calculation basis
The SDMHR table already gives nominal cover.
The tool reads the reinforcing-steel nominal cover for the selected exposure and concrete grade. The 10 mm design-deviation allowance is already included in Table 5.2: it is not added again. A dash in the official table is treated as unavailable, not as zero, and no interpolation is made between concrete-grade columns.
Input guidance and table notes
- Concrete grade: use the project characteristic cube-strength grade, not cylinder fck. The table has Grade 30, 40, 45, 50 and 55 & above columns. Grade 35, for example, needs a separately verified basis; do not interpolate or silently choose another column.
- Exposure: independently classify all relevant concrete faces and environments. The helper checks one exposure at a time and does not approve the entire concrete specification.
- In-situ slab: Table 5.2 note 3 adds 5 mm for thickness greater than 250 mm, with a maximum of 75 mm for that adjusted table requirement. Zero means the member is not an in-situ slab.
- Ground contact: note 4 requires at least 75 mm nominal cover for the underside in contact with ground, and crack width no greater than 0.25 mm. Confirm whether the actual detail includes blinding and the applicable exposure.
- Acidic water: note 5 applies at pH ≤ 4.5: Grade 45 or higher, nominal cover 75 mm and a 0.10 mm crack-width limit. Lower grades receive no recommendation.
- Abrasive seawater: select this only for the corresponding XS3 condition. Its crack-width limit is 0.10 mm rather than the other XS3 row's 0.15 mm.
- Other requirements: enter independently verified bond, fire or project cover requirements as nominal values, already including their applicable deviation. Zero means no additional requirement has been supplied, not that bond or fire has passed.
Worked example: XC3 / Grade 40
Table 5.2 gives 45 mm nominal cover and a 0.25 mm crack-width limit for reinforcing steel at XC3 / Grade 40. With a 300 mm in-situ slab, note 3 gives min(45 + 5, 75) = 50 mm. If the underside is directly in contact with ground, note 4 instead governs at 75 mm. No second 10 mm deviation is added. The crack-width limit is a criterion to use in a separate SLS calculation, not a predicted crack width or a pass.
Interpretation and common mistakes
Use the cover result to revise the section geometry, then recalculate effective depth, strength and serviceability. The separately calculated crack width must satisfy the applicable limit; this lookup does not perform that check. Common errors are mixing SDMHR and generic EC2 cover values, entering cylinder strength as cube-strength grade, treating blank table cells as permissible, using the bracketed prestressing cover for ordinary reinforcing bars, and adding a second execution allowance.
Scope and limitations
This helper implements the stated reinforcing-steel table and notes only. It does not establish concrete composition, bond adequacy, fire resistance, cover tolerance inspection, marine abrasion protection, prestressing provisions or project adoption of the manual. Table adjustments do not override larger independently verified project minima. Different faces may require different exposure checks. A lack of a valid table value is a design-basis issue to resolve with the responsible engineer, not a value to fill in automatically.
Official source
Source: Hong Kong Highways Department, Structures Design Manual for Highways and Railways (2013 Edition), continuously updated December 2025 version, §5.4.2, Table 5.2 on printed page 90, including notes 1–5. Verify the applicable project edition and amendments before use. Numerical values and the specific implemented notes are shown in the calculation output; substantial standard text is not reproduced.
Related CivilKits tools: RC beam section checks and the separately scoped Eurocode cover helper. Do not transfer a highway cover value into a different design basis without verification.
Engineering notice
This tool assists calculations and does not replace professional engineering judgement, independent verification or project-specific review. Users must verify inputs, assumptions, units, parameters, standards and results. A suitably qualified engineer should independently check results before safety-critical use. Read the Terms / Engineering Disclaimer.