Ch. 4 Formwork Properties

Formwork Loads

Wet concrete can weigh from 70 to 160 lbs per cubic foot, depending upon it's makeup. When the concrete's actual weight is unknown, designers use the heaviest possible load based on the dimensions of the structure to ensure that the forms will be strong enough.

The pressure of concrete on a form is measured in pounds per square foot. Concrete places pressure on forms in two directions: The downward pressure caused by the weight of the concrete pressing down on the bottom of a form is called vertical pressure. The pressure exerted by concrete against the sides of a form is called lateral pressure.

Formwork Loads

Lateral pressure changes over time. Concrete places the greatest lateral pressure on form walls while it is wet and as it begins to set. If the concrete is being pumped into place, the pump's force pressurizes the concrete even more. As the concrete hardens, however, lateral pressure gradually decreases to zero.

Example:

0 ft = 0 psf

1 ft = 150 psf

2ft = 300 psf

3ft = 450 psf

4ft = 600 psf

5ft = 750 psf

Many different calculations are used in determining the load on a form, but one simple calculation gives a good estimate of overall lateral pressure:

weight of concrete in lbs. per cubic ft.

x depth of concrete in ft.

Lateral pressure (lbs. per sq. ft.)

weight of concrete in lbs. per cubic ft. x depth of concrete in ft.

Lateral pressure (lbs. per sq. ft.)

Note that the width of the form is not a consideration.

Other factors that affect the actual load on a form:

• Concrete depth-The deeper concrete is, the greater its overall weight. Inside a form, however, lateral pressure is lowest near the top of the concrete and greatest at the bottom. As previously stated pressure is determined by multiplying each foot of the concrete's depth by 160 lbs per square foot. Again, the forms width is not a factor when calculating lateral pressure in this manner.

• The faster concrete is placed into a form, the more pressure is put on the form. During placement, the load is greater at the bottom of the form than the top. The rate of placement is measured in feet per hour.

Other factors that affect the actual load on a form:

• As concrete temperature increases, pressure is reduced. This is one reason why it is desirable to place concrete in warmer weather. Also, concrete tends to set faster at warmer temperatures-about one hour at 70 degrees Fahrenheit.

• If concrete is placed into the form in multiple deposits or (lifts), the upper layers must be mixed with the lower layers in a process called consolidation, by using an internal vibrator. This mixing can be done in various ways, but usually result in additional pressure on the form. Although this pressure is temporary, it can be enough to break a form that has not been designed to withstand it.

Live load

Other factors that affect the actual load on a form:

• A live load is any moving weight, such as people, equipment, or materials that may be placed on the form.

The following are other kinds of stress that formwork must also withstand.

Dead load

• A dead load is any non-moving weight, such as the weight of the formwork itself or of any structural components supported by the formwork.

Strength

All formwork materials are rated according to the amount of stress they can withstand. Of course, some materials (such as steel) are naturally stronger than others (such as plastic).

Successful formwork material supports its load without the following failures:

Manufacturers and industry experts have spent decades studying the strength and stress-resistance of formwork materials. As a result, form designers have access to very specific information about the performance characteristics of various materials.

• Deflection- A form bends when placed under stress. For example, a form panel might bulge outward between its supporting studs, giving the finished concrete a "bowed" appearance.

• Blow out- A form ruptures when panels separate from one another at a joint, or when a break or split occurs within a panel. At best, this type of rupture results in minor concrete leakage or marring of the concrete's surface. At worst, a blow out can lead to total form failure.

Flexability

Plywood's flexability depends largely on its thickness. Plywood's ability to be bent is measured as its minimum bending radius, which is the smallest radius to which the plywood can be curved without damage.

In some cases flexability is as important as strength in formwork. Plywood, for example, is often used to create custom curved wall forms. It is important to use panels that have been rated for such uses to make sure the form will retain its shape without breaking.

For example:

A 1/4" thick dry plywood sheet can be curved to a radius of 2' if the sheet is bent across the grain; the same panel can be curved to a radius of 5' if the sheet is bent parallel to the grain. The thicker the plywood, the larger its minimum bending radius.

Fiberglass-reinforced plastic forms can also be used to create curves. Such forms can be bent to the correct radius by the manufacturer, while some are available as flat sheets and can be curved at the jobsite.

Tighter curves can be achieved by wetting or steaming the plywood before bending. In another technique, called kerfing, shallow grooves are cut into the back side of the panel, allowing it to be bent more freely.

Economy

In some construction projects-especially when large, custom, job-built forms must be used- the formwork can cost more than the concrete itself. At times, formwork is the single most expensive part of the entire job.

Contractors must manage formwork costs in order to be competitive. Here are some methods they use to do so:

Minimizing waste-

Form designers can minimize waste by designing forms to use standard-size components as much as possible. When standard-size materials are used, they require less trimming, reducing waste and lowering labor costs.

Prefabrication-

Time on the jobsite is precious, and workers should spend as little time as possible waiting for form components to be fabricated. This problem can be minimized by making as many form components as possible beforehand. Some contractors accomplish this by setting up workshops on the jobsite to produce formwork before they are actually needed.

