Concrete is strong in compression but weak in tension — roughly 10% of its compressive strength. Steel rebar handles the tension side. But rebar only works if it's in the right place. Two parameters govern placement: cover (distance from the concrete surface to the nearest rebar) and spacing (distance between bars).

Why cover matters: Cover protects rebar from corrosion. Chlorides from deicing salts or seawater penetrate concrete over time. If rebar sits too close to the surface, it rusts, expands (rust occupies ~6x the volume of steel), and spalls the concrete off. ACI 318 specifies minimum cover:

• Concrete cast against earth: 3 inches (75 mm)
• Exposed to weather, #6 bars and larger: 2 inches (50 mm)
• Exposed to weather, #5 bars and smaller: 1.5 inches (38 mm)
• Interior slabs and walls: 0.75 inches (19 mm)

Why spacing matters: Bars need concrete flowing between them to bond properly. If bars are packed too tight, aggregate can't pass through and you get voids — honeycomb — that destroy structural integrity. ACI requires clear spacing of at least the bar diameter, 1 inch, or 1.33 times the maximum aggregate size, whichever is greatest.

Real-world example: A residential garage slab, 4 inches thick, uses #4 rebar on 12-inch centers. The bars should sit in the lower third of the slab to resist bending tension. Contractors often use "chairs" (wire supports) to hold the rebar at the correct height — typically 1.5 inches from the bottom. Without chairs, rebar sinks to the bottom during the pour and provides almost no structural benefit. This is one of the most common field defects in concrete construction.

Quick calculation — development length: Rebar must extend far enough into the concrete to transfer its full tensile load through bond. A simplified rule of thumb for #4 through #8 bars in normal-weight concrete (f'c = 4000 psi): development length ≈ 40 × bar diameter. A #5 bar (diameter = 0.625 in) needs roughly 40 × 0.625 = 25 inches of embedment. If you can't provide that length, you use a hook, which reduces the required straight length by about 40%.

Practical tip for software engineers: If you're ever designing sensor mounts, conduit runs, or equipment bases that get cast into concrete, your embedments follow the same bond-length rules. A threaded rod cast into a footing needs adequate depth or it'll pull out under load.