What are the primary causes and indicators of spalling in concrete bridges?

The primary causes of spalling in concrete bridges are corrosion of reinforcing steel and freeze-thaw cycles. Corrosion of rebar causes it to expand, creating internal pressure that fractures the surrounding concrete. Freeze-thaw cycles occur when water penetrates the concrete, freezes, and expands, also creating internal pressure. Other causes include impact damage, alkali-aggregate reaction (AAR), and poor concrete quality. The indicators of spalling include visible cracks in the concrete surface, rust stains emanating from the cracks, and loose or detached pieces of concrete. The presence of delamination (separation of layers) can also be an early indicator of impending spalling. Spalling exposes the reinforcing steel to further corrosion and weakens the structural integrity of the bridge, requiring timely repair.