Marquages de tête des boulons de structure : identification des ensembles A325, A490 et TC

For receiving inspectors, the practical task is to identify the bolt’s grade, material type, and configuration, then compare those observations with the purchase specification, applicable standard, and supporting documentation. A bolt that looks correct can still be the wrong configuration for the order. Conversely, an unfamiliar supplementary mark is not necessarily evidence of a counterfeit or defective fastener.

ASTM F3125/F3125M consolidates several formerly separate structural bolt specifications into a common product standard. The familiar A325 and A490 designations remain useful for identification. The 2025 RCSC Spécification pour les assemblages structuraux utilisant des boulons à haute résistance, particularly Figure C-2.1, provides illustrated examples of the markings used for these products. Its marking illustrations should be read alongside the edition of ASTM F3125 specified for the actual purchase, rather than treated as an exhaustive substitute for that standard.

Read the Grade and Manufacturer Marks First

The first step is to distinguish the grade designation from the manufacturer’s identification mark. On a typical ASTM F3125 Grade A325 heavy hex bolt, the head carries the A325 designation and a separate manufacturer symbol. An A490 heavy hex bolt follows the same general identification principle, with A490 identifying its grade.

The RCSC 2025 marking illustrations use XYZ to represent a manufacturer’s identification. This is a generic symbol in the reference illustration, not a particular manufacturer’s approved trademark or an additional strength designation. Actual manufacturers use their own identifying symbols, which should be traceable through their documentation when necessary.

The grade markings also correspond to different strength classifications. RCSC 2025 places Grade A325 in Group 120 and Grade A490 in Group 150, reflecting nominal minimum tensile-strength classifications of 120 ksi and 150 ksi respectively. These values characterize the product groups; they are not installation pretension values, proof loads, or allowable loads for a finished connection. Those engineering decisions require the applicable connection design and installation requirements.

A useful identification distinction is that ASTM F3125 defines the product specification, while the head marking provides a visible way to distinguish relevant grades. A bolt does not need to display the complete designation “ASTM F3125” on its head to be recognized as a product within that specification.

This matters during inspection of existing steelwork. An inspector may find A325 markings on bolts supplied years after ASTM F3125 replaced the earlier standalone A325 specification. The continued use of A325 on the head is not, by itself, evidence that the bolt was manufactured to an obsolete specification.

The visible mark still has limits. It does not independently verify chemical composition, heat treatment, tensile properties, thread geometry, coating suitability, or conformity of the associated nut and washer. Neither does the marking alone establish the identity of a production lot.

For a practical first inspection, record the full grade mark, the manufacturer symbol, any underline or supplementary letters, and the bolt’s physical configuration. Compare them with the relevant marking reference before moving to product-document verification.

Distinguish Type 1 from Type 3 Markings

Grade and type identify different aspects of a structural bolt. A325 or A490 identifies the grade, while Type 1 and Type 3 describe material classifications.

In Figure C-2.1 of the 2025 RCSC Specification, Type 1 A325 and A490 examples show the grade marking without an underline. The corresponding Type 3 examples show the grade marking underlined. The same distinction appears in the illustrated twist-off assembly markings.

This gives inspectors a meaningful visual distinction. For example, a bolt marked with the manufacturer’s symbol above an underlined A325 designation corresponds to the illustrated Type 3 form, whereas the comparable non-underlined form corresponds to Type 1.

However, the inspector must examine the actual marking, not merely assume that a mark is absent because it is difficult to see. Coating, dirt, corrosion, mechanical damage, or poor lighting may obscure part of a head stamp. Cleaning or closer examination can help reveal an existing mark, but an uncertain observation must not be converted into a definite material classification.

Historical marking references introduce another complication. Some older identification guides show supplementary radial lines or legacy Type 2 designations. Those illustrations may be useful when investigating older fasteners, but they should not be merged indiscriminately with the current marking rules. The governing product edition and the actual manufacturer documentation remain important.

