Galvanic Cathodic Protection for Steel Rebar in Concrete — ASTM B418, TIS 3029, ISO 12696

In reinforced-concrete structures, the single biggest threat to service life is rebar corrosion — chloride or carbonate ingress reaches the steel, which rusts and expands, cracking the concrete. The damage is most visible on bridge piers, drainage structures, and coastal buildings.

Galvanic Cathodic Protection (GCP) addresses corrosion at the root cause. It embeds zinc anodes — a metal with lower electrochemical potential than steel — into the concrete near the rebar. The anode corrodes "sacrificially" in place of the rebar, allowing the steel to last 25–50 years even in aggressive environments.

How It Works

GCP relies on basic electrochemistry:

flowchart LR
  Zn["Zinc Anode
(-1.10 V)"] -- "galvanic current" --> Concrete["Concrete
(electrolyte)"]
  Concrete --> Rebar["Rebar
(-0.50 V) = Cathode"]
  Rebar -- "bond wire
continuity < 1Ω" --> Zn
  Zn -. "consumed in place of steel" .-> Sacrifice["Zn²⁺ dissolves
(sacrificial)"]
  Rebar -. "excess electrons" .-> Protected["rust reaction stops
25-50 yr life"]

1. A zinc anode (potential ≈ -1.10 V vs. Cu/CuSO4) is embedded in the concrete near the rebar.
2. The rebar (around -0.50 V) becomes a cathode.
3. Galvanic current flows from anode → concrete → rebar → return wire → anode.
4. Zinc is oxidised; the rebar receives excess electrons that suppress the rust reaction.

Crucially, the system needs no external power supply — unlike Impressed Current Cathodic Protection (ICCP), which requires permanent wiring and a control cabinet. Galvanic systems are install-and-forget.

Standards Coverage

Standard	Scope
ASTM B418 Type II	Zinc anode chemistry and physical properties for cathodic protection
TIS 3029-2563	Thai Industrial Standard for sacrificial anodes
ISO 12696:2022	Design, install, test, monitor cathodic protection in concrete
ACI 222R-19	Guide on protecting metals in concrete from corrosion
ASTM C876-22b	Half-cell potential test method for reinforcing steel

Anodes specified for a project should carry a Certificate of Analysis (CoA) showing compliance with ASTM B418 Type II and a test report aligned with TIS 3029-2563.

When to Specify

flowchart TD
  Start{"Project
environment?"}
  Start -->|"marine / coastal
(C5-M)"| GCP["Galvanic Cathodic Protection
(embedded zinc anode)"]
  Start -->|"below-grade / submerged"| GCP
  Start -->|"rehabilitation
corroded piers"| GCP
  Start -->|"urban / low chloride"| Surface["Surface protection
polyurethane primer + topcoat"]
  GCP --> LifeSpan["target service life
25-50 yr"]
  Surface --> LifeSpan2["target service life
10-20 yr + recoat"]

GCP is appropriate where the project demands a long service life and the structure sits in a corrosive environment:

• Marine and coastal structures — bridges, jetties, piers exposed to chloride spray
• Below-grade or submerged structures — deep foundations, sub-grade box culverts
• Brackish water exposure — coastal drainage, Chao Phraya riverside development
• Rehabilitation work — fixing already-corroded bridge piers without full demolition

For new urban builds with low chloride exposure, surface-applied protection (single-component polyurethane primer + topcoat) may be sufficient and more cost-effective.

Practical Guidance

1. Design electrically first — per ISO 12696, calculate required current density (typically 5-20 mA/m² of rebar surface) and anode spacing.
2. Bond reliably — connections between anode and rebar must show electrical continuity below 1 Ω.
3. Measure rebar potential before and after install via the half-cell potential method (ASTM C876). A negative shift > 100 mV confirms protection.
4. Log readings every 6-12 months throughout the project life to confirm anodes remain active.

Summary

Galvanic Cathodic Protection is the standard answer for extending rebar life in aggressive environments under globally accepted ASTM / TIS / ISO frameworks. If your project targets a 50-year service life and sits in C5 (marine/industrial) exposure, GCP pays back from day one.

Sahawatthanakit (1988) has supplied galvanic cathodic protection systems to the Bangkok Metropolitan Administration, the State Railway of Thailand, and private developers. Talk to our engineering team for project-specific guidance.