22-Agric-B2 Structural Design for Agricultural, Biosystems, and Food Industries · May 2018

Question 5 of 6: Manure Storage Tank — Loads and Concrete Durability

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Question 5: Manure Storage Tank — Loads and Concrete Durability (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

a) Loads on the manure tank structure. The below-floor tank is a below-grade concrete box that must be checked for at least three distinct, and partly opposing, load cases. Full-tank / empty-backfill case: immediately after backfilling but before the tank is filled with manure, the exterior backfill applies active or at-rest lateral earth pressure against an unsupported wall with nothing pushing back from the inside — this is often the critical case for wall bending, since there is no internal fluid pressure to counteract the soil thrust. Full-tank / no-backfill (construction) case: conversely, if manure is added before backfill is complete, the internal liquid manure pressure (a fluid load, $p=\gamma_{manure}h$, roughly hydrostatic since liquid manure behaves close to a fluid) pushes outward on an unsupported wall, which can govern for wall design in the opposite direction. Normal operating case: once both backfill and manure are in place together, the two lateral pressures substantially cancel over most of the wall height, but any imbalance (e.g. seasonal water-table rise on the backfill side, or partial tank drawdown during agitation/pump-out) must still be checked. On top of these lateral cases, the structure also carries: vertical loads from the floor slab's own self-weight and any surface live load (vehicle/tractor traffic crossing the tank cover or adjacent slab, per NBCC Part 4 live-load provisions for agricultural yard/traffic areas); buoyancy/uplift on the empty tank floor from a high water table, which can float or crack an empty tank if not checked; and long-term serviceability effects — differential settlement of the long (36 m) tank relative to adjoining slabs, and thermal/shrinkage movement requiring control or expansion joints along its length. Because manure storage is classified low-human-occupancy but represents a significant environmental and safety hazard if breached, the appropriate design considerations also include a minimum liquid-tightness/crack-width serviceability limit (not just ULS strength), freeze-thaw durability for the exposed upper wall band, and a factor of safety against flotation for the empty condition.

b) Concrete durability requirements for the corrosive manure environment. Liquid manure is a chemically aggressive combination of dissolved ammonium/sulfate salts, organic and fatty acids from digestion, and biologically generated hydrogen sulfide and carbon dioxide, all of which attack ordinary Portland cement paste (sulfate attack forming expansive ettringite, and acid attack directly dissolving the calcium hydroxide binder) and promote reinforcement corrosion once chlorides/sulfates reach the steel. CSA A23.1 addresses this directly: cement type — Type HS (high sulfate-resistant, low $C_3A$) portland cement, or an equivalent blended cement with supplementary cementing materials (fly ash or slag) that further reduce permeability and free lime available for sulfate reaction; cement content — a minimum cementitious content (typically $\ge 350$ kg/m$^3$ for this severe exposure class) to ensure enough paste to fully encapsulate the aggregate and reinforcement; water/cement ratio — capped low, $w/cm\le0.45$, since permeability (and hence the rate of ion ingress) falls steeply as w/cm drops, which is the single most effective lever for long-term durability; aggregates — hard, sound, non-reactive aggregate (checked for alkali-silica reactivity) with a well-graded particle distribution to minimize paste demand and shrinkage cracking; admixtures — air-entrainment for freeze-thaw resistance in the exposed upper wall band, plus water-reducing/superplasticizing admixtures to hit the low w/cm target while keeping the mix workable enough to consolidate around closely spaced reinforcement; concrete cover — increased cover (commonly 50–75 mm for this exposure class, versus 30–40 mm for ordinary interior concrete) to keep the corrosion-initiating front from ever reaching the reinforcing steel over the structure's service life; curing — extended moist curing (minimum 7 days, often longer for HS/blended cements which hydrate more slowly) to ensure the low-permeability microstructure the low w/cm ratio was designed to achieve actually develops, since inadequate curing leaves a porous, permeable surface layer regardless of the specified mix; and crack control — tight crack-width limits enforced through closely spaced, adequately sized reinforcement (rather than a few large bars) and properly detailed construction/control joints with waterstops, because manure-facility cracks are both a liquid-tightness (environmental) failure and a fast-track for the ingress that overwhelms all the other durability measures. In short, sulfate resistant concrete (defined in Question 1e) is one necessary ingredient of a durable manure tank, but on its own is not sufficient without the matching w/cm, cover, curing, and crack-control package.