Ten frequently asked questions about wall formwork (part one)

Ten frequently asked questions about wall formwork (part one)

1

Industrialized vertical formwork has become a common part of the construction process on on-site concrete structures. They allow fast and well-defined production cycles, guaranteeing greater safety for operators during assembly, concreting and stripping than traditional formwork.

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Doubts or questions may arise during its use. Its use is simple, but some technical aspects must be taken into account.

Let’s analyze the 10 most-frequently asked questions about wall formwork!

1.- What is the maximum admissible filling height?

The maximum height for concreting walls with industrialized formwork depends on many factors:

• Maximum design pressure: Each panel type has been designed for maximum resistance to pressure. Industrialized formwork is usually designed for maximum pressures between 40 and 80 kN/m².
• Fluidity of the concrete used: More liquid concretes generate higher pressures. Special attention must be paid when using self-compacting concrete “SCC”. (See question #7).
• Filling time: A faster concrete filling speed generates greater pressure on the formwork. When using more fluid concretes at significant heights, it is essential to maintain a low filling speed, which the formwork supplier must indicate in dedicated tables.
• External temperature: influences the curing of the concrete: the lower the temperature, the longer the setting time and the greater the formwork’s exposure to the load.
• Another factor to take into account is the concrete’s vibration, which makes the concrete fluid, maintaining pressures and delaying its setting. Deep vibrations can subject the formwork to very large loads. E.g. Above 6 m in height, it is more difficult to vibrate the concrete with a needle vibrator normally used on site. External wall-mounted vibrators are therefore used, whose positioning must be studied correctly. (See question #9).

2.- What factor most affects the concrete finish?

Sometimes, once the panel has been stripped, stains with different shades or “small holes” may appear in the concrete finish. There may be several reasons for this. Stains are usually caused by the use of a low quality or inadequate release agent, generating reactions with the concrete in some areas. Small holes or cracks are mainly due to poorly executed vibration or poor choice of concrete. Both effects can also be caused by inadequate formwork cleaning.

In general, for architectural or eye-catching finishes, it is advisable to use phenolic finish panels, since they have a glossy finish due to their very-low-porosity surface.

3.- Is it possible to manually handle vertical formwork?

Wall formwork is divided into three main families: manual, lightweight and robust.

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• Lightweight formwork: These are metal-framed panels, with a weight that generally does not exceed 20 kg/m². These panels are manipulable, but have the disadvantage of being less resistant to concrete pressure “around 40kN/m²”, and having a shorter service life. They are mainly used in countries where the crane is still little used or for construction solutions where a crane is not available.
• Manual formwork: these panels are halfway between the Robust and the Lightweight panels, combining factors that define one and the other, usually weighing between 30 and 60kg/m², providing them with manual portability and usually resisting pressures between 40 and 80kN/m².
• Robust Formwork: “Robust” panels are designed to be subjected to heavy loads, and can usually only be handled by mechanical lifting equipment. These panels usually weigh in excess of 60 kg/m².

4.- What is the standard vertical formwork equipment for a construction site?

Often when planning a new construction project, the well-known question arises: How many square meters of formwork do I need to execute my project?

It is difficult, if not impossible, to define equipment that is standard for every job site. Each project has its own unique aspects. However, it is possible to define the criteria for identifying the optimum equipment for each site:

• Number of operators: First of all, it is important to define the number of workers and crews that will handle the formwork. On large construction sites, more teams of workers are employed simultaneously, with formwork team needed for each team.
• Number of cranes: The formwork equipment should not be oversized in relation to the number of cranes on site. Sometimes there is a risk of slowing down production because the formwork equipment is larger than the handling capacity on site.

On-site production times: It is clear that for the execution of walls, times must be respected that, under standard conditions, can be controlled without problems. However, it is true that on some construction sites, the time required by the developer means that these deadlines need to be shortened, so the number of workers and formwork equipment on site has to be increased.

5.- What is the useful life of metal formwork with phenolic resin as formwork surface?

