Five Mechanisms, No Combined Model

Reinforced concrete structures deteriorate through multiple simultaneous mechanisms — carbonation, chloride ingress, freeze-thaw cycling, alkali-silica reaction (ASR), sulfate attack, and reinforcement corrosion. Current service-life prediction models treat each mechanism independently: Tuutti's model for chloride-induced corrosion, the fib carbonation model, and separate empirical models for ASR and freeze-thaw. In reality, these mechanisms interact synergistically — carbonation lowers pH, which accelerates chloride-induced corrosion; freeze-thaw creates microcracks that accelerate both chloride and sulfate ingress; ASR gel expansion increases permeability to all transport processes. No validated model captures these coupled interactions, so infrastructure owners cannot predict when a bridge deck exposed to deicers, freeze-thaw, AND carbonation will fail — they can only predict when each mechanism alone would cause failure.

Concrete infrastructure represents >$10 trillion in replacement value in the U.S. alone. Service-life prediction drives repair-vs-replace decisions for every bridge, parking garage, water treatment plant, and coastal structure. Current single-mechanism models consistently overpredict remaining life because they miss synergistic acceleration. This leads to deferred maintenance: a bridge deck predicted to last 15 more years by chloride-only models may actually need intervention in 5–8 years due to combined degradation. The resulting surprise failures and emergency repairs cost 3–5× more than planned maintenance.

The fib Model Code 2010 provides probabilistic service-life design for new construction but addresses only initiation-phase chloride ingress and carbonation — not propagation-phase coupled deterioration. Multi-physics FEM models (e.g., STADIUM, COMSOL-based) can simulate coupled transport of ions and moisture, but they require input parameters (coupled diffusion coefficients, damage-transport coupling functions) that have never been experimentally validated for real concrete mixtures under combined exposure. Laboratory accelerated tests isolate one mechanism at a time to control variables, creating a data gap: the interaction terms in coupled models have no calibration data. Field monitoring captures actual deterioration rates but cannot disaggregate the contribution of each mechanism.

A systematic experimental program exposing standardized concrete specimens to controlled combinations of deterioration mechanisms (chloride + freeze-thaw; carbonation + chloride + wetting-drying; ASR + sulfate) with sufficient instrumentation to measure transport properties, damage evolution, and steel corrosion simultaneously. This would produce the interaction coefficients needed to validate coupled models. The experimental design challenge is significant — full-factorial testing of 5 mechanisms at 3 severity levels would require 243 conditions — so a fractional factorial or Bayesian adaptive approach is needed.

A team could expose concrete cylinders to paired deterioration mechanisms (e.g., accelerated carbonation + chloride ponding) and compare deterioration rates to specimens under each mechanism alone, quantifying the interaction factor. Even a 2×2 factorial for two mechanisms would produce novel data. A computational team could implement a coupled chloride-carbonation transport model in open-source FEM software and conduct sensitivity analysis on the unknown interaction parameters. Relevant disciplines: materials science, structural engineering, computational mechanics, experimental design.