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

Question 6: Manure Storage Tank — Loads and 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.

6a) Loads on the manure tank structure

The tank walls and base slab must be checked for several distinct load cases, not just one governing case, because the critical condition differs by wall and by construction stage. Internal (liquid) pressure acts outward on the walls whenever the tank holds manure: liquid manure typically has a specific gravity around 1.0–1.03, giving a roughly triangular pressure distribution from zero at the surface to $p=\gamma_m h$ at the 2.4 m base — and because the tank stores 240 days of production, the design must assume the tank can reasonably be found completely full at any time. External (backfill soil) pressure acts inward on the walls once the excavation is backfilled: at-rest lateral earth pressure $p=K_0\gamma_s h$ (or active pressure if the wall is free to yield slightly) plus any surcharge from equipment or stored material driving over the backfill near the wall. Critically, these two pressures do not act together in the governing sense — the empty-tank, backfilled condition (full external soil pressure with no counteracting internal liquid pressure) is often the more severe case for wall bending, and must be checked explicitly as a distinct construction/operating stage, alongside the full-tank case (full internal pressure, backfill in place, which is generally less severe because the two pressures partially cancel).

Hydrostatic uplift on the base slab is a separate and often-overlooked load case: if the local groundwater table rises above the tank invert while the tank itself is empty (for cleaning/maintenance, or before first filling), buoyant uplift pressure acts upward on the slab and can float or crack it unless the slab's self-weight, tank wall dead load, and any counteracting keyed/anchored connection to the walls are sufficient to resist it — this is the "uplift" concept defined in Question 1h applied directly to Question 6's structure. Gravity loads from the barn floor slab above (Question 5's live and superimposed dead load, transferred through the raceway walls) add axial compression to the interior 200 mm walls and must be combined with their own out-of-plane bending from the liquid/soil pressures. Thermal and shrinkage effects from long, continuous 36 m wall runs require movement/control joints or continuous reinforcement sized for restrained shrinkage strain, since an unjointed wall this length will otherwise crack randomly. Finally, corrosive chemical attack from the stored manure itself (Question 6b) is not a structural load in the classical sense, but is a design consideration that governs the durability — and ultimately the long-term structural capacity — of every wall and slab element carrying the loads above.

6b) Concrete durability requirements

Liquid manure is a chemically aggressive environment: it contains dissolved sulfates and organic/fatty acids from anaerobic digestion, ammonium salts, and generates hydrogen sulfide gas at the liquid–air interface, all of which attack ordinary Portland cement paste. The durability design must be addressed on several fronts simultaneously, matching the CSA A23.1 concept of an explicit exposure class rather than a single generic "strong concrete" specification.

Cement type. Specify Type HS (high sulfate-resistant) cement per CSA A3001 — its restricted tricalcium-aluminate ($C_3A$) content limits the ettringite-forming sulfate reaction defined in Question 1j, which would otherwise cause expansive cracking and spalling of the tank walls over years of exposure. A supplementary cementing material (fly ash, slag, or silica fume) blended with the base cement further reduces permeability and improves sulfate/chemical resistance beyond Type HS alone.

Cement content and water/cement ratio. A low water/cementitious ratio (typically $w/cm\le0.40$–0.45 for this severe exposure class) with a correspondingly high minimum cementitious content (commonly $\ge365\ \text{kg/m}^3$) produces a dense, low-permeability paste that slows the ingress of sulfates, chlorides and acids to the reinforcement — permeability, not raw compressive strength, is the controlling durability property here.

Aggregates. Use well-graded, sound, non-reactive aggregate (tested for alkali–silica reactivity) with low absorption, since a porous or reactive aggregate undermines the low-permeability paste regardless of how carefully the cementitious side is specified.

Admixtures. Air-entrainment (typically 5–8% for this severe/freeze-thaw exposure class per CSA A23.1) is essential where the tank is exposed to freeze-thaw cycling near or above grade; a water-reducing (and, where needed, superplasticizing) admixture allows the low $w/cm$ ratio to still be placed and consolidated properly around congested reinforcement, avoiding the honeycombing that would otherwise create direct ingress paths for the corrosive liquid.

Concrete cover. Increased cover over the reinforcement (commonly 50–65 mm for concrete cast against/exposed to a corrosive liquid or backfill soil, per CSA A23.1's C-1/A-1-type severe exposure classes) provides a thicker, less-permeable diffusion path for sulfates and chlorides to reach the steel, directly trading off against Question 5's slab effective depth $d$ — a durability decision with a direct structural-design consequence.

Curing. Extended moist curing (a minimum of 7 days, and commonly extended to a full curing compound or wet-cure regime for a full curing period appropriate to the specified concrete's strength-gain rate) is required to let the low-$w/cm$ paste properly hydrate and develop its intended low permeability before the tank is exposed to manure — inadequate curing can leave the concrete's surface zone permeable and prone to early attack, regardless of how well the mix itself was designed on paper.