Formwork blowouts are rarely a mystery afterwards. The crew poured too fast, the concrete was colder than planned, or somebody sized the form ties for the “average” wall instead of the worst one. All three are pressure problems, and all three are avoidable with one calculation done before the pour.
This guide gives you the ACI 347 lateral pressure equations, the coefficient tables, three worked examples, and a calculator you can use right on the page. It follows ACI 347R-14, Guide to Formwork for Concrete. Always confirm against the edition your project specifies, and have a licensed engineer or formwork specialist approve the final design.
Why concrete pressure on formwork matters
Fresh concrete behaves like a liquid for the first few hours. Every foot of depth adds roughly 150 psf, the same way water adds 62.4 psf per foot. As the mix stiffens, the bottom layers start to carry their own weight and the pressure stops climbing. That stiffening is why a slow pour on a warm day produces far less pressure than a fast pour in cold weather, even with the same wall height.
The design pressure drives four decisions: sheathing thickness, stud and wale spacing, tie size and spacing, and bracing. Undersize any one and the form can bulge, open at the joints, or fail outright.
How the pressure actually develops, step by step
Think of freshly placed concrete as a thick liquid that slowly turns into a solid. In the first hour or two it is heavy enough to push sideways with a pressure equal to its depth times its unit weight, about 150 psf for every foot of depth. That is called the full fluid head, and it is the dashed line in the diagram below.
Then two things happen at once. The concrete near the bottom starts to stiffen as cement hydrates, and aggregate particles begin to lock together and carry their own weight. From that depth downward the sideways push no longer grows. The pressure curve bends away from the dashed line and runs straight down at a constant value. That value is the design pressure the ACI 347 equations estimate.
Pour faster and fresh, still-liquid concrete is added on top before the lower layers can stiffen, so the level-off point moves deeper and the pressure is higher. Pour on a cold day and the stiffening slows down for the same reason. This is exactly what the R and T terms in the equation measure.
ACI 347 lateral pressure formulas
| Case | Formula (psf) | Limits |
|---|---|---|
| Columns (any rate of rise) | p = Cw·Cc·(150 + 9000·R/T) | Min 600·Cw, max w·h |
| Walls, R < 7 ft/hr and pour height ≤ 14 ft | p = Cw·Cc·(150 + 9000·R/T) | Min 600·Cw, max w·h |
| Walls, R < 7 ft/hr with pour height > 14 ft, or R from 7 to 15 ft/hr | p = Cw·Cc·(150 + 43,400/T + 2800·R/T) | Min 600·Cw, max w·h |
| Fully fluid (design ceiling) | p = w·h | w ≈ 150 pcf for normal-weight concrete |
Where: p = lateral pressure (psf), R = rate of placement (ft/hr), T = temperature of concrete in the form (°F), h = depth of fluid or plastic concrete (ft), w = unit weight of concrete (pcf), Cw = unit weight coefficient, Cc = chemistry coefficient.
Coefficient tables: Cw and Cc
| Unit weight of concrete | Cw |
|---|---|
| Under 140 pcf | 0.5·(1 + w/145), but not less than 0.80 |
| 140 to 150 pcf | 1.00 |
| Over 150 pcf | w/145 |
| Cement type or blend | Cc |
|---|---|
| Type I, II or III, no retarder | 1.0 |
| Type I, II or III, with a retarder | 1.2 |
| Other types or blends containing less than 70% slag or 40% fly ash, no retarder | 1.2 |
| Other types or blends containing less than 70% slag or 40% fly ash, with a retarder | 1.4 |
| Blends containing more than 70% slag or 40% fly ash | 1.4 |
A retarder keeps the concrete plastic longer, so the pressure builds higher before the bottom layers set. That is why Cc goes up.
Reading the pressure against pour rate
Two things stand out. First, at slow pour rates the 600 psf minimum controls, so slowing the pour even more does not reduce the design pressure. Second, above roughly 3 to 5 ft/hr (depending on temperature) the pressure climbs in a straight line with the rate of rise, and the cold 50°F mix climbs much faster than the 90°F mix. That is why most blowouts happen on cool days when the crew speeds up.
Free formwork pressure calculator
The calculator picks the right wall formula from your rate and height, applies the tables above, and flags when the minimum or the fluid-head cap controls. Some older editions and secondary sources also quote an extra ceiling of 2,000·Cw·Cc psf, so check the edition your specification names. It does not replace a stamped formwork design.
Worked examples
Example 1: 12 ft wall, normal pour
Normal-weight concrete (150 pcf, so Cw = 1.0), Type I cement with no retarder (Cc = 1.0), R = 4 ft/hr, T = 70°F, h = 12 ft.
p = 1.0 × 1.0 × (150 + 9000 × 4 / 70) = 150 + 514 = 664 psf. Check the limits: minimum 600 psf, maximum 150 × 12 = 1,800 psf. 664 psf governs.
Example 2: Fast wall pour, cold concrete
Same mix, R = 10 ft/hr, T = 60°F, h = 14 ft. Because R is above 7 ft/hr, the second wall formula applies.
p = 150 + 43,400 / 60 + 2800 × 10 / 60 = 150 + 723 + 467 = 1,340 psf. The fluid head is 150 × 14 = 2,100 psf, so 1,340 psf governs. Compare that with 664 psf in Example 1: roughly double the load from a faster pour and cooler mix.
