Compute the design lateral pressure P_max that fresh concrete exerts on formwork using ACI 347R-14. Adjust the pour rate, concrete temperature, cement type, admixture and wall/column shape to see design pressure, equivalent head, plywood allowable span, form-tie spacing and setting time in real time.
Parameters
Pour rate R
m/h
Concrete temperature T
°C
Unit weight w_c
kN/m³
Member shape
Wall / column equations differ
Cement type
Auto-sets cement factor C_c based on hydration speed
Slump
mm
Admixture
Auto-sets C_sp factor by flowability
Pour height h
m
Used for the hydrostatic upper-bound cap w_c·h
Vibrator insertion depth
m
Over-vibrating into set layers re-liquefies and increases P_max
Results
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Design pressure P_max (kPa)
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Equivalent head h_eq (m)
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Plywood allowable span (mm)
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Tie spacing (m)
—
Setting time (hr)
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Flowability factor C_w
—
Formwork, pour and pressure diagram
Left: form with rising pour. Centre: triangular ACI 347 pressure capped against the hydrostatic limit. Right: form-tie layout. Colour shows the P_max safety level (green → orange → red).
Plywood allowable span L_ply (t=18 mm, σ_a=7.5 MPa) and tie spacing s_tie (F_tie=30 kN). They make it easy to see how the design pressure drives the wall and tie layout.
Concrete Formwork Pressure ACI 347 — pour rate & hydration
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When concrete is poured into a form, isn't the pressure just the same everywhere, like water?
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It isn't, no. Right after placing, fresh concrete behaves like a fluid, so the bottom of the form does see hydrostatic pressure w_c·h. But as cement hydrates, the lower layers start to gain stiffness and stop pushing as hard. The real lateral pressure profile is therefore a triangle that flattens at some depth. Try the default values on the left — a wall at R=2 m/h gives a P_max of about 45 kPa, which is the textbook baseline.
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OK, what if we crank R higher?
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Push R up to 5 m/h on the slider. P_max climbs and almost matches the hydrostatic value w_c·h. ACI 347 splits walls into three regimes: low-rate up to 2.1 m/h, medium-rate up to 4.5 m/h, and "too fast for hydration to help" above that, where you simply use w_c·h. Columns use a stiffer coefficient (785 vs 720), and both equations are always capped by the hydrostatic bound — you can't exceed a fluid column.
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Temperature and admixtures matter a lot, too — selecting polycarboxylate suddenly makes P_max jump. Why?
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Hydration is slowed, so the concrete keeps acting like a fluid for longer. Cold temperatures do the same thing. ACI 347 puts polycarboxylate at C_w = 1.4, naphthalene at 1.2, lignosulfonate at 1.0, and no admixture at 1.0. Self-consolidating concrete (SCC) is the headline case: many tie-rod failures have happened because designers assumed standard formwork was fine. In the US the 1971 Skyline Plaza collapse (Virginia, 14 deaths) is the textbook example of underestimated formwork pressure.
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Once you have a design pressure, what do you actually do with it?
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Three things at the same time: plywood thickness and stud span, form-tie size and pitch, and the wales/whalers count. This tool assumes 18 mm plywood (7.5 MPa allowable bending) and 30 kN ties to spit out a quick L_ply and s_tie. In practice you cross-check against catalogue span tables from PERI, Doka, Meva or NOE-Schaltechnik and use the more restrictive value. JASS 5 and the Japanese formwork code play the same role on Japanese sites.
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What is the setting-time number for?
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It tells you how long you can pause a pour and when the next lift can go in. The model is calibrated to about 6 hr at 20 °C, and grows exponentially as temperature drops — about 14 hr at 10 °C. That's why winter pours so often get cold joints if you stop overnight. In summer the opposite problem appears: 4 hr at 35 °C, so long-distance trucking becomes risky.
Frequently asked questions
ACI 347R-14 is an empirical formula that captures how fresh concrete gains rigidity as hydration and setting progress while the pour rises. The standard wall equation is P_max = w_c·(7.2 + 720R/(T+17.8))·C_w·C_c, where R is pour rate (m/h), T is concrete temperature (°C), and w_c is unit weight (kN/m³). It is then capped by the hydrostatic upper bound w_c·h so the result never exceeds a fluid concrete column. Faster pours, lower temperatures and stronger admixtures all push P_max upward.
Walls extend in two horizontal directions, allowing some lateral relief in slow pours, so the wall equation has a smaller coefficient (720). Columns are confined on four sides and are usually poured rapidly, so ACI 347R-14 uses a stiffer formula P_max = w_c·(7.2 + 785R/(T+17.8))·C_w. Wall equations also switch form when R exceeds 2.1 m/h and revert to hydrostatic above 4.5 m/h. Treating a confined wall pocket as a wall instead of a column has caused several failures and is a common pitfall.
Yes. The ACI 347 admixture factor C_w (or C_sp) is set to 1.0 with no admixture, 1.0 for lignosulfonate, 1.2 for naphthalene, and 1.4 for polycarboxylate (high-range water reducer). The reason is delayed setting and very high flowability: the concrete stays liquid longer and keeps pushing the form. Self-consolidating concrete (SCC) pours on tall columns and thin walls have caused several tie-rod ruptures worldwide, and many investigations point to designers leaving C_w at 1.0.
