Materials

Configure concrete grade, reinforcement grade, cover, and stress-strain models for concrete section analysis.

Overview

Material properties drive every calculation in ACS — from section property transformations through to ultimate capacity and serviceability checks. The materials panel in the left sidebar lets you select the design code, concrete grade, reinforcement grade, prestressing strand, cover, and concrete stress-strain model.

Design code

Select the governing design standard from the dropdown. This choice affects:

• Strength reduction factors ($\phi$)
• Ductility limits and checks
• Serviceability stress limits and crack width formulas
• Available concrete grades and rebar grades
• Fire design methods

Concrete grade

Each design code provides a set of standard concrete grades. The grade defines the characteristic compressive strength $f'_c$ (or $f_{ck}$ in Eurocode notation).

Australian grades (AS 3600)

American grades (ACI 318)

Grades are specified by $f'_c$ in psi, displayed in MPa:

Eurocode grades (EN 1992)

Reinforcement grade

Rebar grades define the yield strength $f_y$ and elastic modulus $E_s$.

Australian grades (AS/NZS 4671)

$f_y$ is the lower characteristic yield stress $R_{ek.L}$ that AS/NZS 4671:2001 Table 2 gives for each grade, and the one Clause 5.1(b) defines the strength grade by. The standard specifies no ultimate strength: instead of an $f_{su}$ it bounds the ratio $R_m/R_e$ of a test piece’s own tensile strength to its own yield stress, so no single $f_{su}$ follows from the grade designation.

The three 500-grades therefore share a yield strength and differ only in ductility. Per Table 2:

The dash in R250N’s row is the table’s own answer, not a gap in ours: AS/NZS 4671 specifies no upper characteristic yield for the 250 grade, where it bands all three 500 grades. Nothing in ACS supplies one.

R250N is plain round. Table 2 Note 1 permits Grade 250N to be supplied as plain round reinforcing steel to AS 3679.1, and that is the form the Australian market stocks it in — ligatures, fitments, dowels and helices, normally beside D500N longitudinal bars. Two consequences follow, and ACS applies both:

• Its bar sizes are a different catalogue. See Plain round bars below. The deformed range is not offered on a plain round material, and vice versa.
• Its fitments bend tighter. AS 3600 Cl. 17.2.3.3(a) gives a plain round (or 500L) fitment a minimum internal pin diameter of $3 d_b$, against $4 d_b$ for D500N. ACS applies the $3 d_b$ rule from the grade, with no size threshold. A fitment whose grade you have not declared takes the D500N rule.

Grades above 500 MPa. AS 3600 Cl. 1.1.2(d) admits reinforcement above the Table 3.2.1 listed 500 MPa, up to 800 MPa, where the grade meets the Table 3.2.1 Class N requirements — and it requires that evidence to be declared on the grade, never inferred from a label. The seeded S600N grade (InfraBuild SENSE 600) declares all three: $f_{sy} = 600$ MPa, ductility class N, uniform elongation $\varepsilon_{su} \geq 0.05$ and $R_m/R_e \geq 1.08$. A grade that declares $f_{sy} > 500$ MPa without them is refused, not designed on assumed ductility; so is anything above 800 MPa. See Reinforcement above 500 MPa.

Bar sizes

Bar sizes come from a size catalogue attached to the reinforcement material, not from the jurisdiction alone. Selecting a different rebar material can therefore change the sizes the placement dialogs offer — which is what makes a manufacturer product with its own designations expressible at all.

For the standard AS/NZS 4671 deformed catalogue:

ASTM A615 and the EN 1992 size set (6, 8, 10, 12, 14, 16, 20, 25, 32, 40 mm) are separate catalogues on their own materials.

What a size selection records

Choosing a size records an identity, not a diameter. On save the platform freezes that catalogue row — its designation, published diameter and published area — onto the design, alongside the material snapshot and for the same reason: a later catalogue edit must not move a saved design’s numbers silently.

A bar that names no catalogue size is a dimensional input with no product identity, and its area is the circular idealisation $\pi d^2/4$. Every design saved before size catalogues existed is in that state — no back-fill was performed — so those designs keep exactly the areas they have always had.

Plain round bars

The AS/NZS 4671 plain round catalogue carries the nine sizes the Australian market stocks for the R250N grade. It is a separate catalogue from the deformed range above, and deliberately so: the two ranges are not the same set. R27 and R33 have no deformed twin, and N28, N32 and N40 have no plain round twin.

The range is R10 to R36. There is no R6.5.

