GB/T 14685 Crushed Stone Standard: Gradation, Crushing Value and Quality Control

GB/T 14685 for pebbles and crushed stone for construction is the core basis for aggregate production and sales, yet many quarries treat it as a matter for the laboratory — until a concrete plant rejects a shipment. Across 40 years of crushing equipment and wear parts service since the 1980s, Grand Heavy keeps seeing one pattern: gradation failures usually originate at the crusher, not in the parent rock. This guide explains the key indicators and how to read them in the field.

图：大美智造破碎机耐磨件实物——耐磨件磨损状态直接决定出料级配与粒形

I. The Key Indicators Explained

The standard classifies aggregate into Class I, II and III by application, with limits relaxing across classes. Exact limits must be taken from the current standard text [To be added: current GB/T 14685 version year and class limits]. What follows focuses on the meaning and field significance of each indicator.

1. Particle gradation (the core indicator, and the most frequent failure)

• Meaning: the distribution of particle sizes in the aggregate. Divided into continuously graded (continuous distribution, low void content) and single-size (concentrated in one size band, blended by the user);
• Field significance: gradation drives workability, density and strength of concrete. Failure leads directly to rejection;
• How to read: examine the gradation curve — is it smooth, does it fall within the specified grading zone, are there missing size bands or excess in one band;
• Key insight: gradation drift is usually gradual. A single test may pass while the trend is already off. A periodic sieve analysis log is essential.

2. Crushing value (parent rock strength)

• Meaning: the ability of aggregate to resist crushing under gradually applied load — a measure of the parent rock itself;
• Field significance: a high crushing value means softer rock that may not suit high-grade concrete;
• Diagnostic value: crushing value is set by the parent rock and cannot be improved much by crushing process. A crushing value failure is a raw material problem; a gradation failure is more likely an equipment problem — separating the two points you in the right direction.

3. Flakiness index (particle shape)

• Meaning: the proportion of elongated or flat particles relative to the average dimension of the size fraction;
• Field significance: excess flaky particles reduce concrete strength and workability and are a key concern for concrete plants;
• Key insight: flakiness depends heavily on crushing method and wear part condition. Impact crushing (impactors, hammer crushers, sand makers) produces better shape than compression crushing (jaw crushers); and worn blow bars, hammers or liners change the cavity and markedly increase flakiness.

4. Clay content and clay lump content

• Meaning: fines below a given size for clay content; lumps above a given size that disintegrate when soaked and kneaded for clay lump content;
• Field significance: clay coats aggregate and blocks cement paste bonding, reducing strength; lumps are worse;
• Investigation: mainly related to feed source and washing process (sand washer present, adequate wash water), not to the crusher itself.

5. Soundness and harmful substances

• Soundness: resistance to weathering and freeze-thaw;
• Harmful substances: sulphide and sulphate content, organic content, alkali-aggregate reactivity and similar;
• Field significance: both are set by the feed source and controlled at intake, independent of the crushing process. Failure means changing the source.

II. A Field Lesson: Repeated Rejections Were Not the Rock

A quarry had repeated shipments rejected by a concrete plant for gradation failure. The plant blamed parent rock quality and changed source, yet the problem persisted.

We investigated by separating indicators first, then equipment:

• Step 1, separate indicators: lab data showed the crushing value had always passed — the rock was fine — while the problem was concentrated in gradation drift and high flakiness, both tied to equipment condition and pointing at the crusher;
• Step 2, look at the trend: three months of sieve logs showed gradation changing month by month rather than suddenly — the signature of gradual equipment degradation, not a feed change;
• Step 3, check the machine: on the impact crusher the blow bars were clearly worn with rounded working faces and the impact apron gap had drifted from setting. With rounded bars the impact energy was insufficient, so material was being struck rather than properly broken, reducing breakage along natural grain boundaries and raising flakiness; the changed cavity also shifted overall gradation.

The root cause was worn blow bars degrading cavity geometry and impact conditions, not the parent rock. The fix: replace the blow bars, re-set the impact apron gap, restore cavity parameters, and add a cross-check between sieve logs and wear part condition to routine management. Gradation and flakiness returned to normal and rejections stopped.

