Seafront picnic tables: slats, salt spray and a maintenance budget that has gone
A coastal council is replacing the slats on six seafront picnic tables. The site is 30 m from the sea. The previous oak slats lasted four years before splitting and going green; the steel frames, hot-dip galvanised 12 years ago, are structurally sound but show white rust around the bolt holes. New slats will be 1.8 m long and 90 mm wide, bolted to the frame rails with M8 bolts through 10 mm holes, with rails at 600 mm centres. Surface temperatures range from −5 °C to 35 °C. The supplier lists pressure-treated pine at £11/m, European oak at £26/m, recycled HDPE at £19/m in 40 mm thickness, and anodised aluminium at £31/m. Each table needs about 10.8 m of slat, and the maintenance budget has been cut so the contractor can return only every three years. The client also wants tables usable bare-legged in summer, and resistance to graffiti and knife damage.
1. Recommend a slat material, supporting the choice with stiffness, thermal movement and cost figures, and state the fixing detail you would specify.
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Eliminate timber first: pine rots within two to three years at 30 m from the sea and oak failed at four, so both breach the three-year maintenance interval whatever they cost. Cost is irrelevant if the table is out of service.
Stiffness check, HDPE versus oak: for a 90 mm wide slat, oak at 32 mm gives I = 0.09 × 0.032³/12 = 2.46 × 10⁻⁷ m⁴; with E ≈ 11 GPa, EI ≈ 2 700 N·m². HDPE at 40 mm gives I = 4.80 × 10⁻⁷ m⁴; with E ≈ 0.9 GPa, EI ≈ 430 N·m² - about six times less stiff. An 800 N person on a 600 mm span gives roughly 8 mm immediate deflection, growing to maybe 15–20 mm under creep in a hot sun, so HDPE needs closer rails (≤ 500 mm) or a third rail, not the existing 600 mm.
Thermal movement: ΔL = αLΔT = 150 × 10⁻⁶ × 1.8 × 40 = 10.8 mm per slat. Clamped at both ends this will bow or shear the fixings, so specify 12 mm holes (or 20 mm slots) with domed washers and 3 mm end clearance.
Cost: 6 × 10.8 m = 64.8 m. HDPE ≈ £1 230; oak ≈ £1 685; aluminium ≈ £2 010.
Recommendation: UV-stabilised recycled HDPE slats at 40 mm, rails at 500 mm centres, A4 stainless bolts with isolating washers in oversized holes. It is maintenance-free for well over three years, needs no preservative, gives no splinters on bare legs, and knife damage can be planed out.
2. Explain the white rust at the bolt holes and specify a remedy.
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White rust is zinc corrosion product - zinc oxide and hydroxide - not steel rust. It means the sacrificial coating is being consumed, and once it is gone the steel underneath will start to corrode, so it is an early warning rather than a cosmetic issue.
Why it is concentrated at the holes: three effects combine. First, the galvanising is mechanically damaged when a bolt is tightened and when the hole is drilled or reamed, exposing bare steel. Second, seawater is an excellent electrolyte, and a stainless bolt is well above zinc in the galvanic series, so the zinc near the bolt becomes the anode of a small-anode/large-cathode cell - exactly the geometry that corrodes fastest. Third, water sits in the gap between slat and rail and never dries, so the attack is continuous.
Remedy, in order of preference:
- Isolate the joint - nylon or EPDM washers under the head and nut, and a sleeve through the hole, so the stainless bolt never touches zinc.
- Use hot-dip galvanised or sheradised M8 bolts and dome nuts so both sides of the couple are zinc.
- Seal the joint with a resilient washer so water cannot pond, and re-touch damaged coating with zinc-rich paint at every maintenance visit.
Whatever slat material is chosen, the same rule applies: if two different metals are in contact in a wet, salty environment, isolate them or accept that one of them will be eaten.
3. The client asks why not use the anodised aluminium, since it is the stiffest and will never rot. Evaluate the proposal.
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It is the strongest engineering case on paper and the weakest on this site.
For it: E ≈ 70 GPa, so a much thinner slat gives the same stiffness. A 3 mm aluminium slat has I = 2.0 × 10⁻⁹ m⁴ but EI ≈ 141 N·m²... and thicker sections, say 25 mm box, would let you keep the 600 mm rail spacing. It does not rot, absorb water or support mould, and it can be recycled with a real scrap value.
Against it:
- Coating damage is permanent. Anodising is a thin, brittle oxide. Bolting it up, a dropped bottle or a knife scratch exposes bare aluminium, which then corrodes locally at the joint. On a seafront, a scratched anodised finish is a permanent visual defect that cannot be touched up on site - exactly the opposite of HDPE, which can be sanded.
- Mixed-metal couples again. Bare aluminium in contact with galvanised steel in seawater creates a new galvanic cell, so you must isolate every fixing, adding cost and a maintenance task.
- Thermal and user comfort. Bare aluminium reaches uncomfortable temperatures in direct sun, and it dents rather than yielding, which looks worse than a dented timber or plastic slat.
- Cost. About £2 010 against £1 230 for HDPE, for a benefit that only pays off if you keep the existing rail spacing.
Verdict: specify aluminium only if the frame cannot be re-railed and stiffness is the binding constraint; otherwise HDPE at 500 mm rail centres wins on whole-life cost. If aluminium is used, specify a powder-coated finish and isolating washers in the same sentence.