Read this table differently from the SENSE 600 one below. There, all four quantities are published and none may be derived from another. Here the supplier publishes the designation and the mass only — no diameter column and no area column exist. So:

• The diameter is the designation, which is the supplier’s own reading: the product is described as available “in 250 MPa for diameters 10–36 mm”, the exact span of the designation numbers.
• The area is the circular idealisation $\pi d_b^2 / 4$, at one decimal place — the same treatment the deformed AS/NZS 4671 range gets. R10S and N10 are the same 10 mm round section and carry the same 78.5 mm$^2$. The standard agrees: its own Table 5B publishes 78.5 mm$^2$ for a 10.0 mm bar.
• The published mass is not that area times the density. It carries the allowable manufacturing tolerance — AS/NZS 4671 Cl. 7.3.1 sets that at ±4.5%, and the published column sits at the top of the band. R16S is the clearest row: 1.65 kg/m published against a theoretical $201.1 \times 10^{-6} \times 7850 = 1.579$ kg/m. The published figure is the one recorded, because it is the one the supplier stands behind and the one a delivery docket will show.

Where the standard does and does not help. AS/NZS 4671 Cl. 7.3.1 does publish preferred diameters, areas and masses, in Tables 5A and 5B. Neither covers this product: 5A is Australia only and every row is grade 500 class N; 5B is New Zealand only and its rows are 300E and 500E. So the plain round range itself had to come from the supplier — but the standard still supplies the ±4.5% tolerance and the 7850 kg/m$^3$ density the derivation above uses, and Table 5B’s 10.0 mm row (78.5 mm$^2$, 0.617 kg/m, and $0.617 \times 1.045 = 0.645 \approx 0.64$) independently reproduces the R10S line.

The mass point has a consequence worth stating plainly: a mass, cost or embodied-carbon total that ACS computes from a bar’s area is against the theoretical mass, and will read roughly 3.7% to 4.5% below a figure worked from the supplier’s published kg/m, depending on the bar. The two numbers answer different questions; neither is adjusted to agree with the other.

Product catalogues

InfraBuild SENSE 600 is seeded as a product catalogue of eight deformed bars, with a 600 MPa characteristic yield strength and normal (Class N) ductility:

Three things about this table are easy to get wrong, and ACS is built so that you cannot:

• The S number is not the diameter. S11 happens to be 11.0 mm, but S15 is 14.6 mm and S37 is 36.5 mm. A designation is a product label; read the diameter from the table, never from the digits.
• The published area is not $\pi d^2/4$. The two differ by $-0.877\%$ (S11: 94.2 mm$^2$ against $\pi \times 11.0^2 / 4 = 95.033$ mm$^2$) to $+0.349\%$. ACS analyses a catalogued bar on its published area. The S11 direction is the un-conservative one — deriving from the diameter would over-state the steel — which is why the published number is the one used.
• Neither derives from the other, and neither derives the mass. The manufacturer rounds all three independently: back-solving S11’s diameter from its area gives 10.95 mm, not 11.0, and the masses imply densities spanning 7829–7863 kg/m$^3$. ACS never reconstructs one published quantity from another; where a source publishes no mass, the quantity is reported as absent rather than computed as $A_b \times \rho$.

An area is also not a perimeter. Bond, development-length and crack-spacing terms take $\pi d_b$ or $d_b$ from the nominal diameter, and a declared area does not change them — the manufacturer publishes no rib geometry, so a declared area says nothing about surface.

There is no SENSE equivalent of N10, and the manufacturer’s equivalence column (S11 for N12, S15 for N16, and so on) is reference information about the product range. It is not a design substitution: ACS performs no automatic equivalence, and an equal-force selection does not give the same squash load or the same stiffness. See Reinforcement yield strength in ULS design models for what a 600 MPa grade does and does not buy — and Why the squash load does not rise with the grade for the one check where the answer is “nothing”.

Where the strengths come from

The concrete and rebar pickers record a reference to a catalogue material, not a copy of its numbers. On save, the platform freezes that material’s engineering values into a snapshot stored with the design, and every calculation — the live panels, the derived reads and the PDF report — reads $f'_c$, $f_y$ and $E_s$ from that snapshot.

Freezing is deliberate: a later edit to the catalogue material does not silently move a saved design’s results. Instead the design is flagged as drifted (see Material drift detection), and the new values are adopted only when you choose Recalculate.

Between selecting a material and the design’s next auto-save, no snapshot has been taken yet, so the panels read the strengths straight from the catalogue row you just selected. There is nothing frozen to disagree with at that point; from the first save onwards the snapshot is what every surface reads.