Type 3 is also not a universal guarantee of corrosion resistance. It describes a particular material category with improved atmospheric weathering behavior; it does not automatically establish suitability for marine exposure, aggressive industrial atmospheres, buried applications, or every coating system.

From a procurement perspective, the distinction matters because type can affect compatibility with the specified nuts and washers. Under RCSC 2025, the type of hardened washer must match the bolt type, and permitted nut grades depend on the bolt group, type, and coating condition. A correct-looking Type 3 bolt is not enough if its mating components do not meet the assembly requirements.

For a lot identified as Type 3, the inspector should therefore verify both the material identification and the associated components, rather than accepting the bolt solely because the expected underline is visible.

In an illustrative inspection, a coating-obscured mark is read as Type 1 simply because an underline is not visible. Poor visibility is not evidence that the symbol is absent. Record the uncertainty, inspect without unauthorized coating damage and compare traceable manufacturer records. Use grade-marking guidance to support identification while retaining the governing product-edition requirements.

Recognize Twist-Off TC Bolt Assemblies

Twist-off tension-control assemblies are often recognizable by their splined ends and associated head markings, but the head shape alone is not a reliable identification method.

ASTM F3125 Grade F1852 is the traditional 120-ksi-class twist-off assembly, while Grade F2280 represents the corresponding 150-ksi-class assembly. In the RCSC 2025 marking illustrations, these products are associated with A325TC and A490TC markings respectively.

The “TC” designation is particularly useful because it distinguishes the twist-off assembly form from an ordinary heavy hex structural bolt. It refers to the tension-control configuration, not an independent material grade.

A twist-off assembly normally uses a specialized installation tool that reacts against the splined end while tightening the nut. The spline is designed to separate during the pretensioning operation. This makes the spline a recognizable physical feature before installation, but its presence does not establish the bolt’s grade or the acceptability of the complete assembly.

There is also an important head-shape exception: RCSC 2025 explicitly recognizes that Grade F1852 and F2280 spline-end matched assemblies may have either round heads or heavy hex heads. Consequently, an inspector should not reject a suspected TC assembly solely because it does not have a round head.

The distinction becomes clearer when the visible markings and product configuration are considered together.

These are identification aids based on the cited RCSC examples and related published technical guidance, not a complete reproduction of ASTM F3125/F3125M-26’s normative marking requirements. Unlisted alternative markings should be assessed against the actual governing edition rather than automatically accepted or rejected.

Why a sheared spline is not enough to prove correct pretension

Once a TC assembly has been installed, its spline may no longer be present. This observation is useful, but it answers only a narrow question: the spline was subjected to sufficient torque to separate at some point.

It does not, on its own, show that the joint was properly brought to the snug-tight condition before pretensioning. That sequence matters because tightening fasteners individually without properly snug-tightening the joint can allow relaxation as other parts of the connection are tightened.

RCSC 2025 Section 9.2.2 distinguishes the required inspection activities. The inspector observes the applicable pre-installation verification testing, routinely verifies the snug-tight condition and that splines remain intact at that stage, and then routinely observes their subsequent removal during pretensioning.

Once those specified inspection requirements have been satisfied, the RCSC procedure does not require an additional individual bolt tension measurement merely because the twist-off method was used.

There are therefore two different mistakes to avoid: accepting an installed connection based only on missing splines, and imposing unsupported extra testing after the specified procedure has been properly completed.

This is an installation-inspection issue, separate from whether the delivered bolts and matched components conform to their product specifications.

Avoid Misreading Special and Legacy Markings

Several structural bolt markings look similar but communicate different product configurations. Misreading a short suffix can lead to an incorrect receiving decision, particularly when fully threaded, special-dimension, or twist-off fasteners are supplied for the same project.

A325T is not the same as A325TC.

The A325T designation identifies a fully threaded Grade A325 bolt under a specific supplementary requirement. AISC’s published bolting guidance explains that this provision applies to Grade A325 bolts with nominal lengths not greater than four times their diameter. It does not establish an equivalent A490T provision.

