Introduction
A grade beam is only four materials — concrete, longitudinal steel, stirrups, and the accessories that hold them in place — but each has specifications that matter. The wrong bar size, a mix below the specified strength, or cover measured in hope rather than inches can each compromise the beam. This guide covers what each material does and the standard sizes, so your order matches the structural drawings.
The Grade Beam Calculator converts your beam’s dimensions into concrete volume and steel weight. This guide tells you what to order with those numbers: which bar sizes, which mix, what cover, and which accessories the pour needs on site.
Concrete: Strength, Mix, and What the Numbers Mean
Grade beam concrete is typically specified at 3,000–4,000 psi (21–28 MPa). That number is the design’s requirement for the loads and exposure — not a suggestion or a field decision. Ordering a different strength than specified changes the element the engineer designed.
Beyond strength, the mix ticket carries three more values worth understanding:
- Slump — a measure of workability, typically 3–5 in (75–125 mm) for grade beam work. Too stiff and the concrete won’t flow around the cage; too wet (water added on site) and strength and durability drop. Never add water beyond the mix design’s allowance.
- Maximum aggregate size — must clear the tightest space in the cage: commonly limited to 3/4 in (19 mm) so concrete passes between bars and stirrups without honeycombing.
- Admixtures — air entrainment for freeze-thaw exposure, retarders for hot weather or long hauls, accelerators for cold weather. Specified by the design for the conditions, not chosen at the plant gate.
Confirm the specified mix on the structural drawings before calling the supplier — strength, slump, and any admixtures. The pour-day handling of that mix is in the pro tips guide.
Longitudinal Steel: Bar Sizes and What They Weigh
Longitudinal bars carry the beam’s bending tension — top steel over the supports, bottom steel at midspan, because moments reverse along a continuous beam. Grade beams typically use 4–8 bars in sizes #4 through #8, with the count, size, and layout fixed by the structural design.
| Bar size | Diameter (in / mm) | Weight (lb/ft) | Weight (kg/m) |
|---|---|---|---|
| #4 | 0.500 / 12.7 | 0.668 | 0.994 |
| #5 | 0.625 / 15.9 | 1.043 | 1.552 |
| #6 | 0.750 / 19.1 | 1.502 | 2.235 |
| #7 | 0.875 / 22.2 | 2.044 | 3.042 |
| #8 | 1.000 / 25.4 | 2.670 | 3.973 |
Steel weight for the order is total bar length × unit weight, plus 10–15% for lap splices wherever continuous runs exceed stock bar lengths. The full weight derivation with a worked example is in the formula-explained guide, and the takeoff sequence is in the estimating guide.
Stirrups: Size, Spacing, and Function
Stirrups — the closed rectangular ties wrapping the longitudinal bars — do two jobs: resist shear forces (highest near the supports) and confine the cage so all the bars act together. They are typically #3 or #4 bar, at the spacing the structural design specifies; spacing often tightens near supports where shear demand peaks.
Stirrup detailing has rules that matter on site: hooks properly bent (135-degree seismic hooks where required), spacing measured center to center, and count = length ÷ spacing plus one. Skipping stirrups or widening spacing in the field is a classic silent defect — the beam looks identical from outside (see Mistake 2 in the common mistakes guide).
Concrete Cover: the 2–3 Inches That Protect the Steel
Cover is the distance from the concrete surface to the nearest steel — typically 2–3 in (50–75 mm) for grade beams, with the larger value where concrete is cast against earth rather than forms. Cover does three things: protects steel from moisture and corrosion, provides the bond that lets steel and concrete act compositely, and supplies fire resistance.
Cover is placed, not wished: the cage sits on chairs or concrete dobies (never bricks, wood, or soil that shifts), and the whole cage is tied so it cannot drift during the pour. Verify cover on all faces — including the bottom — after the cage is fully tied, because cages shift during tying. Inspection typically checks this before the pour (see the code compliance guide).
Accessories: the Small Items That Prevent Big Problems
- Chairs and dobies — hold the cage at the correct height for bottom cover. Space them to prevent bar sag between supports.
- Tie wire — every bar intersection in the cage gets tied so the layout survives the pour.
- Formwork and bracing — sized for the lateral pressure of wet concrete at full depth.
