In a coastal climate, a concrete urban bench will typically last 15 to 30 years before it needs major repair or replacement. That number is not fixed, however. I have seen well-detailed precast benches on oceanfront promenades still sound after 25 years, and I have also seen cheaper units with shallow rebar cover start spalling after 8 to 12 years.
The main reason coastal environments are hard on concrete is chloride. Sea spray and wind carry salt onto the surface, and moisture pushes chlorides into the concrete. Once chlorides reach the reinforcing steel, corrosion begins. Rust expands and cracks the concrete from the inside, which shows up as rust stains, hairline cracks along rebar lines, and eventually spalling. Wet-dry cycles make things worse because salt crystals grow in pores and add pressure.
A realistic service life usually falls into three bands. Basic unreinforced or lightly reinforced benches with low-quality concrete may last 10 to 15 years before visible damage. Standard precast benches with reasonable cover and moderate maintenance often last 20 to 25 years. High-performance concrete with low permeability, stainless or coated reinforcement, and good drainage can last 30 years or more.
If you are selecting or maintaining these benches, pay attention to concrete mix design. A low water-cement ratio, supplementary cementitious materials such as fly ash, slag, or silica fume, and minimum 40 to 50 mm of concrete cover over steel make a big difference. Stainless steel rebar or epoxy-coated steel also extends life. Avoid flat horizontal surfaces where salty water can pond, and make sure seat surfaces drain freely. Anchors and embedded bolts should be stainless or hot-dip galvanized, because these small metal parts often fail first.
Maintenance also changes the lifespan. A bench washed with fresh water a few times a year, inspected for cracks, and sealed with a breathable silane or siloxane sealer will last longer. Small cracks should be repaired promptly so chlorides and moisture cannot reach the reinforcement. If rust stains or spalling appear, early patch repair can add years, but once corrosion is widespread, replacement is usually more economical.
So the short answer is: expect around 20 years from a typical concrete urban bench in a coastal climate, with a practical range of about 15 to 30 years depending on quality, detailing, and maintenance.