Service Life Prediction - Carbonation & Chloride Diffusion 2026
Concrete service life defines period until reinforcement corrosion initiation through carbonation front reaching steel or chloride threshold penetration. Fick's second law models carbonation depth x = k√t with k=2-6mm/√year depending concrete quality producing 20-60mm penetration 50 years. Probabilistic design targets 10% failure probability maintaining risk acceptable throughout exposure period balancing initial cost against whole-life economy.
XC3 moderate humidity external walls predicts 40-80 year life 25mm cover C28/35 0.55w/c while XC4 foundations wet-dry contact 50-100 years 40mm cover C30/37 0.50w/c. Chloride exposure XD1/XS1 dramatically reduces life requiring 0.40w/c, 380kg cement, 50mm cover achieving 40-60 years coastal protection preventing premature structural deterioration.
Carbonation coefficient k reduces 50% 0.45w/c versus 0.60 through pore refinement with 10mm cover increase extending life 2.5x √t relationship. Urban CO2 500ppm doubles k versus rural 250ppm significantly affecting exposed elements serviceability. Quality control tolerance ±10mm produces 90% minimum cover maintaining design assumptions despite construction variability.
Chloride diffusion coefficient D=10-50×10^-12m²/s models ingress with surface concentration Cs=0.4-0.6% cement weight predicting time-to-corrosion tc=D(crit-cs)^2/(4Dcrit). XS1 coastal requires D<5×10^-12m²/s achieving 50 years 50mm cover preventing premature spalling and section loss.
Cement content 300-400kg/m³ provides alkaline reserve neutralising CO2 with GGBS/PFA refining pore structure 30% though slower carbonation initially. Crack width control <0.3mm prevents preferential ingress paths accelerating deterioration rates 5-10x continuous cracks. Curing quality affects surface zone permeability dominating first decade performance.
Protective coatings reduce k 50-70% extending life 2-3x though reapplication every 10 years essential maintaining performance. Cathodic protection extends severely deteriorated structures 20-30 years preventing section loss progression. Hydrophobic impregnation reduces water demand 80% significantly slowing carbonation and chloride transport.
| Exposure | Cover | w/c | Life Span | Failure Mode |
|---|---|---|---|---|
| XC1 Internal | 20mm | 0.65 | 100+ years | Low risk |
| XC3 Walls | 25mm | 0.55 | 60-80 years | Carbonation |
| XC4 Foundation | 40mm | 0.50 | 80-120 years | Carbonation |
| XS1 Coastal | 50mm | 0.40 | 40-60 years | Chloride |
| Factor | Good | Poor | Life Multiplier |
|---|---|---|---|
| w/c Ratio | 0.45 | 0.60 | 2.0x |
| Cover Quality | ±5mm | ±15mm | 1.5x |
| Curing | 7 days wet | 1 day | 1.8x |
| Crack Width | <0.2mm | >0.4mm | 0.3x |
Carbonation depth square-root time k=2-6mm/√year concrete quality determines life.
Square-root relationship dramatically extends protection period small increases.
40mm cover C30/37 0.50w/c foundations wet-dry soil contact standard performance.
Capillary discontinuity halves carbonation coefficient versus porous 0.60 mixes.
>0.3mm widths create preferential paths accelerating agent ingress dramatically.
Surface treatments reduce k 50-70% though reapplication essential maintenance.
XC4 specification 40mm cover C30/37 0.50w/c predicts 80-120 years carbonation protection preventing corrosion initiation. Actual performance exceeds predictions with proper construction quality control and maintenance practices throughout service life.
Crack widths >0.3mm reduce life 10x creating preferential carbonation/chloride paths. Poor cover <80% nominal, high w/c>0.60, inadequate curing produce surface weakness dominating first 20 years performance significantly.
GGBS refines pore structure 30-50% reducing permeability though slower early carbonation requires extended cover. Long-term superior performance 56+ days compensates initial period achieving equivalent 50-year protection.
XS1 coastal spray C35/45 0.40w/c 50mm cover predicts 40-60 years chloride protection. XS3 tidal zones require C40/50 60mm cover achieving marginal 30-40 years most aggressive marine environment.
Surface treatments reduce carbonation coefficient 50-70% extending life 2-3x though reapplication every 10 years essential maintaining barrier properties throughout exposure period.
Urban CO2 500ppm doubles k versus rural 250ppm reducing external element life 40-50% significantly. Internal protected concrete achieves 100+ years regardless atmospheric concentration levels.
QC1 ±5mm tolerance produces 95% minimum cover 1.5x life extension versus QC3 ±15mm 80% cover. Construction quality dominates long-term performance exceeding mix design improvements.
Major infrastructure increases cover 5-10mm, w/c 0.05 lower, enhanced QC1 ensuring margin against defects, aggressive exposure, maintenance delays throughout extended service life.
For life-cycle design, visit Concrete Society | Eurocodes | For testing, check BRE