The Recycled Bench
A council runs a competition for 40 park benches. The brief requires at least 60% recycled content by mass, a 10-year outdoor life, and a budget of $420 per bench. A supplier offers three materials. Option A is 100% recycled HDPE lumber: $260 per bench, UV-stabilised, but it creeps under sustained load and expands thermally, so fixings need slotted holes. Option B is a recycled-plastic-and-timber composite: $310 per bench, stiffer, but absorbs some moisture and needs sealing every two years. Option C is virgin aluminium with 30% recycled content: $400 per bench, very durable, but fails the recycled-content rule. A councillor argues Option A is best because it has the highest recycled content and the lowest price.
1. Assess the three options against the brief and recommend one, showing how you weigh the criteria.
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Criteria: recycled content ≥60% by mass, 10-year outdoor life, ≤$420 per bench, plus the implied maintenance burden on a council.
- Option A (recycled HDPE): meets recycled content (100%) and price ($260). Weaknesses are creep under sustained load (a bench seat carries people for long periods, especially in heat) and thermal expansion, which demands slotted fixings to avoid buckling. These are solvable with design (thicker sections, slotted holes, closer supports) but add engineering effort and may shorten life.
- Option B (composite): recycled content likely meets 60% if the plastic fraction is recycled, but this must be verified by mass, not assumed. Price $310 is within budget. Stiffer than A, so less creep, but moisture absorption plus two-yearly sealing means the council carries a recurring cost across 40 benches over 10 years. That maintenance load is a real lifecycle cost the $310 price hides.
- Option C (aluminium): most durable and lowest maintenance, but 30% recycled content fails the 60% rule outright. Disqualified regardless of merit.
Recommendation: Option A with design modifications (slotted fixings, increased section, supports at shorter spacing), because it satisfies both hard rules and the price leaves $160 per bench of headroom to fund the design changes. The councillor is partly right for the wrong reason: recycled content and price are necessary but not sufficient. The 10-year life must be demonstrated by a weathering test or warranty, not assumed.
2. The councillor says recycled content is the only measure that matters. Explain why a lifecycle view changes the ranking, using the maintenance data given.
Read the worked answer
Why single-metric thinking fails: recycled content measures one input at the factory gate. A lifecycle view counts everything from manufacture to disposal, including maintenance, replacement, and end-of-life recovery.
Applying it:
- Option A has the best recycled content but, if creep causes sagging seats, the council may replace components before 10 years. Replacement cost and waste offset the recycled-content benefit.
- Option B has slightly lower recycled content but needs sealing every two years. Across 40 benches and 10 years that is roughly five sealing cycles per bench, a recurring labour and material cost. If sealing is skipped, moisture damage shortens life.
- Option C has low recycled content but very long life and low maintenance, so its whole-of-life impact per year of service may be competitive even though it fails the brief's hard rule.
Conclusion: the ranking depends on which costs are counted. Recycled content is a proxy for material recovery, not for durability or maintenance. A defensible decision records the assumptions (replacement rate, sealing cost, service life) and tests how sensitive the ranking is to them. This is the reasoning the exam rewards: recognising that a sustainability claim is only meaningful when the boundary of the analysis is stated.