Plywood allowable span is the maximum centre-to-centre spacing of vertical wales (whalers); tie spacing is the form-tie pitch. This tool assumes 18 mm plywood with 7.5 MPa allowable bending stress and 30 kN tie capacity to give a quick-look value. In practice cross-check against the published span tables from PERI, Doka, NOE-Schaltechnik and others, and take whichever is more restrictive. For high pressures step up to 24 mm plywood and high-strength ties.
Real-world applications
High-rise RC cores and columns: in tall RC buildings, each lift is typically 3–4 m placed in a single sequence, with pour rates often above 5 m/h. P_max practically equals the hydrostatic bound w_c·h. Jump and slip forms — PERI ACS, Doka SKE — carry the pressure with post-tensioned tie rods. Setting R=5 and h=4 in this tool shows P_max hitting the hydrostatic cap immediately.
Bridge piers, dams and mass concrete: on bridge piers and gravity dams, designers deliberately drop the pour rate below 1 m/h to suppress P_max — a classic "low-rate placing" strategy. At R=0.5 in this tool, P_max falls well under hydrostatic and thinner plywood with fewer ties becomes feasible. The cost is longer construction time, which is a deliberate trade-off.
SCC (self-consolidating concrete) thin walls: SCC is highly flowable and almost always uses polycarboxylate superplasticisers, so even ACI 347's correction underestimates the measured P_max by 20–50% in some studies. The CIRIA Guide 108 (UK) and DIN 18218 (Germany) include dedicated SCC equations, and thin walls are particularly vulnerable. Selecting "polycarboxylate" here applies the C_w=1.4 multiplier immediately.
Formwork failure forensics: the 1971 Skyline Plaza collapse (Virginia, 14 deaths), the 2003 Heathrow car-park failure (UK), and the 2019 Hard Rock New Orleans collapse (US, 3 deaths) all involved formwork-pressure underestimation among other factors. Forensic teams reconstruct the actual R, T and admixture conditions, recompute P_max and compare it to tie and shoring capacities. A quick-look tool like this is exactly what you want for the first pass.
Common misunderstandings and pitfalls
First, "mixing up the wall and column equations". The 720 (wall) vs 785 (column) coefficient is only 9% different, but the column equation has no low-rate cap — it simply scales linearly with R. Several failure investigations have found that engineers treated a confined wall pocket as a regular wall and ended up underestimating P_max by 30%. When the geometry is anything but a clearly long wall, default to the column equation (the conservative choice).
Second, "leaving the admixture factor at 1.0". Almost every modern mix uses polycarboxylate-based high-range water reducers, but the ACI 347 default assumes no admixture (C_w=1.0). Always pull the mix design and confirm the admixture family — for polycarboxylate apply 1.4×, for naphthalene 1.2× — at the start of the project. Picking "None" in this tool will give you a comfortably low P_max that may not match site reality.
Third, "believing over-vibrating cannot push P_max above hydrostatic". The opposite is true: a vibrator driven deep into the already-stiffening lift re-liquefies it and pressure spikes. ACI 347 explicitly says the vibrator depth must not exceed the current lift. The vibrator-depth slider here is for awareness only — make sure your crew enforces the rule on site. Post-Skyline-Plaza investigations suggested over-vibration could have lifted P_max by 30–40%.
How to Use
Enter concrete pour rate (m/hr) between 0.3 and 3.0—typical wall pours range 0.5–1.5 m/hr for standard slump concrete
Input concrete temperature (°C), slump (mm), and unit weight (kN/m³)—use 24 kN/m³ for normal-weight concrete, adjust for lightweight or high-strength mixes
Click Calculate to obtain P_max lateral pressure (kPa) per ACI 347R-14, equivalent hydrostatic head, plywood allowable span, required tie spacing, and setting time
Worked Example
Wall formwork with pour rate 1.2 m/hr, concrete temperature 18°C, slump 150 mm, unit weight 24 kN/m³. ACI 347R-14 equation: P_max = γ_c × h, where h = min(h_cc, 4.8 ft + 61 R) applies unless temperature and rate exceed thresholds. For this case: P_max ≈ 42–48 kPa, equivalent head h_eq ≈ 1.8–2.0 m. Plywood allowable span for Class 1 is ~300–350 mm with 19 mm thickness at 16 in. spacing. Tie spacing required is ~0.6 m on center to resist 48 kPa load on 150 mm thick form facing.
Practical Notes
Cold concrete (below 10°C) extends setting time significantly; reduce pour rate below 0.8 m/hr or use heated concrete and forms to maintain workability and pressure prediction accuracy
High slump (>200 mm) increases lateral pressure proportionally; high slump plus warm temperature and fast pour rate triggers maximum P_max—consider tremie placement for deep walls
ACI 347R-14 P_max formula caps at 4.8 ft + (61 R/T) where R is pour rate ft/hr and T is concrete temperature °F; always verify that formwork tie capacity exceeds computed spacing requirement by 20% safety factor