A section with no material selected refuses

Because the strengths live only on the snapshot, a section that has no catalogue material linked records no $f'_c$, $f_y$ or $E_s$ at all — and every analysis refuses rather than substituting a grade you did not choose:

This is the same principle as the disclosures elsewhere on this page: ACS would rather tell you a value is absent than compute a plausible number on a material nobody selected.

Two consequences worth knowing:

• A new design starts on your jurisdiction’s default catalogue material (for example AS 3600 Grade 40 and AS/NZS 4671 D500N under an Australian project), so it computes immediately. That default is a real catalogue row you can inspect and change — not a hidden value.
• Clearing a picker back to “no material” clears that slot’s strengths with it. The analyses refuse until you select another material. If you want a different strength, pick the material that declares it rather than clearing the link.

A catalogue material that declares no compressive strength refuses on the same terms — the strength is a property of the material, so a material without one cannot be analysed.

Prestressing strand

The PT Strand picker sets the section’s default strand product. It drives the per-strand area $A_p$, the characteristic breaking strength $f_{pb}$, the nominal diameter and the elastic modulus $E_p$ for every tendon that does not carry its own material override.

Strand properties and where each value comes from

Below the picker, ACS shows the strand’s resolved material properties. Each one carries a caption naming where the value came from — because the number alone cannot tell you whether it was measured, prescribed by your design code, or supplied by ACS:

The properties shown are:

$A_{gt}$ shows a value with a platform default — no code caption when the product declares none. No supported design code prescribes an $A_{gt}$, so the figure carries no code authority and says so; ACS uses 0.050, the breaking strain from the same reference strand material its $\sigma_{p1.0}$ and $Q$ defaults come from, taken together so the three cannot describe a strand that does not exist.

It is shown rather than left blank because the constitutive curve consumes it: the knee factor $K$ is derived from $A_{gt}$, so the curve is built to reach $f_{pb}$ at exactly this strain. Displaying an em dash over a value the analysis is integrating would be the more misleading of the two. EN 1992-1-1 Cl. 3.3.6(7)‘s $\varepsilon_{ud} = 0.02$ is still not what is shown — that is a design limit on usable strain, not a measurement of the material.

Declare $A_{gt}$ on the product to replace the default, and the caption changes to declared by grade.

The same $f_{py}$ basis is printed in the PDF report’s Material Properties table, so the panel and the report always state it the same way.

:::note[Why the basis is worth reading] Two design codes can produce the identical $f_{py}$ for the same strand by different routes. Under ACI 318 ACS carries the AS 3600 strand factor forward, because no edition of ACI 318 is in the reference corpus to cite — the number matches AS 3600’s exactly, and only the caption distinguishes a code-prescribed value from an ACS substitution. :::

Cover

Cover is the clear distance from the nearest concrete surface to the outer surface of the outermost reinforcement (including stirrups).

Manual cover

Enter cover values directly for each face:

Code-based cover

Select an exposure class and fire rating, and ACS computes the minimum required cover per the selected design code. The computed value accounts for:

• Durability requirements — minimum cover for corrosion protection based on exposure class (AS 3600 Table 4.10.3.2; ACI 318 Table 20.6.1.3.1; EN 1992-1-1 Table 4.4N)
• Fire requirements — minimum cover for fire resistance based on member type and required FRL
• Construction tolerances — $\Delta c_{dev}$ added per code (typically 5—10 mm)

The governing (largest) value from durability and fire is adopted.

:::note[Prestressing tendons do not change the advised cover] The fire term is derived from the reinforcing bars — cover is what positions them. A tendon is placed by its own profile, so no cover figure positions it, and its AS 3600 Cl 5.3.3 axis-distance requirement is checked separately in the Section 5 fire assessment instead. Adding or removing a tendon therefore leaves this cover unchanged. :::

Concrete stress-strain model

ACS supports multiple concrete compressive stress-strain models for flexural and moment-curvature analysis. The model affects the shape of the stress block and hence the calculated capacity and ductility.

The rectangular stress block ($\alpha_2 f'_c$ over depth $\gamma k_u d$) is the ULS built-in default for AS 3600 and ACI 318 simplified design checks and is not user-selectable in the picker — the rectangular stress block has no defined strain at peak stress, so it cannot be used by the fibre-based analyses the picker drives. The picker chooses among the continuous stress-strain curves above for moment-curvature, stress distribution, and other nonlinear analyses; the rectangular block is applied automatically when the code path is a code-simplified ULS check.

