Concrete Grade & Rebar

Quick pick (TL;DR)

Mild/indoor → C25/30, cover 25–40 mm.
Outdoor/wet → C28/35.
Chloride/marine → C35/45, w/c ≤ 0.40, cover 50+ mm, consider epoxy-coated bars.
Sulphate soil → Type V cement.
Rebar SD40 (fy ≈ 390) is the workhorse.
Then check the section has enough capacity (flexure).

Step 1 — Durability sets the floor

Exposure	Min f'c	Max w/c	Cover
Mild / indoor	C20/25	0.55	25–40 mm
Outdoor / wet	C28/35	0.45	40–50 mm
Chloride / marine	C35/45	0.40	50–65 mm
Sulphate soil	C28/35–C35/45	0.45–0.40	+ Type V cement

Step 2 — Check the section

RC beam flexural capacity

Given f'c = 35 MPa , fy = 390 MPa b = 300 mm , d = 540 mm As = 3-DB25 = 1473 mm² Step 1 — a a = As·fy / (0.85·f'c·b) a = 1473×390 / (0.85×35×300) a = 574,470 / 8,925 = 64.4 mm Step 2 — Mn Mn = As·fy·(d − a/2) Mn = 1473×390×(540 − 32.2) Mn = 574,470 × 507.8 = 291.7 kN·m Step 3 — φMn φMn = 0.90 × 291.7 = 262 kN·m if Mu ≤ 262 kN·m → OK

Step 3 — Detailing essentials

Cover protects rebar — never below the exposure minimum.
Development/lap ≈ 40db; stagger laps.
Min/max steel ratio; crack control.
Epoxy/galvanized bars in chloride.

RC beam moment capacity calculator

Singly-reinforced φMn (ACI 318)

a = As·fy/(0.85·f'c·b); φMn = φ·As·fy·(d − a/2).

f'c (MPa)

fy (MPa)

Width b (mm)

Depth d (mm)

As (mm²)

Mu (kN·m)

Singly-reinforced, tension-controlled. Check ρ limits, shear, development separately.

Common mistakes

Strength-only grade → marine corrosion.
Too little cover.
Wrong cement on sulphate soil.

Requisition & datasheet

Exposure, grade, w/c, cement, cover.
Rebar grade, size, lap, coating; code.

Frequently asked questions

What should I know about Concrete Grade & Rebar?

Mild/indoor → C25/30, cover 25–40 mm. Outdoor/wet → C28/35. Chloride/marine → C35/45, w/c ≤ 0.40, cover 50+ mm, consider epoxy-coated bars. Sulphate soil → Type V cement. Rebar SD40 (fy ≈ 390) is the workhorse. Then check the section has enough capacity (flexure).

Which standards or codes apply?

ACI 318 (Ch.19 durability & Ch.25 detailing); ASTM C150 (cement types); Thai DPT/EIT; Eurocode 2 / EN 206 exposure classes.

Standards — ACI 318; ASTM C150; Thai DPT/EIT; EN 206.