The durability of wall formwork basically on three variables:

• Formwork material: The service life of the frame is different depending on the material and its quality, the materials chosen to define industrialized formwork is usually aluminum or steel. Aluminum is a less impact-resistant material and therefore has a shorter service life than steel.
• Quality and protection: The quality and protection of the frame influences the service life of the frame. Galvanized steel frames guarantee a longer service life compared to painted or galvanized frames.

Use on site: On-site use is of course an important factor that has a direct impact on the service life of the material. The construction site is a place where the formwork is exposed to shocks and extreme weather conditions. The treatment, climate and maintenance have a decisive influence on the durability of the formwork.

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We will be building forms for my wall this weekend. The wall is 4ft high and will be 8" thick. I have some great heavy 3/4" plywood. The plan is to screw 2x4's to the back of the plywood around the perimeter, then two verticals in the middle which will leave about 30" in between them. DO I NEED 3 VERTICALS INSTEAD? Then we plan to use 3/8 all-thread/nuts/washers to keep the forms from spreading. WILL 12 TIES ON EACH 4X8 BE ENOUGH TO KEEP IT FROM SPREADING? Obviiously we will also be blocking the bottom and bracing it.

We will be building forms for my wall this weekend. The wall is 4ft high and will be 8" thick. I have some great heavy 3/4" plywood. The plan is to screw 2x4's to the back of the plywood around the perimeter, then two verticals in the middle which will leave about 30" in between them. DO I NEED 3 VERTICALS INSTEAD? Then we plan to use 3/8 all-thread/nuts/washers to keep the forms from spreading. WILL 12 TIES ON EACH 4X8 BE ENOUGH TO KEEP IT FROM SPREADING? Obviiously we will also be blocking the bottom and bracing it.

I would go with the uprights 24"minimum but 16" on center would be better. Make sure the 2x4 is on edge and not laying flat on the wide side. If you vibrate the wall just be careful because that will put a lot more pressure on the wall. Not sure how long the wall is going to be but what I have done before is to use 3/4" pvc cut to the width of the wall in your case 8" install in the wall and slide your rod through. this way you can tighten the wall up so its not as flimsy. After stripping the wall you can grout the hole from the pvc. The more up rights the stronger the form will be. If you go the 30" I think you might have bows in between them. Thanks everybody so far. Just called and checked everywhere at lunch. The only place here in town that even has the snap ties is the concrete plant and they don't have any wedges or clips to go with them....DUH. 2 local rental companies don't have any forms or attachments for rental.
Plan right now is to screw 2x4's (on edge) at 2 ft spacing and run four rows of three all-thread rod with large washers and nuts. I'm also thinking drilling holes in the footer (3 per each 4x8) and pinning will keep the bottom in place. Maybe concrete nails would also work for this?

Also thinking just spading the walls and not using a vibratory tool. Might be a safer bet. 16" o.c.. Use a 3/16 hammer drill bit, 16 penny duplex nails and wire to hold form to footer. Drill hole, place wire into hole, drive nail down hole. I'd nail on 16" centers. Nail cleats across your tops at 4' o.c.

We use a lot of 14" timber screws instead of Ties. We pull the screws late in the day or first thing the next.

I would put a brace at 2' and run your All thread through it. As close as possible to the vertical studs.

You might ask if the concrete supplier has pencil rod. It is easy to work with and uses different clips to clamp the wire.

I would go with the uprights 24"minimum but 16" on center would be better. Make sure the 2x4 is on edge and not laying flat on the wide side. If you vibrate the wall just be careful because that will put a lot more pressure on the wall. Not sure how long the wall is going to be but what I have done before is to use 3/4" pvc cut to the width of the wall in your case 8" install in the wall and slide your rod through. this way you can tighten the wall up so its not as flimsy. After stripping the wall you can grout the hole from the pvc. The more up rights the stronger the form will be. If you go the 30" I think you might have bows in between them.