The rule recorded: a gradation failure means check the equipment; a crushing value failure means check the source. Get the direction wrong and no amount of source changing will help.

图：骨料检测实验室——周期性筛分台账是发现级配漂移趋势的关键

III. Indicator Diagnosis Table (Save This)

One-line rule: crushing value looks at the rock, gradation looks at the machine, clay looks at the wash plant.

Reading the gradation curve — three checks:

Gradation is tightly linked to wear part condition; see manufactured sand line configuration and screen mesh tensioning and selection.

IV. Insider Gotchas

• Changing the source for a gradation failure: gradation is set by equipment condition and crushing value by the rock. Get the direction wrong and no source change helps;
• Trusting a single passing test: gradation drift is gradual. One pass does not mean the trend is sound; keep a periodic sieve log;
• Ignoring the wear part link: worn blow bars, hammers and liners change the cavity and are a common real cause of gradation drift and rising flakiness, yet raw material usually takes the blame;
• Selling single-size as continuously graded: single-size material must be blended by the user; supplying it as continuously graded fails on missing size bands;
• Non-representative sampling: aggregate must be sampled at multiple points in the stockpile per the standard method. Sampling only from the belt or the pile surface hides the real problem.

V. When NOT to Judge It Yourself

• Acceptance testing or quality disputes: must be performed by an accredited laboratory per the standard method; in-house data cannot serve as acceptance evidence;
• Crushing value, soundness, alkali reactivity: require specialised equipment and controlled test conditions beyond most quarries; commission a laboratory;
• Where specific project requirements apply: hydraulic, highway and building projects require different aggregate classes; follow the project design requirement rather than a generic standard;
• Limit value judgements: as noted, limits must be taken from the current standard text and cannot be judged from remembered figures.

Three actions: 1. Build a periodic sieve analysis log that captures trends, not just single results; 2. On a gradation anomaly, check cavity geometry and wear part condition first, then discharge setting and screen media; 3. Cross-analyse wear part replacement records against sieve data to catch gradation drift early.

FAQ

• Q: Is a gradation failure a material or an equipment problem? A: Usually equipment. Gradation and flakiness are governed by cavity geometry, discharge setting and wear part condition, while crushing value and soundness are governed by the parent rock. Check the machine before the source.
• Q: Why does worn blow bar wear cause gradation failure? A: A rounded working face reduces impact energy, so material is struck rather than properly broken, reducing breakage along natural grain boundaries and raising flakiness, while the changed cavity shifts overall gradation. It is the most common equipment-side cause of gradation drift.
• Q: How can we catch gradation drift early? A: Keep a periodic sieve log and read the trend rather than single results, and cross-analyse it with wear part replacement records to find problems before customers do.
• Q: What is the difference between continuously graded and single-size? A: Continuously graded material has a continuous size distribution with low voids and can be used directly; single-size material is concentrated in one band and must be blended by the user. Supplying single-size as continuously graded fails on missing size bands.
• Q: Does Grand Heavy help improve aggregate particle shape? A: Grand Heavy has worked in crushing equipment and wear parts for 40+ years and can analyse how cavity geometry and wear part condition affect output gradation and shape, and supply blow bars, hammers and liners. See also manufactured sand line configuration.

VI. Final Verdict

Reading GB/T 14685 is not about memorising limits but about knowing what determines each indicator: crushing value looks at the rock, gradation looks at the machine, clay looks at the wash plant. Get the direction right and corrective action means something. Note especially that gradation drift is often an early signal of wear part wear — managing sieve logs alongside wear part condition catches problems before customers do. Foshan Grand Heavy Trading Co., Ltd. has built mining equipment and wear casting expertise for 40+ years, holds 20+ patents, partners with Central South University and Guangdong University of Technology, and is certified as a National High-Tech Enterprise [To be added: certificate number]. We serve CNBM, Conch Cement and clients across 6 overseas markets with 24-hour response from Guangzhou, Foshan and Changsha.

Grand Heavy — Crushing value looks at the rock, gradation looks at the machine!