22 FINAL - TYPE OF FAILURES IN CONCRETE

FLEXURAL FAILURE

SHEAR FAILURE

COMPRESSION FAILURE (CRUSHING)

TORSIONAL FAILURE

STRUCTURAL FAILURES (MECHANICAL STRESS)

IN BEAMS OR SLABS WHEN THE TENSILE STRESS FROM BENDING EXCEEDS THE CAPACITY OF THE REINFORCEMENT (OR THE CONCRETE ITSELF IN UNREINFORCED SECTIONS).

IT TYPICALLY STARTS WITH CRACKS AT THE BOTTOM (TENSION ZONE) OF A BEAM

FLEXURAL FAILURE

MOST DANGEROUS TYPES BECAUSE IT IS OFTEN SUDDEN AND BRITTLE. IT OCCURS DUE TO DIAGONAL TENSION, RESULTING IN INCLINED CRACKS (USUALLY AT 45°) NEAR THE SUPPORTS.

SHEAR FAILURE

THE LOAD EXCEEDS THE COMPRESSIVE STRENGTH. THIS IS COMMON IN COLUMNS OR OVER-REINFORCED BEAMS, CHARACTERIZED BY THE CONCRETE BULGING OR SHATTERING

COMPRESSION FAILURE (CRUSHING)

RESULTING FROM TWISTING FORCES, CREATING A SPIRAL-LIKE CRACKING PATTERN AROUND A STRUCTURAL MEMBER

TORSIONAL FAILURE

CARBONATION

CHLORIDE-INDUCED CORROSION

SULFATE ATTACK

ALKALI-SILICA REACTION (ASR)

DURABILITY & CHEMICAL FAILURES

CARBON DIOXIDE FROM THE AIR PENETRATES THE CONCRETE, LOWERING ITS PH LEVEL. THIS DESTROYS THE PROTECTIVE "PASSIVATION" LAYER AROUND THE STEEL REINFORCEMENT, LEADING TO RUST

CARBONATION

COMMON IN COASTAL AREAS. CHLORIDES PENETRATE THE CONCRETE AND CAUSE RAPID PITTING CORROSION OF THE REBAR

CHLORIDE-INDUCED CORROSION

IN SOIL OR GROUNDWATER REACT WITH THE HYDRATED COMPOUNDS IN THE CEMENT PASTE (SPECIFICALLY C3A), CAUSING EXPANSION, CRACKING, AND EVENTUAL LOSS OF COHESION.

SULFATE ATTACK

OFTEN CALLED "CONCRETE CANCER" THIS OCCURS WHEN REACTIVE SILICA IN THE AGGREGATES REACTS WITH ALKALIS IN THE CEMENT. IT FORMS A GEL THAT SWELLS WHEN IT ABSORBS WATER, CAUSING INTERNAL PRESSURE AND "MAP CRACKING."

ALKALI-SILICA REACTION (ASR)

SPALLING

SCALING

CRAZING

HONEYCOMBING

SURFACE & CONSTRUCTION FAILURES

BREAKING OFF OF LAYERS OR CHUNKS OF CONCRETE FROM THE SURFACE, OFTEN CAUSED BY THE INTERNAL PRESSURE OF RUSTING REBAR OR THE FREEZETHAW CYCLE.

SPALLING

LOCALIZED FLAKING OR PEELING OF THE FINISHED SURFACE, TYPICALLY CAUSED BY REPEATED FREEZING AND THAWING OR THE USE OF DE-ICING SALTS

SCALING

NETWORK OF FINE, SHALLOW CRACKS (RESEMBLING A SPIDER WEB) ON THE SURFACE, USUALLY CAUSED BY RAPID SURFACE DRYING DURING FINISHING

CRAZING

VOIDS LEFT IN THE CONCRETE BECAUSE THE MORTAR FAILED TO FILL THE SPACES BETWEEN COARSE AGGREGATES, OFTEN DUE TO POOR VIBRATION OR A "STIFF" MIX.

HONEYCOMBING