:::important The rectangular stress block cannot be used with fibre-based analyses (moment-curvature, stress distribution, M-$\kappa$ interaction). These analyses require a continuous stress-strain curve. If you select the rectangular model and attempt a fibre-based analysis, ACS displays an error prompting you to switch models. :::

Material drift detection

ACS records a snapshot of the material properties used when analysis results were last computed. If the catalog entry for a material changes after the design was analysed — or if the platform begins recording a field it did not capture before — a drift banner appears above the results panel. The banner distinguishes two reasons a snapshot goes stale, because only one of them makes Keep current a legitimate engineering choice.

Catalogue edited

The material catalog was updated after the design was analysed. The snapshot preserved what the design was actually designed against; the catalog has since moved.

Banner message: “Material ‘X’ has been updated since this calculation.”

Keep current is a valid choice here — the frozen snapshot represents a real, previously-designed-against value. The design stays correct for the material as it existed at analysis time. Click Recalculate to refresh the snapshot and re-run all checks against the catalog’s current values.

Values not recorded when saved

The platform has started recording a field it did not capture when this design was saved — typically because a new data field was added to the catalog schema after the design was created (for example, the strand $\sigma_{p1.0}$, $Q$, and $A_{gt}$ columns added in a platform update). The saved snapshot carries no value for the affected fields; the analysis was therefore run against the platform’s own fallback default, not against anything declared in the catalog or chosen by the engineer.

Banner message: “Material ‘X’ declares values this calculation was never run against.”

Keep current is the wrong choice here. Selecting it pins the engine’s disclosed fallback as the design value — a value that is not a property of the material. The banner shows an additional warning:

“Keep current leaves the engine on its own default for those values, which is not a property of this material. Recalculate to use what the catalog declares.”

Click Recalculate to refresh the snapshot and re-run all checks against the field values the catalog now declares.

Tracked fields

Sweeping stale designs via the API

The ListStaleDesigns endpoint (GET /api/v1/materials/{materialId}/stale-designs) returns every design whose snapshot pre-dates the material’s most recent catalog edit. When called without a since parameter the endpoint derives the cutoff from the most recent material.updated audit-log entry (or from MaterialBase.UpdatedAt as a fallback). Both defaults track catalogue edits only — neither advances when a shape-growth deploy adds a new snapshot field. To sweep designs stale because of a shape-growth deployment, pass since explicitly as the deploy timestamp.

:::note Cover values do not trigger the drift banner — cover affects bar placement geometry, not the material properties used in the stress-state computation. If you change cover, re-running analysis picks up the updated geometry automatically. :::

Section appearance

The section fill colour is a visual annotation. To keep the canvas legible in both light and dark modes, the colour picker offers a WCAG-safe curated palette — every swatch clears a minimum 3:1 contrast ratio against both the light and dark canvas backgrounds (WCAG 2.1 SC 1.4.11). The palette also stays perceptually distinct from the canvas’s semantic overlay colours (selected state, hover, constraint badges) so a filled section is never ambiguous with a UI state.

If your section was assigned a colour before the curated palette was introduced, that colour is preserved without change. Off-palette colours are displayed as-is and are never silently snapped to the nearest swatch — consistent with the §874 principle that incorrect data is worse than no data. The colour remains until you explicitly pick a new one from the palette.

Beyond the fill, two non-colour cues are always rendered regardless of the fill colour:

• Outline — a thin stroke around the section perimeter using the canvas theme’s materialOutline colour. The outline is visible in both light and dark themes, so the section boundary never depends on the fill alone.
• Name label — the material name is drawn at the section centroid using the materialLabelText colour. The label provides a non-colour identifier that remains readable at small scale and when fills are similar across adjacent sections.

Creep environment disclosure

The AS 3600 k4 long-term relative-humidity factor (which spans 0.50–0.70) governs creep and shrinkage in the SLS stress-field, stress-check, crack-width, deflection, moment-curvature, and time-dependent analyses. Its value is set by the SLS Creep Environment saved on the design option — or, where that is absent, by the platform default (interior environment, 50 % RH).

Every derived-read response envelope carries two fields — creepEnvironment and relativeHumidity — stating exactly which values governed the run. This applies to all derived-read quantities, not only those where creep is the primary concern, so a response always answers “which environment did this compute under?” without the caller needing to cross-check the persisted row.

The disclosure is deliberate rather than a refusal: k4 has a defensible platform default (the same value the report and panels have applied since the feature was introduced), so a missing creep environment produces a disclosed result rather than a refusal. To replace the default with a project-specific value, set the SLS Creep Environment in the Analysis panel.

Related pages

• Section geometry — defining the concrete outline
• Prestressing — the strand catalogue, $f_{py}$ per design code, and tendon layout
• Reinforcement — placing bars within the section
• Section analysis — running design checks
• Design standards reference — detailed code comparison