Aggregates make up roughly 60 to 75 percent of the volume of concrete, so their quality sets the ceiling for the strength and durability of the finished member. Checking aggregates before the pour catches problems that no amount of cement can correct later. Field and laboratory testing of coarse and fine aggregates follows standard procedures that produce numbers you can compare against code limits, which turns material selection from guesswork into an engineering decision.
Testing starts with understanding what you are working with. Recycled crushed concrete aggregates behave differently from virgin stone because they carry old mortar on the surface, which changes water demand and strength. Sources that reuse demolition material need tighter testing than quarries, so the same schedule does not apply to every supply.
Sampling of Aggregates
A test result is only as good as the sample it came from. A poorly taken sample produces misleading numbers even when the laboratory work is flawless, which is why sampling procedure comes first in any testing sequence. The goal is a representative portion of the entire supply, not a convenient scoop from the top of the pile.
Sampling from Stockpiles
Stockpiles segregate by size as material rolls down the face, with coarse particles collecting at the base and fines staying near the crest. Samples taken from a single point miss that variation. Take material equally from the top third, the middle third, and the bottom third of the stock height, remove the outer layer at each point to discard weathered and contaminated material, and combine the portions into one representative sample.
Reducing the Sample by Quartering
Field samples are larger than the test quantity requires, so the sample must be reduced without bias. Spread the material on a clean hard surface, mix it thoroughly, divide the pile into four equal quadrants, and discard two opposite quadrants. Repeat the process until the remaining material matches the quantity the test method specifies.
Samples are also taken from moving material during production. The standard locations include:
- Conveyor belts, by sweeping the full width of the belt at regular intervals.
- Barges and railcars, by sampling from several hatches and depths rather than one corner.
- Delivery trucks, by collecting from the top of the load before discharge.
Once the sample size is right, physical properties come next. The bulk density and void percentage test measures how much space the particles occupy in a loose or compacted state, and that number is used to convert concrete mix proportions from weight to volume.
Moisture content matters just as much. Aggregates stored outdoors absorb rain, and the extra water changes the effective water-cement ratio of the mix. Measure the moisture content of each stockpile on pour day and adjust the batch water accordingly, or the slump and strength will drift from the design values.
Aggregate Crushing Value and Impact Tests
Coarse aggregates must resist crushing under the loads transmitted through the concrete. The aggregate crushing value test compresses a sample in a steel cylinder and measures the percentage of material that breaks down to smaller sizes. A lower crushing value indicates a stronger aggregate, and most specifications for structural concrete require a value below 30 percent.
Procedure for the Crushing Value Test
- Dry the sample and fill the test cylinder in three layers, tamping each layer with the specified number of blows.
- Place the cylinder in the compression testing machine and apply the load to reach 400 kN over about 10 minutes.
- Remove the sample and sieve it on the specified mesh, typically 2.36 mm.
- Weigh the material passing the sieve and express it as a percentage of the original sample weight.
Abrasion resistance is checked with a separate machine. The Los Angeles abrasion test tumbles the sample with steel balls in a rotating drum and measures the mass lost to wear. Aggregates that lose less than 30 to 45 percent of their mass, depending on the code and the application, are considered acceptable for concrete work.
Impact Value Test for Toughness
The impact value test drops a standard hammer onto the aggregate from a fixed height and measures the fines produced. It ranks aggregates by toughness, which matters for pavements and other surfaces subjected to repeated impact loads. Values above 30 percent generally fail the requirements for heavy-duty construction, and the test is quick enough to run on site as a screening check.
Choose the test that matches the risk. Crushing and impact tests cover strength under static and dynamic loads, while abrasion tests cover surface wear. A supply that passes all three still needs grading and durability checks before it is accepted for structural work.
Sieve Analysis and Grading
Particle size distribution, or grading, controls how tightly the aggregate particles pack together. Well-graded material fills the voids between large particles with smaller ones, which reduces the paste demand and improves workability without adding cement.
