Repair Materials and Material Selection
Domain focus
This domain tests whether you can select a repair material that is compatible with the substrate, exposure, and structural role, and whether you know the governing specifications and failure modes.
Core rules
- Selection starts with substrate condition, exposure class, repair geometry, and whether the repair is structural. The material must be at least as durable as the surrounding concrete, not necessarily stronger.
- Key properties are bond, compressive and tensile strength, modulus of elasticity, coefficient of thermal expansion, shrinkage, creep, permeability, and curing sensitivity.
- Cementitious repairs are specified under ASTM C928 for packaged dry mixtures and ASTM C1107 for hydraulic cement grouts. Polymer modifications improve bond and reduce permeability but can increase shrinkage and thermal mismatch.
- Epoxy systems are specified under ASTM C881. Type designates service condition, Grade designates viscosity, and Class designates curing-temperature range. Epoxies are generally impermeable and should not be used where vapor transmission is required.
- Rapid-setting magnesium phosphate materials suit fast return to service but have different modulus and curing behavior than portland cement materials.
- Match modulus to avoid stress concentration; use low-shrinkage materials for large patches; use non-sag materials overhead and in vertical repairs.
Worked example
A bridge deck spall is in a chloride environment with 40 mm cover and a 75 mm deep repair. Reasoning: choose a low-permeability, low-shrinkage, silica-fume-modified cementitious repair meeting ASTM C928, with compatible thermal expansion and moderate modulus. Avoid a high-modulus epoxy mortar over weak, chloride-contaminated substrate because failure may occur cohesively in the substrate rather than at the bond line. Verify chloride removal and saturated surface-dry condition before placement.
Traps
- Assuming higher strength always means better repair performance.
- Using the wrong ASTM C881 Type for fresh-to-hardened concrete bonding.
- Ignoring pot life and temperature effects on epoxy.
- Selecting a vapor-impermeable material over a wet slab.