Formwork Pressure Calculator
Work out the lateral pressure of fresh concrete on a wall or column form to ACI 347R-14: enter how high and how fast the concrete is placed, its temperature and the cement, and see the largest pressure, the pressure down the form, the total force and the load on each form tie. Free, with no account.
Largest pressure: 793 psf, on a wall form with 10′ of concrete.
Cw Cc [150 + 9,000 R / T] = 1.00 × 1.0 × [150 + 9,000 × 5 / 70] = 793 psf (ACI 347R-14 Eq. 4.2.2.1a, for walls placed under 7 ft an hour and 14 ft high or less).
The pressure down the form, to scale.
The form
| Largest pressure | 793 psf |
| Where it's reachedBelow the top of the concrete | 5′ 3″ |
| Total force on the formPer foot of form | 5,833 lb |
| Where it actsAbove the bottom | 3′ 10″ |
Form ties
| Tie load793 psf on 24 in by 18 in (3.00 sq ft): 2,379 lb against 3,000 lb safe working load | 79% PASS |
Each tie can carry up to 3.78 sq ft of form at the largest pressure. Ties above 5′ 3″ below the top carry less.
- The formula holds for concrete with a slump of 7 in or less, vibrated no deeper than 4 ft below its surface. Revibrating the full height, or a retarder or admixture that slows setting a lot, calls for the full liquid head.
- The pressure grows with the depth of fluid concrete down to 5′ 3″ below the top, then holds at 793 psf to the bottom.
- Check the sheathing, studs and walers for this pressure too, and the bracing for wind and construction loads (ACI 347R-14 4.2.3). Ties are often checked at their maker's safe working load, which includes a safety factor of 2 (ACI 347R-14 Table 4.3.1).
Wall form pressure by rate and temperature
The largest pressure in psf on a wall form, for normal weight concrete (150 pcf) made with Type I, II or III cement and no retarder, placed up to 14 ft high and vibrated internally. Below 7 ft an hour this is ACI 347R-14 Eq. 4.2.2.1a, from 7 to 15 ft an hour Eq. 4.2.2.1b, and above that the full liquid head. A pour shorter than the pressure's depth (the pressure divided by 150 pcf) never reaches it.
| Rate | 40 °F | 50 °F | 60 °F | 70 °F | 80 °F | 90 °F |
|---|---|---|---|---|---|---|
| 1 ft/h | 600 | 600 | 600 | 600 | 600 | 600 |
| 2 ft/h | 600 | 600 | 600 | 600 | 600 | 600 |
| 3 ft/h | 825 | 690 | 600 | 600 | 600 | 600 |
| 4 ft/h | 1,050 | 870 | 750 | 664 | 600 | 600 |
| 5 ft/h | 1,275 | 1,050 | 900 | 793 | 713 | 650 |
| 6 ft/h | 1,500 | 1,230 | 1,050 | 921 | 825 | 750 |
| 7 ft/h | 1,725 | 1,410 | 1,200 | 1,050 | 938 | 850 |
| 8 ft/h | 1,795 | 1,466 | 1,247 | 1,090 | 973 | 881 |
| 10 ft/h | 1,935 | 1,578 | 1,340 | 1,170 | 1,043 | 943 |
| 12 ft/h | 2,075 | 1,690 | 1,433 | 1,250 | 1,113 | 1,006 |
| 15 ft/h | 2,100 | 1,858 | 1,573 | 1,370 | 1,218 | 1,099 |
How the calculator works out the pressure
Fresh concrete pushes on a form like a fluid until it starts to stiffen. The faster it rises and the colder it is, the deeper it stays fluid, and the higher the pressure gets. ACI 347R-14, the American Concrete Institute's guide to formwork, gives the largest pressure for concrete vibrated internally (section 4.2.2.1):
- Columns, with no side over 6.5 ft, and walls placed under 7 ft an hour and no more than 14 ft high: p = Cw Cc [150 + 9,000 R / T] (Eq. 4.2.2.1a).
- Other walls, placed at 7 to 15 ft an hour or more than 14 ft high: p = Cw Cc [150 + 43,400 / T + 2,800 R / T] (Eq. 4.2.2.1b).
- R is the rate of placement in ft an hour, T the concrete's temperature in °F. The pressure is at least 600 Cw psf and at most the full liquid head, the unit weight times the height.
- Cw, for the unit weight w: 0.5 (1 + w / 145), but at least 0.80, below 140 pcf; 1.0 from 140 to 150 pcf; w / 145 above that (Table 4.2.2.1a).
- Cc, for the cement: 1.0 for Type I, II or III with no retarder, 1.2 with a retarder or for a blend with some slag or fly ash, and 1.4 for a blend with a retarder or one with over 70% slag or 40% fly ash (Table 4.2.2.1b).
The formula only holds for concrete with a slump of 7 in or less, vibrated no deeper than 4 ft into it. The form takes the full liquid head when the concrete is self-consolidating, shaken by external vibrators, revibrated, or placed faster than 15 ft an hour in a wall. Concrete pumped in from the bottom of the form takes the full liquid head plus 25% for the pump's surge.
Down the form, the pressure grows like a fluid's, the unit weight times the depth, until it reaches the largest pressure, and holds at that to the bottom. The calculator gives the total force per foot of form and where it acts, and checks each tie for the largest pressure on the area it holds. It doesn't check the sheathing, studs, walers or bracing.
Formwork pressure questions
What is the pressure of fresh concrete on a form?
At most, the liquid head: 150 psf for each foot of normal concrete above the point. Most concrete starts to stiffen before the form is full, so the pressure stops growing partway down. For a wall placed at 5 ft an hour at 70 °F, it stops at about 793 psf, about 5.3 ft down.
What about self-consolidating concrete?
Design the form for the full liquid head, unless pressure measurements on the job show less. ACI 347R-14 allows a lower pressure only when it's been measured for that mix, form and rate.
Does this work for precast forms?
For a form standing upright, like a column, a wall panel cast on edge or a beam's sides, yes. Precast plants often use self-consolidating concrete or external form vibrators, and both call for the full liquid head. A panel cast flat only pushes on its edge forms with the liquid head of its thickness.
How far apart can form ties be?
Each tie can hold the area of form whose pressure adds up to its safe working load. A 3,000 lb tie at 793 psf holds about 3.8 sq ft, for example ties 2 ft apart across and 18 in apart up the form. Ties near the top, above where the pressure peaks, carry less.