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, 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 $f_y$, ultimate strength $f_{su}$, and elastic modulus $E_s$.

Australian grades (AS/NZS 4671)

Bar sizes

Standard bar diameters and areas are available per jurisdiction. For Australian reinforcement:

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 you change the design code, concrete grade, rebar grade, or prestressing strand grade after running analysis, a drift banner appears above the results panel indicating that the displayed results no longer match the current material selection.

The following fields are tracked for drift:

Clicking Re-analyse in the drift banner discards the stale results and reruns all active checks with the updated material configuration.

:::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.

Related pages

• Section geometry — defining the concrete outline
• Reinforcement — placing bars within the section
• Section analysis — running design checks
• Design standards reference — detailed code comparison