Agreed, but my minimum would be 16" centers for whalers ( studs ) and ties ( all thread ) and must be tied at the very bottom, for this I use perforated steel band strap (available at concrete supply ). Overbuild your forms, when you think you've built them beefy enough, you haven't. If you don't use snap ties use the perf band it is very strong and easy to work with. Just be aware an 8" pour exerts more pressure on the forms than a 12" pour due to what is called the angle of repose and 1yd is greater than lbs. I cannot stress enough how much pressure will be on those forms.

Agreed, but my minimum would be 16" centers for whalers ( studs ) and ties ( all thread ) and must be tied at the very bottom, for this I use perforated steel band strap (available at concrete supply ). Overbuild your forms, when you think you've built them beefy enough, you haven't. If you don't use snap ties use the perf band it is very strong and easy to work with. Just be aware an 8" pour exerts more pressure on the forms than a 12" pour due to what is called the angle of repose and 1yd is greater than lbs. I cannot stress enough how much pressure will be on those forms.

Pstnbly, I hate to call you out on this but you are wrong, the angle of repose has no bearing on liquid, concrete is in a liquid state when placed. Concrete exerts a force of 150 psf per cubic foot on formwork, meaning a concrete wall 4' tall placed at the rate of 4' an hour exerts a pressure of 600 psf at the bottom of the formwork. If you slowed the pour down to 2 feet an hour the pressure would only be 300 Psf. Also the thickness of the wall has no bearing on the pressure exerted on the formwork of a vertical pour. It does not matter if the wall is 6" or 2' thick the liquid pressure pushes equally down and to the sides at 150 psf Per foot of height as a rule.

Pstnbly, I hate to call you out on this but you are wrong, the angle of repose has no bearing on liquid, concrete is in a liquid state when placed. Concrete exerts a force of 150 psf per cubic foot on formwork, meaning a concrete wall 4' tall placed at the rate of 4' an hour exerts a pressure of 600 psf at the bottom of the formwork. If you slowed the pour down to 2 feet an hour the pressure would only be 300 Psf. Also the thickness of the wall has no bearing on the pressure exerted on the formwork of a vertical pour. It does not matter if the wall is 6" or 2' thick the liquid pressure pushes equally down and to the sides at 150 psf Per foot of height as a rule.

So you are trying to tell me that the force exerted on lets say an 8" pour 4' high at the bottom of the form is the same as the force exerted at the bottom of a 24" x 4' pour and that the angle of repose has no bearing on that. You must have read that in some book I don't know about. Have you ever heard of a slump test? If the angle of repose had no bearing on concrete a slump cone would be irrelevant. As someone who has been placing concrete for 40yrs I will tell you that's wrong, concrete in its liquid state is not a pure fluid as the aggregate plays a part. And the aggregate based on the sharpness ( faceting ) and size affects the angle of repose.

So you are trying to tell me that the force exerted on lets say an 8" pour 4' high at the bottom of the form is the same as the force exerted at the bottom of a 24" x 4' pour and that the angle of repose has no bearing on that. You must have read that in some book I don't know about. Have you ever heard of a slump test? If the angle of repose had no bearing on concrete a slump cone would be irrelevant. As someone who has been placing concrete for 40yrs I will tell you that's wrong, concrete in its liquid state is not a pure fluid as the aggregate plays a part. And the aggregate based on the sharpness ( faceting ) and size affects the angle of repose.

That's what I'm telling you. The only time concrete thickness comes into play is when you are pouring a one sided form against a slope. The slope causes the concrete to move more horizontally causing more form pressure, verses straight down as in a form. Once again this is hydrostatic pressure we are talking about.

As for "hearing of a slump test" a cone test measures the fluidity of concrete, again we are dealing with liquid pressure. The slump cone is just a standerization method for measuring concrete wetness- again hydrostatic pressure.

You would be very hard pressed trying to find any mention of " angle of repose" in any concrete literature. Anything you may find will be referring to dry granular material, not concrete.

As for my experience, I am a structural concrete superintendent with 30 years experience with some of the most complex concrete structures in the Pacific Northwest, as well as a member of the American Concrete Institute. In a slow year I would still have placed 10,000 yards of concrete.
And I am writing a book on the subject.