Economy

Contractors must manage formwork costs in order to be competitive. Here are some methods they use to do so:

Ensuring ease of setup-

A detailed project plan can determine the best place to start forming, where and when forms can be moved for reuse, how long individual forms must stay in place, and other important aspects of the job. This kind of planning saves time and labor costs on the jobsite.

Maximizing reuse-

With proper maintenance and storage job-built and manufactured forms can be made to last as long as possible. Contractors should care for manufactured forms following their instruction, while wooden job-built forms should be stripped as soon as possible, cleaned, and oiled regularly, and repaired as needed.

Ensuring fast stripping-

Some types of forms can be stripped in much less time than others. These forms should be used when they are appropriate. Manufactured form systems can simplify the stripping process, and ganged forms can be moved quickly from one place to another.

Forms cannot support themselves-they need additional support and alignment. Workers can use the following methods to add strength and stability to a form:

Aligning and Extra Support

• Usually placed on the side or the end of a form wall, braces provide lateral support to align the wall. Job-built bracing systems are typically made from dimensional lumber. Ideally, the brace rests at a 45 degree angle, and is usually attached to the wall's top plate, a waler, or a strongback.

• Using stakes provides additional support to short form walls. Metal or wooden stakes can be driven into the ground next to the forms, so their exposed shaft rests against the form wall's exterior. They are usually driven flush with the top of the form. Stakes are also used to hold braces and form aligners in place

Aligning and Extra Support

• A shore is a device that provides either vertical or lateral support to a form. When multiple posts are set up in a row and columns under the formwork, diagonal and horizontal bracing is usually installed between them. Shoring can be made from dimensional lumber, but manufactured shoring systems are made from tubular steel or aluminum frames. Shoring is critical to many form systems , but especially to structures such as floor decks, roof decks, girders, and beams.

• Reshoring is the practice of installing new shoring before or as the original shoring is removed, to give the concrete continued support. It is especially important in multistory buildings where the new floor decks must support the dead load of the floor above them in addition to the live loads.

• A manufactured device, called a turnbuckle, can be used with, or instead of, job-built bracing. Once the turnbuckle is attached to the form wall, it can be adjusted to plumb the wall.

Inspecting Formwork

Experienced formbuilders monitor and closely inspect the form system just prior to and during placement. They check every aspect of the formwork and are usually authorized to make repairs immediately if problems are found.

From fabrication to stripping, formwork should be continually inspected for potential problems, such as gaps in seams, loose form ties, or improper shoring. These inspections may be done by an engineer, the contractor's representative, or a licensed inspector.

Form watchers make sure that forms do not shift or deflect out of allowed tolerances. To verify that forms stay put during placement, carpenters may use a telltale, a device (such as a series of marks) that indicates movement of a form.

Strong, well designed formwork resists failure, but even the strongest forms can fail under the right circumstances. Poor workmanship often plays a role in formwork failure.

Reasons For Form Failure

• Insufficient support- If studs are spaced too far, too few ties are used, or shoring is too light weight, then the form may not hold up under the pressure of the placement.

• Inconsistencies in fabrication- Minor weaknesses in a form can cause big problems when the form starts to fill with concrete. Components that must bear high pressures include corners, bulkheads, and the base of sloped form walls. Inconsistencies in nailing or securing form ties can cause some panels to be weaker than others.

• Poor soil conditions- Soil should be tested to make sure it is well compacted, level, and strong before shoring is set up. Water runoff can wash soil out from underneath shoring, causing it to collapse. Contractors should correct drainage problems in the area before shoring is constructed.

Reasons For Form Failure

• Improperly set shoring or reshoring- Shoring must be set up plumb, attachments must be tight, and adequate bracing must be installed. Even the smallest oversight-such as neglecting to nail wedges in place or using undersized mudsills under the shoring posts-can lead to failure.

• Early removal of shoring- As mentioned earlier, shoring must remain in place until the concrete achieves the desired strength. Removing shores too soon can rob the concrete of needed support.

Mudsills are planks placed under shoring to transfer its load across a larger area of ground

Reasons For Form Failure

• Overburdening with live loads- In addition to the weight of the concrete itself, forms must support dead and live loads. Designers use 75 lbs. per Sq. Ft. for live loads, but this amount can easily be surpassed. Too many workers, too much stored materials, or too much equipment can place a highly concentrated load on one area of a form, leading to failure.

• Poor control of placing- Good form design accounts for the amount of concrete to be placed in a form, the rate at which it is placed, and the points where placing starts and stops. If the placing is not controlled properly, one portion of the form can become overloaded and fail or may cause another part of the form to fail.

For Example- When concrete is placed at one end of a deck form, its weight can cause the other end of the deck to rise up. This deflection must be anticipated.

Constantly be on the lookout for signs of trouble. Every carpenter is responsible for safety, and this includes watching for warning signs that forms are failing or endangered.

If a potential problem on a jobsite is spotted, no matter how small it may seem, correct the problem or report it to a supervisor immediately. Actions such as these save lives!