For example, a nominal 3/4-inch-diameter Grade A325T bolt falls within the length condition only when its nominal length does not exceed 3 inches:

4d = 4 × 0.75 in. = 3.00 in.

This calculation illustrates the geometric limit for the referenced supplementary configuration; it does not prove that a particular bolt meets the specification. The actual threading, head marking, product edition and order requirements still require verification.

The engineering consequence is significant. A fully threaded A325T bolt cannot be treated as having an unthreaded shank available at the shear plane. The structural design must account for the specified thread configuration rather than assuming that a conventional A325 bolt and an A325T bolt will behave identically in every connection.

By contrast, the TC suffix in A325TC is associated with the twist-off assembly configuration. A bolt marked A325T should not be identified as a TC assembly merely because its marking contains the letters A325 and T.

What do A325S and A490S mean?

An S suffix is associated with certain special dimensional configurations, including departures from standard thread-length requirements under applicable supplementary provisions.

The marking should prompt an inspector to check the drawing or purchase specification for the exact dimensional departure. It must not be interpreted as confirmation that the special configuration was approved for every structural connection.

The distinction between ordinary and special thread lengths can affect bolt length selection, the location of threads relative to the shear plane, installation assumptions, and inspection of the delivered fastener.

Other designations that may appear

Metric structural bolts use their own grade and dimensional designations. A325M and A490M should not be treated as identical to their inch-series counterparts merely because the grade names look similar.

Current ASTM F3125 also includes Grade 144 classifications, and RCSC 2025 illustrates both heavy hex and twist-off versions of that grade. Their presence does not change the meanings of A325 or A490 markings, nor does it imply that every structural project permits Grade 144 as a substitute.

For older bolts, historical references may contain marking conventions that are not identical to those in the currently published documents. An inspector investigating an existing structure should identify the likely manufacturing period and applicable original specification before deciding whether a marking is anomalous.

The governing principle is to interpret suffixes and historical symbols in their documented context. An unfamiliar marking deserves investigation, not an automatic conclusion that the bolt is unsuitable.

A hypothetical receiving note shortens A325TC to A325T and treats the two deliveries as the same product. The suffixes identify different configurations. Compare the complete marking, physical spline and thread arrangement with the order, then reconcile the product and lot records. Do not re-label stock from an abbreviated purchasing description.

Verify the Bolt Beyond Its Head Marking

A structural bolt may display the expected grade and manufacturer markings but still lack sufficient evidence for acceptance. Identification and specification conformity are related, but they are separate decisions.

The product specification addresses matters such as material requirements, mechanical properties, dimensions and applicable product testing. The structural joint specification addresses the use of the bolting components in connections, including relevant installation and inspection obligations.

RCSC 2025 Section 2.9 requires test reports documenting conformity to the applicable specifications for bolting components and matched assemblies to be available to the Engineer of Record and Inspector before structural steel assembly or erection. The presence of a head marking is not a substitute for these records.

A useful receiving inspection therefore compares three sources of information: the purchase requirement, the physical fastener, and the supporting documentation.

The inspection scope should follow the contract documents and applicable standards. This table is a practical comparison method, not a claim that ASTM F3125 prescribes each listed receiving step in exactly this form.

For heavy hex structural bolts, mating nuts and washers must also be considered. RCSC 2025 identifies permitted nut grades and washer requirements according to bolt group, material type, and coating condition. Matching the nominal thread size is necessary but does not by itself establish a compliant assembly.

For coated bolting assemblies, the relationship becomes more important. Coatings, nut overtapping and lubrication influence assembly behavior. Combining individually plausible components from different supplies without confirming the relevant assembly requirements may produce an arrangement that has not been verified for the intended installation procedure.

