A Look at Hydrostatic Loads on Forms

By Eric Carleton, P.E.

The Hoover Dam holds back a tremendous amount of water, and, depending on the water level in Lake Mead, the resulting pressure can be as high as 45,000 pounds per square foot at its base.

As the dam was built using more than 3.25 million cubic yards of concrete, workers dealt with a similar, albeit far lesser pressure on the formwork. Since fresh concrete is fluid, placing that concrete exerted outward pressure on the forms, which had to be reinforced with outer bracing to keep the form walls from breaking open.

These loads are present whenever a fluid is contained or being restrained from gravity-induced flow. When applied across an area or surface, these loads cause hydrostatic pressure.

Precast manufacturers must consider these loads and the resultant concrete lateral pressure distribution when casting concrete to ensure that the forms resist this pressure to allow the concrete to cure and harden into its designed shape.

Who is Responsible for Form Design Criteria?

Not all precasters have the engineering staff and capacity to construct elaborate formwork on their own, so they must rely on established form manufacturers to design forms that meet the anticipated loading conditions.

Guidance for Specialized Form Design Engineers

ACI Committee 347 – Formwork for Concrete published its first guide on concrete formwork in 1963. This committee has continued to review the latest research and refine the design criteria for formwork with the latest guide, ACI 347R-14, “Guide to Formwork for Concrete.”]

As described within the guide, many factors related to fresh concrete affect the hydrostatic form pressure at any given height and within certain time parameters. The most basic approach and assumption is that fresh concrete is simply a constant heavy fluid for the entire casting. In that case, the concrete lateral pressure distribution can be calculated as:

Ccp = wh where:

• “CCP” is concrete lateral pressure (lb/ft2 or Pa)
• “w” is unit weight of concrete (lb/ft3 or kg/m3)
• “h” is depth of plastic concrete from top of placement to point of consideration in the form (feet or meters)

For normal-weight concrete with a density between 140-150 lbs/ft3 (2,240 to 2,400 kg/m3), the minimum design value of CCP is 600 lbs/ft2 (28.7 KPa), but in no case shall it be greater than wh.

Though easy to calculate, research has shown that the actual developed form pressure of poured concrete can be much less when concrete stiffening begins. Concrete stiffening is when the previously cast concrete in the lower portion of the form begins to set and harden. This stiffening is dependent upon many factors, such as:

• The speed of placement.
• Mix design parameters, including cement type and use of supplementary cementitious materials, retarders, accelerators, etc.
• Concrete temperature at the time of placement.

Formwork for many large cast-in-place building projects constitutes a major expense within the constructed project. Reducing the design pressure of the concrete on the formwork relates to reduced structural bracing to resist this concrete pressure, which in turn simplifies the formwork and reduces the cost of the project.

In addition to the simple formula previously discussed, ACI 347R-14 also provides two more elaborate concrete lateral pressure distribution equations that incorporate concrete stiffening criteria, specifically the rate of concrete placement and the temperature of the concrete at the time of placement.

ACI 347-14 provides guidance that outlines which of the three concrete lateral pressure distribution equations to use. The deciding factors are based on the fresh concrete’s slump, the internal vibration depth and the element’s height.

What About Precast?

Because of the nature of plant-produced precast concrete products, forms typically are not as large or tall as those used in cast-in-place projects. Where cast-in-place routinely can have formwork of 15 to 40 feet and higher, a tall precast form rarely exceeds 10 feet.

Additionally, because the volume of concrete to fill a precast form is relatively small compared to the concrete mixer capacity, the concrete placement is speedy – often measured in minutes – to completely fill a form, while large cast-in-place pours can be measured in feet per hour as provided within the ACI 347R-14 design equations.

Consequently, it is appropriate to assume as a minimum that lateral concrete pressure on precast forms is CCP = wh. However, that is a minimum value for the variety of precast concrete forms and type of concrete being utilized, according to Hamilton Forms Senior Engineer Skip Plotnicki.

Forms for Conventional Wet Cast Conventional

Manufactured wet cast forms from a qualified fabricator are designed to accommodate the fluid concrete pressures described above. Some form producers recommend adding an additional 15% pressure for short-term vibration of lower slump wet cast concrete mixes. Additionally, precast operations often will have the product cast in one location, and the entire formwork and pallet are then moved to another area for curing. In those special cases, the formwork design needs to include considerations for the dynamic loads caused by lifting and moving the precast structures.

Intuitively, large flat rectangular forms in a vertical position require additional steel angle iron reinforcement to resist bending or bowing of the flat form surfaces. Alternatively, forms for manholes and similar products enjoy the benefit of a circular shape, allowing the steel core to handle the loading in ring compression while the exterior jacket is in uniform tension, which reduces the need for extensive structural support.

Regardless of the form shape, it is important to ensure latches and hinges all are in good working order and utilized appropriately. The load path of all the described form pressures is concentrated and passes through the latches. ACI 347R-14 recommends the minimum factor of safety for form hardware to be 2 to 1.

