Concrete & Rebar Diagrams
6 diagrams · 2 calculators
Slab-on-grade cross-sections, control-joint spacing, bag-vs-ready-mix math, and rebar cover, lap, and grid layout — the diagrams behind pouring flatwork that will not crack or corrode.
Calculators in this category
Concrete & Rebar · 6 diagrams
- Concrete & Rebar
How many bags of concrete are in a cubic yard — 45 eighty-pound bags or 60 sixty-pound bags
A cubic yard is 27 ft³, so it takes 45 × 80-lb bags (yield 0.60 ft³ each) or 60 × 60-lb bags (0.45 ft³ each). Past about ½ yard — roughly 25 bags — ordering a ready-mix truck beats mixing by hand.
- Concrete & Rebar
What goes under a concrete slab — gravel sub-base, vapor barrier, reinforcement, and slab thickness
A slab is more than concrete: a compacted gravel sub-base (~4″), a 6-mil vapor barrier, then the slab (4″ standard, 3.5″ IRC minimum) with the steel held at mid-depth on chairs — not on the ground, where it does nothing.
- Concrete & Rebar
Concrete control joint spacing — cut joints so the slab cracks where you want it to
Concrete shrinks and will crack — control joints decide where. Space them (in feet) at about 2–3× the slab thickness (in inches): a 4″ slab gets joints every ~8–12 ft, cut ¼ of the depth within 6–12 hours.
- Concrete & Rebar
Rebar size chart — the bar number is its diameter in eighths of an inch (ASTM A615)
The rebar number is the diameter in eighths of an inch: #4 = 4⁄8″ = ½″ (0.668 lb/ft), #8 = 1″ (2.670 lb/ft). Weight per foot is identical across Grade 40/60/80 — grade is yield strength, not size.
- Concrete & Rebar
How many rebar bars — the “+1” grid formula (subtract cover, divide by spacing, add one)
Count bars with the “+1” rule: subtract cover from both edges, divide the clear span by the spacing, then add one for the far-edge bar. A 20×20 ft slab, #4 @ 12″, 3″ cover → ⌊(240−6)÷12⌋+1 = 20 each way. Keep spacing ≤ 18″.
- Concrete & Rebar
Rebar cover and lap splice — keep bar off the dirt and lap 40× the bar diameter
Two placement details: keep bar up on chairs (3″ cover cast against earth, ¾″ formed — ACI §20.6); and where a run needs splicing, lap 40 × the bar diameter (min 12″), stagger the laps, and tie every one (ACI §25.5).