- Vapor barrier / blinding — where the design calls for it, keeps the beam bottom clean and controls moisture.
- Carton void forms — in expansive soils, the collapsible void under the beam that the design may specify. Not optional where shown.
- Curing materials — wet burlap, curing compound, or covers, on site before the pour, not ordered after.
Worked Example: the Material List for a 24-ft Beam
The beam: 24 ft long, 12 × 18 in, 6 #6 longitudinal bars, #4 stirrups at 12-in spacing, 3,500 psi mix per the (hypothetical) design, 2-in cover, 10% concrete waste, 12% steel lap allowance.
Concrete — imperial: 24 × 1 × 1.5 = 36 ft³ = 1.333 yd³; +10% = 1.467 yd³ → order ~1.5 yd³ of 3,500 psi mix at the specified slump.
Concrete — metric cross-check: 7.3152 × 0.3048 × 0.4572 = 1.0194 m³; 1.333 × 0.7645549 = 1.0194 m³ ✓; with waste 1.121 m³ → order ~1.15 m³.
Longitudinal steel: 6 × 24 ft = 144 lin ft × 1.502 = 216.29 lb × 1.12 = 242.24 lb of #6 (metric: 43.8912 m × 2.235 = 98.10 kg × 1.12 = 109.88 kg; 242.24 × 0.453592 = 109.88 ✓).
Stirrups: (24 ÷ 1) + 1 = 25 #4 stirrups, each ~50 in (4.17 ft / 1.27 m) → 69.64 lb (31.58 kg).
Total steel: ~311.9 lb (141.5 kg). Plus: chairs/dobies for a 24-ft cage, tie wire, formwork for both 24-ft sides, bracing, and curing materials on site before the truck arrives. That is the complete order — concrete, steel, and everything that holds them in position.
Related Guides
- Grade Beam Formula Explained — the weight and volume math behind the material list.
- Grade Beam Cost Guide — turning this material list into a budget.
- Grade Beam Estimating Guide — the takeoff sequence that produces the list.
- Grade Beam Code Compliance — cover, inspection, and ACI 318 requirements.
Frequently Asked Questions
What size rebar is used in grade beams?
Typically #4 through #8 for longitudinal bars (4–8 bars, top and bottom) and #3 or #4 for stirrups. The exact sizes, count, and layout come from the structural design — this guide covers what the sizes mean and weigh, not which to choose.
How much does grade beam rebar weigh?
Total bar length × unit weight, plus 10–15% for laps. Example: 6 #6 bars × 24 ft × 1.502 lb/ft = 216.29 lb (98.10 kg), 242.24 lb (109.88 kg) with a 12% lap allowance; stirrups add ~69.64 lb (31.58 kg).
What concrete strength do I order?
Whatever the structural drawings specify — typically 3,000–4,000 psi (21–28 MPa). Confirm strength, slump, and admixtures on the drawings before calling the supplier; the mix is a design decision.
How much concrete cover does the steel need?
Typically 2–3 in (50–75 mm), larger where cast against earth. Achieve it with chairs or concrete dobies — never bricks or wood — and verify on all faces after the cage is tied.
What are stirrups and why do they matter?
Closed ties wrapping the longitudinal bars that resist shear (highest near supports) and confine the cage. Usually #3 or #4 at design-specified spacing; count = length ÷ spacing + 1. Omitting or widening them is a silent structural defect.
What accessories do I need on pour day?
Chairs/dobies, tie wire, formwork and bracing, and curing materials — all on site before the truck arrives. In expansive soils, carton void forms where the design shows them; vapor barrier or blinding where specified.
Get Your Number
Order what the drawings specify, weigh it with the unit weights above, and verify the arithmetic in both unit systems. The Grade Beam Calculator does the volume and steel math from your dimensions; this guide tells you what those numbers mean on the order ticket. When the materials arrive, check them against the beam schedule before anything goes in the ground — and keep every specification decision with the licensed engineer.
Disclaimer: Calculator results are planning estimates based on the dimensions you enter. Material sizes, strengths, and cover values in this guide are typical ranges for general information — the project specifications govern. Grade beams are engineered structural elements; consult a licensed structural engineer or your local building authority for design, sizing, and code decisions, and verify quantities with your supplier.
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