Example 3: Column with retarder
Normal-weight concrete, Type I with a retarder (Cc = 1.2), R = 6 ft/hr, T = 65°F, h = 10 ft.
p = 1.0 × 1.2 × (150 + 9000 × 6 / 65) = 1.2 × (150 + 831) = 1,177 psf. The fluid head is 1,500 psf, so 1,177 psf governs.
What raises or lowers the pressure
| Factor | Effect on pressure | Site action |
|---|---|---|
| Faster rate of rise | Higher | Slow the pour or place in lifts |
| Colder concrete | Higher (stays plastic longer) | Re-check pressure in cold weather |
| Retarder or high slag/fly ash | Higher | Use Cc 1.2 to 1.4 |
| Internal vibration deeper than 4 ft | Can approach full fluid head | Limit vibrator penetration |
| Self-consolidating concrete (SCC) | Often near full fluid head | Design for w·h unless tested |
| Lightweight concrete | Lower (Cw below 1.0) | Use the Cw table |
One point worth repeating: the ACI 347 equations assume ordinary vibrated concrete. For self-consolidating concrete, the guidance is to design for full fluid pressure unless you have test data showing lower values.
From pressure to tie spacing
Once you have p, the load on one form tie is p multiplied by the tributary area it supports:
Tie load (lb) = p (psf) × horizontal spacing (ft) × vertical spacing (ft)
Example: p = 664 psf with ties at 2 ft by 2 ft supports 664 × 4 = 2,656 lb per tie. Compare that with the tie’s rated working load from the manufacturer, which already includes the required safety factor. If the rating is lower than the load, tighten the spacing or choose a heavier tie. Sheathing and stud checks follow the same idea using bending, shear and deflection limits set by the project specification.
Common mistakes that cause blowouts
- Using the fastest pump rate instead of the planned rate. The calculation only holds if the crew sticks to the rate you used. Agree it in the pre-pour meeting.
- Ignoring temperature changes. A cold morning start can raise the pressure well above the afternoon design value.
- Forgetting the mix. A retarder or a high fly ash or slag blend raises Cc and keeps the concrete fluid longer.
- Treating SCC like normal concrete. Self-consolidating mixes can act close to a full liquid.
- Checking only the ties. Sheathing, studs and wales must also carry the pressure within the limits your specification sets.
- Using the wrong wall equation. Walls taller than 14 ft, or poured at 7 ft/hr or faster, use the 43,400/T equation.
Pre-pour checklist
- Confirm the planned rate of rise with the concrete supplier and the pump crew.
- Record the expected concrete temperature, and re-run the numbers if the weather changes.
- Check whether the mix uses a retarder, slag or fly ash.
- Match tie type, spacing and bracing to the pressure on the formwork drawings.
- Inspect ties, wedges and bracing before the pour and again after the first lift.
- Stop the pour if you see bulging, leaking at joints or ties pulling out.
For the basics of form types and materials, read our guide to formwork definitions, types and significance. If you are sizing the element being poured, see column, beam and footing size for a house, and for the concrete mix side check the slump guide with ACI 211.1 tables. Cover and reinforcement details are in clear cover in RCC.
Frequently asked questions
How much pressure does concrete put on formwork?
It depends on pour rate, temperature and mix. Typical wall pours at 3 to 6 ft/hr and 60 to 80°F give about 500 to 900 psf, with a hard ceiling of about 150 psf per foot of fluid depth. ACI 347 sets a minimum of 600 psf for normal-weight concrete.
What is the ACI 347 formula for wall formwork pressure?
For walls poured slower than 7 ft/hr and no taller than 14 ft: p = Cw·Cc·(150 + 9000·R/T). For taller walls, or any wall poured at 7 to 15 ft/hr: p = Cw·Cc·(150 + 43,400/T + 2800·R/T). Pressure is limited to a minimum of 600·Cw psf and a maximum of w·h.
Does cold weather increase formwork pressure?
Yes. Colder concrete sets slower, so it stays fluid to a greater depth. In the equation, a lower T raises the R/T term and the pressure goes up.
Is the pressure the same for SCC?
No. Self-consolidating concrete can exert close to full liquid pressure, so formwork is usually designed for w·h unless test data supports a lower value.
Which units does the formula use?
The equations here use U.S. customary units: psf, ft/hr, °F and ft. ACI 347 also publishes SI versions with kPa, m/hr and °C.
Key takeaways
- Pressure rises with pour rate and falls with temperature.
- Use p = Cw·Cc·(150 + 9000·R/T) for columns and for walls up to 14 ft poured slower than 7 ft/hr; otherwise use the 43,400/T form.
- Never design below 600·Cw psf or above w·h.
- Check ties as p × tributary area and size to the manufacturer’s working load.
- Treat SCC as full fluid pressure unless tested.
Source basis: ACI 347R-14, Guide to Formwork for Concrete. This article is educational and not a substitute for a stamped formwork design.
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