If a form has four latches and two are broken, do not close properly or simply are not latched, then there is a real potential for harsh physical lessons learned on hydrostatic loads on forms.

Forms for Dry Cast Concrete

It often is said that dry cast forms are built like battleships – stout and sturdy. On the surface, this would seem to contradict the form pressure doctrine that implies a stiff concrete, such as dry cast, would exert less hydrostatic form pressure than a high slump conventional concrete.

According to Dave Stoller, global vice president of engineering at Afinitas, although a dry cast mix is more “solid” than wet cast mix, vibration typically is used to consolidate this type of mix. Vibration essentially liquifies the mix, so we assume that the fresh dry cast concrete acts as a pure liquid with a density equal to that of the concrete mix. Static pressure calculations are based on this assumption.

“An even more significant loading is present, and this load is due to pressing in a joint forming ring ‘header’ or ‘profile ring,’” Stoller said. “Most dry cast forming processes involve placing a top ring under pressure to trap and pressurize the concrete mix under vibration. This pressing or press-heading step subjects the forms to relatively large loads.”

The pressure generated by this pressing process is calculated by dividing the pressing force by the projected area of the top ring. This pressure is added to the static liquid force of the concrete mass because of gravity.

“The other major design consideration is due to the vibration, which can range from 2 to 10 G of acceleration,” Stoller said. “The allowable stresses in the forming structures must often be reduced to remain under the fatigue limit of steel. This is a much more stringent requirement than a simple safety factor against yielding. Consequently, dry cast forming is typically built much heavier than equivalent wet cast forming.”

Engineered Form Panels

Engineered form panels are a popular option among precast concrete manufacturers that produce a variety of different-sized or custom products such as wing walls, special drainage inlets or “one-off” vault structures.

Form panels can be made of a variety of materials, including steel, aluminum, wood and plastic. Each material offers different benefits.

Accordingly, each form type has certain defined structural capacity to resist hydrostatic form pressure. This is based on the panel manufacturer’s specific design and is included within the product information.

A panel’s structure and stiffness are built in by the panel producer, but the form system requires erection by the precast concrete producer. This will include side-by-side panel connections using pins or hardware and wall-to-wall connections using wall ties – also referred to as form ties or snap ties. These ties become the critical structural element holding the facing form panels together under the load and resultant pressure of the fresh concrete.

Similar to other forming hardware, the design guidelines are to provide a 2-to-1 working load factor of safety for wall ties. Damaged or missing ties quickly can increase the stress on the remaining ties to a point of failure. This can lead to a bowing of the form filled with concrete, which is difficult if not impossible to correct, or worse, a panel blowout. Both can have severe ramifications.

What About SCC?

While self-consolidating concrete (SCC) is just starting to gain traction in the ready-mix market, it has become a mainstay product for many precast operations. One reason for reticence within the onsite construction world is the initial lack of understanding and research related to the hydrostatic form pressure on formwork. The current design pressure assumption used today is as written within 347R-14, which states:

“When working with self-consolidating concrete, the lateral pressure for design should be the full liquid head unless the effect on formwork pressure is understood by measurement or prior studies and experience.”

However, researchers at institutions such as the University of Illinois1 and Missouri University S&T2 have investigated SCC form pressure to study data and develop testing methods in areas such as reliable pressure decay curve development.

ACI Committee 237 – Self-Consolidating Concrete soon is expected to publish PRC 237.2-21, “Form Pressure Exerted by Self-Consolidating Concrete: Primary Factors and Prediction Models.” This document can then be utilized by ACI Committee 347 to incorporate appropriate vertical form pressure design criteria for SCC within an updated guide.

Understanding of loads in a precast plant during setup, casting, curing and stripping are critical to safety and quality. Many plants use numerous different mix designs in a single form, so the lateral concrete pressure distribution on the formwork will vary with each mix design.

It’s important to make sure forms are designed for the worst-case scenario. Consult with your formwork supplier to ensure your forms are ready for the next challenge. PI

Eric Carleton was the director of codes and standards at NPCA.

References:

1. Formwork Pressure Model for Self-Consolidating Concrete Using Pressure Decay Signature; Jacob D. Henschen, Daniel I. Castaneda, and David A. Lange, ACI Materials Journal Technical Paper 115-M29, 2018 https://www.concrete.org/publications/internationalconcreteabstractsportal.aspx?m=details&i=51702183
2. Effect of SCC Mixture Composition on Thixotropy and Formwork Pressure – Journal of Materials in Civil Engineering, 2012; Ahmed Fathy Omran, Université de Sherbrooke and Kamal H. Khayat, Missouri University of Science and Technology https://www.researchgate.net/publication/263272770_Effect_of_SCC_Mixture_Composition_on_Thixotropy_and_Formwork_Pressure.