Sieve Sizes for Coarse Aggregates
Sieve analysis passes the sample through a stack of sieves with progressively smaller openings and records the mass retained on each. The table lists the standard IS sieve sizes used for coarse aggregates and the nominal size ranges they define.
| IS Sieve | Nominal size range | Common use |
|---|---|---|
| 63 mm | above 63 mm | mass concrete, heavy foundations |
| 40 mm | 63 to 40 mm | mass concrete and retaining walls |
| 20 mm | 40 to 20 mm | general reinforced concrete |
| 10 mm | 20 to 10 mm | slabs and thin sections |
| 4.75 mm | 10 to 4.75 mm | precast elements and repairs |
The formal name for the sieve procedure is grain size analysis, and it produces the grading curve used to judge particle distribution against code limits.
Interpreting the Grading Curve
Plot the cumulative percentage passing against sieve size on a log scale. A smooth curve that stays inside the code envelope indicates good grading, while a flat or stepped curve means the aggregate has gaps in size that increase void content and paste demand. A gap-graded aggregate can still be used, but the mix proportions must be adjusted to compensate.
On site, a quick hand-sieving check with the two sieves that bracket the nominal maximum size confirms whether a delivered load matches the approved grading. The full stack test belongs in the laboratory, where the balance and the sieves are calibrated.
Fine Aggregate Grading Zones
Fine aggregates are classified into grading zones based on the percentage passing the 600-micron sieve. Zone I is the coarsest and Zone IV is the finest. The delivered zone affects water demand and workability, so the mix design should be rechecked whenever the sand source changes.
Flakiness, Elongation, and Durability Tests
Particle shape and durability matter as much as size. Flaky and elongated particles break easily under load and leave voids in the compacted mass, so codes limit their combined percentage in coarse aggregates.
Flakiness and Elongation Index
The flakiness index measures particles whose thickness is less than 0.6 times their mean dimension, using a slotted gauge. The elongation index measures particles whose length exceeds 1.8 times the mean dimension, using a length gauge. For each test, weigh the material that falls into the rejected class and express it as a percentage of the total sample. Most specifications keep the combined flakiness and elongation index below 30 percent for structural concrete.
The Soundness Procedure
The soundness test per IS 2386 Part 5 exposes aggregate to alternate wetting and drying in sodium sulfate or magnesium sulfate solution. The salt crystallizes inside the pores and expands, which simulates the effect of freeze-thaw cycles and chemical attack. Weight loss after five cycles must stay below the code limit, typically 12 percent for sodium sulfate and 18 percent for magnesium sulfate.
When Soundness Checks Matter
Aggregates from regions with frequent freeze-thaw cycles, marine environments, or chemically aggressive soils should be soundness-tested before acceptance. Marginal aggregates can pass crushing and abrasion tests yet fail the soundness requirement, so the procedure cannot be skipped for exposed concrete.
Specific gravity and water absorption complete the physical picture. Specific gravity feeds the mix design calculations, and water absorption tells you how much of the batch water the aggregate will steal before the cement hydrates. Both tests use the same oven-dried sample and take less than an hour to run.
Compressive Strength Test for Concrete
Aggregate quality shows up in the finished product as compressive strength. The cube test compresses a standard concrete specimen and records the failure load, giving the grade of the concrete in megapascals.
Cube Preparation and Curing
- Cast 150 mm cubes in steel molds in three layers, compacting each layer with 25 to 35 blows of the standard tamping rod.
- Demold the cubes after 24 hours and cure them in water at 27 degrees Celsius until the test age.
- Test the cubes at 7 and 28 days, loading each one at the standard rate until failure.
Aggregate strength feeds directly into concrete strength: the aggregate crushing value test procedure described earlier quantifies the resistance of coarse particles to crushing under load, and concrete made with aggregates that crush easily loses strength regardless of the cement content.
Recording and Reporting Results
Record the failure load, the cross-sectional area, and the calculated strength for each cube. Report the average of three cubes as the test result and compare it against the specified grade. Variation of more than 15 percent between individual cubes usually points to compaction or curing problems rather than aggregate issues.
The 7-day result gives an early warning, typically reaching 65 to 75 percent of the 28-day strength for ordinary Portland cement concrete. A low 7-day value justifies holding the pour schedule and investigating the materials before members are loaded.
No single test answers every question about an aggregate supply. Crushing value judges strength, sieve analysis judges packing, and soundness judges durability, so a complete evaluation combines several procedures.
The aggregate impact value provides a fast toughness check that correlates well with crushing resistance, which makes it a practical field companion to the laboratory series.