Twist-off systems deserve particular attention because they are supplied as matched assemblies. Exchanging a nut, changing lubrication or combining components from different lots without the applicable manufacturer and specification basis can affect installation behavior. RCSC 2025 also restricts the relubrication of spline-end matched assemblies to the manufacturer or its direction.

Another documentary distinction concerns mill test reports and certificates of conformity. A material report, product test report and supplier declaration may serve different evidentiary purposes. Their titles are not interchangeable guarantees. The inspector needs the records applicable to the actual product specification and must be able to establish that those records correspond to the material being accepted.

A bolt’s head marking may remain visible throughout its service life, but the mark does not disclose its entire manufacturing history. That is why traceability should be maintained through appropriate project and supply documentation rather than inferred from the stamp alone.

Handle Missing, Unclear or Conflicting Marks

An unclear head marking should trigger a controlled identification review, not an improvised field decision.

The initial question is whether the mark is genuinely missing or merely difficult to read. Coating buildup, dirt, corrosion or surface damage may obscure characters or manufacturer symbols. Where appropriate, inspect the head under better lighting and record the visible features without damaging the component or removing a protective coating in an unauthorized manner.

If a marking remains unclear, compare the fastener with its packaging, applicable product documents and other traceable information. The receiving inspector should distinguish between a discrepancy that can be resolved through documentation and one that requires the responsible quality or engineering authority to make an acceptance decision.

A supplementary symbol should not be assigned a meaning simply because it resembles a known grade marking. Likewise, a manufacturer symbol that cannot immediately be recognized should not be classified as counterfeit without supporting evidence.

Hypothetical example: Two apparently similar A325 deliveries

Assume a structural steel contractor receives two groups of nominally 3/4-inch bolts for separate connection details.

The first group consists of heavy hex bolts with A325T markings and fully threaded shanks. The purchase requirement identifies a special fully threaded A325 configuration.

The second group has splined ends and heads showing A325 and a manufacturer symbol, without a clearly visible TC suffix. The accompanying paperwork describes these as twist-off matched assemblies.

The first group’s marking suggests that the bolts should be evaluated against the applicable A325T supplementary configuration. The inspector must still check the permitted length, threading and product documentation. At 3/4-inch nominal diameter, a 3-inch bolt would meet the four-diameter length condition, but that alone would not establish conformity.

For the second group, the physical configuration suggests a TC system, but the visible marking does not match the most familiar A325TC illustration. The inspector should not simply assume that the bolts are conventional A325 heavy hex bolts, nor should the entire lot be declared counterfeit on that observation alone.

A reasonable response is to segregate the identification question from the normal acceptance process and obtain the applicable marking basis and traceable manufacturer’s documentation before authorizing use. Whether the marking is acceptable depends on the governing product edition and verifiable evidence, not a visual guess.

This is an illustrative procurement scenario, not a record of a customer complaint, manufacturing defect or test conducted by SUNHY.

A practical inspection decision

When the head marking, ordered product and accompanying records agree, the inspector can proceed with the remaining acceptance checks required for the project.

When a mark is obscured but identity can be established through an authorized and traceable verification process, the material may be evaluated further under the governing inspection procedures.

When the observed grade or configuration conflicts with the purchase requirement, or the required product identity cannot be established, the affected material should be held from use pending disposition by the responsible authority. It should not be substituted, remarked or installed merely to maintain a construction schedule.

The same separation applies after installation. A correctly marked bolt does not prove that the connection was assembled correctly; equally, an apparently completed TC installation does not replace the required product documentation and installation-inspection process.

For structural bolt receiving inspection, the strongest decision comes from combining observable markings, actual geometry, the applicable specification, and traceable documents. Each provides a different part of the evidence. None should be asked to prove what it cannot establish.

Technical basis: ASTM F3125/F3125M-26 official published scope; RCSC 2025 Specification for Structural Joints Using High-Strength Bolts, particularly Figure C-2.1 and Sections 2.4, 2.9, 2.10 and 9.2.2; AISC Bolting FAQ for special threading designations.