauged through it. |
| IS Sieves (63, 50, 40, 31.5, 25, 20, 16, 12.5, 10, 6.3mm) | Separate the bulk sample into individual size fractions before gauging. |
| Weighing balance | Accurate to about 0.1% of the sample weight, used to weigh each fraction and the flaky material recovered. |
| Sample trays / containers | Hold sieved fractions and separated flaky particles during gauging. |
| Oven (optional) | Used to dry the sample to constant weight before testing. |
How To
How to Perform the Flakiness Test: Step-by-Step Procedure
The procedure below follows IS 2386 (Part 1) – 1963 and is the same general sequence used internationally for gauging flaky aggregate.
Collect and dry a representative sample
Obtain the aggregate sample by quartering a larger stockpile so it is representative, then dry it to constant weight if required.
Sieve into size fractions
Pass the sample through the specified IS sieves (63mm down to 6.3mm) and separate it into individual size fractions retained on each sieve.
Weigh each fraction
Weigh the material retained on each sieve size. Discard fractions below 6.3mm and above 63mm, as they fall outside the test’s scope.
Pass each particle through the matching slot
Take each size fraction separately and manually try to pass every particle through the correspondingly sized slot in the thickness gauge.
Collect the flaky particles
Particles that pass through the slot are, by definition, thinner than 0.6 times their mean dimension — set these aside as flaky material for each fraction.
Weigh and calculate the flakiness index
Weigh the total flaky material from all fractions and divide it by the total weight of the sample gauged, then multiply by 100 to get the flakiness index in percent.
Formula
Flakiness Index Formula & Sample Calculation
Worked example: Suppose a 5 kg coarse aggregate sample is sieved and gauged, and a total of 750 grams of particles pass through the thickness gauge slots across all size fractions.
A result of 15% flakiness index would generally be acceptable for most surfacing and premix applications, though the exact acceptable value always depends on the governing specification for that particular layer.
Specifications
Permissible Flakiness Index Limits
Acceptable limits differ by application and governing code (IRC/MoRTH for roads, IS 383 for concrete aggregate). The chart below shows commonly referenced maximum limits used in road construction practice — always confirm against the project’s own specification.
Values shown are typical maximum limits referenced in common road-works practice for illustration; verify against the current IRC/MoRTH specification or client-specific requirement before use in design or QC decisions.
Safety
Is the Flakiness Test Safe?
Evaluation
Advantages and Disadvantages of the Flakiness Test
✔ Advantages
- Simple and quick to perform with inexpensive apparatus.
- Gives a single, quantifiable percentage that is easy to specify and compare against limits.
- Directly linked to pavement and concrete durability, making it a reliable quality-control checkpoint.
- Standardized internationally (IS, BS, ASTM), so results are comparable across labs and projects.
- Non-destructive — the tested aggregate can still be reused for other purposes.
- Helps engineers reject unsuitable quarry sources early, before costly construction begins.
✘ Disadvantages / Limitations
- Measures only thickness relative to mean dimension — it does not fully describe overall particle shape or angularity.
- Manual gauging of every particle is labor-intensive and time-consuming for large samples.
- Results can vary slightly with operator technique and how firmly a borderline particle is pushed through the slot.
- Not applicable to aggregate outside the 6.3mm–63mm range.
- Does not by itself capture elongation; a separate test and the combined index are needed for full shape assessment.
- Gauge slots require periodic calibration to remain accurate.
Applications
Uses of the Flakiness Test
The flakiness index is checked wherever coarse aggregate shape affects strength, workability or durability:
- Cement concrete mix design
- Bituminous road layers (BM, DBM, BC)
- Water bound macadam (WBM)
- Granular sub-base (GSB)
- Railway ballast quality control
- Ready-mix concrete batching plants
- Quarry source approval
- Airport pavement construction
Comparison
Flakiness Index vs Elongation Index
| Parameter | Flakiness Index | Elongation Index |
|---|---|---|
| What it measures | Flat, plate-shaped particles | Long, rod-shaped particles |
| Gauge used | Thickness (slot) gauge | Length gauge |
| Governing ratio | Thickness < 0.6 × mean dimension | Length > 1.8 × mean dimension |
| Applicable size | 6.3mm to 63mm | 6.3mm to 63mm |
| IS Code | IS 2386 (Part 1)-1963 | IS 2386 (Part 1)-1963 |
| Reported together as | Combined Flakiness and Elongation Index | |
Frequently Asked Questions
Flakiness Test: FAQs
It is a laboratory test that measures the proportion of flat, plate-like particles in coarse aggregate. A particle counts as flaky when its thickness is less than 0.6 times its mean dimension, and the result is reported as the flakiness index, a percentage by weight.
IS 2386 (Part 1) – 1963 is the governing standard in India. Internationally, comparable procedures are given in BS 812-105.1 and ASTM D4791, though ASTM uses a different flat-and-elongated ratio.
Flakiness Index (%) = (Weight passing the gauge slots ÷ Total weight of sample gauged) × 100. Aggregate below 6.3mm or above 63mm is excluded from the calculation.
Typical maximums referenced in road works range from about 15% for premix surfacing up to 25–35% for base and binder courses, but the exact limit always depends on the governing IRC/MoRTH clause or project specification.
The flakiness index targets flat particles using a thickness gauge (0.6× ratio); the elongation index targets long particles using a length gauge (1.8× ratio). They are commonly combined into one reported value.
Yes. It is a non-chemical, non-destructive physical test. Standard lab precautions apply — safety glasses, gloves, dust care while sieving, and correct lifting of sample trays.
Flaky particles pack poorly, interlock weakly, break under roller compaction, and absorb excess binder — all of which reduce the strength and durability of the finished layer.
Only on coarse aggregate retained on the 6.3mm sieve and passing the 63mm sieve, since standard gauge slots are defined only for that range.
A set of IS sieves (63mm–6.3mm), a standard thickness (slot) gauge, an accurate weighing balance, and sample trays.
IS 2386 (Part 1) specifies minimum quantities that scale with the aggregate’s nominal size, ranging from roughly a few hundred grams for smaller fractions up to several kilograms for larger nominal sizes, to keep the sample statistically representative.
Yes — it’s used for coarse aggregate in cement concrete, bituminous mixes, WBM, railway ballast and granular sub-base, wherever particle shape affects workability and strength.
Lower is better. Values roughly under 15–25% are generally considered good for most structural and pavement uses, though the exact acceptable figure depends on the applicable code and layer type.